Performance Of Loop Heat Pipe Research Articles

Superior Heat and Mass Transfer Performance of Bionic Wick with Finger-like Pores Inspired by the Stomatal Array of Natural Leaf.

The thermal management of electronics has gained significant attention, with loop heat pipes (LHPs) emerging as an attractive solution for heat dissipation. The heat transfer performance of LHPs is influenced by the heat and mass transfer processes within the wick. However, designing the pore diameter of the wick is challenging due to the different requirements of flow resistance and capillary force. Specifically, the working fluid needs large pores to reduce resistance, while the liquid suction requires small pores to provide a large capillary force. To address this issue, we drew inspiration from the stomatal array of natural leaves used for transpiration and developed an alumina ceramic bionic wick with finger-like pores using the phase-inversion tape casting method. The finger-like pores in the wick resemble the straight hole structure of stomata, which increases the gas-liquid interface area within the wick. This design allows for timely discharge of water vapor generated by boiling, thereby reducing mass transfer resistance. Additionally, numerous micrometer-sized small pores surrounding the finger-like pores provide sufficient capillary force to replenish liquid for the gas-liquid evaporation interface. Experimental results demonstrate that the introduction of finger-like pores in the wick increases gas and water permeabilities by 2.4 and 5.2 times, respectively. Furthermore, the superior heat and mass transfer performance of the bionic wick was demonstrated with an LHP. This work effectively addresses the conflicting demands of capillary force and flow resistance, enhancing the heat transfer performance of LHPs, which holds great promise for addressing heat dissipation challenges in high power density electronic chips and has potential applications in aviation, aerospace, and microelectronics for efficient thermal management.

Investigation on the Influence of the Condenser Length and Its Coupling Effect With Elevation on the Performance of Loop Heat Pipe

Abstract The effects of condenser length and evaporator/condenser elevation on the performance of a loop heat pipe (LHP) in a gravity field were investigated, respectively, as well as their coupling effect. The refrigerant R40 was selected, and the condenser length was altered by changing the number of fins connected to the water-cooled plates. The experiments were conducted at three evaporator/condenser elevations: zero elevation, favorable elevation, and adverse elevation. The experimental results indicated that the evaporator temperatures of LHP with three different condenser lengths are very close when the LHP operates in variable conductance mode (VCM). However, an excessively long condenser will increase the LHP thermal resistance in VCM. In constant conductance mode (CCM), the longer the condenser, the lower the evaporator temperature. As the heat load increases, the LHP first exhibits gravity-driven mode and then capillarity-gravity-driven mode at a favorable elevation. The positive coupling effect on reducing the evaporator temperature by extending the condenser length and running at a favorable elevation manifests that the influence of condenser length is very small in gravity-driven mode, while in capillarity-gravity-driven mode, the effect of operating at a favorable elevation gets weakened, and the positive effects of the two factors cannot be simply calculated by the linear superposition principle. Additionally, reverse flow and flash evaporation were also observed and analyzed in depth. These results and conclusions can be used to guide the design of the condenser and the installation of LHP for ground applications.

Experimental research on the thermal performance of a flat evaporator loop heat pipe with a new composite wick

The heat leakage in evaporator is still a key challenge that impacts the heat transfer performance of the loop heat pipe (LHP). In this study, a new composite wick structure LHP, including sintered copper powder and sintered ceramic powder, was designed and fabricated to reduce the heat leakage into compensation chamber and enhance the heat transfer characteristics of LHP. The copper powder and ceramic powder were sintered as a whole by a low temperature-pressureless dry sintering technology. The heat transfer performance of water/air-cooled LHP with a new composite wick, including startup characteristics, operational characteristics and thermal resistance analysis, were investigated at the horizontal position. The experimental results indicated that the water/air-cooled LHP with a new composited wick can start up successfully when the heating power was not less than 30 W. And the maximum heating power of the new water/air-cooled LHP can reach 550 W and 500 W corresponding junction temperature of 83.42 °C and 94.22 °C, respectively. In addition, compared with the water/air-cooled LHP with a mono wick, the water/air-cooled LHP with a new composite wick has a lower startup temperature at the heating power of 20–50 W, and a lower steady-state junction temperature when the heating power was less than 500 W and 300 W, respectively.

