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Mixing of surface microstructure and wettability is one of the future development trends. Their combination always affects the bubble motion and heat transfer coefficient under pool boiling/flow boiling, but it is difficult to distinguish their respective contributions. This work studies the combination of new microstructure and mixed wettability to improve the flow boiling heat transfer in the channel. The nucleation, growth, coalescence and departure behaviors of R134a boiling bubbles were numerically analyzed. Effects of different heights, gaps and mixed wettability of microstructures and heat fluxes on bubble emission frequency and boiling heat transfer coefficient were discussed. Results showed that this new microstructure provides the bubble with the momentum of lateral motion, which helps the bubble to merge and leave faster. The surface of the hydrophobic groove coupled with the hydrophilicity as the substrate can effectively increase the bubble emission frequency, up to 134.37%. At the same time, the heat transfer coefficient decreases with the increase of the height of the microstructure, and it also increases with the increase of the microstructure gap. Finally, the contribution of each variable was determined by analysis of variance. The research results in this paper provide a reference for heat pipe and vapor chamber. • A creative microchannels with mixed wettability was proposed and optimized. • Synergistic effect of groove microstructure and mixed wettability was studied. • Surface mixed wettability has more influence on boiling heat transfer. • Bubble emission frequency of surface-A is 134.37% higher than surface-B. [ABSTRACT FROM AUTHOR]

In the thermal management of fuel cell vehicles, radiators are subjected to vibrations due to vehicle speed and road surface irregularities. However, there has been no comprehensive study of the impact of these vibrations on the heat transfer performance of radiators. Therefore, this paper employs R141b to conduct three-dimensional transient numerical simulations of flow boiling in small channels under vertical vibrations. The study reveals that the single-phase heat transfer coefficient exhibits periodic variations. At the same vibration frequency, the heat transfer frequency for the vertically oriented heating surface is twice that of the horizontally oriented surface, This is because the thermal boundary layer varies with the vibration velocity. Furthermore, as the frequency increases to 21 Hz, the time-averaged heat transfer coefficient can increase by up to 2.5 times. Following boiling onset, for horizontally oriented heating surfaces, an increase in frequency or amplitude enhances the transient heat transfer performance. Under the conditions of 28 Hz and 5 mm, the heat transfer coefficient can increase by 125.7%. However, for vertically oriented heating surfaces, when the frequency exceeds 14 Hz or the amplitude exceeds 8 mm, the heat transfer performance weakens after 7.143 s due to the promotion of bubble coalescence by the vibrations. [ABSTRACT FROM AUTHOR]

Electrowetting has attracted significant attention in condensation, but its application in enhancing pool boiling heat transfer is very limited. In this paper, the lattice Boltzmann method is adopted to investigate the pool boiling heat transfer on electrowetting surfaces with different electrode configurations. The wettability of the baseline hydrophobic surface above the electrode will change to hydrophilicity once a voltage is applied. The differences in heat transfer and boiling phenomenon between the hydrophilic and hydrophobic surfaces without electrodes are illustrated firstly by the density contours and boiling curves. Then, the possibility of electrowetting in improving the heat transfer coefficient (HTC) and critical heat flux (CHF) is verified by comparing with baseline hydrophobic and patterned-coating surfaces, and different boiling heat transfer regimes are illustrated in detail. Furthermore, based on the heat transfer characteristics of the electrowetting surfaces with different electrode configurations, a surface with temperature-dependent electrodes is obtained to comprehensively enhance heat transfer from the onset of nucleate boiling (ONB) to CHF. • The possibility of electrowetting in enhancing pool boiling heat transfer is verified. • Electrode configurations greatly affect heat transfer characteristics. • Temperature-dependent electrowetting surface enhances pool boiling heat transfer from ONB to CHF. [ABSTRACT FROM AUTHOR]

Nowadays, a bio-inspired heat exchanger incorporating a triply periodic minimal surface (TPMS) structure has demonstrated great potential in the fields of new energy research and aerospace, which necessitates achieving a balance between low volume and high heat transfer efficiency while maintaining low pressure drop. In this paper, the adjustable heat efficiency of the heat exchanger with TPMS in gradient thicknesses is specially designed. A steady-state conjugate heat transfer (CHT) model is coupled with computational fluid dynamics (CFD) analysis. In addition, the temperature profile and velocity streamline are also checked to analyze the fluid flow behavior of the radiator. The results show that the convective heat transfer coefficient of the Gyroid with gradient level set values is 26.02–60.10% higher than that of the uniform Gyroid model, and the pressure drop is decreased by 9.66–18.05%. Both high heat transfer efficiency and low pressure drop can be achieved when the thickness is 0.2–0.3 mm and Re is 100–125. The heat exchangers with a TPMS thickness gradient in the ratio of 2:4:6 demonstrate a remarkable enhancement in overall heat transfer efficiency, achieving a 30.22% improvement compared to those with a TPMS thickness gradient in 6:4:2. [ABSTRACT FROM AUTHOR]

Pool boiling, as an efficient phase-change heat transfer method, has attracted extensive attention in recent years. However, the bubble dynamics behavior (BDB) and heat transfer characteristics of pool boiling at sub-atmospheric pressure (sub-ATM) are significantly different from those at or above standard atmospheric pressure (std-ATM). This paper presents experimental results of BDB and heat transfer characteristics of pool boiling at sub-ATM in the range of 30–120 kPa. High-speed backlight imaging technique was adopted. The results show that with the decrease of boiling pressure, the diameter and near circularity coefficient of bubble departure increase, and the frequency and aspect ratio decrease. With the decline in pressure, the critical heat flux (CHF) of pool boiling on the surface of the electric heater decreases. Specifically, the CHF at 120 kPa is 23.2% higher than that at 40 kPa. At sub-ATM, the boiling heat transfer coefficient (HTC) grows with the increase of liquid height. However, the HTC hardly changes with liquid height at std-ATM and higher. [ABSTRACT FROM AUTHOR]

Coolant two-phase flow in microchannel heat sinks can provide efficient and attractive solutions for disposing of high heat fluxes generated in electronic components and other similar engineering devices. Moreover, the coolant flow in double-layer microchannels has been proven as an also more effective way than a single layer. This paper introduces some useful results of an experimental study with coolant two-phase flow conditions in double-layer microchannels. The double layers microchannels were fabricated by end milling using a CNC machine. The effects of the applied heat flux up to 82 W/cm2, the mass flux in the range of 200 to 426 kg/m2.s, and the maximum microchannel depth of 550 μm, on the thermal behaviour and associated coolant pressure drop, were investigated. The study handles 'in integrated approach' the important topics of the boiling heat transfer characteristics and hydraulic performance. Among these topics, the flow boiling curve and average boiling heat transfer coefficients have received much attention in the present work. Numerical values of this coefficient were predicted using an accurate mathematical model combined with in-house solution code which was developed and reported in this paper. The coolant two-phase flow good square surface temperature uniformity for the double layer microchannels. • The double layers microchannels were precisely manufactured by end milling CNC machines. • The effects of the applied heat flux, the coolant mass flux, and the microchannel depth on the boiling heat transfer characteristics and associated coolant pressure drop were investigated for parallel flow configuration. •. • The study mainly involves visualization of flow boiling pattern in the upper layer microchannel. • The boiling heat transfer coefficients values were predicted using in- house code developed in the present study. • The coolant two-phase flow good square surface temperature uniformity for the double layer microchannels. • The results for the boiling heat transfer coefficients indicated a satisfactory validation of the present in-house code. [ABSTRACT FROM AUTHOR]

