Capillary Evaporator Research Articles

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.

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.

Energy Saving Thermal Management of Space Remote Sensor and Validation

An energy-efficient thermal control management method for space remote sensors using optical, mechanical, electrical, and thermal integration is proposed. The satellite power resources are insufficient, so an energy-efficient loop heat pipe (LHP) is designed for six intermittently operating detectors. The charge-coupled device (CCD) has a total heat generation of 72 W and operates for 8 min per orbital cycle. The LHP includes a capillary pump, six cold plates, and two radiators. The working fluid of the LHP is high-purity ammonia and the material of the wick is ceramic. The drive power on the capillary pump evaporator automatically switches between 30 W and 90 W depending on the operating mode of the remote sensor, resulting in an average power saving of about 58.2% compared to a conventional LHP. For the optical structure, a three-stage insulation technology was developed to save heater power and improve temperature stability. A transient numerical simulation model of the LHP was developed to study the vapor–liquid zone of two radiators under the condition of rapid power change. Vacuum thermal tests were conducted and the test data agreed well with the numerical simulation results. The in-orbit temperature data showed that the temperature fluctuations of the optical structure and CCD were less than ±0.2 °C and ±0.8 °C, respectively.

Design of a thermal control system with a two-phase coolant for a spacecraft with a capacity of 7 kW

This article presents a methodological approach to designing a spacecraft thermal control system with coolant pumping with a cooling capacity of up to 7.0 kW. Two design options are considered. There is no airtight instrument container in the spacecraft layout scheme, and all heat-generating equipment is located directly on the power structure panels, so excess heat is removed from the spacecraft directly from the outside of the instrument panels. With all the attractiveness of a two-phase circuit with a heat pipe circuit, its use in automatic spacecraft is complicated by the need to supply concentrated heat to the capillary evaporator of the circuit. To do this, it is necessary to collect heat from a large surface of the structure, on which a large number of heat sources are installed. A schematic solution of the thermal control system is considered, in which the thermal power of the payload module is distributed between the panel of the service systems module and deployable radiators. At the same time, in the first version, the heat pipes of the payload module are connected to the heat pipes of the service systems module along the profile shelves, the contour heat pipes of the deployable radiator are connected only to the heat pipes of the service systems module. Ums, the heat load of the payload module is transferred to the heat pipes of the service systems module and then to the loop heat pipes. The second option differs from the first one in that to equalize the temperatures of the panels of the payload module, each heat pipe of the North panel is connected to the heat pipes of the South panel. From the results of a comparative analysis of the mass budget and energy efficiency, the conclusion follows: the DFC with capillary pumping is the most preferable option 2. which, with the same other characteristics, has a smaller mass. The specific mass-energy: characteristic of such a system is ~22.9 kg/kW.

Working fluid study for loop heat pipes

Loop heat pipes (LHPs) provide unique applications for high heat flux and long distance heat transfer. Working fluid aspects, including liquid charging ratio, type and fluid dynamics, in loop heat pipe is quite different as compared to conventional heat pipe. This paper will particularly endeavor to explain these differences, and provide optimal design conditions achieved via experimental study. Design considerations and operational aspects of LHP with respect to working fluid is evaluation and discussed in detail. Important thermal-fluid properties with respect to pressure-temperature relation and merit number are outlined for various heat transfer fluids to choose best option for given conditions. LHP with capillary evaporator, 25 mm diameter and 155 mm long, and heat transfer length ∼250 mm was designed to study and optimize different working fluid aspects including optimum working fluid range for different orientations, effect of fluid type on performance and operational dynamics of loop related to fluid (start-up, oscillations, hysteresis and buoyancy effects). Loop utilized copper containment, nickel wick and mainly water as working fluid. As an outcomes of this investigation, it has been concluded that liquid ratio in range of 40 to 60 % of loop volume provide optimum charge, for different orientations, with 50 % charge showing best thermal performance. In summary, considered working fluid type and optimized overall liquid amount, calculated by giving due consideration to volumes of different loop components, can help to get best performance from the loop systems.