Influence of microchannel condenser with different change rate of cross-sections along the flow field on the anti-gravity performance of loop heat pipe

The microchannel condenser has the advantages of high condensation efficiency and low flow resistance. The vapor can liquefy and return to the evaporator rapidly, and the heat transfer performance and the smooth operation of the loop heat pipe can be improved. Suitable change rate of the cross-section along the microchannel flow field can adjust the pressure drop and flow rate of working fluid, and improve the anti-gravity performance of the loop heat pipe. In this paper, microchannels with different change rates of cross-section along the flow field, including 0.5 mm straight channel, 1 mm straight channel, divergent channel, and tapered channel, are designed and applied as condensers in loop heat pipes. The entrance size of the gradient microchannel is 0.5 mm or 1 mm, which uniformly expands to 1 mm or shrinks to 0.5 mm along the flow field. The vapor and cooling water form a countercurrent direction of motion in the microchannel, and the heat exchange is carried out through the bottom of the channel. The influence of the microchannel condenser structure and operating height situation between the evaporator and condenser on the heat transfer performance of the loop heat pipe are experimentally investigated. The results show that the loop heat pipe achieves an extremely low thermal resistance of 0.004℃/W with gravity assist. Under the gravity-free operation situation, the maximum evaporator temperature of the loop heat pipe with 0.5 mm straight channel and 1000 W heat load is 128.1 °C, and the vapor pressure drop fluctuated is in the range of -1000 ∼ 2500 Pa. Under the anti-gravity operation, the loop heat pipe with the tapered channel has effective transfer performance at 500 W, and the influence factor is only 28.1 %.

Experimental study on the influence of surface modification methods of carbon fiber felt on the operation and heat transfer characteristics of loop heat pipes

Under high heat flux, the heat leakage rate of the compensation chamber in a small loop heat pipe is large, which affects the operational stability and heat transfer efficiency of the loop heat pipe. It was proposed for the first time to locally modify the capillary wick thermal conductivity through surface modification, thereby reducing the heat leakage from the compensation chamber. By spraying a high thermal conductivity metal coating on the surface of a low thermal conductivity carbon fiber felt, the contradiction between the heat dissipation rate of the heat source and the heat leakage of the compensation chamber was solved. Static wicking height showed that the metal coating increased the suction force of the carbon fiber felt. Start-up and operation experiments showed that the modified wick significantly improved the start-up stability of the loop heat pipe and reduced the temperature of the heat dissipation surface. The loop heat pipe with a single-sided-coated wick showed the lowest heat dissipation surface temperature, thermal resistance, and heat leakage rate. The loop heat pipe with an aluminum single-sided-coated wick exhibited a heat source temperature of 82.81 ℃, a thermal resistance of 0.26 K/W, and a heat transfer coefficient of 26.44 kW/(m2·K) under a load of 150 W (37.5 W/cm2). This study has significantly contributed to the future application of loop heat pipes in high heat flux electronic cooling.

Experimental Study and Visual Observation of a Loop Heat Pipe with a Flat Disk-Shaped Evaporator under Various Orientations

In this study, visualization treatment was applied to the flat disk-shaped evaporator of a loop heat pipe. By observing the liquid/vapor behavior inside the evaporator and compensation chamber, the effects of orientation on the performances during startup and during a step-increase in heat load were investigated. With water as the working fluid, the loop heat pipe was tested under three typical orientations of φ = −90°, φ = 0°, and φ = +90°. The startup time was the shortest for the φ = −90° orientation but there could be a slight temperature overshoot, resulting in an unsmoothed startup process. The startup speeds under the φ = 0° and φ = +90° orientations were similar, both without any significant temperature overshoot. The orientation could significantly change the heat leak and, therefore, the operating temperature and the heat-transfer limit. For the φ = +90° and φ = −90° orientations, the heat-transfer limits were about 71% and 157% of the value at the φ = 0° orientation, respectively. Based on visual observations, for the loop heat pipe operating in gravity-driven mode, there could be two different paths for the working fluid to return to the evaporator, namely, along the vapor line for low heat loads and along the condenser and liquid lines for relatively large heat loads, respectively.

