Evaporation Heat Transfer Coefficient Research Articles

Comparative thermal model analysis and experimental validation for predicting performance of a pyramidal solar still with integrated pulsating heat pipe

Developing nations face a dire situation as water scarcity and pollution significantly impact various aspects of life. It's crucial to take action now to address these pressing issues and ensure a sustainable future for all. Solar desalination processes emerge as a promising solution to these pressing problems in water purification technologies.This paper reports the thermal modeling and experimental results of two solar still designs, namely (i) Pyramidal solar still with pulsating heat pipe (PHP), Modified pyramidal solar still (MPSS), and (ii) conventional pyramidal solar still (CPSS). This study uses energy balance equations to focus on MPSS and CPSS thermal modeling. In the MPSS, a PHP, in conjunction with a solar collector, provides external heat to basin water. Several models, including Dunkel, Kumar and Tiwari, Chen, and Zheng Hongfei, are utilized to estimate the performance of both MPSS and CPSS. Extensive experiments have been conducted for validation purposes. Key parameters considered for prediction and comparison include evaporative heat transfer coefficient, convective heat transfer coefficient, total heat transfer coefficient, and hourly yield. It is noted that the Kumar and Tiwari model demonstrates a superior ability to predict cumulative yield, with a percentage error of 5.81 % and 5.9 % for MPSS and CPSS, respectively, compared to the experimental cumulative yield. The theoretical average basin water temperature observed in MPSS is 64.56 °C, and CPSS is 58.8 °C. The enhancement in temperature is attributed to the supplementary heat provided by the PHP.

Carbon dioxide evaporation heat transfer coefficient prediction in porous media using Machine learning algorithms based on experimental data

The prediction of the internal heat transfer coefficient during evaporation is vital for vapor-compression refrigeration and closed-loop power cycles. Accurate measurements and understanding of CO2 heat transfer in porous evaporators are essential for optimal system design across various operating conditions. This study utilizes a reference dataset derived from previous experiments that investigated the impact of porous evaporators on CO2′s internal heat transfer coefficient under sub-critical conditions, employing gravel sand as the porous medium. The dataset encompasses key factors: gravel sand porosities ranging from 39.8 % to 44.5 %, evaporator inlet pressures between 3700 and 4300 kPa, CO2 mass flow rates from 10.7x10-5–18x10-5kg.s−1, and porous tube effective diameters spanning 1.53 x10-3–3.4x10-3m. Employing three machine learning techniques (SVM, GPR, OBEM), the study predicts the internal heat transfer coefficient using regression models. The models’ predictions are analyzed and compared to expected values for validation, evaluating their performance using four statistical criteria. Results indicate SVM, GPR, and OBEM models achieved RMSEs of 1.5471, 1.8212, and 3.6978, respectively, while MAE errors were 1.1479, 1.2418, and 2.9787, respectively. Comparison with the dimensional analysis method reveals the effectiveness of the proposed models in accurately predicting internal heat transfer coefficients. The models exhibit low uncertainty and maintain prediction quality on an extended dataset without overfitting concerns. Overall, this research contributes valuable insights for designing heat exchangers and systems in vapor-compression refrigeration and closed-loop power cycles.

Comparative study on productivity of double slope solar still using different wick and energy storage materials

The aim of the proposed study is to evaluate the efficiency of Double Slope Solar Still (DSS) using different wick and energy storage materials. In the study, wick materials such as jute, cotton and sponge were used in combination with energy storage materials such as mild steel, cast iron and copper. The experiments were conducted using DSS with different combinations of wick and energy storage materials to determine the efficient performance combination. The results show that sponge and copper have higher efficiency with wicks and energy storage materials. The results show that the maximum amount of water produced in a single day was 3147 ml when the DSS system was used with a combination of sponge and copper materials. This range shows an increase of 17.47 % and 12.44 % over the DSS with jute and copper and the DSS with the combination of cotton and copper respectively. The DSS showed the highest average energy efficiency when used in conjunction with a combination of sponge and copper, namely 51 % of the average efficiency. This corresponds to an increase of 18.60 % and 10.86 % compared to the combination of DSS with jute and copper and the combination of DSS with cotton and copper respectively. The best evaporation heat transfer coefficient of 345 W/m2K was determined for the combination of DSS with sponge and copper. Average exergy efficiency is also maximum of 3.076 % for the DSS with sponge and copper materials, which is higher than other investigated combinations. The proposed system has an energy payback period of 1.26 years, an energy production factor of 0.78, a life cycle efficiency of 6.9 % and a cost of $0.031 per litre of water produced in the environmental economic analysis.

