Annular Flow Liquid Film Dynamics in Pipes and Bod Bundle
Average liquid film thickness is important for detailed mechanistic modeling of annular two-phase flow in engineering applications. The existing models and correlations either have large relative errors or narrow application range. Because of this, a new liquid film thickness model has been developed for vertical annular flow in pipes based on three databases. The model includes the pressure, liquid and gas velocities, diameter, and viscosity effects on liquid film thickness. Analysis indicates the film thickness to be a function of Weber numbers for both liquid and gas, and the viscosity number. The model is compared with film thickness data which considers a wide range of liquid and gas superficial velocities, system pressure, fluid properties, as well as several pipe diameters. The trend in the current and available film thickness models at various system conditions are analyzed, highlighting the improvement and widening applicability of the new model. The newly proposed film thickness model results in an average relative error of 14% considering the complete database. Interfacial friction factor in annular two-phase flow is essential both for detailed modeling of two-fluid model and the calculation of pressure gradient. Most of the existing correlations on interfacial friction factor are based on Wallis 1969’s correlation, which considers the interfacial friction factor as a function of film thickness. In this research, a new correlation of interfacial friction factor that is based on the wave characteristics has been proposed. The wave characteristics is considered to be a function of a group of non-dimensional numbers. Since the effects of wave characteristics for ripples waves and disturbance waves on interfacial friction factors are different, the correlation is divided into two sub-correlations based on these two wave regimes. The new correlation has been compared with a wide range of data. From the data comparison, the new correlation shows significant improvement on the prediction accuracy compared to other models. In order to develop detailed model of the annular two-phase flow, the average liquid film thickness in rod bundle is an important parameter. It can significantly affect the occurrence of dryout and post-dryout phenomena on heater surfaces. Most research on film thickness has been focused on pipe flows. Data in rod bundle geometry are very limited. However data in rod bundle geometry is much more valuable for reactor safety modeling and calculations. Because of this, an experiment to measure liquid film thickness has been performed for the air-water annular flow in an 8×8 BWR rod bundle. Film thickness data were obtained both on the rods and on the channel wall. Data were recorded at four axial locations within the rod bundle. This included locations just before and just after a spacer grid. This allows the spacer grids effect on the film thickness to be evaluated. The minimum film thickness was measured at the center rod. Also, the presence of the spacer grid results in reduced film thickness. Finally, the models for film thickness and interfacial friction factor in pipes have been implemented into rod bundle geometry with some approaches. These approaches are mainly focused on the length scale and flow conditions. With these methods, the correlations can be successfully used for rod bundle geometry.
Ishii, Purdue University.
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