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Propellant refueling is important for long-term operation of the optical module in orbit. However, its thermal environment is harsher than on previous missions. Therefore, thermal control of the entire process is required. In order to solve the complex heat transfer of propellant refueling process, an integrated thermal model is established, which includes all the equipment such as the compressor, liquid cooler and loop heat pipe. The thermal analysis simulation and system-level thermal tests is presented. Firstly, the heat transfer relationship and temperature variation are analyzed by comparing the results of transient thermal analysis and thermal test in high and low temperature conditions. Then, a variable thermal conductivity simulation method is studied for the transient process from inoperative to operative of the vertical heat pipe due to gravitational factors in the thermal test. Finally, an optimized design scheme for high temperature refueling is proposed and pre-demonstrated in orbit. The results indicate that the transient simulation results are in a good agreement with the test results under the high and low temperatures, which verifies the accuracy and validity of the analysis method and simulation model. When preheating the compressor and starting two sets of loop heat pipes during on-orbit refueling, the maximum temperature of compressor is below 34.1°C, and the total power consumption of preheating is 50 Wh, which meet the design requirement. The investigation provides an important reference for designing the propellant refueling process in the docking of the optical module to the China Space Station (CSS).

Thermal management technology based on loop heat pipes (LHPs) has broad application prospects in heat transfer control for aerospace and new energy vehicles. LHPs offer excellent heat transfer performance, reliability, and flexibility, making them suitable for high-heat flux density, high-power heat dissipation, and complex thermal management scenarios. However, due to limitations in heat source temperature and heat transfer power range, LHP-based thermal management systems still face challenges, especially in thermohydraulic modeling, component design, and optimization. Steady-state models improve computational efficiency and accuracy, while transient models capture dynamic behavior under various conditions, aiding performance evaluation during start-up and non-steady-state scenarios. Designs for single/multi-evaporators, compensation chambers, and wick materials are also reviewed. Single-evaporator designs offer compact and efficient start-up, while multi-evaporator designs handle complex thermal environments with multiple heat sources. Innovations in wick materials, such as porous metals, composites, and 3D printing, enhance capillary driving force and heat transfer performance. A comprehensive summary of working fluid selection criteria is conducted, and the effects of selecting organic, inorganic, and nanofluid working fluids on the performance of LHPs are evaluated. The selection process should consider thermodynamic properties, safety, and environmental friendliness to ensure optimal performance. Additionally, the mechanism and optimization methods of the start-up behavior, temperature oscillation, and non-condensable gas on the operating characteristics of LHPs were summarized. Optimizing vapor/liquid distribution, heat load, and sink temperature enhances start-up efficiency and minimizes temperature overshoot. Improved capillary structures and working fluids reduce temperature oscillations. Addressing non-condensable gases with materials like titanium and thermoelectric coolers ensures long-term stability and reliability. This review comprehensively discusses the development trends and prospects of LHP technology, aiming to guide the design and optimization of LHP. [ABSTRACT FROM AUTHOR]

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.

Using loop heat pipes (LHPs) for thermal management of aerospace and ground electronic devices has become an efficient and attractive method in recent years. A 3-D CFD numerical model was proposed for calculating the thermo-hydraulic behavior in a LHP with a square evaporator and the accuracy was verified by experimental results. The evaporation and condensation were addressed by volume of fluid method and a momentum source term derived from flow resistance analysis was incorporated into wick as the capillary force. The simulation results aligned with experimental trends, and errors of heating surface temperature and system thermal resistance were small. Due to structural constraint and gravity effect, two eddies were formed in compensation chamber, causing uneven distribution of temperature on heating surface. Parametric analysis indicated that higher porosity deteriorated the heat transfer from heating surface to wick and enlarged heating surface temperature and system thermal resistance. Higher wick and shell thermal conductivities ensured more heat conduction to the vapor-liquid interface and enhanced the evaporation intensity. However, the higher the thermal conductivity, the lower the improvement that could be obtained. Additionally, lower heat sink temperature and higher condenser heat transfer coefficient both intensified the LHP working performance by elevating the condensation efficiency.

