Shih-Yang Hsieh

A numerical study of cryogenic fluid injection and mixing under supercritical conditions

The evolution of a cryogenic fluid jet initially at a subcritical temperature and injected into a supercritical environment, in which both the pressure and temperature exceed the thermodynamic critical state, has been investigated numerically. The model accommodates full conservation laws and real-fluid thermodynamics and transport phenomena. All of the thermophysical properties are determined directly from fundamental thermodynamics theories, along with the use of the corresponding state principles. Turbulence closure is achieved using a large-eddy-simulation technique. As a specific example, the dynamics of a nitrogen fluid jet is studied systematically over a broad range of ambient pressure. Owing to the differences of fluid states and flow conditions between the jet and surroundings, a string of strong density-gradient regimes is generated around the jet surface and exerts a stabilizing effect on the flow development. The surface layer acts like a solid wall that transfers the turbulent kinetic energy...

Large-Eddy Simulation of Combustion Dynamics of Lean-Premixed Swirl-Stabilized Combustor

A comprehensive numerical study of the combustion dynamics in a lean-premixed swirl-stabilized combustor is described. The analysis treats the conservation equations in three dimensions and takes into account e niterate chemical reactions and variable thermophysical properties. Turbulence closure is achieved using a largeeddy-simulation technique. The compressible-e ow version of the Smagorinsky model is employed to describe subgrid-scale turbulent motions and their effect on large-scale structures. A level-set e amelet library approach is used to simulatepremixed turbulent combustion. The governing equationsand theassociated boundary conditions aresolvedbymeansofafour-stepRunge‐ Kuttaschemealongwithimplementationofthemessagepassinginterface parallel computing architecture. The analysis allows for a detailed investigation into the interaction between turbulent e ow motions and oscillatory combustion of a swirl-stabilized combustor. Several physical processes responsible for driving combustion instabilities in the chamber have been identie ed and quantie ed, including the mutual coupling between acoustic wave motions, vortex shedding, and e ame oscillations. In particular, the mechanisms of energy transfer from chemical reactions in the e ame zone to acoustic motions in the bulk of chamber are carefully studied.

A Preconditioned Flux-Differencing Scheme for Chemically Reacting Flows at all Mach Numbers

Abstract A new computation procedure is developed for solving time-accurate, chemically reacting flows over a wide range of Mach numbers. The algorithm is based on scaling the pressure terms in the momentum equations and preconditioning the conservation equations to circumvent numerical difficulties at low Mach numbers. The resulting equations are solved using the lower-upper (LU) factorization method in a fully-coupled manner, with incorporation of a flux-differencing upwind TVD scheme to achieve high-order spatial accuracy. The transient behavior of the modeled system is preserved through implementation of the dual time-stepping integration technique. The capabilities of the scheme are illustrated by applying it to selected problems, including one-dimensional nozzle flows, two-dimensional channel flows, rocket-motor internal flows, and acoustic waves in a porous chamber with surface transpiration.

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.

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...

Interactions Between Shock and Acoustic Waves in a Supersonic Inlet Diffuser

The interactions between shock and acoustic waves in a supersonic inlet diffuser are investigated numerically. The model treats the viscous flowfield in an axisymmetric, mixed-compression inlet operating under supercritical conditions. It is solved by means of a finite-volume approach using a four-stage Runge-Kutta scheme for temporal derivatives and the Harten-Yee upwind total-variation-diminishing scheme for spatial terms. Various distinct flow structures, including shock/boundary-layer and shock/shock interactions, are studied under the effects of externally imposed pressure oscillations at the diffuser exit over a wide range of forcing frequencies and amplitudes. As a result of the terminal shock oscillation induced by the impressed disturbances and the cyclic variation of the oblique/normal shock intersection, large vorticity fluctuations are produced in the radial direction. The characteristics of the shock/boundary-layer interactions (such as the size of the separation bubble, the terminal shock configuration, and the vorticity intensity) are also greatly influenced by the acoustic-driven shock oscillation. The overall response of the inlet aerodynamics to acoustic waves can be characterized by the mass-transfer and acoustic-admittance functions at the diffuser exit. Their magnitudes decrease with increasing frequency. A supersonic inlet acts as an effective acoustic damper, absorbing disturbances arising downstream. Severe flow distortion, however, may arise from shock oscillation and subsequently degrade the combustor performance.

A unified analysis of unsteady flow structures in a supersonic ramjet engine

An integrated numerical analysis has been conducted to study the internal flowfield in a ramjet engine. Emphasis is placed on the establishment of a unified numerical scheme capable of treating both the supersonic inlet diffuser and combustor flows. The theoretical model is based on the complete conservation equations of mass, momentum, energy, and species concentration, with consideration of finite-rate chemical reactions and variable properties. Turbulence closure is achieved using a low-Reynolds number ke two-equation model. Calculations have been carried out for the flowfield in a typical ramjet engine consisting of an axisymmetric mixed-compression supersonic inlet and a coaxial dump combustor. The mutual coupling between the inlet and combustor was carefully examined under various operating conditions. In particular, strong vortices arising from the inlet shock/shock and shock/boundary-layer interactions may convect downstream and affect the combustion dynamics. Large vortical motions, coupled with acoustic motions, were observed in the combustion chamber, which in turn modified the inlet flow structures.

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.

Modeling of Jets in Cross Flow with RANS and LES Part I: Momentum Transport for Low R w/ RANS

ABSTRACT Four models (standard k-e, Realizable k-e, Reynolds stress, and Spalart-Allmaras turbulence models) along with the two near-wall treatments (nonequilibrium wall function, and enhanced wall treatment) of software were assesses for low to moderate jet momentum (velocity) ratios of interest in gas turbine combustors. A systematic study was taken to establish RANS capability to be followed by back-to-back assessment of LES/DES capabilities. INTRODUCTION