Condenser Section Research Articles

Experimental study on sodium Screen-Wick heat pipe capillary limit

Heat pipes continuously transport the latent heat of vaporization from the evaporator to the condenser in the heat pipes due to the capillary pressure created by the menisci in the wick pumping the condensed fluid back to the evaporator. The heat pipe heat transfer capacity is then limited by the capillary limit. However, fewer capillary limit experiments were conducted to investigate the transient temperature variations and physical mechanism at the capillary limit for liquid metal wick heat pipes. This work presents capillary limit transient measurements on a sodium screen-wick heat pipe for high heat fluxes, high heating rates during frozen startup, and negative and positive inclination angles. It also investigates the special behavior before and after dryout, including dryout oscillations and the recovery process to understand the cause and physical mechanism at the capillary limit. The direct cause of the capillary limit is that the evaporation rate exceeds the capillary-driven liquid supply to the evaporator. The experiments show that the capillary limit creates a rapid temperature increase of about 5 °C/s at the evaporator end, while the condenser and adiabatic section temperatures decrease. Dryout oscillations were observed with positive inclinations before the capillary limit. After the capillary limit, reducing the power to just below the limit does not lead to heat pipe recovery, but the power needs to be reduced by about 30% of the critical heat flux to make the heat pipe return to its normal operating state.

Experimental operating range evaluation of flat-plate pulsating heat pipes for high-heat flux automotive power electronics cooling

Flat-plate pulsating heat pipes are a promising future technology to passively increase the power density of power electronics in mobile applications by means of a more efficient thermal management. However, in the design phase, it is not known how different thermal boundary conditions are affecting the operation range of a pulsating heat pipe. To experimentally assess this dependency, the present work aims to evaluate the performance of a flat-plate pulsating heat pipe for electric mobility and different operating conditions. To this purpose, a flexible test section was designed allowing to raise different heating and cooling conditions. A novel array of multiple hotspots was employed which realistically represents a power semiconductor layout. This way, a local heat flux of up to 200W/cm2 was introduced to the pulsating heat pipe. In the condenser section, the coolant temperatures were varied between 10°C and 60°C, representing real conditions of a driving cycle. The investigated aluminum pulsating heat pipe consists of 22 rectangular channels and two different newest generation refrigerants R1233zdE and R1234yf were used as working fluids. Based on the new experimental evidence, our previously published thermal resistance model was adapted. A posteriori comparison with previous calculations confirms the validity of the numerical assumptions and shows a good agreement with the obtained results. For the first time, our investigation shows that pulsating heat pipes can robustly achieve heat flux densities above 100W/cm2 at conditions representative for automotive electronics applications. The real implementation potential was demonstrated, as an considerable 5 to 11 times higher effective thermal conductivity than solid aluminum was obtained depending on the integration concept.

Comparative analysis of CLPHP working with binary mixtures in different concentrations

The advanced technologies in design and manufacturing of electronic components have resulted considerable heat flux through component miniaturization. For highly performing microelectronic devices production, thermal management has the priority. Other applications also like material science, fabrication technology, solar applications etc. which directly impact cost, reliability and performance. When it comes to overcome these problems efficient cooling approaches development and use will come into play. The best solution for this is Pulsating heat Pipe (PHP). A simple PHP is a two-phase heat transfer device consists of a tube with a suitable working fluid, along with an evaporator (heating)section, adiabatic section and a condenser (cooling)section. In the present work multi turn Closed Loop Pulsating Heat Pipe (CLPHP) is chosen with capillary dimensions. The experimentation is carried out with different working fluids viz. water, water-based mixtures (75%-25%) at different volumetric fill ratios 50%,60% and 75%. The heat inputs of20W, 40 W, 60 W, 80 W, 100 W applied. At various heat inputs for all the working fluids the variation of thermal resistance is plotted and observed that upon increasing heat input, thermal resistance of fluid decreases. Comparative study is also presented for same experimental setup when the binary mixtures are considered in 50%-50% as mixture concentration. This gives the better understanding of Closed Loop Pulsating Heat Pipe performance when operated at different fill ratios and concentrations for different working fluids considered.

