Experimental study on the passive regulation of a CO2 two-phase thermosyphon loop

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• The passive regulation mechanism of CO 2 TPTL was revealed. • Three typical operation states of the TPTL occurred as the heat load increased. • Each operation state showed different characteristics as the filling ratio changed. • The largest normal operation heat load range was found under 45% filling ratio. Vapor–liquid distribution and flow characteristics in a two-phase thermosyphon loop usually exhibit regular variation under varying working conditions, which can be referred to as passive regulation. Currently, the passive regulation law of a CO 2 two-phase thermosyphon loop has not been well understood. In this study, experiments were performed to examine the passive regulation mechanism of a CO 2 two-phase thermosyphon loop under different heat loads and filling ratios. Local flow visualization was adopted, combined with the measurement of operating parameters. The CO 2 two-phase thermosyphon loop entered the pre-start, oscillatory, and stable operation stages successively with the increase in heat load. Vapor–liquid counterflow was observed in the downcomer in the pre-start stage. Temperature change due to thermal unbalance between the vapor and liquid can be observed along the riser or the downcomer. In the oscillatory stage, different types of oscillation were observed under different filling ratios. In the stable stage, the two-phase thermosyphon loop worked under good conditions until the heat transfer limit was reached. The two-phase thermosyphon loop reached its maximum heat transfer limit of 1200 W at a filling ratio of 45%. The normal operation heat load range of the two-phase thermosyphon loop was 380–650 W, 320–770 W, 700–1100 W, 600–1200 W, and 400–630 W for filling ratios of 20%, 25%, 40%, 45%, and 50%, respectively.