Loop Heat Pipe Research Articles

Experimental studies on the transient characteristics and start-up behaviour of a miniature loop heat pipe

Experimental studies on the transient characteristics and start-up behaviour of a miniature loop heat pipe

The start-up characteristics of a novel loop heat pipe with stainless steel capillary wick

Loop heat pipe (LHP) is an advanced thermal management technology that utilizes liquid–vapor phase change and capillary action to achieve efficient heat transfer across long distances and in various directions. The start-up of an LHP is crucial for its reliable and stable operation. This study aims to explore the start-up characteristics and temperature variations of various components under different heating conditions by designing a cost-effective flat plate evaporator LHP with a stainless-steel casing and acetone as the working fluid. The experimental analysis examined the performance of the LHP under varying heating powers from 28 to 60 W, positions, and gravity angles. The findings revealed that the LHP successfully started at 28 W, and a temperature overshoot occured when the heating position was near the compensation chamber. Notably, the start-up time was 400 s faster compared to the heating condition near the mean pressure chamber. When the evaporator was tilted such that the compensation chamber was above the evaporator’s pressure equalization chamber, circulation within the LHP could not be established. The thermal resistance of the LHP decreased with increasing heating power, achieving a maximum reduction of 51.51 %. When both the compensation chamber and the mean pressure chamber were heated simultaneously, the impact of the gravity inclination angle on thermal resistance was minimized. This study introduces an innovative capillary wick LHP with high start-up success rates and cost-effectiveness, providing both empirical data and theoretical insights for the refinement and optimization of civilian LHP designs.

Experimental study on Start-Up and heat transfer characteristics in loop heat pipes with dual heat sources for battery thermal management system

Electric vehicles represent a breakthrough in sustainable transportation, significantly diminishing global emissions. The efficacy of lithium-ion batteries is significantly influenced by the operating temperature, with elevated temperatures potentially resulting in thermal runaway. A proficient battery thermal management system (TMS) must resolve this concern and uphold safe temperatures. Loop heat pipes (LHPs) offer passive cooling technology and have garnered the attention of researchers. Nonetheless, research on LHPs for battery thermal management systems is scarce, and prior investigations have concentrated on a singular heat source. A liquid heat pipe with a dual heat source evaporator has been engineered to simulate the battery arrangement of electric vehicles, which typically comprises multiple battery cells. This study seeks to examine the starting and heat transmission characteristics of LHP. Two battery simulators are fabricated from steel blocks equipped with cartridge heaters. A copper evaporator (121 × 56 × 20 mm) is equipped with a stainless-steel screen mesh capillary wick, and thermocouples are linked to a National Instruments data acquisition (NI DAQ) system for the LHP. Ethanol serves as the working fluid at a filling ratio of 60 %. The startup characteristics of the LHP are examined by altering the heat load, coolant flow rate, and coolant temperature. The experimental findings indicate that the LHP effectively initiates operation at heat loads ranging from 20 to 100 W. The LHP with multiple heat sources demonstrates reduced temperatures compared to a single heat source. The best performance of coolant flow rate and temperature for the quickest startup times are 1.5 L/min and 25 °C, respectively. The lowest thermal resistance achieved is 0.17 °C/W.

Innovative passive cooling of photovoltaic panel using loop heat pipe technology with passive daytime radiative cooling

This paper proposes an innovative cooling technique that utilizes a loop heat pipe (LHP) and passive daytime radiative cooling. The proposed system uses LHP to move the heat load from the photovoltaic (PV) panel surface facing the Sun to a radiator surface where heat is radiated to the sky and dissipated to the surroundings by convection. The LHP allows heat removal from PV panel surface to the radiator surface with negligible temperature difference. Based on selected PV parameters and conditions, the proposed LHP-radiator cooling system achieves a peak temperature reduction of over 10 °C, leading to a 4.6 % enhancement in PV electrical efficiency compared to the reference efficiency. The selected systems’ results show an annual energy saving of 3.4 % (around 8.7 kWh/m2) for the year of 2022.

Performance and energy consumption study of a dual-evaporator loop heat pipe for chip-level cooling

Performance and energy consumption study of a dual-evaporator loop heat pipe for chip-level cooling

Enhancing Capillary Pressure of Porous Aluminum Wicks by Controlling Bi-Porous Structure Using Different-Sized NaCl Space Holders.

