Numerical Study of Scramjet Combustor with Tandem Dual Cavity for Nonreactive Jets

In the past decades flame out is a major phenomenon that paves way for high fuel consumption in scramjet combustion. For enhancing mixing and flame holding characteristics, different types of cavities are introduced in a scramjet combustion chamber which can hold air for a bit and acts like an atomizer. For increasing combustion efficiency and burnout ratios recirculation is maintained by using cavity and ramp angle techniques. In this paper numerical analysis has been carried out for two dimensional non-reacting flows in the combustor of scramjet engine with tandem dual cavity that creates high turbulent kinetic energy for ensuring combustion instability. This work is an enlightened approach for predicting the flow phenomenon that induces re-circulations after implementing various tandem dual cavities with varying length to diameter ratio and ramp angle. These in turn overcome low mixing rates due to compressibility effects at high convective Mach number.

Effects of Flow Compressibility on Two-Phase Mixing in Supersonic Droplet-Laden Flows

This research addresses a numerical analysis on the effects of flow compressibility on the characteristics of droplet dispersion, evaporation, and mixing of fuel and air according to the simulation of the spatially developing supersonic shear flows laden with evaporating n-decane droplets. A sixth-order hybrid WENO numerical scheme is employed for capturing the unsteady wave structures. The influence of inflow convective Mach number ( M c ), representing the high-speed flow compressibility, on the two-phase mixing is analyzed, in which M c is specified from 0.4 to 1.0. It is found that the shearing vortex is compressed spatially as M c increases, associated with the alternate distributions of compression and expansion regimes in the flow field. The flow compressibility changes not only the vortex structures but also the aerothermal parameters of the shear flows, and further influences the dispersion and evaporation of droplets. The two-phase mixing efficiency is observed to decrease as M c increases.

Jet Noise Reduction in Co-Flowing Jets with Finite Lip Thickness

Passive control for suppressing mixing noise from Co-Flowing Jets (CFJ) is presented in this study. The idea behind this is to reduce the convective Mach number of turbulent eddies that produce intense sound radiation. The present study analyses co-flowing jets with a bypass ratio of 6.3 and the primary nozzle lip thickness of 10 mm. The aim of the study is to find the jet noise level in finite lip thickness in co-flowing jets. CFJ with finite lip thickness forms a recirculation zone (in the near field). The secondary core and recirculation zone are shielding the primary core by reducing the jet noise. A single free jet with a diameter equal to that of a primary nozzle of co-flowing jet is also studied for comparison. The results show that co-flow jet with finite lip thickness of 10mm for various emission angles and the Overall Sound Pressure Level (OASPL) level gets reduced when compared with the single free jet.

Entrainment in a compressible turbulent shear layer

Direct numerical simulations (DNS) of temporally evolving shear layers have been performed to study the entrainment of irrotational flow into the turbulent region across the turbulent/non-turbulent interface (TNTI). Four cases with convective Mach number from 0.2 to 1.8 are used. Entrainment is studied via two mechanisms; nibbling, considered as vorticity diffusion across the TNTI, and engulfment, the drawing of the pockets of the outside irrotational fluid into the turbulent region. The mass flow rate due to nibbling is calculated as the product of the entrained mass flux with the surface area of the TNTI. It is found that increasing the convective Mach number results in a decrease of the average entrained mass flux and the surface area of the TNTI. For the incompressible shear layer the local entrained mass flux is shown to be highly correlated with the viscous terms. However, as the convective Mach number increases, the mass fluxes due to the baroclinic and the dilatation terms play a more important role in the local entrainment process. It is observed that the entrained mass flux is dependent on the local dilatation and geometrical shape of the TNTI. For a compressible shear layer, most of the entrainment of the irrotational flow into the turbulent region due to nibbling is associated with the compressed regions on the TNTI. As the convective Mach number increases, the percentage of the compressed regions on the TNTI decreases, resulting in a reduction of the average entrained mass flux. It is also shown that the local shape of the interface, looking from the turbulent region, is dominated by concave shaped surfaces with radii of curvature of the order of the Taylor length scale. The average entrained mass flux is found to be larger on highly curved concave shaped surfaces regardless of the level of dilatation. The mass fluxes due to vortex stretching, baroclinic torque and the shear stress/density gradient terms are weak functions of the local curvatures on the TNTI, whereas the mass fluxes due to dilatation and viscous diffusion plus the viscous dissipation terms have a stronger dependency on the local curvatures. As the convective Mach number increases, the probability of finding highly curved concave shaped surfaces on the TNTI decreases, whereas the probability of finding flatter concave and convex shaped surfaces increases. This results in a decrease of the average entrained mass flux across the TNTI. Similar to the previous works on jets, the results show that the contribution of the engulfment to the total entrainment is small for both incompressible and compressible mixing layers. It is also shown that increasing the convective Mach number decreases the velocities associated with the entrainment, i.e. induced velocity, boundary entrainment velocity and local entrainment velocity.

Direct numerical simulation of a spatially developing compressible plane mixing layer: flow structures and mean flow properties

The spatially developing compressible plane mixing layer with a convective Mach number of 0.7 is investigated by direct numerical simulation. A pair of equal and opposite oblique instability waves is introduced to perturb the mixing layer at the inlet. The full evolution process of instability, including formation of $ \mrm{\Lambda} $-vortices and hairpin vortices, breakdown of large structures and establishment of self-similar turbulence, is presented clearly in the simulation. In the transition process, the flow fields are populated sequentially by $ \mrm{\Lambda} $-vortices, hairpin vortices and ‘flower’ structures. This is the first direct evidence showing the dominance of these structures in the spatially developing mixing layer. Hairpin vortices are found to play an important role in the breakdown of the flow. The legs of hairpin vortices first evolve into sheaths with intense vorticity then break up into small slender vortices. The later flower structures are produced by the instability of the heads of the hairpin vortices. They prevail for a long distance in the mixing layer until the flow starts to settle down into its self-similar state. The preponderance of slender inclined streamwise vortices is observed in the transversal middle zone of the transition region after the breakup of the hairpin legs. This predominance of streamwise vortices also persists in the self-similar turbulent region, though the vortices there are found to be relatively very weak. The evolution of both the mean streamwise velocity profile and the Reynolds stresses is found to have close connection to the behaviour of the large vortex structures. High growth rates of the momentum and vorticity thicknesses are observed in the transition region of the flow. The growth rates in the self-similar turbulence region decay to a value that agrees well with previous experimental and numerical studies. Shocklets occur in the simulation, and their formation mechanisms are elaborated and categorized. This is the first three-dimensional simulation that captures shocklets at this low convective Mach number.

Numerical Simulation of the Supersonic Planar Mixing Layer

Large eddy simulation (LES) has been used to simulate both non-reacting and reacting supersonic planar mixing layers at convective Mach number Mc=0.3. The different eddy characteristics of two cases have been visualized and discussed based on our calculated results, and the differences of mixing layer structures have also been shown, which can provide some important guide for future relative engineering design.