Spray coolin

The research presented in this dissertation is an experimental and theoretical investigation of spray cooling. A complete study is presented for surfaces maintained between the fluid saturation temperature and the Leidenfrost temperature. The heat transfer is characterized by the range of flow rate. The investigations focus mainly on heat transfer with a flooded surface condition. However, low flow rate dropwise evaporation heat transfer is also considered. The main goal of the research is to provide a fundamental understanding of the very complex heat transfer process. This will allow maximization of the critical heat flux and the heat transfer coefficient. Experiments were conducted to analyze the effects of spray and surface conditions on the heat transfer. The effects of liquid subcooling, noncondensable gases, and surface roughness are also investigated. Phenomenological modeling is conducted to explain the heat transfer and the trends observed in the experimental data. Different mechanisms causing the critical heat flux (CHF) are identified depending on the coolant flow rate. For dropwise evaporation, the CHF results when the heat flux to the surface exceeds the latent heat content of the spray. As the flow rate is increased to overcome this, droplet conglomeration and surface flooding result. For low flow rate cases with a flooded surface, the CHF is caused by a liquid deficiency resulting from droplet expulsion. The droplets are expelled due to nucleating bubbles within the liquid film. For higher flow rate cases, the CHF occurs when the vapor generation rate on the surface is so high that a vapor barrier is formed in a given region in the time interval between droplets impinging in that region. The vapor barrier causes the surface temperature to jump above the Leidenfrost point resulting in a nonwetting surface condition. The experimental data and the modeling provide strong evidence that the proposed mechanisms do control the heat transfer. This study is the first comprehensive work which attempts to fully explain the heat transfer process and relate detailed measurements of the spray and surface conditions to the observed physical phenomena.