Simultaneous Ldv Measurements of Gas and Particle Velocities in Two-phase Flow

An experimental investigation was conducted to simultaneously measure the instantaneous velocities of large particles and air in two-phase flow using Laser Doppler Velocimeter. Measurements at two Photo-Multiplier voltage settings are used to separate the signals from the small seeding particles that trace the air flow and the suspended solid particle based on the difference of scattered light intensity. The method was applied to obtain measurements in a vertical tunnel for 500 micron particles at different slip velocities. The associated air mean velocity, and turbulent intensity for the flow with and without particles show that the particles reduce the air velocity and increase turbulence intensity near the center. INTRODUCTION Two-phase turbulent flow are of considerable engineering importance in a wide range of applications such as rocket motors, ramjet combustion, jet cutting and in jet blast nozzles. Improving the performance of two-phase flow devices requires a knowledge of the instantaneous velocities of gaseous and solid phases. Velocity measurements in two-phase flow fields are complicated by particle-probe interactions and blockage of transmitted and scattered light by the dispersed phase. Different techniques have been developed for the instantaneous two-phase flow velocities measurements using Laser Doppler Velocimeter (LDV). Lee and Dust [1] developed an electronic processing scheme based on amplitude discrimination and filtering to separate the signals of large and small particles. They measured the velocity of air and glass particles ranging from 100 μm to 800 μm diameter in a vertical pipe flow. Their mean velocity profiles of air and glass spheres results showed increased slipvelocity for larger particles, and a reversed slip-velocity region near the wall in the case of small particles. Tsuji et al [2,3] separated the air and particle velocities by setting threshold values against pedestal and Doppler components of the photo multiplier signals. They measured the velocity of air and plastic particles ranging from 0.2 mm to 4 mm diameter in a vertical pipe. They reported that large particles increased air turbulence through out the pipe section while small particles reduced it. Hussainov et al, [4] used two power lasers with displaced probe volumes of two different sizes to measure the gas and 700 μm glass bead velocity in a vertical downward channel flow. They investigated the modulation of grid-generated turbulence and reported that the large particle caused attenuation of turbulence intensity in the streamwise direction, and decreased the energy spectra at high frequencies. Larger particle slip velocities occur in supersonic twophase flows than in the described subsonic flow studies discussed. Hamed et al. [5] predicted particle slip velocities as high as 200 m/s in convergent divergent nozzles, depending on the particle loading and initial conditions. The purpose of the present experimental study is to investigate the separation of large-particle and tracer LDV signals based on light intensity for applications involving velocity measurements in high-speed two-phase flow. EXPERIMENTAL SETUP A schematic of the test facility is shown in Figure 1. It consists of the following components: particle feeder, main air supply pipe, particle injector, acceleration tunnel, test section, exhaust filter and cyclone.