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A multigrid method for convergence acceleration is used for solving coupled fluid and turbulence transport equations. For turbulence closure a low-Reynolds-number q-[Omega] turbulence model is employed, which requires very fine grids in the near wall regions. Due to the use of fine grids, convergence of most iterative solvers slows down, making the use of multigrid techniques especially attractive. However, special care has to be taken on the strong nonlinear turbulent source terms during restriction from fine to coarse grids. Due to the hyperbolic character of the governing equations in supersonic flows and the occurrence of shock waves, modifications to standard multigrid techniques are necessary. A simple and effective method is presented that enables the multigrid scheme to converge. A strong reduction in the required number of multigrid cycles and work units is achieved for different test cases, including a Mach 2 flow over a backward facing step.

Large-Eddy Simulation of a sooting aero-engine model combustor is analyzed by means of proper orthogonal decomposition (POD) to investigate the influence of flow field dynamics on soot evolution. Since classical POD fails to extract the low-frequency intermittent dynamics of scalars such as soot volume fraction, the recently-introduced multiresolution POD (MRPOD) is deployed to treat the LES results instead. MRPOD essentially provides additional dynamic rankings of energetic POD modes via bandpass filters constructed using wavelet packet transform (WPT). In the investigated combustor major dynamics identified in both velocity and scalar fields include a precessing vortex core (PVC) at about 430 Hz and low-frequency dynamics (LFD) at below 100 Hz. Low-frequency dynamics of velocity are present in the whole combustor and affect the secondary air injection and recirculation. While both dynamics are found to influence soot volume fraction, the PVC impact is much smaller compared to the LFD impact. [ABSTRACT FROM AUTHOR]

Byline: Peter Gerlinger, Berthold Noll, Manfred Aigner An assumed probability density function (PDF) approach is used for the simulation of turbulent high speed combustion with finite-rate chemistry. The PDFs employed are a clipped Gaussian distribution for temperature and a joint multi-variate I[sup.2]-distribution for an arbitrary number of species mass fractions. The definition of both PDFs is based on higher order moments obtained from additional transport equations. In the present work, a transport equation for the variance of temperature and the sum of species mass fraction variances is solved. The numerical approach is compared with experimental data of a Mach 2 supersonic hydrogen-air diffusion flame. The focus of this paper is on the investigation of the marginal PDFs of temperature and species molar fractions at representative spatial positions. Moreover, two-dimensional marginal PDFs of pairs of species mass fractions are compared with experimental results. It is shown that the assumed PDF approach achieves a good agreement with experimental data concerning first and second moments and that even the shape of the PDF is met relatively well.

This paper presents time-resolved numerical simulations of a well-characterized sooting swirl flame at elevated pressure. Recently published unsteady Reynolds averaged Navier-Stokes simulations (URANS) are compared here to newly performed large eddy simulations (LES). Finite-rate chemistry, where transport equations are solved for each chemical species, is employed for the gas phase, a sectional approach for polycyclic aromatic hydrocarbons (PAHs), and a two-equation model for soot particles. Feedback effects such as the consumption of gaseous soot precursors by growth of soot and PAHs are inherently captured accurately by a coupled solution of the set of governing equations. The numerical results (velocity components, temperature, and soot volume fraction) compare well with experimental data. No significant differences between URANS and LES are observed for time-averaged temperatures and velocity components, while the prediction of soot is significantly improved by LES. It will be shown that an accurate description of the instantaneous flame structure (especially of the hydroxyl radical distribution) by resolution of turbulent scales is of fundamental importance for accurate soot predictions in confined swirl flames with strong secondary air injection. [ABSTRACT FROM AUTHOR]

