Thermal performance comparison of flat plate evaporator loop heat pipe operating between horizontal condition and gravity assisted condition

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].…”