Recent numerical and experimental studies of Rotating Detonation Engines (RDEs) using air as the oxidizer have primarily focused on the ability to sustain a stable continuous detonation wave when fueled with hydrogen. For RDEs to be a viable technology for land-based power generation it is necessary to explore the ability to detonate natural gas and/or coal-syngas with air in the confines of the annular geometry of an RDE. There are major challenges in obtaining a stable detonation wave for a natural gas–air fueled RDE and to a lesser extent for coal-syngas and air. Recently published computational studies have, however, successfully simulated the underlying flow physics of detonative combustion for two-dimensional (2D) unrolled RDE geometries. In the present work, detonation wave characteristics of a hydrogen-natural gas fueled RDE have been numerically investigated and analyzed to understand the stability of natural gas detonations and detonability limits of fuel blends at relatively low operating combustor pressure. A series of detonation sensitivity studies have been conducted by varying the natural gas content in a hydrogen-natural gas fuel mixture, to assess the stability limit of natural gas detonations in an air breathing RDE. The current study explores the maximum percentage of natural gas content in a hydrogen-natural gas fuel blend that produces self-sustained, stable detonation waves. The simulations have been performed in a 2D unwrapped RDE geometry using the open-source CFD library OpenFOAM employing an unsteady pressure-based compressible reactive flow solver with a k–ε turbulence model in a structured rectangular grid system. Both reduced and detailed chemical kinetic models have been used to assess the effect of the chemistry on the detonation wave characteristics and underlying flow features. A systematic grid sensitivity study has been conducted with various grid sizes to quantify the weakly stable overdriven detonation on a coarse mesh and oscillating features at fine mesh resolutions. The low and high frequency instabilities have been analyzed from the time dependent pressure and temperature collected at various fixed spatial locations within the detonation height region. The results show that the peak pressure oscillates at low frequencies while for the high frequency instabilities, the peak pressure oscillates irregularly. Furthermore, at higher methane content, the high frequency instability leads to detonation extinction due to decoupling of the flame-front from the shock front. Wave speeds, peak pressures and temperatures, and dominant frequencies have been computed from the time histories. 2D contour maps of temperature and species concentrations have been used to visualize the flow structures, and calculate detonation height. Global wave speed and detonation height variations for varying methane content indicate the pathway to detonation failure at higher methane content for the current low pressure RDE. Experimental data from an air-breathing RDE fueled by natural gas-hydrogen fuel blends conducted in a detonation research laboratory at NETL, has been incorporated to verify the numerical findings.

This paper presents a numerical investigation of natural gas combustion in an OPRA OP16 tubular DLE combustor operating in premixed mode. The aim of this study is to compare different CFD models for the accurate prediction of emissions. The numerical results are validated against experimental results obtained in an atmospheric combustion test rig. A CFD workflow using AVL FIRE software has been devised enabling accurate CO and NO x emission calculations. Different turbulence models have been employed for adiabatic flow simulations at nominal load: the k-ζ-f turbulence model, PANS and LES-CSM model. Two detailed reaction mechanisms have been used to model combustion chemistry and thermodynamic properties: the DRM19 mechanism and the GRI 3.0 reaction mechanism. In the adiabatic simulations both the k-ζ-f and LES-CSM turbulence models perform well in combination with the GRI 3.0 reaction mechanism. In the non-adiabatic simulations heat exchange between combustor liner and cooling air is taken into account by using a wall temperature profile derived by scaling the experimentally obtained temperature profile such that the requirement of conservation of energy is satisfied. Reactive flow simulations at nominal load using the k-ζ-f turbulence model and GRI 3.0 reaction mechanism resulted in accurate predictions for time-averaged combustor pressure drop, CO and NO emissions: the predicted CO value deviates less than 20% from its experimentally obtained value, for NO the deviation is less than 10%. Inclusion of heat exchange between combustor and cooling air liner reduces this deviation to less than 10% for CO and less than 2% for NO. At a load equal to 106% of its nominal value, non-adiabatic simulations result in less satisfactory CO and NO predictions: NO is over predicted by approx. 20% while CO is overpredicted by nearly a factor of four. The latter is caused by the fact that experimentally obtained CO emissions reduce significantly when the load is increased from 100% to 106%, while numerically obtained CO emissions just change by a few ppm.

