Two-dimensional quantitative study of boiling flow evolution in vertical inner-heated annulus channel

Boiling flow is a multidimensional phenomenon because of its thermal nonequilibrium characteristic. Studying the evolution of boiling flow in both the axial and radial directions is crucial for the modeling and validation of Computational Fluid Dynamics (CFD) codes. A quantitive analysis is conducted on a boiling dataset with two-dimensional, high-resolution data measured by conductivity probes in a vertical inner-heated annulus. By tracing the bubble layer's development, three stages of the boiling flow evolution are recognized, and the dominant mechanisms of each stage are discussed. New concepts of the bubble sublayers, namely 80%, 50% void layers and 80%, 50% bubble-number layers, are created to identify the region where the majority of the bubbles are located. The previous bubble-layer thickness models are validated, and the flat model is observed to be more accurate in modeling the subcooled boiling flow than the triangular model. Limitations are observed in the results of the bubble-layer thickness model, and modifications are proposed in this paper to overcome the limit. After summarizing the twenty-two conditions in the boiling dataset, the bubble sublayers correspond to the void fraction or to the subcooling number and the boiling number. Correlations are developed to calculate the boundary locations of the sublayers from the parameters. In addition, the development of the bubble sublayers and the transition of the flow regimes are found to be related, where the flow regimes are identified by the criteria and by viewing the high-speed videos.