Abstract This paper is aiming to improve the ultrasonic heat transfer enhancement by controlling the sound intensity distribution and constructing an acoustic focusing field, which can reduce energy dissipation and dramatically enhance the local sound intensity of heat transfer surface. Two design criteria are proposed to construct the acoustic focusing field by utilizing acoustic interference and acoustic resonance. Correspondingly, an elliptical acoustic focusing tank is designed and fabricated. Two-dimensional simulation and experimental tests of sound intensity distribution in the elliptical acoustic focusing tank are performed to verify the theoretical conception and design. In addition, the heat transfer coefficients are measured at different locations in the elliptical tank under natural convection, using a square tank as a control. The experimental results show that the elliptical acoustic focusing tank can promote the intensity of the whole sound field, especially the focus region compared with the square tank. The heat transfer coefficient and heat transfer enhancement rate anywhere in the elliptical tank is higher than that in the square tank. The maximum heat transfer enhancement rate of the elliptical tank is 55.3% in natural convection, which is 47.1% higher than that of the square tank under the same condition. [ABSTRACT FROM AUTHOR]

Abstract In this paper, the vortex-induced vibrational characteristics and heat transfer mechanisms of three tubes with different cross-sectional shapes are numerically studied. The results show that the characteristics of vortex-induced vibration of the vertical elliptical tube and circular tube are obvious. At low flow velocities, the vortex shedding force can affect the vibrational characteristics of horizontal elliptical tube, but the effect is not obvious with increasing flow velocity. When the frequency is locked, the vibrational amplitude of the vertical elliptical tube is the largest, followed in descending order by that of the circular tube, and that of the horizontal elliptical tube. The local heat transfer coefficients of tube at different vibration time are different. High local heat transfer coefficients are produced when vortices form on local walls or—when the relative velocity between the fluid and the heat transfer wall is relatively large. The effects of vortex-induced vibration enhancement heat transfer of these three kinds of tubes are different. The vertical elliptical tube can enhance the heat transfer by 28.6%, the circular tube can enhance it by 21.3%, and the horizontal elliptical tube can only enhance it by 3.7%. [ABSTRACT FROM AUTHOR]

To reduce the energy consumption in data centers, this paper proposes a novel structure that integrates a cabinet with a vertical falling film heat exchanger in data center to perform natural evaporative cooling. First, an experimental setup is built to verify the heat and mass transfer characteristics of a falling film in a vertical tube and to obtain the mass transfer coefficient 0.05. Then, the characteristics of tube length direction and heat and mass transfer are analyzed using compiled user-defined functions for different parameters of moist air and water in the tube. The maximum growth rates of heat transfer coefficient and mass transfer rate are 67.65% and 55.23% at increasing inlet moist air Re ma,i , respectively. Finally, the heat transfer characteristics of the falling film heat exchanger with staggered and in-line are compared for inlet air velocities in the range of 1–3.5 m·s−1. The corresponding results demonstrate that the heat flow rate, heat transfer coefficient, and overall heat transfer performance P EC of the staggered are better than those of the in-line as the inlet air velocity of the heat exchanger continues to increase. The maximum P EC of the staggered heat exchangers is 40.07% higher than that of the in-line heat exchangers. [ABSTRACT FROM AUTHOR]

The third-generation automotive medium-Mn steel (TAMM steel) was newly developed. The medium-Mn steel holds the advantages of fine and even microstructure, low austenizing temperature and wide quenching rate range, and thereby it is possible to contribute to excellent properties, high efficiency and low cost in warm/hot forming. However, research on the forming of this TAMM steel has just started. It is necessary to understand the relation between forming process and heat transfer, and further apply the heat transfer characteristics to numerical simulation. Therefore, this paper explored the interfacial heat transfer rules during warm forming the TAMM steel and determined the interfacial heat transfer coefficient (IHTC) using a self-designed experimental platform and Beck's inverse estimation method. The results indicate that the IHTC is not constant but varies nonlinearly during quenching. Moreover, the influences of several factors, including stamping pressure, stamping temperature and surface coating, on the IHTC were investigated. The IHTC increases with the increase of stamping pressure and stamping temperature. However, it decreases greatly with the increase of coating thickness. It is also found that the microstructure and mechanical properties are not sensitive to the coating and quenching rate, which is beneficial to the achievement of even mechanical properties of the stamped TAMM part, and helpful to the flexible design of quenching method and coating application in actual warm stamping process. [ABSTRACT FROM AUTHOR]

The relative rotation of turbine rotor determines the velocity triangle of the purge flow, which significantly influences the aerothermal performance of turbine endwalls. In this paper, a turbine cascade model is constructed to simulate the relative motion effect between the rotor endwall and the purge leakage. The effects of the swirl ratio of the purge leakage on the aerothermal performance of two endwall schemes (flat endwall and non-axisymmetric endwall) are investigated and compared. The results show that due to the relative motion, the purge leakage's flow direction and migration trajectory shift towards the blade suction side. The cooling jets tend to push the boundary layer towards the suction side, thus enhancing the lateral secondary flows. At a low MFR, with the decrease of swirl ratio, both endwalls' film cooling effectiveness is gradually reduced, with the aerodynamic performance and heat transfer coefficient changing very little. At a high MFR, with the decrease of swirl ratio, the aerodynamic loss and heat transfer coefficient of both endwalls are gradually increased, while the endwall film cooling effectiveness is gradually decreased. Globally, compared to the flat endwall, the non-axisymmetric endwall can dramatically enhance the film cooling effectiveness while effectively reducing the aerodynamic loss and heat transfer levels for all swirl ratios and MFRs investigated. • A numerical model of the turbine cascade was constructed to simulate the relative motion effect. • The effect of swirl ratio on endwall aerodynamic performance and vortex system structure for different endwalls in different mass flow rates were investigated. • The effect of swirl ratio on endwall film cooling for different endwalls in different mass flow rates were investigated. • The effect of swirl ratio on endwall heat transfer characteristics for different endwalls in different mass flow rates were investigated. [ABSTRACT FROM AUTHOR]

