An experimental investigation of droplet impact cooling at controlled surface temperatures

Significantly higher heat fluxes have been achieved by droplet impact cooling compared to conventional heat transfer methods such as pool boiling. Earlier experimental investigations of the droplet impact cooling were conducted by controlling the input power, i.e. heat flux, to the heated surface; water was generally used as the coolant. This thesis deals with experimental investigations of both pool boiling and droplet impact cooling at controlled surface temperatures with HCFC-123 as the coolant. The work has been focused on the heat transfer characteristics of pool boiling and a stream of mono-dispersed liquid droplets impinging on a heated upward facing surface. The heated surface consists of a 4 x 4 array of elements; the temperature of each element has been individually controlled with a current feedback circuit. The droplet impact experiments have been conducted at various droplet frequencies and liquid cooling. The boiling curves generated from experimental result show that the heat fluxes produced by droplet impact cooling are significantly higher than those for pool boiling; droplet impact heat fluxes of 40--50 W/cm2 have been achieved.; The film sensor/heater used in these investigations has the advantage of controlling surface temperatures by means of current feedback circuit. The temperatures of tested surface have been maintained in a required range including nucleate boiling and critical heat flux (CHF) regions. The most important aspect of this research was the development of a controlled temperature heat transfer surface and its application to boiling heat transfer research. Pool boiling data show that the heat transfer regimes clearly. The data for droplet impact cooling also show a clear response to droplet impact, followed by liquid film formation and evaporation. The droplet impact data show that liquid film conduction and evaporation dominate the heat transfer process. The data obtained in this investigation provide a better understanding of the processes taking place during droplet impact. The heated surface developed here will provide the means for more detailed examination of various boiling processes.