Experimental validation of the mechanism and condition for the onset of significant void in subcooled flow boiling

In evaluating the axial void fraction profile in the subcooled boiling region in heated channels, accurate determination of the point of onset of significant void (POSV) is of critical importance. Recently, it has been reported that the POSV can be predicted with high accuracy if the onset of significant void (OSV) is assumed to be caused by a sudden decrease in the interfacial area due to global bubble coalescence (GBC) leading to the formation of large bubbles. Therefore, in this work, experimental investigations were carried out to measure the void fraction, bubble size and bubble number density. The experimental results were utilized to validate the mechanism causing the OSV in subcooled flow boiling. The experimental results were in qualitative and quantitative agreement with the model based on the GBC concept. Thus, it was validated that the sudden decrease in condensation rate due to large bubble formation is the main cause of OSV. It was also confirmed that the GBC occurs when the local void fraction near the heated wall reaches about 0.3, analogous to the flow regime transition from bubbly flow to slug flow in adiabatic gas-liquid two-phase flows.