Leidenfrost evaporation and jet impingement boiling of water with dissolved salt or gas.

摘要

Spray cooling and jet impingement heat transfer are essential in metal processing, electronic cooling, nuclear reactor safety, and fuel injection in internal combustion engines. The investigation of small amounts of liquid evaporating or boiling on a heated surface is helpful for better understanding of the associated mechanisms and further improving those processes. It has been observed that small amount of dissolved salts or gases affect the Leidenfrost evaporation and jet impingement boiling significantly. However, a successful theory to clearly explain these effects has not been proposed.; In this study, two series of experiments were performed to explore the effects of dissolved salt or gas. First, Leidenfrost experiments were conducted for several different liquids at atmospheric pressure. Test liquid was gently deposited on a horizontal heated aluminum surface. The evaporation time at various surface temperatures was recorded and plotted as evaporation curves. Noise emitted during the deposition was recorded to be utilized as a tool to detect liquid-vapor contact. To examine the relationship between bubble coalescence, dissolved salt and the Leidenfrost transition, some test surfaces were fabricated with arrays of small holes serving as artificial nucleating sites. It was demonstrated that the dissolved salt increases but the dissolved gas decreases the Leidenfrost temperature. Mechanisms associated the Leidenfrost transition, such as bubble coalescence prevention, variation of properties and salt deposition during the initial liquid-solid contact, are discussed and reasons of the shifting Leidenfrost temperature are suggested.; In the second series of experiments, carbonated water was used for impinging jet cooling of a heated aluminum block to simulate the quenching process in the metals processing industry. The temperature history at multiple locations inside the block was recorded and analyzed. Surface temperature and heat flux were determined by solving the inverse problem for the temperature field numerically. Cooling curves and boiling curves for several conditions with various initial temperatures, flow rates and CO2 concentrations are presented. The results indicate that dissolved CO2 does significantly reduce the heat flux during the entire transient and slow down the cooling process. The dissolved CO2 decreases the critical heat flux and the critical heat flux temperature.