Performance Of Loop Heat Pipe Research Articles

Numerical research on geothermal energy extraction in backfilled mines by using the excellent heat transfer performance of loop heat pipe

Deep mine harbor substantial geothermal energy. Integrating a heat exchanger within the backfill body mitigates thermal hazards and facilitates the concurrent extraction of mineral and geothermal resources. Inspired by the capabilities of superior thermal conductivity, high heat transfer limit without additional energy consumption of loop heat pipe (LHP), a novel cemented paste backfill system coupled with an LHP heat exchanger (LHPHE-CPB) was developed to effectively improve the thermal conductivity of backfill body and enhance the extraction performance of geothermal energy in backfilled mines. The temperature evolutions of LHPHE-CPB system and the mechanisms of vapor-liquid phase transition and two-phase flow within LHP were numerically analyzed during the stages of heat storage and simultaneous heat storage/heat release. Orthogonal tests meticulously examined the effects of surrounding rock temperature, and the inlet temperature and flow rate of cooling water on the system's heat transfer performance. Optimal operating conditions for the system, in terms of reducing backfill body temperature, achieving favorable temperature differentials between the inlet and outlet cooling water, and enhancing the heat extraction capacity of system, were determined through range and variance analyses. This research establishes a theoretical foundation for the application of LHP in efficiently extracting geothermal energy from backfilled mines.

Study on the startup and heat transfer performance of loop heat pipe under sloshing conditions

Study on the startup and heat transfer performance of loop heat pipe under sloshing conditions

Evaluation of evaporative heat transfer performance based on observation of Thermo-Fluid behavior in porous structures fabricated by additive manufacturing

This study focuses on improving the performance of Loop Heat Pipes (LHPs) by investigating the thermo-fluid behavior within the evaporator. Previous observations using infrared and visible techniques have revealed potential improvement methods, such as modifications to vapor groove dimensions and enhancements in evaporator wettability. In this research, additive manufacturing technology is employed to manufacture both porous media and evaporator casings, with the aim of enhancing performance. The study evaluates four types of evaporator structures, including samples fabricated separately for porous media and heating plate corresponding to the evaporator casing, samples fabricated integrally, and samples with a porous monolayer loaded onto the heating plate. Micro-scale infrared observation is utilized to assess evaporative heat transfer coefficients and maximum heat flux. The experimental results demonstrate a substantial increase in maximum heat flux in structures where the porous body and heating plate are integrated into a single unit. Moreover, the investigation reveals the influence of the contact area structure between the porous media and heating plate on performance. The presence of voids in this area, allowing for improved vapor release during nucleate boiling, was found to enhance heat transfer efficiency. These findings underscore the efficacy of employing additive manufacturing technology for optimized evaporator designs, offering a pathway toward structurally enhanced and more efficient evaporators.

Effect of tilt angle and base plate design on the performance of a loop heat pipe driven by vapor–liquid injector

With the rapid development of microelectronics technology and the increasing heat flux of electronic devices, loop heat pipes have garnered significant attention for their highly efficient passive heat transfer performance. However, the heat transfer performance of conventional loop heat pipes is impaired by the heat leakage, and the maximum heat flux is unable to meet the heat dissipation requirement of high-heat-flux devices. To solve this problem, a loop heat pipe with a vapor-driven injector (LHPI) was previously proposed. However, only the heat transfer performance of the horizontally-placed LHPI had been investigated in the preliminary stage, and the effect of gravity on the heat transfer performance was not further discussed. Now in this paper, a 360° rotatable experimental bench is designed to investigate the influence of the tilt angle on the performance of LHPI. In addition, a capillary wick is integrally sintered with the trough baseplate and the microcolumn baseplate of the LHPI, respectively, to reduce the thermal resistance between them. The results show that the heat transfer performance of LHPI is significantly affected by its tilt angle. In the horizontal condition, LHPI exhibits the lowest baseplate temperature and thermal resistance, and the best heat transfer performance, which verifies its applications in engineering. Integral sintering increases the maximum thermal load from 300 W to 350 W under stable operation conditions of LHPI. The maximum thermal load of the integrated micro pin-finned baseplate reaches 500 W, which improves the heat dissipation effect by 42 % in heat dissipation compared to the trough baseplate under the same conditions., providing an improvement angle for the traditional LHP.