The depletion of fossil fuel resources, as well as the increasing environmental concerns have become the driving forces towards the research and development necessary for the introduction of alternative fuel such as hydrogen into civil aviation. Hydrogen is a suitable energy source primarily because it is free of carbon and other forms of impurities and is also the most abundant element in the universe. The advantages of using Liquid Hydrogen (LH 2 ) for civil aviation extends beyond carbon-free mission level emissions; LH 2 combustion can potentially reduce NO x emission by up to 90%, providing long-term sustainability and unrivalled environmental benefits. The paper presents a simplified parametric analysis to investigate the influence of various injector design parameters on a hydrogen micromix combustor reactive flow field. The main characteristics investigated are the flame structure (shape and position), the aerodynamic stabilization of the flame and the resulting NO x emissions. The design parameters include variations in the air-feed dimensions and the hydrogen injection diameter. A suitable numerical model was established by comparing various turbulence modelling approaches, reaction mechanisms and turbulence-chemistry interaction modelling schemes. The predictive capabilities, and limitations, of each of these modelling approaches, are assessed. The numerical challenges and limitations associated with modelling H 2 /air combustion at high pressure and temperature conditions are detailed. The influence of varying the injector design parameters on the mixing and hence the NO x characteristics is assessed.

A new model approach is presented in this work for including convective wall heat losses in the Direct Quadrature Method of Moments (DQMoM) approach, which is used here to solve the transport equation of the one-point, one-time joint thermochemical probability density function (PDF). This is of particular interest in the context of designing industrial combustors, where wall heat losses play a crucial role. In the present work, the novel method is derived for the first time and validated against experimental data for the thermal entrance region of a pipe. The impact of varying model-specific boundary conditions is analysed. It is then used to simulate the turbulent reacting flow of a confined methane jet flame. The simulations are carried out using the DLR in-house Computational Fluid Dynamics (CFD) code THETA. It is found that the DQMoM approach presented here agrees well with the experimental data and ratifies the use of the new convective wall heat losses model.

Many laboratory-scale combustors are equipped with viewing windows to allow for characterization of the reactive flow. Additionally, pressure housing is used in this configuration to study confined pressurized flames. Since the flame characteristics are influenced by heat losses, the prediction of wall temperature fields becomes increasingly necessary to account for conjugate heat transfer in simulations of reactive flows. For configurations similar to this one, the pressure housing makes the use of such computations difficult in the whole system. It is therefore more appropriate to model the external heat transfer beyond the first set of quartz windows. The present study deals with the derivation of such a model which accounts for convective heat transfer from quartz windows external face cooling system, free convection on the quartz windows 2, quartz windows radiative properties, radiative transfer inside the pressure housing and heat conduction through the quartz window. The presence of semi-transparent viewing windows demands additional care in describing its effects in combustor heat transfers. Because this presence is not an issue in industrial-scale combustors with opaque enclosures, it remains hitherto unaddressed in laboratory-scale combustors. After validating the model for the selected setup, the sensitivity of several modeling choices is computed. This enables a simpler expression of the external heat transfer model that can be easily implemented in coupled simulations.

A combined experimental and numerical study of light-round, defined as the flame propagation from burner to burner in an annular combustor, under perfectly premixed conditions has previously demonstrated the ability of large-eddy simulation (LES) to predict such ignition processes in a complex geometry using massively parallel computations. The present investigation aims at developing light-round simulations in a configuration closer to real applications by considering liquid n-heptane injection. The large-eddy simulation of the ignition sequence of a laboratory scale annular combustion chamber comprising sixteen swirled two-phase injectors is carried out with a mono-disperse Eulerian approach for the description of the liquid phase. The objective is to assess this modeling approach to describe the two-phase reactive flow during the ignition process. The simulation results are compared in terms of flame structure and light-round duration to the corresponding experimental images of the flame front recorded by a high-speed intensified CCD camera. The dynamics of the flow is also analyzed to identify and characterize mechanisms controlling flame propagation during the light-round process.

