Direct Numerical Simulation Of Non-isothermal Gas-Particle Homogeneous Isotropic Turbulence

The mechanism investigation and model prediction on the interaction of solid particles or liquid droplets with velocity and temperature field of turbulent carrier flow are the most important problems in reacting (combusting) multiphase turbulent flows. And many studies have been conducted on gas-particle two-phase turbulent flows both home and abroad. However, researches on non-isothermal two-phase flows are still limited.Direct Numerical Simulation has been utilized to investigate the interaction of particles with velocity and temperature field of homogeneous isotropic turbulent flow in this thesis. The interaction between particles and velocity field, including particle transportation by turbulent flow and turbulence modulation by particles, are first examined in detail, based on which further studies have been done on the heat transfer from fluid to particles and particles′effects on temperature field. The pseudo-spectral technique is used to compute the velocity and temperature field. Taylor-scale Reynolds number ranges from 45 to 24 in decaying turbulence and for stationary flow it is 95. The motion of heavy particles is assumed to be governed by particle inertia (particle time constant) and gravity. Simulations have been performed for a range of particle diameter from 30 to 300μand the corresponding non-dimensional response time from 0.1 to 100.In contrast to fluid particles, finite inertia particles have a tendency to collect in regions with low vorticity and high strain. It is most obvious for particles withτp/τκ~1.0, which is called preferential concentration in the literatures. Preferential concentration greatly affects the dispersion of particles in turbulence. The particle eddy diffusion coefficient is maximal and over that of the fluid about 25% for particles withτp/τκ=1.0. The effect of drift velocity due to body force is to reduce the particle dispersion and due to the continuity effect the dispersion of particle along the drift direction is more affected than that perpendicular to the direction of drift. Comparison of the simulation results on particle dispersion with the Wang & Stock's theory shows good agreement.In decaying isotropic turbulence, small particles (τp/τκ0=0.1) reduce the decay rate of the turbulent kinetic energy and large particles (τp/τk0=5.0) enhance the rate as compared to particle-free turbulence. Intermediate particles (τp/τk0=1.0) are ejected from the high vorticity cores and consequently they have no obvious effect on the velocity field. In forced isotropic turbulence, for all particles considered in this thesis the turbulent kinetic energy and viscous dissipation rate at a mass loading of 1.0 is 50% lower than that at zero mass loading. A spectral analysis shows that there is a non-uniform distortion of the turbulence with a relative increase in small-scale energy while a relative decrease in large-scale energy.The fluid temperature intensities seen by particles, reaches a minimum for particle with response time close to the Kolmogorov time scale of the fluid as a result of the combination of the preferential concentration and the ramp-cliff structure of passive scalar (temperature). The particle seen fluid temperature fluctuation is lower than those of fluid by almost one factor for Pr=0.3. With the increase of the particle inertia, the autocorrelation coefficient of the fluid temperature seen by particles decreased more rapidly than that of the particle temperature. The mean temperature gradient contributes to the correlation between the particles velocity component and temperature fluctuations in the direction of the gradient.The effects of particles on temperature field differ from that on velocity field. With increasing particle mass loading, the fluid temperature intensities and the dissipation rate of the fluid temperature decreases linearly, and the degree of reduction is higher for larger particle specific. At a mass loading of 1.0,the temperature dissipation rate is only 30% of that at zero mass loading and the Lagrangian fluid temperature integrate time seen by particles is more than 80% above that at zero mass loading. The momentum coupling (effects of particles on velocity field) attenuates the effect of the direct interaction between two phases, but enhances the fluid temperature dissipation rate.