Experimental investigation of free-surface jet impingement quenching process

摘要

Liquid jet impingement cooling technique is critical in many industrial processes, such as fuel bundle cooling post-loss-of-coolant-accident in nuclear reactors, heat treatment of metal parts post-hot-processing, etc. The ability of liquid jets to extract high heat flux from metal parts, with temperatures as high as 800-1000 ℃, at moderate flow rates has made them indispensable in these applications. The complex mechanism of flow boiling heat transfer during jet impingement cooling is not been well understood. This study presents a systematic methodology for the measurement and estimation of the temporospatial variation of heat transfer on the impingement surface during jet impingement cooling of extremely hot steel plate. The effect of jet impingement velocity and subcooling variations from 2.5 to 10 m/s and 60 to 87 K, respectively, on the temporospatial heat transfer variation on the impingement surface is reported. A gradually growing circular wetted region, with its periphery named as the wetting front, forms soon after the cooling starts but its velocity decreases as it grows in diameter. A local maximum in the surface heat flux closely follows the wetting front, with the local maximum heat flux reducing with distance from the stagnation point. The wetting front velocity and local maximum heat flux increase with both the jet velocity and subcooling. The enhancement in the local film velocity and subcooling result in a strong suppression of boiling activity and, resultantly, high heat transfer rates at plate surface temperatures in much excess of the critical temperature of the coolant is achieved. This observation confirms the industrial practice of using impinging jets for accelerated cooling of hot steel plates.