Visualization of boiling flow structure in a natural circulation boiling loop

The present study reports vapor-liquid jet flows, flow patterns and their transitions during geysering instability in a natural circulation boiling loop under varied inlet subcooling ΔTsub (30-50 °C) and heater power Q (4-5 kW). Video imaging, voltage measurement using impedance needle probe, measurement of local pressure and loop flow rate have been carried out in this study. Power spectra of the voltage, the pressure and the flow rate reveal that at a high ΔTsub the jet flows have long period (21.36-86.95 s) and they are very irregular with a number of harmonics. The period decreases and becomes regular with a decrease of ΔTsub. The periods of the jet flows at ΔTsub = 30-50 °C and Q = 4 kW are in close agreement with those obtained from the video imaging. The probe was found to be more efficient than the pressure sensor in detecting the jet flows within an uncertainty of 9.5% and in detecting a variety of bubble classes. Both the imaging and the probe consistently identify the bubbly flow/vapor-mushrooms transition or the bubbly flow/slug flow transition on decreasing ΔTsub or on increasing Q.