Peter Habisreuther

A Model for Calculating Heat Release in Premixed Turbulent Flames

Abstract A unified reaction model, being valid in all turbulent combustion regimes, has been developed and tested. Based on a thoroughly validated model for the turbulent burning velocity the Kolmogorov, Petruvski and Piskunuv (KPP) theorem was applied, thus leading to the formulation of the mean reaction rate as a function of local turbulence and kinetic parameters in the flow. Numerical calculations, comprising all flame structures (0.2 Da t [25] that the blow-off limits of strongly swirling flames are determined by chemical kinetic limitation of the mean reaction rate. The results demonstrate the high performance of the reaction model and recommend its application also in complex 3-D flows, because of its simplicity and numerical robustness even in conjunction with higher order turbulence models.

Development and assessment of correlations for predicting stability limits of swirling flames

Abstract Swirling flow systems are being used in many industrial processes like separation of particles in gas flows (cyclones), atomisation and spreading of liquids (water, oil) and fast mixing and high-intensity reaction in reactor systems as, e.g. stable and intense flames of swirl burners. The widespread use of swirl burners in the process and energy industries and, in particular, new concepts for the reduction of NOx-emissions (ultralean premixed combustion) raise the need for simple-to-use models for predicting lean stability limits of highly turbulent flames stabilized by internal recirculation. Based on recently published experimental data of the first author concerning the reaction structures of swirling flames operated near to the extinction limit, different methods for predicting lean blow-off limits have been developed and tested for different burner sizes and fuel gas compositions. The aim of the investigations was to find stabilization criteria that allow predictions of blow-off limits of highly turbulent recirculating flames without the requirement for expensive and time-consuming measurements in those flames. Several similarity criteria based on volumetric flowrates, burner size and material parameters of the cold gases, were found to be capable to predict stability limits of premixed and (in some cases) nonpremixed flames at varying swirl intensities, burner scales and fuel compositions. A previously developed numerical field model, combining a k,e-model with a combined “assumed-shape Joint-PDF”/Eddy-Dissipation reaction model for the determination of the time mean reaction rates in turbulent flows was also tested for its potential for stability prediction. All the methods presented have specific advantages and limitations: the similarity criteria are restricted to geometrically similar systems, but they are easy to use, fairly precise and can take the detailled chemistry into account in an integral manner. The numerical field model necessitates large computational effort and is limited to very much simplified, global reaction mechanisms, but it offers the opportunity to make predictions for different burner geometries and swirl conditions.

Analysis of NOX Formation in an Axially Staged Combustion System at Elevated Pressure Conditions

The objective of this investigation was to study the effect of axially staged injection of methane in the vitiated air cross flow in a two stage combustion chamber on the formation of NOX for different momentum flux ratios. The primary cylindrical combustor equipped with a low swirl air blast nozzle operating with Jet-A liquid fuel generates vitiated air in the temperature range of 1473–1673 K at pressures of 5–8 bars. A methane injector was flush mounted to the inner surface of the secondary combustor at an angle of 30 deg. Oil cooled movable and static gas probes were used to collect the gas samples. The mole fractions of NO, NO2 , CO, CO2 , and O2 in the collected exhaust gas samples were measured using gas analyzers. For all the investigated operating conditions, the change in the mole fraction of NOX due to the injection of methane (ΔNOX ) corrected to 15% O2 and measured in dry mode was less than 15 ppm. The mole fraction of ΔNOX increased with an increase in mass flow rate of methane and it was not affected by a change in the momentum flux ratio. The penetration depth of the methane jet was estimated from the profiles of mole fraction of O2 obtained from the samples collected using the movable gas probe. For the investigated momentum flux ratios, the penetration depth observed was 15 mm at 5 bars and 5 mm at 6.5 and 8 bars. The results obtained from the simulations of the secondary combustor using a RANS turbulence model were also presented. Reaction modeling of the jet flame present in a vitiated air cross flow posed a significant challenge as it was embedded in a high turbulent flow and burns in partial premixed mode. The applicability of two different reaction models has been investigated. The first approach employed a combination of the eddy dissipation and the finite rate chemistry models to determine the reaction rate, while the presumed JPDF model was used in the further investigations. Predictions were in closer agreement to the measurements while employing the presumed JPDF model. This model was also able to predict some key features of the flow such as the change of penetration depth with the pressure.

