EFFECT OF THE BEHAVIOR OF GENERATED VAPOR ON THE MINIMUM-HEAT-FLUX POINT DURING RAPID QUENCHING OF A THIN HORIZONTAL WIRE

摘要

Heat transfer characteristics and minimum-heat-flux point (MHF point) condition during rapid quenching of a thin horizontal wire were studied both experimentally and theoretically. Experiments were conducted with ethanol as a quenching liquid. The wire was either fixed or moving downward at a constant speed in the ethanol bath. When the wire was heated instantaneously to a high temperature, the generated vapor formed bead-like bubbles along the wire as shown in Fig. A-1. This was in contrast to the previous results (Honda et al., 1995) for a heated wire that was dipped into the ethanol bath at a relatively high velocity, where a vapor sheet was formed in the wake of the wire. Consideration was given to the effect of the film boiling mode on the MHF point condition. A linear stability analysis of a vapor film for natural convection film boiling on a horizontal cylinder was presented that considered the effects of liquid inertia, viscosity and compressibility of vapor, and periodic heat transfer. Figure A-2 shows a comparison of the average vapor film thickness at the MHF point δ{sub}(MHF), the modified vapor film thickness at the MHF point based on the effective surface area (thin film region between bead-like bubbles) δ{sub}(MHF,M) and the theoretical prediction of the critical vapor film thickness δ{sub}(cr). It is seen from Fig. A-2 that δ{sub}(MHF) is smaller for higher liquid subcooling ΔT{sub}(sub). The δ{sub}(MHF,M) is not affected by ΔT{sub}(sub) and it increases gradually with increasing wall superheat ΔT{sub}(sat). It is also seen that δ{sub}(cr) compares well with δ{sub}(MHF,M).