As faster computing speed has been increasingly demanded, the industry has been seeking for a newer cooling device with substantially improved cooling performance. One of the plausible and practical solutions can be a closed-loop with microchannels embedded within the chip. Both single phase convective heat transfer and two-phase flow boiling can be considered as the heat transfer mode in the microchannels, the latter having an advantage of larger temperature difference. Given the maximum allowed temperature in the chip for the safe and reliable operation and water being the fluid, the operating pressure should be subatmospheric for flow boiling heat transfer mode, i.e., 31 kPa for 70℃ of inlet water temperature.
The objective of the present work is to experimentally investigate the effect of operating pressure on flow boiling heat transfer and pressure drop in a single microchannel. The experimental apparatus consists mainly of Peristaltic pump, preheater, test section, and vacuum chamber for control of operating pressure. De-ionized water is used as the working fluid. The test section is a round microchannel of 310 μm inside diameter, made of 304 stainless steel.
The experiment has been performed for the conditions of heat flux from 35 to 86 kW/m2, mass flux from 203 to 305 kg/m2s ( 168 to 254 of liquid Reynolds numbers), and the average operating pressure from 11 to 19 kPa.
The measured flow boiling heat transfer coefficients in the microchannel were in the range of 3 to 27 kW/m2 and the experimental showed that the flow boiling heat transfer coefficients in microchannel were affected by the wall heat flux and the operating pressure, while slightly dependent on mass flux. One of the major findings was that the trend of the variation of the heat transfer coefficient in terms of wall heat flux (or vapor quality) was largely changed by the operating pressure. At the lower operating pressure (～ 11 kPa) the heat transfer coefficient increased as the wall heat flux increased, but at the higher operating pressure (～19 kPa ) it decreased as the wall heat flux increased. The cause of such opposing trends by the operating pressure can be attributed to the role of the liquid film in the flow boiling pattern in microchannels and the variation of relevant physical properties such as latent heat of vaporization.