Heat transfer and fluid flow studies in helical–spiral coils of modified cavity receiver for solar dish process heat system

The present study deals with the heat transfer and fluid flow characteristics of heat transfer fluid in helical coil of modified cavity receiver of solar dish collector. A 3-D numerical model is developed to study the effect of fluid flow rate, DNI and opening ratios on the heat transfer and fluid flow from the receiver. The variation of average fluid temperature along the coil, Nusselt number and friction factor has been estimated for the fluid flow along the helical coil under different conditions. At DNI of 500 W/m2 and diameter ratio of 0.48, the rise in average fluid temperature for 100 lph is about 160K and about 17 K for 1000 lph. A steep increase in the average fluid temperature is observed in the spiral section whereas in the bottom helical section, it varies linearly and in the top helical section, the variation is very less. Nusselt number varies between 25 to 75 in the spiral section and 50 to 200 in the helical section for different flow rates. Nusselt number along the coil decreases in the spiral part and oscillates along the helical section. The oscillation of Nusselt number in the helical section is due to the secondary flow developed and the geometry of the coil. It can be noted that the heat transfer rate is comparatively higher in the helical section than the spiral section. The friction factor drastically drops in the spiral section and remains constant in the helical section. The friction factor ranges between 0.01 to 0.05 in spiral section and 0.01 to 0.02 in the helical section for different flow rates. The figure of merit is estimated in terms of ratio of pressure drop and heat transfer rate and it varies between 1 and 540 for different flow rates and DNI. The thermal efficiency of the receiver is found in the range of 70 - 80%. The present model can be used to estimate heat transfer/gain and pressure drop across the helical-spiral coil receiver of solar dish collector used for process heat applications.