Three-dimensional lattice Boltzmann investigation on pore scale liquid–vapor distribution patterns and heat transfer performance of a loop heat pipe heterogeneous porous wick evaporator

The pore scale numerical simulations on evaporation of a three-dimensional heterogeneous porous wick capillary evaporator used in a loop heat pipe (LHP) are realized through using a lattice Boltzmann method with liquid–vapor phase change. The dynamic liquid–vapor interface characteristics are obtained. The steady-state wick state and heat transfer performance as well as the influences of wick porosity and surface wettability are discussed. It is found that as heat flux increases, the wick state changes from the fully saturated state at low heat fluxes, to the partial dryout state at medium heat fluxes, and finally to the complete dryout state at high heat fluxes. The effective heat transfer coefficient of evaporator firstly grows slowly for the saturated wick, then increases rapidly and reaches a maximum value at partial dryout wick state, but afterwards drops sharply when the wick starts to be complete dryout. With the decreasing of wick porosity and contact angle, the heat flux to achieve the maximum effective heat transfer coefficient is increased and the complete dryout state is postponed. In the present range of parameters, the maximum effective heat transfer coefficient is improved by reducing wick porosity, while it is barely affected by contact angle.

The Functioning of The Hybrid Integrated Partially Solar-Vapor-Compression Fridge

The main purpose of our research is to increase the utilization of solar thermal energy to supply a refrigerator with vapor compression and reduce the refrigeration power needed for cooling. Combined Hybrid Solar - the vapor- compression refrigerating unit has been built and operates under Mutah University's environment in Jordan. The systems were made up of a capillary tube, condenser, evaporator, and collector. The vapor-pressure refrigerator was incorporated with the classic water-solar system to minimize the compressor's duty and to reduce power consumption in heating the amount of water held in the pipe to be sent along the tube outside the evaporator. After that, it will be returned to the compressor, But at a lesser temperature, to minimize compressor workload and enhance cooling performance. Before the compressor was developed, a solar collector system had been created to maximize its temperature before reaching the compressor to improve C.O.P, and the difference in temperature was remarkable. The vapor-compression refrigerator unit was powered by many generators: solar collector that has been discharged, photovoltaic system, flat plate solar collector. Two groups of tests have been performed experimentally on the partial solar compression refrigerator integrated into a hybrid system. First in the vapor compression refrigerator only, and the second in the Hybrid solar compression refrigerator incorporated. Total sunlight and different temperatures, current, and voltage were measured for many months each hour of the day. The performance coefficient was determined found 2.019, 2.432 respectively. Many auxiliary instruments are utilized to measure the temperature in irradiation networks, voltage, and night-time current every hour.

Thermal performance of self-rewetting gold nanofluids: Application to two-phase heat transfer devices

This paper presents an experimental analysis of the phenomenon of phase change inside a porous medium using different types of working fluids. It represents the impact of these fluids on improving the characteristics of heat and mass transfer in a Capillary Heat Pipe (CHP). In this study, gold nanoparticles (5 nm in diameter with 1% Cv), a self-rewetting binary solution (butanol with 3% Cv), and a mixture of self-rewetting butanol and gold nanofluid are considered to be the operating fluids within the CHP. The experiments are carried out after designing and developing the capillary heat pipe section. It consists of a water tank with a pump, an evaporator attached to a copper porous medium on which thermocouples and power supplies are placed. The experimental results showed the positive influence of gold nanoparticles on the thermal system's performance by reducing the thermal resistance by 13% compared to pure water as the base working fluid. In addition, a self-rewetting butanol solution showed improvement in the performance of the capillary evaporator by decreasing its casing temperature. While a mixture of self-rewetting butanol solution (3 % Cv) and gold nanofluid (1% Cv) exhibited the best performance of heat and mass transfer performance by reducing the thermal resistance of the system by approximately 22 %. To explain the mechanism for improving heat transfer, the phase change phenomenon was visualized by an infrared camera for the three working fluids. It is shown that as the applied power increases, the shape of the vapor pocket developed in the wick also increases, for pure water, until it reaches a stable form. Whereas, with respect to nanofluid and self-rewetting fluid, the shape of the vapor pockets was smaller than that of pure water allowing more efficient mass and heat transfer. The thermophysical properties of these fluids such as thermal conductivity, stability, surface tension, Marangoni, wettability, and capillary forces were presented to ensure and validate the decrease in the vapor pocket as well as the enhancement of the CHP thermal system.

