Thermomechanical finite-element analysis and dynamics characterization of three-plug oscillating heat pipes

Presented here are a nonlinear thermomechanical finite-element model of U-shaped three-plug oscillating heat pipes (OHPs) and numerical methods that can accurately predict the model's oscillation frequency and calculate the time-varying spatial distribution of temperature and the global heat transfer efficiency. The model accounts for the influences of nonlinear spring effect of vapor bubbles, mass transferring effect, fluid filling ratio, operating temperature, gravity, pressure loss due to pipe bend, and temperature difference between the evaporator and condenser. Dynamics of OHPs is characterized using a newly developed time-frequency analysis algorithm, and an Euler predictor-corrector method with convergence check is used to solve for the temperature distribution within the fluid plug. Results show that an OHP is a parametrically excited nonlinear thermomechanical system. An explicit formula for accurate prediction of the model's oscillation frequency is derived, and it reveals that the oscillation frequency is mainly determined by the fluid plug mass, initial vapor pressure, and the vapor-plug-length/pipe-cross-sectional-area ratio. Parameters that determine the oscillation amplitude include the temperature difference between the evaporator and condenser, heat transfer coefficients, fluid filling ratio, and initial temperature. Moreover, the existence of gravity directing from the evaporator to the condenser increases the oscillation frequency. These results provide better understanding of heat transfer mechanisms of OHPs and can be used to optimize designs of OHPs.