Evaporation Heat Transfer Coefficient Research Articles

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 enhances 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° to 90° to investigate the capillary effects. The temperature measurements showed 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.

Relationship between phase distribution and heat-transfer coefficient of loop heat pipe evaporator investigated by lateral observation of porous media

Loop heat pipes (LHPs) can be passively operated, require no electric power, and can be flexibly arranged, making them promising in thermal control systems for both terrestrial and aerospace applications. Understanding the two-phase state in the porous media of a capillary evaporator induced by nucleate boiling is key to enhancing the evaporator performance. This study visualized the liquid–vapor (L–V) phase distribution on the contact surface between the cylindrical evaporator casing and the wick to investigate the L–V phase distribution and to figure out its relationship with the evaporator heat-transfer coefficient. The visualization was successfully realized under conditions similar to those observed in a practical environment using a sapphire tube as the evaporator casing. The observation results showed that the wick underwent a phase transition from a liquid-saturated state to a two-phase state under the effect of nucleate boiling, and the evolution of the L–V phase distribution on the contact surface with varying heat load could be determined. The heat-transfer coefficient peaked immediately after the nucleate boiling and then decreased with increasing heat load. The three-phase contact line (TPCL) within the casing, liquid, and vapor phases showed a similar trend; however, the saturation on the contact surface was different, decreasing quickly after the nucleate boiling, followed by a gradual decrease. The correlation coefficients between the heat-transfer coefficient and the saturation/TPCL length were −0.61 and 0.83 under all heat load conditions, respectively. The characteristics of the heat-transfer coefficient as a function of the heat load were found to correspond to those of the TPCL length rather than the saturation. This finding can aid in the design of microscopically optimized porous media by additive manufacturing.

Thermal resistance analysis of micro heat pipes incorporated with carbon nanotubes nanocapillaries

The remarkable performance improvement and thermal optimization of micro heat pipes (MHPs) incorporated with carbon nanotubes (CNTs) of different density are analysed using a thermal resistance model. The ultrafast water transport characteristic of CNT-nanocapillaries induces large-surface-area water film where the evaporation, condensation, and condensate circulation processes, which govern the MHP's overall performance, are simultaneously enhanced. By employing a thermal resistance model, the pertinent parameters inside an MHP, which are not measurable due to the miniature size of the microchannels, are estimated. By benchmarking with the uncoated MHP, the condenser and evaporator heat transfer coefficients of the optimal 5 mg/CNTs MHPs are enhanced remarkably by the up to 249% and 264%, respectively. Furthermore, the circulation rate attains a maximum increase of 212%. Consequently, an enhancement up to 250% in the overall performance of MHP gauged by the effective thermal conductivity and a significant temperature drop of 20.8 °C on the heat source as compared to the uncoated MHP case are achieved. By minimizing the total thermal resistance of MHP, the optimized length ratio of the evaporator and condenser, which is useful in the optimal design of the MHP, can be obtained. The thermal resistance model serves as a useful analytical tool to provide insightful thermal analysis and optimized design of an MHP.

Experimental performance evaluation of solar still with zig-zag shape air cooled condenser: An energy–exergy analysis approach

In the present experimental effort is made to increase the performance of a solar still (SS) by including a novel design of a zig-zag-shaped air-cooled condenser (ZZACC) and cuprous oxide (CuO) as a nanomaterial. Research work is conducted in the climatic conditions of Gandhinagar, Gujarat, India, from September to November 2020. A comparison was made to assess the performance of a conventional solar still (CSS) and a solar still equipped with a zig-zag shape air-cooled condenser (SSWZZACC) with CuO. The experiments’ findings showed that adding CuO to SSWZZACC increases the distillate production by 46.83% and the daily energy efficiency by 45.98%, respectively, compared to CSS. Also, SSWZZACC demonstrates a better efficiency of exergy and latent heat of vaporization than CSS because CuO causes an increase in the evaporative heat transfer coefficient of water. In life cycle cost analysis study discovered that SSWZZACC has a 27.77% lower cost per litre of water (CPL) than CSS. The obtained maximum energy and exergy efficiency values for CSS and SSWZZACC were 2.36% & 25.75% and 3.9% & 37.59%, respectively. In economic and environmental aspects, it was found that SSWZZACC with CuO showed a cost-effective desalination unit and was highly effective from a carbon credit point of view (CCP) by CO2 mitigation.

Investigation of Heat Transfer and Pressure Drop for R744 in a Horizontal Smooth Tube of R744/R404A Hybrid Cascade Refrigeration System—Part 2: Low-Temperature Region

This paper studies the evaporative heat transfer characteristics of R744 at low temperatures in a horizontal smooth tube as a cascade refrigeration system (CRS) among hybrid cascade refrigeration systems (HCRSs). There is a lack of research on the low-temperature evaporative heat transfer characteristics of R744 under the operating conditions of evaporators used in actual CRSs used in supermarkets. Therefore, this study aims to provide basic data on the evaporative heat transfer characteristics of R744 in the evaporators of refrigerators used in supermarkets. The tube used in the evaporation experiment conducted herein was a smooth horizontal copper tube with an inner diameter and length of 11.46 mm and 8000 mm, respectively. The experimental parameters were as follows: heat fluxes of 11.8–21.3 kW/m2, mass fluxes of 76.3–175.1 kg/(m2·s), and saturation temperatures of −49.8–−30.2 °C. The main results are summarized as follows: the evaporative heat transfer coefficient of R744 can be predicted well by using the correlation formula of Chen at the evaporation temperature of −40 °C in the CRS.

