液滴撞击加热壁面传热实验研究

Experimental study of heat transfer from droplet impact on a heated surface

本文采用高速摄像仪对水滴和乙醇液滴撞击加热壁面后的蒸发过程进行了实验观测,分析了液滴撞击加热壁面后的蒸发特性参数.实验中,两种液体初始温度均为20℃,不锈钢壁面初始温度范围为68—126℃.水滴初始直径为2.07 mm,撞击壁面时Weber数为2—44;乙醇液滴初始直径为1.64 mm,Weber数为3—88.结果表明,液滴受到重力、表面张力及流动性的影响,在蒸发过程的大部分时间内,水滴高度持续降低而接触直径几乎不变;蒸发后期,液滴发生回缩,水滴的接触直径、高度和接触角出现振荡现象.乙醇液滴的接触角随时间的增加呈现先减小随后保持不变的趋势,而接触直径和高度则持续减小,直到液滴完全蒸发.液滴蒸发总时长与液体物性和壁面温度有关,随壁面温度的升高而减小,与液滴撞击壁面时的Weber数无关.同时,随着壁面温度的升高,液滴显热部分占总换热量的比重增大,显热部分能量不可忽略,本文实验条件下得到水滴的平均热流密度为0.014—0.110 W·mm 2.

Droplets impact on surfaces exist widely in industrial equipments, such as spraying cooling, ink jet printing, oil drops impact on walls in combustion chamber, brine droplets impact on heat transfer tubes in horizontal-tube falling film evaporators etc. In particular, for the droplets impinging on heated surfaces, the contact scale and the heat transfer flux affect the cooling of the hot surfaces greatly. In this work, evaporation processes of water and ethanol droplets impact on a heated surface are observed using a high-speed digital camera with a capacity of 106 frames per second. The corresponding evaporation parameters including the contact diameter, the droplet height, the contact angle, and heat flux are analyzed. The initial liquid temperature keeps constant at 20 ？C, and the initial surface temperature varies in the range of 68—126 ？C. Diameters of single water droplets and ethanol droplets are 2.07 and 1.64 mm, respectively. The impact Weber number of water droplets ranges from 2 to 44 while that of ethanol droplets ranges from 3 to 88. The present results show that due to the coupled effects of gravity, surface tension, fluid flow and evaporation processes, the height of water droplets reduces continuously while the contact diameter almost does not change during the most part of evaporation time. In the later stage of evaporation, the contact diameter, height and contact angle of water droplets oscillate, mainly because of droplet retraction. The critical contact angle for water droplets retraction is in the range of 4？–8？. The contact angle of ethanol droplets first reduces and then remains constant, while the contact diameter and the height decrease continuously. The droplet evaporation time depends on liquid properties and the surface temperature, and the Weber number effect is minor. The evaporation time decreases with the increase in the surface temperature. At the same time, with increasing surface temperature, the ratio between the sensible heat and the total heat increases, and this part of heat cannot be neglected from the total heat transfer calculation. Based on the present experimental conditions, the average heat flux for the water droplets ranges from 0.014 to 0.110 W·mm？2 in this work.