Numerical investigations on metal foam inserted solar parabolic trough DSG absorber tube for mitigating thermal gradients and enhancing heat transfer

Absorber tube in the superheating section of a direct steam generating solar parabolic trough collector field is subjected to large circumferential temperature gradients due to non-uniform concentration of solar flux and low thermal conductivity of steam. The thermal stresses developed due to the thermal gradients can lead to bending and may eventually result in the structural failure of the glass tube enclosing the absorber tube. Therefore, thermal analysis of the absorber tube for identifying the overheated sections and methods to mitigate the thermal gradients are very essential. In the present study, numerical investigations are carried to evaluate the performance of absorber tube with metal foam inserts subjected to non-uniform solar flux. Configurations of absorber tube with circumferential metal foam inserts in the lower half of the absorber tube are considered for the numerical analysis. Moreover, configurations of metal foam inserts with a novel shape based on the nonuniform flux around absorber tube are also analyzed. The metal foam inserted absorber tube configurations are found to be very effective in reducing the circumferential thermal gradients. In comparison to the clear tube, reduction of about 47% to 72% in maximum temperature difference is found for the range of operating conditions considered. The Nusselt number and performance evaluation criterion are found to be enhanced by a factor 10 and 5 respectively. Moreover, maximum gain of 3.71% in the net energy efficiency and 2.32% in exergy efficiency is found for the collector with metal foam inserted absorber tube configurations. The results of the numerical study conclude the insertion of metal foams as an effective method for mitigating thermal gradients around the tube and enhancing the heat transfer performance.