Impact of flow resistance on the operational stability of two-phase thermosyphon loop with different working fluids

When a two-phase thermosyphon loop (TPTL) operates in an oscillatory state, its service life may be shortened due to mechanical impacts. Limited research exists on improving TPTL stability. This study used a needle valve to adjust the flow resistance in the downcomer of a TPTL to analyze its impact on the operational stability of CO2 and R134a TPTLs. Our research revealed that the operating states of the two TPTLs are affected differently by flow resistance. The fluctuating operation of the CO2 TPTL is caused by the periodic changes in the flow regime within the loop. An increase in flow resistance led to higher circulating driving force by increasing vapor quality in the riser and decreasing vapor quality in the downcomer. This suppressed the operational instability of the CO2 TPTL. However, the unstable operation of R134a TPTL resulted from the inherent instability of the stirred flow in the riser. Therefore, the change in flow resistance has almost no effect on the operational stability of the R134a TPTL. The findings of this study provide new insights for improving the operational stability of the CO2 TPTL.