Feasible working-state spaces of heat pipes: Integrated analyses of their isothermality and heat transfer limits

Abstract

Heat pipes (HPs) are promising in advanced energy and power systems, featured by capability to transfer large heat flow over long distances with small temperature drops. However, the feasibility verification of such isothermality is usually ignored while only heat transfer limits (HTLs) are checked when HPs are to be designed and applied. In this work, criteria on feasible working-state spaces of HPs have been established, integrating the classical HTL and the pseudo-wick-thermal-conductivity-based isothermality theories. Calculations and comparisons have been done in wide ranges of working medium species, lengths, temperature and cooling condition. Isothermality is found to improve with increasing temperature and decreasing length as well as cooling coefficient, thus could always be achieved as long as temperature is sufficiently high. With regard to HTLs, differences exist between water HPs and alkali metal HPs. The isothermality and the HTL criteria compete to found the lower bounds of temperature but no upper bound exists for alkali metal HPs, while both upper bounds (determined by the HTL criterion) and lower bounds (determined by the isothermality criterion) exist for water HPs. The ranges of feasible working-state spaces of HPs with different working mediums are primarily determined by the HTL criterion. Once HPs are designed to operate not exceeding HTLs, isothermality can generally be guaranteed.