DEVELOPMENT OF SUPERIOR THERMAL SHOCK TOLERANT MATERIAL FOR NUCLEAR COMPONENTS BY ENGINEERING SURFACE HEAT TRANSFER MODES

摘要

Poor resistance to thermal shock is one of the major limiting factors for ceramic materials to be used as nuclear structural materials. Most past efforts to improve thermal shock tolerance focused on increasing material strength, thermal conductivity. As much as the material aspect of thermal shock tolerance is concerned, convective heat transfer is the other critical component for thermal shock tolerance, as it determines non-uniform temperature fields leading to thermal stresses. Our approach is to achieve thermal shock tolerance by reducing surface heat flux with surface modification. We perform a systematic study of the thermal shock experienced by the alumina during quenching by cold water droplet impingement with heated surface temperature ranging from 125℃ to 475℃ for Weber number =32. Degree of thermal shock is gauged from the residual strength of material post quenching. We find clear sign of thermal shock fracture for as received hydrophilic alumina due to higher heat flux during nucleate and transition boiling mode of heat transfer. Residual strength is nearly constant for surface modified alumina due to the hydrophobic nano-fractal surface that promoted film boiling mode of heat transfer, implying significant improvement in thermal shock tolerance with reduced heat flux. This is a novel approach to reduce thermal shock by controlling the heat transfer with surface modification, different from conventional, yet expensive, method of improving the bulk material properties. The presented method of improving thermal shock tolerance can be applied to various nuclear power plant components, including turbine blades.