Thickness measurements of the thin film in spray evaporative cooling

With electronic circuits becoming increasingly powerful, the limiting factor in their design is frequently related to thermal management. Spray evaporative cooling (SEC) has the capability to remove very high heat fluxes with a low fluid flow rate and can be packaged in a compact design. In order to allow further development of SEC, fundamental mechanisms of heat removal must be understood. One important mechanism is conduction through the thin, two-phase film that develops on the surface. This paper presents measurements of the film thickness obtained using an optical method involving total internal reflection. Ethyl alcohol was dispensed using a full cone spray created by a pressure-swirl atomizer. Two spray nozzle designs were tested. Design 0 employed a single nozzle centrally located over each test die. Design 10 employed 5 nozzles arranged in an "X" pattern over each die. Transparent heaters dies were manufactured out of glass coated with a thin layer of metal. Average film thickness measurements for the dies were measured to be 221 μm. The film thickness measurements were compared with a numerical model of the film based on the special distribution of mass and momentum associated with the spray nozzles.