Thermal performance analysis and optimization of a heat pipe-based electronic thermal management system using experimental data-driven neuro-genetic technique

Heat pipes are highly efficient heat transfer devices and are used widely to dissipate heat generated by electronic components, thereby maintaining their operating temperatures and preventing overheating. The present study deals with the thermal performance prediction of a cylindrical heat pipe (length 380 mm) based electronic thermal management system using steady-state experimental investigation. The experiments are conducted under varying operational conditions, including heat inputs (10–50 W), condenser cooling water inlet temperatures (288.15, 293.15, and 298.15 K), and flow rates (10, 25, and 40 LPH). The study shows that the evaporator temperature of the heat pipe consistently rises with the increase in heat inputs for all tested conditions. The lowest evaporator temperature (311.42 K) is noticed for the cooling water inlet temperature of 288.15 K across all heat inputs. Notably, at 10 LPH, the heat pipe supplied with cooling water at 288.15 K exhibits reductions of up to 1.47 % and 2.29 % compared to 293.15 K and 298.15 K, respectively. Similar trends are also observed at 25 LPH and 40 LPH. The heat pipe’s thermal resistance is lowest (0.95 K/W) at a cooling water inlet temperature of 298.15 K. At 10 LPH, there are reductions of 10.19 % and 5.62 % compared to 288.15 K and 293.15 K, and at 40 LPH, reductions are 15.66 % and 7.89 %. The heat pipe’s evaporator heat transfer coefficient also plays a significant role and is maximum for the cooling water inlet temperature of 288.15 K at a flow rate of 10 LPH. To understand the complex behavior of the heat pipe and for better prediction of its thermal performance, a neuro-genetic (experimental data-driven artificial neural network and genetic algorithm) optimization technique is considered in the study. This predicts the optimal combination of operating parameters (heat input, cooling water inlet temperature, and flow rate) to minimize the heat pipe’s evaporator wall temperature which is found to be 312.54 K at a heat input of 10 W, flow rate of 10 LPH, and cooling water inlet temperature of 289.14 K. These input combinations are further validated through the experiments and the error in the heat pipe’s evaporator temperature is found to be 0.66 % only. Overall, this neuro-genetic technique underscores the importance of optimizing operational parameters for enhanced heat pipe performance and offers valuable insights for electronic cooling systems.

Comprehensive experimental thermal analysis of single bubble nucleation in vertical flow boiling: Whole field temperature and microlayer dynamics

Nucleate boiling, an important heat transfer phenomenon, holds significance in the thermal management of numerous engineering applications. Understanding and predicting its heat transfer characteristics are vital for system optimization. The formation of a microlayer, resulting from rapid bubble growth on a heater substrate, subsequently facilitates further bubble growth through evaporation playing a critical role in overall bubble dynamics. This study examines the intricate relationship between bubble and microlayer dynamics and their impact on heat transfer rates. Experimental work is conducted for varying heat fluxes using rainbow schlieren deflectometry (RSD) and thin-film interferometry (TFI) in tandem for simultaneous mapping of key parameters: bubble dynamics, microlayer dynamics, and two-dimensional temperature field in vertical flow boiling. The analysis of these parameters has provided significant insights into some fundamental aspects of boiling. Firstly, this study emphasizes that the bubble growth rate models need to take into initial microlayer thickness in accurately predicting bubble growth dynamics. The iteratively obtained constant Ceff (describing the initial microlayer thickness) for bubble growth models matches well the value of Ceff obtained experimentally from thin film interferograms. Additionally, this study also validates the applicability of the kinetic theory-based model in predicting the evaporative heat transfer coefficient during the initial stage of flow boiling, while demonstrating that the accommodation coefficient is ∼0.04 for the investigated range of heat fluxes. Subsequently, using the experimentally obtained evaporative heat transfer coefficient, further insights are provided into the dry patch dynamics. Experimental dry patch dynamics were in good agreement with theoretical predictions up to the rewetting phase. The results also indicate a substantial contribution of microlayer evaporation, constituting approximately 34 % of the total heat flux.