Dynamic modelling of a compressed heat energy storage (CHEST) system integrated with a cascaded phase change materials thermal energy storage

Carnot batteries represent an emerging thermo-mechanical energy storage technology based on the conversion of surplus electricity into medium–low temperature heat, and subsequent conversion of the heat into electricity. A promising Carnot battery’s configuration is the Compressed Heat Energy Storage (CHEST) that combines a high-temperature heat pump (charge phase), an Organic Rankine Cycle (discharge phase) and a thermal energy storage (TES) system. As of now, all the literature studies on CHEST were designed to store the thermal energy into a cascaded TES combining a pressurized water storage for the sensible section and latent heat material for the evaporation and condensation sections. The objective of the present work is to numerically investigate a novel configuration of the TES system utilizing a cascade of multiple phase change materials (PCMs) in place of the cascade of sensible heat and phase change materials presented in literature, aiming to enhance both the energy density and the round trip efficiency of the system. The comparative technical analysis has been carried out by developing for the first time a dynamic numerical model of the CHEST system. Indeed, while the current literature studies on CHEST are only focused on preliminary analyses to define the general thermodynamic potential and to identify the limits of the overall system, this paper overcomes this gap and presents a detailed dynamic analysis of the CHEST with focus on TES modelling. Notably, the authors developed a plant model in MATLAB that blends together algebraic and differential sub-models detailing the transient behaviour of the CHEST system. The results are of interest for academia and industry and contribute significantly to the development of a more efficient CHEST system. Indeed, by enhancing the thermal buffer effect typical of PCM media, the cascaded TES augments both the COP of the high-temperature heat pump (4.13 vs 3.79) and the electric efficiency of the ORC (11.69 % vs 11.31 %). As a result, the novel CHEST system based on cascaded PCMs is capable to achieve a round trip efficiency of 47.6 % and an energy density of 6.9 kWhe/m3, simultaneously increasing the respective values of the state-of-the-art solution by 13 % and 100 %, respectively.

Development and thermodynamic analysis of a novel heat pipe vacuum tube solar collector with sensible heat storage

ABSTRACT A novel heat pipe vacuum tube solar collector has been developed and experimentally investigated for sensible heat storage in different climatic conditions. The solar collector has been introduced with a new header where heat transfer fluid (Therminol-55) gains heat directly in contact with the condenser section of heat pipe. This eliminates the application of thermal paste between condenser and manifold in existing systems. Thermal performance of the system has been evaluated with a heating load of 30 L of Therminol-55 in three consecutive months (March, April, and May) to check its maximum and average performance. Maximum temperature has been found as 145.8°C, 153°C, and 151.2°C in March, April, and May respectively. Maximum energy efficiency has been observed as 44.7%, 24.3%, 23.4%, and maximum exergy efficiency has been observed as 3.1%, 3.3%, and 2.6% in March, April, and May respectively. Average energy efficiency for the whole duration of experiment has been determined as 12.1%, 8.3%, 8.4%, and average exergy efficiency has been determined as 2.1%, 1.9%, 1.8% in March, April, and May respectively. The results suggest utility of the developed solar collector for consistent use in daytime for medium temperature solar heating applications.

Design, fabrication and heat transfer performance study of a novel flexible flat heat pipe

In this work, a novel flexible flat heat pipe (FFHP) is developed to dissipate high heat flux in flexible electronic devices. Its evaporation section and condensation section are made of metal cavity structure to guarantee good heat conduction, and flexible copper foil is used as the shell material of the adiabatic section to improve the overall flexibility. Spiral woven mesh adhered to a coarse copper mesh is selected as the liquid-wicking structure due to its excellent flexibility and strong capillary force, and the coarse copper mesh serves as a supportive structure. Deionized water is chosen as the working fluid. A feasible manufacturing process is also proposed. The effects of different liquid filling ratios (30%, 40%, 50%), bending curvature diameters (20 mm, 25 mm, 30 mm), and bending angles (0° ∼ 180°) on the heat transfer performance of the FFHP are investigated. Experimental results demonstrate that the FFHP with the optimum liquid filling ratio of 40% can transport heat up to 25 W in the horizontal operation state with a minimum thermal resistance of 0.99 K/W. Moreover, the FFHP can be bent from 0° to 180°. When bent to 180° with a curvature diameter of 30 mm, it still can transport a maximum heat of 15 W.