In this paper, the thermal load on an actively cooled lobed strut injector for scramjet (supersonic combustion ramjet) applications is investigated numerically. This requires coupled simulations of the strut internal and external flow fields together with the heat conduction in the solid injector body. In order to achieve a fast mixing, the lobed strut is positioned at the channel axis to inject hydrogen into the core of a Mach 3 air stream. There it is exposed to the extremely high temperatures of the high speed flow. While the external air and hydrogen flows are supersonic, the strut internal hydrogen flow is mainly subsonic, in some regions at very low Mach numbers. To enable a simulation of the internal flow field which ranges from very low to very high Mach numbers (approximately Mach 2.25 at the nozzle exit), a preconditioning technique is employed. The compressible finite-volume scheme uses a spatially fourth order multi-dimensional limiting process discretization, which is used here for a first time to simulate a geometrically and fluid mechanically highly complex problem. It will be demonstrated that besides its high accuracy the multi-dimensional limiting process scheme is numerically stable even in case of demanding practical applications. The coupled simulation of the lobed strut injector delivers unique insight into the flow phenomena inside and outside the strut, the heat fluxes, the temperature distribution in the solid material, the required hydrogen mass flux with respect to cooling requirements and details concerning the conditions at the exit of the injector. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This article presents unsteady Reynolds averaged Navier–Stokes simulations (URANS) of a well-characterized aero-engine model combustor with finite-rate chemistry (FRC). The simulations give insight into the complex formation and destruction processes of soot at technically relevant conditions. It will be shown that a recently developed PAH (polycyclic aromatic hydrocarbons) and soot model is able to predict soot under complex combustion conditions at elevated pressure. Finite-rate chemistry is employed for the gas phase, a sectional approach for PAHs and a two-equation model for soot. Thus, feedback effects, such as the consumption of gaseous soot precursors by growth of soot and PAHs, are inherently captured accurately. In agreement with the experiment a precessing vortex core (PVC) is observed in the ethylene fueled combustor. This requires that the computational grid covers swirlers. The PVC intensifies mixing of fuel, primary air, and hot burned gas from the inner recirculation zone, thereby supporting flame stabilization and subsequently influencing soot. The numerical results (velocity components, temperature, and soot volume fraction) compare well with experimental data. Details of soot evolution and remaining differences to the experiment are analyzed. [ABSTRACT FROM PUBLISHER]

This paper investigates some important numerical aspects for the simulation of model rocket combustors. Precisely, (1) a new high-order discretization technique (multi-dimensional limiting process (MLP), low diffusion, and MLP l d) is presented and compared with conventional second-order schemes with different flux limiters. (2) Time accurate unsteady Reynolds-averaged Navier-Stokes (RANS) simulations are performed to assess possible improvements in comparison with steady-state RANS simulations. (3) Fully 3D simulations of an axisymmetric rocket combustor are compared with 2D axisymmetric ones. All studies are based on the Penn State preburner combustor experiment, which uses gaseous oxygen and hydrogen. This comprehensive study offers unique insight into how the mentioned numerical influence factors change the flow field, flame, and wall heat fluxes in the model rocket combustor. Because wall heat fluxes are known from the experiment only, numerical results are compared with LES of other authors, too. It will be shown that the high-order spatial discretization significantly improves the agreement with measured wall heat fluxes at low additional computational cost. In general the transition from simple to more complex numerical approaches steadily improves the qualitative agreement between simulation and experiment. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: In this paper a new soot formation model for gas turbine combustor simulations is presented. A sectional approach for the description of Polycyclic Aromatic Hydrocarbons (PAHs) and a two-equation model for soot particle dynamics are introduced. By including the PAH chemistry the formulation becomes more general in that the soot formation is neither directly linked to the fuel nor to C2-like species, as it is the case in simpler soot models currently available for CFD applications. At the same time, the sectional approach for the PAHs keeps the required computational resources low if compared to models based on a detailed description of the PAH kinetics. These features of the new model allow an accurate yet affordable calculation of soot in complex gas turbine combustion chambers. A careful model validation will be presented for diffusion and partially premixed flames. Fuels ranging from methane to kerosene are investigated. Thus, flames with different sooting characteristics are covered. An excellent agreement with experimental data is achieved for all configurations investigated. A fundamental feature of the new model is that with a single set of constants it is able to accurately describe the soot dynamics of different fuels at different operating conditions. [Copyright &y& Elsevier]

Presents a study on turbulence transport equations and couple fluid, that can be solved by a multigrid low-Reynolds-number q-w turbulence model. Requirements of fine grids in the model; Effects of the use of fine grids on the convergence of most iterative solvers; Results of the study; Conclusion reached in the study.

