Mass-Spring-Damper Model with Steady State Parameters for Predicting the Movement of Liquid Column and Temperature Oscillation in Loop Heat Pipe

In order to investigate the mechanism of the temperature oscillation in loop heat pipes,this paper investigated the movement of the phase interface as the changed input power by a mass-spring-damper model.The model was solved with MATLAB and was used to explain the high-frequency and low-amplitude temperature oscillation.Temperature variation with the input power from 20 W to 75 W was investigated based on a LHP prototype in a literature.The model agreed well with the experimental data in the literature.The simulation results suggested that the movement of the liquid column was caused by the fluctuation of pressure difference applied on the liquid column and the stiffness coefficients of the vapor springs increasing with the input power.According to parameter analyses,the temperature oscillation at the outlet of the condenser can be weakened by increasing the mass of the liquid column and keeping the temperature at the outlet of the condenser steady.

Experimental Investigation of Loop Heat Pipe with Two Evaporators/Two Condensers under Thermal Vacuum Condition

Multiple loop heat pipe is a high-functional thermal transport device. This work was conducted to confirm the working performance of Multiple loop heat pipe under thermal vacuum ambience with the working fluid ammonia. Asmall multiple loop heat pipe with two evaporators and two ra- diators was designed and fabricated. Then thermal vacuum test was conducted. The heaters were fasten on both evaporators, both radiators, both compensation chambers. In the case that both evaporators were heated, the multiple loop heat pipe can transport 120/120 W for 1.5 m, in the case that only one evaporator was heated, evaporator 1 can transport 80 W for 1.5 m, while eva- porator 2 can transport 120 W for 1.5 m. Two flow regulators were installed near the confluence of liquid line to prevent uncondensed vapor penetrating into returning liquid when the tempera- ture difference exists between two radiators. In the case that the heat load at both evaporators were 40/40 W and one radiator was heated, the flow regulator1 can tolerate the 160 W of heat load which was supplied to radiator1 while the flow regulator2 can tolerate the 100 W of heat load which was supplied to radiator2. To demonstrate the multiple loop heat pipe’s startup behavior at lowheat load, each of the compensation chamber was preheated to change the initial distribution of liquid and vapor in the evaporator and compensation chamber, in the result, each evaporator can start up at 5W through preheating.

Microscale Infrared Observation of Liquid-Vapor Phase Change Process on the Surface of Porous Media for Loop Heat Pipe

Loop Heat Pipe (LHP) performance strongly depends on the performance of a wick that is porous media inserted in an evaporator. In this paper, the visualization results of thermo-fluid behavior on the surface of the wick with microscopic infrared thermography were reported. In this study, 2 different samples that simulated a part of wick in the evaporator were used. The wicks were made by different two materials: polytetrafluoroethylene (PTFE) and stainless steel (SUS). The pore radii of PTFE wick and SUS wick are 1.2 μm and 22.5 μm. The difference of thermo-fluid behavior that was caused by the difference of material was investigated. These two materials include 4 different properties: pore radius, thermal conductivity, permeability and porosity. In order to investigate the effect of the thermal conductivity on wick’s operating mode, the phase diagram on the q-keff plane was made. Based on the temperature line profiles, two operating modes: mode of heat conduction and mode of convection were observed. The effective thermal conductivity of the porous media has strong effect on the operating modes. In addition, the difference of heat leak through the wick that was caused by the difference of the material was discussed.

Development and Investigation of a Miniature Copper-Acetone Loop Heat Pipe with a Flat Evaporator

The paper presents the results of development and investigation of a copper miniature loop heat pipe (LHP) with acetone as a working fluid. The device was equipped with a flat evaporator measuring 80 × 42 × 7 mm and vapor and liquid lines with an outside diameter of 3 mm, whose lengths were 145 mm and 175 mm, respectively. The LHP was tested at heat loads from 5 W to 60 W, different orientations in the gravity field and heat-sink temperatures from -40°C to +50°C. It is shown that the LHP retains its efficiency at all testing conditions. It is also mentioned that at a heat-sink temperature of +50°C the device operates in the mode of constant conductivity in the whole range of heat loads, and in this case a minimum thermal resistance of the “heat source-heat sink” system equal to 0.16°C/W is achieved, which is independent of the LHP orientation in the gravity field.

