An Experimental Study on Cryogenic Spray Quenching of a Circular Metal Disk II-Chilldown Efficiency and Effects of Coating and Flow Pulsing

This is the second part of a two-paper series that presents the results of liquid nitrogen spray quenching of a stainless steel (SS) disk plate. The first paper focuses on the heat transfer characteristics of the liquid nitrogen spray quenching, whereas the second paper emphasizes the overall chilldown performance and efficiency. Serving as the baseline information for evaluating the effects of disk surface coating and pulse flow, the results of continuous-flow spray chilldown of a bare surface SS disk are summarized first. We found that for continuous-flow spray chilldown of a bare surface disk, the chilldown efficiency is mainly a function of the average spray mass flow rate with the trend of decreasing efficiency with increasing average mass flow rate. Additional experiments were performed to evaluate the enhancement of cryogenic spray quenching by three techniques: (1) using intermittent pulse sprays on SS bare surface, (2) coating the SS surface with a layer of low thermal conductivity Teflon film, and (3) spraying liquid nitrogen intermittently on the coated SS surface. In general, the results indicate that all three methods effectively produced higher spray efficiencies. However, it was also found that the Teflon coating was more effective than the flow pulsing. The coating produced a quick transition from film boiling to transition boiling. This quick transition shortens the poor heat transfer film boiling period and thus facilitates the switch to much higher heat transfer transition boiling and nucleate boiling periods earlier to expedite the chilldown process and enhance the heat transfer.