Channel height effects on forced-convection boiling and critical heat flux from a linear array of discrete heat sources

Due to the lack of understanding in the literature of channel height effects on forced-convection boiling and critical heat flux (CHF) in rectangular channels, experiments were performed with FC-72 at 1.36 bar for channel heights of 2, 5, and 10 mm. Boiling heat transfer from a linear array of nine discrete heat sources simulating microelectronic chips was investigated for velocity and liquid subcooling ranges of 13400 cm s’ and 3-36°C respectively. Unique to these experiments was the concentration of heat flux on a fraction of the channel perimeter corresponding to the width of the chip, with unheated fluid flowing on either side of the chips. Flow visualization in the 2 mm channel revealed that bubbles in near-saturated flow spanned the entire width of the channel, and increases in velocity and/or subcooling reduced the lateral spread of the bubble layer; on the other hand, bubbles in the 5 and 10 mm channels were primarily confined to the area just above the chips. Using the drift-flux model, an analysis for determining the void fraction in subcooled boiling was developed. Experimental data supported by void fraction predictions and flow visualization suggest the existence of an optimal channel height for which forced-convection CHF is a maximum. For equal flow velocities, maximum CHF values in the present study were obtained with the 5 mm channel.