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· A new numerical model for frosting is developed. · Phase change model based on nucleation theory is modified. · Packing limit value of frost is corrected. · A new method for calculating two-phase average temperature of frost is proposed. · Multiple parameter verification shows that the error of the new model is within 15 %. Cold surface frosting is a complex process involving multi-component phase change heat and mass transfer and variable void structure. It is very challenging to deeply understand the frosting law and mechanism and accurately simulate the frosting process. Existing numerical models of frost growth have many shortcomings, such as a narrow application range, few verification parameters and low reliability. For these reasons, this paper developed a numerical model of frost layer growth based on nucleation theory, modified the phase transition model, and proposed a formula for constraining the picking limit of the frost layer. Additionally, it introduced a formula for calculating the average temperature of the two phases in the frost layer. The simulation results for frost layer thickness, density, temperature, and distribution were extensively verified using experimental data obtained by different scholars under various conditions and compared with simulation results in related literatures. The results show that the simulation results of all parameters under different conditions are in good agreement with the experimental data, with a maximum deviation of less than 15 %. Notably, the simulation results for frost thickness and distribution are more accurate than those found in existing literatures. The model developed in this paper offers a higher number of validation parameters, a wider validation range, and greater accuracy and reliability. This model can provide a solid theoretical foundation for studying frost characteristics under complex conditions and for the optimal design of frost-prone heat exchangers. [ABSTRACT FROM AUTHOR]

• The liquid metal cross flow tube bundle under pulsation condition is simulated. • The effect of different pulsation frequency and amplitude are analyzed. • Circumferential flow field and temperature field of the single tube are analyzed. • The vortex characteristics and turbulent structure are analyzed with PSD and CWT. Liquid metal has a broad application prospect in tube bundle heat exchangers because of its excellent thermal conductivity. However, conventional in-line tube bundle arrangements often exhibit limited secondary flow, so new methods are urgently needed to improve heat transfer. The topic of this paper is to combine two methods, liquid metal cross flow tube bundle and pulsating flow, to improve the heat transfer performance in-line tube bundles. Firstly, the applicability of the model was validated through experimental data from existing literature, encompassing overall flow dynamics, heat transfer performance, and circumferential pressure distribution around the tubes. Subsequently, numerical simulations are conducted involving liquid metal cross flow in-line tube bundles at varying pulsating frequencies, amplitudes, and Reynolds numbers. Circumferential pressure distributions and temperature profiles for individual tubes were analyzed and compared. The amplitude of reduction in thermal resistance compared to the no-oscillation condition increases gradually from a minimum of 8 % in the first row to a maximum of 40 % in the third row, while the reduction in the rear row of tubes is similar to that of the third row. Remarkably, a reversal in the trend of thermal resistance for the first three rows of tubes under pulsating flow were observed, contrasting conventional flow except for the case with an amplitude of 0.1. By analyzing the local transient flow field distribution near single tube and the corresponding circumferential heat transfer performance, the influence of the vortex evolution characteristics on the heat transfer performance at different phases of the pulsating velocity is revealed, which transient Nu is up to three times that of flow with no pulsating. Finally, a series of coherence analyses reveal that as frequency rises, turbulence exhibits a richer spectrum of small-scale vortex structures, intricately linked to enhanced energy cascade efficiency. Amplitude augmentation intensifies velocity fluctuations, particularly at high amplitudes, resulting in substantial energy peak amplification. The integrated heat transfer performance PEC factor varies in the range of 1.25 to 1.51 for all the conditions simulated in this paper, which indicates that the liquid metal coupled pulsating flow approach can significantly increase the comprehensive performance of the tube bundle heat exchangers. These findings hold significant promise for advancing heat exchanger design and enhancing thermal efficiency, with implications for various engineering applications. [ABSTRACT FROM AUTHOR]

• Flow boiling of refrigerants in small to micro tubes and channels. • Two-phase pressure was obtained for different refrigerants, geometries and operating conditions. • Past pressure drop correlations did not compare well with current data bank on two-phase pressure drop. • A new correlation for predicting two-phase pressure drop in flow boiling in small to micro passages is proposed. An accurate and acceptable correlation for the prediction of two-phase pressure drop is considered a crucial step in heat exchanger design. Although many existing models and correlations were developed and proposed in the past, their ability to predict within acceptable error bands when applied in general to flow boiling flows is limited, even within their originally recorded ranges. The discrepancies are worse when predicting two-phase pressure drop in small to micro-scale passages. Therefore, the aim of the work described in this paper is to assess the most well-known models and correlations using a large experimental data-bank. The data-bank includes four refrigerants, namely R134a, R245fa, HFE-7100 and HFE-7200, small to micro heat exchangers made of different metals and different channel configurations. In addition, the data cover a large range of operating conditions, which can allow generally applicability of new correlations developed. This range covers channel hydraulic diameter of 0.46−4.26 mm, heated length of 20−500 mm, system pressure of 1 − 14 bar, mass flux of 50−700 kg/m2 s, wall heat flux ranging from 2 to 234 kW/m2 and exit vapour quality up to one. Twenty six existing models and correlations that were developed and proposed for vertical/horizontal flow, single/multi-channels and circular/non-circular channels were evaluated. Moreover, the effect of using different equations for calculating the two-phase mixture viscosity, void fraction, Fanning friction factor and Lockhart–Martinelli parameter was assessed and discussed. The mean absolute error of the existing correlations when compared with our data-bank was more than 30 %. Therefore, a new correlation for calculating the two-phase multiplier, which was strongly dependent on the Boiling number and the Lockhart–Martinelli parameter, was developed and then the frictional component and total two-phase pressure drop relationships were completed. [ABSTRACT FROM AUTHOR]

• Steady-state measurement set-up for TCR under high-temperature vacuum is built. • Influence of liquid metal, thermal grease and graphite film on the TCR is analyzed. • The TCR between copper and graphite was reduced by 91 % by brazing method. • The TCR between palladium and graphite is reduced by 95 % by adding gallium alloy. • The temperature of the target is successfully controlled within 900 ℃. The high thermal contact resistance (TCR) between the target and the heat exchanger in the Isotope Neutron Source Accelerator will cause the high-energy charged particle beam to stay in the target, resulting in a remarkable increase of temperature which affects the production of stable isotopes. At present, the usage of thermal interface materials (TIMs) can fill the gap at the contact surface and thus reduces the TCR. In this paper, aiming at the actual working condition of neutron source, the steady-state TCR measurement experiment is designed under vacuum and high temperature environment. The TCR at the contact surface was reduced after adding graphite films, thermal greases, liquid metals and preloads. The improvement of TCR of the contact surface between palladium (Pd), copper (Cu) and graphite (Gr) were measured respectively. The results show that the TCR between palladium and graphite is reduced by 95 % by adding liquid metal (Gallium alloy). The TCR between copper and graphite was reduced by 91 % by brazing method. Compared with other TIMs, graphite film has the advantages of high thermal conductivity, high temperature resistance and no fluidity. Adding 0.08 mm thick graphite film between Pd/Gr and Cu/Gr contact surface can reduce the TCR by 50 % and 70 %, respectively. The improvement effect between the two contact surfaces was not obvious when 15.9–318.3 kPa preload was applied. [ABSTRACT FROM AUTHOR]

• An analytical model for heat transfer inside deep borehole heat exchangers (DBHE). • Convolution theorem to address vertically increasing borehole wall temperature. • Relationships between DBHE thermal performance and various parameters. • An effective tool fast and easy enough for the design and optimization of DBHE. Deep borehole heat exchangers (DBHE) provide an effective solution for ground coupled heat pump (GCHP) systems in cold climate region where heating is dominant. Concerning the analytical heat transfer models, the simplification that borehole wall temperature is constant along the depth of ground heat exchangers (GHE) which the existing quasi-three-dimensional models have assumed in the application of shallow borehole GHE, can no longer be accepted in the application of DBHE due to the geothermal gradient in deep ground. Making this simplification cannot give the real temperature distribution of circulating fluid along the depth of DBHE. Therefore, this paper developed a new analytical model for DBHE with coaxial pipes by successfully addressing the increasing borehole wall temperature using the convolution theorem, so that the widely employed quasi-three-dimensional models for shallow borehole GHE is extended for DBHE with coaxial pipes for the first time. The new analytical model is validated by comparing with an existing numerical model. Using the newly developed analytical model, the trends revealing the relationships between thermal performance of DBHE and various parameters are firstly plotted. Because of the high accuracy and quick calculation, this new analytical model can be used as a benchmark for numerical models. More importantly, the proposed analytical model can be an effective tool for the design and optimization of DBHE, since current numerical models are always calculation-demanding, time-consuming and difficult for engineers and designers to use. Also, the method this paper proposed to address the varying borehole wall temperature can certainly be employed to improve the existing quasi-three-dimensional models for shallow borehole GHE so that they can be applicable in some other cases, for example, GHE installed in layered soils, or affected by underground seepage flow in partial depth. [ABSTRACT FROM AUTHOR]