Performance of a novel loop heat pipe coupled with micro vapor-driven jet injector – An experimental and numerical study

Loop heat pipe (LHP) is an efficient phase-change heat transfer device which has been widely applied in cooling high-power electronics on ground and in spacecraft. In previous work, a novel LHP coupled with vapor-driven jet injector (VDJI) has been proved to be feasible and shows excellent performance, namely LHPI. However, due to the lack of pressure data during LHP operation, the coupling mechanisms of VDJI and LHP remain unknown. Especially, the heat-mass transfer between high-speed vapor and subcooled liquid inside VDJI, which has great influence on the LHPI, need to be investigated to guide the design of the LHPI. In this paper, the start-up process, steady-state operation characteristic of LHPI and internal flow field of VDJI were investigated experimentally and numerically. By using the pore-forming sintering and integrated sintering method, the bi-porous wick was prepared and embedded on the base plate surface of evaporator, which brought a significant improvement in its performance. The results indicated that the LHPI could operate under a power of 600 W (50.0 W/cm2) without dry-out. Maintaining the heating wall temperature within 85 °C, the minimum thermal resistance of the LHPI with bi-porous and integrated sintered wick was below 0.18, and the maximum heating power can reach up to 404 W, which are 60% lower and 52.5% higher than mono-porous wick, respectively. In addition, four operation modes of VDJI were classified as low-efficiency mode, normal-injection mode, restricted expansion mode and superheated mode. The operation modes of VDJI had great influence on the performance of LHPI. In normal-injection mode, the entrainment ratio of VDJI exceeded 100, which resulted in the lowest thermal resistance of LHPI. For long-term operation of LHPI, the VDJI is suggested keep operating in normal-injection mode. Moreover, by using 3D numerical simulation method, the internal flow structure of the VDJI was obtained to gives an insight into mechanisms of these four operation modes. The numerical results showed the profile of compression waves and confirmed the existence of condensation shock wave in VDJI.

Demonstration of heat switch function of loop heat pipe controlled by electrohydrodynamic conduction pump

Loop heat pipes (LHPs) are attractive devices to cool electronic devices in spacecraft. The LHP operation must be shut down in some cases to avoid considerable reductions in the temperature of such electric devices. A new active technology capable of heat switching is developed for LHPs using an electrohydrodynamic (EHD) conduction pump that operates to facilitate liquid flow in the reverse direction against the normal LHP flow. To demonstrate our proposed approach, an EHD conduction pump with rod–ring electrodes was developed and evaluated to obtain generated pressure with varying applied voltages. The LHP with R134a as a working fluid was shut down successfully when a sufficiently high voltage was applied. The LHP shutdown continued until the EHD conduction pump was turned off. A comparison with a test in which a manually controlled mechanical valve caused the capillary limit showed that the EHD pump worked differently from the valve: the shutdown by the EHD pump involved a reverse flow. The consumed electric power for the EHD pump was 10–30 mW for 6–8 kV of applied voltage during the LHP shutdown. Furthermore, pressure loss through the EHD pump during normal LHP operation was approximately 0.5 % of the capillary pressure.

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.

Review of “cold shock” cases in operation of loop heat pipes and related thermal instabilities

Loop Heat Pipes (LHP) are passive two-phase heat transfer devices, driven by capillary pumps that source energy for circulation of working fluid solely from the heat source. They have a proven track record in spacecraft thermal control as well as terrestrial applications. However, LHP may experience partial or complete wick dryout and thermal instabilities at rapid power-up steps or (and) rapid condenser cooling conditions change due to a rapid inflow of subcooled liquid from the condenser into the evaporator. Such cases do not have very wide coverage in literature and are usually presented as a little nuisance in testing of a particular LHP, for which some design workaround was found. Oftentimes suggested solutions trade performance or other design merits for robustness of operation. This article aims to review such case studies where phenomena similar to the described “cold shock” affected LHP performance. The aforementioned instabilities become even more pronounced in grid-like multievaporator LHP having multiple parallel connections between capillary pumps and compensation chambers, as heat and mass transfer between those elements becomes less definite and can eventually increase the heat leak into compensation chambers. Vulnerability of LHP with distributed network of evaporators to seemingly “safe” transient regimes fosters deeper analysis of the “cold shock” phenomenon. With comprehensive definition to the problem and a list of research questions, technical measures for getting rid of it in perspective LHP designs can be foreseen.

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.