Effects of heater length and orientation on the trigger mechanism for near-saturated flow boiling critical heat flux—II. Critical heat flux model

Abstract

A critical heat flux (CHF) model is presented that accounts for both heater length and orientation effects. The model is verified with FC-72 data obtained for 10-, 30- and 110-mm long heaters that were flush-mounted in a 10-mm × 5-mm channel and orientations including vertical upflow, 45-degree inclined flow, and horizontal flow, with liquid flowing above the heaters. The inlet liquid velocity was varied between 25 and 200 cm s −1 while maintaining the inlet temperature constant at 4°C subcooling. CHF was found to decrease with heater length but showed little sensitivity to orientation. Formulation of the model was based on flow visualization and photomicrography of the vapor-liquid interface along the heaters. Just prior to CHF, a wavy vapor layer engulfed the heater, allowing liquid access only through wetting fronts where wave troughs touched the heater surface. The distance between wetting fronts increased in the stream-wise direction, decreasing the number of wetting fronts available for liquid replenishment. Lifting of the most upstream wetting front was found to catastrophically cause CHF. The CHF model incorporates the observed stream-wise reduction in wetting fronts with a criterion for lift-off heat flux to obtain a simplified set of equations for CHF. Local information such as pressure, phase velocities, and average vapor layer thickness along the heater were also incorporated in the CHF model using the assumption of separated two-phase flow. The model predicts the CHF data for the 0°, 45° and 90° orientations with mean absolute errors of 12.6, 13.6 and 17.5%, respectively.