The use of oscillating heat pipe to the aeronautic world is of relevance due to its high rate of heat transfer, small size and low cost. Hence, a good understanding of its overall performance is needed. Previous research performed on studying its overall performance has included findings on its operating limit and optimal filling ratios. However, more effort needs to be done as these researches have not provided a comprehensive knowledge of optimum heat transfer capabilities of the oscillating heat pipes.
Naoko Iwata and Hiroyuki Ogawa from Japan Aerospace Exploration Agency and Yoshiro Miyazaki from Thermal Control Laboratory Inc., discussed the impact of operating limit at certain operating temperatures via the usage of HFC-134a as the working fluid at various filling ratios when passed in a 15-turn oscillating heat pipe. The research is now published in Journal of Heat Transfer.
The results of the experiments conducted on the oscillating heat pipe when set in a thermostatic chamber indicated that the values observed at maximum heat transfer due to changes in ambient temperature at similar filling ratios was only dependent on changes in cooling section temperature. While the other section, i.e. heating section temperature had constant temperatures at the operating limit at similar filling ratios as the ambient temperature varies. This result shows that the heating section temperature determines the operating limit of oscillatory heating pipes.
The authors also observed that at high filling ratios of 61.wt% and 85.wt%, the calculated liquid volumes at the operating limit reach and surpassed the total volume of the oscillating heating pipes respectively. The liquid volumes were estimated by the weight of the working fluid and the working fluid is assumed to be in saturation in OHP. The operating temperature at the operating limit with filling ratios of 61.wt% was 970C while that of 85.wt% was 670C as a result of saturation conditions. Further results, however indicated the occurrence of highest operating temperature of 1010C at the operating limit at filling ratio of 50.wt%. The high filling ratio also favored the increase of liquid volume in the oscillating heat pipes as the heat section temperature increases.
At the low operating limit of filling ratios; 45.wt% and 41.wt%, the authors noticed that the total liquid volume decreases as a result of evaporation when the heating section temperature increases. At a certain point, the liquid evaporates totally, leading to the sudden failure of the oscillating heat pipes.
Visualization experiments on a modeled oscillating heat pipe with the same working fluid coupled with different filling ratios and heat input backed the results gotten in view of the changes in liquid and vapor phases of the working fluid. The authors also provided the thermodynamic process of the oscillating heat pipes with the use of a pressure-volume diagram of the working fluid.
This study provided the relevant knowledge at which the oscillating heat pipes can have an improved maximum heat transfer.
Iwata, N1, Ogawa, H2, Miyazaki, Y3. Maximum Heat Transfer and Operating Temperature of Oscillating Heat Pipe, Journal of Heat Transfer 138 (2016) [DOI: 10.1115/1.4034054].Show Affiliations
- Japan Aerospace Exploration Agency, 2-1-1, Sengen, Tsukuba 305-8505, Ibaraki, Japan.
- Japan Aerospace Exploration Agency, 3-1-1, Yoshinodai, Chuoku, Sagamihara 252-5210, Kanagawa, Japan.
- Reinetsu, 7-6-13-608, Bunkyo 910-0017, Fukui, Japan.
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