Effect of PTFE wick thickness on Loop heat pipe performance

Heat leakage has long been a problem in the development of Loop heat pipe (LHP). Among the strategies to suppress heat leakage, the use of low thermal conductivity polymer materials and adjustment of wick thickness are considered as one of the effective methods. However, although there are many recent reports on the effect of metal wick thickness on performance, there are few studies on the effect of polymer wick thickness such as PTFE wick. Therefore, this paper hopes to integrate the two factors and present the best results. This paper investigates the effect of PTFE wick thickness (1 to 2.75 mm) on LHP performance and proposes the thickness effects on both thermal resistance and flow drag. In addition, since PTFE is hydrophobic, water, with its high surface tension, cannot be used as working fluid. Therefore, in this study, a tiny amount of Butanol (6 wt%) is added to water to create self-rewetting fluid to be used as working fluid, overcoming the flow drag of PTFE wick and allowing the system to operate smoothly.Regarding the wick’s porosity and permeability, the best capillary effect and flow drag occurs at wick thickness of 2 mm. The LHP performance result shows, compared to using PTFE wick with water as working fluid where the system was unable to start up, the system using self-rewetting fluid can smoothly operate, overcome heat leakage and enhance performance. The best performance occurs at wick thickness of 2 mm, the maximum heat load reaches 600 W and heat flux 30 W/cm2; the results are all consistent with the performance predictions based on the wick parameters.

Experimental thermal characterization of the in-wheel electric motor with loop heat pipe thermal management system

Current trends in technology toward the elimination of fossil fuels are increasing the value of electric vehicles (EVs). In turn, one of the main components of EVs is an electric motor (EM). The lifetime, reliability and output power of the EMs depend on the performance of their cooling systems. The cooling systems that are based on two-phase technologies of heat transfer, such as loop heat pipes (LHPs), have important advantages over traditional air- and liquid systems. On the other hand, LHP EM thermal control application is slowed down by the complication of their manufacturing and high production expenses. However, in recent years, new technologies are constantly being developed that can reduce the customization of LHP and, therefore, the cost of its production. In this paper the technology based on modular standardized approach for LHP design and manufacturing is applied to demonstrate the cooling performance of LHP in an in-wheel motor of a personal mobility vehicle (electric bike, wheelchair etc.) and compare it to the standard cooling solution. For this purpose, a double-capillary pump unit evaporator LHP was produced and coupled with an EM. EM thermal characteristics were studied and compared to the values obtained prior to LHP installation. Studies have shown that the use of two-phase cooling can increase motor performance up to 2.5 times.

Analysis of the thermal resistance of a loop heat pipe based on the P-T diagram of the working fluid operating cycle.

A thermodynamic analysis of the operation of a loop heat pipe (LHP) as a two-phase heat transfer device with the circulation of a working fluid in a closed loop due to capillary forces is presented. The conditions for the LHP operation as part of the “heat source - LHP - heat sink” system are formulated. Based on the P-T diagram of the working fluid operating cycle, a graphical interpretation and analysis of the thermal resistance of the LHP and its main components as well as the thermal resistance of the “heat source - LHP - heat sink” system are given. The effect of vapor overheating on the evaporator thermal resistance is demonstrated.

Numerical and experimental study on the vapor-liquid distribution of loop heat pipe with varying density of working fluid

Loop heat pipes (LHPs) are effective passive heat transfer devices that are used in the thermal management systems for satellite payloads and electronic devices. The vapor–liquid distribution in the LHP determines the LHP's operating characteristics and heat transfer mechanism. This work developed a two-dimensional transient model for an LHP based on CFD methodology. The model includes a capillary force model and a phase change model, and considers the change in vapor and liquid densities during LHP operation. The steady-state vapor–liquid distribution in the condenser under different heat loads can be simulated. To validate the calculation results, a visualization experiment was conducted on the condenser. The simulation and the experimental results agree well. It was discovered that when the LHP works in steady-state conditions, the increase in vapor density and decrease in liquid density at high heat loads is a major cause for the the growth of liquid volume fraction in the condenser. When the condenser is entirely filled with liquid, the LHP fails to operate. The simulation model can be employed for theoretical research and design optimization of LHP.