Abstract: In this paper the method used in a recently published paper to measure the latent heat of the martensite transformation during quenching of a steel is analysed. The arrangement of the experiment made it possible to obtain reasonable values for the latent heat of transformation, but cannot be expected to produce reasonable values for the heat transfer coefficient. Improved methods are discussed, including the novel probe design of the present authors. [Copyright &y& Elsevier]

This paper presents numerical analysis on microchannel laminar heat transfer and fluid flow of nanoencapsulated phase change materials (NEPCM) - water slurry as coolant, with two different melting ranges, 5 °C and 20 °C. In this study n-Octadecane for NEPCM and water is used as base fluid. With the purpose of evaluating best thermal performance, NEPCM–water slurry was compared to Cu-water nanofluid. The diameter ratio of the micro-tube was D i / D o = 0.3/0.5 mm with a tube length L = 100 mm with fixed heat transfer rate Q = 2 W. NEPCM-water slurry and Cu-water nanofluid were considered with various volume concentrations ϕ = 1,3 and 5% and diameter of the particles d p = 36 nm. Two different approaches were used concerning fixed Re and pumping power Π, in terms of average heat transfer coefficient and maximum temperature of the substrate. The results revealed that NEPCM-water slurry, with melting range of 5 °C, presents better thermal performances than base fluid and NEPCM with melting range of 20 °C. Besides, Cu-water nanofluid has much higher heat transfer coefficient ( h ave ) and lower maximum temperature of substrate ( t max ) for all values of volume concentrations, in cases of both comparison criteria ( Re or Π). [ABSTRACT FROM AUTHOR]

In recent years, mini-channel water flow boiling has been proven to have favorable cooling capability, especially for electronic component heat dissipation. Nevertheless, the fundamental principles related to flow boiling in mini-channel water lack clear comprehension, and the relevant experimental data on heat transfer performance and CHF visualization need to be supplemented. The behavior of vapor bubbles was explored in detail through visualization experiments to elucidate the mechanism of flow boiling and heat transfer in mini-channels. Meanwhile, the effects of mass flux, heat flux and tilt angle on the flow boiling of water and the CHF characteristics were revealed clearly though a series of crossover experiments. The results showed that in mini-channel water flow boiling, mass flux and heat flux will have effects on vapor bubble behavior, especially the merging of vapor bubbles, and then modify the heat transfer coefficient, and the mass flux in the optimal range exists. The tilt angle has less effect on the heat transfer coefficient, but affects the rate of vapor bubble growth. After local dryness the heat transfer mechanism transitions directly to film boiling instead of transition boiling, the boiling curve grows linearly with the wall superheat, and the CHF increases with increased mass flux. The research content of this paper helps to fill the gap in the experimental study of flow boiling in mini-channel and provides guidance for related practical applications. • Water boiling heat transfer and CHF in a mini-channel conduit is experimentally investigated. • The effects of mass flux and heat flux on bubble growth behavior and on heat transfer mechanisms were clarified. • The effect of tilt angle on the heat transfer of water flow boiling in mini-channel is not significant. • A new criterion for the classification of bubble growth and heat transfer properties was proposed. [ABSTRACT FROM AUTHOR]

This paper introduces the historical background about the development of water based, ethylene glycol (EG) based and EG:water mixture nanofluids for the past 20 years. The primary consideration is to review the salient of research work related to EG:water mixture nanofluids and their applications. Nowadays, the fundamental studies of nanofluids are increasing rapidly for engineering applications. The determination of the forced convection heat transfer and pressure drop was reviewed for nanofluid flow in a tube. The experimental and numerical heat transfers of nanofluids were presented. A review of other relevant research studies is also provided. Substantial heat transfer literature has been studied on water based nanofluids used in the fundamental study for engineering applications. However, there are limited studies that use EG:water mixture nanofluids in evaluation of forced convection heat transfer. A number of research studies have been performed to investigate the transport properties of EG:water mixture nanofluids either in experimental or numerical approach. As the performance of EG:water mixture nanofluids could be verified through experimental studies, researchers have conducted the experimental works using several types of potential nanofluids. As a result, nanofluids have been used in certain engineering applications such as in automotive, transportation, cooling of electronics components, solar, and nuclear reactor coolant. [ABSTRACT FROM AUTHOR]

In this work, a three-dimensional analysis is used to study the heat transfer and entropy generation inside a helically coiled tube-in-tube heat exchanger in laminar flow regime using two different types of nanofluids. Overall heat transfer coefficient, effectiveness, Nusselt number, and entropy generation of helically coiled tube-in-tube heat exchanger were investigated considering the nanoparticle volume concentrations between 0 and 2.0 vol.%. The mass flow rate of the nanofluid from the inner tube was kept and the mass flow rate of the water from the annulus was set at either half, full, or double the value. The present contribution is a companion paper of Huminic and Huminic [1]. The numerical results reveal that the use of nanofluids in a helically coiled tube-in-tube heat exchanger improves the heat transfer performances. Thus, the maximum effectiveness was 91% for 2% CuO nanoparticles and 80% for 2% TiO 2 nanoparticles. Also, the increase of nanoparticles volume concentration leads to the Nusselt number increase and the reduction of the entropy generation due heat transfer effects, the viscous effects to entropy generation being negligible. [ABSTRACT FROM AUTHOR]

This paper describes in detail the measurement of the overall heat transfer coefficient and phase transformation latent heat by modified pattern search method (PSM). By introducing the pattern search algorithm into finite element analysis, the measurement of heat transfer coefficients and phase transformation latent heat can be simplified as recording the temperature with time. Comparison of the measurements by PSM and temperature gradient method (TGM) shows that PSM can evaluate the overall heat transfer coefficient accurately. Experiments and simulations show that bainite transformation latent heat in 26NiCrMoV10-10 steel is about − 90.5 J/g, and the latent heat increases the surface temperature of a cylindrical specimen by a maximum of 76 °C. Error analysis shows that the highest difference between calculated temperature and measured temperature is 11.7 °C, which verifies that the heat transfer coefficient and phase transformation latent heat estimated by PSM are accurate, and that the pattern search method is reliable. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer characteristics and flow visualization of the hydrocarbon refrigerant R-600a during flow boiling inside horizontal smooth tube are investigated. For this purpose the well instrumented experimental apparatus including pump, flow meter, pre-heaters, test evaporator, by-pass and condenser was designed, fabricated and installed in order to measure the experimental data on flow boiling heat transfer and flow pattern of hydrocarbon refrigerant. The test section was a plain copper tube with an inside diameter of 8.7 mm and a wall thickness of 0.41 mm. The experimental tests were carried out varying: (i) the refrigerant mass fluxes within the range of 130–380 kg/m 2 s; (ii) the vapor qualities up to 0.70; and (iii) the heat fluxes from 10 to 27 kW/m 2 . The results indicate that, intermittent and annular flow patterns are the dominant flow patterns in the ranges of operating parameters of the present study. Also, the flow boiling heat transfer coefficient increased as the mass velocity increased in the fixed vapor quality. Moreover, heat transfer coefficient increased by the increase of vapor quality as the mass velocity was fixed. Most of the flow boiling heat transfer and flow pattern prediction methods have been developed for HFC and HCFC and CFC refrigerants and there are no general methods which can be applicable for all fluids. Therefore the present study experimental data for flow boiling heat transfer coefficient and flow pattern were compared with some existing models in order to find out which method could predict the convective boiling heat transfer and flow pattern characteristics of R-600a as a hydrocarbon refrigerant more accurately. The results indicate that Kattan–Thome–Favrat [23] flow map was closer to the flow patterns observed in this study. In addition, Gungor–Winterton [8] correlation had the best consistency with the present study experimental heat transfer data. [ABSTRACT FROM AUTHOR]

