HEAT TRANSFER FROM FINNED SURFACE IN DOWNWARD-FACING SUBCOOLED FLOW BOILING

摘要

Severe Accident Management Guidelines exist to mitigate the effects of severe accidents. A particular strategy that may prevent RPV failure for PWRs is In-vessel Retention (FVR). During IVR, a large amount of heat may be transferred to the water from the vessel surface. If excessive boiling conditions are reached, Critical Heat Flux (CHF) conditions may occur. Therefore, increasing the safety margin for the CHF limit is an important aspect that should be considered especially for future larger-power Light Water Reactors. Although many different enhancement techniques may be available, a simple method of enhancing heat transfer can be performed through fins. Therefore, an experimental study was performed to investigate the heat transfer effect from a pin fin during flow boiling conditions. The finned surface was compared to a bare surface in order to observe any effects in the heat transfer. The experiments were performed for different mass flux: 244 kg/m~2 -s, 215 kg/m~2-s, and 177 kg/m~2-s. CHF conditions occurred at the lowest flow rate. Results showed the finned surface was able to enhance the heat transfer for 244 kg/m~2-s case. The ability of the fin to enhance the heat transfer reduced as CHF conditions were approached. Upstream and downstream temperatures on the finned surface varied, and a significant rise in the downstream temperatures was observed as the flow rate was reduced. Bubble accumulation was also observed downstream after the fin. In some cases the bubble accumulation occurred for a relatively long period, posing a potential risk for the heat transfer.