真空度对脉动热管传热性能的影响

实验研究了真空度对以去离子水和Al_2O_3/H_2O纳米流体为工质的脉动热管传热性能的影响,其中真空度的变化范围在-0.014～-0.098 MPa之间。研究发现真空度对脉动热管的启动和加热功率大于90 W的稳定运行过程的传热均有重要影响。蒸发段的启动功率随真空度的上升而降低。同时,加入Al_2O_3纳米粒子更有利于脉动热管的启动,其蒸发段启动热阻低于同等条件下去离子水的启动热阻。此外,加热功率大于90 W时,去离子水蒸发段的温度会发生剧烈的脉动,且随真空度不断上升,剧烈脉动的现象更加明显,原因可能是高真空度影响了液膜蒸发,使脉动热管的流型提前由弹状流转变到环形流。但加入Al_2O_3纳米粒子后并没有发生剧烈的脉动,可能是纳米粒子的加入延缓了这一现象的产生。

Vacuum effect on heat transfer performance of the pulsating heat pipe

An experimental investigation was conducted to explore vacuum effect on the heat transfer performance of pulsating heat pipe (PHP) charged with de-ionized water or Al2O3/water nanofluid. In the experiment, the vacuum degree ranged from - 0.014 MPa to - 0.098 MPa. Experimental results show that vacuum degree has an important effect on the starting-up and heat transfer performance of PHP as heat load is more than 90 W. The starting-up heat load decreases with the increasing of internal vacuum degree of the PHP. For nanofluid as working fluid, it can be found the starting-up thermal resistance of evaporation section is lower than that of the PHP charged with de-ionized water under the same conditions. Furthermore, when the heat load exceeds 90 W, the evaporation section temperature of the de-ionized water will fluctuate violently, and the phenomenon of intense pulsation will be more obvious with the increasing vacuum degree. One possible explanation is that the flow pattern in PHP changes from slug flow to annular flow in advance due to the effect of high vacuum on evaporation of thin liquid film with the increase of heat load. But there is no severe pulsation after adding Al2O3 nanoparticles. It may be that the nanoparticles delay this phenomenon.