Biporous Wick Research Articles

Experimental investigation of loop heat pipe with flat evaporator using biporous wick

Abstract In order to solve heat dissipation of electronic equipment, an experimental investigation was carried out on the thermal performance of a miniature stainless-steel-ammonia loop heat pipe (LHP) with a flat disk-shaped evaporator. A biporous wick made from nickel powder was used for developing the capillary force. Tests demonstrated that the device could start up at heat load as low as 2.5 W. Meantime, the maximum heat load the LHP could transfer reaches 130 W (heat flux 12.8 W/cm2) at the allowable evaporator temperature of below 60 °C. The LHP showed a very fast response to variable heat load and operated stably without obvious temperature oscillation. The evaporator surface has very high isothermality while the monitored temperature difference between the maximum and minimum value on the evaporator surface does not exceed 3 °C for heat load below 130 W. The operation modes of variable conductance and constant conductance are found in the whole tested heat load range. The total thermal resistance varies between 1.42 and 0.33 °C/W at heat load ranging from 10 to 130 W.

Operational characteristics of two biporous wicks used in loop heat pipe with flat evaporator

Two special biporous wicks are adopted in stainless-steel–ammonia loop heat pipes (LHPs) with flat evaporator to enhance their heat transfer performances. The experimental results demonstrate that thermal and hydraulic characteristics of the wick with porosity of 69% (in LHP 2) are better than that of the wick with porosity of 65% (in LHP 1). The maximum heat loads of LHP 1 and LHP 2 could, respectively, reach 120W (heat flux 11.8W/cm2) and 130W (12.8W/cm2) at the allowable evaporator temperature below 60°C. Meanwhile, they can start up at heat load as low as 2.5W. The LHPs show very fast and smooth response to heat load and operate stably without obvious temperature oscillation. The total thermal resistances of the LHPs vary between 1.47 and 0.33°C/W at heat load ranging from 10 to 130W.

Investigations of Biporous Wick Structure Dryout

Dryout in a heat pipe evaporator is caused by insufficient condensate supply through the wick structure. Dryout is generally considered a failure of the heat pipe operation. However, traditional dryout theory may not fully explain the heat and mass transport limitations in the biporous (biwick) wick structure due to new mass transfer mechanisms, such as liquid splash at high heat flux, and vapor bubble/jet occupation of liquid transport passages. This article investigates the dryout phenomenon in carbon nanotube (CNT) based biwick structure. The incipience and expansion of the dryout zone on the CNT biwick structure are visualized. Variation of the evaporator temperatures at various heat fluxes is measured to characterize the temperature responses on the biwick dryout. Results based on both visualization and measurement show that dryout of CNT biwick structures is affected by vapor flow induced droplet splash and vapor occupation of liquid transport passages, which reduces the liquid supply to the hottest region and creates a local dry zone. On the curves of heat flux versus the evaporator temperature, dryout can be defined as the appearance of the inflexion point during the heating period, and associated with the existence of a large temperature hysteresis in a heating and cooling cycle. Experimental measurement also shows that over 12% of the liquid by volume is lost without being phase changed, due to high-speed vapor flow induced liquid splash. Liquid splash and interactions between vapor and liquid flows also increase the pressure drop weight in the evaporator over the system loop and result in more notable heating area effect on biwick structures when compared with traditional monowick structures.

Development of biporous wicks for flat-plate loop heat pipe

Two different methods, cold pressing sintering and loose powder sintering, are adopted to fabricate the biporous nickel wicks for loop heat pipes (LHPs) in the present study. Porosity of the wicks is measured by Archimedes method and radius and distribution of pores is observed by Scanning Electronic Microscope (SEM), and permeability of wicks is calculated by empirical equation. The effect of different sintering method, proportion of pore former, and sintering temperature on the wicks is investigated experimentally. Result shows that wicks are successfully fabricated, the optimal wicks are found to be sintered at 700 °C, using cold pressing sintering method, with pore former content 30% by volume; these wicks could reach the porosity of 77.40%, the permeability of 3.15 × 10 −13 m 2, and have sufficient mechanical strength to meet the machining requirements. The effect of lathing and wire electro discharge machining on surface pores of wick is analyzed by SEM. In order to verify the performance of the biporous wick, a flat plate type of LHP is designed, fabricated and tested in this paper, and the results presents that the LHP can startup and run reliably under different heat loads.

