By direct numerical simulations, the particle dispersion is systematically investigated in a three-dimensional spatially developing compressible mixing layer. The convective Mach number is 1.2 and particles interact with fluid through both the one- and two-way coupling. Six simulations are conducted with different particle diameters (Stokes numbers) or particle back-reaction. The compressible mixing layer is characterized by various vortical structures and unsteady shocklets, which both have significant effects on the dispersion of particles. The particles tend to accumulate in the peripheries of the vortical structures with high density, low vorticity, and high strain rate inside the mixing layer, as well as the high-density regions behind the shocklets outside the mixing layer. Due to the sweep and ejection effects, the particles from the high-speed side cluster in the high-speed streaks while those from the low-speed side collect in the low-speed steaks. Also, the particle mixing between the two streams skews towards the low-speed side. Compared with the other cases, medium particles show the strongest preferential concentration in the peripheries of vortices while small particles exhibit the most significant tendency to accumulate behind the shocklets. As the Stokes number decreases, the particle mixing is enhanced with more significant deviation towards the low-speed side. Besides, the particle back-reaction on fluid attenuates the preferential concentration. Outside the mixing layer, the number of the compression regions of the particle dispersion decreases, but the particles have stronger concentration behind the residual shocklets. The particle mixing as well as the skewness towards the low-speed side is also attenuated under the two-way coupling, which is attributed to the reduction of the vortex number and the centrifugal effects on particles.

Combustion instability is one significant issue in the development of aero-engines and gas turbines. In practical combustors, the exit boundary of the combustion chamber is complex and the mechanisms of thermoacoustic coupling become more complicated. In order to study the effect of outlet boundary on combustion instabilities, the premixed swirling combustor was built and several different damping rings were adopted to obtain the outlet contraction of combustion chamber. Acoustic impedances corresponding to different configurations were measured in cold flow, and reactive experiments were conducted in different conditions. The results show that combustion instabilities were tightly related to both the equivalence ratio and exit boundary. The quarter wave mode would be triggered at a certain interval of equivalence ratio and suppressed in different levels while applying various damping rings. The outlet contraction left an effect on both the frequency and amplitude of thermoacoustic mode. It affected the resonant frequency by influencing the gas temperature and modifying the acoustic boundary. Oscillation amplitudes were weakened due to the change in acoustic energy generated from unstable heat release. Moreover, the helical mode corresponding to the precessing vortex core was detected, which was also affected by the outlet boundary. These results are rarely reported in previous work and may provide useful ideas in the prediction and control of combustion instabilities.

With the Eulerian–Lagrangian point-source method, turbulence modulation by dispersed particles is systematically investigated in a three-dimensional spatially evolving compressible mixing layer with the convective Mach number up to 1.2. Particles interact with the mixing layer through two-way coupling, and three simulations with different particle diameters are conducted and compared with the particle-free simulation. The underlying mechanisms responsible for turbulence modulation are revealed by analyzing the transport equations of the Reynolds stresses and turbulent kinetic energy, especially the two-way coupling terms. The compressible mixing layer turbulence is significantly anisotropic with strong three-dimensionality. The addition of particles augments turbulence anisotropy of the shear layer, and the augmentation becomes greater as the particle inertia increases, which is attributed to the different particle responsive features to the fluid fluctuations in the streamwise, normal, and spanwise directions. Particles respond fast to the fluid streamwise fluctuation but slowly to the normal and spanwise fluctuations because the streamwise turbulent intensity is larger compared with the normal and spanwise turbulent intensities. Consequently, the streamwise fluctuating velocity and the Reynolds shear stress are augmented and the normal and spanwise velocity fluctuations are attenuated. Besides, small particles slightly enhance the growth rate of the mixing layer, while large particles reduce the shear layer growth rate in the fully developed turbulence, which is due to the quick response of small particles and the slow response of large particles to the total fluid fluctuation.

Direct numerical simulations of non-premixed swirling fuel-rich/fuel-lean flames within a high-pressure model gas turbine combustor are conducted to investigate the flow and flame structures, as well as the transport mechanisms of both turbulent kinetic energy (TKE) and enstrophy. The effects of non-premixed flames upon these characteristics are also analyzed through comparison with the corresponding non-reacting swirling flows. We demonstrate that the turbulence state in the swirling flows behaves axisymmetrically overall in the current cylindrical laboratory-type combustor and is more likely to be cigar shaped in the presence of combustion. The analysis of TKE budgets within non-reacting swirling flows indicates that TKE is predominantly produced by mean shear in the shear layers and redistributed by transport effects from the inner shear layer (ISL) to the internal-recirculation zone; however, these transport effects are suppressed by combustion in fuel-lean non-premixed flames. Although the total pressure effects consume TKE with a similar magnitude in all cases, the essential cause is different. The influence of combustion upon TKE budgets is more significant for fuel-lean flames than for fuel-rich flames as a result of the stronger burning intensity in the ISL of the former. Analysis of enstrophy dynamics shows that dilatation and baroclinic torque play relatively noticeable roles in swirling non-premixed flames, unlike their negligible effects in high-intensity homogeneous isotropic turbulence. The augmentation of baroclinic torque caused by non-premixed swirling combustion mainly arises from the remarkable decrease in density and enhancement of preferential alignment between the vorticity and baroclinic torque vectors.

Nonreacting swirling flow behavior through a novel double swirl burner has been investigated in this paper using Detached Eddy Simulation (DES) calculations and experimental measurements. The effect of mixing both the swirling flow of the inner jet (primary air or inner swirler) and the swirling flow of the annular jet (secondary air or outer swirler) on the flow structure has been analyzed. Moreover, pressure losses and flow field characteristics have been investigated at different inlet air mass flow rates and combustor diameters. Due to the drawbacks of general pressure loss to predict the pressure drop through high turbulence swirl flow as well as the complex geometry, the time-averaged viscous dissipation field function is used to predict the pressure drop through the burner. This function is based on the relationship between viscous dissipation and pressure losses. DES flow fields are compared with high-speed particle image velocimetry measurements at different inlet air mass flow rates. It is found that there is good agreement between numerical and experimental results, and both the central toroidal recirculation zone and the corner recirculation zone are well captured. In the case of a single inner swirler, the general pressure drop is very high compared to single outer and double swirl generators, and 84% of the inlet air pass through the outer swirler and just 16% of the air pass through the inner swirler in the case of a double burner (partially premixed). The time-averaged viscous dissipation field function is very important to identify the locations that lead to pressure drop inside the burner, which induces to select the appropriate design for the best performance. In addition, it is concluded that the performance of the double swirl burner is more efficient compared with the other two single swirlers in terms of pressure losses and flow structure.