Improvement and validation of the Tanabe model to simulate human thermal behaviors in hot environments at high altitudes

Exposure to hypobaric and hyperthermic environments can easily induce heat stress and affect operational efficiency. Thermal manikins represent advanced instruments for simulating human thermal behaviors in complex ambient conditions. However, currently, no thermal manikins are coupled with a human thermoregulatory model adapted for high-altitude environments. This study aimed to develop the traditional Tanabe model by incorporating air pressure into the calculation of the convective and evaporative heat transfer coefficients, therefore local skin temperature and thermal sensation at high altitudes could be predicted. To validate the model, these predicted variables were also measured in human thermal physiology experiments at the altitude of 0 m (101.3 kPa) and 4000 m (56.04 kPa) with an ambient temperature of 35 °C. The results demonstrated that local and weighted average skin temperatures were consistent between simulation and experimental results. The maximum deviation between experiments and simulation did not exceed 0.7 °C at segments except hand and foot, and 2.5 °C at each segment, at the altitude of 0 m and 4000 m respectively. The R2 values of Gaussian distribution and linear regression results concerning the experimental and simulation weighted average skin temperature and the PMV score at the altitudes of 0 m and 4000 m were 0.966, 0.975, 0.857, and 0.851. These findings suggest that differences in skin temperature and thermal sensation at different altitudes are influenced by variations in evaporation and dry heat exchanges. The improved Tanabe model in this study was validated and could be further utilized in investigating heat transfer processes and coefficients between the skin surface and the environment and developing control algorithms for thermal manikins.

Design and evaluation of a modified basin-type stepped solar- still under Malaysia climate conditions

ABSTRACT Distillation represents one of the earliest methods of treating water, and it remains as such in many parts of the world. A basin-type stepped solar-still remains the simplest desalination technology. It is simple, easy to construct, requires minimal maintenance, and is cost-effective. The objectives of this study were to design, fabricate and evaluate the performance of a modified stepped solar-still. The theoretical analysis was conducted to determine the optimum design of the still. The productivity and solar-still efficiency were obtained by solving the energy balance equations for the absorber plate, saline water, and glass cover, the temperature difference between saline water and the glass cover, and evaporation, convective, and radiative heat transfer coefficients under the climate of Malaysia. The experimental setup was constructed, and experiments were conducted at the Universiti Kebangsaan Malaysia (UKM), Selangor (Latitude 2°56‘22“North and Longitude 101°47’16” East). The basin area of the stepped solar-still was 1.m2 (0.5 m × 2.0 m). The basin of the still was constructed from a black-painted galvanized steel sheet to increase the absorptivity. The cover of the still was made up of glass. The absorber plate of the stepped still consisted of 5 steps (0.1 m × 2 m). The stepped solar-still has been modified to include external and internal reflectors, fins, and an external condenser. The performance of the modified stepped solar-still design has been investigated and compared with the unmodified stepped solar-still. The theoretical results show a significant improvement in the performance of the modified stepped solar-still. The productivity was higher than that for stepped solar-still before modification by approximately 29% under the same climate conditions. The experimental results have shown that the maximum temperature of saline water and glass cover were 62.1°C and 56.2°C, respectively. The maximum distillate water in the still was 6.10 kg/m2. The daily efficiency of the modified stepped solar-still was approximately 52.3%. Based on the experimental and theoretical results, the proposed design significantly enhanced the stepped solar-still thermal performance. A statistical t-test was conducted to establish the significance of the difference in the stepped solar-still experimental. It was found that there was a significant improvement in the stepped solar-still performance after modification. The techno-economic analysis showed that the product cost would be around USD0.07 per liter of distillate.

Micro-grooved circular miniature heat pipe for thermal management: Experimental and analytical investigations

In this paper, thermal performance of a cylindrical micro-grooved heat pipe is experimentally and mathematically investigated. The experimental tests are conducted included the optimum working fluid filling ratio, heat pipe inclination, and input heat load. The new correlations of micro-grooved heat pipe condensation and evaporation heat transfer coefficients are proposed basing on the experimental data. A coupled thermal and hydraulic model is developed to predict the heat pipe local thermal resistances and operating temperature regarding the effect of the architectural size. The Experimental results show that the heat pipe thermal performance is not affected by the inclination angle. Whereas the heat pipe performance is better at horizontal position. It showed that the micro-grooved heat pipe could provide an efficient performance even if the condenser is longer that 40% of the total length and the heat pipe microgrooves aspect ratio higher than 37%.

Experimental and analytical investigation of applying an asymmetric plate heat exchanger as an evaporator in a thermally driven adsorption appliance

This communication presents an experimental and analytical study on the evaporation mechanism in a closed-structured asymmetric plate heat exchanger (PHE) employed as a stagnant water evaporator for the application in an adsorption heat transformation appliance. To this aim, an experimental unit is constructed, which comprises two identical PHEs, one acting as an evaporator/condenser and the second, as an adsorber/desorber. Two endoscopes are mounted inside the investigated evaporator to visualize the evaporation mechanism when performing adsorption-evaporation processes under different boundary conditions. It turned out that the evaporation mechanism is a partially covered, thin film evaporation. A heat transfer analysis is performed to evaluate the heat transfer coefficient of the thin film evaporation (hf) inside the investigated evaporator, resulting in hf-values between 1330 and 160 [W∙m−2∙K−1] over the investigated adsorption-evaporation time. Correlating the obtained (hf) to the film thickness δ and the wetted area Awet results in δ-values between 0.34 and 0.78 [mm] and wetted to total area ratios Awet/Atotal of 0.78 to 0.16. Besides, an analytical model has been developed and introduced to correlate the overall evaporator heat transfer coefficient with the adsorption potential and the time rate of change of the water uptake.