Experimental study of bubble behaviors and CHF on printed circuit board (PCB) in saturated pool water at various inclination angles

Experiments were performed to investigate bubble behaviors and pool boiling Critical Heat Flux (CHF) on a thin flat rectangular copper heater fabricated on Printed Circuit Board (PCB), at various inclination angles. The surface inclination angles were 0°, 45°, 90°, 135°, and 180°. Results showed the Onset of Nucleate Boiling (ONB) heat flux increased with increasing heater orientation from 0° to 90°, while early ONB occurred when the heater faced downwards (135° and 180°). The nucleate boiling was observed to be unstable at low heat flux (1–21% of CHF) and changed into typical boiling when the heat flux was above 21% of CHF. The result shows the CHF decreased with increasing heater orientation from 0° to 180°. In addition, the bubble departure diameter at the heater facing upwards (0°, 45°, and 90°) was more prominent compared to that of the heater facing downward (135°). The nucleation site density also observed increased with increasing heat flux. Moreover, the departed bubbles with larger size were observed to require a longer time to re-heat and activate new nucleation sites. These results proved that the ONB, CHF, and bubble dynamics were strongly dependent on the heater surface orientation