An experimental study on evaporator heat transfer performance of radially rotating heat pipes with water as working fluid

The utilization of radially rotating heat pipes (RRHPs) provides a novel method for gas turbine cooling due to the high thermal conductance and simple structure. Centrifugal force is utilized to return the condensate from the condenser to the evaporator. In this paper, the evaporator heat transfer performance of a RRHP, including heat transfer regime and the effect of centrifugal acceleration on heat transfer, are investigated experimentally. The RRHP evaporator with 45 % filling ratio was heated by electric heating wire with heat input ranging 50 W to 200 W and the condenser was cooled by air. Centrifugal acceleration at the evaporator center ranged from 117 g to 1457 g. Temperature distribution along the RRHP was measured by thermocouples. Experimental results revealed that non-boiling regime and boiling regime could occur in the RRHP evaporator according to heat input while natural convection and nucleate boiling were the dominating heat transfer mechanisms respectively. Geyser boiling phenomenon at 117 g to 344 g and temperature overshoot at 682 g to 1457 g were first observed in RRHPs as the transition stage at which boiling started to occur and the heat transfer performance was enhanced by 23 % in average. The evaporator heat transfer coefficient ranged 5180 to 9866 W/(m2·K) in non-boiling regime and 10,621 to 15438 W/(m2·K) in boiling regime with a boundary around 10000 W/(m2·K). With increase of centrifugal acceleration, transition stage was delayed to higher evaporator temperature and heat input. Nucleate boiling was suppressed by 20.3 % while natural convection was enhanced by 14.9 % as accelerations increased from 117 g to 682g. Interestingly, it was first observed that boiling was not entirely suppressed at 1457 g and the RRHP evaporator showed high heat transfer coefficient of 13065 W/(m2·K) at 200 W under such a high acceleration.

Modelling of evaporative falling-film heat transfer over a horizontal tube

This article focuses on evaporative falling film heat transfer through numerical simulations. A two-dimensional symmetric liquid film flowing outside a single horizontal tube was simulated using the open-source CFD software OpenFOAM. The Tanasawa model was employed to consider mass transfer at the liquid–vapour interface, and the isoAdvector VOF method was used for interface tracking. The simulations explored the impact of film flow rate, tube surface heat flux, and tube diameter on the liquid film thickness and surface heat transfer coefficient. Additionally, the critical heat flux and operational limits were considered. The results demonstrate the accurate reproduction of evaporative falling film heat transfer performance by the present model. The film thickness exhibits a similar profile to adiabatic and sensible falling film heat transfer but provides smaller values. Similarly, the evaporative heat transfer coefficient exhibits significantly higher values and a gentle peripheral variation. The influence of heat flux on evaporative falling film heat transfer is contingent on the film flow rate; it can either enhance or weaken the process. Notably, a larger tube diameter negatively affects heat transfer, suppressing the impact of heat flux, and also increases the risk of liquid film dryout. For a given film flow rate, the average heat transfer coefficient increases initially, reaches a maximum value, and then decreases as the wall heat flux is increased. The peak heat flux, termed critical heat flux, increases with film flow rate but decreases with tube diameter. An operational limit exists where the heat flux is lower than the critical heat flux and the film flow rate is higher than the critical film flow rate; under these conditions, evaporative falling film heat transfer operates in a fully wetting status.

Experimental investigation of a 10 kW-class flat-type loop heat pipe for waste heat recovery

A flat-type 10 kW-class loop heat pipe (LHP) with a box-type wick was designed and developed to handle the higher heat loads in waste heat recovery applications, such as industrial processing and internal combustion engines. Additionally, a numerical model was developed to predict the overall thermal performance of the LHP. The LHP used a stainless steel wick with a pore diameter of 2.0 µm and pure water as the working fluid. The LHP was tested using two types of cooling for the condenser (forced and natural convection), two types of evaporator orientations (horizontal and vertical), and with and without gravity assist (0.3 m and 0 m). For all the tests, the maximum heat load ranged from 5 kW to 10 kW, with the test with a gravity assist of 0.3 m, vertical evaporator orientation, and natural convection performing the best. This test sustained a 10 kW heat load at a steady-state temperature of 182 °C for the evaporator. During the same test, a maximum evaporative heat transfer coefficient of 92,000 W/m2/K at 4.5 kW and a thermal resistance between the evaporator and condenser value of less than 0.007 K/W was achieved. A numerical model was developed to compare the experimental results with the numerical temperature results for the heater block, evaporator, vapor line, condenser, liquid line, and compensation chamber. Overall, the average temperature difference for all components ranged from 7.1 °C to 10.6 °C, with the horizontal orientation without gravity assist and natural convection test predicting the best. The findings demonstrate that LHPs can handle the higher heat loads that are found in waste heat recovery applications for industrial processing and internal combustion engines.

