Flow and heat transfer characteristics in slip regime for an annulus of evacuated collector tube subjected to non-uniform solar flux boundary condition

Abstract

Due to the accumulation of residual gas, vacuum failure may occur in the annulus of evacuated collector tube in solar trough thermal power system, resulting in the changes of flow and heat transfer mechanism and characteristics. In this paper, based on the actual situation of solar radiation in summer of Chongqing, China, the flow and heat transfer characteristics of the annulus in slip regime were numerically studied. The effects of gas pressure, gas types (N2, H2 and He), heat transfer fluid temperature were analyzed and discussed. The results show that the temperature jump under lower gas pressure directly affects the heat loss and temperature distribution of the annulus. The temperature jump is most obvious for the residual gas of H2 at gas pressure of 130Pa, while it basically disappears for N2 and He (the corresponding Knudsen numbers Kn are 0.0019 and 0.0040; 0.0022 and 0.0046; 0.0026 and 0.0052 respectively under heat transfer fluid temperatures of 473.15 K, 573.15 K and 673.15 K). The heat loss and the outer wall temperature of glass envelope rise with the increasing of heat transfer fluid temperature and gas pressure, and are related to the thermo-physical properties and thermal adaptation coefficient of gas. For gas pressure being less than 70Pa (Kn > 0.0048) or greater than 20000Pa (continuous gas flow regime), the heat loss of N2 keeps increasing with gas pressure, and the heat losses of H2 and He are even more than 1200 W·m−1 under heat transfer fluid temperature of 673.15 K. When the temperature of heat transfer fluid is 573.15 K and the pressure increases from 1.83Pa to 130Pa, the declining relative values of first law efficiency (thermal efficiency) and second law efficiency (exergic efficiency) for N2, H2 and He are 1.59%, 10.07% and 8.48% respectively.