In the design and optimization of ambient and cryogenic loop heat pipes (ALHPs and CLHPs), the cost for optimizing every parameter may be very high. Additionally, in cryogenic spacecraft applications, the resources allocated to the thermal control system are very limited. Therefore, it is crucial to identify and optimize the parameters that have a greater impact on the overall performance. In this paper, a thermal resistance network model for LHP was established. The model was validated experimentally using an ethylene-CLHP and by using experimental data of an ammonia-ALHP and a nitrogen-CLHP reported in the literature. The model was then used to comprehensively investigate the impact of boundary and inherent parameters on the operational performance of nitrogen-CLHPs in the 70–110 K temperature range, ethylene-CLHPs in the 150–250 K temperature range, and ammonia-ALHPs in the 250–400 K temperature range. Standardized coefficients were obtained by multiple linear regression to compare the relative importance of the influence of each parameter on the evaporator temperature, serving as an optimization criterion for identifying the most significant parameters for LHP optimization with different working fluids. It was found that most parameters affect the operating performance in the same manner for ALHPs and CLHPs, while the length of wick and compensation chamber had opposite effects on ALHPs and CLHPs. Finally, modified strategies for parameter optimization and design of ALHP and CLHP were proposed and an optimization case for an ethylene-CLHP was presented. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Dalian University of Technology / Dalian Ligong Daxue Xuebao is the property of Journal of Dalian University of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The aircraft surface icing changes the flight dynamics characteristics of the aircraft and seriously threatens flight safety. Therefore, aircraft anti-icing technology is of great significance for safe flight. The basic principle, type, and influence of aircraft icing methods were analyzed and the existing anti-icing technology methods were summarized and compared. At the same time, the research status of superhydrophobic materials was comprehensively reviewed, and its specific conditions ( superhydrophobic property failure) were analyzed, especially the existing problems and challenges. Secondly, based on the defects of a single anti-icing system, a composite anti-icing system was proposed, and several composite anti-icing systems were introduced and compared with a single antiicing system. In addition, aiming at the possible problems in the application of superhydrophobic materials, an aircraft anti-icing system combining loop heat pipe and superhydrophobic materials was proposed. Finally, the problems of superhydrophobic coating surfaces and composite anti-icing and deicing systems were analyzed and prospected. It can be seen that the hybrid anti-icing and deicing system has good application potential. [ABSTRACT FROM AUTHOR]

This paper discusses the transient model of loop heat pipes in detail. A new model was constructed using an implicit method for fluid calculations and an explicit method for heat transfer calculations between pipe walls and radiators. In addition, the effectiveness of this model was verified using a loop heat pipe used for demonstration purposes on International Space Station. Good agreement was shown with the results of thermal vacuum tests. On the other hand, a comparison with the on-orbit test results revealed differences in vibration phenomena. It was concluded that this was because the heat input in orbit, such as solar heat input, was not accurately estimated. [ABSTRACT FROM AUTHOR]

Loop Heat Pipes (LHPs) are efficient heat transfer devices, providing a highly efficient and energy-saving thermal control integrated management method for Unmanned Aerial Vehicles (UAVs) with their flexible structural design. In this paper, a conceptual design of LHP anti-icing system was proposed, using LHP to transfer waste heat to wing's leading edge. The outer surface temperature of wing skin is one of the main indicators to measure the anti-icing effect. As a result, this paper mainly focused on the heat transfer process of the condenser section by experimental and numerical simulation methods. A stainless steel-nickel LHP was fabricated and tested at different conditions. Volume of Fluid (VOF) method and Lee model were adopted to simulate the condensation process. Results showed that increasing the heating power (from 35 W to 60 W) slowed down the condensation and lengthened the two-phase zone. However, when the angle of attack changes within the range of 0°-10°, the liquid-vapor distribution in the tube remains almost unchanged. The average error of surface temperature between experiment and simulation results is 3.6% and 2.5% for the heating power of 180 W and 60 W respectively. Additionally, because the volume of water droplets collected is the largest at the leading edge of the wing, we recommend arranging the LHP condenser tube inlet near the leading edge of the wing and increasing the density of pipe arrangement to achieve better antiicing effect. [ABSTRACT FROM AUTHOR]

The present study aims to evaluate the feasibility of a novel dual-evaporator loop heat pipe (DE-LHP) in battery thermal management systems (BTMS). A 3S4P (3-series and 4-parallel) Li-ion battery module with a 12.6 V and 10 Ah capacity is made and tested under various C rates for different environmental conditions. The battery generates an average heat of 11.28 W@1C and 24.44 W@1.5C with 100% discharge and 43.36 W@2C with 50% discharge at an ambient temperature of 30 °C. An increase in temperature from 30 to 40 °C results in a decline in the depth of discharge. The proposed DE-LHP is evaluated using deionized water with filling volumes of 12, 16, 20, and 24 mL. The DE-LHP started working above 5 W regardless of the filling volume, and before the evaporator reached dry-out, the DE-LHP transferred heat loads of 10, 30, 35, and 55 W with filling volumes of 12, 16, 20, and 24 mL, respectively. At a heat load of 30 W, the evaporator section of the 20-mL filled LHP demonstrated a minimum thermal resistance of 0.274 KW-1, while the condenser section exhibited a minimum thermal resistance of 0.372 KW-1 at a heat load of 35 W. The DE-LHP is found to be effective in transferring heat of 35 W to keep the module temperature below 60 °C. Based on these initial findings, it is suggested that incorporating the multi-evaporator LHP-based BTMS could be a feasible and efficient solution for the thermal management of battery modules/packs for electric vehicles. [ABSTRACT FROM AUTHOR]

