Detailed visualization experiments on the start-up process and stable operation of pulsating heat pipes: Effects of internal diameter

In the present work, detailed flow characteristics in glass pulsating heat pipes (PHPs) are pioneeringly organized into category charts based on careful visualization during the start-up and the steady operational periods, respectively. Two IDs of 6 mm and 3 mm are studied and compared. The larger ID of 6 mm weakens the effect of surface tension that all the water sinks to the bottom after the filling. Whereas, sections of liquid slugs are formed initially in the 3 mm-ID PHP. Pulsation is observed to operate on and off under low values of heat input (Qin), and gradually develop into steady oscillation with increasing Qin. In the low-Qin start-up period, different pulsation motions exist in separate tubes, which is associated with the initial distribution of liquid slugs. Geyser pulsation, which tends to occur for ultra-long liquid slugs, recurs irregularly in the 6 mm-ID PHP. For the 3 mm-ID PHP, the smaller ID and the lighter weight of working fluid intensify the surface tension effect against gravity that the liquid slug trains can be easily driven around, preventing the geyser pulsation. During the operation period, in either the 6 mm-ID or the 3 mm-ID PHP, continuous up-and-down pulsations switch between the liquid and vapor states with increasing Qin. The critical Qin for the operational period with continuous pulsation is related to the filling ratio: 70 W for FR = 35 %, 60 W for FR = 50 %, and 40 W for FR = 65 % in the 6 mm-ID PHP.