Manfred Aigner

Validation of a Multicomponent-Fuel Model for Spray Computations

A multicomponent-fuel droplet evaporation model based on the theory of continuous thermodynamics is presented. It was implemented in the framework of a multiphase flow Eulerian-Lagrangian code and computations were validated using measurements of a monodisperse spray injected in a hot turbulent air flow. The continuous description of real fuel composition is briefly presented for two models, one based on a presumed distribution function, namely the gamma distribution function and the second one based on harmonic decomposition, namely the Fourier series. Then, the validation test case is described and the computation results pertaining to the continuous phase mean velocity and temperature fields are validated against measurement results. Finally, based on these validated computations of the surrounding flow, the monodisperse spray computation results concerning the evaporation of light heating oil are compared with the experimental results. The agreement is very good. The two-way coupling implementation methodology, which is necessary for spray combustion applications is also demonstrated.

An Improved Lobed Strut Injector Concept for Supersonic Combustion

A major problem in supersonic combustion is the short residence time of the fluid inside the combustor due to high flow velocities. Thus techniques for mixing enhancement have to be used to achieve a fast and efficient fuel-air mixing. In the present paper, different types of strut fuel injectors are investigated numerically. All strut injectors use lobed structures to create streamwise vorticity to achieve a faster mixing. A new concept for a single wedge lobed strut injector (SWLS) will be presented which is compared with an older double wedge strut injector concept. It will be shown, that the SWLS injector improves mixing and combustion efficiency significantly. Additionally the new injector is able to achieve a complete combustion at higher equivalence ratios.

Impact of Fischer-Tropsch fuels on aero-engine combustion performance

In this paper a detailed numerical investigation of an aero-engine single sector combustor model is carried out. The main goal of this study is to compare its combustion characteristics when fueled with a single component surrogate Jet A-1 fuel to the ones when fueled with a multicomponent Jet A-1 as well as with multicomponent Fischer-Tropsch (F-T) fuels under the same operating conditions. The combustor geometry under investigation mirrors at smaller scale many salient features of modern aero-engine combustors such as high swirling flow, eusion cooling at the walls, head cooling flow and, dilution flow. The numerical simulations are performed by means of an Eulerian-Lagrangian method for the gas and liquid phase, respectively. A finite-rate chemistry combustion model, which explicitly takes all species into account and an assumed-PDF approach for the turbulence-chemistry interaction are used to model the turbulent reacting flow. A multicomponent-fuel droplet evaporation model is implemented and enables to study real-fuel eects on the overall combustion process. A former numerical study had shown good results when comparing the reacting velocity and temperature fields and spray pattern for Jet A-1 combustion with LDA, CARS and Mie scattering measurements respectively. In the present study, a fully synthetic jet fuel which contains 50 % Coal-to-Liquid (CtL) fuel and a 100 % Gas-to-Liquid (GtL) fuel, both synthesized from a Fischer-Tropsch process are substituted to Jet A-1. The simulations are repeated under the same nominal operating conditions. A comparison in terms of temperature distribution, OH radical concentration and, main reactants concentration is carried out in order to outline the influence of the fuel composition on the combustor performance. Additionally, an analysis of certain soot precursors concentration and spray evaporation characteritics is delivered.

Numerical Investigation of the HyShot Supersonic Combustion Configuration

A multivariate assumed PDF approach together with finite-rate chemistry is used for the simulation of the HyShot supersonic combustion configuration. Because the combustor entrance conditions of scramjets at low flight Mach numbers (Ma 8) are close to the ignition limit of hydrogen air mixtures, detailed kinetic schemes are required and an accurate simulation of temperature and temperature fluctuations is essential. In the present paper, experiments of the HyShot supersonic combustion configuration performed at the High

Numerical Simulation of the Internal and External Flowfields of a Scramjet Fuel Strut Injector Including Conjugate Heat Transfer

The use of fuel strut injectors is a promising concept to achieve good mixing and combustion performances in supersonic combustors. In the present paper a lobed strut injector is investigated. The chosen geometry creates streamwise vorticity to enhance the mixing of fuel and air compared to a planar strut. The injector is located in the center of the hot supersonic flow field at the entrance of the combustor. To avoid burning, the strut has to be cooled which is done by the injected fuel. Especially at high flow Mach numbers, the thermal load for the strut may be severe. In the present paper, the strut surface heat fluxes and the temperature distribution in the strut are calculated. Conjugate heat transfer to the strut surface is considered both, from the simulated external air flow to the strut and from the strut to the cold internal hydrogen flow. An all-Mach number preconditioning is used in the numerical code to compute supersonic as well as subsonic flows. The strut investigated is assumed to be out of copper. The coupled simulations provide a survey of the internal strut temperature distribution and delivers accurate boundary and injection conditions (for hydrogen) in cases, where the interior fuel flow and heat transfer are neglected.

