Simulation Of Combustion Research Articles

A hybrid chemical source treatment for non-premixed combustion simulations

Different approximations based on the exact point implicit (EPI) chemical source treatment, including diagonal approximation (DA), multiscale diagonal approximation (MDA), and single-scale approximation (SA), are numerically assessed and theoretically analyzed with zero-dimensional and one-dimensional combustion simulations in detail. The results show that EPI performs the best in stability, accuracy, and mass conservation. For DA and MDA methods, the ignition delay time, equilibrium temperature, and mass conservation are predicted with a significant error when the time step is large due to the diagonally non-dominant coefficient matrix. However, the error is relatively small when the reactions are weak, thus rendering the accuracy acceptable. SA method performs best in the stability perspective, but the temporal evolution of ignition can not be predicted appropriately. Based on the above conclusions, a hybrid chemical source treatment(HCST) is proposed, where the EPI method is used in chemically active regions while the MDA method is adopted in chemically inactive regions. HCST is first examined with one-dimensional non-premixed flame of various fuels. Then, a Reynolds-averaged Navier–Stokes (RANS) simulation of a supersonic hydrogen/air combustion case is conducted. The same accuracy is achieved by HCST as that using the EPI method for the whole flowfield. In terms of efficiency promotion, for the hydrogen/air 9 species 19 reactions mechanism, an overall promotion of 17% is achieved, while for methane/air 53 species 325 reactions mechanism, a promotion of 75% is observed.

High-temperature corrosion behavior of carbon steel subjected to simulated combustion atmosphere

This research aimed to study the high-temperature oxidation of carbon steel in simulated combustion atmosphere. The comparative oxidation rate of sample substrate carbon steel was experimentally investigated. The comparative high temperature oxidation rate studies were carried out using different atmosphere of gas and salt at 800 °C with different time 1, 25, and 100 hr. The kinetics of oxidation rate of carbon steel were established with weight change measurements. The oxidation rate was measured by the ratio of weight loss and quantity of gas and gas-salt atmosphere. The morphology surface of oxidation in form of spallation and peeling of unprotected oxidation and sulphurization. The structure morphology and chemical composition were characterized by Scanning Electron Microscopy (SEM) and X-ray diffractometry (XRD). The oxidation behavior was further discussed in this research.

Simulation of Liquid Fuel Combustion of a Rocket Engine

For a successful ignition, the ignition should happen at the right time and place. If the ignition is not performed correctly, then substantial damage can occur both within and outside the engine surface. During ignition, the propellants should mix in a proper proportion. During the injection of propellant, the pressure and temperature are generally in subcritical condition. When they are injected inside the combustion chamber, they rapidly grow and transform into supercritical. The combustor shape is utilized to reduce the intention mixture rate and the computational domain encompasses the portion of this design. This is studied in this work by varying the inlet angles of the fuel. By changing the inlet angles (2.5, 5 and 7.5), the initial ignition time can be reduced. The simulations are done using ANSYS Fluent. The initial turbulence is reduced up to a certain angle and then, it starts decreasing. In the same way, the difference is observed in chamber pressure which rapidly increases, followed by a decrease. It can be concluded that 5 angle shows better performance in terms of pressure and velocity turbulence.

Thermodynamic Simulation of Producer Gas Combustion from Biomass Gasification

The use of new and renewable energy for the application on gas burners is beneficial for the environment. This study aims to determine the effect of excess air during the combustion of producer gas from biomass gasification to release heat used as a fuel gas burner. Simulations are carried out by applying mass and energy balance, showing that an increase in excess air will decrease the non-adiabatic and adiabatic flame temperature. The result showed that an increase in excess of air reduces the amount of heat released to the environment for the same flame temperature. The maximum adiabatic flame temperature is at 1725.430C, while the non-adiabatic ranges from 600 to 8000C. Furthermore, heat is released in the range of 20.1 kW to 28.8 kW, and excess air from 0 to 40%.

