LDA Measurements in a Turbulent Diffusion Flame with Axial Pressure Gradient

Abstract

Earlier work on the turbulent diffusion flame of a hydrogen jet issuing into a co-flowing air stream has been extended to include the effects of axial pressure gradients. Pressure gradients with parameters of − 1.1 × 10⁻³, — 0.41 × 10⁻³ and +0.09 × 10⁻³ were produced by converging and diverging the floor and roof of the working section. Laser-Doppler anemometer (LDA) measurements including mean velocities, turbulence intensities and Reynolds stress were made with only the jet seeded so that the measurements are of a turbulent zone average nature. Both favourable (negative) and adverse pressure gradients shorten the flame. A flame of elliptical cross-section was produced but the distortion is fully accounted for by the free stream strain and the results are presented in pseudo-axisymmetric form. For favourable pressure gradient, the mean velocity profiles in the radial direction are closely similar to those found earlier for near-zero pressure gradient. For the adverse pressure gradient, the mean profiles show no such similarity and eventually show a negative excess or wake profile. This flame also shows a significant secondary flow due to buoyancy. Both favourable and adverse pressure gradients produce increases in turbulence intensity with the velocity scale ratio increasing to 0.35 and more. Structure parameters such as skew-ness, kurtosis and R_uv at the maximum shear stress point are much the same as in isothermal jets and the near-zero pressure gradient flame. The results should prove to be a good test for second order closure models of-turbulence as the ratio of advection to production varies considerably with pressure gradient. There is also apparently a significant direct effect of the mean pressure gradient on the kinetic energy of the turbulence.