Lean-Blow-Out Simulation of Natural Gas Fueled, Premixed Turbulent Jet Flame Arrays With LES and FGM-Modeling

To ensure compliance with stricter regulations on exhaust gas emissions, new industrial burner concepts are being investigated. One of these concepts is the matrix burner, consisting of an array of premixed, non-swirling jet flames. For the design of such burners, the prediction of fundamental burner properties is mandatory. One of these essential quantities is the lean blowout limit (LBO), which has already been investigated experimentally. This study investigates the possibility of numerical LBO prediction using a tabulated chemistry approach in combination with Large-Eddy-Simulation turbulence modeling. In contrast to conventional swirl burners, the numerical description of blowout events of multi jet flames has not yet been studied in detail. Lean blowout simulations have therefore been conducted for multiple nozzle variants, varying in their diameter and global dump ratio for a variety of operating conditions, showing their general applicability. A procedure to induce LBO is introduced where a stepwise increase in total mass flow is applied. LBO is determined based on the temporal progress of the mean reaction rate. A comparison with measurements shows good agreement and demonstrates that the procedure developed here is an efficient way to predict LBO values. Further investigations focused on the flame behavior when approaching LBO. The flame shape shows a drastic change from single jet flames (stable conditions) to a joint conical flame approaching LBO, which increases in length for increasing inlet velocity, showing the importance of jet interaction at LBO

3D direct pore level simulations of radiant porous burners

Inside porous burners, chemical combustion reactions coincide with complex interaction between thermo-physical transport processes that occur within solid and gaseous phase and across phase boundary. Fluid flow, heat release and resulting heat flows influence each other. The numerical model used in this work considers gaseous and solid phases, includes fluid flow, enthalpy transport, conjugate heat transfer, and radiative heat transfer between solid surfaces, as well as combustion kinetics according to a skeletal chemical reaction mechanism, fully resolved on the pore scale in three-dimensional space (Direct Pore Level Simulation, DPLS). The calculations are performed based on the finite volume method using standard applications implemented in the OpenFOAM library. The present study presents simulations of three different structures, each at four settings of specific thermal power. Results indicate that specific surface area of the porous structure is a major influencing parameter for increasing radiation efficiency, whereas no correlation of the orientation of an anisotropic structure on radiation efficiency was observed

Super-adiabatic flame temperatures in premixed methane-oxygen flames

Oxy-fuel combustion differs significantly from conventional air combustion. The absence of nitrogen leads to higher flame temperatures and larger concentration of major species as well as intermediate species. In the present work, freely propagating methane-oxygen flames were numerically calculated using a 1D model from lean to rich conditions in order to investigate the appearance of super-adiabatic flame temperatures (SAFT). The calculations were performed for equivalence ratios of 0.5 < Φ < 3.0 with an increment of 0.1, different inlet temperatures from 300 K to 700 K and a pressure range of 0.1 MPa to 1 MPa. Additionally, selected results were investigated with different detailed chemical reaction mechanisms. The results showed that the maximum flame temperature exceeds the equilibrium temperature for equivalence ratios Φ > 0.9. Two different regimes were identified, where SAFT phenomenon appears. The first regime was found in slightly rich conditions (1.0 2.1). A first maximum of temperature difference is observed at an equivalence ratio of Φ = 1.5. Approximately 120 K to 180 K higher temperatures than the equilibrium ones at standard inlet conditions are locally observed, depending on the applied reaction mechanism. The first maximum at Φ = 1.5 correlates with the maximum concentration of the Hradical, which plays a key role in the first SAFT regime. A minimum over-temperature of 50 K was identified at an equivalence ratio of Φ = 2.1. By significantly increasing the equivalence ratio, the maximum flame temperature exceeded the equilibrium up to almost 400 K at Φ = 3.0 in the second SAFT regime. An increased preheating temperature enhanced the occurrence of SAFT in the first regime and degraded it in the second regime. Elevated pressure leads to the opposite effects with decreased SAFT in the first and increased SAFT in the second regime

