Experimental and numerical analyses for boiling model selection and nonuniform void behavior at the onset of flow instability

Numerical analyses were conducted to simulate the flow boiling phenomenon using different boiling models under natural circulation conditions. From the analyzed results, bubble behaviors were investigated at the onset of nucleate boiling (ONB) and the onset of flow instability (OFI). At the ONB, the predictions obtained for the bubble diameter, nucleation site density, and void fraction were compared with the experimental data. At the OFI, the bubble diameter, nucleation site density, void fraction, and bubble movement with respect to time were investigated. Among 4 cases with different combinations of boiling models, Case 1 was selected because the wall temperature, bubble diameter, nucleation site density, and void fraction were well predicted considering the agreement between the experimental and computational fluid dynamics results. When flow instability started as the heat flux increased, bubble behavior was nonuniform although heat flux was uniformly generated on the heated surface.