Particle concentration measurements by phase-doppler anemometry in complex dispersed two-phase flows

Abstract

Particle concentration or mass flux measurements by phase-Doppler anemometry are based on counting the number of particles crossing the probe volume. In complex particulate flows this requires the knowledge of the particle size-dependent cross-section of the measurement volume perpendicular to the instantaneous particle velocity for each sample. A new method is presented which allows to estimate the instantaneous particle velocity using a one-component PDA-system with the aid of the integral value under the envelope of the Doppler signal. The envelope is reliably determined by a recently developed analogue burst detection electronic circuit using narrow band-pass filters and a synchro detector. The particle size-dependent measurement volume is obtained by the mean log-arithmic amplitude method introduced by Qiu and Sommerfeld (1992). The special signal processing requirements for realizing this method were implemented in a novel signal processor which additionally involves a burst detection based on an online estimation of the signal-to-noise ratio (SNR) and hence the data aquisition is only triggered for signals above a pre-set SNR level. The estimation of the signal frequency and phase is based on the calculation of the cross-spectral density using a hardware FFT (Fast Fourier Transform) chip. In order to demonstrate the accuracy of the novel method for particle concentration and mass flux determination, measurements were performed in a liquid spray and a particle laden swirling flow. Especially in the swirling flow, the particles exhibit random trajectories through the measurement volume and the powerfulness of the present method is demonstrated especially for this complex flow.