Two-Phase Flow in Pulsating Heat Pipes

Pulsating heat pipes (PHPs) use phase change phenomena including evaporation and condensation to transfer heat from a high-temperature source to a low-temperature one. A working fluid inside the device changes its phase and transfers the heat. PHPs have three sections which are called evaporator, condenser, and adiabatic section. In the evaporator section, heat is applied to the device, and in the condenser section, heat is removed from it by the working fluid. The adiabatic section stands between these two sections and has no heat transfer with the surroundings. Below I have brought some of my observations from the two-phase flow in two one-turn closed-loop PHPs used in this study. One of the PHPs was fully made of Pyrex glass. The other one was made of copper in most parts and of Pyrex glass in one side of the adiabatic section. All images are from top/left side to down/right side.

Vapor plug motion from evaporator (lower section) to condenser (upper section) (filling ratio=70%, heat input=10 watts, working fluid=TiO2/water nanofluid). The time span between each pair of images is 30 msec

Vapor plug motion from evaporator (lower section) to condenser (upper section) (filling ratio=30%, heat input=10 watts, working fluid=TiO2/water nanofluid). The time span between each pair of images is 19 msec

Rapid contraction of vapor plugs (filling ratio=50%, heat input=40 watts, working fluid= graphene/water nanofluid). The time span between each pair of images is 3 msec

Circulation of vapor plugs and liquid slugs from condenser (upper section) to evaporator (lower section) (filling ratio=50%, heat input=70 watts, working fluid= graphene/water nanofluid). The time span between each pair of images is 15 msec

Rapid nucleation and expansion of vapor bubble in evaporator (filling ratio=50%, heat input=40 watts, working fluid= Fe2O3/water naonofluid). The time span between each pair of images is 32 msec

Oscillations of liquid slugs and vapor plugs (FR = 50%, heat input = 20 W, working fluid = TiO2/water nanofluid with a concentration of 1 mg/ml). The time span between each pair of adjacent images is 162 ms

video_2022-07-22_10-37-47.mp4

Here, the working fluid is graphene/water nanofluid. One can observe the strong oscillating motions of liquid slugs and vapor plugs at a filling ratio of 50% at a moderate heat input.

video_2022-07-22_10-33-20.mp4

Here, the working fluid is TiO2/water nanofluid. One can observe the oscillating motions of liquid slugs and vapor plugs. These motions help the device to transfer heat from the evaporator section to the condenser section with high flux.

video_2022-07-22_10-44-22.mp4

Here, fast circulation of liquid slugs and vapor plugs can be observed. This mode of motion usually happens in heat inputs greater than or equal to 40 W when the filling ratio is equal to 50% and it can be seen from 20 W when the filling ratio is 70%. The working fluid here is TiO2/water nanofluid.

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