Numerical Study of Thermal Performance of a Capillary Evaporator in a Loop Heat Pipe with Liquid-Saturated Wick

Heat transfer of a capillary evaporator in a loop heat pipe was analyzed through 3D numerical simulations to study the effects of the thermal conductivity of the wick, the contact area between the casing and the wick, and the subcooling in the compensation chamber (CC) on the thermal performance of the evaporator. A pore network model with a distribution of pore radii was used to simulate liquid flow in the porous structure of the wick. To obtain high accuracy, fine meshes were used at the boundaries among the casing, the wick, and the grooves. Distributions of temperature, pressure, and mass flow rate were compared for polytetra-fluoroethylene (PTFE) and stainless steel wicks. The thermal conductivity of the wick and the contact area between the casing and the wick significantly impacted thermal performance of the evaporator heat-transfer coefficient and the heat leak to the CC. The 3D analysis provided highly accurate values for the heat leak;in some cases, the heat leaks of PTFE and stainless steel wicks showed little differences. In general, the heat flux is concentrated at the boundaries between the casing, the wick, and the grooves;therefore, thermal performance can be optimized by increasing the length of the boundary.

Investigation of Geometric Factors of Convex Platforms in the Flat Evaporator of Loop Heat Pipes

This paper investigated the influence of geometric factors of vapor groove structures on the performance of flat evaporator of a loop heat pipe system. COMSOL multiphysics software was employed to simulate the heat transfer in the evaporator with convex platforms of different shapes,sizes and area ratios(φ)between convex platforms and the heated surface. The maximum temperature and temperature distribution of each model were obtained. The results showed that the decrease of the size of platforms and the increase of φ can lower temperatures and improve temperature distribution homogeneity of the heated surface. Compared with circle and oval platforms,square platforms achieved lower temperature. The results also indicated that φ had the most significant impact on the performance of the evaporator.

Experimental Investigation on the Effection of Flow Regulator in a Multiple Evaporators/Condensers Loop Heat Pipe with Plastic Porous Structure

Multiple loop heat pipes which have two evaporators and two condensers in one loop are a kind of active heat transfer device. Since they have two evaporators and two condensers, the operating mode also becomes multiple. This work discusses the cases that multiple loop heat pipes were operated with one condenser at high temperature and the other at low temperature. To avoid the high temperature returning liquid and keep the multiple loop heat pipes work properly, the flow regulator which was made of polyethylene was designed, fabricated and applied in this test. The effect of flow regulator was confirmed and analyzed. In the test that large temperature difference existed between two sinks, it can be found according to the result that the flow regulator worked effectively and prevented the high temperature vapor to enter the inlet of common liquid line, which can keep the evaporators and returning liquid to operate at low temperature. With the increment of heat loads and the temperature difference between two sinks, the pressure difference between two condensers became larger and larger. When the pressure difference was larger than the flow regulator’s capillary force, the flow regulator could not work properly because the high temperature vapor began to flow through the flow regulator. According to the test data, the flow regulator can work properly within the sinks’ temperature 0°C/60°C and the two evaporators’ heat load 30/30 W.

Testing of a Low-Cost Loop Heat Pipe Design

This paper presents and describes the test campaign of a low-cost Loop Heat Pipes (LHP) design. LHP have been around for many decades now. Their potential as passive heat transfer devices has been widely demonstrated in numerous both ground- and space-based applications. One of the major disadvantages of LHP is their inherent high manufacturing cost;this is the main factor why LHP are still confined to niche/high end applications such as thermal management of spacecrafts. This paper proposes to use an alternative manufacturing design for the LHP evaporator, which is the main contributor to the overall LHP cost. Preliminary thermal results are also reported and briefly explained. Future work is needed to confirm the promising results discussed in this paper and address fully other issues such as tolerance of this LHP design to vibrations and accelerations typical of space missions.

