Characteristics Of Loop Heat Pipe Research Articles

Operational Characteristics of Loop Heat Pipe in Microgravity and Normal Gravity Environments

Operational Characteristics of Loop Heat Pipe in Microgravity and Normal Gravity Environments

Steady-state performance analysis of loop heat pipe radiator in space reactor

The loop heat pipe has a good application prospect in the field of space reactor radiator because of its characteristics of the separation of gas/liquid pipelines and the long-distance efficient heat transfer. The comprehensive performance analysis of loop heat pipe radiator for space reactor is carried out by the heat balance calculation method based on energy coupling iteration proposed in this paper. The loop heat pipe radiator models of different configurations were developed using the mass and energy conservation theory. The models were validated by experimental data of the loop heat pipe under various heat loads. The model in this paper can correctly simulate the steady-state operation characteristics of loop heat pipe, and evaluate the design of loop heat pipe radiator for space reactor. The performance of loop heat pipe radiator was analyzed in this paper under various design conditions such as power, evaporation temperature, and heat sink temperature. The results show that the thermal operating parameters have significant effect on characteristics of loop heat pipe radiator. The design length of the condenser pipe was considered as an important factor to achieve steady state heat balance under different design power. Meanwhile, the operation characteristics of loop heat pipe radiator with different configurations were compared, which demonstrated that the performance of panel-type radiator was better than fin-type radiator.

Experimental study on the characteristics of loop heat pipe with modified carbon fiber felt wick

In order to achieve accurate and effective thermal management of high-performance electronic components, the performance of a small flat loop heat pipe was investigated. A flexible composite capillary wick material has been applied in a flat loop heat pipe. Surface modification of carbon fiber felt was carried out using arc spraying technology to make it a more suitable material for producing capillary wicks. The physical properties of the modified carbon fiber felts were determined by measuring their wettability, porosity, capillary force, and thermal conductivity. A small flat loop heat pipe assembled with carbon fiber felt wick was designed and fabricated, and experimental testing was conducted. The experimental data indicated that the performance of methanol was better than that of acetone and water. The loop heat pipe with a 60% filling ratio maintained the heat source temperature at 84.47 °C under a load of 120 W (30 W/cm2). Conditions where the heat source starts before the cooling system are more favorable for the quick and stable startup of the loop heat pipe. The experimental results demonstrated that the loop heat pipe with a modified carbon fiber felt wick exhibited excellent operational and heat transfer performance.

Experimental study on the influence of surface modification methods of carbon fiber felt on the operation and heat transfer characteristics of loop heat pipes

Under high heat flux, the heat leakage rate of the compensation chamber in a small loop heat pipe is large, which affects the operational stability and heat transfer efficiency of the loop heat pipe. It was proposed for the first time to locally modify the capillary wick thermal conductivity through surface modification, thereby reducing the heat leakage from the compensation chamber. By spraying a high thermal conductivity metal coating on the surface of a low thermal conductivity carbon fiber felt, the contradiction between the heat dissipation rate of the heat source and the heat leakage of the compensation chamber was solved. Static wicking height showed that the metal coating increased the suction force of the carbon fiber felt. Start-up and operation experiments showed that the modified wick significantly improved the start-up stability of the loop heat pipe and reduced the temperature of the heat dissipation surface. The loop heat pipe with a single-sided-coated wick showed the lowest heat dissipation surface temperature, thermal resistance, and heat leakage rate. The loop heat pipe with an aluminum single-sided-coated wick exhibited a heat source temperature of 82.81 ℃, a thermal resistance of 0.26 K/W, and a heat transfer coefficient of 26.44 kW/(m2·K) under a load of 150 W (37.5 W/cm2). This study has significantly contributed to the future application of loop heat pipes in high heat flux electronic cooling.