Freezeback behavior of a cast-in-place pile foundation surrounded by two-phase closed thermosyphons in permafrost regions

The heat of hydration of cast-in-place pile foundations significantly warms, and even thaws, the surrounding permafrost, resulting in a substantial loss of bearing capacity. How to reduce the freezeback time is crucial to the construction schedule of superstructures; however, this has not been fully explored. To accelerate the freezeback process, we innovatively induced two-phase closed thermosyphons (TPCTs) to cool a cast-in-place pile foundation during construction. In this study, a laboratory model experiment and numerical simulations were conducted to highlight the refreezing behavior, cooling mechanism, and structural optimization of the pile-TPCTs system. The results indicate that the TPCTs can significantly accelerate the water–ice phase change, and increase the cooling rate of the subsequent cooling process by 1.75–2.8 times for the soil adjacent to the evaporator section. The pile-TPCTs system completed the freezeback approximately half the freeze–thaw cycle ahead of the traditional pile. Two TPCTs can satisfy the demand of freezeback under the conditions that 1) the condenser and evaporator section lengths must exceed 40% and 68% of the length of the underground pile, respectively, and 2) the distance from the TPCT to the pile side should be less than 5.26 times the TPCT diameter. The findings of this study will be valuable for controlling the construction schedule of cast-in-place pile foundations in permafrost regions.

Experimental study on performances of flat-plate pulsating heat pipes without and with thermoelectric generators for low-grade waste heat recovery

Low-grade waste heat recovery has significant importance to energy saving and emission reduction. Due to the lack of cost-effective heat recovery technology, a lot of low-grade heat is wasted. In this paper, performances of 3D-printed flat-plate PHPs and computerized numerical control (CNC) machined flat-plate PHPs without and with thermoelectric generators (TEGs) under four recycling modes were studied. The effects of the type and number of flat-plate PHPs, condenser section length, recycling mode, flow arrangement, velocity and temperature of hot/cold air, the number and layer number of TEGs on the performances of the waste heat recovery system were studied to maximize its performance. Results indicate that the performance of the heat recovery system with CNC machined flat-plate PHP is generally better than that with 3D-printed flat-plate PHP, and their maximum effective thermal conductivities can reach 585 and 541 W/(m·K), respectively. Performances of flat-plate PHPs based waste heat recovery system can be improved by using slotted fins, increasing the condenser section length and flat-plate PHPs number. For the air-to-air heat recovery system with three flat-plate PHPs, the maximum heat transfer rate and heat recovery rate can be up to 303 W and 85.3%, respectively. As for heat-to-power waste heat recovery under Mode 3, flat-plate PHP coupled with three TEGs can achieve its maximum open-circuit voltage of 7.6 V and output power of 1.98 W. Preliminary economic analysis proves that flat-plate PHPs based waste heat recovery system displays good economic benefits. These results may be helpful in designing a cost-effective flat-plate PHPs based low-grade waste heat recovery system.

Numerical Evaluation of Thermal Performance of Two-Phased Closed Thermosyphon for Solar Applications

A heat pipe is an effective passive heat transfer device with a highly efficient heat transfer rate and thermal conductivity through evaporating and condensing a working fluid circulating in a vacuum-sealed container. For solar applications, heat pipes are usually used in evacuated tube collectors (ETCs). However, the use of heat pipes in solar photovoltaic/thermal (PVT) collector is limited. Generally, a heat pipe composes of three sections: the evaporator section, the condenser section, and the adiabatic section. A wickless heat pipe, also known as a closed thermosyphon or gravitationally assisted heat pipe, relies on gravity to return the working fluid to the evaporator. Various parameters could greatly influence the thermal performance of wickless heat pipes such as initial volume filling ratio (VFR), inclination angle, inner diameter of the heat pipes, ratio of evaporator/diameter and type of working fluid used. In this study, 2D transient computational fluid dynamics (CFD) simulations were performed using ANSYS Fluent to simulate the two-phase flow (water/steam) and to study the effect of different parameters on the heat transfer processes of a two-phase closed thermosyphon (TPCT) to increase its thermal performance. The 2D CFD simulations using multiphase volume of fluid (VOF) with two Eulerian phases could replicate the mass and heat transfer processes in comparison with published experimental data. A good agreement can be observed between the present CFD study with the published data in comparing the temperature profiles and thermal performance of the heat pipe. The numerical simulation of the evaporation and condensation process indicated that the thermal performance and characteristics of the TPCT are influenced by certain key parameters. In the future, full-scale 3D CFD simulations are planned to be performed to evaluate the thermal performance of heat pipes for solar photovoltaic/thermal (PVT) applications.

