Study of the Effect of the Reduced Pressure on a Mechanistic Heat Transfer Model for Flow Boiling of CO2 in Macroscale and Microscale Tubes

Abstract

The effect of the reduced pressure on flow boiling of CO2 in macroscale- and microscale-tubes was analyzed in this paper. First, flow boiling heat transfer at high and low reduced pressures were simulated using a general mechanistic heat transfer model for flow boiling CO2 to analyze the heat transfer mechanisms. Then, the simulated heat transfer coefficients were compared to an experimental database at the reduced pressure from 0.1332 to 0.9082 (the corresponding saturation temperature from -40.6 to 26.77 °C), the tube diameter from 0.529 to 9.52 mm, the heat flux from 2 to 72 kW/m2 and the mass flux from 100 to 1500 kg/m2s. The model predicts 81.3% of the data before the dryout inception within ±30% while it only predicts 21% of data in the dryout and mist flow regimes within ±30%. The poor prediction of the heat transfer in these two regimes are because the model does not capture the dryout inception and completion. Furthermore, the heat transfer mechanisms are discussed from the standpoint of the flow regime variations and unstable flow boiling phenomena in macroscale and microscale tubes. It is recommended that the model should be improved by considering the mechanisms at low and high reduced pressures.