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

In this study, visualization treatment was applied to the flat disk-shaped evaporator of a loop heat pipe. By observing the liquid/vapor behavior inside the evaporator and compensation chamber, the effects of orientation on the performances during startup and during a step-increase in heat load were investigated. With water as the working fluid, the loop heat pipe was tested under three typical orientations of φ = −90°, φ = 0°, and φ = +90°. The startup time was the shortest for the φ = −90° orientation but there could be a slight temperature overshoot, resulting in an unsmoothed startup process. The startup speeds under the φ = 0° and φ = +90° orientations were similar, both without any significant temperature overshoot. The orientation could significantly change the heat leak and, therefore, the operating temperature and the heat-transfer limit. For the φ = +90° and φ = −90° orientations, the heat-transfer limits were about 71% and 157% of the value at the φ = 0° orientation, respectively. Based on visual observations, for the loop heat pipe operating in gravity-driven mode, there could be two different paths for the working fluid to return to the evaporator, namely, along the vapor line for low heat loads and along the condenser and liquid lines for relatively large heat loads, respectively.