Study of heat dissipation characteristics of loop heat pipe with heat sink of composite material

In this study, a new heat dissipation device with a composite heat sink of a loop heat pipe (LHP) system was evaluated for efficient cooling of electronic components. To improve the heat dissipation ability of the high heat flux chip and its local hot spot area, LHP, which had a gas–liquid phase change as the main mechanism of heat dissipation, was selected to remove heat. The experimental results showed that the average temperature performance was improved after the graphene copper foil heat sink was fitted, and the maximum temperature under the heat loads of 80 W, 90 W, 100 W, and 120 W was reduced by 3.8 °C, 5.7 °C, 6.5 °C, and 7.7 °C, respectively, compared with without a heat sink. The hot spot temperature increased linearly with an increase in the inlet water temperature. Increasing the ambient temperature within a certain range had little effect on the heat-dissipation effect of the hot spots. To interpret the experimental results, the hot spot heat transfer performance of the evaporation section of the LHP with a composite heat sink was set up by the equivalent thermal resistance model. The CFD simulated results were in good agreement with the experimental values. This indicated that the heat sink significantly reduced the hot spot temperature and improved the cooling uniformity largely. When the transverse thermal conductivity of the heat sink is sufficiently high, the increase in longitudinal thermal conductivity is more effective in reducing the hot spot temperature than the transverse thermal conductivity. Moreover, it has been suggested that the hot spots and heating power play a major role in the actual electronic structure and heat dissipation design. In summary, the study proposes a valuable method for improving the uniform cooling of chips, which was conducive to the heat management of the system.

Experimental Study on Thermal Performance of a Loop Heat Pipe with Different Working Wick Materials

In loop heat pipes (LHPs), wick materials and their structures are important in achieving continuous heat transfer with a favorable distribution of the working fluid. This article introduces the characteristics of loop heat pipes with different wicks: (i) sintered stainless steel and (ii) ceramic. The evaporator has a flat-rectangular assembly under gravity-assisted conditions. Water was used as a working fluid, and the performance of the LHP was analyzed in terms of temperatures at different locations of the LHP and thermal resistance. As to the results, a stable operation can be maintained in the range of 50 to 520 W for the LHP with the stainless-steel wick, matching the desired limited temperature for electronics of 85 °C at the heater surface at 350 W (129.6 kW·m−2). Results using the ceramic wick showed that a heater surface temperature of below 85 °C could be obtained when operating at 54 W (20 kW·m−2).

Effects of External Heat Flux during Orbital Period of Spacecraft on Operating Characteristics of Loop Heat Pipe

Effects of external heat flux during orbital period on operating characteristics of loop heat pipe were discussed through a thermal vacuum experimental study based on the orbit of remote sensing satellite. Results demonstrate that changes external heat flux of orbit causes periodic fluctuation of temperature on the heat delivery surface of loop heat pipe (LHP), thus causing period changes of state of working media in the LHP. Changes of state of working media further lead to “backflow” of working media, thus causing adverse impacts on stable operation of LHP. The adverse effects of backflow on operation of LHP were verified through a thermal vacuum experiment. Research conclusions have high guidance to the in-orbit application of LHP.

Numerical and experimental investigation of the steady-state performance characteristics of loop heat pipes

A numerical model to analyze the steady-state operational characteristics of loop heat pipes (LHPs) is presented. The mathematical model uses a new approach and is developed in a way that it can be easily extended to a transient model. The LHP components are discretized into control volumes. The one-dimensional mass, momentum and energy conservation equations are solved by using a staggered grid. The model uses both single-phase and two-phase correlations to determine the heat transfer coefficient and friction factor for each control volume. All the fluid interfaces in the LHP is tracked by using a phase-check algorithm. Four main experimentally-determined accommodation parameters are considered in the model. Later, it is shown that two of these parameters can be ignored without a significant effect on the results. The model results are validated by comparing them with experimental values obtained from the two different LHPs and the predictive capability of the model is demonstrated. Finally, a detailed sensitivity analysis is presented to study the effects of various essential variables including the accommodation parameters on the model results.