Numerical Investigation with Experimental Validation of Heat and Mass Transfer during Evaporation in the Porous Wick within a Loop Heat Pipe

The heat transfer performance of the evaporator significantly affects the heat transfer capacity of the loop heat pipe (LHP). The vapor blanket can be formed once the vapor penetrates the wick especially at high heat flux, resulting in an unsaturated state of the wick and deteriorating the evaporator performance. It is crucial to understand the liquid–vapor behavior for enhancing the LHP performance by investigating the fundamental heat and mass transfer in the wick with phase-change. However, previous modeling studies only considered a single-phase flow or complete saturation in the wick, and the capillary effect on the fluid states was rarely taken into account. The present work developed two mathematical models based on the assumptions of saturated and unsaturated wicks. The fluid states were analyzed at the liquid–vapor interface under the consideration of the capillary effect, and a pore-scale evaporation model was applied to study the phase change behavior and interfacial heat and mass transfer. The relative permeability was introduced to describe the two-phase flow in the porous wick, and the capillary force was modeled as a function of the local saturation in the two-phase region. The temperature results calculated by the models were compared with the experimental results, and the assumption that the vapor penetration leads to deterioration of evaporator performance at high heat flux was validated. Vapor blanket thickness can be estimated through the saturation profile, which provides a simple and effective method. It was also found that the capillary number ω was the key factor affecting the thickness of the vapor blanket. The greater the ω, the faster the vapor blanket thickness increases.

Experimental study on the heat transfer performance of loop heat pipe with different particle morphology and wettability of porous wick

Particle morphology and wettability of porous wick affect the movement and distribution of the working fluid in wicks, which further affects the phase change heat transfer. So the heat transfer performance will be significantly different for loop heat pipe (LHP) with different porous wicks. To study the effects of particle morphology and wettability of porous wick on the heat transfer performance of LHP from the perspective of phase separation, four types of porous wicks were fabricated and applied in LHP. Porous wicks were sintered with spherical and dendritic particle copper powder, respectively. Superhydrophilic wicks can be obtained by H2O2 oxidation. The results showed that capillary performance of dendritic particle wick was better than that of spherical particle wick. At t = 470 s, the climbing height of distilled water in the hydrophilic dendritic and spherical particle wicks were 19.8 and 18.1 cm, respectively. Compared with hydrophilic wick, superhydrophilic wick has a maximum 2.0 cm enhancement in climbing height at present experiments. The particle morphology of porous wicks had great effects on the heat transfer performance of LHPs. For LHPs with spherical or dendritic particle wick before H2O2 oxidation, their operating temperatures were 73.5 and 68.9 °C under θ = 90°, Q = 220 W situation. The latter has a lower operating temperature. For LHP with superhydrophilic dendritic particle wick, it had the best heat transfer performance among four kinds of LHP. The minimum thermal resistance can reach 0.07 K/W at Q = 300 W. Two diffusion modes of liquid in porous wicks were found in the visualization experiment, the distribution of the working fluid in dendritic particle wick was more uniform than that of spherical particle wick, which can explain well that the temperature of evaporator bottom surface is more uniform for LHP with dendritic particle wick. These results have a good guiding role for the preparation of high-performance LHP.

Parametric Study on Ammonia-Based Loop Heat Pipe

The loop heat pipe (LHP) is a two-phase heat transfer device used for thermal management in applications like space, mobile, laptop, electronic cooling. In the present study, modeling and parametric analysis of LHP was done by applying energy balance approach at different sections using Dev-C++ software. The effects of various parameters like sink temperature (5 °C, 10 °C, 22 °C), condenser effectiveness (0.4–0.9), vapor line diameter (0.0043 m, 0.0053 m, 0.0063 m), wick thickness (0.0073 m, 0.0048 m) and wick porosity (40%, 60%, 80%) were studied on the performance of LHP. The results obtained were validated with the results published in the literature. With increase in condenser effectiveness from 0.40 to 0.90, steady state operating temperature got decreased by 4.5 °C at a heat load of 600 W. The studies on wick porosity revealed that it has significant effect on the heat leak from evaporator to compensation chamber and the lower porosity leads to higher heat leak. From the analysis, an optimum performance of LHP was observed with 60% wick porosity, 0.0073 m wick thickness, 5 °C sink temperature, vapor line diameter of 0.0053 m and condenser effectiveness as 0.9 among the various parameters under investigations.