An experimental study was conducted to perform an analysis of the effect of the geometric modifications of the venturi on the non-reactive and reactive flow behavior using a counter-rotating radial-radial swirler. In the non-reactive flow tests, measurements were taken in a central vertical plane and horizontal (cross-sectional) plane at the exit of the swirler, using a High-Speed, Two Dimensional, Particle Image Velocimetry (2D PIV) system. The size of the swirler used in the non-reactive flow tests is a 4.76X scaled size of the swirler used in combustion. The 4.76X swirler models were tested in air flow seeded with olive oil at Re = 51,500, corresponding to the pressure drop across the 1X swirler models of 4% of atmospheric pressure at ambient conditions. Compared with the 1X swirler models, the 4.76X swirler models provide high spatial and temporal resolutions from the enhanced visibility of the flow characteristics and lower velocities at the same Re. Four swirler configurations of high swirl number (SN ≈ 1.0) were used, with no modification for the baseline configuration (configuration 1), and with the chevrons on the venturi for the straight chevrons configuration (configuration 2). The design of the inclined venturi was used for the converging venturi configuration (configuration 3), and chevrons were added on the converging venturi for the converging chevrons configuration (configuration 4). In the combustion tests, the 1X swirler models were tested using 478K preheated air at 4% pressure drop across the swirler, and different chamber lengths. Measurements were conducted using a regular camera to capture the flame image, and dynamic pressure transducers to obtain the acoustic pressure oscillations. Four configurations were studied and compared in the non-reactive and reactive flows with the objective of understanding the mechanisms responsible in reducing the extent of the combustion instabilities. Results of this study show that the converging venturi in configuration 3 appears to be the best design in eliminating the combustion instabilities in the fuel-lean region as compared to the other configurations. This indicates that the prevention of the frequencies coupling between the heat release rate and acoustic oscillations has been achieved by using the design of the converging venturi.

Aircraft engine components are subject to hostile thermal environments. The solid parts in the hot stages encounter very high temperature levels and gradients that are critical for the engine lifespan. Combustion chamber walls in particular exhibit very heterogeneous thermal fields. The prediction of this specific thermal field is a very complex task as it results from complex interactions between fresh gas injections, cooling flow distributions, combustion, flame stabilization and thermal transfers to the solids. All these phenomena are tightly coupled and do not evolve linearly. Today, the design phase of a combustion chamber is strongly enhanced by the use of high fidelity computations such as Large Eddy Simulations (LES). However, thermal boundary conditions are rarely well known and are thus treated either as adiabatic or as approximated isothermal conditions. Such approximations on thermal boundary conditions can lead to several errors and inaccurate predictions of the combustion chamber flow field. With this in mind and to foresee the potential difficulties of LES based Conjugate Heat Transfer (CHT) predictions, the effect of the wall temperature on a laminar premixed flame stabilization is numerically investigated in this paper for an academic configuration. The considered case consists of a squared cylinder flame holder at a low Reynolds number for which several wall-resolved Direct Numerical Simulations (DNS) are performed varying the bluff-body wall thermal condition. In such a set-up, the reactive flow and the flame holder interact in a complex way with an underlying strong impact of the wall temperature. For a baseline configuration where the flame holder wall temperature is fixed at 700K, the flow field is steady with a flame stabilized thanks to the recirculation zone of the flame holder. As the wall temperature is decreased, the position of the stabilized flame moves further downstream. The flame remains steady until a threshold cold temperature is reached below which an instability appears. For solid temperatures above 700 K, the flame is seen to move further and further upstream. For very hot conditions, the flame even stabilizes ahead of the bluff-body. The various flow solution bifurcations as the flame stabilization evolves are detailed in this paper. Heat flux distribution along the bluff-body walls are observed to be dictated by the flame stabilization process illustrating different mechanisms while integration of these fluxes on the whole flame holder surface confirms that various theoretical equilibrium states may exist for this configuration. This suggests that computation of more realistic cases including thermal conduction in the bluff-body solid part could lead to different converged results depending on the initial thermal state.