On course to maximization and optimization of a two-phase closed thermosyphon, this paper expounds on multiple impactful factors in an experimental and numerical setting. Among various fluids, 1-Butanol with 6% concentration, due to having the largest difference in surface tension with water, engendered the most favorable outcome. Response Surface Method has been employed to develop two correlations for efficiency and the thermal resistance of the thermosyphon paving the way for more accurate quantifications without necessarily running costly experiments. Other studied factors based on heat transfer coefficients of evaporator and condenser have been analyzed. The resultant numbers suggest an increase up to roughly 40% and 9% by changing power input and fluid, respectively. Also, enhancements falling within the range of 16 to 20% with the employment of 6% water-Butanol were observed by manipulation of filling ratio. However, coolant mass flow rate influence was limited to less than 10% when exposed to a 40% jump in flow rate if the working fluid was the same, implying its poor dominance in thermal performance of thermosyphon. As 1-Butanol proved superior among enhancing determinants, two numerical cases based on this factor were analyzed, confirming experimental findings while concurrently revealing distinctive phase-change patterns arising from the self-rewetting fluid. • Effect of different self–rewetting fluids on TPCT performance has been investigated. • The 1-Butanol, 1-Pentanol, and 1-Hexanol have been selected as self–rewetting fluids. • Effect of heat input, FR and coolant flow rate on TPCT performance have been studied. • Water/1-Butanol 6% has augmented the h e by 8.6% at 250 W comparing with pure water. • Numerical simulation has been done for water and Water/1-Butanol 6% charged TPCT. [ABSTRACT FROM AUTHOR]

Hydrofluoroolefins are environment-friendly working fluid and used as replacements for hydrofluorocarbons. A plate heat exchanger with a zeotropic mixture as working fluid has been widely used as a condenser in various thermodynamic systems. However, only very few studies investigated the zeotropic mixture condensation in the plate heat exchanger, and none of these studies addressed mixtures based on hydrofluoroolefins. In this paper, the hydrofluoroolefin mixture condensation in a plate heat exchanger were experimentally tested, and the characteristics of heat transfer and pressure drop were analyzed. We tested the condensation of R1234ze(E)/R1233zd(E) with four different mass fractions at wide ranges of temperatures and flow rates. The test results suggest the R1234ze(E)/R1233zd(E) mixture with the 0.352/0.648 mass fraction presents a 54% maximum degradation in heat transfer coefficient, compared with results obtained using the ideal mixing rule of pure fluids. A modified Silver-Bell-Ghaly model and a pure-fluid correlation can predict the heat transfer and pressure drop data with a mean absolute percentage deviation of 4.1% and 16.1%, respectively. In addition, for the same temperature glide, the heat transfer coefficient and frictional pressure drop of the R1234ze(E)/R1233zd(E) are up to 13.5% and 5.4% higher than those of the R134a/R245fa, respectively. [ABSTRACT FROM AUTHOR]

In this paper transient condensation and heat transfer been investigated on a printed circuit board (PCB) and a surface mounted (SMD) capacitor during a vapour phase reflow soldering (VPS) process. Classic VPS processes provide rapid heating with high heat transfer coefficient rates due to condensation that forms a continuous liquid film on the surfaces. Our aim was to develop a simple and fast computable method that, instead of relying on static descriptions, can reproduce the transient dynamics of the process. It was defined as an inverse heat conduction problem (IHCP) and proposed a solution based on finite difference modelling (FDM) with an alternating direction implicit (ADI) scheme. The layer dynamic was described with a parametric differential equation, and it was coupled to the concentrated and distributed thermal models of the PCB and the capacitor. The parameter values were determined by fitting to measured data. It was found that the heat transfer coefficient is inversely proportional to the layer thickness, and it changes over time according to the dynamic properties of layer growth. It was corroborated that the heat transfer depends on the geometry and thermal properties of the heated surfaces in VPS ovens. The presented method shows good agreement with the validation data. The new method provides a significant update over previous modelling approaches describing the condensation-based heating of the VPS process. [ABSTRACT FROM AUTHOR]

This paper presents the flow boiling heat transfer coefficient and pressure gradient measurements of R32 and R1234yf mixtures in a microchannel tube at three concentrations: 15/85, 50/50, and 85/15 by mass. R32/R1234yf mixture has a temperature glide curve that the 15/85 R32/R1234yf mixture has low-temperature glide (0.5 K) while the opposite concentration (85/15) has the highest glide (7 K). The experiment was conducted on a 24-port microchannel tube with an average hydraulic diameter of 0.643 mm. The experiment covers mass flux from 100 to 200 kg-m−2 s−1, heat flux from 0 to 6 kW-m−2, and vapor quality from 0 to 1. Mass flux has a strong effect on both HTC and dP/dz. Heat flux affects HTC but not dP/dz. In addition, the results are compared to pure fluid to discuss the effect of temperature glide. When the glide is higher, the HTC is lower and flatter. At fixed quality, as R1234yf concentration increases, HTC of the mixture decreases slightly first and then increases dramatically. Furthermore, two experiment methods are compared: evaporation in one pass and in one test section. The non-equilibrium in two-phase flow affects concentration gradient in the two phases and further affects the HTC of mixtures with high-temperature glide. • Heat transfer coefficient and pressure gradient of R32/R1234yf are measured. • Three mixtures with different compositions are studied. • Non-equilibrium in two-phase flow affects heat transfer coefficient measurements. • Heat transfer coefficient is lower when the mixture has a larger temperature glide. [ABSTRACT FROM AUTHOR]

