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They found that this novel loop heat pipe can start up and run successfully under different working modes and the heat dissipation capacity of a set of loop heat pipes can reach 250 W at room temperature. The series of UK Heat Transfer Conference (UKHTC) has been organized under the auspices of the UK National Heat Transfer Committee since 1984. In the seventh paper, Ren et al. conducted the parametric analysis of a novel photovoltaic/thermal system using amorphous silicon cells and micro-channel loop heat pipes. [Extracted from the article]

Pulsating heat pipes are widely used in cooling electronic devices, automotive radiators, refrigeration, and air conditioning systems. The addition of nanoparticles is one of the effective ways to improve the heat transfer characteristics of fluids. This paper, based on large eddy simulation (LES) numerical method and field synergy theory, investigates the heat transfer characteristics of Al2O3/Cu mixed nanofluid in a self-excited oscillating heat exchanger tube. Through multi-objective optimization of field synergy angles, field synergy factors, and structural parameters, the enhanced heat transfer mechanism of mixed nanofluids in a self-excited oscillating heat pipe is investigated. It is found that there is a significant negative correlation between the heat transfer coefficient h at the wall surface of the heat exchanger tube and the synergistic angle β. The incorporation of nanoparticles improves the synergistic properties between the velocity and temperature fields. The optimum structural parameters are L/D = 0.469931, α = 128.9904° and d2/d1 = 2.047937, and the optimization increases Nu by 1.92%, decreases f by 10.46%, and decreases the temperature synergy angle β by 2.24°. At the same time, the orthogonality or lack of orthogonality between isotherms and streamlines determines the extent of synergistic performance in the countercurrent vortex. [ABSTRACT FROM AUTHOR]

Miniaturization and enhanced performance of microchips has resulted in powerful electronic devices with high heat flux components. For these advanced electronics, the current heat transfer method of single-phase forced convection is reaching its thermal limit and more effective cooling solutions are needed. A pumped two-phase loop, in which a pump circulates a working fluid that evaporates to absorb heat, can offer a solution. In this paper the cooling performance of a pumped two-phase loop is discussed and validated. A numerical tool has been developed to aid in designing a fit-for-purpose pumped two-phase loop and to predict its behaviour to changing system parameters and heat inputs. Results from the numerical model are compared with temperature, pressure and flow velocity measurements obtained from a prototype setup. The effects of applying varying heat loads on both a single evaporator and on multiple evaporators simultaneously either in series or in parallel have been investigated. Heat transfer coefficients between 7 and 10 kW/m2K were obtained during the experiments. Model predictions correspond well to the measured performances and findings on the two-phase boiling behaviour are presented. The model is particularly useful for the rapid assessment of the layout of a pumped two-phase loop for high heat flux electronics cooling. [ABSTRACT FROM AUTHOR]

The current technologies to treat hypersaline produced water (PW), such as thermal evaporation, are usually energy-intensive and cost-prohibitive. This study developed a low-cost, robust, solar-driven carbon black and airlaid paper-based evaporator (CAPER) for desalination of PW in the Permian Basin, United States. The study aims to better understand the removal of aromatic organic compounds and heavy metals during solar distillation, water output, and heat transfer. Outdoor experiments using CAPER assisted with polystyrene foam in a single slope, single basin solar still achieved an enhanced average evaporation rate of 2.23 L per m2 per day, 165% higher than that of a conventional solar still. Analysis of heat transfer models demonstrated that CAPER solar evaporation achieved an evaporative heat transfer coefficient of ∼28.9 W m−2·K−1, 27.9% higher than without CAPER. The maximum fractional energy of evaporation and convection heat transfer inside the solar still with and without CAPER was ∼81.4% and ∼78.2%, respectively. For the PW with a total dissolved solids concentration of 134 g L−1, solar distillation removed 99.97% salts and over 98% heavy metals. The high removal efficiency of 99.99% was achieved for Ca, Na, Mg, Mn, Ni, Se, Sr, and V. Organic characterization revealed that solar distillation removed over 83% aromatic compounds. Solar desalination using CAPER provides a low-cost and high-performance process to treat PW with high salinity and complex water chemistry for potential fit-for-purpose beneficial uses. Image 1 • A carbon black and airlaid paper-based solar evaporator (CAPER) was developed. • Outdoor distillation of produced water (PW) achieved evaporation of 2.23 L m−2·d−1. • The evaporation rate of PW with CAPER is 165% higher than without CAPER. • Heat transfer coefficient with CAPER is 27.9% higher than without CAPER. • 99.97% salts, 98% metals, >83% aromatic compounds removed from 134 g L−1 TDS PW. [ABSTRACT FROM AUTHOR]

In hot and dry climatic zones, direct evaporative cooling (DEC) system is used as an economical and efficient alternative to traditional air conditioning systems. The present paper concerns comprehensive experimental investigations and analysis of heat and mass transfer characteristics and thermal performance parameters of a bee-hive construction of corrugated-cellulose papers as a new cooling pad material. The performance parameters used to evaluate the effectiveness of the cooling pad materials are the outlet air temperature and relative humidity, pressure drop, humidifier effectiveness, rate of the evaporated water, sensible cooling capacity, specific cooling capacity (SCC), coefficient of performance (COP), specific water consumption (SWC), and Nusselt and Sherwood numbers. The cooling pad performance was investigated for a wide range of air and water temperatures and flow rates and pad thicknesses. The results show the enhancements of heat and mass transfer coefficients and the performance parameters with rising air temperature and water flow rate to humidifier. High values of the performance parameters (h c = 45 W/m2oC and h m = 0.23 m/s, ε h u m = 0.85 and COP = 170) are obtained compared to the traditional pad materials. Experimental correlations for the evaporative cooler performance parameters in terms of air and water temperatures and flow rates pad thicknesses are presented. • Cellulose paper, bee-hive structure has efficient thermal to be used as evaporator cooling pads. • Good thermal-hydraulic characteristics at a wide range of operating and design conditions are obtained. • Presented empirical correlations predict the performance parameters very well. • High values of the performance parameters of the proposed cooling pad were obtained. • The highest saturation efficiency is 84% with associated temperature and pressure drops 18 °C and 8 Pa. [ABSTRACT FROM AUTHOR]

In this paper, the effect of solar intensity on the heat pipe tip temperature in a heat pipe type—evacuated-tube solar collector (HP-ETSC) was investigated. A simple relation was proposed, relating the solar intensity to the heat pipe tip temperature generated from the experimental data. This simple empirical relation was applied in a set of heat transfer equations derived to predict the heating medium temperature at the manifold outlet of the evacuated-tube solar collector. The calculated results corresponding to two types of heating medium, i.e., palm oil and water, were compared with experimental results from the literature. The results show that the average error was 6.41% for the case of palm oil and 4.66% for the case of water. Based on the case of water as a heating medium fluid, it was found that the flow rate of the heating medium fluid affected the accuracy of prediction, as the percentage error increased with the heating medium flow rate. The maximum percentage error increased from only 1.83% for a water inlet flowing at a Reynolds number of about 2.4 × 103 to 15.23% for a water flow rate at a Reynolds number of about 2.6 × 104. The correction factor was added into the correlation to predict the heat transfer coefficients of heating medium fluids. With this correction factor, the maximum error could be reduced from 11.78% to 7.29% for the palm oil case and from 15.23% to 5.57% for the water case. The average errors corresponding to palm oil and water cases could be reduced to 0.74% and 1.26%, respectively. [ABSTRACT FROM AUTHOR]

