Metal quenching is a commonly used heat treatment technique, e.g. Direct Chill aluminum casting, quenching of steel for obtaining desired micro-structures. Film boiling, transition boiling, nucleate boiling and forced convection are the mechanisms of heat transfer during quenching. When the coolant strikes the hot metal surface during quenching, the surface can be divided into two distinct zones which are dry and wet zones. Heat transfer in dry zone is dominated by film boiling and the wet zone is influenced by transition boiling, nucleate boiling and forced convection. Wetting front is the boundary zone which separates the dry and wet regions. Wetting front is a thin region of coolant in which the transition and nucleate boiling occurs. Within a wetting front, the heat flux leaving from the hot surface reaches it global maximum. The speed of the wetting front indicates the quench ability of the hot surface for the corresponding flow conditions and the coolant. Wetting front tracking is more important for the prediction of surface temperature during quenching. This research works presents the combined numerical and experimental aspects of the heat flux estimation during the quenching process. At any instant, the position of the wetting front is simply assumed as the location of maximum heat flux. This assumption implicitly treats the wetting front as a line instead of area. The location of wetting front and its velocity at every instant are determined by using the experimental temperature data and the inverse algorithm. Experimental setup and temperature measurement technique are explained in detail. The developed inverse algorithm predicts the quenched side temperature and heat flux from the measured side temperature. A two-dimensional Inverse Heat Conduction Problem (IHCP) is solved through the non iterative Finite Element Method (FEM). The considered quenching technique for the study, based on the method of coolant supplied which is array of water jets. One kind of coolant used in this study is tap water. Aluminum 2024, Inconel, and Nickel are the three different materials considered for the analysis. A rectangular plate made of Nickel with dimension 140 × 70 × 2 mm, using the same dimensions of the Inconel. As in the case of the use of Aluminum, the thickness is the only change to 3 mm, the plate quenched by array of water jets with velocities 0.9 m/s, 1.2 m/s, 1.5 m/s and 1.8 m/s. The measured temperature data are further processed through the inverse finite element technique for the estimation of heat flux leaving from the quenched surface. The position of maximum heat flux changes with time which indicates the movement of wetting front. In this work, four different coolant velocities are employed, and the change in coolant velocity strongly affects the heat flux and wetting front movement.
Boiling of FC-72 on Surfaces with Open Copper Microchannel