Heat switches are devices for controlling heat flow in various applications, such as electronic devices, cryogenic cooling systems, spacecraft, and rockets. These devices require non-linear transient thermal simulations, in which there is a lack of information. In this study, we introduce an innovative 1D thermo-hydraulic lumped parameter model to simulate loop heat pipes as heat switches by regulating the temperature difference between the evaporator and the compensation chamber. The developed thermo-hydraulic model uses the continuity, energy, and momentum equations to represent the behaviour of loop heat pipes as heat switches. The model also highlights the importance of some thermal conductance parameters and correction coefficients for accurately simulating the different operational states of a loop heat pipe. The simulations are conducted using the proposed 1D model, solved through the application of the Mathcad block function. The numerical model presented is successfully validated by comparing the temperatures of the evaporator and condenser inlet nodes with those of a referenced loop heat pipe from the literature. In conclusion, in this research, the mathematical modelling of loop heat pipes as heat switches is presented. This is achieved by incorporating correction coefficients with Boolean logic that results in non-linear transient simulations. The presented 1D thermo-hydraulic lumped parameter model serves as a valuable tool for thermal system design, particularly for systems with non-linear operational modes like sorption compressors. The graphical and nodal representation of this proposed 1D thermo-hydraulic model further enhances its utility in understanding and optimising loop heat pipes as heat switches across various thermal management scenarios. [ABSTRACT FROM AUTHOR]

The ever-increasing power density of electronic devices fosters development of cooling systems. Among them, two-phase heat transfer devices in general and Loop Heat Pipes (LHP) in particular offer advantages by being completely passive while being able to cope with high heat fluxes. However, LHP manufacturing and especially integration is complicated by the loop architecture, which is not found in conventional Heat Pipes (HP). In an attempt to fit LHP layout into HP-like housing, a novel Heat Loop Pipe (HLP) device was designed, manufactured and tested in application to cooling a portable high-power (80 W) LED luminaire with a focusing reflector. The HLP utilizes co-axial layout of vapor and liquid lines and an additional wick in the condenser. The butane-charged, 350 mm-long HLP showed unexpectedly high resistance in the adverse orientation (evaporator above the condenser), which was an order of magnitude higher than in reflux orientation (0.8 K/W vs. 0.03 K/W). A possible explanation of this behavior in terms of vapor superheating in the evaporator channels is given. Analysis of the test results with help of p-T diagram justifies this assumption. Finding the root cause for such behavior requires further study.

Thermal management technology based on loop heat pipes (LHPs) has broad application prospects in heat transfer control for aerospace and new energy vehicles. LHPs offer excellent heat transfer performance, reliability, and flexibility, making them suitable for high-heat flux density, high-power heat dissipation, and complex thermal management scenarios. However, due to limitations in heat source temperature and heat transfer power range, LHP-based thermal management systems still face challenges, especially in thermohydraulic modeling, component design, and optimization. Steady-state models improve computational efficiency and accuracy, while transient models capture dynamic behavior under various conditions, aiding performance evaluation during start-up and non-steady-state scenarios. Designs for single/multi-evaporators, compensation chambers, and wick materials are also reviewed. Single-evaporator designs offer compact and efficient start-up, while multi-evaporator designs handle complex thermal environments with multiple heat sources. Innovations in wick materials, such as porous metals, composites, and 3D printing, enhance capillary driving force and heat transfer performance. A comprehensive summary of working fluid selection criteria is conducted, and the effects of selecting organic, inorganic, and nanofluid working fluids on the performance of LHPs are evaluated. The selection process should consider thermodynamic properties, safety, and environmental friendliness to ensure optimal performance. Additionally, the mechanism and optimization methods of the start-up behavior, temperature oscillation, and non-condensable gas on the operating characteristics of LHPs were summarized. Optimizing vapor/liquid distribution, heat load, and sink temperature enhances start-up efficiency and minimizes temperature overshoot. Improved capillary structures and working fluids reduce temperature oscillations. Addressing non-condensable gases with materials like titanium and thermoelectric coolers ensures long-term stability and reliability. This review comprehensively discusses the development trends and prospects of LHP technology, aiming to guide the design and optimization of LHP.