The cooling system is the auxiliary equipment that consumes the most energy in a data center, accounting for about 30 to 50% of the total energy consumption. In order to effectively reduce the energy consumption of a data center, it is very important to improve the heat exchange efficiency at the chip level. Compared with air cooling, single-phase cold plate liquid cooling, and immersion liquid cooling, the flat loop heat pipe (FLHP) is considered to be a better chip-level cooling solution for data centers. It has extremely high heat transfer efficiency and heat flux variability, and it can avoid the operation risk caused by liquid entering the server. In this paper, a FLHP with an evaporator designed with a "Tesla valve" flow channel configuration is developed. Experiments on the FLHP are carried out, focusing on the installation angles and cooling condition factors. The results show that an inclination angle of 20° is the critical point of the influence of gravity on the performance of the FLHP; to ensure good operation of the FLHP, the installation angle should be greater than 20°. The equivalent heat transfer coefficients of the FLHP condenser under different cooling conditions are calculated. It is found that water cooling can provide higher cooling heat transfer coefficients with lower energy consumption and operating noise. Additionally, the heat transfer limit, operating temperature uniformity, and start-up stability of the FLHP are significantly improved under water cooling conditions. The maximum heat load of the FLHP is up to 230 W, and the temperature difference of the evaporator surface can be controlled within 0.5 °C, under 20 °C water cooling. Finally, using the FLHP for thermal management of the chip, its heat transfer efficiency is 166 and 41% higher than that of air cooling and water cooling, respectively. [ABSTRACT FROM AUTHOR]

This paper examines experimentally the effect of jet vortex technology on enhancing the heat transfer rate within a double pipe heat exchanger by supplying the heat exchanger with water at different vortex strengths. A vortex generator with special inclined holes with different inlet angles was designed, manufactured, and integrated within the heat exchanger. In this study, four levels of Reynolds number for hot water in the annulus (Reh) were used, namely, 10,000; 14,500; 18,030; and 19,600. Similarly, four levels of Reynolds number for cold water in the inner tube (Rec) were used, namely, 12,000; 17,500; 22,500; and 29,000. As for the inlet flow angle (θ), four different levels were selected, namely, 0°, 30°, 45°, and 60°. The temperature along the heat exchanger was measured utilizing 34 thermocouples installed along the heat exchanger. It was found that increasing the inlet flow angle (θ) and/or the Reynolds number results in an increase in the local Nusselt number, the overall heat transfer coefficient, and the ratio of friction factor. It is revealed that the percentage increase in the average Nusselt number due to swirl flow compared to axial flow was 10%, 40%, and 82% for an inlet flow angle of 30°, 45°, and 60°, respectively. [ABSTRACT FROM AUTHOR]

A recent study compared experimentally the hydraulic and thermal activity of twisted tape inserts for two types, metal foam twisted tape (MFTT) and traditional twisted tape (TTT), in a double pipe heat exchanger. The investigation goal of the innovatively designed MFTT is to enhance the heat transfer process, which provides a higher thermal enhancement factor over those of TTT under the same conditions. Heat transfer activity in terms of Nusselt number (Nu) and Nusselt number ratio (Nu/Nuo) as the heat exchanger equipped with the proposed MFTT and TTT inserts with various tape‐twist ratios (y/W = 3.6, 5.2, and 6.8) are considered over various values of Reynolds numbers (16,300 < Re < 32,500). The results reveal an improvement in heat transfer rate with both MFTT and TTT and this improvement is proportional inversely to the twisted ratio, but the MFTT showed the highest value of heat transfer compared to TTT and clear tube. Also, the Nu of MFTT augments averaged about 175%–230% compared to the TTT depending on the twist ratio and about 275% and 500% over that of the clear tube. At a twist ratio of 3.6, the heat transfer coefficient is recorded at 37% and 45% for MFTT and TTT models, respectively, with an increase in Re. However, friction factor (f) increases about 35% for MFTT with decreasing in twist ratio (from 6.8 to 3.6). The effectiveness of the proposed MFTT in the range of Re used is found to be about 12%, 15%, and 18% higher than those induced by TTT at twisted tape ratio (y/w = 3.6, 5.2, and 6.8), respectively. [ABSTRACT FROM AUTHOR]

Numerical modeling of the multi‐turn closed‐loop pulsating heat pipe (CLPHP) in the bottom, horizontal, and top heat mode is presented in this paper, with water as working fluid. Modeling is carried out for 2‐mm ID CLPHP having 5, 16, and 32 turns at different orientations for 10 different cases. Momentum and heat transfer variations with time are investigated by numerically solving the one‐dimensional governing equations for vapor bubble and liquid plugs. Instead of considering all the vapor bubble at saturation temperature, vapor bubbles are allowed to remain in super‐heated condition. Film thickness is found using a correlation. Two‐phase heat transfer coefficient is calculated by considering conduction through the thin film at liquid–vapor interface. Liquid plug merging and splitting result in continuous variation in the number of liquid plugs and vapor bubble with time, which is also considered in the code. During the merging of liquid plugs, a time step‐adaptive scheme is implemented and this minimum time step was found to be 10−7 s. Model results are compared with the experimental results from literature for heat transfer and the maximum variation in heat transfer for all these cases is below ±39%.. Numerical modeling of the multi‐turn closed‐loop pulsating heat pipe (CLPHP) in the bottom, horizontal, and top heat mode is presented in this paper, with water as working fluid. Modeling is carried out for 2‐mm ID CLPHP having 5, 16, and 32 turns at different orientations for 10 different cases. Momentum and heat transfer variations with time are investigated by numerically solving the one‐dimensional governing equations for vapor bubble and liquid plugs. Instead of considering all the vapor bubble at saturation temperature, vapor bubbles are allowed to remain in super‐heated condition. Film thickness is found using a correlation. Two‐phase heat transfer coefficient is calculated by considering conduction through the thin film at liquid–vapor interface. Liquid plug merging and splitting result in continuous variation in the number of liquid plugs and vapor bubble with time, which is also considered in the code. During the merging of liquid plugs, a time step‐adaptive scheme is implemented and this minimum time step was found to be 10−7 s. Model results are compared with the experimental results from literature for heat transfer and the maximum variation in heat transfer for all these cases is below ±39%.. [ABSTRACT FROM AUTHOR]

Surface modification is an important means to improve boiling heat transfer. Based on the research of surface wettability, this paper briefly describes the experimental and numerical simulation research on enhancing heat transfer of gravity heat pipe by changing surface wettability. According to the microscopic nature and mesoscopic characteristics of lattice Boltzmann method, the gas-liquid model of pseudo-potential lattice Boltzmann method was used to simulate the heat transfer process of gravity heat pipe with different wettability, and the corresponding morphology diagrams of boiling and condensation phenomena were obtained. It not only overcomes the disadvantage that it is difficult to study the bubble nucleation mechanism by arranging seed bubbles or seed droplets in advance when simulating phase transition in the macro flow model, but also overcomes the defect that Lennard-Jones potential in molecular dynamics cannot accurately describe the force between fluid molecules and wall solid molecules. Then, combining the simulated visual images and experimental results, the heat transfer mechanism of gravity heat pipes with different wetting properties is analyzed in detail. The results show that compared with ordinary heat pipes, the equivalent convection heat transfer coefficient of fully mixed wettability gravity heat pipe is increased by about 45.6 % and the total thermal resistance is reduced by about 40.2 %, which provides a theoretical basis for practical application. [ABSTRACT FROM AUTHOR]

