Local measurement of air-water velocity field in rectangular channel with porous plate using LDV

Abstract

Sufficient knowledge of velocity field is indispensable to understand a turbulence structure and to develop a turbulence model in a boiling two-phase flow. Furthermore, bubble effects on CHF mechanism are related with two-phase turbulence structure. There is, however, little information on the turbulence structure in the boiling two-phase flow. Though some researchers reported important knowledge on the turbulence structure in a boiling bubbly flow, this knowledge was not sufficient for understanding bubble effects and turbulence structure. In this study, time-mean axial and radial velocities and axial and radial turbulent fluctuation intensities were measured in turbulent upflow of water through a rectangular channel with porous plate. Turbulent intensities were also measured with various porous conditions and channel geometry in order to know the porous diameter effects and channel width effects. A two-component laser Doppler velocimeter was used for the local velocity measurements. Experiments are conducted for water mass fluxes of 588~2667Kg/㎡s at pouring air mass flow rates of 0, 2, 4 and 6 lpm. From the experiments, total 239 data of velocity and its velocity fluctuation intensity were obtained. It was found that dramatic increases of the axial and radial velocity intensities occurred near the bubbly layer. For the radial velocity intensity, the effect of bubbly layer on the entire velocity field is small in present air-water two-phase experiment. It was also found that for a channel with smaller porous diameter and smaller width, the velocity fluctuation intensities are larger. Comparison of the present data with boiling flow data showed that the turbulence length scales calculated have similar values with those of boiling flow, though the entire velocity field exhibits somewhat differently.