An Innovative Design for a Solar Water Heating System Utilizing a Flat-Shaped Heat Pipe

Abstract In this work, an innovative design for a solar water heating system using a flat-shaped heat pipe as a heat transfer device is presented to pave the way for a substantial increase in the thermal performance of these systems. An analytical study is utilized to investigate the thermal performance of the solar water heating system. The analytical results of the flat-shaped heat pipe system are compared with the results of the evacuated tube solar water heating system with a U-tube, thermosyphon, and closed-loop pulsating heat pipe. It is found that the water temperature difference between the inlet and outlet of the flat-shaped heat pipe system is substantially higher than the U-tube, thermosyphon, and closed-loop pulsating heat pipe system by as much as 31.4, 22.5, and 18.5 °C, respectively, at a nominal 0.6 l/min mass flowrate. Furthermore, utilizing the flat-shaped heat pipe in the solar water heating system optimizes the thermal conductivity of the solar setup due to a reduction in the condenser section length. These reductions also lead to a large reduction in the weight and cost of the system.

Heat transfer performance of a two-phase closed thermosyphon with different inclination angles based on the core-tube monitoring

Two-phase closed thermosyphons (TPCTs) have been widely used in permafrost regions because of their good cooling effect. The variation in the inclination angle of TPCTs significantly affects the heat transfer process and temperature distribution inside TPCTs, which is still not fully understood. This study experimentally investigated the heat transfer performance of TPCTs with inclination angles varied from 0° to 90° with 10° intervals. In addition to the outer wall temperature and heat flux, the thermal regime of the working medium inside TPCT was simultaneously monitored based on the core-tube. Experimental results show that the temperature distributions inside TPCT along the axial direction vary regularly with inclination angle. The heat transfer rates of the evaporator and condenser sections change nonmonotonically and nonlinearly with the inclination angle. The influence of the inclination angle on the thermal resistance is greater in the evaporator and the adiabatic sections than in the condenser section. Consequently, a TPCT at 10° produces the best heat transfer performance, corresponding to the best isothermal characteristics, the maximum heat transfer rates of the evaporator and condenser sections, and the minimum total thermal resistance. This study provides a reference of monitoring method for further research on TPCT.

Investigation into the effects of passive check valves on the thermal performance of pulsating heat pipes

A passive check valve without a subsidiary channel that effectively increases the thermal performance of pulsating heat pipes (PHPs) is proposed. To realize this, topology optimization is employed to maximize the diodicity (Di) of the check valve. The proposed passive check valve consists of a pair of hook-shaped structures and has a Di value of 2.2. PHPs with overall dimensions of 53.5×80×1.5 mm3 are fabricated to investigate the effects of the passive check valve on the thermal performance and the flow behaviors of PHPs. A ten-turn closed-loop serpentine square channel with a height and an average width of 0.6 mm and 2 mm is engraved on a silicon plate and then covered with a glass plate for flow visualization. A total of twenty passive check valves are implemented into the entire channel at the locations where the condenser section and the adiabatic section meet. HFE-7000 is used as the working fluid and the filling ratio is fixed at 48% by volume. The experimental results on PHPs with the single-diameter channel show that the proposed passive check valves significantly improve the thermal performance of PHPs: 48% and 81% reduction in the thermal resistance in the vertical orientation and the horizontal orientation, respectively. Also, the experimental results on PHPs with the dual-diameter channel show that the passive check valves effectively improve the thermal performance over the wide operating range regardless of the orientation of PHPs: 33% and 34% reduction in the thermal resistance in the vertical orientation and the horizontal orientation, respectively. It is found through high-speed photography that this enhanced performance occurs because the passive check valves significantly increase the time-averaged volumetric fraction of vapor in the condenser section by 5 times from 0.09 to 0.45. This stems from the fact that the passive check valves effectively make the working fluid flow 3.5 times more preferentially in one direction over the other.

Heat transfer performance of flat micro-heat pipe with sintered multi-size copper powder wick

The heat flux rises dramatically as electronic circuits become more integrated. Flat micro-heat pipe (FMHP) is a popular device for high heat flux management. A novel sintered multi-size copper powder wick (SMW) is designed in this paper. The evaporator and condenser section of the SMW is a double-layer structure with two copper powder diameters. The adiabatic section is a single-layer structure. Copper powder sintering is a compelling method to achieve graded pore size design. SMW and six sintered single-size copper powder wicks (SSWs) are prepared by vacuum sintering of copper powder. The capillary pressure of wicks and heat transfer characteristics of FMHPs are experimentally investigated. The results show that the liquid transfer time of the SMW is shortened 16.59% compared to that of the SSW. The dynamic response time constant of the SMW FMHP is decreased by 0.9 s, and the overall thermal resistance is reduced by 14.22%. The heat pipe made in this study is more agreeable to heat dissipation and is of importance for thermal management in power devices.