Loop Heat Pipes (LHPs) are efficient heat transfer devices, providing a highly efficient and energy-saving thermal control integrated management method for Unmanned Aerial Vehicles (UAVs) with their flexible structural design. In this paper, a conceptual design of LHP anti-icing system was proposed, using LHP to transfer waste heat to wing’s leading edge. The outer surface temperature of wing skin is one of the main indicators to measure the anti-icing effect. As a result, this paper mainly focused on the heat transfer process of the condenser section by experimental and numerical simulation methods. A stainless steel-nickel LHP was fabricated and tested at different conditions. Volume of Fluid (VOF) method and Lee model were adopted to simulate the condensation process. Results showed that increasing the heating power (from 35 W to 60 W) slowed down the condensation and lengthened the two-phase zone. However, when the angle of attack changes within the range of 0°–10°, the liquid-vapor distribution in the tube remains almost unchanged. The average error of surface temperature between experiment and simulation results is 3.6% and 2.5% for the heating power of 180 W and 60 W respectively. Additionally, because the volume of water droplets collected is the largest at the leading edge of the wing, we recommend arranging the LHP condenser tube inlet near the leading edge of the wing and increasing the density of pipe arrangement to achieve better anti-icing effect.

The challenge of dissipating heat from light-emitting diode (LED) poses a limitation on LED's application. Loop heat pipe (LHP) is an efficient heat transfer device that is used in the thermal management of electronic components. Therefore, in this article, an LED with an LHP as the heat dissipation solution was designed and investigated for the replacement of halogen street lights without changing other accessories of the existing street lights. The prototypes with sintered nickel wick and polyethersulfone (PES) wick were studied at the tilt angle from 0° to 12° and the ambient temperatures from 16 °C to 50 °C. In addition, five prototypes were tested for 50 days to verify the stability of the PES wick. The solution could solute the heat dissipation problem of the LED in a limited space and inclination. From the results, the PES wick showed a better performance. When the power was 110 W and the angle was 12°, the temperature of the light source could be below 62 °C and the thermal resistance could be below 0.37 °C/W. The results demonstrate that the ability of the prototype with PES wick can well solute the heat dispassion problem of the LED.

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. [ABSTRACT FROM AUTHOR]

Loop Heat Pipe (LHP) is an efficient two-phase heat transfer device, which can be used in waste heat recovery, electronics cooling, aerospace and other fields. The wick, the core component of LHP, plays an important role in its start-up and operation. In this paper, the wick fabricated by 3D printing technology had uniform and interconnected pores. In the experiment, the position of the parallel vapor removal grooveswas always fixed towards the vapor outlet. When the cylindrical wick was placed in the evaporator, the rotation angle relative to its central axis could be changed, thus changing the number and shape of the pores facing the vapor removal grooves. Thewick deflection angle represented its change in spatial position relative to the fixed vapor removal grooves. The effect of the wick deflection angles on the heat transfer characteristics of the flat LHP was experimentally investigated. It was found that with the change of deflection angle, the number of pores in the evaporation-oriented zone would also change, which had a significant impact on the start-up process and heat transfer performance of LHP. When the deflection angle was 30°, LHP could start fastest at a low heat load of 20 W and operate stable at a high heat load of 180W. [ABSTRACT FROM AUTHOR]

To better analyzing the temperature oscillation and the two-phase behavior inside a flat loop heat pipe, visual studies were conducted. Under the 20°C water cooling and horizontal orientation, the effects of the filling ratio and heat loads on the temperature oscillation were analyzed. Based on the experimental data, the results indicate that owing to the increased system pressure, the temperature oscillation decays as the filling ratio increases from 34% to 58%. Meanwhile, during the startup process, temperature oscillation tends to occur during the boiling and steady stages due to the more violent two-phase behavior, while the temperature curves are smooth during the slow evaporation stage. Moreover, as the heat load increases, the evaporation becomes more intense at the active zone of evaporator, leading to a faster startup process and a higher oscillation frequency. Besides, owing to the synergistic effect of two-phase flow in the compensation chamber caused by heat leak and subcooled liquid backflowing, a "breathing" oscillation behavior of the vapor-liquid interface is observed at the compensation chamber, which further leads to the unstable operation behavior of the loop heat pipe system. [ABSTRACT FROM AUTHOR]

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).