Capillary pressure and permeability of porous media are important for heat transfer devices, including loop heat pipes. In general, smaller pore sizes enhance capillary pressure but decrease permeability. Introducing a bi-porous structure is promising for solving this trade-off relation. In this study, the bi-porous aluminum was fabricated by the space holder method using two different-sized NaCl particles (approximately 400 and 40 μm). The capillary pressure and permeability of the bi-porous Al were evaluated and compared with those of mono-porous Al fabricated by the space holder method. Increasing the porosity of the mono-porous Al improved the permeability but reduced the capillary pressure because of better-connected pores and increased effective pore size. The fraction of large and small pores in the bi-porous Al was successfully controlled under a constant porosity of 70%. The capillary pressure of the bi-porous Al with 40% large and 30% small pores was higher than the mono-porous Al with 70% porosity without sacrificing the permeability. However, the bi-porous Al with other fractions of large and small pores did not exhibit properties superior to the mono-porous Al. Thus, accurately controlling the fractions of large and small pores is required to enhance the capillary performance by introducing the bi-porous structure.

Experimental and numerical investigations of a diffusion bonded flat-type loop heat pipe for passive thermal management

Previous diffusion bonded loop heat pipes have been researched due to the benefits of this manufacturing technique, which includes direct sealing and maintaining material properties. The previous single-material diffusion bonded loop heat pipes were made of copper, leading to a high heat leak due to high thermal conductivity, causing a reduction in efficiency. This research used a new approach that separated the evaporator from the loop heat pipe while using diffusion bonding to manufacture a loop heat pipe using two materials, copper and stainless steel. This separation allowed for lower thermal conductivity materials to be used to help reduce the heat leak. Two evaporators with different wicks were tested, one made of copper and the other made of polytetrafluoroethylene. Both designs were tested in multiple orientations and operated for a range of 15 W to 45 W with an evaporator to condenser thermal resistance ranging from 2.18 °C/W to 0.004 °C/W. Additionally, a numerical model was developed to predict the performance of both evaporators, with an average temperature difference ranging from 5.5 °C to 6.6 °C for all thermocouple locations. The heat leak ratio was analyzed using the model, and a heat leak ratio of 7.6% to 8.0% and 13.6% to 16.0% for the copper wick and polytetrafluoroethylene wick loop heat pipe was calculated, respectively. This research demonstrates that diffusion bonding copper with a lower thermal conductivity material and using low thermal conductivity sealing material can help reduce the heat leak within diffusion bonded loop heat pipes.

Thermofluidic behavior of a 2-m nitrogen-charged cryogenic capillary pumped loop during its supercritical startup

A cryogenic capillary pumped loop (CCPL) is a highly efficient two-phase capillary-force-driven heat transport device that operates at cryogenic temperatures. CCPL satisfies the demands for space applications in cryogenic regions as it can transport heat over long distances without mechanical moving parts. In this study, the transient internal flow during the supercritical startup of CCPL was predicted, and various temperature relationships were used to determine whether CCPL starts up or not. The utilized CCPL comprised a wick (pore radius = 1.0 μm), exhibited a heat transport distance of 2 m, and was filled with nitrogen as the working fluid. The supercritical startup experiments were performed at a temperature range of 77–300 K; the startup procedure was initiated when the maximum temperature of CCPL decreased to ∼150 K. Three different liquid supply cycles were tested during the supercritical startup, and the startup time was reduced (a maximum and minimum of 4.1 and 1.9 h, respectively). CCPL started when the evaporator temperature was below the cold reservoir temperature. Thus, the temperature relationship between the cold reservoir and evaporator at the time of applying the heat load to the evaporator could be used to determine the possibility of starting CCPL. The startup was considered successful when the cold reservoir temperature was higher than the evaporator temperature, as the cold reservoir, which exhibited a two-phase state, supplied sufficient liquid to the evaporator, filling the inside of the evaporator with liquid.

