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, 467μm wide and 4052μm deep while the second has 29 channels, 199μm wide and 756μm 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 342W/cm2 for mass velocities from 100 to 4000kg/m2s. 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 H=4052μm channels, inlet subcooling seemed to play less a role as the channel size decreased. CHF showed reversed tendency with increasing inlet saturation temperature (10⩽Tsat⩽50°C) 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 DheDhe and the actual mass velocity GeqGeq to implement the circular tube correlations.