The classical flamelet method, the new Flamelet Generated Manifolds method (FGM), and the hybrid BML/flamelet approach are assessed in the context of the Reynolds-averaged Navier-Stokes (RANS) equations on a large range of configurations for both gaseous and spray flames. The conceptual differences, advantages, and shortcomings of the models are discussed in detail both from a theoretical and a practical point of view. In order to assess the models under gas turbine like conditions, the reactive flow in TU Darmstadt’s Generic Gas Turbine (GGT), DLR Stuttgart’s PRECCINSTA burner, and a premixed industrial combustor are computed. The computational results are compared to available measurement data and are used to discuss the strengths and the weaknesses of each of the aforementioned combustion models. In the current study it is shown that the hybrid BML/flamelet method globally performs well, but that it can be difficult to obtain a burning solution with this method, especially when the combustion process is operated close to the flammability limits. While the flamelet method is very robust, it is outperformed by the FGM method even for purely non-premixed configurations. It is demonstrated that the FGM approach can be used for the whole range of combustion modes, from non-premixed over to premixed combustion processes. Since the model did not lead to any difficulties with attaining a burning solution, and is computationally as efficient as the flamelet approach, the authors recommend the usage of this model over the other models investigated.

A rich burn/quick mix/lean burn (RQL) combustor concept for reducing pollutant emissions is currently under investigation at the NASA Lewis Research Center (LeRC). A numerical study was performed to investigate the chemically reactive flow with liquid spray injection for the RQL combustor. The RQL combustor consists of an airblast atomizer fuel injector, a rich burn section, a converging connecting pipe, a quick mix zone, a diverging connecting pipe and a lean combustion zone. For computational efficiency, the combustor was split into two sub systems, i.e. the fuel nozzle/rich burn section and the quick mix/lean burn section. The current study investigates the effect of varying the mass flow rate split between the swirler passages for an equivalence ratio of 2.0 on fuel distribution, temperature distribution, and emissions for the fuel nozzle/rich burn section of an RQL combustor. The input conditions used in the study were chosen based on tests completed at LeRC. It is seen that optimizing these parameters can substantially improve combustor performance and reduce combustor emissions. The optimal mass flow rate split for reducing NO x emissions based on the numerical study was the same as found by experiment at LeRC.

A numerical model to solve three-dimensional turbulent reactive flows in arbitrary shapes is presented. The conservative form of the primitive-variable formulation of steady density-weighted Navier Stokes equations written for a general curvilinear system is adopted. Turbulent transport is described by the k-ε model. The reactions associated with heat release are assumed sufficiently fast for chemical equilibrium to prevail on an instantaneous basis and the influence of local turbulent fluctuations in mixture strenght accounted for by a β-probability density function. The numerical scheme is based on a non-staggered grid (cartesian velocity components and pressure located at the same grid-points) and a special interpolation technique is used to avoid checkerboard oscillations. The present model was used to simulate an annular combustion chamber for which experimental results were available. The agreement between calculation and experiments ranges from fair to good.

A numerical study was performed to investigate the chemically reactive flow with liquid spray injection in a staged combustor concept for reducing pollutant emissions. The staged combustor consists of an airblast atomizer, a rich bum section, a converging connecting pipe, a quick mix zone, a diverging connecting pipe, and a lean combustion zone. For computational efficiency, the combustor was split into two subsystems, i.e. the fuel nozzle/rich burn section and the quick quench/lean bum section. The current study investigates the effect of wall geometry and swirl direction, i.e. co- or counter-rotating swirl, on fuel distribution, temperature distribution, and emissions for the fuel nozzle/rich bum section at a cruise condition. At an equivalence ratio of 1.9, the nozzle-combustor (dome) interface geometry was varied from a flat wall (normal to the combustor wall) to a sloped wall of 45 degrees. It is seen that the sloped wall with co-rotating swirl direction had a substantial effect on combustor performance and reducing pollutant emissions.

