Comparison of some theoretical concentration profiles for solid particles in a turbulent jet with the results of measurements using a laser-doppler anemometer

Abstract It has previously been shown, (Davidson and McComb, 1975), that the motion of small solid particles entrained in a turbulent air jet may be characterized by a dimensionless parameter = AU o t d d x 2 Here τ is the particle momentum relaxation time, d is the diameter of the jet nozzle, U o is the nozzle velocity and A is a constant which takes the value 7.02 for a round, free jet. Mean concentration and velocity profiles for particles have been obtained as power series in ϵ, the limit ϵ → 0 being the case of infinitesimal (tracer) particles which move as part of the fluid. A laster-Doppler anemometer (LDA) was used in the real-fringe mode to measure radial concentration profiles of small particles of metallic tungsten, in a round free air jet. The fringe spacing was adjusted to be much larger than the particle diameter and this permitted the use of a relationship between number of scatterers in the probe volume and the LDA signal modulation depth. Concentration ratios (relative to centre-line values) were measured for various radial and downstream positions. Normalized concentration profiles were obtained for ϵ = 0.1, 0.2 and 0.3; and compared with the theoretical predictions. An axial mean velocity profile (for particles) was measured for ϵ = 0.2 and also compared with the theoretical predictions. In all cases, the theoretical curves were found to agree with the measured values, within experimental error.