Film boiling in quench cooling with high-temperature jets

Abstract

In the steel industry, quench cooling is used in the Run Out Table (ROT) to define the steel microstructure and mechanical properties. Film boiling is the preferred boiling regime in the ROT since its characteristic homogeneous and constant heat flux leads to a stable and controllable operation. The collapse of the vapor layer formed during film boiling, or rewetting, leads to a sharp increase in heat flux resulting in process complications such as uneven cooling, poor product quality, and deformation of the steel strips. In this study, the use of high-temperature water jets during the quenching of hot steel plates resulted in the formation of a stable vapor layer and long periods of film boiling. High-speed recordings are used to observe the dynamics of the vapor layer collapse. Surface heat flux estimations during quenching indicate an increase of vapor layer thickness with increasing distance to the jet location. The vapor layer collapse occurs at a surface temperature comparable to the Thermodynamic Limit of Superheat of water (302°C). The effects of surface roughness, initial surface temperature, and water jet temperature in the stability of the film boiling regime are studied. Surface roughness is found to increase the rewetting temperature and the surface heat fluxes after the collapse of the vapor layer. Increasing water jet temperature leads to decreasing rewetting temperatures and surface heat fluxes and therefore has the potential to widen the ROT stable operation window.