The heat transfer characteristics for two working fluids, a 50–50 ethylene glycol—water mixture and water, on a small flat heater in a rectangular flow channel were examined for operating conditions in typical internal combustion engines. A particular effort was made to present robust heat transfer data without the constraints of the cross-section of the channel in the regions of forced convection and nucleate boiling, including the onsets of nucleate boiling. The effects of velocity, pressure, and subcooling were discussed using both boiling curves and visual observations.

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A 20-mm-width × 30-mm-length flat copper heater in a 30-mm-width × 20-mm-height cross-sectional rectangular channel was used to simulate the forced convection flow and subcooled flow boiling in standard engine cooling systems. To visualize the effects of velocity, subcooling, and pressure, a high-speed camera was used. The subcooling ranged from 10°C to 50°C, the velocity from 0.5 m/s to 4 m/s, the pressure from 1 atm to 2.6 atm, and the heat flux from 0 W/cm2 to 80 W/cm2. The measured forced convection and boiling curves of water were presented along with high-speed photographs.

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