• Condensation from hot and humid air is investigated experimentally • Relevance to condensing economizers in the industry is made • The impact of the non-condensable gasses is investigated Condensing economizers are used in industry and power plants for waste heat recovery. Despite widespread use, their thermal design is still not optimized due to the complexity of the condensation process especially when non-condensing gasses are prevalent. This paper presents an experimental study of water vapor condensation on the vertical tubes of a serpentine bundle in a humidified air crossflow at water vapor mass fractions of 10% and 20%. The analysis showed a clear dependence of the efficiency of the condensation process on the humidified air inlet temperature and Reynolds number, and the efficiency increases as these parameters decrease. Condensation efficiency also depends highly on flow humidity especially in the region of higher Reynolds numbers. A comparison of the average Nusselt number in the case of dry air with the experimentally determined average Nusselt number is also presented, and showed a uniform increase in the Nusselt number as the inlet temperature of the humidified air decreased. [ABSTRACT FROM AUTHOR]

· The sheet-network and solid-network TPMS structures considering porosity and other parameters are constructed. · The solid-network TPMS structures with larger through-hole and skeleton concentration ratio have higher overall performance. · Under the same pressure drop, the heat transfer performance of TPMS with D-type sheet structure is better. Triply periodic minimal surface (TPMS) structures are widely used in the field of flow and heat transfer. In this paper, four TPMS structures, Schwartz Diamond-sheet (D-sheet), Schwartz Diamond-solid (D-solid), Schoen IWP-sheet (IWP-sheet), and Schoen IWP-solid (IWP-solid), are designed based on two structure generation strategies. The above structures all are manufactured through additive manufacturing, and the material is AlSi10Mg. The samples were characterized by X-ray scanning. The flow and heat transfer performance of different TPMS structures was studied by computational fluid dynamics (CFD) and experimental methods. The results show that the pressure drop of sheet-network is larger than that of solid-network for the same TPMS type. With the same structure generation strategy, the pressure drop of Schwartz Diamond (D-type) is greater than that of Schoen IWP (IWP-type). The heat transfer performance of the TPMS structure shows the same results as the pressure drop. The performance enhancement criterion (PEC) of the solid-network TPMS is better than that of the sheet-network structure, and the D-type TPMS structure is better than the IWP-type structure. The average PEC of D-solid and IWP-solid is 31 % and 44 % larger than that of D-sheet and IWP-sheet, respectively. The average PEC of D-sheet and D-solid is 11.8 % and 1.7 % larger than that of IWP-sheet and IWP-solid, respectively. The mechanism of the higher PEC of the solid-network structure is the low pressure drop caused by the high through-hole ratio and relatively concentrated skeleton. Under the same pressure drop, the heat transfer performance of TPMS with D-type sheet structure is better. The research content of the article is mainly applied to fin-enhancement, which can replace fins in traditional heat exchangers. In actual engineering applications, the TPMS structures type is reasonably selected according to the requirements. [ABSTRACT FROM AUTHOR]

Highlights • The theory of Karman vortex street was adopted to explain the reason of weak region formation. • Two tube configurations of which the section was semicircular and semielliptical were discussed in this paper. • The velocity vector, the flow streamline and the temperature contour of the modified configurations were displayed. Abstract The main objectives were to develop a method for designing an effective finned heat exchanger through changing the configuration to ensure the tubes be streamlined shape. Kinds of inserts including, vortex generator, louver fin, and serrated fin in the heat transfer was used to increase the effect of thermal performance, but also was harmful to the flow characteristics. Consequently, the comprehensive performance was obviously lower than the heat exchangers with no inserts. In order to solve this problem, streamlined tube configuration was adopted in this paper to search a new method to enhance the comprehensive performance by reducing the Karman vortex street effect. Two modified configurations of tube were investigated in this paper. The modified configurations were both made of semicircular tube and semielliptical tube. The simulation was validated by Wang's experiment and Arafat's simulation. The numerical data were presented at Reynolds number in the range between 1300 and 2000. Based on the numerical results, the effect of elliptical axis ratio on the thermal hydraulic performance was discussed. The results showed that the comprehensive performance of the modified configurations was greater than that of the circular tube. In addition, the velocity vector, flow streamline and temperature contour of the configurations were displayed. The Karman vortex street effect was adopted to explain the reason of wake region formation. It can be concluded that long and thin section of heat transfer tube, which was coincident with streamlined design, was beneficial to improve the comprehension performance of the heat exchanger. [ABSTRACT FROM AUTHOR]

Highlights • A quantitative study of the frost distribution and growth on FTHX was reported. • Frost pattern between windward and leeward sides of FTHX was compared. • Frost thickness, density and mass on different parts of FTHX were analyzed. • The effects of frosting conditions on frost distribution and growth were studied. Abstract Recently, the authors have published a paper which provided the frost distribution and growth along the airflow direction of finned tube heat exchangers (FTHXs), and demonstrated that some frost was accumulated on the edge of windward fins at a given frosting condition. As a follow-up to the work reported in the previous paper, in the current paper, the effects of frosting conditions on frost distribution and growth characteristics of FTHXs with different fin pitches were experimentally investigated. The results showed that the protruded thickness of frost on the edge of windward fins and the ratio of the frost mass on the edge of windward fins to that on the entire FTHX were both increased with the increases in air temperature (T a) and relative humidity (RH a), but decreased with an increase in initial air face velocity (v a). In addition, the effects of RH a on the mass ratio were more significant than those of T a and v a. Furthermore, the averaged frost density on the edge of windward fins was increased with an increase in RH a ; and that on the surfaces of fins and tubes was increased with an increase in v a , but decreased with an increase in RH a. [ABSTRACT FROM AUTHOR]

Graphical abstract This paper presents an experimental investigation of heat transfer performances including heat transfer coefficient along flowing direction, log-mean temperature difference and differential pressure of pure R123 and four volume concentrations 20 nm Al 2 O 3 -R123 nanorefrigerants with various volume concentrations 0.03%, 0.13%, 0.18%, 0.23% flowing inside a double-tube counter flow heat exchanger of an organic Rankine cycle system under the conditions of various heat source temperatures and flow rates. A comparison has been made between pure R123 and four kinds of nanorefrigerants to find out the influences of different nanoparticles volume concentrations on local heat transfer coefficient and variation tendency (straight lines) along flowing direction in evaporator. By using polynomial fitting where polynomial order is 3, the polynomial fitting curves (dash lines) and the optimum of volume concentration has been acquired, which indicates the loading carrying capacity in each operation condition. Highlights • Effect of Al 2 O 3 nanoparticles on R123 heat transfer performances. • Variation tendency along flowing direction in evaporator of ORC. • Polynomial fitting curve. • Comparison of differential pressure and log-mean temperature difference of R123 and nanorefrigerants. Abstract This paper presents an experimental investigation of variation tendency of heat transfer coefficient, log-mean temperature difference and differential pressure of pure R123 and 20 nm Al 2 O 3 -R123 nanorefrigerants with four various volume concentrations, 0.03%, 0.13%, 0.18%, 0.23% flowing inside the evaporator of organic Rankine cycle system under the conditions of various heat source temperatures and flow rates. Heat source temperatures are in the range of 50–90 °C at an interval of 10 °C, and heat source flow rates are 0.7 m3/h, 1.3 m3/h and 1.8 m3/h. Results show an increment of heat transfer coefficient along flowing direction for both pure R123 and four Al 2 O 3 -R123 nanorefrigerants with the increment of heat source temperature and flow rate, and that of four nanorefrigerants are higher than that of pure R123. There is no optimum value of heat transfer coefficient when operation condition is changed for the loading carrying capacity increasing with the increasing intensity of operation condition. Meanwhile, suspending nanoparticles and increasing heat source temperature can change the variation tendency of heat transfer coefficient along flowing direction except pure R123 and 0.18 vol% nanorefrigerant. In addition, 0.13 vol% as a whole is the optimum volume concentration for both log-mean temperature difference and differential pressure. [ABSTRACT FROM AUTHOR]

The theoretical basis for the calculation of shell side condensation of the spiral coil heat exchanger is weak. In order to solve the problem of pure steam film condensation heat transfer of the shell side of the spiral coil heat exchanger, the theoretical model of the homogeneous flow was established. Considering the centrifugal force, two different mathematical models were established and the heat transfer characteristics were studied. The solution formula of the heat transfer coefficient and thickness of first layer liquid film, meanwhile, was obtained. The theoretical formula of the membrane-like condensation heat transfer of the heat exchanger was worked out, based on the calculation model of the heat transfer coefficient of one-component working fluid outside the tube bundle, considering the effect of tube bundle. The two calculated results in this paper and the theoretical result of Bays and McAdams were compared with experimental values respectively. The results show that the relative error between the calculated values obtained by the two theoretical models and experimental values is 7.48% (ignoring centrifugal force), 3.05% (considering centrifugal force), while the relative error between Bays and McAdams’s result and the experimental value is 61.3%. The correctness of the theoretical models proposed in this paper are verified. The theoretical models can be used to calculate the film condensation heat transfer of the spiral coil heat exchanger. [ABSTRACT FROM AUTHOR]

