M. G. Mungal

Mixing, structure and scaling of the jet in crossflow

The mixing of the round jet normal to a uniform crossflow is studiedn for a range of jet-to-crossflow velocity ratios, r , from 5 to 25. Planar laser-induced fluorescence (PLIF) of acetone vapour seeded into the jet is used to acquire quantitativen two-dimensional images of the scalar concentration field. Emphasis is placed on r =10n and r =20 and a few select images are acquired up to r =200.n The Reynolds number based on the jet exit diameter, d , and the exit velocity varies from 8400 to 41 500. Images are acquired for conditions in which the product rd isn held constant, requiring decreasing d for increasing r . Results from this experimental study concern structural events of then vortex interaction region, and mixing and mean centreline concentration decayn in the near and far fields. The results cover all three regions of the transverse jet,n and suggest that the jet scales with three length scales: d , rd and r 2 d . Events within the vortex interaction region display d -scaling,n including the crossflow boundary layer separation and roll-up. Over the range of velocity ratios studied, the vortex interaction region shows r -dependent variations in then flow field, including the emergence of jet fluid in the wake structures for r >10 andn a slower development of the counter-rotating vortex pair (CVP) in higher- r jets. The trajectory and physical dimension of the jet in both the near andn far field display rd -scaling. The near field is characterized by a centreline concentration decay along the centreline coordinate s of s −1.3 ,n different from the decay rate ( s −1 ) of the free jet. When normalized by rd , the decay of each velocity-ratio jetn branches away from the s −1.3 decay, approaching a decay of s −2/3 , a rate predicted by modelling efforts.n The branch points represent a transition in the flow field from enhancedn mixing to reduced mixing compared to the free jet. When normalized by r 2 d , the branch points occur at a uniform jet position, s / r 2 d =0.3,n which is viewed to be the division between the near and far fields. Self-similarity is not seen in the near field,n but may be present in the far field. The view of the branch points as a place of transition in the flow isn supported by the probability density function (p.d.f.) of concentration along the uppern edge of the jet. Before the branch points, the p.d.f.s are non-marching in character,n and after the branch points, they are tilted in character. Instantaneously, the CVP is asymmetric in shape and concentration. Endn views reveal extensive motion of the CVP and plan views show this motion cann occur in both axisymmetric and sinusoidal motion. Ensemble-averaged images shown the jet concentration is asymmetric about the centreline plane.

Large-scale structure and entrainment in the supersonic mixing layer

Experiments were conducted in a two-stream planar mixing layer at convective Mach numbers, M c , of 0.28, 0.42, 0.50, 0.62 and 0.79. Planar laser Mie scattering (PLMS) from a condensed alcohol fog and planar laser-induced fluorescence (PLIF) of nitric oxide were used for flow visualization in the side, plan and end views. The PLIF signals were also used to characterize the turbulent mixture fraction fluctuations. Visualizations using PLMS indicate a transition in the turbulent structure from quasi-two-dimensionality at low convective Mach number, to more random three-dimensionality for

Instantaneous flame-stabilization velocities in lifted-jet diffusion flames

M_cgeqslant 0.62

A Study of the Laminar Flame Tip and Implications for Premixed Turbulent Combustion

. A transition is also observed in the core and braid regions of the spanwise rollers as the convective Mach number increases from 0.28 to 0.62. A change in the entrainment mechanism with increasing compressibility is also indicated by signal intensity profiles and perspective views of the PLMS and PLIF images. These show that at M c = 0.28 the instantaneous mixture fraction field typically exhibits a gradient in the streamwise direction, but is more uniform in the cross-stream direction. At M c = 0.62 and 0.79, however, the mixture fraction field is more streamwise uniform and with a gradient in the cross-stream direction. This change in the composition of the structures is indicative of different entrainment motions at the different compressibility conditions. The statistical results are consistent with the qualitative observations and suggest that compressibility acts to reduce the magnitude of the mixture fraction fluctuations, particularly on the high-speed edge of the layer.

Time evolution and mixing characteristics of hydrogen and ethylene transverse jets in supersonic crossflows

Liftoff height and velocity measurements are presented for turbulent, lifted methane and ethylene flames. A range of Reynolds numbers from 3800 to 22,000 is investigated, and the effect of coflow velocity is examined. The mean liftoff height is shown to increase with jet-exit and coflow velocity, while the rms fluctuation about the mean increases with distance from the jet exit. Particle image velocimetry (PIV) is used to provide instantaneous, two-dimensional velocity fields in the region of the lifted flame base. The results show that the instantaneous flame base is anchored primarily in the low-velocity regions of the jet, with axial and radial movement of the flame to meet this criterion. The fluid velocity conditioned on the instantaneous flame base location is less than three times the laminar flame speed, and the velocity profiles through the flame base are similar in nature to those predicted by recent simulations of triple flames.

Simultaneous measurements of scalar and velocity field evolution in turbulent crossflowing jets

Abstract Flame surface curvature is a significant geometrical parameter that affects the structure and propagation of premixed laminar and turbulent flames. In this study, the flame tip of a two-dimensional laminar Bunsen burner is investigated using a quasi-one dimensional model, direct numerical simulations and experimental results. The laminar flame tip is a simple prototype of curved flamelets embedded in a turbulent flow field. It is shown that two characteristic flame speeds are necessary to give a local description of a given flamelet: the consumption speed associated with the structure of the reaction zone, and the displacement speed of the flame front relative to the unburned flow. The quasi-one dimensional model shows that three different mechanisms affect the displacement speed of a curved flame in a non-uniform flow field: a chemical mechanism associated with the expansion of the reaction zone structure, a hydrodynamic mechanism due to isothermal area modification by lateral flow divergence an...

A planar Mie scattering technique for visualizing supersonic mixing flows

We report an experimental investigation that reveals significant differences in the near-flowfield properties of hydrogen and ethylene jets injected into a supersonic crossflow at a similar jet-to-freestream momentum flux ratio. Previously, the momentum flux ratio was found to be the main controlling parameter of the jet’s penetration. Current experiments, however, demonstrate that the transverse penetration of the ethylene jet was altered, penetrating deeper into the freestream than the hydrogen jet even for similar jet-to-freestream momentum flux ratios. Increased penetration depths of ethylene jets were attributed to the significant differences in the development of large-scale coherent structures present in the jet shear layer. In the hydrogen case, the periodically formed eddies persist long distances downstream, while for ethylene injection, these eddies lose their coherence as the jet bends downstream. The large velocity difference between the ethylene jet and the freestream induces enhanced mixing...

Transverse jets and jet flames. Part 1 : Scaling laws for strong transverse jets

Simultaneous planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) yield measurements of two-dimensional jet fluid concentration and velocity fields in turbulent crossflowing jets. The jet-to-crossflow velocity ratio is

Direct measurement of entrainment in reacting/nonreacting turbulent jets

r = 5.7

Transverse jets and jet flames. Part 2. Velocity and OH field imaging

and the jet exit Reynolds number is approximately 5000. The measurements are focused on the developing region of the flow. Two flow configurations are studied, one in which the jet nozzle is flush with the tunnel wall and the other where the nozzle protrudes into the uniform region of the tunnel flow. The jet nozzle in both cases is a simple pipe. The averaged scalar and velocity fields show a strong similarity in growth rates and centreline decay rates between the two nozzle configurations when using the centreline downstream coordinate