Shanwu Wang

Large-eddy simulations of turbulent swirling flows injected into a dump chamber

Turbulent swirling flows injected into a coaxial dump chamber at different swirl numbers were studied using large-eddy simulations. The Favre-filtered conservation equations of mass, momentum, and energy in three dimensions were solved numerically by means of a finite-volume approach. Results have been validated against experimental data in terms of mean flow velocity and turbulence properties. The work provides insight into several salient features of swirling flows, including vortex breakdown, shear-layer instability, and vortico-acoustic interactions. The dominant acoustic mode in the chamber was found to be sensitive to unsteady vorticity evolution, which in turn strongly depends on the swirl number. Low-frequency acoustic oscillations may arise from large-scale coherent motions in the central toroidal recirculation zone at high swirl numbers. In contrast, the shear-layer instability downstream of the backward-facing step results in high-frequency acoustic waves at low swirl numbers.

Large-eddy simulations of gas-turbine swirl injector flow dynamics

A comprehensive study on confined swirling flows in an operational gas-turbine injector was performed by means of large-eddy simulations. The formulation was based on the Favre-filtered conservation equations and a modified Smagorinsky treatment of subgrid-scale motions. The model was then numerically solved by means of a preconditioned density-based finite-volume approach. Calculated mean velocities and turbulence properties show good agreement with experimental data obtained from the laser-Doppler velocimetry measurements. Various aspects of the swirling flow development (such as the central recirculating flow, precessing vortex core and Kelvin-Helmholtz instability) were explored in detail. Both co- and counter-rotating configurations were considered, and the effects of swirl direction on flow characteristics were examined. The flow evolution inside the injector is dictated mainly by the air delivered through the primary swirler. The impact of the secondary swirler appears to be limited.

Systematic analysis of lean-premixed swirl-stabilized combustion

A systematic data-analysis procedure is established to explore the underlying mechanisms responsible for driving unsteady flow motions in gas-turbine combustors. Various data processing and analysis approaches are developed and implemented. These include triple decomposition of flowfield, vortex identification, spectral analysis, linear acoustic modal and hydrodynamic stability analyses, and proper orthogonal decomposition. The work allows for a detailed investigation of the mechanisms of energy exchange between the mean, periodic, and turbulent flowfields in a combustion chamber, as well as their collective interactions with chemical heat release. As a specific example, the combustion dynamics in a lean-premixed swirl-stabilized combustor operating under a variety of conditions is carefully examined, based on an avalanche of time-resolved numerical data obtained from large-eddy simulations.

Unsteady flow evolution in swirl injectors with radial entry. II. External excitations

Our previous study on turbulent flows in a gas-turbine swirl injector was extended to explore the effects of externally impressed excitations on the unsteady flow evolution. Three-dimensional large-eddy simulations were conducted to investigate the responses of the injector flowfield by imposing periodical oscillations of the mass flow rate at the entrance over a wide range of frequencies. Results show that the impressed disturbances are decomposed and propagate in two different modes because of their distinct propagating mechanisms in swirl injectors. The flow oscillation in the streamwise direction travels in the form of acoustic wave, whereas the oscillation in the circumferential direction is convected downstream with the local flow velocity. The vortex breakdown is mainly controlled by the dynamics in the core region near the axis, not so much by the excitation in the main flow passage surrounding the central recirculation zone. External excitations only exert minor influences on the mean flow proper...

Unsteady flow evolution in swirl injector with radial entry. I. Stationary conditions

The vortical flow dynamics in a gas-turbine swirl injector were investigated by means of large eddy simulations. The flow enters the injector through three sets of radial-entry, counter-rotating swirl vanes. The formulation treats the Favre-filtered conservation equations in three dimensions along with a subgrid-scale model, and is solved numerically using a density-based, finite-volume approach with explicit time marching. Several methods, including proper orthogonal decomposition, spectral analysis, and flow visualization, are implemented to explore the flow dynamics in the complex three-dimensional flowfields. Various underlying mechanisms dictating the flow evolution, such as vortex breakdown, the Kelvin–Helmholtz instability, and helical instability, as well as their interactions, are studied for different swirl numbers. The flowfield exhibits well-organized motion in a low swirl-number case, in which the vortex shedding arising from shear instabilities downstream of the guide vanes drives acoustic o...

Numerical Simulation of Gas Turbine Swirl-Stabilized Injector Dynamics

A comprehensive numerical analysis has been conducted to investigate the vortical flow dynamics and acoustic response of a gas-turbine swirl- stabilized injector. The theoretical formulation is based on the complete conservation equations of mass, momentum, and energy in three dimensions. Turbulence closure is achieved by means of the large-eddy-simulation (LES) technique. The compressible version of the Smagorinsky eddy- viscosity model is employed to describe the subgrid- scale turbulent motions and their effect on large-scale structures. The governing equations and the associated boundary conditions are solved by a finite- volume, Adam-Bashforth predictor-corrector scheme along with the implementation of the message passing interface (MPI) parallel computing architecture. Detailed flow structures are studied for two different swirl numbers. Results show that the internal flowfield in the injector is intrinsically unsteady and subject to shear and centrifugal instabilities. The unsteady flow evolution and vortex breakdown are clearly visualized and can be explained on theoretical bases. The unsteadiness may be related to periodic vortex shedding, vortex breakdown and breakup, mode competition, and other phenomena that are sensitive to the swirl number.