Utilization of Advanced Working Fluids With Biporous Evaporators

A novel fluid for use as a working fluid in a heat pipe has been tested at UCLA. The fluid was discovered originally in use with a device consisting of a metal tube charged with the patented inorganic aqueous solution (IAS), which is evaporated when the tube is evacuated before use. According to the patent, this evaporation leaves a thin film that allows the tube to carry high heat flux loads with low temperature drop across the tube in a solid state mode. However, various experiments with these tubes have produced inconsistent results, and there are some questions as to whether the fluid is completely evaporated. The research on which this work is based is focused on testing whether the charging fluid will operate as the working fluid in a heat pipe, in order to determine the nature of the IAS fluid. A heat pipe apparatus was charged with a biporous wick in order to investigate if the fluid plays a role in heat transfer. There are extensive data for this experiment using water as the working fluid, which will be used to compare the two sets of results. Testing has shown a reduction of the superheat required to drive heat fluxes through a wick compared to water by approximately 40%. Some experiments have shown that the operating (temperature) range of the IAS is much larger than a standard heat pipe. It is theorized that the increase in performance of the IAS is due to an increased thermal conductivity of the wick and increased capillarity. If this fluid is proven to be effective, it would lead to more effective and tunable heat transfer devices.

ADVANCES IN BIPOROUS WICK DESIGN AND TESTING FOR THERMAL GROUND PLANES

Investigation of bi-porous wicks has yielded an effective method for increasing surface heat transfer when the heat flux is high. Tests at UCLA have been aimed at augmenting biporous wicks in an effort to maximize their performance in thermal ground plane devices (TGP). The researchers have developed methods to more closely simulate the working environment inside of a TGP as well as added a monoporous layer between the biporous wick and the heater interface. In this work, it will be shown that both these assertions have been proven experimentally.

Heat and Mass Transfer in Evaporator of Loop Heat Pipe

Investigation of heat-exchange processes in the evaporator of a loop heat pipe is important for the development of heat transfer devices with low thermal resistances. A two-dimensional mathematical model of the evaporator active zone is presented. Three modes of vapor generation in the wick have been examined, where each differs in the mechanism of the vapor phase formation and in the saturation of the capillary structure: 1) evaporation to the vapor grooves, 2) volumetric evaporation in the two-phase zone, and 3) volumetric evaporation in the two-phase zone separated from the heated wall of the evaporator by dried zones. Conditions identifying changes between modes have been formulated. Structural characteristics of the wick with different pore sizes have been taken into account. Using a numerical-analytical method, results were obtained for three copper loop heat pipes with biporous wicks, where the working fluid was water for one of the loop heat pipes and methanol for the other two. The heat-load dependent temperature drop between the evaporator wall and the vapor in the vapor grooves has been presented. Additionally, a comparative analysis of calculated and experimental results was performed.

Experimental study of biporous wicks for high heat flux applications

Biporous wicks are wicks with two distinguished characteristic pore sizes while monoporous wicks are wicks with a single characteristic pore size. In this work three monoporous and 19 biporous wicks were tested. Thermophysical properties for the biporous wicks were measured. Thin biporous wicks distinguish from thick biporous wick by mechanism of heat transfer that occurs inside the wick. Thin biporous wicks remove heat similar to monoporous wicks by evaporation from menisci formed inside pores at liquid–vapor–solid interfaces. Thin biporous wicks were found to reach higher critical heat flux (CHF) than monoporous wicks because they develop evaporating menisci not only on top surface of the wick but also inside the wick. Thick biporous wicks were found to reach even higher CHF than thin biporous wicks because they continue to operate although the vapor blanket (film boiling) exists on the heated surface. This is possible because the top layer of the wick continues to supply liquid to the evaporating menisci above the vapor blanket region and vapor jets form between large pores of the wick and vent the vapor out of the wick. It was also found that for thick biporous wicks operating at very high heat fluxes, the heat conducts radially into the wick. The best monoporous wick tested had CHF at 300 W/cm 2 (21 °C superheat), the best thin biporous wick tested had CHF at 520 W/cm 2 (50 °C superheat), and the best thick biporous wick tested had CHF at 990 W/cm 2 (147 °C superheat). Thick biporous wicks can be used for 600–1000 W/cm 2 applications where high superheats and heat spreading into the wick are acceptable. For applications below 600 W/cm 2 are recommended thin biporous wicks and for applications below 300 W/cm 2 are recommended monoporous wicks.

