Theoretical Investigation Of Oscillating Motion And Heat Transfer In An Oscillating Heat Pipe

Oscillating heat pipe (OHP) is a heat transfer device which has unique features of simple structure, low cost and high performance. However, the heat transfer process in an OHP is involving liquid-vapor two-phase flow and oscillated forced convection. Fluid flow and heat transfer mechanisms have not been fully understood. In the current investigation, an analytical study of oscillating flow in a capillary tube was conducted to determine the oscillatory flow effect on the heat transfer coefficient and find whether there exists an optimum condition for the maximum heat transfer coefficient Based on a uniform heat flux boundary condition, the analytical solutions of temperature distribution and Nussek number for a pulsating laminar flow are obtained to analyze the effects of thermal and mechanical properties on the heat transfer performance. The reseach are foucus on the sine waveform and triangular waveform to find how the waveform affects the heat transfer coefficient in a capillary tube. Furthermore, based on the fact that pressure wave speed in vapor phase is different in liquid phase, a mathematical model predicting the filling ratio effect on the startup of oscillating motion in an OHP is established to slove the question about the relation of the mininmu heat toad vs. the filling ratio. The analytic solutions obtained in the current investigation show that not only the dimensionless pulsation frequency, ω*, amplitude, Y, and Prandtl number, Pr, are primary factors affecting the heat transfer performance of an oscillating flow in a capillary tube, but also fluid properties and oscillating waveform. More importantly, the triangular waveform of oscillating motion can result in a higher heat transfer coefficient by comparison to the sine waveform. Furthermore, a mathematical model that predicts the filling ratio effect on the start-up power of a one-turn OHP is presented. It shows that the heat input needed to start the oscillating motion in an OHP depends on the filling ratio. When the filling ratio increases, the heat input required to start up the oscillating motion increases. And there exists an upper limit, which is dependent on the properties of the working fluid.