Особенности расчета нестационарного теплообмена в стенах вращающихся печей

A refined calculation of the non-stationary temperature of the walls of a rotary kiln is proposed, taking into account the non-stationary thermal conductivity in the enclosing walls and the accumulation of heat in them, intended for use in numerical simulation of combustion and heat transfer processes. The influence on heat transfer of the alternating contact of the inner surface of the walls of a rotary kiln with a gaseous medium and a layer of technological material is considered. The adequacy of the method was verified by numerical solution of the differential equation of non-stationary thermal conductivity of walls in the conditions of a computational experiment.

Numerical Simulations of Spray Combustion in Jet Engines

The aviation sector is facing a massive change in terms of replacing the currently used fossil jet fuels (Jet A, JP5, etc.) with non-fossil jet fuels from sustainable feedstocks. This involves several challenges and, among them, we have the fundamental issue of current jet engines being developed for the existing fossil jet fuels. To facilitate such a transformation, we need to investigate the sensitivity of jet engines to other fuels, having a wider range of thermophysical specifications. The combustion process is particularly important and difficult to characterize with respect to fuel characteristics. In this study, we examine premixed and pre-vaporized combustion of dodecane, Jet A, and a synthetic test fuel, C1, based on the alcohol-to-jet (ATJ) certified pathway behind an equilateral bluff-body flameholder, spray combustion of Jet A and C1 in a laboratory combustor, and spray combustion of Jet A and C1 in a single-sector model of a helicopter engine by means of numerical simulations. A finite rate chemistry (FRC) large eddy simulation (LES) approach is adopted and used together with small comprehensive reaction mechanisms of around 300 reversible reactions. Comparison with experimental data is performed for the bluff-body flameholder and laboratory combustor configurations. Good agreement is generally observed, and small to marginal differences in combustion behavior are observed between the different fuels.

The Influence of Design Geometry on the Operational Performance of Helmholtz Pulse Combustor

The objective of this study is to build a pulse combustion simulation model in order to determine the design parameters that influence the pulse combustor’s optimized performance. The Helmholtz valveless pulse combustor type is the simple design of pulse combustors. It has many advantages over conventional combustion, so it has been used in a variety of applications. There are many parameters that need to be studied in order to design this combustor type to achieve optimal performance. This research presents numerical and experimental investigations of pulse combustion. FLUENT software is used to conduct numerical simulations of the design parameters and pulse combustor performance. These simulations are then experimentally validated and confirmed to be accurately optimized in order to be applicable. The performance of the pulse combustor is studied for different inlet and tailpipe length and combustion chamber volume. This research accurately predicts how the frequency and pressure amplitude vary as the lengths of the inlet and tailpipe increase, as well as the combustion chamber volume.

Lattice Boltzmann method with nonreflective boundary conditions for low Mach number combustion

The paper presents a lattice Boltzmann (LB) method for premixed and nonpremixed combustion simulations with nonreflective boundary conditions, in contrast to Navier–Stokes solvers or hybrid schemes. The current approach employs different sets of distribution functions for flow, temperature and species fields, which are fully coupled. The discrete equilibrium density distributions are obtained from the Hermite expansions thus thermal compressibility is included. The coupling among the momentum, energy and species transport enables the model to be applicable for reactive flows with chemical heat release. The characteristic boundary conditions are incorporated into the LB scheme to avoid numerical reflections. The multi-relaxation-time collision schemes are applied to all the LB solution procedures to improve numerical stability. With detailed thermodynamics and chemical mechanisms for hydrogen-air, the LB modelling framework is validated against both premixed flame propagation and nonpremixed counterflow diffusion flame benchmarks. Simulations of circular expanding premixed flames further demonstrate the capability of the new reactive LB method. The developed LB methodology retains the advantages of classic LB methods and extends the LB capability to low Mach number combustion with potential applications in mesoscale and microscale combustors, catalysis, fuel cells, batteries and so on.

