EFFECTS OF HEATER LENGTH AND ORIENTATION ON THE TRIGGER MECHANISM FORNEAR-SATURATED FLOW BOILING CRITICAL HEAT-FLUX .2. CRITICAL HEAT-FLUXMODEL

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