Volume 5, Issue 10 (November 2008)
Enhancement and Local Regulation of Metal Quenching Using Atomized Sprays
In atomized spray quenching, the spraying water is atomized into fine droplets of the size of about 20 μm by compressed air and sprayed onto a hot surface. Only single droplets touch the surface, become deformed, and transfer heat. The drops partially evaporate and then move away from the superposed airflow. Thus, the vapor film is avoided as it is the case for other quenching techniques. It is demonstrated that the unintended collapses of the vapor film (Leidenfrost problem) at edges, corners, roughness peaks, etc., is eliminated by this technique. In this way, mass lumped regions of work pieces can be cooled more intensively than the edges. Consequently, a uniform temperature distribution with reduced thermal stresses can be obtained. The heat transfer was measured by infrared thermography. One side of the metallic sheet of 3 mm thickness was quenched by atomized spray and the surface temperature was measured on other side by an IR camera. The heat transfer coefficient is mainly determined by the impingement density. Heat transfer coefficient of 10,000 W/m2/K can be achieved within film boiling regime. This value is about three times higher than that of conventional spray quenching.