TURBULENCE MODULATION IN JET DIFFUSION FLAMES - MODELING AND EXPERIMENTS

Turbulence in a diffusion Aame is modulated by thermal expansion, buoy ancy effects and lift-off. Aside from simple shear-generated sources d ensity-velocity correlations, <(rho'u ''(i))over bar> represent additi onal source terms in the Reynolds-stress equations that distort the sh ear-generated turbulence anisotropy. A modeled transport equation and several zero-order models of <(rho'u ''(i))over bar> are analyzed and their effect on flame predictions with a Favre averaged second-order m oment closure for velocity and scalar transport is investigated. The c hemical reaction is described by chemical equilibrium and laminar flam elet modeling. The latter is shown to have limitations in application due to differential diffusion. Two attached, vertical H-2/N-2-air flam es with the same Reynolds numbers but different Froude numbers are inv estigated numerically and experimentally. The desired data base for an overall comparison is provided by comprehensive 3D-LDV, coherent anti -Stokes Raman spectroscopy and spontaneous Raman spectroscopy measurem ents. The calculations yield correct results in all measured profiles of velocity, temperature and species concentrations. It is shown that only one zero-order model and the transport equation of<(rho'u ''(i))o ver bar> are adequate. The neglect of those terms will falsify the pre diction of decay rates, fluctuations and flame shapes. The magnitude o f errors depends on the local Froude number which decreases downstream . The increase in the influence of buoyancy leads to smaller decay rat es of axial velocity and to enhanced scalar mixing. Furthermore, turbu lence intensities are reduced, and scalar fluctuations and anisotropy are enlarged. The experimentally observed visible flame length shorten ing with decreasing, density weighted Froude number is reproduced by t he presented model.