Feichi Zhang

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 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.

Numerical Simulation of the Ignition of Fuel/Air Gas Mixtures Around Small Hot Particles

Abstract This study presents the simulation and detailed analysis of the ignition of initially quiescent fuel/air mixtures by small, stationary, laser-heated spherical particles. Our simulations cover a wide parameter space by varying the kind of fuel, stoichiometry, heating rate, radical surface destruction efficiencies as well as particle size. The results agree well with experimentally determined particle surface temperatures at the time of ignition over the whole range of parameters. The surface temperatures required for ignition strongly depends on the kind of fuel and increases in the order hydrogen, acetylene, ethylene, ethane, propane and methane. It also increases with decreasing particle size. By contrast, mixture stoichiometry and heating rate have a minor influence on the ignition temperatures. Comparisons with two-dimensional direct numerical simulations show that fast, but fully coupled one-dimensional simulations are sufficient to capture the details of the ignition event, permitting a systematic investigation for large number of conditions. At small particle radii (r≤2 mm) there exists a simple mapping of only two parameters, an apparent activation energy and a factor comprising thermo-physical properties of the gas phase that is able to estimate the particle surface temperature required for ignition. Such a map might be used for the safety assessment of ignition hazards by small hot particles as function of fuel, stoichiometry and particle size.

Thermoacoustics of a turbulent premixed flame

Quantitative analyses of noise induced by turbulent combustion processes are essential for the design of efficient combustors. To understand the noise generating mechanisms detailed thermoacoustic source mechanisms for the acoustic perturbation equations are deduced from the governing equations of compressible reactive fluids. A generic burner configuration operated with a turbulent premixed flame is experimentally and numerically investigated to identify relevant source mechanisms and to show the dependence of the noise radiation on the operating conditions. Besides direct combustion noise mechanisms by heat release fluctuations indirect mechanisms by acceleration of entropy inhomogenities and non-isentropic mixing processes are identified as major noise sources.

Impact of Grid Refinement on Turbulent Combustion and Combustion Noise Modeling with Large Eddy Simulation

For Large Eddy Simulation (LES) of turbulent combustion, as the turbulent flow as well as the thin flame front are directly filtered by the cut-off scale, resolution of the computational grid plays a very important role in this case and represents always a quality determining factor. In addition, the fluctuation of heat release is found to be the main source for noise generation from turbulent combustion, which is attributed to the interaction of the turbulent flow and the combustion reaction. As the flame is thickened or filtered respectively by the computational grid, it becomes less sensitive to fluctuations of the flow as well, so that the emitted noise from the flame due to unsteady heat release is affected by the grid resolution in LES combustion modeling as well. The current work represents an investigation with respect to these aspects. For this purpose, LES and DNS simulations for a realistic jet flame at moderately turbulent condition have been carried out. The LES calculations have been performed on computational grids with different resolutions (0.4/3.2/10.7 million cells) on the HP XC4000 cluster of the Steinbuch Centre for Computing (SCC) at the KIT. In order to assess predictability of the LES methodology, a three-dimensional DNS simulation on a grid with 54 million cells has been carried out on the Cray XE6 (HERMIT) of the High Performance Computing Center Stuttgart (HLRS). The comparison of the LES solution with experimental and DNS data allows an evaluation of the influence of the grid refinement to a great extent.

Ignition of combustible mixtures by hot particles at varying relative speeds

ABSTRACT Detailed numerical simulations have been performed to study the ignition behavior of hot spherical particles at atmospheric conditions. The particles move relative to a combustible gas with a velocity of 0–30 m s, which spans different flow regimes, from creeping flow to unsteady vortex shedding. The temperature of the particles’ surface increases linearly over time and is recorded at ignition for methane/air and hydrogen/air mixtures. For low relative velocities m s or Reynolds numbers , increases proportionally to or and the flow field is axisymmetric. For higher relative velocities, an unsteady vortex street forms behind the particle so that three-dimensional simulations are required. A correlation employing the van’t Hoff criterion yields linear correlations based on the Nusselt number and for both the low- and high-velocity ranges. For rich hydrogen flames at high velocities, the flame temporarily stabilizes near the hot particle in the recirculation zone downstream. As the surface temperature increases further, the flame suddenly starts to propagate downstream, leading to two distinct ignition events: local ignition at the particle’s surface and start of the propagation into the surrounding gas. The latter yields a much steeper increase of ignition temperature with incoming flow velocity.

