An experimental investigation of flow boiling heat transfer characteristics and the triggering mechanisms of CHF in a natural circulation loop under moderate to low driving forces

Flow boiling heat transfer, flow instability, and critical heat flux (CHF) were investigated experimentally in a vertical R134a natural circulation loop with an integrated reservoir. Tests on flow heat transfer characteristics and flow visualization were conducted at filling ratios of 25–85% and cooling water temperatures of 10–22 °C. Higher peak heat transfer coefficients were obtained in the filling ratio range of 25–45%, with a maximum measured value of 16,145.5 W·m −2 ·K −1 . At a cooling water temperature of 14 °C, regular pressure and temperature oscillations were first detected at heat fluxes of 49.10–71.71 kW·m −2 , with frequencies of 0.453–0.628 Hz. The measured CHF increased by approximately 50% across filling ratios of 25–75%, rising from about 100–150 kW·m −2 . At a 75% filling ratio, the CHF increased from approximately 132–151 kW·m −2 as the cooling water temperature rose from 10 to 14 °C, with no further increase observed up to 22 °C. The CHF was not determined at an 85% filling ratio due to equipment pressure constraints. The onset of sustained wall-temperature excursion was distinguished from subsequent post-CHF heat transfer deterioration. Flow-pattern observations and circulation measurements suggested that insufficient liquid supply relative to evaporation induced the occurrence of CHF. The Benkheira correlation predicted all CHF measurements within the range of −22.72% to + 23.31%, providing an approximate estimation for the present dataset. As indicated by the above-mentioned results, the conditions yielding higher heat transfer coefficients differed from those maintaining higher limiting heat loads.