The paper presents investigation results on heat transfer in falling films of refrigerant R21 on an array of horizontal tubes with a coating deposited by the method of microarc oxidation (MAO). Different MAO-coatings were applied on the surface of tubes of aluminum alloys AD31 and D16T with a diameter of 10 mm in silicate-alkaline, acidic, and phosphate electrolytes. Two coated test tubes were copper-plated additionally in a copper sulfate solution. The enhancement of heat transfer on the tubes with a porous coating depends in a complex manner on the parameters of MAO-coating surfaces defined by their treatment regimes. The highest 3-fold heat transfer enhancement in comparison with plain tube in the nucleate boiling regime was achieved on the surface of a tube made of alloy D16T with the MAO coating of 30-μm thickness, obtained in a silicate-alkaline electrolyte. Additional deposition of copper particles on the MAO-coatings leads to an enhancement of heat transfer by (20–30) % compared to plain tube. In the studied range of Reynolds number 600–1500, the effect of irrigation density on the heat transfer coefficients is not observed. [ABSTRACT FROM AUTHOR]

Abstract: Nanofluid is a heat transfer fluid that can improve the performance of heat exchanger systems. Different parameters such as particle size, shape, and volume concentration affect the performance of these systems. The objective of this paper is to study the effect of different nanoparticle shapes (such as cylindrical, bricks, blades, platelets, and spherical) on the performance of a shell and tube heat exchanger operating with nanofluid analytically. Boehmite alumina (γ-AlOOH) nanoparticles of different shapes were dispersed in a mixture of water/ethylene glycol as the nanofluid. The thermodynamic performance of the shell and tube heat exchanger that is used in a waste heat recovery system was analysed in terms of heat transfer rate and entropy generation. Established correlations were used to measure the thermal conductivity, heat transfer coefficient and rate and entropy generation of nanofluid. The results show an increase in both the heat transfer and thermodynamic performance of the system. However, among the five nanoparticle shapes, cylindrical shape exhibited better heat transfer characteristics and heat transfer rate. On the other hand, entropy generation for nanofluids containing cylindrical shaped nanoparticles was higher in comparison with the other nanoparticle shapes. However, the increased percentage of entropy was below 1%. Therefore, this greater entropy generation could be deemed negligible and cylindrical shaped nanoparticles are recommended to be utilized in heat exchanger systems working with nanofluids. [Copyright &y& Elsevier]

Abstract: In this paper, experiments are carried out to obtain convective heat transfer coefficients of turbulent flow and transition flow of molten Hitec salts in a circular tube. The present experimental data together with experimental data of four kinds of molten salts from the existing literature are correlated for transitional and turbulent convective flow respectively. In addition, the Prandtl number dependence of convective heat transfer with different working fluids is examined. It is shown that the present experimental data are in good agreement with existing correlations. [Copyright &y& Elsevier]

Abstract: The nucleate pool boiling heat transfer characteristics of TiO2 nanofluids are investigated to determine the important parameters'' effects on the heat transfer coefficient and also to have reliable empirical correlations based on the neural network analysis. Nanofluids with various concentrations of 0.0001, 0.0005, 0.005, and 0.01vol.% are employed. The horizontal circular test plate, made from copper with different roughness values of 0.2, 2.5 and 4μm, is used as a heating surface. The artificial neural network (ANN) training sets have the experimental data of nucleate pool boiling tests, including temperature differences between the temperatures of the average heater surface and the liquid saturation from 5.8 to 25.21K, heat fluxes from 28.14 to 948.03kWm−2. The pool boiling heat transfer coefficient is calculated using the measured results such as current, voltage, and temperatures from the experiments. Input of the ANNs are the 8 numbers of dimensional and dimensionless values of the test section, such as thermal conductivity, particle size, physical properties of the fluid, surface roughness, concentration rate of nanoparticles and wall superheating, while the outputs of the ANNs are the heat flux and experimental pool boiling heat transfer coefficient from the analysis. The nucleate pool boiling heat transfer characteristics of TiO2 nanofluids are modeled to decide the best approach, using several ANN methods such as multi-layer perceptron (MLP), generalized regression neural network (GRNN) and radial basis networks (RBF). Elimination process of the ANN methods is performed together with the copper and aluminum test sections by means of a 4-fold cross validation algorithm. The ANNs performances are measured by mean relative error criteria with the use of unknown test sets. The performance of the method of MLP with 10-20-1 architecture, GRNN with the spread coefficient 0.7 and RBFs with the spread coefficient of 1000 and a hidden layer neuron number of 80 are found to be in good agreement, predicting the experimental pool boiling heat transfer coefficient with deviations within the range of ±5% for all tested conditions. Dependency of output of the ANNs from input values is investigated and new ANN based heat transfer coefficient correlations are developed, taking into account the input parameters of ANNs in the paper. [Copyright &y& Elsevier]

Abstract: In this paper, heat transfer of single cylindrical particle affected by wall has been investigated numerically and experimentally for Reynolds number range 2000 to 6000. The heat transfer in two different orientations, axial and cross flow over the particle has been considered in simulation with MultiPhysics Software FEMLAB version 2.3. The heat and mass transfer analogy technique has been applied for validation of the simulation results. The coated particle with naphthalene was sublimated to obtain the corresponding Sherwood numbers. The results show that the CFD model can predict the particle-to-fluid heat transfer for two situations due to trivial error (an average error of 6%) compared to experimental values. Influence of wall on heat transfer of particle in seven different bed-to-cylinder diameter ratio (N=1.66, 2.65, 2.75, 5, 6.66, 12, and 18) have been discussed in different velocities. According to obtaining results, with increasing the bed-to-cylinder diameter ratio over the 12 wall have no significant consequence on Nusselt number. Due to this fact, a CFD based correlation has been proposed to consider the wall effects on particle-to-fluid Nusselt number with an average error of 2.19%. [Copyright &y& Elsevier]

Abstract: The numerical modeling of the conjugate heat transfer and fluid flow of Al2O3/water nanofluid through the micro-tube was presented in the paper. The laminar flow regime was considered along with viscous dissipation effect. The diameter ratio of the micro-tube was D i/D o =0.1/0.3mm with a tube length L =100mm. The heat transfer rate was fixed to Q =0.5W with three different Br=0.1, 0.5 and 1. The water based Al2O3 nanofluid was considered with various volume concentrations of Al2O3 particles ϕ =1, 4, 6, 9% and two diameters of the particles D p =10nm and 47nm. The analysis was performed on the results for local heat transfer coefficient. [Copyright &y& Elsevier]

Abstract: In this study, forced convection flows of nanofluids consisting of water with TiO2 and Al2O3 nanoparticles in a horizontal tube with constant wall temperature are investigated numerically. The horizontal test section is modeled and solved using a CFD program. Palm et al.''s correlations are used to determine the nanofluid properties. A single-phase model having two-dimensional equations is employed with either constant or temperature dependent properties to study the hydrodynamics and thermal behaviors of the nanofluid flow. The numerical investigation is performed for a constant particle size of Al2O3 as a case study after the validation of its model by means of the experimental data of Duangthongsuk and Wongwises with TiO2 nanoparticles. The velocity and temperature vectors are presented in the entrance and fully developed region. The variations of the fluid temperature, local heat transfer coefficient and pressure drop along tube length are shown in the paper. Effects of nanoparticles concentration and Reynolds number on the wall shear stress, Nusselt number, heat transfer coefficient and pressure drop are presented. Numerical results show the heat transfer enhancement due to presence of the nanoparticles in the fluid in accordance with the results of the experimental study used for the validation process of the numerical model. [Copyright &y& Elsevier]