Novel hybrid structures to improve performance of miniature flat evaporator loop heat pipes for electronics cooling

• Two novel hybrid structures have been proposed to improve performance of miniature LHPs. • A new and simple fabrication process has been proposed for fabrication of hybrid structures. • Hybrid structures have high bond strength, over 100 kPa. • The maximum heat flux is around 7 times higher than that of other polymer heat pipes. Two miniature loop heat pipes (LHPs) with novel hybrid structures of evaporators are developed to cool electronic devices. These structures ensure good heat conduction between evaporators and heat sources and low heat leak from evaporators to compensation chambers through these cases. LHP 1 uses a polymer-glass hybrid structure while LHP 2 uses a polymer-stainless steel hybrid structure. Hydrophilic polytetrafluoroethylene membranes are used as wicks and the working fluid is pure water. The bond strength of these hybrid structures was evaluated quantitatively. Then the effect of hybrid structures on the LHP thermal performance was investigated by measuring temperature at each point. The maximum heat transfer capacity reached 9 W (4.5 W / c m 2 ) and 14 W (7.0 W / c m 2 ) while the thermal resistances were 4.8 K / W and 2.1 K / W for LHP 1 and LHP 2 respectively. Using the polymer-stainless steel structure, the maximum heat flux is around 7 times higher while the thermal resistance is smaller than the corresponding values of other polymer heat pipes.

Visual studies of the operation of loop heat pipes by neutron radiography

• Visual studies of two loop heat pipes were conducted. • Visual studies were carried out by neutron radiography. • The loop heat pipes were made of stainless steel with a titanium wick. • Ammonia and methanol were used as working fluids. Neutron radiography was used to visually study the operation of two compact loop heat pipes (LHPs) having a similar configuration, made of stainless steel and equipped with a titanium wick. One was filled with ammonia and the other with methanol. The experimental setup made it possible to obtain a visual picture of the distribution and movement of a working fluid simultaneously in all the structural components of the LHPs. Unique data was obtained on the operation of the devices at three different orientations in the range of heat loads from 10 to 110 W, which allow to understand the features of their operation in more detail. Visual observations were supplemented by the results of thermal tests, which were presented in the form of heat load dependences of the temperature at the heating (thermal contact) surface of a heat source.

Experimental study on long-distance anti-gravity loop heat pipe with submicron-scale porous structure

• An anti-gravity type loop heat pipe (LHP) which can transport heat for a few meters against gravity was proposed. • To generate a capillary force of several hundred kPa, a submicron-scale porous material made of glass was fabricated as a primary wick. • To reduce the heat leak, hydrophilic polytetrafluoroethylene wick was installed at the rear side of the primary wick. • The LHP with the vapor and liquid heat transport length of 4 m was fabricated, and the LHP operation under 4 m anti-gravity condition was accomplished. Although a loop heat pipe (LHP) is capable of transporting heat over a long distance under anti-gravity conditions in principle, it has not been demonstrated yet. This paper reports an experimental study of an anti-gravity type LHP that can transport heat for a few meters against gravity. For a LHP to generate a capillary force of several hundred kPa, a submicron-scale porous material made of glass was selected as the primary wick. The wick properties such as pore radius, permeability, and wettability with the working fluid were evaluated. A flat evaporator-type LHP with a few meters of vapor and liquid transport lines was designed, fabricated, and tested. Firstly, a preliminary experiment was conducted with water as a working fluid under horizontal conditions. The length of the vapor and liquid lines were both 10 m. It was found that there was a large heat leak from the evaporator to the compensation chamber. In addition, the vapor did not reach the condenser because the vapor was condensed in the vapor line. Secondly, experiments were conducted under the horizontal (0 m), 2 m, and 4 m anti-gravity conditions. In order to reduce the heat leak, a hydrophilic polytetrafluoroethylene wick was installed at the rear side of the primary wick. The vapor and liquid transport length were 6.5 m, and the ethanol was selected as a working fluid. These experimental results demonstrated that the LHP could transport heat under horizontal and 2 m anti-gravity conditions, whereas the LHP cannot transport heat under 4 m anti-gravity conditions. Finally, a LHP with a vapor and liquid transport line length of 4 m was fabricated, which was able to operate under 4 m anti-gravity conditions.

Current Trends in Wick Structure Construction in Loop Heat Pipes Applications: A Review.

