Abstract:

In the nucleate boiling zone of the spray cooling, the impact of the droplets with the liquid film and the bubbles in the liquid film has an important effect on the process heat transfer. A two-dimensional numerical model of a droplet impacting liquid film with bubbles was established. The model using water as the cooling fluid was used to simulate the impact process and heat transfer laws. The results showed that when the We was 6.94 and the quantity of dimension one liquid film thickness was 0.5(corresponding droplet velocity 0.5m/s, liquid film thickness 1mm), the liquid film was not disturbed significantly, and the motion shape was similar to ripples. When the We increased to 111.11(corresponding droplet velocity 2m/s), the gauge pressure at the junction of the droplet and the liquid film reached 6000Pa, which became the driving force of the neck jet phenomenon and gradually developed into a crown splash. The existence of bubbles hindered contact between the droplet and the heating surface, but as the bubbles burst, the droplet directly contact the heating surface, which made the surface heat transfer coefficient near the impact point much larger than other areas. The peak value of the surface heat transfer coefficient increased with the decrease of the liquid film thickness, or with the increase of the droplet velocity. The research results can provide a basis for further research on spray cooling system.

Key words: droplet impact, spray cooling, two-phase flow, bubble entrainment, numerical simulation

摘要：

在喷雾冷却过程的核态沸腾区，液滴与液膜及液膜内气泡的撞击对过程传热有重要影响。本文建立以水为冷却工质的单液滴撞击带气泡液膜的二维数值模型，模拟研究过程现象和传热规律。结果表明，We为6.94、量纲为1的液膜厚度为0.5（对应液滴速度0.5m/s、液膜厚度1mm）时，撞击过程中液膜扰动不显著、运动形态近似波纹；当We增大到111.11（对应液滴速度2m/s）时，撞击过程中液滴与液膜接合处的表压达到6000Pa，成为颈部射流现象的推动力，并逐步发展形成冠状水花；撞击过程中气泡的存在会阻碍液滴与加热表面的直接接触，但随着气泡的破裂，液滴与加热表面直接接触换热，使撞击点附近表面传热系数远大于其他区域，提高了传热能力，且液膜厚度越小、液滴速度越大，表面传热系数峰值越高。研究结果可为喷雾冷却系统的进一步研究提供理论依据。

关键词: 液滴撞击, 喷雾冷却, 两相流, 气泡夹带, 数值模拟