The present work explores the capability of the transported probability density function (PDF) method to predict nitric oxide (NO) formation in turbulent combustion. To this end a hybrid finite-volume/Lagrangian Monte Carlo method is implemented into the THETA code of the German Aerospace Center (DLR). In this hybrid approach the transported PDF method governs the evolution of the thermochemical variables, whereas the flow field evolution is computed with a Reynolds-averaged Navier-Stokes (RANS) method. The method is used to compute a turbulent hydrogen-air flame and a methane-air flame and computational results are compared to experimental data. In order to assess the advantages of the transported PDF method, the flame computations are repeated with the "laminar chemistry" approach as well as with an "assumed PDF" method, which are both computationally less expensive. The present study reveals that the transported PDF method provides the highest accuracy in predicting the overall flame structure and nitric oxide formation. [ABSTRACT FROM AUTHOR]

Abstract: A new soot model is presented, which has been developed for CFD applications, combining accuracy and efficiency. While the chemical reactions of small gas phase species are captured by a detailed chemical kinetic mechanism, polycyclic aromatic hydrocarbons (PAHs) and soot particles are represented by sectional approaches. The latter account for important mechanisms such as the formation of sections, their oxidation, the condensation of acetylene, and the collisions between sections. The model has been designed to predict soot for a variety of fuels with good accuracy at relatively low computational cost. Universal model parameters are applied, which require no tuning in dependence of test case or fuel. Soot predictions of ethylene, propylene, kerosene surrogate, and toluene flames are presented, which show good agreement with the experimental data. Furthermore, the importance of the correct choice for thermodynamic data of PAHs and soot is highlighted and the impact of heat radiation is discussed. [Copyright &y& Elsevier]

A consistent real gas framework based on a preconditioning scheme is implemented into the CFD code TASCOM3D to simulate nonreacting and reacting turbulent flows with transcritical injection. All thermodynamic properties and partial derivatives are rigorously evaluated from the reduced residual Helmholtz energy by use of fundamental thermodynamic relations. The Soave–Redlich–Kwong equation of state with an optional volume translation is employed in combination with high-pressure models for the calculation of fluid transport properties. Three test cases are simulated to validate the implemented real gas framework using different turbulence modeling strategies. A good match between numerical and experimental data is obtained. [ABSTRACT FROM AUTHOR]

Abstract: A sooting C2H4/air jet diffusion flame was investigated experimentally by laser measuring techniques and the results are compared to CFD calculations. The target flame (C2H4 10.4g/min, bulk exit velocity 44m/s, RE=10,000) exhibits well-defined boundary conditions and presents a good test case for model validation. Flow velocity, temperature and soot volume fraction in this flame has been measured previously. In this paper, further experimental results from Raman scattering and laser-induced fluorescence (LIF) measurements are presented to expand the validation data base. Raman scattering is used to measure the fuel/air mixing prior to combustion, while LIF of PAHs monitors the soot precursor region and successive planar OH–LIF serves to map the flame front position and its statistics. Furthermore, a numerical simulation of this flame was performed based on the DLR in-house code THETA. Within the scope of the test case presented here, the code combines a relatively detailed description of the gas phase kinetics coupled with a detailed yet computation-efficient soot model, suitable for CFD applications. This model has been designed to predict soot for a variety of fuels and flames with good accuracy at relatively low computational costs. Universal model parameters are applied, which requires no tuning for the dependence of test case or fuel. The experimental and numerical results are compared and discussed with special emphasis on the pre-flame region of the jet and up to the downstream position where significant soot concentrations are present. Validation shows the general applicability of the CFD code with implemented soot model to rather complex systems like the target sooting turbulent jet flame. Identified discrepancies are analyzed and can be explained, while opening up the field for future optimization of parts of the CFD code. [Copyright &y& Elsevier]