Heat transfer mechanisms of single droplet impacting onto flowing film

Abstract

The experimental study of heat transfer and film dynamics of droplet impacting onto flowing film using Novec-7100 as the cooling liquid was presented in this article. The droplet velocity and the liquid film velocity could be adjusted in this experimental study. The Reynolds number of the flowing liquid film varied from 1257 to 6290. In this study, the substrate of heater was Zinc Selenide (ZnSe), and the heating element was a thin layer of Indium Tin Oxide (ITO). The overflow plate was used to make the liquid film thickness fixed at 1 mm. The heat transfer mechanisms of flowing liquid film (flowing film only), droplet impinging on the dry surface (droplet-dry surface) and droplet impinging on the flowing liquid film (droplet-flowing film) were discussed in this paper respectively. The results showed that when the averaged temperature of the heated surface does not reach the liquid saturation temperature, bubbles appear in the liquid film, and the bubbles are more dense downstream. When the averaged heat flux of the heated surface ranges from 6.2 × 103 W/m2 to 3.4 × 104 W/m2, the averaged heat transfer coefficient of the droplet-flowing film can be calculated by the superposition of the averaged heat transfer coefficient of the flowing film only and the averaged heat transfer coefficient of the droplet-dry surface, the maximum error is 14%. When the averaged heat flux of the heated surface ranges from 3.4 × 104 W/m2 to 6.8 × 104 W/m2, the Reynolds number of the flowing liquid film and the effect of the coupling between droplet impingement and flow boiling should be considered in the calculation of the averaged heat transfer coefficient of the droplet-flowing film. In addition, boiling heat transfer is the main heat transfer mechanism of droplet impinging on thin liquid film when the averaged heat flux of the heated surface ranges from 2 × 104 W/m2 to 6.8 × 104 W/m2.