Evaporation heat transfer characteristic of porous wick in an open capillary evaporator

An open capillary evaporator is proposed to study the evaporation heat transfer performance of porous wick. Both temperature and evaporating mass are measured in this system. Besides, the temperature and relative height of the supplied fluid can be adjusted. A composite porous wick is tested through the constant and variable heat load operation, and it reaches a maximum heat load of 320 W, which corresponds to a heat flux of 25.5 W/cm2. Two types of the evaporator wall temperature curve are observed, and the effect of the initial condition is considered. Intermittent fluid supply appears and causes a temperature overshoot of the evaporator wall. But continuous intermittent fluid supply and small amount of fast-replenishing fluid does not cause the temperature overshoot. The influence of adding water on temperature overshoot is also studied. The boiling and evaporation occur at different heat loads, and the two heat transfer modes coexist at moderate heat load. The transition in heat transfer modes leads to that the evaporation heat transfer coefficient first increases and then decreases with the increase of heat load. This study is helpful to analyze the evaporation heat transfer performance of porous wick and understand the operating characteristics of loop heat pipe.

Analysis of a flat capillary evaporator with a bi-layered porous wick

A numerical evaluation of the heat and mass transfer concerning a flat capillary evaporator provided by a bi-layered porous wick is presented. The wick has a shape of a flat disc and is assembled between the liquid feeding channel and the vapor chamber. An external heat input is applied into the upper surface of the bi-layered wick, where the working fluid evaporates. The mass and heat transfer are modeled using the mass and energy conservation equations. The model allows to verify the effect of design variables, such as working fluids, dimensions, permeability, average pore radius and thermal conductivity of the wick, in the performance of the capillary evaporator. It can be used as a predictive tool to design similar capillary pumping systems for thermal control of satellite or electronics systems in general.

Effect of surface properties of capillary structures on the thermal behaviour of a LHP flat disk-shaped evaporator

In this work, capillary structures are manufactured by sintering of copper and zirconia powders using different sintering parameters, in order to obtain porous samples with various hydrodynamic characteristics and surface properties. A specific test bench is designed to reproduce a capillary evaporator of a LHP decoupled from the remaining of the loop and enables the determination of the evaporator heat transfer coefficient and optimum heat flux at the occurrence of the operating limit. The influence of the surface properties on the evaporator thermal behaviour is investigated by polishing the copper wick surface in contact with the evaporator fins, or by coating the zirconia wick with a thin gold layer. It is shown that polishing decreases the thermal performance when the size of the gap between the evaporator fins and the wick becomes lower than the pore size. The coating enables to enhance the evaporator thermal performance, probably by a reduction of the thermal contact resistance or by a favourable surface wettability effect, reducing the strong effect of the wick topography on the optimum heat flux value. Finally, a data mining approach is proposed which highlights the preponderant effect of the surface quality on the evaporator heat transfer coefficient.

Investigation on liquid-vapor interface behavior in capillary evaporator for high heat flux loop heat pipe

This paper reports a theoretical and experimental study on a thermo-fluid dynamics in an evaporator of a loop heat pipe (LHP). A steady-state model that includes liquid–vapor interface behaviors in the evaporator was developed. In this model, the changes in the heat transfer coefficient of the evaporator were considered. A flat-type evaporator was designed on the basis of the LHP model. The main characteristics of the evaporator are as follows: the number and width of the vapor grooves that were processed in a porous wick were 85 and 0.3 mm, respectively. As results of the experiments, the LHP transported heat up to 260 W (18.2 W/cm2) for the heat transport length of 550 mm. The minimum thermal resistance of the LHP was 0.13 K/W. The operating temperature of the experimental result agrees well with the calculation result. The liquid–vapor interface behaviors in the evaporator during the LHP operation were discussed. On the basis of the model, the relation between the liquid–vapor interface behaviors and the heat transfer performance of the LHP was quantitatively explained. In addition, it was found that the contribution of the nucleate boiling heat transfer increased with the applied heat. This phenomenon enhanced the heat transfer performance at high heat flux.

