(479j) Turbulence Collapse in a Dilute Particle-Gas Suspension

Turbulence modification in a particle-gas suspension is usually considered to be due to one of two methods, turbulence attenuation due to the increased drag force exerted by the particles for relatively small particles, and turbulence enhancement due to the wake behind the particles for relatively large particles. The process of turbulence attenuation is usually considered to progress continuously as the particle volume fraction is increased. We have carried out direct numerical simulations (DNS) of turbulent particle-gas suspensions with Reynolds number based on the channel width and average flow velocity in the range 3000-5000. The particles are modeled as point particles with volume fraction in the range 0-0.003, mass loading in the range 0-10 and particle Stokes number in the range 0-377. The Reynolds number based on the particle diameter and the particle mean velocity is about 42. It is found that the turbulence attenuation is a discontinuous process, where there is a discontinuous decrease in the turbulence intensity by 1-2 orders of magnitude when the particle volume fraction is increased by 10^-4 for particle volume fraction in the less than 0.002. There is a reduction, by one order of magnitude, in the mean square fluctuating velocities in all directions and in the Reynolds stress. Though there is a modest increase in the energy dissipation due to particle drag, this increase is smaller than the decrease in the turbulent energy production; moreover, there is a decrease in the total energy dissipation rate when there is turbulence collapse. The flow at particle volume fraction as low as 0.001 resembles a laminar flow with small fluctuation due to the presence of the particles, suggesting that a small increase in the particle volume fraction can completely relaminarise the flow. Thus, turbulence attenuation appears to be due to a disruption of the turbulence production mechanism, and not due to the increased dissipation due to the particles.