Operating limit of heat transport in two-phase thermosyphon with connecting pipe (Heated surface temperature fluctuation and flow characteristics)

An experiment of heat transfer characteristics in a two-phase thermosyphon with an adiabatic connecting pipe of inner diameter Dp = 2 to 8 mm and of length L = 250, 500 and 1000 mm has been carried out at atmospheric pressure employing water as an operating substance. As the heat is loaded into the thermosyphon, the temperature of heated surface starts fluctuating at a heat flux at which unstable vapor-liquid counter current flow is generated in the connecting pipe. A feature of bubbles at the upper end of the connecting pipe was photographed when the temperature fluctuation starts. It was found that the heat flux at the onset of the temperature fluctuation is increase with an increase in Dp and then can be predicted well by the Wallis equation with Cw = 0.7 for 5 ≤ Dp ≤ 8 mm. Furthermore, we clarified that the cause of this fluctuation comes from the inlet effect of the connecting pipe using a bell mouth, which is placed to either bottom end or both ends of the connecting pipe.