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 capillaritygravity 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.

Experimental Study and Visual Observation of a Loop Heat Pipe with a Flat Disk-Shaped Evaporator under Various Orientations

“…Compared to the other two orientations (i.e., ϕ = 0 • and ϕ = +90 • ), the evaporator temperature exhibited more overshoots under an orientation of ϕ = −90 • ; for example, during the heat load increment from 30 W to 45 W and during the stepwise increase of the heat load from 90 W to 165 W. Throughout the augmentation of the heat load, the steady-state vapor temperature exhibited an "N-shaped" pattern of variation and the FLHP performances were much better than those at an orientation of ϕ = 0 • . During the heat load increment test, the vapor temperature would rise gradually over a heat load range of 30-90 W and then experience some decrease as the heat load further increased to 105 W. The vapor temperature would reach a peak at a heat load of 90 W, of which the value was 52.3 • C. After the heat load exceeded 105 W, the vapor temperature increased again with the heat load, reaching a maximum of 61.7 • C at a heat load of 165 W. Regarding the φ = −90° orientation, the condenser was located above the evaporator-CC assembly, which was usually called the positive-elevation [22,23,50,51] or gravity-assisted attitude [30,[52][53][54]. Under the circumstances, the general analysis determined that FLHP operation could be divided into gravity-driven mode and capillarity-gravity codriven mode, depending on the value of the heat loads [22,23,50,52,54].…”

“…Regarding the operation of the gravity-driven mode, the circulation path of the working fluid is shown in Figure 11; the working fluid flow was driven entirely by the pressure gain from the liquid head between the liquid line and the vapor line. Regarding the ϕ = −90 • orientation, the condenser was located above the evaporator-CC assembly, which was usually called the positive-elevation [22,23,50,51] or gravityassisted attitude [30,[52][53][54]. Under the circumstances, the general analysis determined that FLHP operation could be divided into gravity-driven mode and capillarity-gravity co-driven mode, depending on the value of the heat loads [22,23,50,52,54].…”

Startup characteristics of an ammonia loop heat pipe with a rectangular evaporator

“…As an advanced two-phase heat transfer device, loop heat pipe (LHP) makes use of the evaporation and condensation cycle of a working fluid to realize efficient heat transfer over a long distance [1][2][3]. It typically consists of an evaporator, a vapor line, a condenser, a liquid line and a compensation chamber (CC) [4][5][6].…”