The paper addresses the problem of coupled heat and moisture transfer in porous materials with the time-periodic boundary conditions applied. The solution of Luikov equations [1] , which describe coupled heat and moisture transfer, is presented. Laplace transform is used, where some terms of the inverse Laplace transform ought to be solved by Gaussian quadrature, meaning that the solution is semi-analytical. The time-periodic boundary conditions are applied to simulate the humidity and temperature oscillations of natural environment. Therefore, the proposed solution is appropriate to evaluate the distribution of moisture and temperature within the porous material exposed to everyday natural cycles. The paper presents convergence tests, validation of semi-analytical solution and application to different building materials are presented in the paper. [ABSTRACT FROM AUTHOR]

This paper presents a comparative study of a counter-cross flow plate heat recovery exchanger, which operates as an indirect evaporative air cooler in cooling conditions. In the study, thermal performance of this exchanger with different placement modes (vertical and horizontal) was investigated by numerical simulation based on original mathematical model. To validate the numerical model, vertically and horizontally positioned prototypes with the same exchangers were made and tested on the test rig. According to the test results, the numerical model has a satisfactory accuracy. For these two types of prototypes, influence of supply and exhaust air inlet parameters (temperature and relative humidity) on the thermal performance of the two prototypes in both constant inlet airflow rate and variable inlet air flow rate was researched. The results show that, under conditions of constant airflow rate with variable supply air parameters, the outlet supply air temperature of the exchanger set vertically is 1.41–2.40 °C lower than that of exchanger set horizontally, meanwhile, 24–44% more cooling capacity is obtained by the vertical exchanger. It indicates that placement mode has important effects on thermal performance of the exchangers and the exchanger set vertically always shows a better performance. The main reason is considered to be the existence of larger dry areas in exhaust air channels of the horizontal exchanger that reduce the heat transfer between supply air and exhaust air. Therefore, characteristics of the heat transfer in horizontal exchangers was especially studied in this paper. [ABSTRACT FROM AUTHOR]

Compact high-effectiveness counter-flow heat exchangers (CFHEX) are an essential building block of remote cooling systems and Reverse Turbo-Brayton coolers. This technology enables a wide variety of on-ground and space applications, including the cooling of infrared detectors, cryogenic chains of quantum computers and ultra-sensitive superconducting SQUID sensors. A mesh-based CFHEX design was previously proposed for such applications and demonstrated a great potential to meet the high effectiveness and compactness needs [1]. This paper presents a novel improved mesh-based CFHEX concept with a "transparent" inner wall and outlines the methods used for its manufacturing. Two of such CFHEXs are constructed and integrated in a technology demonstrator of a remote cooling system. Their performance is tested in the ▪ temperature, 1 bar–5 bar pressure and 50 mg/s–250 mg/s helium mass flow rate ranges. An effectiveness of ▪ (NTU=39–124) has been experimentally achieved with a pressure drop of less than 10 mbar per stream at the nominal operating conditions of the system. The experimental effectiveness and pressure drop results are analysed and compared to numerical predictions. The implications of the achieved high effectiveness on the final performance of the designed remote cooling system are also discussed. • Novel transparent-wall heat exchanger (HEX) design based on woven metal mesh reaches a remarkably high effectiveness. • Developed additive manufacturing method allows to fabricate mechanically sound mesh-based high-effectiveness cryogenic HEXs. • Proposed friction factor models for different woven mesh enable accurate HEX pressure drop predictions in various conditions. • Remote cooling systems based on novel HEXs have a great potential to reduce disturbances transported to the cooled object. [ABSTRACT FROM AUTHOR]

• A hybrid heat exchanger which enables sole air or liquid cooling as well their combination simultaneously is proposed. • The performance of the proposed heat exchanger is documented experimentally. • Hybrid heat exchanger keeps the battery cell temperature below 30°C for continuous 3C discharge rate. • Hybrid cooling increases temperature uniformity and the maximum temperature difference is 2.7°C. • The applicability of the proposed heat exchanger for electronic cooling is validated experimentally with resistance heaters. The temperature of cells varies greatly during dis/charge while their performance and lifetime are greatly affected by this fluctuation. Elevated temperatures may yield battery fire due to thermal runaway as well they accelerate ageing and capacity fade of cells. Thermal management systems are a necessity for electric vehicles to extend the lifetime of battery cells and eliminate any fire risks, especially for fast dis/charging applications. Here, we document a hybrid cold plate with a working fluid(s) of sole air or liquid as well as both of them. Hybridization of air and liquid cooling promises to minimize energy consumption requirements during a charge/discharge cycle by combining the benefits of both thermal management strategies if energy management is controlled accordingly. The temperature of each cell can be kept below 30°C with the proposed hybrid cooling heat exchanger, and the temperature difference between the cells is reduced by 30 % relative to liquid cooling. The maximum temperatures are decreased by 18 % and 3 % in hybrid cooling when compared to air and water cooling, respectively. Furthermore, a step function combining various discharge rates (1C and 3C) was employed in experiments to mimic a realistic situation, i.e. variable C-rate rather than constant. The results show that the temperature of the battery cells can be kept below 30°C with air cooling for variable discharge rate and the effect of contact resistance should not be overlooked for liquid cooling. Furthermore, the possible use of the proposed hybrid cold plates is surveyed in the cooling of electronic devices which produce more and continuous heat than cells. Therefore, three resistance heaters with a capacity of 50W are used in experiments as well. The results show that the proposed cold plates could be used in both electronics cooling and battery thermal management with a control algorithm to switch between sole working fluid and combination modes which could be developed based on the results of this paper. [ABSTRACT FROM AUTHOR]

• Trinary micro-porous 32/4/4 MgO/PVDF/PMMA blend with atmospheric window emissivity and solar reflectivity exceeding 97% is developed. • Compared to the baseline radiative cooling materials, trinary micro-porous 32/4/4 MgO/PVDF/PMMA blend enhanced surface temperature suppression by 1.6 ℃ at daytime. • Functionalized glass-PDMS micro-pillar heat exchanger chills water by 2.5 ℃ at an efficiency of 6.3% at nighttime. • Compared to the saturation limits, reduced temperature reduction and efficiency for low Peclet number low is attributed to axial conductive viscous dissipation. Chilled water harvesting is a fundamental application of passive radiative cooling and promotes energy conservation for space cooling in buildings potentially, which relies on well-designed radiative cooling materials and heat transfer interface. This paper reports a scenario leading to viscous dissipative thermo-photonic energy conversion, which takes place in low Peclet number regime of an order of magnitude of 100, where heat transfer is non-Graetzian. Compared to benchmarked glass-polydimethylsiloxane radiative cooler and barium sulphate coating, a newly developed trinary micro-porous 32/4/4 magnesium-oxide/poly(vinylidene-fluoride)/poly(methyl-methacrylate) radiative cooling blend, featuring high atmospheric window emissivity and solar reflectivity, both exceeding 97%, demonstrated a superior cooling performance with additional temperature reduction of 1.6 °C at daytime. Meanwhile, it chilled water at a flow rate of 6.3 µL/s by 1.3 °C upon coating on a glass-polydimethylsiloxane micro-pillar heat exchanger. Quantitative evaluation on the chilled water capacity was carried out at nighttime when the system ran pseudo-steadily. Cooling power measurement on a radiative cooler of same materials recorded a cooling power of 134 W/m2 which is close to the ideal limit. And measured water temperature reduction and cooling efficiency were 2.5 °C and 6.3% respectively. They were significantly lower than the saturation limit. Degraded thermal and energy conversion performances, attributive to extended Graetzian viscous dissipation, were discussed theoretically. [ABSTRACT FROM AUTHOR]

• Thermal performance of novel polymer nanocomposite coatings towards anti-fouling and anti-wetting behaviour on steel substrates is analysed. • The incorporation of reduced graphene oxide lead to a significant reduction in the wettability and surface roughness of coatings. • It is found that even small thickness of bio-foulant layer can significantly impede the heat transfer across the substrate. • In case of the epoxy-based nanocomposite coatings an ∼18° increase in contact angle was observed with the addition of rGO. This paper presents an experimental study on the thermal performance of novel polymer nanocomposite coatings under the fouling environments with particular emphasis on their ability to exhibit anti-fouling and anti-wetting behaviour. Two different weight fractions at 0.3%, and 0.5% of the reduced graphene oxide/Epoxy (rGO-ER7004) and reduced graphene oxide/polyurethane (rGO-UR9002) coatings were applied to stainless steel substrates by a facile spray coating technique. To evaluate the anti-fouling performance of the coatings, the specimens were immersed in the Modified Baar's Medium for 672 h. The antifouling performance was evaluated by comparing the overall heat transfer effectiveness of the nano coated and bare specimens at different immersion intervals. Three different flow rates, viz. 4 lpm, 6 lpm and 8 lpm were used to compare the heat transfer effectiveness. The incorporation of rGO lead to a significant reduction in the wettability and surface roughness of coatings. amongst all the investigated coatings, the heat exchange with rGO/UR9002 0.5 wt.% coatings showed the highest heat transfer effectiveness. A small reduction (∼10%) in the heat transfer effectiveness after an immersion in the fouling media for 672 h was noticed. This can be attributed to the accumulation of salt particles on the coatings, observed visually. Finally, the 1-D heat transfer analysis is performed using finite element method based approach to evaluate the effect of the nano coatings and foulants on the temperature drop. It is deduced that a small foulant layer can significantly impede the heat transfer across the substrate. A strong correlation (∼0.99) between the heat transfer effectiveness and temperature drop is observed. [ABSTRACT FROM AUTHOR]