Experimental and analytical examinations of a single-glazed solar still desalination with a spray-feeding water system with an artificial neural network

Using solar stills to desalinate salt water effectively provides clean water in remote areas at affordable prices. It is well-established that a crucial factor in enhancing the efficiency of solar desalination systems is increasing brackish water's surface evaporation rate by minimizing surface tension and boosting thermal energy. However, recent studies and proposed ideas indicate a limited and insufficient focus on this aspect. As a response, this research introduces a novel approach to raising the temperature of saline water. This is achieved through a system that involves simultaneous saline water spraying and circulation, affecting all factors and effectively reducing the surface evaporation rate. This approach impacts factors influencing surface evaporation rates and offers advantages such as low thermal inertia, reduced surface tension, and enhanced thermal energy within the desalination system. Therefore, the main goal of this study will be to improve the performance of the solar desalination system through circulating preheated salt water spray and using artificial neural networks and machine learning algorithms to obtain the function of predicting the amount of fresh water produced. The results of this study show that the evaporative heat transfer coefficient of the introduced system is significantly larger than that of the conventional solar still unit, with an average difference of about 43.98%. Also, the results of daily water output in both systems show a significant increase and superiority of 28.75% in freshwater production for the modified system. Also, the efficiency of the improved system is 65.18% higher than that of the passive solar distillation system, and the cost per liter was 0.059 ($/L/m2). The results of this study highlight the effectiveness of the saltwater spraying technique in enhancing the performance of solar desalination systems, making it a valuable prospect for future industrial applications in desalination systems and into energy systems for smart cities as a sagacious strategy towards clean and sustainable process.

An Experimental Investigation of Sintered Particle Effect on Heat Transfer Performance in an “Annular Flow” Evaporation Tube

Abstract Wicking structures have been widely used within passive heat transfer devices with high heat fluxes, such as heat pipes, to enhance their thermal performance. While wicking structures promote capillary pumping of the working fluid and thin film evaporation, they can result in capillary evaporation and further enhance the evaporation heat transfer. In this study, a 0.5 mm thick layer of 105 µm sintered copper particles was added to the inner wall of a copper tube, aiming to form an “annular flow” and enhance the heat transfer characteristics by taking advantage of thin film and capillary evaporation. Acetone was chosen as the working fluid, and the performance of an evaporation tube was tested for power inputs of 10, 30, 50, and 70 W. For each power input, trials were run at inclination angles varying from −90 deg to 90 deg to investigate the capillary effects. The temperature measurements showed that the temperature distribution along the evaporation tube is always downward sloping, meaning the temperature at the fluid inlet is larger than the outlet. Results show that an “annular flow” formed by a thin layer of sintered particles can promote thin film and capillary evaporation and, therefore, boost the evaporation heat transfer coefficient.

Effects of Heat Transfer Characteristics of R32 and R1234yf with Al2O3 Nanoparticle through U-Bend Tube Evaporator

This study used the Ansys Fluent® computational fluid dynamics code in conjunction with a volume of fluid multiphase model and phase-change model to analyze the flow boiling evaporation heat transfer coefficient and flow patterns of R32 and R1234yf with Al2O3 nanoparticle through the U-bend tube with a curvature ratio for downward-oriented flow. The volume of fluid (VOF) model was used to follow the patterns at the interface, while the SST k-omega model was used to simulate the gas-liquid flow. This work has been validated by utilizing a R134a refrigerant. Simulations were performed at various mass fluxes, vapor qualities, and temperatures to determine the effects of these variables on heat transfer and frictional pressure decrease in the tube. R1234yf shows much better performance than the other pure refrigerants in terms of heat transfer and vaporization. The addition of nanoparticle Al2O3 with the refrigerants R32 and R1234yf significantly improved the heat transfer coefficient and increased the vapor fraction. The frictional pressure drop increases with increasing mass flux and decreases with increasing vapor quality due to a significant decrease in the liquid film thickness. The heat transfer coefficient, on the other hand, increases with increasing mass flux and decreases with vapor quality up to a point. There are certain changes in the heat transfer coefficient at the bend. After the bend, the frictional pressure drop increased at a higher rate than before the bend, and the vapor fraction increased at a higher rate.