An improved stiff-ODE solving framework for reacting flow simulations with detailed chemistry in OpenFOAM

The integration of stiff ordinary differential equation (ODE) systems associated with detailed chemical kinetics is computationally demanding in practical combustion simulations. Despite the various approaches in expediting the computational efficiency, it is still necessary to optimise the cell-wise calculation in operator-splitting type simulations of reactive flow. In this work, we proposed an improved stiff-ODE solver framework targeting to speed up the simulation of reactive flow in OpenFOAM. This framework combines the Radau-IIA and backward differentiation formula (BDF) ODE-integration algorithms, the pyJac-based fully analytical Jacobian formulation, and dense-based LAPACK and sparse-based KLU sophisticated linear system solvers. We evaluate the performance of the efficient solver framework on various benchmark combustion problems across a wide range of chemical kinetic complexities. A comprehensive investigation of the key elements of stiff ODE solvers is conducted in the homogeneous reactor, focusing respectively on the influences of error tolerance, integration time interval, Jacobian evaluation methodology, and linear system solver on the accuracy and efficiency trade-off. More realistic simulation results are presented regarding the one-dimensional laminar flame and three-dimensional turbulent flame. The results indicate that the Radau-IIA is more preferable in both efficiency and accuracy compared with the widely used BDF and Seulex methods for large integration interval, whereas the differences between three methods diminish as the integration time interval decreases. In all cases, it is found that the full analytical Jacobian is more advantageous for small mechanisms of species number around 50–100 while the approximated formulation of Jacobian is recommended for larger ones. Furthermore, the more robust linear system solvers provide significant improvement on computational efficiency with the dense-based LAPACK solver being more suitable for small to moderate-scale mechanisms while sparse-based KLU being superior for large-scale mechanisms. The proposed efficient solver framework in its optimal configuration obtains more than 2.6 times speedup in realistic high-fidelity flame simulation with a 57 species combustion mechanism.

Automated combustion model classification for char particle distributions using 3-D morphology analysis and pore-resolving CFD simulations

Combustion of pulverized coal involves millions of particles with a distribution of morphologies. A workflow is presented to characterize, classify, and model distributions of highly porous coal char particles. To understand the impacts of morphology, 3-D pore-resolving combustion simulations for 150 particles imaged with micro-CT are first performed. To facilitate modeling distributions of morphology in reactor-scale CFD, a machine learning algorithm is then trained to classify particles according to their expected combustion behavior and to select an accurate, computationally efficient 1-D particle-scale combustion model. Whereas existing approaches classify particles solely according to morphology and use 2-D measurements, the present approach classifies particles according to combustion behavior, using 3-D morphological data as input and 3-D pore-resolving combustion simulation data for training. To facilitate application of the workflow to other highly porous coal chars, an automated image analysis routine is developed to filter and segment particles and to measure their morphological parameters for use in classification. When using the most appropriate 1-D combustion model for each particle as determined by the 3-D simulations, the average relative error in 1-D effectiveness factors for the 130 particles tested compared to their 3-D effectiveness factors was 9.9%, while the average relative error in 1-D effectiveness factors predicted by the proposed workflow with automated image analysis and a pretrained random forest classifier was 13.7%.

Numerical Simulation of Methane Combustion in Two-Layer Porous Media Under Oxy-Fuel Condition

Numerical Simulation of Methane Combustion in Two-Layer Porous Media Under Oxy-Fuel Condition

Experimental study and molecular simulation of spontaneous combustion of coal gangue by oxidation under different water contents

ABSTRACT To determine the effect of water contents on the oxidation spontaneous combustion of coal gangue, the properties and combustion characteristics of coal gangue were studied using industrial and elemental analyses and Thermo Gravimetry-Differential Scanning Calorimetry (TG-DSC). The molecular dynamics simulation (MDS) and giant regular Monte Carlo simulation (GCMC)were used to investigate the adsorption isotherms of oxygen and coal gangue (kaolinite) under different water molecules (400, 500, 600, 700). The results of thermogravimetric showed that with increasing moisture content, the characteristic temperature points T3-T5 of CG4 were much lower than those of the other samples, the combustion conversion rate is the highest and the spontaneous combustion effect is the best at the same temperature. The molecular simulation results show that under 700 water-oxygen system, the adsorption of oxygen molecules and kaolinite (001) surface is the strongest. The interface has the fewest water molecules and the largest diffusion coefficient.