Effect of self-rewetting fluids on the liquid/vapor phase change in a porous media of two-phase heat transfer devices

This paper presents an experimental study of the phase change phenomenon in a porous medium. The tested working fluids are pure water and butanol aqueous solutions with different concentrations. In contrast to ordinary fluids, the surface tension of self-rewetting fluids exhibits a positive gradient beyond a certain temperature value. The experimental results indicate that the use of self-rewetting fluids (water/butanol) as working fluid significantly improves the performance of the capillary evaporator by decreasing the casing temperature. To explain the heat transfer enhancement mechanism, the phase change phenomenon is visualized for the two working fluids. It is shown that as the applied power increases, the shape of the vapor pocket that developed within the porous wick also increases for pure water until it reaches a stable shape. With respect to self-rewetting fluids, the shape of the vapor pocket decreases with increasing applied power allowing more efficient mass and heat transfers. Wettability, capillary pressure and Marangoni forces are the factors related to surface tension and contact angle that seem to be responsible for this heat transfer improvement for self-rewetting fluids.

Static and Dynamic Liquid-Vapor Phase Distribution in the Capillary Evaporator of a Loop Heat Pipe

The liquid–vapor phase distribution and displacement in the capillary evaporator of a loop heat pipe (LHP) are key phenomena affecting the steady state and transient operating characteristics. This study intends to analyze the liquid-vapor interface behavior in the capillary evaporator that causes operational instability and enhances the heat-transfer while performing optical observation in the transparent cylindrical evaporator during the LHP operation. A quartz wick-acetone LHP system was designed and fabricated, which operated successfully with a maximum heat flux of 5.9 W/cm2. Phase displacement in various operations, such as the start-up involving nucleate boiling, capillary limit, hysteresis, and step-up of the heat load were observed. Binarized image quantitatively processed revealed dynamic characteristics on the contact surface between the wick and case. Comparison of the phase displacements during the start-up involving nucleate boiling and nucleate boiling after normal start-up showed that the equilibrium vapor phase on the contact surface between the evaporator case and wick is formed by both imbibition and drainage. On the step-up-down test of the heat load, a visual evidence of hysteresis of the evaporator heat-transfer coefficient due to the phase distribution in the wick was noticed. The simulation results showed that the residual liquid phase along the three phase contact line within the case, wick and grooves, observed by the visualization experiment, is of low temperature. Therefore, the distribution of the residual liquid can enhance the evaporator heat-transfer coefficient. This characteristic is the key aspect of optimizing the porous structure of the wick.

Characteristics of phase-change heat transfer in a capillary evaporator based on microscale infrared/visible observation

In this study, the characteristic of liquid–vapor interface behavior in a capillary evaporator of loop heat pipes (LHPs) is studied experimentally and theoretically. The behavior of a phase–change heat transfer at the surface of a fine porous medium that is called a wick, is evaluated using microscopic infrared camera and microscope. Four distinct samples made using four different materials (polytetrafluoroethylene and stainless steel (SS1.0 μm, SS4.5 μm, SS22 μm)) were used to simulate a part of the wick used in the evaporator. The changes in a heat transfer coefficient were caused by three types of liquid–vapor interface behaviors. In the analysis, a one-dimensional model of the liquid–vapor interface behavior in the wick is proposed based on thin liquid film evaporation theory. The predicted heat transfer coefficient was in good agreement with the experimental data.

Experimental study of the liquid/vapor phase change in a porous media of two-phase heat transfer devices

The study presents an experimental examination of heat and mass transfer with phase change in a porous media composed of copper foam. The aim of the study involves separately examining the effect of the adverse hydrostatic head, subcooling at the wick inlet and wick properties (porosity and average pore diameter) on the heat and mass transfer inside porous media. The experimental results indicate that an increase in the adverse hydrostatic head from Δh=0.5 cm to Δh=2 cm increases the casing temperature of 5 °C for low heat loads and 40 °C for high heat loads owing the vapor breakthrough toward the wick inlet. The results also suggest that the heat transfer coefficient and the critical heat flux are reduced by approximately 27% (for a heat load of 106 W) and 17%, respectively for the same adverse hydrostatic head variation. The increase in the subcooling at the wick inlet reduces the thermal performance of the evaporator. With respect to wick property effects, the results indicate that the porosity and average pore diameter significantly impact the phase change phenomenon inside the porous wick and the casing temperature. There are optimal values of porosity and average pore diameter leading to the best thermal performance of the capillary evaporator.