Operating Characteristics of Multiple Evaporators and Multiple Condensers Loop Heat Pipe with Polytetrafluoroethylene Wicks

This paper presents fabrication and testing of a multiple-evaporator and multiple-condenser loop heat pipe (MLHP) with polytetrafluoroethylene (PTFE) porous media as wicks. The MLHP has two evaporators and two condensers in a loop heat pipe in order to adapt to various changes of thermal condition in spacecraft. The PTFE porous media was used as the primary wicks to reduce heat leak from evaporators to compensation chambers. The tests were conducted under an atmospheric condition. In the tests that heat loads are applied to both evaporators, the MLHP was stably operated as with a LHP with a single evaporator and a single condenser. The relation between the sink temperature and the thermal resistance was experimentally evaluated. In the test with the heat load to one evaporator, the heat transfer from the heated evaporator to the unheated evaporator was confirmed. In the heat load switching test, in which the heat load is switched from one evaporator to another evaporator repeatedly, the MLHP could be stably operated. The loop operation with the large temperature difference between the heat sinks was also tested. From this result, the stable operation of the MLHP in the various conditions was demonstrated. It was also found that a flow regulator which prevents the uncondensed vapor from the condensers is required at the inlet of the common liquid line when one condenser has higher temperature and cannot condense the vapor in it.

Experimental Study on Cooling Down Process of a Nitrogen-Charged Cryogenic Loop Heat Pipe

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.

Visualization Investigation on Temperature Oscillation and Two-Phase Behaviors of A Flat Loop Heat Pipe

To better analyzing the temperature oscillation and the two-phase behavior inside a flat loop heat pipe,visual studies were conducted.Under the 20℃ 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.

Experimental Study on a Multi-Evaporator Loop Heat Pipe with a Dual-Layer Structure Condenser

A loop heat pipe(LHP)is a kind of passive heat transfer device that uses the latent heat of the working fluid and the capillary forces of the capillary wicks.It demonstrates high heat transfer efficiency,long-distance heat transfer,and high pipeline flexibility.The multi-evaporator loop heat pipe(MeLHP)is a special loop heat pipe with multiple evaporators so that heat collection and emission from multiple heat sources can be achieved.In this paper,a new type of the multi-evaporator loop heat pipe prototype with a dual-layer condenser was designed,which can ensure the uniform and symmetrical layout of pipelines.The working temperature was 20℃,and propylene was used as the working fluid.The performance of the same evaporator in a single-loop LHP was considered as a reference.The experiment was conducted under two heating modes,i.e.single-evaporator heating and multi-evaporator heating,and the working stability of the prototype was verified by applying periodic heating power change and adverse elevation condition.It was observed that the prototype can be successfully started in different heating modes with a heat transfer limit of 230 W.In the test,the four loops were different in heat transfer limit due to the differences of flow resistance,and less power distribution to the loop with lowest heat transfer limit was considered to be beneficial to the prototype’s performance.Meanwhile,the prototype showed good heat sharing characteristic as the maximum temperature difference is low(smaller than 2 K in single-evaporator heating mode and 0.5 K in multi-evaporator heating mode).The prototype was of good operational reliability and found to be adaptable to the adverse elevation and cyclic variation of the heating power to a certain extent.

Experimental investigation on operating behaviors of loop heat pipe with thermoelectric cooler under acceleration conditions

An experimental study was carried out in this article to investigate the transient operating performance of a Dual Compensation Chamber Loop Heat Pipe(DCCLHP) with Thermoelectric Cooler(TEC) under acceleration conditions and ammonia was selected as the working fluid.For the purpose of comparison, experimental work was conducted under terrestrial gravity.Sensitivity analysis was performed to explore the effect of several control parameters such as the heat load, acceleration magnitude and TEC assist on the startup and operating performance of the DCCLHP.Experimental results indicate that the DCCLHP can get to a steady-state operation when the heat load changes from 25 W to 300 W under terrestrial gravity.While under acceleration conditions, the DCCLHP can work at a high operating temperature or even fail to operate, which shows the acceleration effect plays a significant impact on the loop operation.The TEC assist with power of 10 W can improve the operating performance and reduce the operating temperature for the case of small heat load and acceleration magnitude.When the acceleration exceeds 3 g at large heat load, the effect of TEC assist on the operation at large heat load can be ignored.