In this paper, the effects of integrated fins on the thermal performance of the concentric tube heat exchanger with a variable flow rate of hot and cold water are discussed. The inner pipe is made of copper pipe of length = 800 mm, OD=30 mm, ID=26 mm, t=2 mm. the integrated fines of the geometry 1.25 mm and 1.5 mm pitch length, 1 mm of the depth of cut are considered in the study. The thermal performance of the heat exchanger with a flow rate of hot and cold fluids ranges from 0.014 kg s-1 to 0.07 kg s-1 and the effects of parallel and counter flow direction are also discussed in detail. The experimental result confirms that the thermal performance of the concentric tube heat exchanger is more in the case of the tube with integrated fins and counter flow direction with the highest flow rate of water in the pipe and annulus of the heat exchanger. [ABSTRACT FROM AUTHOR]

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]

The heat transfer characteristics of air-atomized water spray cooling on the hot metallic surface are presented and discussed in this paper. The controlling parameters mainly investigated are air pressure and spray height. The effects of these parameters on the important thermal characteristics such as interfacial heat transfer coefficient, cooling rate, and wetting layer evolution attained by experiment and inverse heat conduction method. The value of interfacial heat transfer coefficient is proportional to the air pressure and inversely proportional to the spray height. As the air pressure is 0.2 MPa, and the spray height is 40 mm, the maximum cooling rate is 85.08 ℃/s. There is no film boiling stage under this condition. At the spray height is 80 mm, and air pressure is 0.3 MPa, the maximum cooling rate is 62.6 ℃/s. In addition, transition boiling and nucleate boiling always exist, but their retention time is different under different conditions. The temperature-dependent interfacial heat transfer mechanism of air-atomized water spray cooling is explored according to the thermal characteristics and photographs taken by the high-speed camera. The results show that air pressure and spray height both have an influence on the interfacial heat transfer. [ABSTRACT FROM AUTHOR]

Heat pipes are silent heat transfer devices that work on the motion of boiling and condensation process. They have been used in space crafts, heat recovery and ventilation, power conversion, energy, and electronics cooling applications. Over the past few eras, several necessary upgradations in heat pipe technologies have happened to implement new advanced fluids, design modification, and modified wick structures. Heat transfer enhancement due to these upgradations/implementations has been deliberated in many studies. In this paper, performance studies of heat pipes with deposition of nanoparticles and suitable coating made on the wick structure are reviewed. Various heat transfer mechanisms involved in heat pipes with nanoparticles deposition on the evaporator are summarized. Also, the various heat transfer mechanisms in heat pipes while using a nanofluid and a porous coating in the evaporator are summarized. This review shall offer a superior comprehension of the improvement required in cooling devices and help future research fraternity to develop advanced cooling gadgets. [ABSTRACT FROM AUTHOR]

Heat pipe is a two-phase heat transfer device with high effective thermal conductivity and transfer huge amount of heat with minimum temperature gradient in between evaporator and condenser section. This paper objective is to predict the thermal performance in terms of thermal resistance, R, and heat transfer coefficient, h, of screen mesh wick heat pipe with deionized water-TiO2 as working fluid. The input process parameters of heat pipe such as heat load, Q, tilt angle, θ, and concentration of nanofluid, ϕ, were modeled and optimized by utilizing response surface methodology with MINITAB-17 software to attain minimum thermal resistance and maximum heat transfer coefficient. The minimum thermal resistance of 0.1764 °C per W and maximum heat transfer coefficient of 1411.52 W/m² °C was obtained under the optimized conditions of 200 W heat load, 57.2° tilt angle, and 0.159 vol.% concentration of nanofluid. [ABSTRACT FROM AUTHOR]

Flat thermosyphon (FTS) has gained wide attention in solving heat dissipation problems of data centers. The miniaturization of densely packed electronics has led to a demand for compact heat sinks. In this paper, a confined FTS consisting of an evaporator, condenser and customized quartz-glass chamber was developed. Bubble behaviors were investigated by the visualization approach under vacuum conditions. Effects of the space height, nominal heating power, and liquid filling ratio on the thermal characteristics were involved. Experimental results found that the bubble behaviors with 10 mm height are different from others. For 10 mm height, bubble contacts the condensation surface and then bounces back to the evaporator. The quantitative analysis shows that the boiling heat transfer coefficient with 10 mm height is 65.5 % of that with 25 mm height , and a condensation heat transfer coefficient with 10 mm height is 58.9 % of that with 20 mm height. Increasing heating power deteriorates the condensation heat transfer via forming a bubble film on the condensation surface. A small filling ratio (30 %) with 10 mm height is beneficial for the phase change heat transfer. It is hoped that these results can offer guidance for designing effective cooling devices. [Display omitted] • Bubble behaviors of confined flat thermosyphon (FTS) is explored. • Effects of space height, heat power, and filling ratio are involved in analysis. • The bubble directly bounces back to the evaporator of FTS with 10 mm height. • Small filling ratio (30 %) is beneficial for phase change of FTS with 10 mm height. • Reduced space height attenuates the thermal performance of confined FTS. [ABSTRACT FROM AUTHOR]

• Synergistic enhancement spray cooling use micropillars and capillary-driven wicking hybrid structures. • The coolant can self-supplied reach the target surface under the capillary force. • The circulation heat transfer enhancement process of wicking-rewetting-vapor evacuation. • HTC and maximum heat flux were improved 79.6% and 80.0%, respectively on the hybrid structure surface. • A maximum reduction of approximately 30.8 °C was obtained for ONB. Spray cooling, which has a fast thermal response and low flow rate consumption, is a common and effective thermal management approach for high power electronic systems. Typically, to improve the heat transfer coefficient (HTC) for spray cooling, the target surface wettability is decreased, promoting nucleation boiling by reducing the surface energy barrier for bubble nucleation. However, the merging of bubbles and the formation of a vapor film have limitations in further increasing the critical heat flux (CHF). Hence, maximizing both the HTC and CHF of spray cooling can be mutually exclusive on the same heated surface. This paper presented a novel design for a vapor-liquid separation structure surface. It consisted of lower micropillars that serve as the vapor separation channel. On top of that, there was an upper multi-layer copper micromesh that wicks and spreads the coolant. Additionally, nano-grass attached to the micromesh to enhance capillary force and increase the nucleation sites simultaneously. Subsequently, we calculated the forward velocity and pressure drop of the droplets passing through the copper mesh to study the effects of droplet velocity and capillary force on heat transfer performance. The results show that the forward velocity of the droplets decreases rapidly as the increase of copper mesh layers and it drops to zero when passing through the third mesh. Therefore, the capillary force generated by the copper mesh is another important factor for the atomized droplets to pass through the copper mesh and reach the target surface. We have proven that all the vapor-liquid separation structure surfaces satisfy the condition ΔP cap > ΔP pore + ΔP local. Hence, the synergistic effects of the micropillar, copper micromesh, and nano-grass enable a circulation process of wicking-rewetting-vapor evacuation, which improves the onset of nucleate boiling (ONB), enhances the HTC, and increases the maximum heat flux dissipated by the system. Enhanced HTC (7.94 W/cm2/K) and maximum heat flux (817 W/cm2) were achieved at 1.50 ml/s on the vapor-liquid separation structure surfaces (#200 2-layer), representing an enhancement of 79.6% and 80.0% compared with that for the flat surface and the temperature of the heated surface at the ONB maximum decreased by approximately 30.8 °C. [ABSTRACT FROM AUTHOR]