Effect of the wick characteristics on the thermal behaviour of a LHP capillary evaporator

The present work aims at investigating the effect of the characteristics of porous wicks integrated in a flat disk-shaped LHP evaporator on their thermal performance and operating limits. Several wicks were manufactured by sintering of copper powder following a design of experiment, and characterised in terms of permeability, porosity, pore radius and thickness. A specific test bench was designed, consisting of a LHP evaporator uncoupled from the condenser. Water and pentane were used as the working fluids. The theoretical capillary and boiling limits of the wicks in the test bench were studied and compared to the experimental data. It was observed that a high permeable wick (K > 10−13 m2) is likely to reach the boiling limit before the capillary limit because the fluid easily percolates through it. The heat transfer coefficient between the evaporator wall and the evaporating fluid is significantly higher with pentane than with water. It reaches a maximum value of 2340 W m−2. K−1 with water and 5310 W m−2. K−1 with pentane, but no clear tendency could be highlighted concerning the effect of the wick characteristics on this parameter. The maximum dissipated heat transfer rate before the wick dry-out varies in a smaller range with pentane than with water. The highest measured values are equal to 88 kW m−2 with water and 56 kW m−2 with pentane.

Investigation on refrigerant transport by capillary effect with fleeces in an evaporator for a high temperature cooling machine

The work described in this paper is part of development of a compact cooling machine for electronics in well environments with ambient temperatures above 200 °C. It includes an evaporator, a compressor and a condenser such that the cooling machine can be operated permanently. The evaporator is designed as a capillary evaporator. Instead of metal or polymer wicks, different fleece materials based on cellulose and wool have been investigated regarding capillary wetting on heat transfer surfaces.Characteristic fleece properties have been determined experimentally. The tests include effects supporting the wetting rise, such as multiple layers, compressed layers, pre-wetted fleeces and increased temperature.Results of evaporation tests show the functionality of the evaporator principle and confirm sufficient cooling effect.The presented work can serve as basis for further investigation of wetting under evaporation influence and for optimization of functionality of evaporator design using passive capillary wetting with fleece materials.

Liquid–vapor phase behavior and operating characteristics of the capillary evaporator of a loop heat pipe at start-up

The liquid–vapor phase distribution and displacement in the capillary evaporator of a loop heat pipe (LHP) are key phenomena affecting the steady-state and transient operating behavior. This study conducts optical observation of liquid–vapor phase behavior in the evaporator using a transparent glass tube heated by far-infrared radiation during start-up of the LHP. A quartz wick-acetone LHP system is designed, fabricated, and operated successfully with a heat flux of 1 W/cm2 transported by the cylindrical evaporator. To investigate effect of the liquid–vapor phase distribution in the evaporator at the initial state, start-up experiments with eight phase patterns—which are either liquid-saturated or two-phase in each of the inlet of the vapor line, grooves, and compensation chamber (CC)—are performed. The phase distribution affects the temperature profile although it has no effect upon the steady-state characteristics, and nucleate boiling occurs in the wick or groove simultaneously with evaporator-temperature drop when both the grooves and the inlet of the vapor line are completely filled with liquid. Liquid–vapor displacements from start-up to the steady-state are classified into four transient behaviors, which include boiling in the CC and meniscus oscillation at the three phase contact line within the case, wick, and groove.

Numerical Analysis of the Impact of Nanofluids and Vapor Grooves Design on the Performance of Capillary Evaporators

This paper discusses the impact of nanofluids and vapor grooves position on the performance of capillary evaporators. The phase change that occurs within the wick structure is simulated using a 2D transient mathematical model. The model combines Darcy’s law, Langmuir’s law, and energy equations to describe the heat and mass transfers inside the wick and the metallic wall. A comparison with experimental visualizations and numerical simulations is proposed. The analysis was performed for water and aluminum oxide nanoparticles with three volume concentrations of nanoparticles. Also, the effect of the vapor grooves configuration on the capillary evaporator performance is studied. The numerical results show that substituting water with nanofluids had a significant effect in reducing the evaporator temperature and the pressure drop in the whole loop. The present work has also shown that the contact between the metallic wall and the wick is an important element that must be taken into account in the design of the evaporator.