In this study, a prototype of a flat-disk global visualisation loop heat pipe was constructed, and the two-phase working phenomenon in the system operation was elaborated. This study focuses on a new method for global visualisation of loop heat pipes. The evaporator, condenser, and pipeline are integrated into a single piece of glass. Deionised water was used as the working fluid. Simultaneously, the airtightness of the system was guaranteed. During the experiment, an infrared thermal imager and a high-definition camera were used to collect optical information. The heat and mass transfer phenomena in the system under different working conditions were experimentally studied. The three stages of the loop heat pipe start-up were analysed according to the change in the vapour–liquid state. Through global observation of the heat and mass transfer process, the effects of condenser working efficiency, heat leakage, and non-condensable gas on the thermal performance of the loop heat pipe were studied. During the test, the temperature of the heating surface was lower than 92 °C, and the heat load range of the system was 25–45 W. The condenser was designed as a fan-tube condenser with forced convection heat dissipation. The system achieved self-regulation and stability through the cooperation of various components. The significance of this study is that the global visualisation of the loop heat pipe comprehensively describes the heat and mass transfer process of the system, and the operating characteristics of the loop heat pipe were studied intuitively.

The existing work deals with the evaluation of compact loop heat pipe by means of a low thermal conductivity sintered chrysotile wick to avoid large heat leaks as of the evaporator to the compensation chamber. Accordingly, a wick with low thermal conductivity (0.068–0.098 W/mK) chrysotile powder of a mean particle diameter of 3.4 μm is fabricated through sintering. Nine chrysotile wicks are sintered with different compositions of binders (bentonite and dextrin) and pore‐forming agent NaCl at sintering temperatures of 500°C, 600°C, and 700°C with a sintering time of 30 min. The wick properties, for instance, porosity, permeability, wettability, and capillary rise are studied owing to sintering temperature. Consequently, it is observed that a pure chrysotile powdered wick at a sintering temperature of 600°C exhibits a high porosity of 61.8% with permeability 1.04 × 10−13 m2 and a capillary rise of 4.5 cm in 30 s and is considered optimal. This optimal wick is used for performance evaluation in compact loop heat pipe and a decrease of 36.1% in thermal resistance is found when compared with copper mesh wick in a loop heat pipe. The lowermost thermal resistance originates to be 0.147 K/W at 120 W with wall temperature 57.7°C. This indicates that loop heat pipe with sintered chrysotile wick can operate at lower heat loads efficiently when compared with copper mesh wick and as heat load increases a chance of dry out condition occurs. The highest evaporative heat transfer coefficient obtained is 65.7 kW/m2 K at a minimum heat load. [ABSTRACT FROM AUTHOR]

Copyright of Bulletin of the Chinese Ceramic Society is the property of Bulletin of the Chinese Ceramic Society Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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. [ABSTRACT FROM AUTHOR]

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.

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

Cryogenic loop heat pipes are highly efficient heat transfer devices at cryogenic temperature range, which have promising application prospects in satellites, spacecrafts, electronics, and so on. Cooling down process is a most critical process for a CLHP before startup. At present, secondary loop is a major way for a CLHP to fulfil cooling down and most studies are concentrated on heat transfer characteristics during normal operation. However, few investigations have been carried out on the cooling down process. In this paper, the cooling down process of a nitrogen-charged CLHP assisted with a secondary loop was experimentally investigated. A simple qualitative approach to estimate the cooling down time was proposed according to the law of conservation of energy. The two flow paths of the working fluid in the CLHP during the cooling down process were described. Experimental studies on the cooling down process with various secondary heat loads and working fluid inventory were presented in detail. With the increase of secondary heat load, the elapsed time of Stage III decreased significantly due to the larger mass flow rate in Path I. In addition, the effect of the working fluid inventory on the cooling down time was generally small in the range from 2.99 MPa to 3.80 MPa. However, with 2.80 MPa working fluid inventory, it required much longer cooling down time, which was because of the lack of liquid in the CLHP with low working fluid inventory. Moreover, the influence of gravity on the temperature variation of the components during the experiments was analyzed. This work is beneficial for better understanding of the cooling down process and optimizing of CLHPs. [ABSTRACT FROM AUTHOR]

Among all types of heat pipes, loop heat pipes stand out for their ability to generate a significant capillary pressure and can rightfully be called antigravity heat pipes. At the same time, the loop heat pipe has a number of specific features, which shall be provided for the reliable start-up of the loop heat pipe. Features of heat pipes' design and operation in the thermal control system of the landing module are presented in this paper. [ABSTRACT FROM AUTHOR]