This paper presents a simple straight tube with different types of inserts in which there is a flow of water + Al2O3. The experimental and computational studies were performed for different inserts, such as simple insert, twisted‐I, twisted‐II, and simple straight tube. The coefficient of heat transfer and Nusselt number is computed for mass flow rate ranging between 0.06 and 0.20 m/s. The simple straight tube is used as a means of validating the experimental results with computational results. The work concluded that twisted‐II inserts provide enhanced thermal performance in comparison to all other process conditions. In this study, statistical analysis is performed by statistical software and by generating model equations. Furthermore, the effect of a twisted insert in heat pipe for two types of fluids, that is, water and nanofluid (Al203 + water) is experimentally and computationally designed using the Taguchi method and orthogonal array concept. Moreover, temperature and velocity contours are developed for all four types of insert designs in the tube. [ABSTRACT FROM AUTHOR]

This paper considers the main parameters of the solar collector with thermosyphon circulation for the solar heat supply system. There was developed a flat solar collector construction with thermosyphon circulation wherein the heat transfer coefficient is increased by removing the additional partitions between a panel and heat insulation. The solar collector's efficiency reached by the availability of a tank and thermal pump in the construction where a condenser and evaporator executed the form of a heat exchanger of «spiral in spiral» type and heat exchanger pipe connections are one above another, which allows increasing the square and heat exchange intensity. The outcome of the study is a description of the mathematical analysis of the heat transmission process in the tank accumulator. We have shown a heat carrier thermophysical parameter dependence on the temperature. [ABSTRACT FROM AUTHOR]

Thermal and hydrodynamic models were developed for an axial swallow-tailed micro-grooved heat pipe and solved numerically. The axial heat conduction, the variation of the axial meniscus radius and the thin liquid film heat transfer in the evaporator and condenser sections were included in the proposed model. The results showed that the liquid film thickness increases along the axial direction of the microgrooves. The evaporation heat transfer coefficient of the liquid-vapor interface in the evaporator section is greater than the condensation heat transfer coefficient of the liquid-vapor interface in the condenser section. Additionally, the evaporation/condensation mass flow rates in the adiabatic section are not all zero. The wall, liquid and vapor temperatures are almost the same in the adiabatic section. In addition, an experimental study was performed to verify the heat transfer model in a heat pipe with axially swallow-tailed microgrooves. Moreover, in this paper, the obtained numerical results are accordant with the obtained experimental data. [ABSTRACT FROM AUTHOR]

This paper describes the experimental investigations conducted on a closed loop pulsating heat pipe (CLPHP) for assessing the thermal performance. The pulsating heat pipe has a single closed loop made of copper. The working fluids used are water and titanium dioxide nanofluids with varying concentrations of TiO2 nanoparticles (1.5% and 1%) on weight basis. The TiO2 particles are mixed in water to form a stable suspension using a sonicator. The heat input is varied between 40 W and 100 W in steps of 20 W. All experiments are conducted in the bottom heating mode (evaporator at the top) in the vertical and horizontal orientations. The parameters considered for evaluating the thermal performance are the temperature difference between evaporator and condenser, thermal resistance, heat transfer coefficient, and thermal conductivity. The results of the investigation reveal that the vertical orientation and increase in nanoparticle concentration favors better heat transfer performance of the single closed loop pulsating heat pipe. [ABSTRACT FROM AUTHOR]

This paper deals with vaporization heat transfer in a small diameter closed two-phase thermosyphon with a long evaporator and a short condenser, filled with water as operating fluid. The internal diameter of the evaporator is equal to 6.4 mm and the length-to-diameter ratio at the evaporator is equal to 166. A similar geometry is commonly used in vacuumed tube solar collectors. In the present investigation, the input power to the evaporator is provided by means of an electrical resistance wire wrapped around the external wall of the tube, while a water jacket is built at the condenser to reject the heat. The performance of the thermosyphon is described by using the wall temperature and the overall thermal resistance for different operating conditions: input power at the evaporator, cooling water temperature at the condenser, and inclination of the thermosyphon (30 deg, 60 deg, and 90 deg tilt angle to the horizontal plane). The present experimental data cover a range of heat flux between 1700 and 8000 W/m² and saturation temperature between 28 °C and 72 °C. The vaporization heat transfer coefficients are compared with some correlations for closed two-phase thermosyphons displaying large disagreement. A new correlation is presented, which accurately predicts the present experimental values and other data by independent labs taken in closed two-phase thermosyphons, varying geometry and operating fluid (water, R134a, and ethanol). [ABSTRACT FROM AUTHOR]

This paper discusses a prototype of the ORC, which is developed utilizing a novel multi-coil evaporator and condenser for the waste heat source with a temperature below 100 °C. R123 acts as a working fluid for the ORC. The thermal performance of a multi-helical coil is evaluated experimentally and compared to the single-helical coil heat exchanger available in the literature. The performance of a novel multi-coil heat exchanger depicts that when the Reynolds Number varies from 60000 to 120000, the heat transfer coefficient varies from 1560 W/(m2K) to 2900 W/(m2K) for a single coil heat exchanger and 3014 W/(m2K) to 4531 W/(m2K) for multi helical coil evaporator. The Nusselt Number varies from 300 to 570 for single-coil heat exchangers and 715 to 1076 for multi-helical coil heat exchangers. The results show that the expander's power output is around 2.14 kW and proportional to the evaporator temperature and pressure increase. The high condensing temperature decreases the expander power output. The main contribution of the present work is developing a small ORC system with novel multi-helical coil heat exchangers and testing them on a lab scale for the most probable application to fulfill the electricity requirement in remote areas in the future. [Display omitted] • The novel multi-helical coil heat exchanger (MHCHE) designed, manufactured, and tested. • Novel MHCHE utilized in small-scale organic Rankine cycle's lad scale testing. • Impact of experimental parameters on power output and thermal efficiency of ORC-MHCHE assessed. • Usefulness of a small-scale ORC-MHCHE demonstrated at low temperature (<100 °C). • R123 and water acts as working and heat transfer fluid in a small-scale ORC-MHCHE. [ABSTRACT FROM AUTHOR]

• Subcooled flow boiling heat transfer of water in the porous metal foam tubes was investigated experimentally. • Filling a tube with metal foam could improve the heat transfer and also pressure drop. • The thermal efficiency of metal foam tubes was investigated. • Based on experimental data, a correlation was presented to predict heat transfer in the metal foam pipes. This paper has investigated the effect of different porosities (0.80–0.90) of the metal foam pipes on heat transfer coefficient and pressure drop of subcooled flow boiling of water experimentally. The metal foam pipe with 0.80 porosity has increased the Nusselt number and pressure drop by 41 % and 21 % compared to the 0.90 metal foam pipe, respectively. The heat transfer of water phase change in a porous medium has received less attention due to the complexities of the problem. One of the challenges in this field is the absence of a correlation for predicting heat transfer in metal foams. In this paper, a systematic method based on dimensionless numbers considering the essential parameters, like mass flux, subcooling, and heat flux, was used to provide a correlation for calculating the Nu-number based on 106 experimental points. The mean absolute deviation (MAD) of the results predicted by the new correlation is 8.9 %, and it predicts 95 % of the database with ±20 %. [ABSTRACT FROM AUTHOR]