Thermodynamic Process Analyses of SOFC/GT Hybrid Cycles

This paper presents the description of a numerical design tool for the steady state thermodynamic process analysis of SOFC/GT hybrid systems. The tool includes models for all gas turbine components and for a SOFC stack, based on the tubular fuel cell design of Siemens. The modular structure of the code allows the simulation of all kinds of hybrid system configurations with gas turbines from different manufacturers and of varying performance ranges. Furthermore a selection of atmospheric and pressurized hybrid systems based on the Turbec T100 micro gas turbine and a tubular SOFC stack is discussed. Also parameter studies are shown which focus on the operating conditions of the chosen system configurations.

Development of a Projection-Based Method for the Numerical Calculation of Compressible Reactive Flows

The ability to calculate compressible reactive flows enables the computation of thermoacoustic interactions in gas turbine combustor systems. A new projection-based numerical method able to compute compressible reactive flows is developed within this work. This computational scheme is based on a modified Helmholtz decomposition, by which an arbitrary vector field is split up into a field with a so-called divergence constraint and an irrotational field. This leads to a fractional step scheme which consists of a predictor and a corrector step. The Poisson equation solved for the pressure in case of incompressible flows is extended to a Helmholtz equation for the computation of compressible flows. After solving the corrector step, the mass and momentum balances are fulfilled. This results in a fast numerical scheme, since no further iterations need to be computed. Based on the modified Helmholtz decomposition, it is shown that the created method can be understood as an extension of the incompressible projection scheme. Moreover, the spatial and temporal order of accuracy of incompressible projection-based methods are discussed and the ones of the compressible scheme are determined. The created compressible projection-based method is further on validated against a one-dimensional linear acoustic test case from the literature, whereby an analytical solution can be derived.

Ignition and Flame Propagation along Planar Monodisperse Droplet Streams

An experimental and numerical study is presented concerning the ignition of a fuel spray under well defined conditions. The development of a flame kernel that follows the generation of a plasma by a focused laser pulse and the subsequent flame propagation along five co-planar monodisperse streams of fuel droplets is investigated. High-speed video of the broadband luminosity and simultaneous fuel- and OH-PLIF provide qualitative and quantitative experimental data. Numerical simulations have shown that the focused laser pulse not only provides an energy/radical source for the ignition but is also responsible for transforming the droplets present in a small region into fuel vapor. Actually, without the quasi-instantaneous phase change there would not be enough vapor fuel to sustain the flame kernel under these time scales and surrounding conditions. The simulations are performed using an Eulerian-Lagrangian turbulent spray combustion code. Detailed chemistry is used to compute the laminar flame speed under relevant conditions, and a turbulent flame closure model (TFC) is then adopted for turbulence-chemistry interaction.

Numerical Investigations of Model Scramjet Combustors

In the present paper different types of scramjet (supersonic combustion ramjet) combustors are investigated. Thereby the main difference between the combustors is the way of injecting the fuel into the combustion chamber. The first investigated concept of fuel injection is the injection by strut injectors. Here the injection of fuel is realized by a lobed strut that is located in the middle of the combustion chamber. The second concept for fuel supply is the wall injection of hydrogen. Here the fuel is injected by several holes in the wall of the combustor. Both concepts of fuel injection have different advantages and disadvantages which are explained in detail. Although different performance parameters for both scramjet combustors are introduced this paper will not compare the different techniques among each other. Because of the high Reynolds numbers in scramjet combustors, the need to resolve the boundary layers and the necessity of detailed chemistry, the simulation of scramjets is extremely CPU time demanding.

Numerical Simulations of Soot and NOx Distributions in a Full Scale Aero-Engine Combustor at Two Different Flight Altitudes

Numerical simulations of a full-scale aero-engine combustor at two different flight altitudes are presented. Turbulence is treated by the SST model. A finite-rate chemistry model, where a separate transport equation is solved for each species, is applied in order to treat combustion accurately. Chemistry-turbulence interactions are modeled by an assumed probability density function model (APDF). A sectional model is applied for polycyclic aromatic hydrocarbons, while soot is modeled by a highly efficient two-equation model. The liquid phase of the fuel is calculated by a Lagrangian spray code which is coupled to the CFD code. Comparatively good agreement between experiment and simulation is observed at the reference operating point. At a reduced flight altitude, lower temperatures and lower NOx emissions are predicted, which result from a higher air-fuel ratio. Soot emissions at the combustor exit are due to cold regions in the vicinity of the combustor walls.