• An innovative heat exchanger based on channels-in-tube principle is proposed. • Experimental set-up and procedure to measure pressures and temperatures. • Measured pressure drop and temperature profiles in the exchanger are presented. • A recently proposed CFD model is successfully validated against measured data. In our previous work, an innovative heat exchanger design, the so-called internally channeled tube (ICT), was proposed based on a channels-in-tube principle. The new geometry provides a significantly larger heat transfer area than similar conventional heat exchangers. Furthermore, a three-dimensional numerical model was developed to govern fluid dynamics and heat transfer in the ICT. The focus of the present paper is an experimental investigation of the ICT under turbulent flow conditions. The experimental set-up is presented in detail and the measurement procedure is highlighted. In particular, the maldistribution issue is considered. Measured pressure drop and temperature values are used to validate the numerical model. The deviation between simulated and experimental values is below 6.5% for the pressure drop and below 13% for the heat transfer coefficient. [ABSTRACT FROM AUTHOR]

In this paper, a novel geometry is proposed for evaporators that are used in Supercritical Organic Rankine Cycles. The proposed geometry consists of successive plenums at several length-scale levels, creating a multi-scale heat exchanger (HX). The channels at the lowest length-scale levels were considered to have their length determined by the thermal entrance-length. Numerical simulations based on turbulent flow correlations for supercritical R134a and water were used to evaluate the performance of heat exchangers. Using the data on pumping power and area of heat exchange, the total present cost was evaluated using a cost model for shell-and-tube heat exchangers. With respect to the shell-and-tube baseline case, the cost per heat load and total costs of new HXs is lowered by approximately 20–26% and 15–30%, respectively. This reduction in present costs of the new HXs were found to be attributed to higher operational costs for the shell-and-tube HXs, as evidenced by the higher pumping power, as well their capital investment costs. The cost savings in the new HX designs compared to those of the shell-and-tube HXs, at similar heat load performance, indicate that the new HX architectures proposed in this paper are valid alternatives to traditional HX designs. [ABSTRACT FROM AUTHOR]

• A novel heat exchanger with latent heat storage was investigated. • The optimized method was proposed to enhance heat transfer rate of system. • The thermal property of material and geometry configuration were studied. In this paper, an optimization method is proposed to improve the performance of a novel heat exchanger with latent heat thermal energy storage. Paraffin/expanded graphite composites were used as phase change material filled in annular tube, maintained contact with the shell and tube flow channels simultaneously. The heat transfer and flow characteristics of this novel heat exchanger were investigated in a previous study, which indicated that further research was required to enhance thermal transfer rates. Therefore, in this work, the thermal property of the material and geometric parameters of the heat exchanger, including compression density, tube diameter, and fins and baffle configuration, were studied numerically to improve the thermal behavior of the system. It was found that a composite with density of 600 kg/m3, a tube diameter of 19 mm, and an extended surface and baffle space of 100 mm, can be used for the final optimization result. The results show that, after optimization, the thermal conductivity of the phase change material can reach 5.16 W/m K and the latent thermal energy stored in system could be twice that previously obtained. The solidification rate can increase to a maximum of 50% while the shell side Nu/Pr1/3 can reach 29.98. The improved heat exchanger shows better energy storage capacity and working power. [ABSTRACT FROM AUTHOR]

• Comprehensive experimental dataset for a plate-fin heat exchanger is presented. • Issues relating to the current heat exchanger manufacturing methods are stressed. • Novel heat exchanger corrugation modelling approach is developed. • Good aggrement of novel CFD is found in comparison to the experiments. The paper compares two serrated plate-fin Heat Exchanger (HE) corrugation modelling methods using Computational Fluid Dynamics (CFD). The first method follows closely recent literature studies and models a finite length single channel of a corrugation layer inside the HE core. The second method utilises the conjugate heat transfer methodology and models a section of the HE core with both cold and hot fluid streams separated by a solid conducting wall (HE corrugation). The results of latter model are then extrapolated for the full dimensions of a HE core layer to obtain flow and heat transfer characteristics. The conjugate heat transfer analysis methodology presented is novel and eliminates the need for analytical/empirical modelling currently widely used within industry. Furthermore, it provides more detailed information about the flow and heat transfer inside the HE core enabling potential for more efficient HE designs. Predictions at the corrugation level were carried out at 88 ⩽ Re corrug ⩽ 2957 with mesh independence studies completed for all the computational domains. The results obtained in the HE corrugation predictions were then implemented to the multi-scale HE unit model where the flow inside the HE core was modelled using two porous media simplifications whilst the heat transfer was simplified using the effectiveness source term. The HE unit predictions were validated against industrial experimental data with good agreement found between the numerical and experimental results. All the simulations were completed using the open-source CFD package OpenFOAM. [ABSTRACT FROM AUTHOR]

• An experimental device of SWHE based on dual DMR liquefaction process is built. • A multi-phase four-stream code for LNG SWHE is developed. • Heaving motion has a larger influence on heat transfer characteristics than surging. As one of the core equipment of floating liquid natural gas (FLNG), the spiral wound heat exchanger (SWHE) will be affected by feed gas flow and sea conditions, which will lead to more complex flow and heat transfer process inside the heat exchanger. In this paper, the experimental device of SWHE based on dual mixed refrigeration (DMR) liquefaction process is built to test the adaptability of feed gas flow and sea conditions on SWHE. Meanwhile, a multi-phase four-stream code for liquid natural gas (LNG) SWHE is developed. The following conclusions are obtained: (1) the parallel heat transfer calculation program can predict the change of the heat transfer coefficient and the temperature inside the heat exchanger well. (2) The temperature decreasing rate of feed gas inside SWHE decreases gradually. When the feed gas is liquefied completely, the temperature decreasing rate increases gradually. (3) The heat transfer performance of SWHE is greatly affected under sea conditions. [ABSTRACT FROM AUTHOR]

• Heat transfer coefficient and pressure drop of R1234ze(E) measured in one pass in a 0.643 mm microchannel tube. • Real pressure drop as in a heat exchanger. Flow pattern recorded at exit of each data point. • Result compared to R134a and R32 measured in the same facility. This paper presents heat transfer coefficient and pressure gradient of R1234ze(E) in a tube of 0.643 mm. In addition, visualization sections are added for evaluation of flow patterns. Both heat transfer coefficient and pressure gradient are presented against real saturation pressure, while flow pattern captures at exit of data points are presented in the same plot (Fig. 8). Experiments are conducted on a 24-port microchannel tube with a hydraulic diameter of 0.643 mm. The single-phase friction factor shows fRe = 64 in laminar and underestimation of Churchill prediction in transition (1700 < Re < 3000). It seems that Gorenflo and Kenning (2010) predicts boiling HTC in a nucleate boiling dominated part very well since the prediction is close to HTC measurement at quality close to 0.1. The pressure gradient of R1234ze(E) is slightly higher than R134a, and the heat transfer coefficient is almost the same at the same condition. The pressure gradient increases with quality and mass flux but decreases with reduction of saturation pressure. R1234ze(E) heat transfer coefficient increases when heat or mass flux rises, decreases when saturation temperature increases. Heat transfer coefficient of R1234ze(E) first increases when quality increases due to the enhancement of convective effects and then drops at moderate quality due to the reduction of nucleate boiling and dry-out. The local maximal value of HTC increases from 3.65 to 3.95 and the quality shifts from 0.55 to 0.7 as mass flux increases from 100 to 200 kg-m−2 s−1. Supported by the captures of flow patterns, HTC reaches local maximal with turbulence at G = 200 kg-m−2 s−1. Comparing measurements to the existing models these two models are recommended: Mishima and Hibiki (1996) has an MAE (Mean Absolute Error) of 11.7% for pressure gradient and Bertsch et al. (2009) has an MAE of 25.1% for heat transfer. [ABSTRACT FROM AUTHOR]