Experimental analysis on the loop heat pipes with different microchannel evaporators

Thoroughly understanding the influence of evaporator design on the characteristics of loop heat pipes (LHPs) is of great significance to develop optimal microchannel structures for high-power microprocessors. In the current work, the parallel microchannel loop heat pipe (PMLHP) and the self-similar fractal microchannel loop heat pipe (SFMLHP) were designed, fabricated and experimentally analyzed from the aspects of filling ratio and inclined angle, and then the heat transfer performances were compared with the dendritic bionic microchannel loop heat pipe (DBMLHP) under the same experimental conditions. The results show that PMLHP had four operation phenomena similar to DBMLHP while SFMLHP only had two operation phenomena. The start-up performances of PMLHP and SFMLHP were the best when the inclined angle was 90°. When the inclined angle was fixed at 90°, the optimal filling ratios for PMLHP and SFMLHP to yield the optimal thermal resistance were rather different. Through comparative analysis, it can be concluded that the overall heat transfer characteristic of DBMLHP was the best of the three under specified conditions. However, the average temperature fluctuation range in DBMLHP evaporator was the largest in the stable operation. The results provide valuable guidance on the choice of better and more suitable evaporator structure for microchannel loop heat pipes.

Evaporation heat transfer characteristic of porous wick in an open capillary evaporator

An open capillary evaporator is proposed to study the evaporation heat transfer performance of porous wick. Both temperature and evaporating mass are measured in this system. Besides, the temperature and relative height of the supplied fluid can be adjusted. A composite porous wick is tested through the constant and variable heat load operation, and it reaches a maximum heat load of 320 W, which corresponds to a heat flux of 25.5 W/cm2. Two types of the evaporator wall temperature curve are observed, and the effect of the initial condition is considered. Intermittent fluid supply appears and causes a temperature overshoot of the evaporator wall. But continuous intermittent fluid supply and small amount of fast-replenishing fluid does not cause the temperature overshoot. The influence of adding water on temperature overshoot is also studied. The boiling and evaporation occur at different heat loads, and the two heat transfer modes coexist at moderate heat load. The transition in heat transfer modes leads to that the evaporation heat transfer coefficient first increases and then decreases with the increase of heat load. This study is helpful to analyze the evaporation heat transfer performance of porous wick and understand the operating characteristics of loop heat pipe.

Operating characteristics of an anti-gravity loop heat pipe with a flat evaporator that has the capability of a loop thermosyphon

This paper deals with the operating characteristics of a loop heat pipe (LHP) under 0.52-m anti-gravity conditions at a constant heat sink temperature of 80 °C. To clarify the operating characteristics, an LHP with a flat evaporator was designed and fabricated. Two kinds of wicks with different characteristics, a steel use stainless (SUS) wick and a polytetrafluoroethylene (PTFE) wick, were used for the experimental tests. During the preheating stage to set the sink temperature of 80 °C, the evaporator and condenser changed the roles passively, and the LHP behaved as a loop thermosyphon (LTS). The proposed LHP had at least the capability of LTS. However, when the heat load was applied to the evaporator, then the LHP was driven by capillary forces (i.e., LHP mode). Under the LHP mode, the steady-state temperature could be predicted using the proposed numerical model. Furthermore, we quantitatively verified that the increase in the heat leak (equivalent to the decrease in the evaporation) leads to an increase in operating temperature and as well to an increase in the minimum start-up heat load using the direct measuring of the flow rate in the LHP. Below the heat source temperature of 180 °C, the LHP with the SUS wick performed 700-W heat transport, and the thermal resistance was 0.073 K/W. For the PTFE wick, we obtained the heat transport of 650 W and the thermal resistance 0.071 K/W.

