Heat transfer and eccentric effect investigation on heat pipe used annular heat exchangers with densely longitudinal fins

Abstract

Within the annular heat pipe heat exchangers, heat transfer capacity and assembly eccentricity are two crucial issues for the overall thermal performance (such as heat transfer capacity, flow friction, and temperature distribution). This study aimed to investigate: 1) the heat transfer and fluid flow characteristics of two annular heat exchangers for the purpose of configuration optimization; and 2) the influence of an assembly clearance and the resulting eccentricity on heat transfer characteristic and temperature distribution. Two types of densely longitudinal fins with different heights were designed to enhance heat transfer. Three eccentricities were compared under uniform heat flux and uniform temperature conditions, and two mediums of air and helium were considered to fill the assembly clearance. The results indicated that due to the larger heat transfer area, the average heat transfer coefficient and friction factor produced by the high fins equipped annular heat exchanger were about 20% and 28% higher than the ones with short fins within Re = 2000–15000. The thermal resistance of the clearance varied with the assembly eccentricity. The larger the eccentricity, the smaller the average thermal resistance. Significant temperature gradients appeared in assembly clearance under uniform heat flux conditions. Under uniform temperature conditions, the temperature gradients are outstanding in the annular heat exchanger. A filling medium with greater thermal conductivity is suggested since it can effectively improve the circumferential temperature nonuniformity. This study provides a design guideline as well as heat transfer performance optimization for the heat pipe heat exchangers.