• A DCCLHP with sequential cooling to the CCs is proposed. • Startup under both favorable and unfavorable attitudes is experimentally studied. • Significantly improved startup performance of the DCCLHP is achieved. • Strong temperature fluctuation under unfavorable attitudes is observed. • Heat leak from the evaporator to the CC is quantitatively analyzed. Dual compensation chamber loop heat pipe (DCCLHP) holds great application potential in efficient heat dissipation of avionics due to its flexible heat transport, fast temperature response, and effective liquid replenishment in all orientations. In this work, a DCCLHP with sequential cooling to the compensation chambers (CCs) was proposed, aiming to enhance its startup performance especially under unfavorable attitudes. Comprehensive experimental studies were conducted mainly focusing on its startup performance with the evaporator/CCs under five typical attitudes. Experimental results show that the DCCLHP could provide effective cooling to the CCs, enabling the DCCLHP to realize successful startup under small heat loads of 0–100 W in all orientations, and no startup failure occurred. For the startup under 80 W at -90° tilt angle, it took only 13 s from applying heat load to the evaporator to the vapor's entering the condenser, and the lowest system thermal resistance was only 0.193 °C/W. The heat load taken away by the wrapping line from the CC1 was also quantitatively analyzed based on the energy balance of the CC1. The wrapping line under unfavorable attitudes effectively takes more heat load away from the CC1. In the startup under 100 W at -45°, the heat load taken away by the wrapping line reached a maximum of 3.71 W. This work well demonstrates that the DCCLHP can achieve orientation-free startup in attitude-varying application scenarios. [ABSTRACT FROM AUTHOR]

• A novel loop heat pipe that provided passive thermal management up to 10 kW was developed. • A peak evaporative heat transfer coefficient of 92,000 W/m2/K was achieved. • Natural convection recorded the lowest system thermal resistance of 0.007 K/W. • Numerical model predicted performance with an average temperature difference of 7.1 °C to 10.6 °C. 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. [ABSTRACT FROM AUTHOR]

• Coupling mechanisms of vapor-driven jet injector (VDJI) and loop heat pipe (LHP) are investigated. • Biporous wick was prepared and sintered integrated with the base plate of evaporator. • Operation modes of VDJI are classified in the operation of LHP. • Internal flow structure of VDJI at different operation modes are revealed. 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. [ABSTRACT FROM AUTHOR]

• The effect of filling pressures on a gravity-assisted cryogenic loop heat pipe was experimentally studied. • Filling pressures were determined based on the presumption of vapor-liquid distribution. • Heat transfer behavior and hysteresis phenomena under different filling pressures were investigated. • Test results show a good agreement with the presumptions regarding vapor-liquid distribution. This paper investigated the effect of filling pressure on the operating characteristics of a gravity-assisted cryogenic loop heat pipe(CLHP) for use in gravity environments such as terrestrial and lunar environments. The CLHP wick is made of sintered stainless-steel fibers with a pore radius of 1.56 μm and designed with a heat transport distance of 2.05 m. The experiments were conducted under gravity-assisted conditions (the condenser was placed 0.1 m higher than the evaporator). Notably, the filling pressure originated from the assumed vapor-liquid distribution in the CLHP under steady-state conditions. The filling pressure was varied from 2.9 MPa to 3.4 MPa in 0.1 MPa increments for six different conditions. Specifically, (1) 2.9 MPa and (2) 3.0 MPa are conditions where the heat leakage due to the vapor phase in the evaporator core is large, while (3) 3.1 MPa and (4) 3.2 MPa are conditions where there is no vapor phase in the evaporator core and the surplus vapor phase escapes to the CC. In general, this condition is considered to be the optimum amount of working fluid for room-temperature LHPs when designing. (5) 3.3 MPa and (6) 3.4 MPa are overfilling conditions that cause the CC to be filled with liquid. The results revealed that the higher the filling pressure, the more obvious the variation in operating temperature caused by the transition of drive modes. The maximum heat transfer capability reached 25 W in cases (1)-(4). In cases (5) and (6), the heat transfer capabilities increased to 30 W, although the operating temperature was higher. Furthermore, the hysteresis effect under different filling pressure conditions was newly confirmed. The power cycling experiments demonstrated that hysteresis in the operating temperature occurred at high heat loads and showed a similar trend to the room-temperature LHP. [ABSTRACT FROM AUTHOR]

Heat switches are devices for controlling heat flow in various applications, such as electronic devices, cryogenic cooling systems, spacecraft, and rockets. These devices require non-linear transient thermal simulations, in which there is a lack of information. In this study, we introduce an innovative 1D thermo-hydraulic lumped parameter model to simulate loop heat pipes as heat switches by regulating the temperature difference between the evaporator and the compensation chamber. The developed thermo-hydraulic model uses the continuity, energy, and momentum equations to represent the behaviour of loop heat pipes as heat switches. The model also highlights the importance of some thermal conductance parameters and correction coefficients for accurately simulating the different operational states of a loop heat pipe. The simulations are conducted using the proposed 1D model, solved through the application of the Mathcad block function. The numerical model presented is successfully validated by comparing the temperatures of the evaporator and condenser inlet nodes with those of a referenced loop heat pipe from the literature. In conclusion, in this research, the mathematical modelling of loop heat pipes as heat switches is presented. This is achieved by incorporating correction coefficients with Boolean logic that results in non-linear transient simulations. The presented 1D thermo-hydraulic lumped parameter model serves as a valuable tool for thermal system design, particularly for systems with non-linear operational modes like sorption compressors. The graphical and nodal representation of this proposed 1D thermo-hydraulic model further enhances its utility in understanding and optimising loop heat pipes as heat switches across various thermal management scenarios.