Direct Numerical Simulation of Chemically Reacting Flows with the Public Domain Code OpenFOAM

A new solver for direct numerical simulation (DNS) of chemically reacting flow is introduced, which is developed within the framework of the open-source program OpenFOAM. The code is capable of solving numerically the compressible reactive flow equations employing unstructured grids. Therewith a detailed description of the chemistry, e.g. the reaction rates, and transport, e.g. the diffusion coefficients, has been accomplished by coupling the free chemical kinetics program Cantera. The solver implies a fully implicit scheme of second order for the time derivative and a fourth order interpolation scheme for the discretization of the convective term. An operator-split approach is used by the solver which allows solutions of the flow and chemistry with time scales that differ by orders of magnitude, leading to a significantly improved performance. In addition, the solver has proved to exhibit a good parallel scalability. The implementation of the code has first been validated by means of one-dimensional premixed flames, where the calculated flame profiles are compared with results from the commercially Chemkin code. To demonstrate the applicability of the code for three-dimensional problems, it has been applied to simulate the flame propagation in an explosion vessel of laboratory-scale. A computational grid with 144 million finite volumes has been used for this case. The simulation has been performed parallel on 8192 processors from the HERMIT cluster of HLRS. The calculated burning velocity agrees well with the experimental data.

Experimental Study on Combustion Characteristics of Conventional and Alternative Liquid Fuels

Gas turbine combustor design relies strongly on the turbulent flame velocity over the whole turbine operation range. Due to the fact that turbulent flame velocity depends strongly on the laminar one, its characterisation at different thermodynamic conditions is necessary for further optimisation of gas turbines. The Markstein number, which quantifies the response of the flame to the stretch, also has to be considered. Additionally, the Markstein number can be utilised as an indicator for laminar and turbulent flame front stability.The current attempts to replace conventional fuels, such as kerosene, with alternative ones, obtrude their comparison in order to find the most appropriate substitute. Additionally, significant differences in the flame behaviour, which could be recognised through different combustion characteristics, can lead to modification of currently used gas turbine design. Even so, the experimental data of alternative fuels are scarce, especially at elevated pressure conditions.So, the combustion characteristics, laminar burning velocity and Markstein number of kerosene Jet A-1 and several alternative fuels (GTL and GTL blends) are investigated experimentally in an explosion vessel. For this purpose an optical laser method is employed based on the Mie-scattering of the laser light by smoke particles. Within this experimental study the influence of three crucial parameters: initial temperature, initial pressure and mixture composition on the burning velocity and Markstein number are investigated. The experiments were performed at three different pressures 1, 2, 4bar; three different temperatures 100°C, 150°C, 200°C; and for a range of equivalence ratio 0.67–1.67. The observed results are compared and discussed in detail.Copyright

Experimental and Numerical Investigation of a Turbulent Premixed Flame in an Anechoic Environment

The turbulent jet emanating from an unconfined, premixed burner is investigated by numerical simulation using Large Eddy Simulation (LES) and experimentally by means of optical (OH chemiluminescence), acoustic (microphone) and laser-optical measurement techniques (Laser Doppler Anemometry, Particle Image Velocimetry) for non-reacting and reacting flow, respectively. While 2D-LDA data of the non-reacting flow field are in good agreement with calculated results, 2D-PIV data of the reacting flow field, burning a methane-air mixture, differ significantly from the LES data. This is caused by a falsely chosen seeding injection location, which is apparently located too far downstream. In addition, a large percentage of the total air mass flow has to be used for proper seeding injection and, therefore, is not well mixed with the fuel from the actual fuel injection located further upstream. Consequently, this leads to a non-premixed flame which differs from the partially premixed flame obtained in case using LES. The strong seeding jet itself impacts on the velocity distribution, as well. The emitted noise spectrum has a tonal shape with peaks at the burner’s resonance frequency for the non-reacting flow which changes to broadband noise and is raised in amplitude in case of the reacting flow.

Flow Investigation and Acoustic Measurements of an Unconfined Turbulent Premixed Jet Flame