Current progress in gas turbine performance is achieved mainly by increasing the turbine inlet temperature. At high temperature levels (>2000K), the hot combustion gases can no longer be considered as chemically inert, and it becomes important to account for dissociation and recombination reactions occurring not only in the combustion chamber but also within the expanding gas stream in the turbine. In this paper, the authors present a two-dimensional numerical study of chemically reactive flow of hot combustion gases through the first guide vane of a gas turbine. For this initial study, simplified boundary conditions are assumed: blade cooling air mixing is neglected, the blade wall temperature is assigned a fixed value, and uniform inlet conditions are assumed. This study investigates the effect of turbulence on chemical reaction kinetics and presents pollutant emission levels at the nozzle exit. Particular attention is also focussed on chemical reactivity near the pressure and suction sides of the turbine guide vane blades.

Current progress in gas turbine performance is achieved mainly by increasing the turbine inlet temperature. State-of-the-art military aircraft gas turbines operate with turbine inlet temperatures exceeding 2000 K, and future development plans call for even higher temperature levels. At such high temperatures, the hot combustion gases can no longer be considered as chemically inert, and it becomes important to account for the chemically reactive nature of the expanding flow. In this paper, the authors present a one-dimensional model of the chemically reactive flow through the first turbine stage of an aircraft turbo-jet engine. The model is used to study the impact of chemical reactivity on pollutant emission characteristics and engine performance (i.e., overall efficiency and specific thrust). Three different flight conditions are considered: sea-level static operation (take-off), subsonic cruising at 10000 meters altitude, and supersonic flight at 20000 meters altitude. The results of this study show that typical flight conditions and operating parameters of turbo-jet engines produce high pollutant emission levels and decrease overall efficiency and specific thrust.

The chemically reacting flow in the mixing and secondary zone of the improved RQL combustor sector of the DLR has been numerically investigated. The combustion model is adapted to meet experimental conditions and proves to underpredict temperature fluctuations. The pressure dependence of the NO x emission index for the quench and lean zone of the combustor can be described with an exponential factor of 0.66. Although the level of agreement between measured and calculated results is not consistently good, the code can be used to study the effect of various operating conditions as well as the effect of geometrical changes on the flow and the emissions.

Numerical techniques are commonly used during both design and analysis processes, mainly considering single components. Technological progress asks for advanced approaches that include real-machine conditions and analyze components interaction, especially considering the combustor/turbine coupling. Modern combustors operate with strong swirl motions in order to obtain an adequate flame stabilization, generating a very complex flow field characterized by high turbulence level. These aspects affect performance of downstream components which are subjected to very aggressive inlet flow conditions: non-uniform total temperature, non-uniform total pressure, swirl and high turbulence intensity. For these reasons coupled analysis of combustor and turbine is necessary to accurately predict aero-thermal aspects that influence performance and reliability of these two components. From a numerical point of view the simulation of a single domain characterized by a reactive flow with very different Mach number regimes (from low-Mach flow in combustion chamber to transonic flow in turbine) is problematic due to the different numerical requirements needed, especially concerning stability and accuracy. These problems could be overcome using coupled methods to simultaneously simulate combustor and turbine in separated domains which are managed by different solvers that communicate with each other. A coupling method for the study of combustor/turbine interaction using the RANS methodology is proposed. In the first part of the paper the method is described and validated. The second part is dedicated to the application of the proposed coupling methodology to a realistic test case consisting of a model annular combustor and the Nozzle Guide Vane (NGV) of the MT1 high-pressure turbine stage. A commercial solver and an in-house code are respectively used for the simulation of combustor and NGV. Results are presented and analyzed highlighting the importance of such type of simulations in understanding aero-thermal phenomena that characterize combustor/vane interaction.