Surface functionalization enabling highly effective phase-change heat transfer of R1234yf: A molecular dynamics study

Phase-change heat transfer has been demonstrated to be an effective thermal management approach for high-power electronics and power generators. In this work, we propose a strategy using surface functionalization to enhance the phase-change heat transfer of an environmentally friendly refrigerant, i.e., hydrofluoroolefins (HFO)-based R1234yf. Using molecular dynamics simulations, we find that the evaporation (condensation) heat transfer coefficient (HTC) at self-assembled monolayer (SAM) functionalized gold (Au) surfaces can be increased by ∼7 (∼5) times compared to that at the planar Au surfaces. The improvement in phase-change heat transfer of R1234yf at functionalized Au surfaces arises from the high thermal conductance across functionalized Au/R1234yf interfaces and the strong vibrational couplings between SAM molecules and R1234yf molecules. On the one hand, the introduced SAM layer can increase the interfacial thermal conductance (ITC) between Au and R1234yf owing to the strong bonding between Au and the SAM molecules, as well as the strong vibrational couplings at low-frequency modes (0–6 THz) among Au, SAM, and R1234yf. On the other hand, the thermal energy exchange during the evaporation (condensation) process can be improved by surface functionalization through the vibrational couplings between SAM and R1234yf at high frequency (∼12 and ∼30 THz) regions. This dual vibrational coupling will then allow the SAM layer to work as an intermedia to bridge the vibrations between Au and R1234yf molecules, which in return benefits the phase-change heat transfer. By further characterizing the evaporation and condensation processes, we find that the evaporation rate of R1234yf is increased by ∼50% at most at the wall superheat <70 K when the Au surfaces are functionalized using SAMs. When the wall superheat is above 70 K (i.e., 100 K for the planar Au surface), the evaporation of R1234yf becomes like film evaporation in which clusters of R1234yf molecules move off the surface together, which limits the enhancement (i.e., ∼100% at most) of evaporation HTC at the functionalized Au surfaces. At the same time, the condensation rate at functionalized Au surfaces is found to be increased by 15%–100% compared to that of the planar Au surfaces. By elucidating the mechanisms governing phase-change heat transfer on functionalized surfaces, our results provide an efficient strategy to enhance the cooling efficiency of the HFO refrigerants, which may benefit the design of surfaces with high phase-change heat transfer capabilities.

Analysis of the factors affecting heat transfer performance and prediction of heat transfer coefficient for MVR evaporator: A case study

The heat transfer coefficient of evaporation is one of the key factors affecting the design and management of evaporators. This paper presented an industrial case to investigate the influencing factors on the evaporator's heat transfer coefficient, as well as the interaction between the influencing factors. Besides, a comparison of accuracies was made between the heat transfer coefficients predicted by physical models and the support vector regression (SVR) model. The study results showed that due to the interaction between the influencing factors, changes in the heat transfer coefficient varied greatly from the laboratory results. The impact of other variables must be controlled for in industrial research. There were big errors between the results predicted by the early experimental or numerical models and the observed results. The root mean square error (RMSE) values and mean absolute percentage error (MAPE) values of the physical models were all greater than 150 and 5%, respectively, and their R2 values were all lower than 0.50. The data mining-based SVR predictions were more accurate than the predictions by physical models, with a maximum relative error of 5.38% and a minimum relative error of −5.60%.

