Impact of a drop burst flow on a film flow cooling a hot surface

To better understand the drop impact on film flow in spray cooling, we study the impact of a drop burst flow on a film flow that is cooling a hot surface. Focus is put on the cooling enhancement by the drop burst flow. The drop burst flow is generated by periodically interrupting a drop train flow (formed by the breakup of a capillary jet) into a flow of drop groups. Each drop group is called a drop burst, and each burst contains a constant number of drops, which is called the burst size. The frequency at which drop bursts are generated and impact is called the burst frequency. An infrared camera is used to record the spatiotemporal temperature distribution on the surface during the impact of the drop burst flow. The temperature is measured to fluctuate at the burst frequency. The minimum temperature is found to be related to the burst size, while the peak-to-peak amplitude is related to the burst size and the time interval between bursts. The mean temperature is found to depend on the average number flow rate of the burst flow. Although the temperature fluctuates below the film cooling temperature, the heat transfer coefficient analysis shows that the local flow has recovered to the original film flow in case of long time interval between bursts. The impact of drop bursts generates liquid sheet rising upward around the impact area, thereby expelling coolant from local surface. Increasing drop velocity may reduce cooling enhancement due to the loss of coolant caused by the rising liquid sheet. The cooling performance is also related to the observed flow impact dynamics. (C) 2018 Elsevier Ltd. All rights reserved.