Hybrid eulerian-lagrangian approach for dense spray simulations

In this work, a hybrid Euler-Lagrangian solver for dense spray systems is developed specifically for cases where film creation by accumulation of liquid droplets at the walls plays a crucial role. EulerLagrangian solvers are commonly used to describe the spray with predefined spray characteristics. The Lagrangian particles represent liquid drops moving along the continuous gaseous phase. This approach assumes a small particle size compared to the cell size and it is unable to capture the breakup behavior of liquid jets in the presence of instabilities. VOF methods, on the other hand, are not a computationally feasible option when it comes to small droplet sizes as a result of liquid atomization because they have to be fully resolved by the computational mesh. Hence, multiscale simulations are required to bridge the gap between the two methods combining subgrid droplets in Lagrangian approaches and large liquid structures in VOF methods. In the present work, a multiscale approach is developed where Lagrangian particles representing small droplets are tracked through the continuous phase until they hit a wall or a liquid-gas interface represented by a continuous VOF field. At the time of impact, the Lagrangian particles are removed and the mass and momentum of these particles are transferred to the VOF field. This allows having a spray consisting of subgrid droplets computed with a Lagrangian particle tracking (LPT) approach and liquid films at the walls with VOF method. The method represents a one-way coupling, converting Lagrangian particles to Eulerian liquid phase (VOF) and has been implemented into the open-source CFD code OpenFOAM. Subsequently, the solver has been tested in different scenarios to ensure mass and momentum conservation. Positive test results encouraged its use to gain insight on the fluid flow in a real cylindrical compression-dissolution unit where the liquid is sprayed from the top while simultaneously the gas is compressed from the bottom. Dynamic mesh technique is used to account for the compression with a moving piston

Experimental determination of the laminar burning velocity of methane-oxygen-flames stabilized with cylindrical burners at various preheating temperatures

The laminar burning velocity of pure methane-oxygen flames stabilized with a cylindrical tube burner was determined experimentally via optical methods (Schlieren-technique and CH* chemiluminescence). Both flow exit profiles were examined: a fully developed and a plug flow profile. The experiments were conducted for an equivalence ratio range of 0.5 < Φ < 2.2, while the inlet temperature of the educts was set to 293 K. The results of the CH4-O2 flames show a maximum laminar burning velocity of approx. 360 cm/s near stoichiometric conditions

Identification of Flame Regimes in Partially Premixed Combustion from a Quasi-DNS Dataset

Identifying combustion regimes in terms of premixed and non-premixed characteristics is an important task for understanding combustion phenomena and the structure of flames. A quasi-DNS database of the compositionally inhomogeneous partially premixed Sydney/Sandia flame in configuration FJ-5GP-Lr75-57 is used to directly compare different types of flame regime markers from literature. In the simulation of the flame, detailed chemistry and diffusion models are utilized and no turbulence and combustion models are used as the flame front and flow are fully resolved near the nozzle. This allows evaluating the regime markers as a post-processing step without modeling assumptions and directly comparing regime markers based on gradient alignment, drift term analysis and gradient free regime identification. The goal is not to find the correct regime marker, which might be impossible due to the different set of assumptions of every marker and the generally vague definition of the partially premixed regime itself, but to compare their behavior when applied to a resolved turbulent flame with partially premixed characteristics

Detailed Transport and Performance Optimization for Massively Parallel Simulations of Turbulent Combustion with OpenFOAM

This work describes the implementation of two key features for enabling high performance computing (HPC) of highly resolved turbulent combustion simulations: detailed molecular transport for chemical species and efficient computation of chemical reaction rates. The transport model is based on an implementation of the thermo-chemical library Cantera [1] and is necessary to resolve the inner structure of flames. The chemical reaction rates are computed from automatically generated chemistry-model classes [2], which contain highly optimized code for a specific reaction mechanism. In combination with Sundials’ [3] ODE solver, this leads to drastic reductions in computing time. The new features are validated and applied to a turbulent flame with inhomogeneous mixing conditions on a grid with 150 million cells. The simulation is performed on Germany’s fastest supercomputer “Hazel Hen” [4] on 28,800 CPU cores, showing very good scalability. The good agreement with experimental data shows that the proposed implementations combined with the capabilities of OpenFOAM are able to accurately and efficiently simulate even challenging flame setups

The Eulerian Stochastic Fields Method Applied to Large Eddy Simulations of a Piloted Flame with Inhomogeneous Inlet

Large Eddy Simulations of the Sydney mixed-mode flame with inhomogeneous inlet (FJ200-5GP-Lr75-57) are performed using the Eulerian Stochastic Fields (ESF) transported probability functions method to account for the sub-grid scale turbulence–chemistry interaction, to demonstrate the suitability of the ESF method for mixed-mode combustion. An analytically reduced 19-species methane mechanism is used for the description of the chemical reactions. Prior to the reactive case, simulation results of the non-reactive setup with cold and hot pilot stream are presented, which show differences in the jet breakup and radial species mass fluxes. The reactive case simulations are compared to experimental data and a recently conducted model free quasi-DNS (qDNS), showing very good agreement with the qDNS in terms of scatter data and radial mean values of temperature and species distribution, as well as mixture fraction conditional statistics. Further analysis is dedicated to sub-grid scale statistics, showing that mixture fraction and reaction progress variable are strongly correlated in this flame. The impact of the number of stochastic fields on the filtered temperature and species distribution is investigated; it reveals that the ESF method in conjunction with finite-rate chemistry is very insensitive to the number of employed fields to obtain highly accurate simulation results