Pressure gradient modeling during subcooled boiling in high heat-flux and high mass flux systems

A historical overview of the methods used to predict the channel pressure drop in subcooled boiling is given in this study. It is recommended for the conditions of the Advanced Neutron Source reactor that the drift-flux model developed by Avdeev [1982-1988] be used to predict the local pressure gradient. This reactor has fuel cooling channels with a 1.27 mm gap, 80 mm span, and 0.507 m length. The average heat flux is 6 MW/m², the channel inlet temperature is 45°C, the nominal mass flux is 25,000 kg/m² s, and the exit pressure is 1.7 MPa. The model predictions using the recommended correlations are about 6% high on average with a standard deviation of 0.10 in the single-phase and partially developed subcooled boiling regions. When the fully developed boiling region is included, the average prediction of pressure drop is 13% high with a standard deviation of 0.12. Three mechanistic approaches are developed that predict the increase in wall shear due to bubble ebullition. These models are shown to produce trends consistent with existing data. It is also shown that the combination of high heat flux, high mass flux, and small equivalent diameter causes the bubble departure size and the laminar sublayer to be roughly equal based on conventional models. It is demonstrated experimentally that bubble ebullition at the channel surface of the THTL test section disrupts the oxide layer and creates a surface with a roughness scale two or three times the predicted laminar sublayer thickness. It is established that under low pressure and high Reynolds number flows, the onset of net vapor generation (ONVG) is a good approximation to the location of the minimum in the channel demand curve. Alternatively, ONVG precedes the minimum in the demand curve significantly for conditions of high pressure and low Reynolds number, as long as the flow Peclet number remains above 70,000.