• Oscillatory surface is proposed to enhance direct contact heat transfer. • Influence of oscillation parameters on heat transfer are analyzed. • The reasons for enhancement of heat transfer are demonstrated. Direct contact heat transfer between stratified immiscible fluids is commonly encountered in solar salt industry, desalination, petrochemical engineering, etc. Usually, the mutual mixing and penetration between stratified flows are weak, and hence the heat and mass transfer rates are quite limited and act as an obstacle in their wider applications. Considering the fact there is always clear surface (interface) between immiscible fluids, this study proposes to stimulate interface oscillations artificially to reinforce the mixing between fluids and thus enhance the transfer processes. The artificial oscillations employed in this paper are sinusoidal wave and their amplitude, period and wave length on the direct contact heat transfer between stratified airstream and water surface are investigated respectively. Heat transfer was found to be enhanced to a great extent by artificial oscillations, ranging from one to three times higher than that without oscillations. With the amplitude, period and wave length covered in this study ranging from 1 mm to 4 mm, 0.1 s to 2 s, and 40 mm to 100 mm, respectively, the heat transfer enhancement effect was found to increase with increase of oscillation amplitude and decrease of wave length, and this effect became more distinct at higher airstream velocity. However, the change of oscillation periods covered in this study shows minor difference on the heat transfer enhancement. Without using special solid inserts or alternation to the heat exchanger structure, artificial oscillation of phasic interface can be a simple and efficient technique to enhance transfer process between stratified flows. [ABSTRACT FROM AUTHOR]

• A method is proposed to assess the efficiency of heat transfer enhancement techniques. • Reynolds number and ln(Nu e / Nu r)/ln(f e / f r) were taken as the abscissa and ordinate. • The data in higher levels in the coordinate will be more effective in energy saving. In this paper, a revised performance evaluation method was proposed to estimate the thermal hydraulic performance of various heat transfer enhancement techniques. The energy saving effectiveness for the enhanced structures can be identified via a plot. In the plot, Reynolds number and the efficiency index ln(Nu e / Nu r)/ln(f e / f r) are taken as the X and Y coordinates, respectively. The efficiency can be ranked into four different levels. For experimental data or simulation results in Level 1, enhanced heat transfer is accompanied with more power consumption. The data in Level 2 is that the heat transfer can be intensified at the same pumping power. In Level 3, it is characterized by the augmented heat transfer at the same pressure drop. The efficiency in Level 4 is the highest. The ratio of heat transfer augmentation is higher than the increment ratio of friction factor at the same Reynolds number. Four levels correspond to four horizontal regions with Level 1 at the bottom and Level 4 at the top. The data at higher levels in the ordinate will be more competitive for energy saving. The examples on how to use the method for evaluating the performance of enhanced surfaces and typical heat exchangers were also presented. [ABSTRACT FROM AUTHOR]

• Compound porous media model is proposed to simulate performance of U-tube HX. • The complicated U-tube bundle is divided into three porous media sections. • For each section, porous coefficients are obtained from empirical correlations. • Results of compound porous media approach are in good agreement with test results. • The proposed model shows the ability to correctly model a complex geometry HX. This paper describes a compound porous media model approach for numerical analysis to investigate the aero-thermal performance and characteristics of a flat top U-tube heat exchanger. The heat exchanger was considered as a combination of three sections; vertically straight tube section, bent tube section, and horizontally straight tube section. The porous media coefficients were obtained by using empirical correlations for the pressure drop and heat transfer for each section. The numerical results of the compound porous media model were compared to those of a conjugate heat transfer (CHT) CFD analysis, considering real tube geometry and experiments in order to validate the proposed compound porous media approach. The validation revealed that the proposed approach provides reasonable flow and heat transfer characteristics as well as the overall aero-thermal performance of the flat U-tube heat exchanger, with a considerable reduction of the required computational power. [ABSTRACT FROM AUTHOR]

• The construction of the enhanced plate used in the AHU is proposed. • The highest enhanced effects are obtained for "H12-R10-h4" (PEC=1.45 at Re=5000). • All nine enhanced channels can enhance heat transfer at the same pressure drop. • The DMTD is proposed for the cross-flow AHU heat exchanger design. • The heat exchanger picked up is "H7.5-R10-h3" which has the maximum ratio of the heat exchange rate (40.5 kW) over the flow loss power (3.25 kW). In this paper, the construction process of the enhanced plate, the heat transfer and fluid flow characteristics, and the domain decomposition method for AHU performance estimation are presented. The AHU heat exchanger 3D model is built from plate modeling to heat exchanger assembling. The heat transfer plate modeling starts from a basic flat plate, elliptic cylindrical dimples (ECDs) are firstly added to the fixed location, and then spherical crown dimples (SCDs) are applied to the flat surface among the ECDs. For the enhanced channels, the effect of channel height is weaker than SCD depth on the Nusselt number and friction factors. For all enhanced channels, the PECs are larger than 1.1 within Re from 5000 to 35,355. The performance of a deeper depth of SCD is superior to that of a shallow one when the channel height is the same. Among the nine channels studied, the highest enhanced effects are obtained for "H12-R10-h4" (PEC=1.45 at Re=5000). And all nine enhanced channels can enhance heat transfer at the same pressure drop among the Reynolds number of 5000 to 35,355. The energy saving performance decreases with Re, and the channels with smaller SCD depth can save more energy when channel height is identical at the higher Reynolds number region. A decomposition thermal design method is proposed for the cross-flow AHU heat exchanger design, and the typical AHU design result is picked based on the heat exchange rate and flow loss power. The recommended heat exchanger is "H7.5-R10-h3" whose heat exchange rate is 40.5 kW when flow loss power is 3.25 kW. [ABSTRACT FROM AUTHOR]

An increasing number of high-rise buildings are being designed, enabling the use of pile foundation to also act as ground heat exchangers (GHEs) of ground source heat pumps (GSHPs). These so-called energy piles include heat exchange pipes already cast into the pile foundation to produce a new type of GHE. Groundwater seepage, by means of advection, plays an important role in improving the heat transfer performance of energy pile. Existing research has studied heat transfer mechanism of energy pile when groundwater is considered to flow in one direction only, and in two directions also. No studies, however, have been introduced explaining the effects of three-dimensional (3-D) groundwater seepage (i.e. in three directions) on the heat transfer between pile and underground medium. This paper presents a new mathematical model about the heat transfer of energy pile with 3-D groundwater seepage, whereby the analytical temperature responses solutions induced by both infinite and finite models are obtained. Comparisons between pure conduction and combined heat transfer were made, and the temperature distributions in a 3-D seepage environment are explained. In addition, the characteristics involved in the new model were explored and then the factors exerting influence on heat transfer are described. The 3-D groundwater seepage is more advanced and realistic, and the research described in this paper is usefully relevant to promoting the development of energy piles of GSHP. [ABSTRACT FROM AUTHOR]

Heat transfer enhancement technology has been widely used in heat exchangers. The six-start spirally corrugated tube is a novel heat transfer tube, and its flow resistance characteristics have not been studied. In this paper, the flow resistance characteristics of the six-start spirally corrugated tube are studied. Specifically, the effect of influencing factors such as geometric parameters (pitch and corrugation depth), fluid properties and Reynolds number on flow resistance of the six-start spirally corrugated tube are studied. The results show that, firstly, the six-start spirally corrugated tube has a better heat transfer performance than both the circular tube and four-start spirally corrugated tube, and its flow resistance is situated between circular tube and four-start spirally corrugated tube. Secondly, with the increasing of the pitch p of the six-start spirally corrugated tube, both the pressure drop Δ P and resistance coefficient f are decreasing gradually. Thirdly, the increase of corrugation depth e can lead to a large flow resistance, so it is not recommended to adopt an excessive increase in the corrugation depth e . The effect of fluid properties on flow resistance in tube is not obvious, and the changing rate of ratio f e / f s of all working media are almost the same. Moreover, with the increasing of Reynolds number Re , resistance coefficient f both in the six-start spirally corrugated tube and circular tube are decreasing gradually. Finally, based on the above research data, the criterion correlation for flow resistance calculation in the six-start spirally corrugated tube is obtained. It can be concluded that the six-start spirally corrugated tube studied in this paper has a smaller flow resistance than the commonly used four-start spirally corrugated tube. This work can provide some advices for the future research of reducing flow resistance in spirally corrugated tube or other related devices. [ABSTRACT FROM AUTHOR]

In this paper, we investigate a coupled velocity and temperature phase changing problem for a spinning extruded micro-column. Molten polymer is squeezed out from a spinning container in the form of a symmetrical column. The extruded and spinning fluid is being solidified through the heat exchange between the polymer in the solid state and the ambient environment resulting in a two phases (solid and liquid) problem. Temperature profile has previously been determined analytically using the boundary layer and lubrication theory techniques. However, the study on the effect of the moving boundary on the fluid flow in the liquid phase and the induced stress on the condensed shell remains absent. In this paper, we analytically deduce the velocity and stress profiles for different stages during the micro-extrusion process. Several simulations have also been performed to validate the assumptions made in the current mathematical model. We find that the velocity profile alters from certain quadratic flows into linear flows when the moving boundary retreats, and the surface stress alters from shear stress into the principal stress on the outer-shell accordingly. In conjunction with the centrifugal force induced by the spinning fluid acting on the shell, these provide some crucial factors for ensuring extruded products in their highest quality. [ABSTRACT FROM AUTHOR]