Experimental investigation on the performance of a high capacity dual compensation chamber loop heat pipe under the effect of acceleration

As an efficient two-phase heat transfer technology, the application of loop heat pipe (LHP) technology in the aerospace industry has received more attention in recent years. This paper presents a performance study of a high capacity dual compensation chamber loop heat pipe (DCCLHP) under acceleration environment. The experimental study investigated accelerations up to 8 g in five directions, while the DCCLHP operated close to the heat transfer limit in the corresponding directions. The experimental results indicated that the effect of acceleration on the performance of the DCCLHP could be either favorable or unfavorable, depending on the direction of acceleration. The operating temperature of the DCCLHP varied between −6 °C and +14 °C under different acceleration conditions. The thermal resistance changes of the DCCLHP ranged from −25 % to 116 %. For the high capacity DCCLHP, the effect of acceleration on its performance was mainly realized by affecting the thickness of the condensate film inside the condenser, which in turn affected the condenser performance. The research findings presented in this paper might aid in the advancement of DCCLHP technology for aircraft applications.

Experimental study on the flat loop heat pipe with minichannel wick under low heat flux

Stable operating under low heat flux condition is crucial for loop heat pipe. To investigate the performance of the flat loop heat pipe under low heat flux, a visual structure of evaporator with longitudinal replenishment characterized by minichannel wick is proposed and fabricated. The minichannel wick increases the permeable area and reduces flow resistance to enhance the heat transfer capacity. However, under low heat flux condition, excessive liquid permeation into the vapor line and the local reverse vapor flow may deplete the liquid storage in the compensation chamber, resulting in a wick temperature increase of 4.7 °C under extreme condition. Lowering the heat sink temperature does not necessarily lead to a lower stable wick temperature in specific cases but rather poses operational challenges and further raises the stable wick temperature by 9 °C. By visualizing the flow structure, it is found that arduous vapor–liquid prestart distribution leads to prolonged circulation establishment time and higher wick temperature, resulting in a higher stable wick temperature. As the heat flux increases, the effect of prestart distribution on the stable wick temperature weakens as the redistribution capability improves.

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.

Selective laser melting manufacturing and heat transfer performance study of porous wick in loop heat pipe

Porous wicks, characterized by high capillary pumping force, lightweight, and excellent permeability, have been widely used in heat transfer. Currently, porous wicks are fabricated using powder sintering, porous foaming, and solvent volatilization techniques. However, there is the problem that the shape, size, and pore distribution of the porous structure are difficult to control. This paper proposes a novel 3D printing method to fabricate porous wicks using selective laser melting. The unit diameter and length are optimized to obtain a porous wick with high porosity and capillary pumping force. The effect of laser power on the surface morphology and porosity of the porous wick is investigated by modulating the 3D printing process parameters. The results show that the porosity of the porous wick can be regulated by changing the diameter and length of the pore unit. Decreasing the laser processing power and attaching unmelted solid-phase particles to the pore surface can further increase the porosity. When the laser power is 125 W, the unit diameter is 0.4 mm, and the unit length is 0.6 mm, the porous wick has strong capillary pumping force. The 316L porous material is modified by heat treatment to obtain super-hydrophilic properties. The porous wick is applied to the evaporator of the loop heat pipe, which significantly improves the heat transfer performance. When the heat load is 200 W, the evaporator temperature is 57.09 °C, and the thermal resistance is 0.025 °C/W.

Comparison of the thermofluidic behaviors of 2-m nitrogen-charged cryogenic loop heat pipe under anti-gravity and horizontal conditions

One of the main advantages of a cryogenic loop heat pipe (CLHP) is its heat transfer capability over long distances and operability under anti-gravity conditions. However, there are only a few studies on the thermal characteristics of long-distance CLHPs. It is essential to investigate the effect of a hydraulic head on CLHP performance to enhance the utilization of CLHPs in various applications. This study investigated the thermofluidic behaviors of a 2-m nitrogen CLHP with a capillary starter pump (CSP) under horizontal and anti-gravity conditions where the evaporator was 350 mm higher than the condenser. The novelty of the study is to reveal the heat transfer characteristics and operating mechanisms under anti-gravity conditions based on comparisons with experimental results under horizontal conditions. In the CLHP, a fine stainless-steel porous wick with a pore radius of 1.0 μm and permeability of 1.3 × 10−13 m2 was used for an evaporator and the CSP. The lengths of the vapor line, condenser, and liquid line were 2000, 1500, and 2000 mm, respectively. When a heat load of 4 W was applied to the CSP and evaporator, the CLHP successfully started with an initial cooling condition called a supercritical startup under anti-gravity conditions. The startup temperature behaviors were compared under horizontal and anti-gravity conditions. The thermal resistance of the CLHP with a stepped-up evaporator heat load and various CSP heat loads was evaluated for two CLHP orientations. The CLHP stably operated under evaporator heat loads of 4–24 W (horizontal) and 4–20 W (anti-gravity) for three CSP heat loads of 0, 2, and 4 W. The effect of the CLHP orientation on the thermal resistance with various CSP heat loads is discussed. This study enhances the applicability of the long-distance CLHP to various applications with a high degree of postural freedom by revealing the operating mechanism and thermal characteristics of the long-distance CLHP under anti-gravity and horizontal conditions.

