Effect of evaporator/condenser elevations on a loop heat pipe with non-condensable gas

Abstract

The coupling effect of evaporator/condenser elevations and non-condensable gas (NCG) on the performance of a loop heat pipe (LHP) operating in gravitational field was investigated experimentally. Ammonia and nitrogen were selected as the working fluid of LHP and the simulated gas of NCG, respectively. The experiments were conducted at three kinds of evaporator/condenser elevations, namely zero elevation, adverse elevation and positive elevation. Experimental results show that NCG will cause an increase in operating temperature, but the trends varies at different evaporator/condenser elevation elevations. The temperature rises caused by NCG at the zero and adverse elevations are negatively correlated with heat load, and the maximum temperature increments are both at the minimum heat load of 15 W, but the influence of NCG is less at adverse elevation. On the other hand, at favorable elevation, the temperature rise exhibits different characteristics in different LHP operation modes, i.e., positively correlated with heat load in gravity driven mode and negatively in capillarity-gravity co-driven mode, and the transition heat load is 60 W. For an LHP that has already contained a certain amount of NCG, functioning at favorable elevation could eliminate the adverse effects of NCG on operating temperature and heat transfer performance to some extent. Furthermore, it is found that the presence of NCG and adverse elevation appears to inhibit the backflow during startup and improve startup stability. These results might have reference significance for the design and installation of the LHP for terrestrial applications.