Influence of initial pressure on confinement characteristics in two-phase closed thermosyphons

초록

Understanding confinement-induced instability in small-diameter two-phase closed thermosyphons (TPCTs) is essential to improve the thermal reliability and stability of compact thermal management systems. However, initial pressure, confinement transition, pressure imbalance, and thermal performance degradation in TPCTs remain unexplored. This study investigates the influence of initial pressure on the confinement characteristics and thermal performance of an 8 mm inner-diameter TPCT, using simultaneous flow visualization and pressure-temperature measurements under various heat input conditions. The results reveal that increasing the initial pressure significantly alters the internal flow regime and confinement behavior due to changes in vapor density, surface tension, and saturation temperature. Four distinct confinement operations are tested: unconfined, intermittently confined, consecutively confined, and fully confined. The fully confined operation was observed at confinement <0.315 and bubble release >0.08. Under high-pressure and high-heat-input conditions, liquid accumulates in the condenser and does not return to the evaporator, resulting in pressure imbalance, evaporator dry-out, and thermal performance degradation. The maximum temperature difference between the upper and lower condenser sections reaches 2.8 degrees C at a reduced pressure of 0.0077 and a heat input of 50 W. The evaporator and condenser heat transfer characteristics are analyzed using heat transfer coefficients and thermal resistance. This shows confinement-induced slug/plug flow-enhanced liquid mixing in the evaporator, whereas excessive confinement deteriorates condenser heat transfer due to temporary liquid blankets and flow blockage. This provides insight into relationships between confinement transition and thermal degradation in compact TPCTs and offers guidelines for preventing confinement-induced thermal instability under varying pressure conditions.

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