Experimental Characterization of Geysering Instabilities in a Small-Scale Closed Loop Thermosyphon

Abstract

Abstract. Closed loop thermosyphons (CLTs) are passive heat transfer devices, that when operating in a buoyancy-driven two-phase regime, can transfer heat nearly isothermally and thereby achieve excellent thermal performance. However, CLTs are susceptible to flow instabilities, particularly geysering, which can lead to transient fluctuations in pressure and temperature. This work presents an experimental investigation of geysering instabilities during the powering up of a small-scale CLT. Experiments are conducted for heat loads from 45 W to 260 W and water fill ratios from 48% to 81%. The experimental facility uses transparent tubing, enabling flow visualization. Loop performance is characterized through loop thermal resistance, dominant oscillation frequency of the evaporator outlet temperature, and standard deviation of the transient temperature data. Flow visualization revealed that the geysering instability follows a repeating cycle of subcooled boiling in the evaporator, followed by a buoyancy-driven increase in fluid column height in the riser, which draws subcooled liquid back into the evaporator causing bubble collapse and subsequent flow reversal, before the cycle begins again with subcooled boiling in the evaporator. Two distinct geysering oscillation behavior were observed: low-frequency, high amplitude and high-frequency, low amplitude modes. Lower fill ratios transition from low- to high-frequency oscillations with power, while higher fill ratios maintain low-frequency oscillations. Results show that increasing heat load decreases thermal resistance for all fill ratios, with the 62% fill ratio achieving the lowest values. These findings provide insight into the underlying mechanism of geyser boiling and establish trends in loop behavior with fill ratio and heat load.