Temperature and flow fluctuations in a quartz-glass oscillating heat pipe

An experimental investigation was conducted on the oscillatory flow in a vertical oscillating heat pipe (OHP). The OHP was made of a quartz-glass tube (of inner diameter 1.5 mm and outer diameter 3.5 mm) and coated with a transparent heating film on the outer surface in its evaporating section. The working fluid flow inside the OHP at different heat loads were recorded using a CCD camera, and the outer-wall temperatures in the evaporating section were simultaneously monitored by an infrared image camera. When the heat load exceeded 6 W, flow oscillation became prominent and was divided into two stages: slow oscillation at small amplitude and fast oscillation at large amplitude. In the former stage, liquid slugs oscillated in local regions. A part of heat load was transported to the condensing section and the other stored in the fluid and tube wall of evaporating section. The outer-wall temperature in the evaporating section increased to some critical values, at which quick nucleation and drastic phase change happened suddenly, forming the primary pushing force and causing the transition to the latter stage. In the latter stage, quick nucleation and drastic phase change occurred intermittently; liquid and vapor slugs travelled frequently and drastically between the evaporating and condensing sections. Outer-wall temperatures in the evaporating section decreased quickly, and then the flow returned to the slow oscillation stage again. The slow oscillation stage lasted much longer than the fast oscillation stage at relatively low heat loads. As the heat load increased, the duration time of the slow oscillation stage decreased while that of the fast oscillation stage was almost unchanged.