The paper presents numerical investigations of a three fluid heat exchanger (TFHE), which is an improvement on the double pipe heat exchanger, where a helical tube is inserted in the annular space between two straight pipes. The helical tube side fluid, that is, hot water continuously transfers heat to the outer annulus side fluid and innermost tube side fluid. The heat transfer and pressure drop characteristics of the TFHE are assessed for different flow rates and inlet temperatures. With an increment in the volumetric flow rate of the helical tube side fluid and outer annulus side fluid, the overall heat transfer coefficient increases, and the effectiveness decreases for heat transfer from the helical tube side fluid to outer annulus side fluid in both parallel flow and counter flow configurations. It is also observed that with increment in the helical tube side fluid inlet temperature, the overall heat transfer coefficient and effectiveness increases for heat transfer from the helical tube side fluid to outer annulus side fluid in both flow configurations. The parameter, JF factor, has been proposed to evaluate the thermohydraulic behavior of the TFHE, where it is obtained that the behavior of the TFHE is better at a lower helical tube side fluid velocity and higher outer annulus side fluid velocity. [ABSTRACT FROM AUTHOR]

At this age, engineering applications are demanding effective ways of heat recovery and energy savings for their optimum performance. Among other cooling techniques, pulsating heat pipes have emerged as a convenient and cost effective thermal design solution due to its excellent heat transfer capability, high thermal efficiency and structural simplicity. The paper presents an experimental study on the operational limit of an aluminum closed-loop pulsating heat pipe (CLPHP) charged with ammonia. It consists of total 14 turns of aluminum pipe with 3 mm inner and 4 mm outer diameter. Ammonia was used as working fluid with 3 different filling ratios such as 0.4, 0.6 and 0.8. Operation orientations were vertical, 30°, 45°, 60°, 90° and 180° inclinations. Constant electric heat input of 36 W was applied to the heating block and temperature rise in various sections was monitored till steady state was reached. Temperature was measured at different locations of the CLPHP by using thermocouples. The effects of operational orientations and filling ratios were investigated on heat transfer by working fluid Q̇php (Watt), overall heat transfer coefficient U (W/m2 °C) and thermal resistance R (°C/W) considering the measured temperature. The result shows that, 0.4 and 0.6 fill ratios and inclination angle of 30º give better result than any other arrangements for CLPHP. [ABSTRACT FROM AUTHOR]

An extensive development of two-phase thermal control systems based on two- and multi-phase devices (standard heat pipes, loop heat pipes, heat switches, etc.) offers a potential to assume an increasingly important position in the instrumental thermal control for a number of space and earth applications. However, experimental data regarding long-life testing for such systems is insufficient. The first ammonia loop heat pipe (LHP) was developed in 1976 in the Ural Polytechnic Institute under supervision of Prof. Gerasimov, Iu. F. (1921–2001) (Kiseev, 1977; Gerasimov et al., 1981; Maydanik, 2005) [1–3]. This paper features its long-life test, covering 41 years with varying operation times. The life test consists of two parts: permanent and periodic LHP operation (approx. 5000 h) and LHP stand-by storage (approx. 152,600 h). Experimental investigations were performed using the developed loop heat pipe based on composite nickel wick with effective pumping pores of 0.5–10 µm in radius and 1 × 10 −14 m 2 to 4 × 10 −12 m 2 in permeability. We have provided experimental data demonstrating the impact of orientation, varying heat load and temperature on the LHP operation. In addition, a qualitative and quantitative chemical element analysis of the distilled water in a loop heat pipe without capillary structure (loop thermosyphon) was performed before and after its long-term operation (one year) to identify any possible interaction between the working fluid and the pipe materials. [ABSTRACT FROM AUTHOR]

In this paper, heat transfer analysis for an air-air heat exchanger was experimentally carried out to find its thermal performance and effectiveness. Air-air heat exchanger equipped with finned heat pipes was considered for the experimentation. Mass flow rate of air was considered in between 0.24 to 0.53 kg/s. The temperature at the condenser side of the heat pipe heat exchanger was kept constant at around 23 °C and at the evaporator part it was varied from 88 to 147 °C. The performance of heat pipe heat exchanger was evaluated at different mass flow rate of air, in terms of effectiveness and compared with its corresponding value found by theoretical analysis. [ABSTRACT FROM AUTHOR]

• Nanomaterial deposition on heating surface are used to attain reduction in wall superheat, enhancing nucleate boiling HTC and CHF. • Different working fluid is used such as de-ionized water, fluorocarbon fluid, hydofluoroethers and refrigeration-based fluid for enhancing CHF. • The surface modification enhanced the CHF due to the reduction in wall superheat temperature, increased nucleation site, porosity rise, enhanced surface wettability, small nano-pores/cavities generation, rise in bubble release frequency. • Nanoparticle size, nanofluid preparation techniques and concentration of nanofluid mainly influenced the CHF. Boiling is the most significant phase change phenomena for various applications such as cooling of electronic devices, heat pipes, cooling in nuclear reactor, different industrial application and plenty more. Due to its wide scope of application, any enhancement in this domain leads to remarkable economic and energy efficient consequences. Augmentations in boiling heat transfer performance are very crucial which establishes the various industrial applications further energy efficient. To reduce the energy dissipation in heat transfer equipment, boiling heat transfer augmentation is one of the significant techniques used by researchers. This paper demonstrates a thorough review and investigation of published articles describing the various surface modification method (nanomaterial coating, nano/micro porous coating, nano/micro structured surface) for pool boiling and flow boiling heat transfer enhancement as well as the effect of surface modification on wettability, porosity, surface roughness, nucleation sites are studied. Various augmentation methods are utilized to attain three apparent heat transfer desire: reducing wall superheat temperature, enhancing pool boiling heat transfer coefficient (HTC) and improving critical heat flux (CHF). [ABSTRACT FROM AUTHOR]

Visual and instrumental investigations of the processes of condensation and redistribution of a working fluid in a loop heat pipe have been carried out. This paper presents the results of an experimental investigation of the heat transfer and hydrodynamics during the condensation of water vapor in a flat gap condenser measuring 80×40×1mm. Investigations have been conducted at a condenser cooling temperature of 20, 40 and 60°С. During all operating modes a stratified two-phase flow and film condensation have been observed. The temperature field in the condenser has been measured, and the heat-transfer coefficients and the thermal resistances have been determined. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the performance analysis of a finned U-shape heat pipe used for desktop PC-CPU cooling. The experiments are conducted by mounting the system vertically over a heat source situated inside a rectangular tunnel, and force convection is facilitated by means of a blower. The total thermal resistance (Rt ) and heat transfer coefficient are estimated for both natural and forced convection modes under steady state condition, by varying the heat input from 4W to 24W, and the air velocity from 1m/s to 4m/s. The coolant velocity and heat input to achieve minimum Rt are found out and the corresponding effective thermal conductivity is calculated. The transient temperature distribution in the finned heat pipe is also observed. The experimental observations are verified by simulation using ANSYS 10. The results show that the air velocity, power input and heat pipe orientation have significant effects on the performance of finned heat pipes. As the heat input and air velocity increase, total thermal resistance decreases. The lowest value of the total thermal resistance obtained is 0.181°C/W when heat input is 24W and air velocity 3m/s. The experimental and simulation results are found in good agreement. [Copyright &y& Elsevier]