Investigation of thermal characteristics of a loop heat pipe in a wide range of external conditions

A study has been made of thermal characteristics of an ammonia loop heat pipe (LHP) with an effective length of 421 mm equipped with a cylindrical evaporator 10 mm in diameter in a wide range of external conditions. The slope angle was varied from −90° to +90°, the temperature of the liquid cooling the heat sink from 0 to +60 °C, and the liquid flow rate from 0.1 to 4 L/min. It is shown that the most favorable position an LHP is the horizontal one. At a cooling temperature of 0 °C and a flow rate of 4 L/min the maximum heat load reached 500 W. In this case the thermal resistance of the “heat source – cooling liquid” system was 0.147 °C/W. It is also shown that changes in the value of the maximum heat load at all slope angles did not exceed 19% of the maximum value. The greatest effect on the LHP maximum heat load, operating temperature and thermal resistance was exerted by the heat sink temperature and the efficiency of its cooling. LHPs tests in a real cooling system of 1U computer server demonstrated energy savings of 8%.

A lean approach of modeling the transient thermal characteristics of Loop Heat Pipes based on experimental investigations

A mathematical model capturing the steady-state and transient thermal characteristics of a Loop Heat Pipe (LHP) is presented. It is intended to provide valuable insights into the static and dynamic LHP system behavior, to be used e.g. for the design of sophisticated thermal control strategies by LHP end users. The model is set up in such a way, that its parameters are both physically meaningful and accessible to a determination based on a non-destructive and pragmatic experimental characterization of the considered LHP. The transient behavior of the LHP is predicted by the model based on separate calculation and update of the internal mass flow rates and phase distributions. It has been applied successfully to two different LHPs, as used in common satellite applications. Numerical results of the steady-state characteristics show an excellent agreement with corresponding experimental results, proving the reliability and transferability of the approach proposed. An in-depth transient analysis of one selected LHP is presented using characteristic heat load profiles, common for LHP space applications.

Development of composite wicks having different thermal conductivities for loop heat pipes

In this paper, a composite wick having different effective thermal conductivities for loop heat pipes is proposed. The composite wick is designed and sintered to have higher thermal conductivity on the side close to the vapor channels, and have lower thermal conductivity on the side close to the liquid in the compensation chamber in order to achieve better thermal performance for a loop heat pipe. The wick parameters including the effective thermal conductivity, porosity and pore radius have been measured and calculated. The composite wick is proved to be able to be sintered in one time by applying different ratio of Ni-Cu fine powders in different layers of a wick and desired features have been achieved. A preliminary test was performed by comparing the thermal characteristics of loop heat pipes with composite wick and without composite wick.

Operational characteristics of loop heat pipes with porous copper fiber sintered sheet as wick

Porous copper fiber sintered sheets (PCFSS) as wick are fabricated for loop heat pipes (LHP) using low-temperature solid-phase sintering technology and smooth and rough copper fiber as the manufacturing material. The operational characteristics of LHP with varied wick surface morphologies and filling ratios are analyzed. This study focuses on evaluating the following characteristics: thermal resistance characteristics under increasing and decreasing heat load patterns, evaporator surface thermal homogeneity, thermal inertia, and limit operational characteristics. Experimental results demonstrated that LHP with rough PCFSS exhibited lower thermal resistance. As compared to decreasing heat load conditions, relatively lower thermal resistance of LHP under increasing heat load conditions was obtained. The evaporator surface thermal homogeneity yielded improvement with rough PCFSS and increased filling ratio. A larger thermal inertia of LHP was observed when the low filling ratio was applied, and it will be improved when the filling ratio was increased and the smaller heat load variation intensity was selected. When rough PCFSS wick with 70% porosity and a deionized water filling ratio of 30% were selected, the LHP was able to effectively operate under 5–200W heat load conditions and yielded prompt response to heat load variation.

