Numerical and experimental investigation of a multiple air jet cooling system for application in a solar thermal reciever

The transparent quartz glass window of a high temperature solar receiver (1000 degrees C air outlet temperature, 15 bars) has to be protected from overheating. The window is an axially symmetric part that can be approximated by a hemisphere with a cylindrical extension (diameter 0.31 m, height 0.42 m). The cooling is accomplished by impinging several air jets onto the concave window surface. Due to concentrated solar radiation, the air supply nozzles can only be installed at the circumference of the cylindrical extension. Symmetric configurations with six or nine nozzles, equally distributed around the window circumference, are examined. A second configuration generates a swirl in the window cavity by inclining the nozzles. In a third, asymmetric configuration, only nozzles on one side are simultaneously charged with mass flow, while a spatial homogenization of heat transfer is reached by periodically modulating the air flows with time. Computational fluid dynamics (CFD) calculations and laboratory measurements of the heat transfer have been carried out. In the performed 3-D simulations, the realizable k-epsilon model, the k-omega model, and the SST-k-omega model are compared. For measuring the heat transfer coefficient, a periodic-transient measurement technique with high spatial resolution is used. For the application of cooling of the solar receiver window, the jet cooling system with periodically modulated air flows is identified as the best solution.