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Accepted: 2018-05-26, 資料番号: SA1180054000

Achieving a high level of pointing accuracy in spacecraft requires that vibration and thermal distortions not be relayed to the structure. Therefore, a flexible thermal strap is essential to dissipate heat from heat-and-vibration-generating components, such as cryocoolers. This study developed a flexible and highly conductive thermal strap for space application with a micro-Oscillating Heat Pipes (OHP). So far, few studies have focused on the flexibility of a micro-OHP, particularly that of the tube type. This paper presents the results of testing strap’s thermal performance and dynamic stiffness, which is a key parameter of flexibility but has been rarely evaluated for heat pipes, including OHP. To achieve the triaxial flexibility and resistance against space environment, the OHP consists of a 10-turn micro-OHP composed of metal tubes with an inner diameter of 0.4 mm or less. It is filled with HFC-134a as the working fluid. Ten electroformed check valves are mounted in the OHP. The liquid slug is induced to flow from the condenser to the evaporator, and the OHP operates stably even in a horizontal position. Thermal performance tests have shown the maximum thermal conductance of the strap to be 0.8 W/K and the heat transfer rate to be at least 7 W. The thermal vacuum test (TVT) showed that, below 20 ℃, the thermal conductance decreases with the condenser temperature and that the OHP does not operate below −15 °C. Dynamic stiffness tests were performed on the OHP to determine the dynamic stiffness in the two directions perpendicular to the flow (along the Y and Z axes). The stiffness parallel to the flow was obtained by a finite element methods analysis using a structural model of a bent OHP based on earlier test results. The dynamic stiffness is less than 0.2 N/mm in Y and Z axis, which is less than that of the graphite thermal strap used for space application. During and after the stiffness test, the OHP continued to operate stably without any change in the temperatures of the evaporator and condenser.

It is reported that the operating temperature of an oscillating heat pipe (OHP) at an operating limit is not dependent on the ambient temperature but that the maximum heat transfer is dependent on this. In this study, using different ambient temperature conditions, a 15-turn OHP filled with HFC-134a as a working fluid was operated until it dries out. The maximum heat transfer was found to vary with changes in the ambient temperature, but the operating temperature at an operating limit, which depends on the filling ratio (FR) of the working fluid, was found to be constant. At the operating limit, the operating temperature decreased with an increase in the FR when the ratio was greater than 50 wt.%. Visualization experiments and calculations were used to confirm that there is an increase in the liquid volume in the OHP in accordance with an increase in the heat input and that ultimately the OHP fills with the liquid, resulting in the failure of OHP operation. In contrast, at the operating limit, when the FR was less than 50%, the operating temperature increased in line with an increase in the FR. In this case, it is assumed that the volume of liquid slugs decreases as the heat input increases, thus causing the OHP to dry out. This theory is explained using a P–V diagram of the working fluid in the OHP. The OHP thermodynamic cycle reaches a saturated liquid or vapor line before it reaches a critical point if a specified volume is shifted from the specified volume at the critical point. The optimum FR for maximum heat transfer is therefore decided by the void ratio at the critical point of the working fluid.

資料番号: SA1004955000

Accepted: 2011-02-07, 資料番号: SA1002879000

A meter-scale Oscillating Heat Pipe (OHP) has been developed for the General Anti-Particle Spectrometer (GAPS) project. Two types of OHP routing, U-shaped and O-shaped, have been investigated. For the operation at low temperature, R410A was used as the working fluid. As the result of the investigation, we verified for the first time that both the meter-scale U-shaped and O-shaped OHPs can transfer heat under gravity in a wide temperature range between 20°C and -60°C. Generally, the O-shaped OHP showed better performance than the U-shaped OHP. Both OHP models showed good thermal conductance and a good amount of heat transport under the particular sets of conditions which meet the design requirements. In order to clarify the drive force to operate OHP to further improve the OHP design, the performance difference between the U-shaped and the O-shaped models has been interpreted in terms of the gravity effect and the pressure loss.

Meter-scale Oscillating Heat Pipe (OHP) has been developed for the General Anti Particle Spectrometer (GAPS) project. Two types of the OHP routing have been investigated. One is a U-shaped routing and the other is an O-shaped routing. For the operation at low temperature, R410A was used as the working fluid. As the result of the investigation, we verified for the first time that both the meter-scale O-shaped and U-shaped OHPs can transfer heat in gravity at low temperature. Between the heat input section and the radiator section, the thermal conductance larger than 10 W/K was achieved under particular sets of conditions. In terms of thermal characteristics, the O-shaped OHP showed better performance than the U-shaped OHP. The O-shaped OHP started up more quickly than the U-shaped OHP at low temperature. The temperature uniformity of the O-shaped-OHP heating section was also better without drying-out at around the top of the heating section. The U-shaped OHP could transfer heat at temperatures above around 240 K, and the O-shaped OHP could transfer heat at temperatures above around 210 K. We consider the O-shaped OHP utilizes the gravity to assist the circulation of the working fluid resulting in better performance.

Thermal performance of a 6m-long Oscillating Heat pipe (OHP) for balloon-borne General Anti-Particle Spectrometer (GAPS) experiment has been investigated. The developed OHP is a closed loop type and U-shaped due to the mission detector requirements. R410A is selected as the working fluid for the use at low temperature. We achieved a thermal conductance of 18.7 W/K for 150 W heat input with 16 turns at +20 deg-C. We also found for the first time that this large-scale OHP can be operated at low temperatures when the cooling section is cooled from +20 deg-C to -60 deg-C. In addition to the well-known advantages of OHP that it can be simply fabricated and has low sensitivity to the orientation, it is found that the OHP has an adjustability and flexibility to 3-dimensional alignment and wide temperature range.

The pressure oscillation in oscillating heat pipes is caused by mutual excitation between pressure and void fraction. The pressure oscillation forms a progressive wave traveling in the heat pipe. The wave velocity is determined by the mechanism of self excited oscillation. The amplitude and the frequency are determined by the heat transport performance, that is, the heat load and the heat transport length. Based on the theory, pressure oscillation characteristics are obtained as a function of design parameters and operating conditions of the heat pipe.

著者人数: 23名, 資料番号: SA1160215000

An oscillating heat pipe consists of a micro channel which turns many times between the heating section and the cooling section. Herein, a developmental study on flexible oscillating heat pipes for cooling notebook personal computers is presented. The heat pipe functions to transport heat from the CPU to the rear surface of the folding display, which serves as a radiating surface. The heat pipe tubes at the hinge are flexible so that the heat pipe may fold. In order to evaluate the thermal performance of the cooling system, flexible oscillating heat pipes were fabricated and tested. The heat pipes consist of copper capillary tubes and Teflon flexible tubes. Excellent thermal performance was obtained in the test: the thermal resistance was 0.3 K/W and the maximum heat transport capability was 100 W.Copyright © 2005 by ASME

This paper describes some experimental values of aerodynamic resistance of perforated plates in natural convection, providing basic thermal data necessary for designing electronic equipment casings. In the conventional measuring method which measures only in the higher region of Reynolds number, there is a disadvantage that the utmost precision cannot be attained in measurement due to the values of pressure loss and velocity being unidentifiably small. Io our new method, however, it is designed to evaluate a fluid resistance coefficient depending on the air temperature elevated by the combined effect of motive power of natural convection and fluid resistance and consequently to elucidate the influence of Reynolds number, coefficient of porosity, and thickness of plates/diameter of holes on resistance coefficient of perforated plates.