Numerical investigation on necessary condition for temperature oscillation in loop heat pipe

A loop heat pipe (LHP) is a two-phase heat transfer device that utilizes the latent heat and capillary force of a wick. Many spacecraft have been equipped with LHPs because of the ability of LHPs to transfer heat efficiently. However, temperature oscillations may sometimes occur and cause deviations from the allowable temperature range. A recent numerical analysis of temperature oscillations has revealed some sufficient conditions for temperature oscillations. However, the necessary conditions, which are required to prevent temperature oscillations, are not yet understood. Here, a parametric study was performed using a transient mathematical model to understand the necessary conditions for temperature oscillations. The parametric study showed that a small heat capacity of the reservoir, low-thermal-conductivity wick, or high evaporation heat transfer coefficient led to temperature oscillations. These results indicated that temperature oscillations occur when the temperature difference between the reservoir and condenser becomes small. Therefore, a small temperature difference between the reservoir and condenser is a necessary condition for temperature oscillations. This is because the small temperature difference causes the penetration of the two-phase flow into the liquid line. To prevent temperature oscillations, a large temperature difference should be maintained. One method to maintain the large temperature difference is to increase the thermal capacity of the reservoir, such as by increasing the filling ratio. Heating the reservoir is another method to keep the temperature difference high.