Reacting and non-reacting, three-dimensional shear layers with spanwise stretching

A three-dimensional, steady, laminar shear-layer flow spatially developing under a boundary-layer approximation with mixing, chemical reaction, and imposed normal strain is analyzed. The purpose of this study is to determine conditions by which certain stretched vortex layers appearing in turbulent combustion are the asymptotic result of a spatially developing shear flow with imposed compressive strain. The imposed strain creates a counterflow that stretches the vorticity in the spanwise direction. Equations are reduced to a two-dimensional form for three velocity components. The non-reactive and reactive cases of the two-dimensional form of the governing equations are solved numerically, with consideration of several parameter inputs, such as the Damköhler number, the Prandtl number, chemical composition, and free-stream velocity ratios. The analysis of the non-reactive case focuses on the mixing between hotter gaseous oxygen and cooler gaseous propane. The free-stream strain rate κ* is predicted by ordinary differential equations based on the imposed spanwise pressure variation. One-step chemical kinetics are used to describe diffusion flames and multi-flame structures. The imposed normal strain rate has a significant effect on the width of downstream mixing layers as well as the burning rate. Asymptotically in the downstream direction, a constant width of the shear layer is obtained if the imposed normal strain rate is constant. The one-dimensional asymptotic result is an exact solution to the multicomponent Navier–Stokes equation for both reacting and non-reacting flows, although it was obtained using the boundary-layer approximation. A similar solution with the layer width growing with the square root of downstream distance is found when the imposed strain rate decreases as the reciprocal of downstream distance. The reduced-order asymptotic solutions can provide useful guidance in developing flamelet models for simulations of turbulent combustion.

Influence of multiple structural parameters on buffer performance of a thin‐walled circular tube based on the coupling modeling technique

AbstractPyrotechnic devices are widely used in the aerospace and defense industries. However, these devices generate high‐frequency and high‐amplitude shock responses during their use, compromising safe operation of the system. In this paper, the application of a thin‐walled circular tube as the energy absorber in pyrotechnic devices is investigated. To accurately predict the shock load and the buffer performance of the thin‐walled circular tube, a coupled model connecting the energetic material combustion and finite element simulation is established. The validity of the coupled model is verified by comparing with experiments. Then, the collapse mechanism of the thin‐walled circular tube is studied, and the influence of multiple structural parameters on its buffer performance is analyzed. The results show that the thin‐walled circular tube effectively reduces the shock overload. The maximum shock overload reduced from 572 612g to 11 204g in the studied case. The structural parameters of the thin‐walled circular tube mainly affect the deformation process and the maximum shock overload. The order of importance of structural parameters to the maximum shock overload is determined, among which the wall thickness has the most significant effect.

Large Eddy Simulation of Combustion for High-Speed Airbreathing Engines

Large Eddy Simulation (LES) has rapidly developed into a powerful computational methodology for fluid dynamic studies, between Reynolds-Averaged Navier–Stokes (RANS) and Direct Numerical Simulation (DNS) in both accuracy and cost. High-speed combustion applications, such as ramjets, scramjets, dual-mode ramjets, and rotating detonation engines, are promising propulsion systems, but also challenging to analyze and develop. In this paper, the building blocks needed to perform LES of high-speed combustion are reviewed. Modelling of the unresolved, subgrid terms in the filtered LES equations is highlighted. The main families of combustion models are presented, focusing on finite-rate chemistry models. The density-based finite volume method and the reaction mechanisms commonly employed in LES of high-speed H2-air combustion are briefly reviewed. Three high-speed combustor applications are presented: an experiment of supersonic flame stabilization behind a bluff body, a direct connect facility experiment as a transition case from ramjet to scramjet operation mode, and the STRATOFLY MR3 Small-Scale Flight Experiment. Several combinations of turbulence and combustion models are compared. Comparisons with experiments are also provided when available. Overall, the results show good agreement with experimental data (e.g., shock train, mixing, wall heat flux, transition from ramjet to scramjet operation mode).

