Reacting Shear Layer Research Articles

The effect of heat release on various statistical properties of a reacting shear layer

Three-dimensional direct numerical simulations were used to study the effect of heat release from a binary, single-step chemical reaction on the statistical properties of a temporally developing turbulent mixing layer. Various statistical moments, probability density functions, power spectral densities, and autocorrelations of a conserved scalar, and the velocity field are presented. Scalar-velocity and pressure-velocity correlations, and joint probability density functions, which are extremely difficult to measure experimentally, were also calculated from the simulations. Many features of the calculated statistics compare qualitatively well with results reported from related experimental studies. Significant changes in the vortex structure occur with moderate heat release, resulting in more diffuse vortices than in the isothermal simulation. Consequently, slower rotation rates of the coherent structures occur with moderate heat release. This effect has previously been shown to be caused by the baroclinic torques and thermal expansion in the mixing layer. The statistics in this study reflect these changes in the vortex structure due to moderate heat release.

Effect of Damkohler number on the reactive zone structure in a shear layer

Numerical simulation is used to study an unstable, two-dimensional, two-stream, spatially developing, confined, reacting shear layer. The physical model is based on a low-heat-release, temperature-independent chemical reaction and is used to investigate the effect of the flow field on the burning rate at high Reynolds number and moderate Damkohler numbers. The unsteady mass, mometum, energy, and species conservation equations are integrated using a grid-free, Lagrangian field method in which gradients of the primary variables are transported along particle trajectories to preserve the numerical accuracy as strong strains arise in the flow. Results reveal a strong similarity between the distributions of product concentration and vorticity at a wide range of Damkohler numbers. This similarity is due to the entrainment field associated with vorticity amalgamation into large-vortex eddies. At low Damkohler numbers, products form in distributed zones within the large eddies, following the mixing of entrained reactants. At high Damkohler numbers, products form within thin zones around the outer edges of the eddies and then get entrained into the their cores. Between the large eddies, the strain field reduces product concentration by thinning the braids and reducing the time allowed for mixing. The effect of chemical kinetics rates on product formation is strongest around a Damkohler number of 1, and reaches its limit around Damkohler number of 20. Product formation is strongly governed by the entrainment field, and is thus weakly dependent on the Reynolds number. The reactants' ratio across the layer affects the rate of product formation, through the mechanism of asymmetric entrainment, by enforcing stronger potential of chemical activity.

Calculation of supersonic turbulent reacting coaxial jets

The mixing and subsequent combustion within turbulent reacting shear layers is examined. To conduct this study, a computer program has been written to solve the axisymmetric Reynolds-averaged, Navier-Stokes equations. Turbulence is modeled using three algebraic turbulence models, and the chemical kinetics is modeled using a seven-species, seven-reaction, finite-rate chemistry model. Three separate flowfields are investigated. The effect of turbulent mixing upon the extent of combustion is demonstrated. No single turbulence model considered accurately predicted the degree of mixing for all three cases.

Effects of heat release in a turbulent, reacting shear layer

Experiments were conducted to study the effects of heat release in a planar, gas-phase, reacting mixing layer formed between two free streams, one containing hydrogen in an inert diluent, the other, fluorine in an inert diluent. Sufficiently high concentrations of reactants were utilized to produce adiabatic flame temperature rises of up to 940 K (corresponding to 1240 K absolute). The temperature field was measured at eight fixed points across the layer. Flow visualization was accomplished by schlieren spark and motion picture photography. Mean velocity information was extracted from Pitot-probe dynamic pressure measurements. The results showed that the growth rate of the layer, for conditions of zero streamwise pressure gradient, decreased slightly with increasing heat release. The overall entrainment into the layer was substantially reduced as a consequence of heat release. A posteriori calculations suggest that the decrease in layer growth rate is consistent with a corresponding reduction in turbulent shear stress. Large-scale coherent structures were observed at all levels of heat release in this investigation. The mean structure spacing decreased with increasing temperature. This decrease was more than the corresponding decrease in shear-layer growth rate, and suggests that the mechanisms of vortex amalgamation are, in some manner, inhibited by heat release. The mean temperature rise profiles, normalized by the adiabatic flame temperature rise, were not greatly changed in shape over the range of heat release of this investigation. A small decrease in normalized mean temperature rise with heat release was however observed. Imposition of a favourable pressure gradient in a mixing layer with heat release resulted in an additional decrease in layer growth rate, and caused only a very slight increase in the mixing and amount of chemical product formation. The additional decrease in layer growth rate is shown to be accounted for in terms of the change in free-stream velocity ratio induced by the pressure gradient.