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

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

Startup characteristics of a water loop heat pipe with dual heat sources for battery thermal management system in electric vehicle

Abstract The usage of electric vehicles has significantly reduced emissions of greenhouse gases and other pollutants. However, the high heat release generated by the electric vehicle batteries poses a challenge. To solve this problem, scientists have created a passive cooling thermal management system specifically for electric vehicle based on heat pipes, particularly loop heat pipes. A battery pack often consists of several battery modules, which results in multiple heat sources being dispersed according to their power capacity. Startup behavior of loop heat pipe has been investigated extensively in the literature. However, most of the studies use only one heat source. This paper aims to fill the research gap, particularly when the system is implemented in dual heat sources managed by only one evaporator. To achieve the research objectives, a custom loop heat pipe was constructed. This cooling system’s design is briefly described. The evaporator is made of copper, deionized water was selected as the working fluid because of its high merit number, which indicates strong performance as a heat pipe working fluid and the stainless-steel wire mesh serves as the porous wick. Battery simulator was built using aluminum material and a cartridge heater to mimic the heat produced by the battery. Two case studies were done. First, only one battery simulator was used. Second, two battery simulators were placed on both sides of the evaporator. A type-K thermocouple attached to the NI DAQ 9214 module was used to measure the temperature while the electric heat load varied between 10 W and 50 W. The study investigated the interaction between the heat load distribution and the startup behavior of the loop heat pipe. Startup behavior is crucial for the performance of the loop heat pipe. Based on the experimental results, the loop heat pipe demonstrates outstanding startup performance. It can effectively initiate operation even at a minimal heat load as low as 30 W for the first and second case study. The findings of the study indicate that the dual heat source arrangement effectively mitigates overshoot temperatures and enhances heat transfer performance by increasing the contact area.

Thermal dynamics aspect identification of loop heat pipe with capillary tube wick using nonlinear autoregressive exogenous neural network

The loop heat pipe (LHP) has the potential to be used as a passive cooling system in small modular reactors. The research objective is to study the thermal dynamics of LHP with capillary tube wick using a non-linear autoregressive exogenous (NARX) based on a neural network. The neural network identification of LHP with capillary tube wick was carried out on the MATLAB platform. The experiment data obtained is used to identify the neural network of LHP with capillary tube wick. The temperature of the water as an evaporator heat source was varied at 60, 70, 80, and 90 °C. The LHP was charged with demineralized water with a filling ratio of 100 %. The air as a coolant in condenser section was blown at velocity of 2.5 m/s. The LHP was vacuumed with an initial pressure of 2690 Pa. The result confirmed that NARX based on the neural network model can predict the temperature of the condenser section with a given input set under the steady-state and transient conditions. The coefficient of determination is higher than 0.998 and Mean Square Error (MSE) is below 0.0082. The result obtained shows that the NARX neural network model can predict thermal dynamics phenomena in LHP quickly and precisely.

Variable Switching System for Heat Protection and Dissipation of Ultra-LEO Satellites Based on LHP Coupled with TEC

Ultra-low Earth orbit (LEO) satellites are widely used in the military, remote sensing, scientific research, and other fields. The ultra-LEO satellite faces the harsh aerothermal environment, and the complex variable attitude task requires the radiator of the satellite to not only meet the heat dissipation requirements of the load but also to resist aerothermal flux. In this study, the aerothermal flux of 160–110 km was calculated, and the loop heat pipe (LHP) coupled with a thermo-electric cooler (TEC) and multi-layer insulation (MLI) were applied to ultra-LEO satellites to determine the variable switching and fast response of heat dissipation and heat protection. An aerothermal flux simulation test platform was built. After the assessment of the ultra-LEO aerothermal flux test, even when the head temperature was as high as 350 °C and the side radiator temperature was as high as 160 °C, the temperature of the internal heat source could be controlled within 22.5 °C through the efficient work of the thermal variable switch system. The study confirms the accuracy and feasibility of the system, which provides an important reference for the subsequent actual on-orbit mission.

