Flow and heat transfer of liquid plug and neighboring vapor slugs in a pulsating heat pipe

A model of fluid flow and heat transfer on liquid slug and neighboring vapor plugs in a pulsating heat pipe (PHP) is proposed. A new energy equation for the liquid slug is built by aid of Lagrange method. The shear stress term related with the fluid flow state is included in the motion equation of the liquid slug. A sensitive heat term is replaced by a phase change term in the energy equation of the vapor plug. Based on our analysis on the displacement variation of the liquid slug with time, it is known that the harmonic force acting on the liquid slug in PHPs is the pressure difference between the vapor plugs. The flow oscillation can be considered as a forced damping vibration of one degree of freedom system. The phase difference of the oscillating flow between with and without the gravity effect can reach 45°. The amplitude and angular frequency of flow oscillation is irrespective with the initial displacement of liquid slug. If the flow pattern remains strictly slug flow in the entire system, the contribution of the sensible heat exchange to the total heat transfer of the PHP is about 80%.