Now-a-days fin-and-tube heat exchangers are employed in a wide variety of engineering applications such as modern heat exchangers, automotive radiators, automotive air conditioning evaporators and condensers. In order to improve the performance, further innovative designs, additional applications, and advanced geometries are expected. Researchers and academicians had been working on this topic for last few decades. A number of experimental and numerical studies have been conducted and various correlations were developed. The optimum design parameters with the maximum overall heat conductance and minimum pressure drop needs more focus and research. In this paper, different experimental/numerical studies performed are reviewed, grouped and summarized based on the types of heat exchangers, heat transfer and pressure drop performance, effects of geometrical parameters under different flow conditions. Overall, this paper will highlight on the existing technologies and emerging trends in designing of finned-tube heat exchangers considering different arrangements and geometric parameters under variable flow conditions which will be helpful for selecting appropriate design depending on the requirement. [ABSTRACT FROM AUTHOR]

Highlights • Novel modelling approach for thermal energy storage with immersed coil heat exchangers. • Discretizing the storage tank into only three fully-mixed control volumes. • Experimental validation of the numerical model using a full-scale testing facility. • Numerical model validated for storage tanks with and without phase change materials. • Numerical model captures different thermal behaviors of thermal storage tanks. • Satisfactory agreement between the experimental and numerical results. Abstract A novel modelling approach is presented for a thermal energy storage system with immersed coil heat exchangers. The energy store consists of a water tank in which rectangular phase change material (PCM) modules are submerged. The immersed coil heat exchangers are placed at the bottom and top sections of the tank for charging and discharging, respectively. This design promotes thermal mixing inside the tank when charged from the bottom coil and discharged from the top coil. In addition, the location of the coil heat exchangers favorably limits the heat transfer when charged through the top coil or discharged through the bottom coil giving rise to the thermal diode effect. PCMs can offer large storage capacity since they store large amounts of thermal energy in the form of latent heat of phase change (solid-liquid). This paper proposes a simplified physics-based numerical model of the heat transfer and phase change in the thermal storage tank. The model discretizes the storage tank one-dimensionally into three fully-mixed control volumes; two control volumes surrounding the immersed coil heat exchangers and one control volume in the middle of the tank including the PCM modules. All the heat transfer dynamics between the control volumes, the heat exchangers and the PCM modules are considered. Experimental validations, using a full-scale test facility, are carried out for systems with and without PCMs under various operation scenarios and the results are found to agree well within the experimental uncertainties. Such model potentially provides high computational efficiency that is desirable in simulating the performance of storage systems under long-term operations. [ABSTRACT FROM AUTHOR]

Highlights • A visualized scaled IRWST with PRHR HX was applied based on the H2TS method. • The overall velocity flied was acquired in three axials by the PIV measurement. • An obvious upwelling flow was observed near the PRHR HX rod bundle. • The mutual influence between the thermal stratification and natural convection was investigated. Abstract Passive Residual Heat Removal Heat Exchanger (PRHR HX) immerged in the In-containment Refueling Water Storage Tank (IRWST) plays an important role in removing the core decay heat under non-Loss of Coolant Accident (LOCA) accident. In the initial stage of the Passive Residual Heat Removal System (PRHRS) activating, the natural convection is the main way to exchange heat between the secondary side of the PRHR HX with IRWST. However, it is almost impossible to acquire the thermohydraulic performance in IRWST since the influence of the natural convection and thermal stratification in IRWST happens simultaneously. In this paper, Hierarchical Two-Tiered Scaling (H2TS) method is employed to get scaling criteria based on control equations. Anda visualized water tank with five C-shape electrical heating tubes was scaled-down and applied to investigate the thermohydraulic performance for IRWST of CAP1400 Nuclear Power Plants (NPPs). Particle Image Velocimetry (PIV) measurement was used to investigate the evolution of the flow field of total twenty-five surfaces in three axial directions under three steady- and two variable-heating conditions. Meanwhile, three thermocouple bundles were adopted to acquire temperature data in three regions. The experimental results show that the temperature deviation in the same height of the IRWST is flattened by plenty of vortexes. As a result, local circulation and the temperature difference is less than 1 °C. While the thermal stratification along the vertical direction is obvious, the distribution of flow field and temperature demonstrate that a "dead zone" exists in bottom of the IRWST. Besides, a "thermal interface" region forms in the lower middle region as hot and cold fluid mixing. An obvious upwelling flow is observed near the PRHR HX heating rod bundle from flow fields of three axial directions. At the same time, the maximum height of the upwelling flow decreases with time increasing. On the one hand, the natural convection is induced by the thermal stratification. What is more, the thermal stratification will impair the scale and strength of natural convection, which will intensify the thermal stratification conversely. [ABSTRACT FROM AUTHOR]

Highlights • Teardrop dimple/protrusion is novelly introduced into supercritical CO 2 channels. • Flow structures, overall thermal performance and entropy generation are analyzed. • Effects of dimple/protrusion eccentricity, Re and CO 2 properties are investigated. • Positive eccentricity dimple (PED) produces the best overall thermal performance. • Maximum TP greater than 6 is achieved near the temperature 305K and pressure 8MPa. Abstract Supercritical carbon dioxide (SCO 2) is an excellent candidate for refrigeration system, energy utilization and new power cycle due to its easy-to–reach pseudo-critical point and the advantages of thermo-physical properties. As an efficient and passive thermal control technology, teardrop dimple/protrusion has great potential in heat transfer enhancement and resistance reduction. In this paper, three types of teardrop dimple and protrusions are novelly introduced into the SCO 2 rectangular channels. Numerical calculations have been carried out to obtain the detailed flow structures, friction and heat transfer characteristics, overall thermal performance and amount of entropy generation under full turbulence regions. Moreover, the effects of geometrical structure, eccentricity of teardrop dimple/protrusion, Re , temperature and pressure of SCO 2 are discussed with in-depth analysis. According to the results, complex and different evolution and development patterns of vortices are presented for three different structures. Positive eccentricity dimple (PED) produces the best overall thermal performance, owing to significant heat transfer enhancement and less friction, and the maximum TP reaches 1.17, indicating a 17% improvement compared with the smooth case. The variation of eccentricity has little effect on overall thermal performance and entropy generation for PED structure, while plays a major role in positive eccentricity protrusion (PEP) and negative eccentricity protrusion (NEP) cases. However, the increase of Re is unfavorable to the overall thermal performance. In addition, the temperature and pressure of CO 2 have significant effects on the overall thermal performance for PED cases, and corresponding high- TP and thermal performance deterioration regions have been captured, which is advantageous to fast positioning the specific ranges of temperature and pressure, and whose coupled mode. Specifically, the maximum TP greater than 6 can be obtained near the temperature 305 K and pressure 8 MPa. [ABSTRACT FROM AUTHOR]

Highlights • Summary of an experimental database including 2376 heat transfer data and 1590 frictional pressure drop data. • Survey and classification of heat transfer and frictional pressure drop correlations. • Assessment of correlations with the experimental database. • Development of a new frictional pressure drop correlation, cross validation and sensitivity analysis. Abstract The corrugation channels of plate heat exchangers enhance the heat transfer and complicate the prediction of heat transfer coefficients and frictional pressure drop. This paper reviews the heat transfer and frictional pressure drop correlations for condensation in plate heat exchangers, and classifies the correlations into basic forms. An experimental database is developed including the data of HFCs, hydrocarbons, HFOs and CO 2. The mass fluxes are in the range of 2–150 kg·m−2·s−1. The chevron angles and hydraulic diameters are distributed in 25.7°–70° and 3.23–8.08 mm. The saturated temperatures are −34.4 to 72.1 °C, while the reduced pressures are from 0.03 to 0.49. Eight heat transfer correlations are assessed with the database. The correlation of Longo et al. [48] predicts the experimental data best, while the correlation of Kuo et al. [16] shows the second best performance. Six frictional pressure drop correlations are compared with the database. The prediction of frictional pressure drop is relatively poor, and a new correlation is developed using multi-variable regression analysis with non-dimensional numbers. This new correlation predicts 87.5% of the experimental data within ±50%. [ABSTRACT FROM AUTHOR]

