An experimental study on critical heat flux in vertical annulus under low flow and low pressure conditions

An experimental study was carried out to investigate the phenomenon of critical heat flux (CHF) at low flow and low pressure conditions in an internally heated vertical annulus. The rod heater of the test section was made of solid stainless steel with diameter of 6 mm and heated length of 454 mm. The unheated outer glass tube had an inner diameter of 16.3 mm. The pressure of the water coolant ranged between 116 and 228 kPa, the mass flux was varied in the range of 49.88-108.53 kg/(m2 s) and the inlet subcooling was kept at 3 °C below the saturation temperature. Among the 111 measured CHF data points 107 was detected at the uppermost thermocouple position, but four CHF were detected at lower thermocouple positions. Despite the uniform heating, the measurements showed that the CHF-location may shift from the uppermost position at L/Dhe = 11.86 (heated length/heated equivalent diameter) ratio if the mass flux is lower than 63.22 kg/(m2 s). Five selected correlations for the low flow and low pressure range were compared with our CHF data points.