Yuanzineng kexue jishu (Aug 2022)
Study on Interfacial Drag Model Based on Air-water Two-phase Flow Experiment in Vertical Pipe
Abstract
The study of interfacial drag characteristics in two-phase flow is of great significance for the closure of key constitutive models in system program. In this paper, the air-water two-phase flow experiment covering bubbly flow and slug flow regimes was carried out in vertical pipe with 25 mm diameter. The four-sensor conductivity probe was used to measure local radial distribution of bubble parameters. The probe included one long needle and three short needles, resulting in four pairs of positive and negative electrodes respectively with the stainless-steel tube at perpendicular angle. The local void fraction could be obtained based on the time proportion with high voltage. The bubble velocity and chord length could be calculated according to the time gap between bubbles contacting two adjacent needle tips in turn. The interfacial area concentration could be calculated with the bubble velocity at normal direction. The measurement uncertainties from four-sensor conductivity probe are about 10%, guaranteeing the accuracy of experimental results. The experimental results present that the void fraction and bubble chord length are both characterized with central peak distribution due to the lift force. While the relationship between interfacial area concentration and flow velocity is not monotonous. This is because that the interfacial area concentration results are affected by both factors. The interfacial area concentration is directly proportional to void fraction and inversely proportional to bubble chord length. The interphase drag models in bubbly flow and slug flow are further developed. The effects of liquid superficial velocity and pipe diameter on bubble size distribution are considered. And the relationship between critical Weber number and different liquid velocities is established. The calculated interfacial area concentration is compared with experimental data and RELAP5 predicted results. The RELAP5 predicted results are obviously lower than the experimental data. The present model greatly improves the calculation accuracy in predicting the interfacial area concentration and reduces the original average relative error from 35% to 19%. The modified model is also applied to calculate the void fractions from CISE experiment with steam-water two-phase flow in vertical tube. The calculated results of void fractions from present model are in well comparisons with results from the widely used drift model in both bubbly flow and slug flow. Based on the above validation results, the calculated interfacial area concentration and void fraction are in good agreement with experimental data, proving the validity of improved model. The present study can provide reference for further study of interphase force characteristics in two-phase flow.