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Possible configurations of loop heat pipe evaporators have been analyzed, and the current state of experimental and theoretical studies on the choice of their capillary structures with a real pore size distribution function has been presented. On this basis, a methodology for selecting capillary structures for the primary and secondary loop heat pipe wicks with various operational parameters has been proposed. The operating conditions of loop heat pipes and the method of analytical calculation of the temperature field for a model with pseudoconvection in capillary structures have been formulated. The calculated and experimental data have been compared. [ABSTRACT FROM AUTHOR]

Loop heat pipe (LHP) is widely used in high-power electronic cooling as a passive cooling device. In this paper, the LHP system was designed with a flat evaporator and a capillary biporous wick with cupric oxide nanofluids as working fluid in LHP. The capillary biporous wick was sintered by nickel powder and the nanofluids were prepared by a two-step method. The startup tests, variable heat load tests, and performance degradation tests of LHP were conducted respectively and the thermal resistance was further analyzed. The results showed that, compared with distilled water, the maximum heat load increased from 290 W to 310 W and the minimum startup heat load decreased from 40 W to 20 W with nanofluids as working fluid. Both the temperature of the evaporator wall and the thermal resistance were reduced in startup tests. At the heat load of 50 W, the performance improvement was the most noticeable, where the temperature of the evaporator wall was reduced by 12.7 °C, the thermal resistance of the evaporator was reduced by 26.9%, and the thermal resistance of the LHP system was reduced by 17.6%. With a variable heat load ranging from 50 W to 290 W, the response speed of LHP became faster and the temperature fluctuation was smaller. The LHP with nanofluids as working fluid still operated normally and the change of thermal resistance with the varies of heat load had the same tendency as the previous experiment in performance degradation tests of 2 months.

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.

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. [ABSTRACT FROM AUTHOR]

Since the onset of the 21st century, the number of data centers has been increasing continuously, and the energy consumption of these data centers has aroused people's attention. Many experiments have been conducted to reduce the energy consumption in data center cooling, and free cooling utilization based on the mechanically driven two-phase cooling loop (MTCL) system has been proven to be an effective solution for energy saving. This paper reviews the research on and developments in the practical application of the MTCL system for data center cooling in recent decades. It presents a detailed summary of the progress of research on the MTCL system in data centers. The aim is to provide an improved understanding of the system and promote the application of this system in data center energy saving. [ABSTRACT FROM AUTHOR]

The deep understanding of characteristics and mechanism of liquid-vapor phase change and capillary pumping in porous wick is necessary for the optimal design of efficient heat transfer devices such as loop heat pipes. This paper develops a toolbox based on OpenFOAM for simulating the two-phase flow and phase change heat transfer in the hybrid domain using the micro-continuum model, which can asymptotically converge to Darcy model in porous domain and NS equation in solid-free domain. The toolbox also provides choices for the phase change model, capillary pressure model, and specific boundary conditions. The toolbox is validated by comparing the simulation results with analytical solutions or published results through several example problems. The simulation of the LHP evaporator indicates the receding of liquid-vapor interface into the porous wick with the increasing of heat flux, and its thermal performance is in good agreement with previous experimental studies. The developed toolbox can be anticipated to do theoretical and optimized studies on phase change heat transfer in porous media.

Dual compensation chamber loop heat pipe (DCCLHP) is a promising technology for future avionics cooling. In this work, the effect of orientations on the steady state and startup performance of a DCCLHP with kW class heat transfer capacity was investigated experimentally. Experimental results show that for high capacity DCCLHP, the variation of condenser performance at different orientations is a decisive factor in system performance, which could result in up to a 50 % decrease or 10 % increase in the heat transfer limit. In addition, the lack of condenser performance could also have a potential negative impact on the startup performance. By establishing a method to analyze the operating temperature, the authors provided an in-depth analysis of the related mechanisms. The effect of gravity could change the force condition and therefore the thickness of the condensate in the condenser, which in turn affects the condenser capacity and the working fluid temperature flowing into the compensation chamber. As a result, for practical applications, it is necessary to focus on the installation of the condenser to avoid the orientations that tend to cause the increase in the thickness of the condensate film, such as the condensate flow inside the condenser was opposite to gravity. The research content of this paper could deepen the understanding of LHP technology and contribute to the realization of application in acionics cooling in the future. [ABSTRACT FROM AUTHOR]