1D/3D simulation procedure to investigate the potential of a lean burn hydrogen fuelled engine

In recent years hydrogen, especially the one generated by renewable energy, is gaining increasing attention as a clean fuel to support the future mobility towards efficient and low emission solutions for propulsion systems. In this scenario, the present work deals with the virtual conversion of a single-cylinder Diesel engine, conceived for marine applications, into a hydrogen Spark Ignition (SI) unit. A simulation methodology is adopted, combining 1D and 3D Computational Fluid Dynamics (CFD) methods. First, experiments are realized on the original Diesel engine mounted on a test bench, collecting main performance indicators and emissions. A complete 1D engine model (GT-Power™) is developed and validated against measurements. Then, a 3D model of the cylinder (STAR-CD) is set-up and the related combustion outcomes are compared both with 1D and experimental results, showing an overall good agreement. In the second stage, the Diesel unit is converted into a port-injected hydrogen SI engine; the 3D model is re-arranged and utilized to reproduce pre-mixed hydrogen combustions under ultra-lean air/fuel (A/F) mixtures. Also, the 1D model is partly modified and coupled to an advanced combustion sub-model integrated with fast tabulated chemical kinetics to predict the knock. In particular, 1D combustion evolution is calibrated against the results of 3D CFD hydrogen combustion simulation. Finally, the calibrated 1D model is applied to investigate the advantages of ultra-lean hydrogen combustion in terms of efficiency, NO, and unburned H2 formation at medium/high loads.

Large Eddy Simulation of a Pistonless Constant Volume Combustor: A New Concept of Pressure Gain Combustion

Abstract Classical gas turbine thermodynamic cycle has undergone no change over the last decades. The most important efficiency improvements have been obtained by reducing thermal losses and raising the overall pressure ratio and peak temperature. Pressure gain combustion (PGC) represents an increasingly interesting solution to break out current technological limits. Indeed cycle models show that a pressure raise across the combustion process would reduce fuel consumption, increasing efficiency. Providing an efficiency close to the corresponding detonative technological concepts, constant volume combustion (CVC) represents a viable solution that still needs to be studied. In this work, the CV2 (constant-volume combustion vessel) installed at the Pprime laboratory (France) is numerically investigated using the high-fidelity compressible large eddy simulation (LES) solver AVBP. All the successive phases of the CVC cycle, i.e., air intake, fuel injection, spark-ignited combustion, and exhaust, are considered in the LES. Intake and exhaust valves are properly represented by novel boundary conditions able to mimic the valves impact on the flow without the need to directly consider their presence and dynamics during the simulation, reducing the computational costs. The spark ignition is modeled as an energy deposition term added to the energy equation. The combustion phase is treated by the dynamic version of the thickened flame model (DTFLES) extended to deal with nonconstant pressure combustion. Time-resolved particle imaging velocimetry (PIV) and pressure measurement inside the chamber reveal that cold and reactive turbulent flow are well captured in all the phases, showing the reliability of the approach and the models used.

Numerical Simulation of HCCI Combustion Fuelled with Gasoline + Methyl Tert-Butyl Ether Blend

Background: Homogeneously charged compression ignition (HCCI) engines are generally popular for low production of NOx and Soot formation, also consuming lower fuel as compared with conventional gasoline and diesel engine. Objectives: In this study, the compression ignition engine (CI) was simulated in HCCI mode fuelled with Gasoline and Methyl tert-butyl ether (MTBE) blends, and the analysis was carried out in similar operating conditions as experimentation carried out. Methods: The chamber pressure was maintained at 2 bar, and the air-fuel ratio was maintained at2.75. The optimal blend for MTBE was fixed as 15 vol % (MTBE 15), and the measured properties of the blends were used in the simulation. ICE simulation module in ANSYS WORKBENCH was used for this current simulation. Both cold flow and combustion simulations were carried out and the results indicate velocity indie the cylinder, cylinder pressure, and temperature distributions for two cases such as gasoline and gasoline with MTBE15. Findings: The average velocity was found high with the MTBE blend, whereas in-cylinder pressure was found low with MTBE blend than with gasoline. The temperature inside the cylinder pressure was found higher with gasoline than MTBE blend. The deviation between the experimentation and simulation was found low. Novelty: The simulation provides a better understanding of the combustion phenomena when gasoline is mixed with MTBE and utilized in a diesel engine at HCCI mode. Keywords: CFD; MTBE; