Michael Rachner

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.

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.

Modelling of soot and NOx in a full scale turbine engine combustor with detailed chemistry

A numerical simulation of spray atomization, combustion and soot formation in a full scale turbine engine combustor is presented and discussed in this work. Turbulence has been captured by a two equation turbulence model, chemistry by a detailed kinetic mechanism and turbulence chemistry interactions have been accounted for by an APDF-approach (assumed probability density function approach). For the kerosene fuel a surrogate has been used, consisting of n-decane, isooctane and toluene. The injection of the liquid fuel spray has been included by coupling the gas field CFD code with a spray code. Apart from the n-decane and isooctane reaction paths, the chemical kinetic reaction mechanism accounts for species as heavy as toluene. Polycyclic aromatic hydrocarbons heavier than toluene are represented by a sectional approach, while soot is calculated via soot volume fraction and particle number density. Main flow features are investigated and good agreement with the experimental measurements can be reported.Copyright

Validation of an Ignition and Flame Propagation Model for Multiphase Flows

This paper presents a numerical investigation of a generic lab scale combustor with focus on the ignition characteristics. The test case has been examined thoroughly in a comprehensive measurement campaign to provide a detailed set of boundary conditions and a profound data base of results. The experimental setup comprises five parallel-aligned mono-disperse droplet chains which are ignited, using a focused laser beam. One aspect of the experimental study is the ignitability with respect to the imposed boundary conditions. The second covers the growth and the propagation of the flame after the establishment of an initial kernel. The outcome of the numerical simulations is compared to the experimental results which allows an in-depth assessment of the employed numerical models. The chemistry and, thus, the flame propagation behavior is captured by a turbulent flame speed closure approach with an adaptation to render the model suitable to multiphase flows. For the dispersed phase a Lagrangian particle tracking scheme is employed in combination with a continuous thermodynamics fuel model for the evaporation. The overall good agreement demonstrates the capability of a multiphase flow CFD solver in the field of ignition modeling.Copyright

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.

Experimental and numerical investigation of spray characteristics in a new FLOX® based combustor for liquid fuels for Micro Gas Turbine Range Extender (MGT-REX)

A liquid fuel combustor based on the FLOX ® gas turbine burner concept has been developed for application in a Micro Gas Turbine (MGT) Range Extender (REX) for next generation cars. The characterization of this combustor was performed at the High Pressure Optical Test rig (HIPOT) at DLR Stuttgart. Spray characteristics were measured using droplet mie scattering and phase Doppler interferometry in flames of a stable burner operation point (BOP) at a pressure, preheat temperature, global lambda (λG), and jet velocity of 3.5 bars, 300 °C, 1.45 and 120 m/s respectively. The experimental results showed long flames with deep penetration of the spray into the combustion chamber. A comprehensive data set of the spray characteristic with well-defined boundary condition was made available for CFD simulations. The CFD simulation of the two-phase flow was performed by coupling the DLR liquid phase simulation code SPRAYSIM with the commercial CFD-code ANSYS CFX-16.1. The comparison of axial and radial velocity profiles between simulation and experiment clearly showed that the turbulence model used in the numerical simulation was unable to predict the measured turbulence appropriately. The calculated and measured spray behavior in the combustion chamber showed satisfying agreement. The observed differences were mainly due to the simple 1-step global combustion model, which predicted an early onset of the heat release. The simulation showed that even though a large portion of the evaporation happened already inside the nozzle, the remaining spray droplets penetrate deep into the combustion chamber.

Euler-Lagrange Simulation of a LOx/H2 Model Combustor with Single Shear Coaxial Injector

In this paper a mixed Euler-Lagrange approach is used for a 3D simulation of a LOX/H2 model rocket combustor with a single shear coaxial injector. The specific test case presented is the MASCOTTE combustor at 10 bar pressure in the so called A-10 configuration. The simulation of the gas phase is conducted with the scientific code TASCOM3D which works in an Eulerian mode while the liquid fuel droplets are treated by the scientific code SPRAYSIM in a Lagrangian framework. The two codes and the coupling mechanisms are explained and results of a preliminary simulation will be presented. At the end an outlook is given focusing on how to obtain an even more accurate representation of the experiment in subsequent simulations. Finally some comments on the computational costs of the calculations and the performance of the two codes on the NEC SX-9 are given.

Jet A-1 Fuel Spray Evaporation in a Turbulent Flow: Experimental Investigations and Validation of Numerical Models

An experimental system was built to study the evaporation of sprays in well controlled environment. Appropriate measurement techniques were used to study in detail the evaporation of a single component fuel and the multicomponent-fuel Jet A-1. Boundary conditions and initial conditions, necessary to perform accurate computations are determined precisely. The single component fuel is used for a basic matching of computation with experiments. To describe the evaporation of Jet A-1 a single component surrogate model and a continuous thermodynamics model (CTM) are employed. The composition of the Jet A-1 fuel used in the experiments is known and thus it was possible to determine accurately the distribution parameters of the CTM model. Finally the experimental data inferred about the evaporation of Jet A-1 is used to validate the models.