Comparative Performance Analysis of Exhaust Air Heat Recovery Devices Based on Microchannel heat pipe

In response to the problem of low recovery efficiency of sensible heat recovery devices, this paper designs a microchannel heat pipe heat recovery device (MHPHRD) and compares its heat transfer performance during summer use through experiments and simulation calculations. The experimental results show that the microchannel heat pipe (MHP) has strong temperature uniformity, and the temperatures of the evaporation and condensation sections are basically the same. When the inlet temperature of fresh air changes between 32°C and 40°C, the recovery efficiency of the device varies between 67%-80%, and the linear relationship between the two is obvious. The established calculation model has high accuracy, with a maximum error of 0.7% between measured and simulated values. Simulation calculations show that the cooling capacity recovery efficiency of the MHPHRD is 4%-16% higher than that of the cylindrical heat pipe heat recovery device (CHPHRD), and 24%-30% higher than that of the plate heat recovery device (PHRD). The research in this article can provide reference for the practical application of the device.

Sustainable dropwise condensation enabled ultraefficient heat pipes

AbstractHeat pipes play a critical role in determining the operations, safety, and energy efficiency of electronics. The main focus to improve the heat pipe performance is on the evaporator design or wicking structures. However, the intrinsic limitation comes from the condenser, which is fundamentally constrained by inefficient filmwise condensation (FWC). In this study, we successfully achieved a peak effective thermal conductivity (keff) of ~140 kW/(m·K) on widely used groove heat pipes by implementing sustianble dropwise condensation (DWC) and integrating with enhanced evaporator. To better understand the working mechanisms of the ultraefficient heat pipe, both the evaporator and condenser of the heat pipes have been modified accordingly. Our results show that up to 296% enhancements on the keff can be achieved under various inclination angles by only inducing DWC in the condenser section. The drawback of temperature fluctuations induced by DWC in smooth heat pipes appears to be effectively solved using the grooves‐wicking structure. Furthermore, by integrating the nanostructured evaporator, the keff of the heat pipe can be boosted up to 517% compared to conventional groove heat pipes. This study, for the first time, demonstrates the huge potential of engineering both the condenser and evaporator simultaneously in developing ultraefficient heat pipes.

FLUID FLOW AND HEAT TRANSFER CHARACTERISTICS OF A CENTERED-WICK HEAT PIPE WITH ADDITIONAL WORKING FLUID: EXPERIMENTAL STUDY WITH VISUALIZATION

This article describes the experimental study of the centered-wick heat pipe with additional working fluid. A semi-transparent heat pipe is employed, and the heat transfer experiments are conducted in three orientation modes: horizontal, vertical, and sideways. The wick structure is a sintered copper powder, and the working fluid is water. The heat pipe has a small gap between the wick structure and the container wall. In each orientation mode, a condenser section of the heat pipe is water-cooled with a cooling jacket, and an evaporator section is heated with a heater. Fluid-flow and phase-change phenomena in the heat pipe are captured by using a video camera, and the temporal changes in the temperatures of the heat pipe are obtained by using thermocouples. Due to the additional working fluid, liquid slugs are found in vapor flow channels. The liquid slugs can be categorized into two types: a dynamic liquid slug and a static liquid slug. The experimental results demonstrate that the dynamic and static liquid slugs are distributed more effectively in the horizontal orientation mode. Thus, the thermal resistance of the heat pipe tends to be smaller and the maximum heat input to the heat pipe is larger in the horizontal orientation mode than in the other two orientation modes. The experimental results also confirm that the small gap is effective in the horizontal orientation mode. The additional working fluid is stored in the gap, which increases the thermal performance of the heat pipe.

Fluid Flow and Heat Transfer Characteristics of a Centered-wick Heat Pipe with Additional Working Fluid: Experimental Study with Visualization

This article describes the experimental study of the centered-wick heat pipe with additional working fluid. A semi-transparent heat pipe was employed, and the heat transfer experiments were conducted in three orientation modes: horizontal, vertical, and sideways orientation modes. The wick structure was a sintered copper powder, and the working fluid was water. The heat pipe had a small gap between the wick structure and the container wall. In each orientation mode, a condenser section of the heat pipe was water-cooled with a cooling jacket, and an evaporator section was heated with a heater. Fluid-flow and phase-change phenomena in the heat pipe were captured by using a video camera, and the temporal changes in the temperatures of the heat pipe were obtained by using thermocouples. Due to the additional working fluid, liquid slugs were found in vapor flow channels. The liquid slugs can be categorized into two types: a dynamic liquid slug and a static liquid slug. The experimental results demonstrated that the dynamic and static liquid slugs were distributed more effectively in the horizontal orientation mode. Thus, the thermal resistance of the heat pipe tended to be smaller and the maximum heat input to the heat pipe was larger in the horizontal orientation mode than in the other two orientation modes. The experimental results also confirmed that the small gap was effective in the horizontal orientation mode. The additional working fluid was stored in the gap, which increased the thermal performance of the heat pipe.

