Prediction of the critical heat flux in water subcooled flow boiling usinga new mechanistic approach

A thorough examination of the results of existing models based on the liqui d sublayer dryout theory suggested the need to postulate a new mechanism to predict the CHF in subcooled water flow boiling. Considering that we have local boiling with bulk subcooled conditions, there will be a distance from the wall at which the fluid temperature is equal to the saturation value. This distance is called 'superheated layer', and is the only region where a bubble may exist. Because of the accumulation and condensation of the vapo ur generated from the heated wall, a thin elongated bubble, called a 'vapou r blanket', is formed, rising along the near-wall region as vertical distor ted vapour cylinders. The CHF is postulated to occur when the vapour blanke t replenishes the superheated layer, coming into contact with the heated wa ll (superheated layer vapour replenishment model). The vapour blanket thickness, assumed to be equal to the bubble diameter at the wall detachment, is independent of the heat flux, depending on physica l properties, thermal-hydraulic and geometric parameters. The superheated l ayer depends on the heat flux, physical properties, thermal-hydraulic and g eometric parameters. The heat flux for which the superheated layer is equal to the vapour blanket thickness will be the CHF. The comparison of new model predictions with fusion reactor relevant data ( 0.1 less than or equal to p less than or equal to 8.4 MPa, 0.3 less than or equal to D less than or equal to 25.4 mm, 0.0025 less than or equal to L l ess than or equal to 0.61 m, 1 less than or equal to G less than or equal t o 90 Mg m(-2) s(-1), 25 less than or equal to Delta T-sub,T-in less than or equal to 255 K) is pretty good, as more than 85% of the 1968 data are pred icted within +/-25%, with a standard deviation of +/- 16.6%. Besides, becau se of its structure, based on the heat balance method, the model is applica ble to both peripheral uniformly and non-uniformly heated channels. (C) 199 8 Elsevier Science Ltd. All rights reserved.