In the framework of cooling technology applied to electronics, multiple setups and coolants have been used to achieve effective methods to remove high heat density fluxes. In this paper, an innovative device based on the combination of two fluids in a closed loop two-phase thermosiphon is proposed as an effective passive system to cool electronic components. The cooling device is an external flow boiling system that uses NOVEC 7000 as a circulating boiling fluid to transport heat from an evaporator capsule to a condenser. The bubble lifting effect in the closed-loop thermosiphon accelerates the fluid circulation and creates a natural jet impingement effect in the evaporator. In the condenser, the NOVEC bubbles ascend through a water pool, achieving a highly effective heat exchange between the fluids. The advantage of this two-fluid condenser system is highly efficient condensation since the vapour of NOVEC is condensed in a water pool, which produces a high heat transfer between the fluids and is not sensitive to the presence of non-condensable gases. In addition, the movement of the NOVEC bubbles produces high agitation in the water that enhances the heat transfer to the condenser walls. The system was tested under different working conditions with several power generation rates in the heat source and operating at atmospheric pressure and in a vacuum. The system was tested to assess the effect of water in the condenser, and heat transfer coefficients were calculated for the setup operating with and without water. In addition, a numerical model was developed to predict the different temperatures in the system and extrapolate the results for different operating conditions. Several tests were performed, and the results showed robust working of the system and good control of temperatures in the element to be cooled. The system was able to maintain a temperature lower than 70 °C in the aluminium block working with a heat flux of 220 kW/m2 without any extra energy consumption. The effect of water in the condenser was shown to be effective in improving heat transfer and coolant condensation. The heat transfer coefficients in the condenser can be improved by 22 to 133%. The model for temperature prediction showed reasonably good results for most of the experiments regarding both transient behaviour and stabilization temperatures. • A 2-fluid CLTPT is designed to operate as a passive cooling system. • A flow boiling evaporator produces a highly stirred flow and effective heat transfer. • The condenser contains NOVEC 7000 and water combined to improve the condensation efficiency. • The biphasic condenser prevents the negative effects of non-condensable gases. • A numerical model is proposed to predict the system behaviour under different operating conditions. [ABSTRACT FROM AUTHOR]

• Study the effect of condenser temperature on the performance of a wickless heat pipe. • Performance in microgravity declined as the condenser temperature decreased. • Temperature data was used to extract a heat transfer coefficient in the evaporator. • Nusselt number was found to scale with Marangoni number to the -1/3 power. • A single correlation using the Marangoni number collapsed all the experimental data. The Constrained Vapor Bubble (CVB), a simple, wickless, heat pipe design that depends on interfacial forces to drive corner flow in a square cuvette, was studied in the microgravity environment aboard the International Space Station (ISS). In this paper, we consider the effects of different condenser temperatures on the heat transfer and fluid flow behavior using pentane as the working fluid. As the condenser temperature was decreased, the performance of the system decreased. This performance decrease using the pure working fluid was opposite to the behavior observed when using a mixture of 94 vol% pentane and 6 vol% isohexane. The mechanism for the decline in performance as the condenser temperature was lowered was a stronger than expected increase in the apparent strength of Marangoni flows at the heater end of the system. A simple mathematical model was fit to the experimental data and used to extract an evaporator heat transfer coefficient for experiments where we held the condenser temperature constant while increasing the heater power and where we held the heater power constant while decreasing the condenser temperature. All the results could be collapsed onto a single Nusselt number vs. Marangoni number curve. In this formulation, the Nusselt number was found to decrease with increasing Marangoni number to the 1/3 power. [ABSTRACT FROM AUTHOR]

This paper proposes a methodology aiming at determining the most influent working variables and geometrical parameters over the pressure drop and heat transfer during the condensation process of several refrigerant gases using heat exchangers with pipes mini channels technology. A multi-criteria decision making (MCDM) methodology was used; this MCDM includes a mathematical method called SIMUS (Sequential Interactive Modelling for Urban Systems) that was applied to the results of 2543 tests obtained by using a designed refrigeration rig in which five different refrigerants (R32, R134a, R290, R410A and R1234yf) and two different tube geometries were tested. This methodology allows us to reduce the computational cost compared to the use of neural networks or other model development systems. This research shows six variables out of 39 that better define simultaneously the minimum pressure drop, as well as the maximum heat transfer, saturation pressure fluid entering the condenser being the most important one. Another aim of this research was to highlight a new methodology based on operation research for their application to improve the heat transfer energy efficiency and reduce the CO2 footprint derived of the use of heat exchangers with minichannels. [ABSTRACT FROM AUTHOR]

• Critical review of morphological and thermal-hydraulic performance of the conventionally and additively manufactured lattices is conducted. • Conventionally manufactured wire-meshes can provide heat transfer enhancements similar to stochastic metal foams and relatively easy and cheap to manufacture. • Additively manufactured lattices demonstrate higher interfacial heat transfer coefficient values than stochastic metal foams, but the role of inherent surface roughness needs more investigation. The advantages offered by the stochastic metal foams when used in high-performance compact heat exchangers is evident in the open literature. Recently, there has been a surge in investigations on regular metal lattices which can be manufactured additively, because of their "potential" to provide multifunctional and tunable characteristics to heat exchangers. These lattices are also reported to lend superior mechanical properties as well. The literature, however, lacks in the critical assessment of these additively manufactured regular lattices in terms of the quality of printed parts (defects, surface roughness, etc.), their compatibility in compact heat exchangers, flow and thermal transport characteristics, and whether or not they are viable alternatives to the commercial metal foams conventionally manufactured. This paper reports different manufacturing routes for the fabrication of conventional stochastic and regular lattices and additively manufactured regular lattices, and how these technologies have evolved over the years. The additively manufactured lattices different from the commercial TKD may result in significantly higher interstitial heat transfer coefficient. Further, the design freedom allows different porosities through which higher effective thermal conductivity can be achieved. The usage of regular lattices in tube-heat exchangers, as wicks in heat pipes, heat transfer enhancement features in boiling heat transfer and transpiration cooling mechanisms of turbine blades as an alternative to conventional technologies is promising. The role of inherent surface roughness in dictating the thermal and flow transport in additively manufactured lattices, especially for multi-phase flow applications, should be studied in more detail. [ABSTRACT FROM AUTHOR]

In this paper, the performance of a multi-pass heat pipe-based heat exchanger (HPHE) is investigated experimentally and theoretically. The heat pipe system consists of copper heat pipes in a specific equatorially staggered configuration to facilitate heat transportation from a hot gas (air) to a water flow, which cools the condenser section of these heat pipes. The effect of the Reynolds number on the heat transfer rate was studied by altering the number of passes for the evaporator section for the same system by the incorporation of various baffles and by varying the water flow rate. The experimental results have highlighted the strong correlation between heat exchanger performance and the Reynolds number. By increasing the number of passes from one to five, the effectiveness of the HPHE was improved by more than 25%. It has been demonstrated that increasing the number of passes increases the Reynolds number of the flow, leading to higher heat transfer coefficients and lower thermal forced convection resistances. The HPHE overall performance, as well as, the outlet temperatures of the fluids were predicted through two theoretical models, based on the Log Mean Temperature Difference (LMTD) method and the Effectiveness-Number of Transfer Units (ε-NTU) method. The predictions were compared with experimental results and the accuracy of the models reported. The validation showed that the developed iterative LMTD model predicted the performance of the HPHE within ±15.5% error. In comparison, the ε-NTU model predicted the total effectiveness with a maximum error of 19% and was able to predict the outlet temperatures of both air and water streams within an accuracy of ±0.7 °C. The reported research is of importance for the application of heat pipe heat exchangers in waste heat recovery. Finally, knowledge is provided on the accuracy of the available prediction models. [ABSTRACT FROM AUTHOR]