Highlights • Performance of DS-RBHX and SS-RBHX is compared experimentally. • DS-RBHX has better comprehensive performance than SS-RBHX. • Three guide shells are proposed and investigated numerically. • All guide shells play a positive role in heat transfer enhancement of outlet zone. • DS-RBHX-GS2 is the best one among the three. Abstract In this paper, the heat transfer and flow performance of a double shell-pass rod baffle heat exchanger (DS-RBHX) is investigated experimentally. Likewise, a single shell-pass rod baffle heat exchanger (SS-RBHX) is set as the control. Water serves as the working fluid both in the shell side and tube side. Experimental results indicate that the overall heat transfer coefficient of the DS-RBHX is higher than that of the SS-RBHX for all measurements. As the shell-side volume flow rate varies from 2.8 to 15.2 m3/h, the shell-side heat transfer coefficient and pressure drop of the DS-RBHX increase by 33.5–54.0% and 34.0–74.3%, respectively. From the perspective of the comprehensive performance, the shell-side heat transfer coefficient of the DS-RBHX is 14.4–24.3% higher than that of the SS-RBHX under the same shell-side pressure drop. Consequently, it is proved that the DS-RBHX has better comprehensive performance compared with the SS-RBHX. On the basis of experimental results, numerical studies are conducted to analyze the shell-side behaviors of the DS-RBHX further. According to numerical results, three kinds of guide shells, arranged at the end of the sleeve, are proposed to reduce the flow dead zone in the shell-side outlet zone. The behaviors of DS-RBHXs with the guide shell (DS-RBHX-GSs) are obtained numerically. The results show that all three guide shells improve the heat transfer performance of the shell-side outlet zone, particularly in the outer side. Moreover, the guide shell of the DS-RBHX-GS2 has more significant effects than the others. [ABSTRACT FROM AUTHOR]

Highlights • A correlation for flow boiling was proposed with the effect of boundary layer. • The correlation is for horizontal circular channels with diameters of 1.0–8.0 mm. • The standard error and γ 30 of new correlation are 18.9% and 91.3%, respectively. Abstract Flow boiling in small channels has attracted much attention due to its promising application in compact phase change heat exchangers. In this paper, a flow boiling heat transfer database of light hydrocarbons in horizontal small channels is established. It is found that the heat transfer coefficient (HTC) for propane in the channel with 1.0 mm diameter depends on heat and mass fluxes in either the low or high quality region, which is different from the corresponding HTC in conventional channels. To explore the special phenomenon in small channels, a new model is proposed based on boundary layer theory by considering the function of bubble growth in flow boiling. The relationship between the boundary layer thickness and the bubble diameter is emphatically discussed. Based on the theoretical model, a new dimensionless parameter (Rtd) is introduced to evaluate the effect of channel dimension on the flow boiling heat transfer. Rtd reveals that the bubble growth is enhanced in both the low and high quality regions due to the flow confinement of the small channel. With the aid of Rtd, a modified heat transfer correlation is developed based on the database. Within the whole database, the overall standard error and γ 30 of the modified correlation are 18.9% and 91.3%, respectively. The prediction performance is superior to those of existing correlations. [ABSTRACT FROM AUTHOR]

Highlights • State-of-the-art overview for enhanced surface manufacturing technique was given. • Boiling heat transfer characteristics of low-GWP refrigerants were compared and analyzed. • Predictive methods for pool boiling heat transfer were critically reviewed. • Existing shortfalls are identified and suggestions for future studies were proposed. Abstract Global warming mitigation efforts have stimulated investigations of a new generation of low-GWP refrigerants. Because some of the proposed low-GWP refrigerants have unfavorable characteristics (e.g., flammability and toxicity), a small refrigerant charge is desirable. Compact heat exchangers with enhanced surfaces facilitate small refrigerant charge by having a large heat exchanger surface area to heat exchanger volume ratio. This paper reviews the current state-of-the-art for pool boiling heat transfer of low-GWP refrigerants on enhanced surfaces. An overview for the enhanced surface manufacturing technique is given along with detailed reviews of the heat transfer measurements and predictions for many of the low-GWP refrigerants, including the hydrofluoroolefin (HFO) class, the hydrocarbon (HC) class, carbon dioxide (CO 2), and ammonia (NH 3). The overview of the predictive methods includes mechanistic models and correlations for pool boiling on enhanced surfaces. Based on the surveyed literature, existing shortfalls are identified and suggestions for future studies are proposed. [ABSTRACT FROM AUTHOR]

Highlights • The algorithm is capable of sovling the heat exchanger in which super-critical fluids exist. • The algorithm is capable of evaluate the wall temperature of the heat exchanger. • The accuracy of the algorithm is increased relative to the MBM method. Abstract In this paper, a coupling algorithm which combines the advantages of MBM (Moving Boundary Method) and FVM (Finite Volume Method) is presented. In this algorithm, the heat exchanger is divided into MB cells on the basis of the heat transfer rate and inlet parameters of HSF and CSF, and then FVM is used to solve each MB cell to obtain the heat transfer area required for MB cells. When the heat transfer area required for each MB cell is obtained, the residual function based on the ratio of the required to the available heat transfer area of the heat exchanger is derived, and then Brent's method is used to adjust the heat transfer rate for the derived residual function to be zero. The MB-FV coupling algorithm is suitable for solving the heat exchanger with or without supercritical fluids, which not only extends the application range of the original MBM, but also increases the accuracy of the heat exchanger computational results. [ABSTRACT FROM AUTHOR]

Highlights • Constructal design of a disc-shaped heat exchanger is implemented. • Maximum thermal efficiency is taken as optimization objective. • Total tube volume is taken as constraint. • Optimal tube lengths are obtained. • Effect of mass flow rate on its optimal construct is analyzed. Abstract Constructal design of a disc-shaped heat exchanger (DSHE) is implemented in this paper. Maximum thermal efficiency and total tube volume are taken as the optimization objective and constraint, respectively. Optimal tube lengths of the DSHE are obtained, and the effect of mass flow rate on its optimal construct is analyzed. The results show that for the first order DSHE, its thermal efficiency has the maximum and the elemental tube angle has the corresponding optimal value. For the higher order DSHEs, the optimal constructs of the DSHEs are not varied when their mass flow rates are small and large enough, and the DSHE structure is simplified into the structure of the last order DSHE when the mass flow rate is large enough. For the same tube volume and area of the DSHEs with 4 central tubes and a large mass flow rate, the thermal efficiency of the third order DSHE is increased by 7.33% than that of the first order DSHE. Therefore, the performance of the DSHE is increased when its structure becomes complex in this case. The obtained results can offer some instructions for the optimal designs of various thermal systems, such as power, refrigeration and heat pump plants. [ABSTRACT FROM AUTHOR]

Highlights • Heat transfer and multi-phase pressure drop of CO 2 near the critical point. • Homogeneous equilibrium model approach is used to evaluate the 2-phase flow. • Heat transfer and pressure drop test was conducted with a PCHE. • A new set of correlations is suggested for CO 2 2-phase flow near the critical point. Abstract In this study, condensation heat transfer and multi-phase pressure drop of CO 2 near the critical point are investigated occurring in a Printed Circuit Heat Exchanger (PCHE) for a supercritical CO 2 power cycle application in mind. Homogeneous Equilibrium Model (HEM) approach is used to evaluate and develop appropriate heat transfer and pressure drop correlations. The experiment was performed with the CO 2 test facility called SCO2PE (Supercritical CO 2 Pressurizing Experiment) in KAIST. The heat transfer and pressure drop test was conducted with the PCHE in the facility. Existing correlations were compared to the test data and a new set of correlations was necessary to be developed. A new set of correlations is newly suggested in this paper which captures physical characteristics reasonably well. [ABSTRACT FROM AUTHOR]

Highlights • Shock wave impact on transpiration cooling for sintered porous plate was investigated. • The shock wave obstructs the coolant flow through the porous plate. • The cooling efficiency recovers with high coolant blowing ratios. Abstract This paper experimentally and numerically investigates the effect of shock waves on transpiration cooling of a porous plate. The experiments were conducted in a wind tunnel of Mach number 2.8, wherein oblique shock waves were impinged onto the porous plate. A numerical simulation model was used to obtain detailed temperature and flow field distributions near the porous plate surface. The effects of different coolant blowing ratios and shock wave intensities on the transpiration cooling efficiency were studied. The results show that the transpiration cooling efficiency reduces because of the impinging oblique shock waves with the decrease in the fluid velocity near the porous wall, resulting in an increase in the local fluid static temperature. Moreover, the oblique shock waves increase the local static pressure, thus obstructing the coolant outflow and increasing the local wall temperature. [ABSTRACT FROM AUTHOR]

Highlights • Experimental results at high condensation temperatures. • R245fa and its HFO replacements R1233zd are tested. • Different working fluids show diverse heat transfer mechanisms. • New heat transfer and pressure drop correlations are developed. Abstract The fundamental understanding of the thermal–hydraulic performance of working fluids during condensation is important for the optimal design of the condenser in various thermodynamic cycles. This paper is aimed at obtaining flow condensation heat transfer and pressure drop characteristics in a plate heat exchanger during the working conditions of the condenser of either organic Rankine cycle power systems or heat pump units. The selected working fluids are two hydrofluorocarbons, R134a and R245fa, as well as their hydrofluoroolefin replacements, R1234ze(E) and R1233zd(E). Measurements of heat transfer coefficients and pressure drops were carried out with varying saturation temperature, mass flux, and liquid Reynolds number, ranging from 30 °C to 70 °C, 16 kg/m2s to 90 kg/m2s and 65 to 877, respectively. Based on commonly used existing correlations, new heat transfer and pressure drop correlations were developed, including the effect of the surface tension. The experimental data indicate that different heat transfer mechanisms occur at low liquid Reynolds number with the different working fluids. The results suggest higher heat transfer coefficients and pressure drops for R1234ze(E) and R1233zd(E) than for R134a and R245fa at the same working conditions. The new correlations enable significantly better prediction accuracies for the experimental results in this study than existing correlations, indicating that the surface tension is a suitable parameter to consider in mini and micro-scale condensation heat transfer. [ABSTRACT FROM AUTHOR]

