Experimental Study on Heat Transfer Characteristics of Vertical 5×5 Heated Rod Bundles around Critical Pressure with R-134a

Abstract

With the aim of researching the heat transfer characteristics around the critical pressure in rod bundle geometry, the behaviors of the heating surface temperatures at the vertical 5Â5 heated rod bundles with R-134a were experimentally investigated under both pressurizing and depressurizing processes, where the pressure constantly decreases from supercritical to subcritical and constantly increases from subcritical to supercritical under constant inlet thermal conditions. As the pressure approaches the critical pressure, a DNB type or a dryout type of CHF was induced, and then the post-CHF was kept until the vicinity of the critical pressure. Under the supercritical pressure, the post-CHF disappeared due to the extinction of boiling phenomena, but the heat transfer deterioration was induced in a certain pressure range. In the depressurizing process, the heating surface temperatures followed the trajectory of temperature behaviors as in the pressurizing process except for the quenching point. The quenching point from the post-CHF moved to the lower pressure side than the pressure as for the CHF in the pressurizing process. This phenomenon was considered as the hysteresis phenomenon frequently observed in pool boiling curve characteristics.