Determination of stagnation and convective zones in a solar cavity receiver

The convective heat loss mechanism in a solar cavity receiver is influenced by the presence of the stagnation and convective zone within the receiver. This paper focuses on the experimental and numerical studies carried out to identify these zones in a downward facing cylindrical cavity receiver of length 0.5 m, internal diameter 0.3 m and having a wind skirt of 0.5 m in diameter. This design is different from the receiver used normally for dish-Stirling systems. It is meant for providing low and medium temperature process heat and does not have stringent constraints on tube volume and pressure drop. The experiments are conducted for low and medium fluid inlet temperatures between 50 degrees C and 150 degrees C for receiver inclination angles of 0 (side ways facing cavity). 30, 45. 60 and 90 degrees (vertically downward facing receiver). Water is used as the working fluid within the receiver tubes during the low temperature tests (50-75 degrees C) while compressed air is the working fluid for the medium temperature tests (130 and 150 degrees C). The numerical investigations have also been carried out for various conditions. The air velocity and temperature profiles obtained from the numerical and experimental studies are analysed to determine the zone boundary, which is found to be nearly a horizontal plane passing through the topmost point of the cavity aperture. This validates the results reported in the literature. This paper proposes a quantitative estimate for identifying the zone boundary and a term called critical air temperature gradient is defined for this purpose. The locations within the cavity having the air temperature gradient less than the critical air temperature gradient represent the stagnation zone. The locations having air temperature gradient more than the critical air temperature gradient represent the convective zone. A non-dimensional parameter psi is defined to represent the values of the air temperature gradient. It is observed that a value of about 0.3 for psi corresponds to the critical air temperature gradient for all the tests carried out. The stagnation zone is observed at regions having psi <= 0.3 and convective zone having psi > 0.3. (C) 2009 Elsevier Masson SAS. All rights reserved.