Abstract: In this study, the best artificial intelligence method is investigated to estimate the measured convective heat transfer coefficient and pressure drop of R134a flowing downward inside a vertical smooth copper tube having an inner diameter of 8.1mm and a length of 500mm during annular flow numerically. R134a and water are used as working fluids in the tube side and annular side of a double tube heat exchanger, respectively. The ANN training sets have the experimental data of in-tube condensation tests including six different mass fluxes of R134a such as 260, 300, 340, 400, 456 and 515kgm−2 s−1, two different saturation temperatures of R134a such as 40 and 50°C and heat fluxes ranging from 10.16 to 66.61kWm−2. The quality of the refrigerant in the test section is calculated considering the temperature and pressure obtained from the experiment. The pressure drop across the test section is directly measured by a differential pressure transducer. Input of the ANNs are the measured values of test section such as mass flux, heat flux, the temperature difference between the tube wall and saturation temperature, average vapor quality, while the outputs of the ANNs are the experimental condensation heat transfer coefficient and measured pressure drop in the analysis. Condensation heat transfer characteristics of R134a are modeled to decide the best approach using several artificial neural network (ANN) methods such as multilayer perceptron (MLP), radial basis networks (RBFN), generalized regression neural network (GRNN) and adaptive neuro-fuzzy inference system (ANFIS). Elimination process of the ANN methods is performed by means of 183 data points, divided into two sets randomly, obtained in the experiments. Sets of test and training/validation include 33 and 120/30 data points respectively for the elimination process. Validation process, in terms of various experimental conditions, is done by means of 368 experimental data points having 68 data points for test set and 300 data points for training set. In training phase, 5-fold cross validation is used to determine the best value of ANNs control parameters. The ANNs performances were measured by means of relative error criteria with the usage of unknown test sets. The performance of the method of multi layer perceptron (MLP) with 5-13-1 architecture and radial basis function networks (RBFN) were found to be in good agreement, predicting the experimental condensation heat transfer coefficient and pressure drop with their deviations being within the range of ±5% for all tested conditions. Dependency of outputs of the ANNs from input values is also investigated in the paper. [Copyright &y& Elsevier]

Abstract: This paper reports on the investigation of pool boiling heat transfer of γ-Al2O3/CMC non-Newtonian nanofluids. To prepare nanofluids, γ-Al2O3 nanoparticles were dispersed in CMC solution (carboxy methyl cellulose in water) using ultrasonic mixing and mechanical mixer. Different concentrations of CMC non-Newtonian fluids and γ-Al2O3/CMC non-Newtonian nanofluids were tested under nucleate pool boiling heat transfer conditions. Experiments were carried out at atmospheric pressure. Results show that the pool boiling heat transfer coefficient of CMC solutions is lower than water. The decrease in boiling heat transfer is more pronounced at higher CMC concentrations and, as a result, higher solution viscosity. Adding nanoparticles to CMC non-Newtonian solutions results in an improved boiling heat transfer performance. The enhancement in the boiling heat transfer coefficient increases with the nanoparticle concentration; at a concentration of 1.4wt.%, the boiling heat transfer coefficient increases by about 25% when compared to the base fluid. [Copyright &y& Elsevier]

Abstract: In order to understand the heat transfer characteristics of molten salt and testify the validity of the well-known empirical convective heat transfer correlations, an experimental study on turbulent convective heat transfer with molten salt in a circular tube was conducted in this paper. Molten salt circulations were realized and operated in a specially designed system over 1000 h. The flow rates and temperatures of molten salt and mineral oil at the inlet and outlet in the test section were measured and the average forced convective heat transfer coefficients of molten salt were determined by least-squares method. Finally, heat transfer correlations of turbulent flow with molten salt in a circular tube were obtained. Good agreement was observed between the experimental data of molten salt and the existing well-known correlations. The experimental data of molten salt in the present work are consistent with experimental results reported by different references in a wide range of Prandtl numbers from 0.7 to 59.9. [Copyright &y& Elsevier]

Abstract: An experimental apparatus for evaluating carbon dioxide local heat transfer coefficients during flow boiling has been set up at University of Naples Federico II. In this paper, the local heat transfer coefficient measured is evaluated relative to the measurement error, varying the type and costs of the instrumentation. The analytical analysis has been performed prior to construction of the test facility. The main aim of the work is to provide a methodology that allows one to reach the best compromise between measurement accuracy and financial resources in the selection of the instrumentation. [Copyright &y& Elsevier]

Abstract: Two- and three-dimensional CFD modeling of heat transfer from discrete circular cylindrical particles in four different situations including A) infinite cylinder in cross-flow, B) cross-flow on finite cylinder with different aspect ratio in a rectangular duct, C) axial-flow on finite cylinder and D) axial-flow on finite cylinder with upstream turbulence have been investigated with the commercial CFD software, FEMLAB. The results were validated using experimental data from different research papers and also experimental correlations and show good quantitative and qualitative agreement with each other. In case B, a correction term has been proposed from CFD work which is applied to an experimental correlation to consider the aspect ratio influence on predicting the Nusselt number with an average error of 3.7%. [Copyright &y& Elsevier]

There is a scarcity of available data on boiling process in vertically oriented tube bundles in accessible sources. Lack of systematic studies is limiting further expansion of this highly efficient process of heat transfer into heat recovery field. In this paper boiling process of three triangular pitched and vertically oriented tubes has been studied in ethanol at 78 °C. The main focus of this work was to study the effect of tube spacings on heat transfer coefficient (HTC) and bubbles behavior (bubble departure diameter in particular) that were visualised with the help of a high speed camera. Experiments were performed in a wide range of tube spacings (from 10.75 to 0.25 mm) and heat flux densities (from 3 to 70 kW/m2). The obtained results show that, long spacings i.e., much longer than bubble departure diameter, have no influence on HTC as well as on bubbles behavior. On the contrary, the spacings on the order of the bubble departure diameter tend to create slug flow in the bundle, that is very beneficial for the heat exchange at low heat fluxes. Finally, narrow spacings that are much shorter than the bubble departure diameter have shown the potential to enhance the HTC in tube bundles with low length to diameter ratios. • Long spacings have no influence on heat transfer in tube bundles. • Slug flow in a bundle is very beneficial for heat exchange at low heat fluxes. • Spacings shorter than bubble departure diameter enhance heat exchange at low length to diameter ratios. [ABSTRACT FROM AUTHOR]