Enhancement of heat transfer coefficient of water desalination in multi-stage flushing using hematite nanofluid

This study is the first step toward the enhancement of the evaporation amount and heat transfer coefficient (HTC) in multi-stage flushing (MSF) desalination processes by incorporating hematite nanoparticles (Fe2O3 NPs) into saltwater. The study involved the application of a laboratory experiment aimed at determining various physical properties of saline water (SW) nanofluid (NF). These properties included stability, thermal conductivity (TC), evaporation volume and rate, boiling temperature (BT) and time, and local HTC in stagnant conditions. The investigation took into consideration the impact of salinity, NPs (with regard to both size and volume fraction (φ)), and pressure. The utilization of stable SW NF resulted in reduced BT and expedited boiling. In the actual MSF process, the preheating of the cooling SW occurs rapidly and at low temperatures prior to its arrival at the brine heater. The determined decline in usage of steam was 2 % in the least favorable scenario. Meanwhile, the measurement of the amount of evaporation was conducted by evaluating the condensation content under ideal circumstances, which indicated a 15 % increase under atmospheric pressure and a rise of 25 % under vacuum circumstances. A significant enhancement of the local HTC, reaching to 134 %, was observed under stagnant conditions when utilizing NPs with a size of 20 nm and φ of 0.05 %. This denotes the initial stage in the estimation of the augmentation in output of water through the utilization of SW NF. The deposition of Fe2O3 NPs with high thermal conductivity is expected to have a significant impact on the mitigating surface thermal resistance caused by inorganic scale in MSF plants.

Thermodynamic performance study on the novel efficient flow pattern global construction evaporator

Based on the fluid-efficient flow pattern global construction heat transfer enhancement mechanism, a novel evaporator, called the efficient flow pattern global construction evaporator (EFGE), is presented to improve the in-tube evaporation heat transfer at low quality. The numerical analysis and experimental study of EFGE's thermodynamic performance are performed. Results show that the temperature distribution characteristic of the fluid-efficient heat transfer flow pattern construction is better than that of the initial fluid. The evaporation heat transfer coefficient (HTC) of EFGE is 0.34–1.04 times that of a common parallel flow evaporator (PFE), and the pressure drop of EFGE is only 80%–116 % of that of a common PFE at the quality of 0.9. The theoretical nonuniformity of the evaporation HTC between low- and high-quality flow is approximately 12%–67 %, which is 55%–72 % of the pressure drop. The numerical analysis results agree with the finding that EFGE has better thermodynamic performance than PFE in terms of friction power reduction and minimum entropy generation number.

Investigation of aqueous alcohol solutions as working fluids in an ultra-thin vapor chamber

ABSTRACT An ultra-thin vapor chamber (1.5 mm thick) has been developed for experimental investigation. Aqueous solutions of propanol exhibiting concentration and thermal Marangoni effects are investigated as potential vapor chamber charge fluids, and their impact on the thermal performance is investigated and compared against pure water and 2-propanol. The vapor chamber thermal resistance and evaporator temperature are lower at low charges with pure 2-propanol and 2.4 M and 4.2 M 2-propanol-water solutions compared to pure water. A 4.2 M propanol-water mixture shows an almost 4.5 X increase in the evaporator heat transfer coefficient compared to pure water.

Numerical investigation on necessary condition for temperature oscillation in loop heat pipe

A loop heat pipe (LHP) is a two-phase heat transfer device that utilizes the latent heat and capillary force of a wick. Many spacecraft have been equipped with LHPs because of the ability of LHPs to transfer heat efficiently. However, temperature oscillations may sometimes occur and cause deviations from the allowable temperature range. A recent numerical analysis of temperature oscillations has revealed some sufficient conditions for temperature oscillations. However, the necessary conditions, which are required to prevent temperature oscillations, are not yet understood. Here, a parametric study was performed using a transient mathematical model to understand the necessary conditions for temperature oscillations. The parametric study showed that a small heat capacity of the reservoir, low-thermal-conductivity wick, or high evaporation heat transfer coefficient led to temperature oscillations. These results indicated that temperature oscillations occur when the temperature difference between the reservoir and condenser becomes small. Therefore, a small temperature difference between the reservoir and condenser is a necessary condition for temperature oscillations. This is because the small temperature difference causes the penetration of the two-phase flow into the liquid line. To prevent temperature oscillations, a large temperature difference should be maintained. One method to maintain the large temperature difference is to increase the thermal capacity of the reservoir, such as by increasing the filling ratio. Heating the reservoir is another method to keep the temperature difference high.