A DNS Analysis of the Correlation of Heat Release Rate with Chemiluminescence Emissions in Turbulent Combustion

The essential correlation of heat release rate and chemiluminescence emission from turbulent combustion is quantitatively analyzed by means of direct numerical simulation (DNS) of premixed methane/air flames, employing a detailed reaction mechanism with 18 species and 69 elementary reactions, and the mixture-averaged transport method. One-dimensional freely propagating laminar flames have first been studied for different stoichiometries varying from fuel-lean to fuel-rich conditions. There, the local generation of the chemiluminescent OH* species correlates strongly with the heat released by the combustion reaction, especially in the fuel-lean range. Three-dimensional DNS have then been applied to calculate a synthetically propagating flame front subjected to different turbulent inflow conditions. Joint probability density functions of OH* concentration and heat release rate have been generated from the DNS results, showing a stronger scattering of the correlation curve compared to the corresponding laminar flame. As the chemiluminescence measurement gathers light only along one viewing direction, the line-of-sight integrated values of heat release and OH* concentration have been evaluated from the DNS, where the domain has been decomposed into a number of rays defined by a fixed viewing direction and a specific area. A quasi-linear relationship has been identified for these integral values, where the correlation becomes stronger for flames subjected to lower turbulence intensities or larger cross-section areas of the rays. A computational grid with 16 million finite volumes has been used for the DNS of the turbulent flames and the simulations have been performed in parallel with 3,600 processor cores from the Hazel Hen cluster of HLRS. Scale-up performance of the DNS code, which is based on the open-source program OpenFOAM, has been evaluated.

Numerical Simulation of Turbulent Combustion with a Multi-Regional Approach

The current work uses a multi-regional method for improving the computing performance of large-scale combustion simulations. In this manner, the solution of the isothermal flow within the burner is treated separately from the domain with combustion reaction. For the fresh gas flow within the nozzle only the Navier-Stokes equations for a non-reactive, fixed composition flow are solved, whereas the combustion model accounting for the chemical reactions is enabled in the ignition zone downstream of the burner. Because the chemistry solution takes a major part of the total computing time, the approach saves that part of execution time for the computing nodes located within the nozzle, where no chemical reaction occurs. In the present study, the potential of this methodology has been assessed by large eddy simulation (LES) of a model burner operated with a premixed methane/air flame. The multi-regional simulation showed consistent results with data obtained from the conventional single-regional computation. It however has been proven to be considerably faster than the comparable single-zonal LES, denoting an improved computing performance.

Application of the Unified Turbulent Flame-Speed Closure (UTFC) Combustion Model to Numerical Computation of Turbulent Gas Flames

The current work presents the numerical computation of turbulent reactive flow by means of three different classes of flame: a premixed, a non-premixed and a partially premixed flame. The aim thereby is to validate the unified turbulent flame-speed closure (UTFC) combustion model developed at our institute. It is based on the presumption that the entire turbulent flame can be viewed as a collection of laminar premixed reaction zones (flamelets) with different mixing ratios. The mixing process is controlled by the mixture fraction ξ and the subsequent chemical reaction by the progress variable θ. The turbulent flame speed S t is used to describe the flame/turbulence interaction as well as the finite rate reaction. Complex chemistry is included and the pressure dependency (elevated pressure) of the combustion process is included in the model as well. The applicability of the model is explored by means of RANS (Reynolds averaged Navier-Stokes approach) and LES (large eddy simulation) methodologies at a wide range of Damkohler number Da. The results of all simulations show reasonable good agreement with the experiments.