Height effect on nucleation-site activity in microchannel convective boiling

Forced convection boiling in microchannels is studied experimentally under uniform heat flux boundary condition. Several microchannel heat sinks, ranging in height between 5 and 20 μm, have been fabricated using standard micromachining techniques with nucleation sites varying in size etched at the channels bottom surface. The heat flux was provided by a heater integrated at the device back side, while constant water flow rate was supplied by a syringe pump. Boiling curves of device temperature as a function of the input power have been measured for a variety of conditions. The points corresponding to the onset of nucleate boiling and critical heat flux (CHF) are clearly distinguishable. Furthermore, the boiling curves for increasing and decreasing heat flux exhibit a hysteresis loop. The activity of the nucleation sites is found to depend on the channel height. The critical size, above which nucleation sites are active, increases exponentially with the microchannel height, asymptotically approaching a theoretical value.