PARTICLE DYNAMICS IN AN ELECTROHYDRODYNAMIC FLOW FIELD INVESTIGATED WITH A 2-COMPONENT LASER-DOPPLER VELOCIMETER

A laser-Doppler instrument has been used to measure the migration velocity of NaCl particles in an electrohydrodynamic flow field of an electrical precipitator. The measured average migration velocity of 1.40-mum particles (number distribution median with a geometric standard devitation of 1.46) is approximately five to six times higher than the calculated steady-state velocity for a 1.40-mum particle, provided there is a saturation charge of at least 90%. Further, the particle velocities in the main flow direction are also influenced by the electrical operation conditions. Both observations demonstrate the important role of the state of the electrohydrodynamic flow field (super-position of moving gas ions and neutral gas molecules) on the particle transport, characterizied by the dimensionless electrohydrodynamic number N(EHD). A comparison between six different electrohydrodynamic states revealed that N(EHD) almost-equal-to 1 is a critical value for the mutual interactions between the gas ions and the neutral gas phase. Whereas for N(EHD) values >1 the stochastic particle motion is chiefly determined by the non-steady-state character of the negative corona, for N(EHD) values <1 the particle velocity fluctuations are governed by the turbulence level of the neutral fluid. These findings might be helpful in adjusting the operating conditions in electrical precipitators for an optimized particle separation.