Investigation on Convective Heat Losses from Solar Cavities under Wind Conditions

Numerical three dimensional studies of forced convective heat loss from cavity receiver of different shapes have been investigated under wind conditions. The cavity shapes used are: cylindrical, conical (frustum of a cone), cone-cylindrical (combination of frustum of cone and cylindrical shape), dome-cylindrical (combination of hemispherical and cylindrical shape) and hetro-conical. These studies are carried out for three isothermal wall temperatures (523, 723 and 923 K) and five inclinations: theta = 0 degrees (cavity aperture facing sideways), 30, 45, 60 and 90 (cavity aperture facing down). Besides, effects of mouth blockage (mouth blockage area 36% and 64 %) on forced convective heat loss are also investigated. Three wind directions viz., head-on, side-on and back-on and wind speed of 1 to 5 m/s are considered. The ratio of convective losses occurring under wind and no-wind conditions shows minimum value 0 = 0 and it increases with cavity inclination. As expected, the convective heat loss under wind conditions is higher than the no-wind case. The convective heat losses are higher for head-on wind condition for all shapes in the range (1 to 5 m/s) of wind speed considered. Among the different shapes under study, conical cavity yields the lowest convective losses for both categories of cavities (with and without mouth blockage). Under wind condition, convective losses reduce marginally with mouth blockage. Increase of mouth blockage from 36 % (D-ap = 0.4 m) to 64 % (D-ap = 0.3 m) does not significantly alter the magnitude of convective loss. The mouth blockage is found to be more effective for conical cavity where reduction in convective heat loss is observed to be 7 and 16 % respectively for wind speed of 1 and 5 m/s as compared to fully open conical cavity (D-ap = 0.5 m). Generalized Nusselt number correlation is proposed based on the forced convective heat loss data from cavities of different shapes and sizes with and without mouth blockage for head-on wind condition. It correlates 83% of data within 11%, 95% of data within 15% and 100% of data within 21%. (C) 2014 The Authors. Published by Elsevier Ltd.