Assessment of a Mist Cooling System for Aluminum Alloys

Wrought aluminum alloys can be heat treatable following a three-stage cycle that consists of solution, cooling, and aging. The cooling rate at which the heat-treated parts are subjected to is a critical parameter; if the rate is slow, the dissolved elements will have enough time to precipitate during cooling, affecting the mechanical properties after aging, whereas with a high cooling rate, it will be possible for pieces of the complex geometry to exhibit distortion or, in some cases, fracture. A mist cooling system prototype is presented in this work. The system was developed by mixing forced air that is produced by a blower with atomized water within. The cooling rate was measured in 6061-T6 aluminum alloy cylinders by varying the air velocity and volume of atomized water; the results were compared to cooling in still air. The temperature profiles during cooling were obtained using K-type thermocouples that gathered data from the inside and from surface locations. Cooling rates were determined by a first-order derivative of the measured temperatures, and the heat transfer coefficients (HTC) were calculated by the inverse method using 2-D transient axial symmetrical analysis with commercial software. HTC values were found in a range of 250 to 590 W/m²·K. The results showed that the HTC increased with the amount of atomized water. The HTC does not seem to be affected by the higher range values when plotted against surface temperature.