Thermal control systems have been introduced as an important part of electronic devices, enabling thermal management of their electronic components. Loop heat pipe (LHP) is a passive two-phase heat transfer device with significant potential for numerous applications, such as aerospace applications, high-power LEDs, and solar central receivers. Its advantages are high heat transfer capability, low thermal resistance, long-distance heat transfer, and compact structure. The essential role of wick structures on the performance of LHPs has already been highlighted, but no comprehensive review is available that deals with different parameters such as LHP design and wick size, which are largely decisive and effective in achieving a practical level of thermal transmission governed by wick structures. To rely on this necessity, this article summarizes, analyzes, and classifies advancements in the design and fabrication of wick structures. The main conclusion to be drawn after careful monitoring and weighing of the related literature is that LHPs with composites and additively manufactured wicks show a higher heat transfer coefficient than other conventional structures. Indeed, future works should be focused on the design of more structurally efficient wicks, which may allow us to optimize materials and geometrical parameters of wick structure for higher heat transfer through LHPs.

Experimental investigation on the start-up performance of a novel flat loop heat pipe with dual evaporators

Thermal management for multiple heat sources has been becoming increasingly important, especially under high heat flux conditions. As a passive heat transfer device, loop heat pipes (LHP) with multiple evaporators does not consume extra energy, demonstrating a great potential in thermal management. This paper proposed a flat plate LHP with dual evaporators for the first time. Two flat plate evaporators were adopted, and both of them had their own porous wick, compensation chamber (CC) and vapor line. The condensed liquid converged, then moved along the main liquid line. The CC of two evaporators was joined by a tube, which can ensure the sharing of the returning liquid. To verify the performance of novel flat plate LHP, the start-up test was investigated under different heat load arrangements, involving only a single evaporate was applied heat load, two evaporators were applied with the equal or unequal heat load. Meanwhile, the operation stability was determined by testing the performance with the successionally variable heat load. This system has been shown to operate successfully at heating source temperature below 100 °C with heat load ranging from 10 W–10 W to 130 W–130 W (corresponding to the maximum heat flux of 13.52 W/cm2).

Experimental study on temperature uniformity and heat transfer performance of nitrogen loop heat pipe with large area and multi-heat source

• A large area loop heat pipe with multiple heat sources is designed. • The temperature uniformity and heat transfer performance are tested. • The heat pipe starts up and operates smoothly in anti-gravity. With the development of space technology, the requirements of thermal control on spacecraft are increasing. In space missions, due to the improvement of the detection accuracy, the diameter of deep space telescope is becoming larger and larger. These detection instruments need efficient and reliable thermal management. Thus, the application of the large-scale cryogenic cooling technology, which can provide heat dissipation for the continuous and long life of the detector, has become a difficult problem. In the present study, a large-area cryogenic loop heat pipe is designed for the optical telescope. It is a good combination of multi heat source and large-area cryogenic loop heat pipe. The main evaporator is cylindrical due to fit the lens barrel. The heat exchange area can reach 1 m 2 , and the average temperature performance is less than 5 K. The condenser adopts the form of parallel pipeline to further reduce the system resistance and ensure the smooth operation of loop heat pipe. Its operation stability and temperature uniformity of the main evaporator under different total loads are experimentally studied and analyzed. It is proved that the loop heat pipe is feasible for large area multi-heat source cryogenic loop heat pipe in engineering. It can also start up and run normally under anti-gravity conditions. The experimental results show that the large area multi-source heat collection cryogenic loop heat pipe can be started and run normally in the temperature range of 80–120 K.

Heat transfer performance of loop heat pipe for space vehicle thermal control under bypass line operation

This study analyzed the experimental results regarding the heat transfer performance of a loop heat pipe (LHP) with a vapor bypass line and a polypropylene fiber capillary wick. The LHP with the bypass tube between the vapor channel and liquid chamber was fabricated, tested, and analyzed. A control valve was attached to the inlet of the bypass line to manually switch between the bypass line operating mode and the normal operating mode. The polypropylene capillary wick generated the capillary pressure to operate the LHP. The container and tubing system of the LHP were of stainless steel, and methanol was used as the working fluid. The planar dimensions of the LHP with a flat evaporator were 40 mm × 50 mm. The study investigated the effect of the auxiliary loop on the LHP. The effects of the bypass line were investigated in terms of the minimum start-up thermal load, start-up thermal performance, and steady-state performance. The minimum start-up thermal load of the LHP was reduced by up to 63% under the bypass line operation compared with the normal operation. The temperature overshoot in the evaporator outer wall was eliminated during the start-up under the bypass line operation. Furthermore, the steady-state heat transfer performance was favorably enhanced at high thermal loads under the bypass line operation.