A turbulent jet emanating from an unconfined, premixed burner is investigated by using large eddy simulation (LES) and direct numerical simulation (DNS) as well as experimentally by means of optical (OH* chemiluminescence), acoustic (microphone), and laser-optical measurement techniques (Particle Image Velocimetry). Comparison of the results obtained through experiments, LES, and DNS indicate a reasonable agreement. In order to analyze the impact of mesh refinement on the resolved flame properties and acoustic radiations, computational grids with varying resolutions are used for the LES. As large coherent flow motion exists in the considered flow case, due to an over-predicted diffusion the flame length calculated with LES is underestimated. On the other hand, DNS exhibits a similar intensity distribution for OH* as the experiment and, hence, the flame length is predicted accurately by DNS. The emitted noise spectrum has a tonal shape with peaks at the burner’s resonance frequency for the non-reacting flow which changes to broadband noise and, in general, is raised in amplitude for reacting flows. In addition, it is shown that an increase in Reynolds number, preheat temperature, or a decrease in equivalence ratio close to stoichiometric ratios yields more noise being emanated from the burner. The latter indicates the fact that direct combustion noise is linked to interactions of turbulent fluctuations with the flame front. When using an equivalence ratio closer to stoichiometric ratio, a thinner reaction zone is expected and the intrinsic interaction between the flame and turbulent flow is more pronounced.

Flame Stabilization and Emissions of a Natural Gas/Air Ceramic Porous Burner

Increasingly stringent regulations for limiting pollutant emissions for both aircraft and industrial gas turbines enforce further reduction of NOx emissions while maintaining flame stability. Application of premixed flames offers the possibility to reduce these emissions, but nevertheless it is strongly connected with flame instability risks. A possible solution to ensure the stability of premixed flames is to provide enhanced heat recirculation employing porous inert material. Experimental determination of flame stability and emissions of a porous burner containing a reticulate ceramic sponge structure are reported and the influence of the structural properties of the porous matrix on stable operating range was investigated. It was found, that the flame stability limit was significantly higher compared with free flame burners and nitric oxide (NOx) emissions were below 10 ppm for all cases.

Experimental and numerical study concerning stabilization of strongly swirling premixed and nonpremixed flames

The wide-spread application of swirl-stabilized flames in industrial combustion devices necessitates an improved understanding of the stabilization mechanisms and stability limits, particularly for the design of modern combustors with high volumetric heat release. Due to the complex mixture and reaction structures in these flames, the predictions of simplified stability models are not satisfactory. We have carried out measurements on unconfined strongly swirling premixed and nonpremixed flames supplying detailed temperature and velocity data as well as mean concentration values of most stable species. Furthermore, significant modifications of the turbulent reaction structure with increasing flowrate are observed. A numerical model for the prediction of the relevant aerodynamic and chemical processes has been developed. A k ,e-model which was corrected for turbulent exchange in swirling flows provides improved prediction of flowfield and mixing, which is crucial for reaction modeling. The mean reaction rates are determined by integration of an “assumed shape joint-PDF,” which is based on experimental findings concerning the joint probability betwen temperature and mixture fraction and on a global 2-step reaction mechanism. Numerical results agree fairly well with experiments and indicate that combustion takes place mainly at low turbulent Damkohler numbers. It can be stated that the stability limits of strongly swirling flames are controlled by finite kinetic rates associated with residence times dependent on flowrate rather than by turbulent mixing rates; these interactions between reaction kinetics and turbulent mixing are correctly reflected by the numerical model.

Numerical verification of the similarity laws for the formation of laminar vortex rings

From analytical investigations it is well known that the roll-up of an inviscid plane vortex sheet which separates at the edge of a body is a self-similar process which can be described by scaling laws. Unlike plane vortices, ring vortices have a curved rotational axis. For this special vortex type experimental investigations as well as calculations in the literature suggest that the scaling laws are only partially valid. The main goal of this work is to clarify how far these similarity or scaling laws are also valid for the formation of viscid laminar vortex rings. Therefore, the formation process of laminar vortex rings was investigated numerically using a CFD (computational-fluid-dynamics) code. The calculations refer to an experimental setup for which detailed experimental data are available in the literature. In this setup, laminar ring vortices are generated by ejecting water from a circular tube into a quiescent environment by means of a piston. First, a case based on a constant piston velocity was investigated. Comparing calculated and measured data yields a very good agreement. Further calculations were made when forcing the velocity of the piston by three different time-dependent functions. The results of these calculations show that the formation laws for inviscid plane vortices are also valid for the formation process of viscid ring vortices. This applies to the normalized axial and radial position of the vortex centre as well as the normalized diameter of the vortex spiral. However, the similarity laws are valid only if the process is considered in a special frame of reference which moves in conjunction with the front of the jet and if the starting time of the formation process with respect to the starting time of the ejection is taken into account. Additionally, the formation of a ring vortex, which occurs during the start-up process of a free jet flow, was calculated. The results confirm a dependence for the motion of the jet front, which is known from analytical considerations and allows some interesting features to be identified.