• Experimental analysis on hysteresis phenomenon during flow boiling heat transfer • Dedicated experimental procedure with variation of the imposed heat flux over time • Detection and isolation of parameters affecting the hysteresis phenomenon. • Hysteresis effect related to activation of surface cavities with higher heat fluxes This paper presents an experimental analysis on the hysteresis phenomenon occurring during flow boiling heat transfer in a horizontal stainless-steel tube having an internal diameter of 6.0 mm. Pure R134a is the working fluid employed, working at a saturation temperature and a vapor quality fixed to 40°C and 0.30, respectively, whereas two different mass velocities of 400 and 800 kg/m2s are studied. A dedicated experimental procedure has been carried-out to highlight the variation of the two-phase heat transfer performance due to the hysteresis effect. Specifically, tests performed at the same operating conditions but having a different thermal history (a higher imposed heat flux in the transient phase) were seen to provide lower wall superheat values and therefore higher heat transfer performances. The nucleate boiling contribution is isolated from the overall heat transfer and the parameters affecting the boiling curve behavior are highlighted and discussed, pointing out the differences when increasing and decreasing the imposed heat flux once a peak value is reached. The experimental trends are finally upheld by the boiling theory, considering the hysteresis effect as the activation of the surface cavities undergone a higher given thermal boundary condition. [ABSTRACT FROM AUTHOR]

• Condensation of R134a/R245fa in a plate heat exchanger is experimentally tested. • Heat transfer and pressure drop characteristics of mixtures are analyzed. • Prediction methods of heat transfer and pressure drop are developed. Zeotropic mixtures have shown great potential to improve the system efficiency compared with the use of pure fluids as the working fluids. However, there is a lack of analysis of the heat transfer during the phase change of zeotropic mixtures in plate heat exchangers, which is a common heat transfer device used as the evaporator and condenser in these systems. This paper presents an experimental analysis of the flow condensation heat transfer and pressure drop characteristics of zeotropic mixtures in a plate heat exchanger. The objectives are to obtain prediction methods for the heat transfer coefficient and frictional pressure drop and to quantify the degradation of the heat transfer coefficient by using mixtures. Zeotropic mixtures of R134a/R245fa with five different quantitative compositions were experimentally tested with the bubble point temperatures of 30°C to 50°C and various mass fluxes. The experimental results indicate that condensation of the mixtures is governed by a shear-controlled process, similar to what happens with pure fluids. Moreover, the heat transfer degradation of the zeotropic mixture caused by the mixture effects increases with decreasing condensation temperature and mass flux. There is a 48% maximum decrease in the heat transfer coefficient for the mixture of R134a/R245fa with 0.431/0.569 in mass fraction. A modified Silver-Bell-Ghaly method provides a good prediction for the heat transfer data with a mean absolute percentage deviation of 12.2%. A pure-fluid correlation developed in a previous work by the authors enables a prediction with an 8.6% mean absolute percentage deviation for the pressure drop data. [ABSTRACT FROM AUTHOR]

The research on working fluids of refrigerant/absorbent mixture is important for the development of the absorption refrigeration area. R290/POE-oil mixture is a new potential working fluid, in which R290 (propane) is the refrigerant and POE oil is the absorbent. This refrigerant/oil working fluid has a wide operating range, low generation temperature, and no corrosion to metals. Flow boiling heat transfer performance of the new working fluid in smooth horizontal tubes has been experimentally investigated in this paper. The R290 concentration for the experimental working fluid was selected as 11 wt%. The test sections were horizontal smooth stainless steel annular tubes with outer diameters of 6, 8 and 10 mm. The boiling saturation pressures in the test sections coincided with the actual operation of the absorption refrigeration cycle, which were set as 1.2 MPa, 1.3 MPa, 1.4 MPa, respectively, corresponding to boiling temperatures 55.3 °C, 58.7 °C, 62.5 °C. The mass fluxes were 166 kg m−2s−1, 221 kg m−2s−1, 276 kg m−2s−1, 332 kg m−2s−1 and 387 kg m−2s−1. The effects of heat flux, vapor quality, mass flux, boiling temperature and tube diameter on the flow boiling heat transfer coefficient were displayed and analyzed. Experimental results show that heat flux, mass flux and vapor quality have obvious effects on the coefficients; the nucleate boiling mechanism plays an important role, and the convective boiling mechanism plays a significant role in the boiling process. The present study is of good significance to the application of R290/POE-oil working fluid and the design and optimization of the finned-tube generators in absorption refrigeration systems. • R290/POE-oil working pair has obvious advantages in absorption refrigeration. • The flow boiling heat transfer performance of R290/POE-oil mixture are studied. • The influences of different operating conditions on the coefficient are analyzed. • Both nuclear boiling and convective boiling mechanisms play an important role. [ABSTRACT FROM AUTHOR]

Coolants are widely used to dissipate grinding heat in conventional grinding. This process, however, is not satisfactory as coolants often lose efficacy in grinding due to film boiling and can result in adverse health and environment effects. The present paper put forward the concept of a rotating heat pipe grinding wheel, attempting to reduce or eliminate the coolant amount and realize green machining. The heat transfer performance of rotating heat pipe grinding wheel was studied by using volume of fluid method in ANSYS/FLUENT. The influence of the input heat flux, filling ratio and rotational speed were investigated by a simulation method. Results show that the appropriate heat flux range for the rotating heat pipe grinding wheel was from 2000 to 100,000 W/m2, the ideal filling ratio was 50% and the rise of the rotational speed turned out to weaken the heat transfer coefficient. Finally, dry grinding experiments on Ti-6Al-4V were performed and the temperatures in both the rotating heat pipe and the grinding contact zone were monitored. The new designed rotating heat pipe grinding wheel showed a good prospect for application to green grinding of difficult-to-cut materials. [ABSTRACT FROM AUTHOR]

• Effect of tilt and length on porous miniature heat pipes thermal performance is investigated experimentally. • Thermal resistance network of 1D of miniature heat pipes with fin is detailed for thermal modelling. • Prediction of evaporation and condensation heat transfer coefficients using experimental results. • Effect of adiabatic length on capillary pressure of the two studied heat pipes using the developed hydraulic model. • Effect of porous media parameters on thermal resistance and heat pipes thermal performance and evaluation of optimal porosity. Miniature heat pipes are considered as an innovative solution able to dissipate high heat fluxes with low working fluid charge, temperature control, and operating with minimum energy consumption and less noise. In this paper, results show that tilt ranging [10°:60°] has not an impact on heat transfer meanwhile the system thermal resistance is the lowest at horizontal position. A theoretical analysis on heat pipe thermal performance has been carried out based on experimental data by predicting heat pipe evaporation and condensation heat transfer coefficient. The measured heat transfer coefficients are close to the predicted results with a mean deviation less than 10%. A hydraulic mathematical model is developed and shows a good agreement with Kaya's study using water as working fluid. This work comprises a parametric study that investigates the effect of particles size, porosity, permeability, heat pipe length on heat transfer and capillary pressure. The obtained results show that the heat pipe operating temperature rises when particles size increases from 1 μ m to 90 μ m and the optimum porosity range is between [10%–40%]. [ABSTRACT FROM AUTHOR]