Highlights • Conjugate heat transfer method was adopted to modelling resistance furnace. • PID method was proposed to define heat generation rate of the energy source. • The prediction of equilibration time for GH4169 superalloy was validated. Abstract The simulation of heating process of superalloys is of great significance to predict the temperature distribution and equilibration time of workpiece. In this paper, a new method was proposed to simulate the heating process of superalloy billet in resistance furnace for all strategies of thermal schemes using commercial CFD software FLUENT. In the model, numerical analysis of natural convection and surface thermal radiation in a chamber electric furnace having heat-conducting solid walls of finite thickness with a heat source located in the side and top walls of the furnace in conditions of convective heat exchange with environment has been carried out, and such conjugate heat transfer process was solved by FLUENT solver. Furthermore, a PID methods was proposed to control the energy source of the resistance wires based on FLUENT UDF, in order to regulate the furnace temperature according thermal scheme. This model was validated through temperature measurement experiment in a small chamber resistance furnace. Compared with the measured data, the model was adequate for predicting the temperature profile and equilibration time of superalloy. [ABSTRACT FROM AUTHOR]

Highlights • Relative humidity has little effect on the temperature distribution of the fin. • No condensation occurs near the inlet area. • The maximum condensation mass flux is detected in the second row of pipe. • Relative humidity has little influence on the sensible heat transfer rate. • But manifests profound effects on the latent and total heat transfer rate. Abstract The heat and mass transfer of a five-row fin-and-tube heat exchanger under dehumidifying conditions is numerically studied in this paper, and the distributions of vapor condensation mass rate for both fin and tube surfaces are obtained. The influences of vapor mass percent of humid air and inlet velocity on heat and mass transfer are investigated. Numerical results reveal that relative humidity has little effect on the temperature distribution of the fin under humid conditions. It is also noticed that no condensation occurs near the inlet area, and the maximum condensation mass flux is detected in the second row of pipe; this phenomenon is more obvious at higher inlet velocities. Relative humidity has little influence on the sensible heat transfer rate; however, it manifests profound effects on the latent heat transfer rate and total heat transfer rate. The dimensionless heat and mass transfer factors are also obtained from the numerical study and compared with the correlations in literature. [ABSTRACT FROM AUTHOR]

Highlights • 2D low Mach approximation has been used to simulate cooling effect in thermoacoustic device. • Investigating the impact of the heat exchangers plates inter-spacing on the device performance. • The model used agrees well with DeltaEC software results. Abstract In this paper, a numerical study is carried out to assess the effect of heat exchangers plates inter-spacing on the performance of a thermoacoustic refrigerator (TAR). Considering that typically most of the heat loss occurs near the heat exchangers, a great emphasis will be placed on the space between the latter and the stack. Several geometric arrangements of heat exchangers plates inter-spacing were tested. The numerical method used is based on two-dimensional low Mach number model. This model assumes that the length of the assembled heat exchangers and stack is less than the wavelength. The results are in agreement with those provided by the DeltaEC software. It can also be observed that the larger the heat exchangers plates inter-spacing, the greater the cooling impact through the thermoacoustic effect. [ABSTRACT FROM AUTHOR]

Highlights • The trapezoidal baffle was proposed to enhance thermodynamics performance of STHX. • Characteristics in shellsides of two kinds of heat exchangers were analyzed. • Effects of twist-flow heat exchanger's structural parameters were investigated. Abstract Periodic whole cross-section computation models of shutter baffle heat exchanger and twisty flow heat exchanger were established respectively. The heat transfer coefficient, flow resistance and thermal performance in the shell side are numerically studied. Compared with that of the shutter baffle heat exchanger, the results show that the heat transfer coefficient of twisty flow heat exchanger is improved by 7.3–10.2%, pressure drop decreased by 18.5–21%, and the thermal performance factor TEF enhanced by 14.9–19.2%. The correctness and accuracy of simulation method and results are confirmed by experiments. In addition, the influence of inclination angle of trapezoidal baffle, baffles width, baffles pitch and number of baffles on the heat transfer performance are studied. The results show that the inclination angle of trapezoidal baffles and baffles pitch have a significant effect on heat transfer performance, the effect of baffle width has a secondary effect, and the number of baffles has a less significant effect. The results of this paper provide a new scheme for the structural improvement of the shell and tube heat exchanger, and provide the references for further performance optimization of the twisty flow heat exchanger. [ABSTRACT FROM AUTHOR]

Highlights • Coal water slurry can be effectively preheated in a new type of heat exchanger. • Coal water slurry is a shear-thinning fluid and simulated with Bingham model. • The thermo-performance of non-Newtonian fluid is different from Newtonian fluid. • Both Nu and f increase with the helical angle and reduce with the overlapped degree. • Comprehensive performance of fold helical baffle is superior to segmental baffles. Abstract Pre-heating process of coal water slurry (CWS) in Integrated Gasification Combined Cycle (IGCC) systems is beneficial to enhance the energy efficiency. In this paper, shell-and-tube heat exchangers with fold helical baffles (STHXsFHB) were firstly used as pre-heating exchanger. Using numerical simulation method, the flow and heat transfer performance of STHXsFHB with different helical angles and overlapped degrees was investigated. CWS with concentration of 62 wt% was shell-side fluid, which is characterized as non-Newtonian fluid and simulated with Bingham model. The numerical results show the temperature of CWS rises obviously, both Nusselt number and friction coefficient increase with helical angle at same shell-side mean velocity, Nu · f−1/3 decreases when helical angle increases. Nusselt number and friction coefficient have a negative correlation with overlapped degree, and Nu · f −1/3 with 50% overlapped degree is 13.8–33.1% and 3.9–8.8% higher than that with 1% and 30% overlapped degree, respectively. The variation of Nusselt number and friction coefficient versus helical angle and overlapped degree of CWS differs from thermal-hydraulic performance of Newtonian fluid in STHXsFHB. It is also found that the comprehensive performance of STHXsFHB is superior to shell-and-tube heat exchangers with segmental baffles. [ABSTRACT FROM AUTHOR]

Performance of shallow geothermal systems such as borehole thermal energy storage (BTES) and ground source heat pump (GSHP) mainly depends on the thermal properties of the subsurface and proper design of borehole heat exchangers (BHE). This paper introduces a novel hybrid approach for measuring the effectiveness of BHEs and surrounding subsurface thermal properties, which combines traditional thermal response test (TRT) with the borehole temperature relaxation method (BTR), based on two dimensional radial conductive heat transfer. The new method allows for: (1) evaluation of how convective heat loss at groundwater layers influence estimation of subsurface thermal properties; (2) examination of non-uniform heat transfer through a BHE to stratified subsurface layers; and, (3) calculation of depth-dependency of thermal properties of unsaturated subsurface layers. The hybrid approach was tested using a 50 m U-type BHE, the results of which indicated that convective heat transfer at the groundwater level altered the real value of effective thermal conductivity from 0.45 to 1.56 W/m K. The non-uniformity of heat transfer along the BHE was confirmed by calculations that showed subsurface thermal conductivities were depth dependent, varying between 0.34 and 0.61 W/m K. [ABSTRACT FROM AUTHOR]

Performance estimation of heat exchanger apparatuses requires a proper knowledge of the thermo-fluid dynamic interaction between fluid and device. One of the aspects to take into account is related to the estimation of the internal convective heat transfer coefficient. This requires the knowledge of the internal temperature and the internal wall heat flux, although these quantities are complicated to be measured. Therefore, the use of inverse problem techniques may be very useful for estimating these quantities. This approach deals with the estimation of the local internal properties, given some external temperature measurements, possibly by means of contactless experimental methodologies (i.e. infrared camera imaging). The solution strategy here presented, for a 2D model, is based on the Reciprocity Functional approach (RF), which requires the solution of two auxiliary problems that are solved, in this paper, by means of the Classical Integral Transform Technique (CITT). The RF approach coupled with CITT, presents some advantages over other techniques since its solution is completely analytical and therefore computationally fast, and it does not require any numerical simulation. Moreover, it deals with the noise content of the experimental data, filtering it and overcoming the documented limits of the classical RF approach. This original method, named Filtered Reciprocity Functional (FRF) approach, was applied to different test cases using simulated measurements, and the results were compared to both the exact and the reconstruct solutions obtained by the Truncated Singular Value Decomposition (TSVD) method, showing a good agreement. Finally, the FRF approach was applied to an experimental case in order to test its robustness when dealing with real data. [ABSTRACT FROM AUTHOR]