Experimental Investigation and Visualization Study of the Condensation Characteristics in a Propylene Loop Heat Pipe

At present,the structural design of a loop heat pipe(LHP)condenser mostly relies on experience.To analyze the condensation flow pattern in an LHP and to find a more accurate empirical correlation that can better guide condenser design in existing empirical correlations,a test system was designed and fabricated to observe the condensation phenomenon in the condenser and to evaluate the LHP’s heat transfer performance.Since propylene has a wide temperature range to different temperature control requirements,it was chosen as the working fluid.In this paper,the condensation process in the condenser was observed,and the operating parameters of a propylene LHP were measured at 283 K.The condensation flow patterns in the 2.5 mm×2.5 mm channel mainly included stratified flow,wavy flow and intermittent flow.By comparing the experimental results with four typical condensation flow regime maps,it was found that the Breber and Cavallini flow regime maps can successfully predict most of the condensation flow patterns of the LHP even with ultra-small mass flux.In addition,four condensation heat transfer coefficient correlations were selected to compare the predicted values of heat transfer coefficient with the experimental results.The study showed that the predicted values from the correlation by Cavallini et al.matched 90%of the experimental values well with an error of less than 20%,and this correlation is recommended for highly accurate LHP design.

Numerical Study on Surface Wettability Gradient Enhanced Ultra-Thin Loop Heat Pipe

Loop heat pipes(LHPs),as high-efficiency heat dissipation components,are considered to be superior thermal conductors beyond any known materials.To apply LHPs to mobile electronics,a small,thin and compact system needs to be designed.However,with the trend of miniaturization,the heat transfer performance of LHPs degrades rapidly due to the significant increase of working fluid backflow resistance.This work aims to propose an effective solution to this problem.In this work,the surface wettability gradient(SWG)is introduced into the ultra-thin LHP,and the influence of SWG on mass and heat transfer performance is studied comprehensively by using a transient three-dimensional numerical model.It is observed that the SWG can significantly increase the vapor-liquid circulation efficiency and improve heat transfer performance.Numerical experiments have been performed to compare the two kinds of LHPs with and without SWG.At the heat load of 4–6 W,the start-up time for LHP with SWG is shortened by 11.5%and the thermal resistance is reduced by about 44.3%,compared with the LHP without SWG.This work provides a solution for the performance-degradation problem caused by miniaturization,as a numerical reference for experiments.

Experimental Investigation on Thermal Characteristics of Long Distance Loop Heat Pipes

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.

Development of sintered Ni-Cu wicks for loop heat pipes

The present study fabricated a series of capillary wicks for loop heat pipes(LHPs),using two different methods,the cold-pressing sintering and direct loose sintering,and experimentally investigated the effect of different methods,compositions and sintering parameters on their properties in terms of porosity,permeability and pore radius.Porosity and pore radius were measured by the Archimedes method and Scanning Electron Microscope(SEM),respectively.Permeability of the wicks was compared by calculation using empirical equation.Results show that capillary wicks were successfully fabricated by using two different methods;the optimal capillary wick was found to be sintered at 650℃ for 30min,using direct loose sintering technique,with 90% nickel and 10% copper.The wicks could reach the porosity of 70.07% and the permeability of 10-13m2 order,with mean pore radius of 0.54μm.

Experimental investigation on operating instability of a dual compensation chamber loop heat pipe

The operating instability of a dual compensation chamber loop heat pipe (DCC-LHP) including temperature hysteresis, reverse flow and temperature oscillation is described and explained in this paper. Test results indicate that the steady state operating temperature under the variable conductance mode is not the same during the power cycle tests with the same heat load, and it is lower during the power reduction cycle than that during the power increase cycle. Orientation has an effect on the heat load range when temperature hysteresis occurs, and the effect of power variation amplitude can be ignored. Reverse flow tends to occur in some of the startups at low heat loads, even if vapor existed in the vapor grooves initially, which is caused by a higher pressure inside the wick due to evaporation in the evaporator core or vapor penetration into it. Temperature oscillation tends to occur in some of the startups at low head loads or some steady-state operations at high heat loads. Especially when the compensation chamber with the bayonet through is above the evaporator, the incidence rate of temperature oscillation is high.