• A numerical model of R290 condensation in minichannels was proposed and verified. • The effect of diameter on heat transfer and frictional pressure drop was analyzed. • The refrigerant charge of R290 in different pipe diameters was analyzed. • Reducing the diameter can reduce the refrigerant charge of R290 by 94.8% • Wall effects have a remarkable influence on HTC in small diameter and mass flux. In order to enhance the heat transfer performance and decrease the refrigerant charge of R290 in condensers, this paper numerically studied the condensation and refrigerant charge of R290 in minichannels (not the entire system) with different diameters to analyze the effect of diameter on them. The VOF multiphase flow model and SST k − ω turbulence model were used for 3D transient calculation in the simulation. The simulation results show that the diameter has a remarkable effect on the flow pattern, heat transfer coefficient, friction pressure drop and refrigerant charge. When the mass flux is small, a remarkable eddy current appears between vapor and liquid on the circular section, which can sharply enhance the heat transfer. At high vapor quality (x >0.8), the heat transfer coefficient and frictional pressure drop do not almost increase and even decrease with the increase of vapor quality under some working conditions, which is due to the influence of wall effect. Under the same heat transfer rate and heat transfer temperature difference, with the decrease of diameter, the maximum reduction of refrigerant charge reaches 94.8%. But when the diameter is reduced to a certain value, continuing to reduce the diameter will have little effect on the refrigerant charge. [ABSTRACT FROM AUTHOR]

In this paper, a transient model that describes the heat transfer processes in the wellbore during circulation is presented. A simulation of the transient effect on these heat transfer processes was achieved by using a numerical approach to solve the thermal balance equations that describe the heat flow in that system. The present model can be applied for both laminar and turbulent flow regimes through the convective heat transfer coefficient (CHTC). The run time of the present model is relatively short, hence it can be applied for real-time simulation applications. This was achieved by simplifying the model and reducing the calculation complexity. Moreover, the present model has gone through different validation processes. First, the present model is reduced to a steady state condition and solved analytically. This solution was used as a reference to verify the results of the numerical model. Additionally, a grid sensitivity analysis was performed. Second, the analytical solution was validated through a comparison with the known literature. Finally, the assumptions that were made to simplify the model were verified by comparing the results with the comprehensive version of the model. The effect of the flow rate and the flow regime on the wellbore temperature profile are examined. The results showed a reverse effect of the flow rate on the bottom hole temperature within each flow regime. However, a significant increase in the bottom hole temperature is observed as the flow regime changed from laminar flow to turbulent flow. Changing the flow regime from laminar to turbulent will affect the value of the CHTC leading to a significant increase in wellbore fluid temperature. It is also shown that the maximum temperature at the annulus is located where the rate of heat exchange between the annulus and the pipe and the heat from the formation to the annulus is equal. • Modeling of the wellbore thermal activity during circulation is performed. • Two transient numerical models and a steady state analytical model are presented. • Three different convective heat tranfer coefficients have been used. • The location of the maximum temperature at the annulus is physically explained. • The models are verified through different validation procedures. [ABSTRACT FROM AUTHOR]

• HTC and pressure drop rises with mass velocity and quality in plain and TTI tubes. • Gravity-dominated regimes (stratified-wavy) are more sensitive to vapor quality. • Shear-dominated regimes (intermittent/annular) are more sensitive to mass velocity; • Tapes increase HTC and pressure drop, the less twist ratio, the more increment. • Twist ratio of 9 has the highest thermal performance, usually less than one. This paper deals with condensation flow inside plain and twisted tape inserted (TTI) tubes with diameter and length of 8.7 mm and 700 mm, respectively. R1234yf, identified as a viable alternative for R134a with low global warning potential (GWP), is examined as the working fluid. The heat transfer coefficient and the pressure drop of the flow are measured in different mass velocities and vapor qualities ranging 160–310 kg m−2 s−1 and 0.12–0.84, respectively. A well-equipped test rig consisting of a test condenser and other necessary equipment is employed to precisely adjust the test conditions. Experimental results for the plain tube are compared with some well-known correlations. As a result, Shah and Souza et al. equations are the most reliable equations to predict the flow heat transfer coefficient and its pressure drop, respectively. Different twisted tape inserts with three twist ratios of 6, 9, and 12 are exploited to study the effect of twisted tape geometry. The results indicate that up to 42% and 235% increment in the heat transfer coefficient and the pressure drop, respectively, is associated with implementing twisted tape inserts. In addition, it was shown that, among some correlations proposed previously in the literature, the experimental heat transfer coefficient and pressure drop in TTI tubes can be predicted by Akhavan Behabadi et al. and Hejazi et al. correlations with reasonable accuracy. According to the results, the twist ratio of 6 gives the highest increase in the heat transfer coefficient, while the ratio of 9 is associated with the highest overall enhancement ratio. [ABSTRACT FROM AUTHOR]

• Experimental study of evaporation from asymmetric microdroplets. • Increasing levels of droplet asymmetry significantly enhance evaporative transport. • Asymmetric droplets have higher heat transfer performance than hemispherical droplets. • Asymmetric droplets yield smaller thicknesses near regions with high curvature. Droplet evaporation is ubiquitous both in everyday life and numerous engineering, biomedical, and environmental applications. For example, the large amount of latent heat associated with the liquid-vapor phase change process makes droplet evaporation an ideal solution for cooling high-powered electronic devices. Over the past 100 years, many new theories and models of droplet evaporation have been proposed. Still much of the fundamental transport physics remains elusive, which makes it challenging to describe diverse evaporation behavior by a universal mechanism. In particular, evaporation from asymmetric droplets remains grossly unexplored due to their rarity in nature. However, recent advances in micro- and nanoengineering technology have made it possible to tune and maintain droplets in non-spherical geometries, where the interfacial transport rate becomes highly anisotropic along the circumferential direction. Such a characteristic represents a distinctive feature from evaporation of capped-spherical droplets and has not been comprehensively explained by existing theories. This paper exams the evaporation rate and heat transfer performance of continuously–fed water microdroplets confined on heated silicon micropillar structures with circular, square, and triangular cross sections. The evaporation experiments are performed at substrate temperatures ranging from 60 °C to 98 °C. The droplets exhibit a capped spherical shape on the circular micropillar but an asymmetric geometry on non-circular micropillars. Our experimental results show that increasing levels of droplet asymmetry significantly enhance evaporative transport. Specifically, for a 60 °C substrate, a triangular micropillar has 45% greater heat transfer coefficient than a circular one; at 98 °C, the increase reaches 71%. This enhancement is validated by multiphase numerical simulations which show agreement with experimental values. The enhanced heat transfer coefficient from asymmetric droplets is attributed to a reduced conduction resistance inside the droplet and smaller diffusion resistance at the droplet interface, both of which result from the non-uniform geometric features of the droplet along the circumferential direction. Finally, a thermal resistance network is developed to demonstrate the variation in the local droplet thickness and interfacial curvature along the circumferential direction for droplets confined on circular, square, and triangular micropillars. Asymmetric droplets yield smaller thicknesses near regions with high curvature, which results in a smaller conduction resistance and higher fraction of heat transport near the contact line region compared to a capped spherical droplet. Furthermore, with increasing droplet temperature, the thermal resistance associated with vapor diffusion is reduced significantly, leading to greater enhancement of evaporative heat transport. These findings not only provide insights in the evaporation behavior of asymmetric droplets, but also provide important guidance for relevant applications, such as the design of evaporative cooling systems. [ABSTRACT FROM AUTHOR]