Uniform Mems Chip Temperatures in the Nucleate Boiling Heat Transfer Region by Selecting Suitable, Medium Boiling Number Range

Abstract

The not only lower but also uniform MEMS chip temperatures can be reached by selecting suitable boiling number range that ensures the nucleate boiling heat transfer. In this article, boiling heat transfer experiments in 10 silicon triangular microchannels with the hydraulic diameter of 155.4 μm were performed using acetone as the working fluid, having the inlet liquid temperatures of 24–40°C, mass fluxes of 96–360 kg/m²s, heat fluxes of 140–420 kW/m² , and exit vapor mass qualities of 0.28–0.70. The above data range correspond to the boiling number from 1.574 × 10⁻³ to 3.219 × 10⁻³ and ensure the perfect nucleate boiling heat transfer region, providing a very uniform chip temperature distribution in both streamline and transverse directions. The boiling heat transfer coefficients determined by the infrared radiator image system were found to be dependent on the heat fluxes only, not dependent on the mass fluxes and the vapor mass qualities covering the above data range. The high-speed flow visualization shows that the periodic flow patterns take place inside the microchannel in the time scale of milliseconds, consisting of liquid refilling stage, bubble nucleation, growth and coalescence stage, and transient liquid film evaporation stage in a full cycle. The paired or triplet bubble nucleation sites can occur in the microchannel corners anywhere along the flow direction, accounting for the nucleate boiling heat transfer mode. The periodic boiling process is similar to a series of bubble nucleation, growth, and departure followed by the liquid refilling in a single cavity for the pool boiling situation. The chip temperature difference across the whole two-phase area is found to be small in a couple of degrees, providing a better thermal management scheme for the high heat flux electronic components. Chen's [1] widely accepted correlation for macrochannels and Bao et al.'s [2] correlation obtained in a copper capillary tube with the inside diameter of 1.95 mm using R11 and HCFC123 as working fluids can predict the present experimental data with accepted accuracy. Other correlations fail to predict the correct heat transfer coefficient trends. New heat transfer correlations are also recommended.

KEY WORDS:

• microchannels
• nucleate boiling heat transfer
• uniform chip temperature
• flow pattern

∗This article was supported by the National Natural Science Foundation of China (50776089), the Natural Science Foundation of Guangdong Province (7000742), and the National Key Laboratory on Bubble Physics and Natural Circulation of Nuclear Power Institute.