Critical heat flux in multi-microchannel copper elements with low pressure refrigerants

New saturated critical heat flux (CHF) data have been obtained experimentally in two different multi-microchannel heat sinks made in copper with three low pressure refrigerants (R134a, R236fa, R245fa). One of the test sections has 20 parallel rectangular channels, image wide and image deep while the second has 29 channels, image wide and image deep. The microchannels are 30 mm long in the flow direction where a 20 mm length in the middle is heated with an electrical resistance deposited on a silicon plate. Base CHF values were measured from 37 to image for mass velocities from 100 to image. When increasing the mass velocity, CHF was observed to increase while the rate of increase was slower at high velocities. While CHF increased moderately with large inlet subcooling (e.g. 20 K) in the image channels, inlet subcooling seemed to play less a role as the channel size decreased. CHF showed reversed tendency with increasing inlet saturation temperature image depending on the flow condition and the channel size. The experimental data were compared with existing prediction methods. The data demonstrated good agreement with several predictive methods using the heated equivalent diameter image and the actual mass velocity image to implement the circular tube correlations.