In this paper, an adaptive neuro-fuzzy inference system (ANFIS) with fuzzy C-means clustering (FCM) structure identification is proposed to model condensation heat transfer and flow regimes in flattened smooth tubes with different aspect ratios. The FCM-ANFIS model was trained by using experimental data points for six effective chosen parameters of saturation temperature, heat flux, mass flux, aspect ratio and hydraulic diameter of the flattened tube, and vapor quality. Three flow regimes of annular flow, stratified, and intermittent flow were linked to the effective parameters based on the experimental data. Two models were proposed to predict the condensation heat transfer coefficient and the flow regime of R134a and R410a in flattened smooth tubes. Three statistical criteria were used to ascertain the accurateness of the models compared to the experimental results. It is found that while among benchmarked cases, the proposed model for the condensation heat transfer coefficient performs well, the best result with the lowest error (MAE = 0.029, RMSE = 0.036 and MRE = 2.83%) is when T sat = 45 °C, q″ = 10 kW/m2, G = 100 kg/m2. s, β = 6 and D h = 2.3 mm. On the other hand, in the worst-performing case when the errors are MAE = 0.239, RMSE = 0.239 and MRE = 13.36%, the predicted results are still in the uncertainty range of the experimental result when T sat = 45 °C, q″ = 5 kW/m2, G = 200 kg/m2. s, β = 6 and D h = 2.3 mm. [ABSTRACT FROM AUTHOR]

In this paper, the 3D numerical calculations of the water flow inside a pin-fin heat sink (PFHS) to determine the influence of tip clearance on the hydrothermal behavior and frictional & thermal irreversibilities of the fluid flow. The numerical simulations were conducted for five different fin heights (ξ = 1.5 mm to 2.5 mm) considering four different Reynolds numbers (Re =500, 1000, 1500, and 2000). The results demonstrated that the highest heat transfer coefficient is associated with ξ = 2.25 mm due to more uniform vorticity distributions, while the minimum central processing unit (CPU) average temperature is obtained for ξ = 2.5 mm due to the higher heat transfer surface area. The minimum thermal and frictional entropy generation rates are associated with PFHS with ξ = 1.5 mm. However, the PFHS with ξ = 2.25 mm has the highest Performance Evaluation Criterion (PEC) of nearly equal to 1.28 as compared to the other ξs. Besides, the fluid flow from the pin-fin base to the top and vice versa leads to create the vortices especially at the entrance of the PFHSs with tip clearance. [ABSTRACT FROM AUTHOR]

When the membrane wall water tubes and immersed tubes heat exchanger cannot effectively control the furnace temperature, arranging the horizontal tubes in the dilute phase is a method to solve the limited layout of the heat exchanger area. In order to study the heat transfer characteristics of the horizontal tubes in the dilute phase, the local and average heat transfer coefficients (HTC) were measured based on the pressurized fluidized bed apparatus under different pressures, particle sizes and gas velocities. The experimental results showed the improved heat transfer at elevated pressure and high gas velocities. The heat transfer tube with a lower height position had higher heat transfer coefficients because of the particle concentration effect. Under the same system pressure, the circumferential HTC of the horizontal tube were slightly higher at 0° and 180° circumferential positions, but the overall distribution tended to be uniform. Based on the experimental results, an empirical model for the average heat transfer coefficient prediction was developed and can predict 97.4% of the experimental data from this study and other different sources within 25% error with the absolute average deviation of 8.18%. This work can provide reliable experimental data for the design of heat exchanger tubes in dilute phase zone of pressurized fluidized bed and existing model verification. Also the correlation proposed in the paper can be translated to CFD code for further application. [ABSTRACT FROM AUTHOR]

In this paper, 3D analysis of forced convection of Newtonian and non-Newtonian nanofluids through a horizontal pipe using single- and two-phase mathematical models is performed. The models include the Newtonian single-phase model, non-Newtonian single-phase model, Newtonian two-phase mixture model and finally non-Newtonian two-phase mixture model. The geometry of the problem is a horizontal pipe that is filled with Al 2 O 3 /water nanofluid. The effects of particle volume fraction and the Reynolds number on the heat transfer and hydrodynamic characteristics of the flow are examined. Results indicate that for the volume fraction of 0.01, the non-Newtonian mixture model gives more accurate results in comparison with the other models. By increasing the solid volume fraction, the mixture models give a higher heat transfer coefficient in comparison with that of the corresponding single-phase model; however, by comparing the results of Newtonian and the corresponding non-Newtonian models, no meaningful relationship can be obtained. Furthermore, for all the investigated models, the Nusselt number is increased with increasing the Reynolds number. [ABSTRACT FROM AUTHOR]

New and efficient mixing heat technology is increasingly demanded for many industrial fields. This paper proposed a novel technique of enhancing convective heat transfer by dynamic controlling rib and investigated several cases of vibration rib controlling flow. The study results of heat transfer coefficient, temperature and turbulent intensity indicate that dynamic flow control is an efficient heat transfer enhancement technique. The dynamic flow control breaks through the current enhancement heat transfer technologies such as general stationary rib and other vortex generators, and achieves a better heat transfer effect. Comparing traditional rib technologies, vibration rib does not only increase heat transfer performance but leads a lower flow resistance. More importantly, it can be dynamically adjusted to achieve better flow turbulence. As a conclusion, the enhancement heat transfer performance can be timely dynamically adjusted to meet the different heat transfer demand by changing timely the frequency and amplitude of vibration rib, which achieves the optimization of whole system thermal protection. New enhancing heat transfer technique effectively improve the comprehensive heat transfer performance, and provides a reference for intelligent thermal control protection system [ABSTRACT FROM AUTHOR]

This paper numerically qualifies the effect of rib orientation on heat transfer and flow performances of mist/steam in square channels with inclined ribs. Four types of rib orientations are investigated. The study is conducted in contrast to steam with Reynolds number ranged from 10,000 to 60,000. Secondary flow patterns and droplet trajectories are analyzed to reveal the underlying reasons bringing about heat transfer enhancement due to mist/steam. The results indicate that rib orientation greatly determines secondary flow pattern in channels, and the distribution of secondary flow triggered by N type ribs is opposite to that by P type ribs. Such secondary flow forces droplets at different locations to assemble near different side walls. The places where water drops gather in a large quantity provide splendid heat transfer enhancement. And this effect gets more obvious at higher Reynolds number. For mist/steam, both heat transfer coefficient and overall heat transfer performance in the first passage are remarkably stronger than that in the second one. In general, NN channels and PP channels filled with mist/steam have the best heat transfer performance among the investigated cases. [ABSTRACT FROM AUTHOR]

