Transient cooling experiment of laminar jet impacting on cylinder

Abstract

Liquid jet impact cooling is found in many industrial applications. In transient process of liquid jet impacting on surface, there are correlations among liquid film flow, heat transfer regime and surface temperature when phase transition is involved. Aiming at application scenarios of laminar jet impacting on cylindrical cooling targets, an experiment of laminar jet impacting on cylinder was performed to investigate transient cooling characteristics featured by temperature dropping curves. Liquid film evolutions were acquired by high-speed camera, while surface temperatures were monitored by thermocouples, and effects of jet outlet velocity u (outlet Reynolds number Re) and impact height H were explored. Results reveal that, transient process of jet impact is categorized into non-wetted (film boiling, duration of τ1), developing wetted (mainly transition boiling and nucleate boiling, duration of τ2) and fully wetted stages (mainly non-phase transition convection). Among them, τ1 and τ2 are decreased with rising of Re. Jet exhibits steady, perturbed and fractured states at different H. When u = 0.44 m/s (Re = 680), for steady state jet, τ1 decreases from 119 s to 103 s, while variation of τ2 is insignificant as H increases from 27 mm to 37 mm. When jet is unstable (H = 42–47 mm), τ1 drops remarkably while τ2 rises with increasing H. Effect of H on temperature dropping curves is weakened with increasing Re. Unlike cases of liquid impacting on flat surface, there are clear distinctions in durations of each transient stage, and there is a secondary inflection point for surface temperature variation in present experiment due to secondary mixing of liquid film.