Comparative and sensitive analysis on the filling, operating and performance patterns between the solar gravity heat pipe and the traditional gravity heat pipe

When integrating the gravity heat pipe with solar collector, the diameters of the evaporator and condenser are usually unequal, namely unequal diameter gravity heat pipe (UDGHP). Compared with the equal diameter gravity heat pipe (EDGHP), there are few studies focusing on operating pattern of the UDGHP. In this work, a numerical model for the UDGHP is firstly developed based on that of the EDGHP. Sensitivity analysis of the geometries of the evaporator and condenser, wall temperature, and working fluid on the filling and operating envelopes and the thermal resistance of the UDGHP is carried out under the criteria of local dryout and flooding limit. Particularly, comparisons between the UDGHP and EDGHP are simultaneously presented and discussed. The results reveal that the UDGHP has the expanded filling and operating envelopes and a smaller thermal resistance. Enlarging the inner diameter of the condenser, shortening the length of the evaporator, and raising the wall temperature can effectively improve the filling and operating envelopes and the heat transfer performance; lengthening the condenser improves the operating envelope and the heat transfer performance but shrinks the filling envelope. Variation patterns in the filling and operating envelopes and the thermal performance with different working fluids are disorders.