In simulating chemically reacting flows, the differential entropy inequality (the local form of the second law of thermodynamics) must be satisfied in addition to the differential mass, momentum and energy balances. Previously we have shown that entropy violations occur when using a global/reduced mechanism. Herein we show that entropy violations also occur when using a detailed/skeletal/reduced mechanism. Using a recent theorem [1], we illustrate how to modify a reduced chemical kinetics model to automatically satisfy the differential entropy inequality. The numerical solution of a methane laminar flame was improved when using reduced chemical kinetics modified in this way. In addition, an ad hoc temperature limiter is no longer necessary.

A 3D CFD simulation of turbulent reactive flows is discussed. The original compressible version of the solver HybFlow designed for turbine rows investigation is here applied for low speed burning flow. A conserved scalar approach is considered to simulate the turbulent reacting flow field of non-premixed flames. The spatial discretization is based on an upwind finite volume method for unstructured grids using the Roe’s Riemann solver with a non-linear TVD scheme. The steady state solution is computed by means of an implicit relaxed Newton method. The linear solver is GMRES coupled with an ILU(0) preconditioning scheme. The turbulence chemistry interaction is described using a presumed β-PDF Flamelet approach. Two test applications are here presented to verify the methodology characteristics for a pilot-jet turbulent flame and a bluff-body stabilized flame both using CH4. A model combustor supplied with propane is also briefly shown as an example of application to a more realistic configuration.

Combustors with fuel-spray atomisers are susceptible to a low-frequency oscillation, particularly at idle and sub-idle conditions. For aero-engine combustors, the frequency of this oscillation is typically in the range 50–120Hz and is commonly caused ‘rumble’. In the current work, Computational Fluid Dynamics (CFD) is used to simulate this self-excited oscillation. The combustion model uses Monte Carlo techniques to give simultaneous solutions of the Williams’ spray equation together with the equations of turbulent reactive flow. The unsteady combustion is calculated by the laminar flamelet presumed pdf method. A quasi-steady description of fuel atomiser behaviour is used to couple the inlet flow in the combustor. A choking condition is employed at turbine inlet. The effects of the atomiser and the combustor geometry on the unsteady combustion are studied. The results show that, for some atomisers, with a strong dependence of mean droplet size on air velocity, the coupled system undergoes low frequency oscillations. The numerical results are analysed to provide insight into the rumble phenomena. Basically, pressure variations in the combustor alter the inlet air and fuel spray characteristics, thereby changing the rate of combustion. This in turn leads to local ‘hot spots’, which generate pressure fluctuations as they convect through the downstream nozzle.

Steady-state Reynolds Averaged Navier-Stokes (RANS) equations are solved in the present numerical investigation to simulate the reactive two-phase flow in a model aero-engine combustor, and the reactive flow field with NOx emissions is analyzed. The gaseous phase is modeled by the modified SST turbulence model, and the liquid phase is modeled by Lagrangian tracking method considering the droplet breakup, collision and evaporation. Turbulence-combustion interaction is modeled by the extended coherent flame model, and NOx emissions are modeled by solving the species transport equation based on the assumption of frozen temperature. The fuel system of the present simulated combustor is radially staged, with a main stage employing the principle of lean prevaporized and premixed (LPP) concept to reduce pollutant emission, and a pilot stage burning a diffusion flame for flame stability. For the exit temperature quality improvement, dilution air is assigned with little amount of airflow. Detailed numerical results including exit temperature distribution, dominant burning performances and species distributions are evaluated for the combustion with and without dilution air. The influence of upstream burning characteristics to downstream temperature distribution is assessed. Numerical prediction of NOx emission demonstrates its capability of a reasonable reduction, and the exit temperature pattern with the dilution air is also able to fulfill its design target.