Effect of Wick Characteristics on the Thermal Performance of the Miniature Loop Heat Pipe

Two phase heat transfer devices based on the miniature version of loop heat pipe (LHP) can provide very promising cooling solutions for the compact electronic devices due to their high heat flux management capability and long distance heat transfer with minimal temperature losses. This paper discusses the effect of the wick properties on the heat transfer characteristics of the miniature LHP. The miniature model of the LHP with disk-shaped evaporator, 10 mm thick and 30 mm disk diameter, was designed using copper containment vessel and water as the working fluid, which is the most acceptable combination in electronic cooling applications. In the investigation, wick structures with different physical properties including thermal conductivity, pore radius, porosity, and permeability and with different structural topology including monoporous or biporous evaporating face were used. It was experimentally observed that copper wicks are able to provide superior thermal performance than nickel wicks, particularly for low to moderate heat loads due to their low heat conducting resistance. With monoporous copper wick, maximum evaporator heat transfer coefficient (hev) of 26,270 W/m2 K and evaporator thermal resistance (Rev) of 0.06–0.10°C/W were achieved. For monoporous nickel wick, the corresponding values were 20,700 W/m2 K for hev and 0.08–0.21°C/W for Rev. Capillary structure with smaller pore size, high porosity, and high permeability showed better heat transfer characteristics due to sufficient capillary pumping capability, low heat leaks from evaporator to compensation chamber and larger surface area to volume ratio for heat exchange. In addition to this, biporous copper wick structure showed much higher heat transfer coefficient of 83,787 W/m2 K than monoporous copper wick due to improved evaporative heat transfer at wick wall interface and separated liquid and vapor flow pores. The present work was able to classify the importance of the wick properties in the improvement of the thermal characteristics for miniature loop heat pipes.

Heat transfer analysis of a loop heat pipe with biporous wicks

Because the evaporative heat transfer of a wick structure in a loop heat pipe is exceedingly sensitive to the internal volume fractions of liquid and vapor phases, the purpose of this study was to investigate the evaporative heat transfer of various biporous wick parameters by controlling the particle size of pore former, the pore former content, and the sintering temperature. A statistical experiment was carried out to analyze the evaporative heat transfer of the biporous wicks and to understand the effects of the parameters more effectively. The statistical analysis indicated a clear and strong relationship between the effect of the pore former content and the evaporative heat transfer of a biporous wick. This is because the pore former content had a great influence on the probability of large interconnecting pores and an extended surface area for liquid film evaporation in a biporous wick. Experimental results also showed that, at the sink temperature of 10°C and the allowable evaporator temperature of 85°C, the evaporative heat transfer coefficient of the biporous wick, which reached a maximum value of 64,000W/m2K, was approximately six times higher than that of the monoporous wick.

A study of loop heat pipe with biporous wicks

The purpose of this study is to investigate the effects of various bimodal pore size distributions of biporous wicks for a loop heat pipe (LHP). The study was conducted following a statistical method using a two-level factorial plan involving three variables (particle size of pore former:74–88 and 125–149 μm Na2CO3, pore former content:20% by volume and 25% by volume, sintering temperature:700 and 750°C). Finally, the heat transport capability of the LHP between monoporous wicks and biporous wicks has been investigated. Experimental results show that, at the sink temperature of 10°C and the allowable evaporator temperature of 80°C, the heat transfer capacity of the better biporous wick achieved 200 W and the total thermal resistance was 0.31°C/W. The performance is enhanced about 60%, compared to a monoporous wick for 125 W and 0.53°C/W. Therefore, LHPs with biporous wicks are very attractive for high heat flux applications in the future.

Use of biporous wicks to remove high heat fluxes

Three monoporous wicks with average powder diameters 60 μm, 300 μm, and 600 μm and five biporous wicks with average cluster (μm) to powder (μm) ratios and wick thickness (mm) 600/60/4, 300/60/4, 600/40/1, 300/60/1, and 300/40/1 were tested with degassed distilled water. Dryout values for monoporous wicks were: 223 W/cm 2 at 37 °C superheat (60 μm), 209 W/cm 2 at 31 °C superheat (600 μm), and 222 W/cm 2 at 73 °C superheat (300 μm). Dryout values for biporous wicks were 236.5 W/cm 2 at 45 °C superheat (300/40/1), 160 W/cm 2 at 18 °C superheat (600/40/1), and 150 W/cm 2 at 32.5 °C superheat (300/60/1). Dryout of thick wicks was not seen. They were able to remove 202 W/cm 2 at 81 °C superheat (300/60/4), and 494 W/cm 2 at 128 °C superheat (600/60/4). The 300/40/1 wick showed the highest heat transfer coefficient, 10.4 W/cm 2 K, at 180 W/cm 2. For high heat flux removal is most desired 600/60/4 wick, which can continuously remove heat at almost constant heat transfer coefficient, 4.2 W/cm 2 K, without seeing a dryout. All wicks were tested at a partial pressure of the water vapor 0.1 atm.