Origin and manifestation of flow-combustion interactions in a premixed shear layer

The interactions between the flow field and the combustion process in a premixed shear layer are investigated using the results of numerical simulation. The reaction is governed by a finite-rate Arrhenius kinetics, the flow is compressible and at high Reynolds number, heat release is moderate and molecular heat and mass diffusivities are finite. The thickness of the reaction zone and that of the vorticity layer are approximately the same. Lagrangian simulations are obtained using the vortex and transport element methods. Results indicate that at the early stages, a reacting shear layer behaves like a laminar flame. During the growth of the roll-up eddy, the rate of burning is strongly enhanced by the entrainment fluxes that lead to the swelling of the reaction zone, and the total rate of product formation can be approximated by the unstrained laminar burning velocity times the flame length measured along the line of maximum reaction rate. Following the burning of the eddy core, the strain field along the eddy boundaries causes a noticeable thinning of the reaction zone and reduces the rate of burning. Baroclinic vorticity generation due to the acceleration of fluid elements in the density gradient is the most important mechanism by which combustion affects the flow field. It augments the overall volumetric entrainment into the eddy core, and causes an entrainment asymmetry with a bias towards the products. The generated vorticity extends the growth period of the eddy and imparts on it an extra mean convective motion.

Flame extinction in a temporally developing mixing layer

Nonequilibrium effects leading to the local quenching of a diffusion flame were investigated by examining the evolution of large-scale structures in a two-dimensional temporally developing mixing layer. Pseudospectral calculations of a temperature-dependent, nonpremixed, constant-density, reacting shear layer indicate that the primary important parameter to be considered for flame extinction is the local instantaneous scalar dissipation rate, conditioned at the scalar stoichiometric value (Xst). At locations where this value is increased beyond a critical value (Xq), the local temperature decreases and the instantaneous reaction rate drops to zero. This is consistent with the results of perturbation methods employing large activation energy asymptotics for the study of flame extinction in nonpremixed flames.

Turbulent mixing and combustion in a reacting shear layer

The temperature field is investigated in a gaseous mixing layer consisting of low-concentration hydrogen and fluorine. The results show the presence of large, hot structures separated by tongues of cool fluid that enter the layer from either side. The cores of the structures appear to be well mixed. the usual bell-shaped mean temperature profiles result from a duty cycle whereby a given point sees alternating hot and cool fluid, which results in the local mean. The adiabatic flame temperature is not achieved on average at any point across the layer. It is found that, in general, two different mean temperature profiles result from a given set of reactant compositions if the sides of the layer on which they are carried are reversed. These observations are not consistent with gradient diffusion concepts.

The Effects of Turbulent Mixing in Clouds

Abstract Turbulent mixing of cloudy and cloud-free air may play an important role in determining the overall dynamic and microphysical behavior of warm clouds. We present a model of turbulent mixing based on laboratory and theoretical studies of chemically reacting shear layers, extended to include the effects of buoyancy instabilities and droplet sedimentation. It is found to be consistent with recent observations of microphysical variability in natural clouds.

Experimental Study of the Flowfield of a Two-Dimensional Premixed Turbulent Flame

A turbulent reacting shear layer in a premixed propane/air flow has been studied in a two-dimensional combustor, with a flame stabilized behind a rearward facing streamlined step. Spark shadowgraphs show that in the range of velocities (7.5-22.5 m/s corresponding to Reynolds numbers of 0.5xl0 4-1.5xl04 cm1) and equivalence ratios (0.4-0.7) studied, the mixing layer is dominated by Brown-Roshko type large coherent structures in both reacting and nonreacting flows. High-speed schlieren movies show that these eddies are convected downstream and increase their size and spacing by combustion and coalescence with neighboring eddies. Tracing individual eddies shows that on average eddies accelerate in the reacting shear layer as they move downstream, with the highest acceleration close to the origin of the shear layer. Combustion is confined to those large structures which develop as a result of vortical action of the shear flow. On the average, the reacting eddies have a lower growth rate than nonreacting eddies. A turbulent boundary layer created by means of a tripping wire upstream of the edge of the step virtually eliminates the large coherent structures in the shear layer, while for the case in which the wire could not trigger the transition to turbulence, the large coherent structures dominated the reacting and nonreacting flows.