Hydrodynamics of quenching with impinging free-surface jet

The hydrodynamics of jet impingement quenching of a stainless steel specimen has been studied experimentally. The specimen is heated to an initial temperature of about 900 °C and then quenched by a subcooled free-surface water jet. High-speed imaging shows that the free-surface of the water film in the wetted region is smooth. The water film outside the wetted region is deflected away from the surface and then breaks into droplets due to surface tension and shear forces. The splashed droplet velocity is found to be low up to a wetting front radius of 6 mm (r/d_J ≈ 2), beyond which it increases rapidly before reaching a constant value at a wetting front radius of about 8 to 10 mm (2.67 ? r/d_J ? 3.34). The water film velocity at the wetting front is calculated using the single-phase boundary layer model suggested by Watson [2]. At moderate subcooling, the splashed droplet velocity up to a wetting front radius of 10 mm (r/d_J ≈ 3.34) is found to be much lower than the estimated single-phase film velocity. The study shows that although the wetted region may appear devoid of any bubbles, strong two-phase flow occurs within this region.