Experimental study on evaporation heat transfer characteristics of multi-composite porous wick with three different powders

Based on our previous research, the composite porous wick with spherical-dendritic powders owns biporous structure and uneven local thermal conductivity. In this paper, a multi-composite porous wick sintered with three different materials and structures of spherical copper, spherical nickel and dendritic nickel is tested. The spherical copper is beneficial to expansion of the evaporation heat transfer area, the dendritic nickel can provide sufficient liquid for evaporation, and the spherical nickel is used to regulate the distribution of spherical-dendritic powders and large pores. Through the evaporation heat transfer experiment, it is found that there are five different types of operating temperature and evaporation mass curves, including continuous and intermittent overshoot, single overshoot, stable operation and temperature losing control. The occurrence of the overshoot is affected by the bubble behavior in the bottom of the porous wick. And two types of the vapor–liquid interface behavior occur in the overshoot operation, in which the porous wick is penetrated by a single bubble or the evaporating interface, respectively. The multi-composite porous wick can reach a maximum heat load of 520 W (the corresponding heat flux is 43.5 W/cm2), and the highest heat transfer coefficient of the open capillary evaporator can reach 2776.1 W/(m2·K), with the lowest total thermal resistance is 0.3089 K/W at 500 W heat load. As a reference using a composite porous wick prepared with spherical copper powder and dendritic nickel powder of the same volume ratio, its maximum heat load is only 210 W (the corresponding heat flux is 17.6 W/cm2). The multi-composite porous wick shows better evaporation heat transfer performance. After the evaporation heat transfer experiment, the oxidized multi-composite porous wick still shows good wettability. It is inferred that a mixed wetting structure is formed inside it due to the composite structure sintered by three kinds of metal powders, which may be beneficial for improving the evaporation heat transfer performance of the multi-composite porous wick. In addition, the conditions such as lower effective thermal conductivity, more uneven local thermal conductivity, and large pores provided by the unchanged spherical nickel powder may also play a role.

Pore network simulation of loop heat pipe evaporator with different pore size distribution

To investigate the relationship between the loop heat pipe evaporator heat-transfer coefficient and wick pore size distribution, this work simulated the two−phase thermal hydraulics in the evaporator by pore network model in three dimensions. The simulation considered a liquid–vapor (L–V) two-phase state in the wick, which is induced by nucleate boiling and local pore-scale fluid dynamics. Some wick samples, while having the same macroscopic porous characteristics such as permeability, were created with different pore size distributions. Heat transfer and fluid flow of the evaporator were solved with ammonia and R134a as working fluids, along with stainless-steel and polytetrafluoroethylene (PTFE) porous materials. Each evaporator showed a different evaporator heat-transfer coefficient and maximum applied heat flux. Some characteristics with respect to the L–V phase distribution in the wick were selected to investigate their correlation with heat-transfer coefficients. As a result, the correlation coefficients with the three-phase contact line (TPCL) length ranged from 0.70 to 0.91. Only the TPCL showed a correlation with the heat-transfer coefficients in all conditions, as opposed to the vapor pocket maximum depth and liquid saturation. It was discovered that the TPCL length is proportional to the heat-transfer coefficients. In all cases, the TPCL length and heat-transfer coefficients reached a maximum at moderate pore size dispersion. In addition, The heat-transfer coefficients of the PTFE wick were more sensitive to the TPCL length than that of the stainless-steel one. The discovery helps enhance the thermal performance of the evaporator porous structural design by additive manufacturing.

Numerical Study on Heat and Mass Transfer of Evaporated Binary Zeotropic Mixtures in Porous Structure

This study investigates the heat and mass transfer characteristics of a binary mixture (R134a/R245fa) evaporated in a porous medium. The Eulerian model coupled with the multiphase VOF model and species transport equations is employed to establish a multi-component evaporation model. The effects of heat flux ranging from 200 kW/m2 to 500 kW/m2, porosity ranging from 0.4 to 0.6, and mass fraction ratios (R134a/R245fa) of 3:7, 5:5, and 7:3 are explored. The results indicate that an increase in heat flux contributes to an increase in the evaporation rate. For the overall evaporation rate, the evaporation rates of R134a and R245fa improve by 11.3%, 6.9%, and 16.3%, respectively, while the maximum improvement in heat transfer coefficient is only 1.4%. The maximum evaporation rate is achieved at intermediate porosity in the porous medium, and the highest heat transfer coefficient is obtained at a porosity of 0.4. With the increase in mass fraction, the evaporation rate of the corresponding species also increases, while the overall evaporation rate and heat transfer coefficient remain almost unchanged.