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Engine cooling systems contain multiple parallel flow paths, which under sub-cooled boiling conditions can experience excursive flow instability. This study presents an analytical model describing the pressure drop in a channel subjected to flow boiling. Using water as the working fluid, results are presented showing the effect of inlet velocity and surface heat flux on the overall pressure drop. The accuracy of this model for water has been verified. The calculations show that for typical engine operating conditions, the cooling passage can contain regions of partially developed and fully developed sub-cooled boiling.

The performance of 50/50 (vol) propylene glycol/water and ethylene glycol/water mixtures in the convective and forced flow boiling regimes has been evaluated and compared with water and previously published data for ethylene glycol/water mixtures. The fluid velocities examined ranged from 0.1 to 0.5 m/s at heat fluxes between 50,000 and 750,000 W/m2. The results indicate that the heat transfer of 50/50 (vol) propylene glycol/water coolants is slightly less effective in the convective regime compared to 50/50 (vol) ethylene glycol/water mixtures, but superior once nucleate boiling is the dominant mechanism. The experimental data showed good agreement with the predictions of the Chen correlation for boiling heat transfer to saturated fluids in convective flow. It is concluded that a 50/50 (vol) propylene glycol/water mixture will be at least as effective in providing cooling within an automobile engine as a 50/50 (vol) ethylene glycol/water mixture.