This paper designs the treelike networks with the wedge-shaped channel from the idea of bionics. Pool boiling heat transfer experiments of FC-72 under different liquid subcoolings were conducted on the networks with different branch levels. The bubble phenomenon was recorded to reveal the boiling mechanism. The bubble pumpless movement towards the diverging direction in the wedge-shaped channel under the action of capillary force was observed. Under the saturated condition, the surfaces are easier to be completely covered by bubbles and the effect of treelike networks is hindered, which results in similar heat transfer performance. With the increase of liquid subcoolings, the bubble fraction decreases and the effect of the branch level comes into effect. On the surface with high branch level, the bubbles are transported immediately at small radius by the narrow wedge-shaped channel. The moving bubbles disturb the fluid and coalesce with other bubbles, which increases the heat transfer coefficient. There are also more liquid supply pathways between small bubbles, which increases the critical heat flux. The importance of this research is to provide a solution to the difficulty of bubbles vertical departure caused by the lack of buoyancy under microgravity. [ABSTRACT FROM AUTHOR]

This paper presents a detailed discussion on the hysteresis phenomenon caught during the flow boiling heat transfer of R245fa on a Carbon/Carbon surface. Due to the partial hydrophilicity of the Carbon/Carbon surface, a hysteresis in flow boiling heat transfer is observed and studied to understand the underlining heat transfer mechanisms on the basis of the surface activation. A combined experimental approach is developed by monitoring the local wall temperature and contemporarily recording the two-phase flow patterns using a high-speed video camera. The observed dewetting/rewetting phenomenon is linked to the activation process of the Carbon/Carbon surfaces giving new insights on the boiling behavior of hydrophilic surfaces. The results show that after activation the heat transfer coefficients increases up to 20%. The experimental results are compared against the estimations of a model recently proposed by the present Authors and based on R134a measurements. The proposed model is able to predict the heat transfer coefficients within ±20%. • R245fa flow boiling on a Carbon/Carbon surface is studied. • A hysteresis of boiling heat transfer is observed and studied. • A combined experimental methodology is applied to study the surface activation process. • The dewetting/rewetting phenomenon during flow boiling was linked to the activation process. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer coefficients and critical heat fluxes of the R134a fluid in two-phase flow was studied in a 27-channel, 421 μm hydraulic diameter and 40 mm long microchannel experimental set-up. Data were collected at constant mass flux of 1000 kg m−2 s−1, saturation inlet temperature range of 20–28 °C and at inlet vapor quality range of 0.01–0.20. The heat flux was initiated at 250 kW m−2 and gradually increased until reaching the critical heat flux (CHF). The effect of inlet vapor quality, saturation inlet temperature and heat flux on heat transfer coefficient and critical heat flux were investigated in detail and presented graphically. It can be observed that the CHF decreases with the increasing of saturation inlet temperature and inlet vapor quality. However, the heat transfer coefficient at the CHF point increased with the increasing of saturation temperature and the heat transfer coefficient at the CHF point was decreased with the increasing of vapor quality. The test results from the correlations, suggested in well-known articles, were compared with the experimental data, where the Kosaz et al. proposed correlation was showing well fit with the experimental data having absolute error of 9.86%. In this investigation, the new correlations for the heat transfer coefficient for the heat transfer coefficient and the critical heat flux have been proposed by taking into account the effect of the vapor quality. [ABSTRACT FROM AUTHOR]

In order to design a new vortex generator (VG) and effectively induce and properly control the vibrations of elastic coil pipes (ECPs) in heat exchanger (HE), the effects of the fluid flow parameters, vortex element size and external flow field on vortex flows in a fluid domain with variable cross-section were analyzed. The vibration and heat transfer characteristics of the ECPs were studied based on the new VG. The numerical results show that the fluid flow parameters, vortex element size and external flow field have influence on the frequency and intensity of vortex flow. The VG designed in this paper can generate vortex flows with basically the same frequency and intensity at each branch outlet. Under the action of the new VG, the vibration amplitudes of the ECPs are significantly enhanced and the vibration uniformity is improved. Meanwhile, the new VG can increase the heat transfer coefficients of the ECPs and improve the consistency of the heat transfer performances of the ECPs. [ABSTRACT FROM AUTHOR]

This paper explores machine learning approach as a heat transfer correlation. Machine learning significantly reduces the effort to develop multi-variable heat transfer correlations, and is capable of readily expanding the parameter domain. Random forests algorithm is used to predict the convection heat transfer coefficients for a high-order nonlinear heat transfer problem, i.e., convection in a cooling channel integrated with variable rib roughness. For 243 different rib array geometries, numerical simulations are performed to train and test ML model based on six input features. Machine learning model predicts closely to numerical simulation data with high determination of coefficient (R2), e.g., R2 > 0.966 for the testing dataset. The capability and limitation of random forests algorithm are discussed with validation dataset. [ABSTRACT FROM AUTHOR]

This paper investigates experimentally heat transfer during evaporation of n-pentane inside a parallel multiport mini-channel. The experimental tests are conducted in a test section within five rectangular stainless steel minichannels. The local heat transfer coefficient is measured for different testing conditions to study the influence of mass flux (G = 113–187 kg/m2·s) and heat flux (200 W–320 W). For low vapor quality region [0–0.4], the domination of nucleate boiling is significant while the forced convective boiling is obtained for high vapor quality region. A comparison of the experimental results with existing correlations is carried out by calculating mean absolute deviation which led to some diverging correlations >100% while others present a good estimation around 30%. Thus, a new correlation is developed to predict local heat transfer with MAE of 22% based on the experimental results. A considerable attention is also paid to pressure drop effects by measuring experimentally pressure drop in the minichannels and develop a new correlation to evaluate frictional pressure drop. [ABSTRACT FROM AUTHOR]

This paper aims to evaluate the hydrothermal and irreversibility behaviour of a biological water-Ag nano-fluid in a new heatsink with helical microchannels. Two-phase mixture model is applied to precisely simulate the behavior of nanofluid in the nanoadditive concentration (φ) range of 0–1% and Reynolds number (Re) range of 500–1500. The influences of φ and Re on the convective heat transfer coefficient, CPU surface temperature, pumping power, as well as the irreversibilities due to heat transfer and fluid friction are examined. The findings depict that boosting the Re and φ augments the performance of heatsink by intensifying the convective heat transfer coefficient of the working fluid which favourably declines the CPU surface temperature and the heat transfer irreversibility and importantly results in the temperature uniformity of the CPU surface. However, intensification in Re adversely affects the pumping power, the fluid friction and total irreversibilities in the system. Furthermore, it is revealed that the nano-fluid always has a superior cooling performance as compared with the pure water. Finally, it is found that the best hydrothermal performance of the nano-fluid in the proposed heatsink occurs at Re = 1500 and φ = 1%, while the minimum total irreversibility occurs at Re = 500 and φ = 1%. [ABSTRACT FROM AUTHOR]