Experimental study on flow optimization and thermal performance enhancement of an ultrathin silicon-based loop heat pipe

Excessive heat flux of integrated circuits (ICs) leads to the failure of electronic devices and the consumption of extra energy. Silicon-based loop heat pipe (sLHP) offers a simple, reliable and easily-integrate method for IC thermal management. In this study, the influence of two common wicks (micropillar arrays (MP) wick and microchannel (MC) wick) on fluid flow and performance of sLHP are systematically compared and studied through visual experiment. And two advanced wicks (in-line long rib (IR) wick and staggered long rib (SR) wick) are proposed to further optimize the sLHP. The results show that the fluctuant vapor-liquid interface in MP wick is beneficial to the supplement of working fluid from adjacent flow channel, but it also introduces the uncertainty of flow. The MC wick features the preset flow channel and stable fluid flow but limited heat transfer capacity. Notably, the IR wick and SR wick effectively enhance that capability of supplementing fluid by combining the advantages of the MP wick and MC wick showing better thermal performance. The effective thermal conductivity of SR-sLHP and IR-sLHP is better than that of MP-sLHP and MC-sLHP, with the maximum values of 860 W/(m·K) and 848 W/(m·K), respectively.

Visualization experimental study on evaporation heat transfer characteristics of an open evaporator with a tower-like porous wick

Temperature overshoot often occurs in loop heat pipe, which is caused by the large bubble in the compensation chamber. But it is still uncertain whether this large bubble grew from parasitic bubbles inside porous wick. A multi-layer tower-like porous wick is designed to understand the growth and expansion of parasitic bubbles. The parasitic bubbles are induced to occur inside porous wick by inserting a copper pillar on the evaporator wall, and the multi-layer porous wick is used to eliminate parasitic bubbles. The pumping stagnation occurs under certain heat loads, but temperature overshoot of the evaporator wall no longer occurs in the open evaporator with the tower-like porous wick, which guides the design of evaporator to avoid temperature overshoot. It is found that a lot of small bubbles only appears on the lower surface of porous wick immersed in liquid in the evaporator, while not appears on the upper surface. Three groups of repeated experiments are carried out sequentially and the performance of the evaporation heat transfer gradually deteriorates, which is speculated to be caused by a gap between porous wick and the copper pillar, larger pore size next to copper pillar, or poor wetting caused by copper pillar oxidation. Two different types of the evaporator overheating are discovered, including the overall retreat of the evaporating interface and dry-out region expanded from the internal of porous wick to the external. At high heat load, the evaporating interface expands along the copper pillar, causing an increase in the heat transfer coefficient of the evaporator. The research results indicate that in order to better utilize porous wick for evaporation heat transfer, it is necessary to reliably adhere porous wick to the heating surface and avoid the formation of the large bubble in the compensation chamber.

Novel Lightweight Loop Heat Pipe Radiator for High-Power Space Systems

High-power space systems reject a large amounts of waste heat via medium- or high-temperature radiators. For this application, the heat pipe radiator is desired for its simplicity, efficiency, and reliability. It is commonly composed of cylindrical heat pipes and planar fins. Although typically made of advanced materials with low density and high thermal conductivity, the pipe–fin combination suffers from a series of drawbacks, such as large thermal resistance in component joints, thermal expansion rate incompatibility, and high acquisition cost. This study innovates the heat pipe radiator configuration by developing an all-metal loop heat pipe (LHP) radiator. The evaporator and compensation chamber are formed in a cylindrical container with traditional sintered wick layers and a plug. The LHP applies dozens of parallel capillaries arranged in two planar arrays as condensers to spread heat to radiating panels, realizing lightweightness and efficient heat transfer. A theoretical study was performed to analyze the nonuniform liquid–vapor distribution in multicondenser lines. Experimental studies were carried out to investigate startup performance, quasi-steady-state operation, and temperature hysteresis. The proposed design is proven to be capable of fulfilling the heat rejection requirement of space systems and exhibits technical benefits over existing ones.