Experimental research on thermal characteristics of loop heat pipe with liquid guiding holes

In the present study, the thermal characteristics of a loop heat pipe (LHP) with four lines of liquid guiding holes (LGHs) distributed along the bayonet were experimentally investigated. The loop was made of aluminum with nickel wick, and the working fluid was water. Experiments were conducted on the start-up, transient performance and the temperature oscillation of the LHP with LGHs at the heat loads from 70 W to 220 W, and the tilt angles between the evaporator and compensation chamber (CC) from 0° to 10°. It was found that with the increase of tilt angle from 0° to 10°, the working temperature of the LHP increases by approximately 4.6–8.2% for the heat load less than 180 W, while decreases by 3.5% for the heat load over 180 W. Obvious temperature oscillations of every component of the LHP, especially the condenser inlet, were observed when the temperature of CC was changed and the tilt angle was decreased from 5° to 0°. Under the present experimental conditions, the thermal resistance of the LHP ranges from 0.31 to 0.59 °C/W.

Transient behaviors of loop heat pipes for alpha magnetic spectrometer cryocoolers

Startup and oscillation characteristics of loop heat pipes (LHPs) for alpha magnetic spectrometer (AMS) cryocoolers during thermal vacuum and thermal balance (TVTB) testing were determined. The mechanism of the physical process during LHP startup was analyzed in detail. Factors including liquid/vapor distribution in the evaporator, filling rate of the compensation chamber, and LHP elevation were investigated. Startup proved easier when vapor was present in the vapor grooves, whereas vapor in the evaporator core had an adverse effect. When the vapor grooves are liquid-filled, the startup process can be divided into three stages: heating of LHP components before superheating establishes, nucleate boiling after superheating is satisfied, and successful loop circulation. When the vapor grooves are vapor-filled, there are only two stages in LHPs: nucleate boiling and loop circulation establishment. Examination of the effect of compensation chamber filling rate on startup characteristics showed that reverse flow occurred when vapor was present in the compensation chamber and startup proved difficult. Filling rate of the compensation chamber should be sufficient to provide thorough wetting of the capillary structure, as otherwise wick dry-out may occur and prevent LHP startup. Temperature oscillations normally occurred at high sink temperature and/or heat load. Inclusion of a bypass valve was an effective measure to suppress temperature oscillations, because vapor at the condenser was not saturated when the bypass valve was open.

Temperature Oscillation of Loop Heat Pipe for AMS Cryocooler

Temperature oscillation characteristics of loop heat pipes (LHPs) for alpha magnetic spectrometer (AMS) cryocoolers during thermal vacuum and thermal balance (TVTB) testing were determined. Temperature oscillation only occurred for only one of eight LHPs. Temperature oscillations normally occurred at high sink temperature and/or heat load. Inclusion of a bypass valve was an effective measure to suppress temperature oscillations, because vapor at the condenser was not saturated when the bypass valve was open.

Study on start-up characteristics of loop heat pipe under low-power

A series of experiments based on a flat loop heat pipe under low-power have been conducted in this work. It is found that the LHP can start up unfailingly under low heat power ( Q = 6 W) and its thermal resistance is rather high in some cases of low-power. With the thickness of the sintered capillary interlayer increasing, the performance of start-up in the LHP becomes better because heat leak to compensation chamber is reduced and the temperature difference between compensation chamber and the evaporation room is increased. When compensation chamber in the LHP has a heat sink installed, the temperature difference and pressure difference between compensation chamber and evaporation room are augmented. As a result, it is beneficial to the improvement of start-up characteristics.

3D heat transfer analysis in a loop heat pipe evaporator with a fully saturated wick

A practical quasi three-dimensional numerical model is developed to investigate the heat and mass transfer in a square flat evaporator of a loop heat pipe with a fully saturated wicking structure. The conjugate heat transfer problem is coupled with a detailed mass transfer in the wick structure, and incorporated with the phase change occurring at the liquid–vapor interface. The three-dimensional governing equations for the heat and mass transfer (continuity, Darcy and energy) are developed, with specific attention given to the wick region. By comparing the results of the numerical simulations and the experimental tests, the local heat transfer mechanisms are revealed, through the obtained temperature distribution and the further derived evaporation rates along the liquid–vapor interface. The results indicate that the model developed herein can provide an insight in understanding the thermal characteristics of loop heat pipes during steady-state operation, especially at low heat loads.