Heat transfer performance of the L-shaped flat gravity heat pipe used for zero-carbon heating houses

The L-shaped flat gravity heat pipe (LFGHP) is applied to the building envelope to effectively improve solar heat gain and achieve zero-carbon heating due to its ultrahigh equivalent thermal conductivity (ke) and thermal diode effect. In this paper, the forward and reverse heat transfer models of LFGHP are developed and experimentally verified. An experimental platform is built to experimentally study the thermal start-up time and the key factors on forward heat transfer, including the working fluid and filling ratio, the bending angle, the inclination angle, the length ratio of the condenser to the evaporator, the cooling temperature, and the heat flux. The ke of an LFGHP in practical use during a whole day is obtained. The results show that: (1) The heat transfer model of LFGHP can be regarded as three series thermal resistances in forward heat transfer and three parallel thermal resistances in reverse heat transfer, and the thermal diode effect is experimentally verified, with a rectification ratio over 170. (2) The acetone heat pipe with a volume filling ratio of 10% is of the highest ke and is the optimum combination of working fluid and filling ratio. (3) The ke in forward heat transfer increases with the decrease of bending angle and inclination angle, and increases with the increase of heat flux, cooling temperature, and length ratio of the condenser to the evaporator. (4) The thermal start-up time shortens with the increase of heat flux and inclination angle and can be neglected in applications. This study is helpful for the selection and design of the LFGHP used in solar houses and has guiding signifi-cance for the improvement and optimization of the practical applications of zero-carbon heating houses.