Investigation of heat transfer and flow characteristics in two-phase loop thermosyphon by visualization experiments and CFD simulations

As an efficient heat transfer device, two-phase loop thermosyphon (TPLT) has a broad application prospect in many engineering fields, and its internal gas-liquid phase change and flow characteristics are the key factors affecting the heat transfer performance. In order to get a better thermal design and performance, it is necessary to clarify the heat transfer mechanism of the TPLT, especially under the condition of high filling ratio. In this study, the visualization experiment was combined with the CFD method to study the heat transfer and flow characteristics of TPLT. Due to the strong coupling relationship among the boiling process and the condensate process, the Lee model is optimized, and the relationship between saturation temperature and local pressure, and the parameter of subcooling degree in the Lee model were both introduced to accurately simulate the phase change process. Results shows that the optimized model has good prediction performance for the heat and mass transfer simulation of the loop thermosyphon. The two-phase flow in the loop thermosyphon changes from slug flow to churn flow with the increase of heating power at low filling ratio (50%), while the bubble size decreases and the number of bubbles increases with the increase of heating power (20W-80W) at high filling ratio (70%). The heat transfer mechanism before and after model optimization has changed, from saturated flow boiling to subcooled flow boiling at high filling ratios. At low filling ratio (50%), the fluid in heating section is saturated state, and the flow velocity at the bottom of the loop is low (0.018m/s). At high filling ratio (70%), the fluid in the heating section is supercooled state, and the fluid flow velocity at the bottom of the loop is high (0.171m/s). This indicates that the heat transfer mechanism of the loop thermosyphon at low filling ratio was saturated pool boiling, and the filmwise condensation occurs in the condensation section. However, the heat transfer mechanism is subcooled flow boiling at high filling ratio, and the gas-liquid two phases in the condensation section are in a mixed state.

Experimental and numerical investigation of solid-solid phase change material assisted heat sink with integrated heat pipe for electronic cooling

Advancements in phase change materials (PCM) technology offer adaptable solutions for challenging high heat dissipation problems in electronic devices. In this work, the influence of Neopentyl Glycol (NPG) as PCM having solid-solid phase transition on the functioning of heat pipe assisted heat sink is studied. Experimental and numerical investigation was performed on the transient thermal performance of heat pipe integrated PCM-assisted heat sink application with forced air cooling. The heat pipe's evaporator section is in contact with PCM- assisted heat sink, while condenser section is exposed to the ambient. Investigations were carried out at power levels of 9–13 W, corresponding to an equivalent heat flux range of 2.5–3.5 kW/m2 and with PCM filling of 33, 66 and 99 % of the heat sink volume where without integration of PCM and heat pipe in heat sink as the reference configuration. PCM-assisted heat sink and heat pipe configurations outperformed other configurations based on charging period for all experimental parameters with a maximum enhancement ratio of 3.29. Heat sink assembly comprising 99 % PCM with heat pipe gives the most significant results on comparison with all other configurations, whereas the assembly without heat pipe showed the best results for 66 %.

Theoretical and experimental investigations on the supercritical startup of a cryogenic axially Ω-shaped grooved heat pipe

The successful supercritical startup of a cryogenic grooved heat pipe is the premise for its normal operation, and the startup time determines the working efficiency of the payload. Based on this demand, a supercritical startup model of grooved heat pipe is established in the paper. The model applies more practical boundary conditions of the condensation section, without the need of knowing the temperature change of condenser in advance. With known performance curve of the cryocooler, the startup performance of heat pipe can be predicted, including the temperature distribution, startup time, etc. The predicted results by the model showed good agreement with the startup experiment results of the cryogenic grooved heat pipe. Additionally, the effects of different condenser lengths and structural parameters on the startup of heat pipe were analyzed by using the model. The results show that longer condenser length, larger groove diameter can accelerate the flow rate of condensate and the startup process. The influence of the groove depth to width of an Ω-shaped grooved heat pipe and a trapezoidal grooved heat pipe was also compared. This model provides a theoretical basis for the design and optimization of cryogenic grooved heat pipe.