Transient microscale flow boiling heat transfer characteristics of HFE-7000

A detailed experimental study was conducted to identify the important parametric trends governing the temperature response of a microdevice to transient heat loads for flow boiling of HFE-7000. The microdevice consisted of a microgap etched on a silicon wafer and placed centrally over a thin-film heater deposited on a Pyrex wafer. A step change in heat flux and a rectangular pulse were applied to the heater. The effects of mass flux, heat flux (pulse amplitude), and pulse width on the heater temperature response and boiling dynamics were investigated in detail. Conditions at which onset of boiling occurred were identified and the repeatability of the boiling process was studied. Onset of boiling and the subsequent bubble dynamics was recorded with a high-speed video camera. Boiling initiated at very high wall superheat due to the smoothness of the heater surface and low surface tension of HFE-7000. At high heat fluxes, onset of boiling resulted in the formation of a vapor film on the surface and rapid heater temperature rise was observed. Time taken to initiate boiling decreased rapidly with increasing heat flux and then reached a constant value. The wall superheat at which boiling started increased with increasing heat flux and subsequently reached a constant limit.