Experimental Study Of Flow And Heat Transfer During Droplet Impact On Heated Surface

Due to the high heat and mass transfer capabilities, the process of a droplet impacting on heated surface and its vaporization process is widely applied in the industry. Besides, the droplet impact on heated surface contains a series of complicated and specific physical changes which is different from the droplet impact on the normal temperature surface. In this work, in order to study the flow and evaporation characteristics, a systematic research was carried out for a droplet impacting on heated surface.The droplet impact on heated wall was observed using a high speed camera, and the results were analyzed. At relatively low wall temperature, the rise of droplet Weber number can not only increase the contact diameter, but also ahaed the time of largest contact diameter. When Weber number is at a high level, non-dimensional contact diameter has a trend of increasing first then decreasing later with the increasing temperature in the process of spreading. Experience formula in function of the oscillation frequency of the non-dimensional height and the Weber number is proposed. The results show that droplets rebound after impacting under the high initial wall temperature condition. The non-dimensional contact diameter is affected mainly by the droplet impact velocity and the maximum non-dimensional contact diameter increases linearly with impact velocity.The evaporation after the droplet impacting on the heated wall was analyzed, and the results show that, the height and contact angle of droplets with larger surface tension coefficient reduces continuously while the contact diameter is almost invariable during initial stage of the evaporation process. In the later stage of evaporation, the contact diameter, height and contact angle of water droplets oscillate. The critical receding contact angle for water droplets ranges in 4-8°. The contact angle of droplets with smaller surface tension coefficient reduces firstly 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, but the Weber number effect is minor. The evaporation time decreases with the increment in the surface temperature. The addiction of surfactant can enhance the rate of evaporation. 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 1.4 x 104 to l.lX105 W/m2 in this work.The impact point transient temperature was measured using thin-film thermocouple, it is found that the wall temperature drop decreases with the increasing impact velocity during initial stage of the impact. Thereafter, the influence of impact velocity on wall temperature gradually disappear with time. The wall temperature drop is determined by the droplet property and decreased with the increasing surface temperature and the increasing droplet size, while the maximum temperature drop decreases with the increasing Weber number. The liquid surface tension influences evaporation process by changing the shape of droplet and contact area during evaporation. In addition, droplet with smaller sensible heat and latent heat of vaporization can maintain its temperature to balance the wall temperature. The droplet with smaller surface tension coefficient or latent heat of vaporization has a little influence on the evaporation time when its droplet size changes. Under the similar condition of the droplet flow, the droplet with greater latent heat of vaporization has larger wall temperature drop on the wall...