· Different change rate of cross-sections along the flow field is investigated. · Microchannel condenser is used in loop heat pipe. · Effects of operating height situation and microchannel structure are studied. · Smooth operation and anti-gravity performance are effectively improved with the help of tapered channel. 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 %. [ABSTRACT FROM AUTHOR]

Loop heat pipes (LHPs) are attractive two-phase thermal control devices for satellites, electronics and many other applications. They are capable of transporting heat efficiently for long distances up to several meters at any orientation. This paper investigated the heat transfer characteristics of loop heat pipes with long distances and small diameter transport lines. Small stainless steel tubes of 2 mm and 3 mm in inner diameters were chosen as liquid lines and vapor lines of the LHPs. The local thermal resistances in the evaporator of the 6 m-LHP were researched and analyzed, which indicated that the thermal resistance between the aluminum block and the vapor in the vapor channel accounted for a major proportion of the total thermal resistance. The effect of heat sink temperatures on the performance of the 6 m-LHP were compared with 10°C, 15°C, 20°C and 25°C cooling water temperatures. Moreover, the thermal characteristics of LHPs with transport distances of 2 m and 16 m were also experimentally investigated. The 16 m-LHP could achieve a heat transfer capacity of 100 W and the 2 m-LHP could reach more than 339 W, on the premise of the evaporator temperature below 100°C. The thermal resistance of the 2 m-LHP could achieve 0.125°C/W. [ABSTRACT FROM AUTHOR]

The potential applications of loop heat pipes (LHPs) are the nuclear power space systems, fuel cell thermal management systems, waste heat recovery systems, medium temperature electronic systems, medium temperature military systems, among others. Such applications usually operate in temperature ranges between 500-700 K, hence it is necessary to develop an LHP system that will meet this requirement. Such a thermal management device require to meet various technical problems and challenges currently existing in the development of LHP working in medium temperatures, including: (1) selection of appropriate working fluid; (2) selection of appropriate LHP construction material; (3) construction of suitable test rig capable of testing at elevated temperatures; (4) development of new testing methods. Currently, there are no proven working fluids that can be used in LHPs in medium temperature ranges. Water can be applicable only at temperatures up to 570 K. Caesium can be applicable at temperatures above 670 K. Organic fluids usually tend to generate non-condensable gasses and/or decompose at elevated temperatures and their viscosity dramatically increases. For halides, most of them are very reactive or toxic and their full property data are not available or the majority of the physical properties are predicted, also live tests and their environmental impact data are not adequate. As for casing/LHP construction material, there are no full chemical compatibility tables with most of the medium temperature working fluids and the reactivity of fluids significantly limits the potential materials. Also, testing such an LHP is an endeavour as the reactivity of medium temperature fluids and the use of obscure metals create new challenges. Altogether creates multiple challenges in the development, testing, handling and operating of LHP in the medium temperature range. [ABSTRACT FROM AUTHOR]

Loop heat pipes (LHPs) have all the advantages of conventional heat pipes, are less susceptible to gravity effects, and offer better heat-transfer performance. They can readily be used in confined areas. As to temperature maintenance, they are self-regulating devices thereby preventing a considerable decrease in the temperature of the mounting seat of heat-releasing equipment on a heat load reduction. Miniature LHPs can replace conventional heat pipes in promising spacecrafts, while substantially increasing the ability to remove heat and reducing their weight. To develop effective methods for the design of LHPs, physical and mathematical models of conjugated heat and mass transfer in the device are required. This paper presents a procedure for the calculation of the steady-state performance of an LHP. This procedure is to be used to calculate the hydraulic resistance to the motion of liquid and vapor in LHP elements. The heat-transfer ability is determined by the ability of the capillary structure to transport the coolant against the hydraulic resistance. In the second part of the problem, the thermal condition of all elements is calculated using an approach based on the balance of heat fluxes. The model considers the potential vapor superheating in a vapor pipeline. Thermophysical properties of the coolant is calculated in each transport section as a function of the local temperature. The predictions describe the LHP performance quite accurately and agree with the results of testing an experimental loop heat pipe under various conditions at an evaporator temperature from –35 to +25°C. The tests were carried out at room temperature or in a climatic chamber, when the temperature of the radiator was almost equal to the temperature of the environment. The maximum deviation of the predicted evaporator temperature from the measured one was less than 5°С with the average deviation being below 3°С. [ABSTRACT FROM AUTHOR]