Numerical study of surface heat transfer enhancement in an impinging solar receiver

Sammanfattning: During the impinging heat transfer, a jet of working fluid, either gas or liquid, will besprayed onto the heat transfer surface. Due to the high turbulence of the fluid, the heat transfer coefficient between the wall and the fluid will be largely enhanced. Previously, an impinging type solar receiver with a cylindrical cavity absorber was designed for solar dish system. However, non-uniform temperature distribution in the circumferential direction was found on absorber surface from the numerical model, which will greatly limit receiver's working temperature and finally affect receiver's efficiency. One of the possible alternatives to solve the problem is through modifying the roughness of the target wall surface. This thesis work aims to evaluate the possibility and is focusing on the study of heat transfer characteristics. The simulation results will be used for future experimental impinging solar receiver optimization work. Computational Fluid Dynamics (CFD) is used to model the conjugate heat transfer phenomenon of atypical air impinging system. The simulation is divided into two parts. The first simulation was conducted with one rib arranged on the target surface where heat transfer coefficient is relatively low to demonstrate the effects of rib shape (triangular,rectangular, and semi-circular) and rib height (2.5mm, 1.5mm, and 0.5mm). The circular rib with 1.5mm height is proved to be most effective among all to acquirerelatively uniform temperature distribution. In the second part, the amount of ribs is taken into consideration in order to reach more uniform surface heat flux. The target wall thickness is also varied to assess its influence.