R134a低压闪蒸喷雾冷却实验研究

Experimental Investigation of R134a Flash Spray Cooling Under Reduced Pressure

低压闪蒸喷雾冷却技术是指压强和温度相对较高的液体工质经喷嘴雾化形成微细液滴,射入低压闪蒸腔中,并以较高的速度冲击高热流密度被冷却表面,依靠液体迅速相变吸热的一种新型、高效散热技术。本文通过实验研究了R134a冷却剂在低压和常压环境中的喷雾冷却规律。结果表明,低压条件下喷雾冷却换热能力随着压强降低而降低,但与常压工况对比存在强化或弱化两种可能。由于剧烈闪蒸带来的气液不稳定性,闪蒸喷雾冷却的流量波动对于表面温度波动和热流密度影响较大。随着热流密度的增大,R134a喷雾冷却存在液膜完全覆盖、部分干涸和周期性完全干涸三种现象,前两种现象对应于较低的温度波动,而后一种对应于较大的温度波动和恶化的传热。闪蒸喷雾冷却距离较远的大表面时,闪蒸导致的沿程制冷剂损失会导致冷却能力和冷却均匀性下降,在布置喷嘴和表面时应加以考虑。

Flash spray cooling is an efficient cooling technique in which liquid working fluid is atomized and sprayed onto a heated surface under reduced pressure conditions,cooling the surface with fast evaporation.The spray cooling heat transfer of R134 a are experimentally investigated under reduced and atmospheric pressures.The results show that the heat transfer at low pressure decrease with decreasing chamber pressure,while it may be better or worse than the atmospheric scenario.Due to the flash evaporation,flow instability may lead to severe temperature fluctuation and heat transfer decrease.Three regimes exist with increasing heat flux,namely,film covered,partial dry-out and unstable periodic fully dry-out cooling,with the former ones corresponding to low temperature fluctuation and the latter one corresponding to large temperature fluctuation.Besides,the cooling of large distant surface with low pressure flash spray may be not as efficient as atmospheric cases due to the loss of coolant along the spray by flash evaporation.