Transient heat transfer characteristics in spray cooling

An experimental system of swirl spray cooling is designed to study the transient heat transfer efficiency of spray cooling with spray flow evolution. At an initial temperature of 300 & DEG;C, the spray cooling process is observed on a hot wall using a 20 mm x 20 mm square of aluminum alloy as the cooled specimen and water as the cooling medium. To measure the temperature variation, five high-precision thermocouples are used at five different locations along the vertical axis of the cooling specimen. Using the finite difference method, the one-dimensional unsteady heat conduction equation is solved, and the surface heat flux curve is obtained by inversion. The transient heat flux evolution curves under different working conditions are found by varying the spray height and flow rate. As a result, the spray cooling process is divided into four stages: I. the Leidenfrost effect stage (where the heat flux rises slowly), II. the liquid film formation stage (during which the heat flux rises sharply), III. the boiling stage (during which the heat flux decreases gradually), and IV. the convective evaporation stage (where the heat flux tends to equilibrium). As the heat flux reaches its peak value, the cooling process changes from the liquid film forming stage (stage II) to the boiling stage (stage III). The effect of spray height on liquid film is more significant when compared with the effect of pressure, which demonstrates that the heat transfer capacity at a spray height of 5 mm is significantly higher than at 10 mm and 15 mm, leading to the conclusion that the appropriate spray height is an important factor in maximizing the efficiency of spray cooling.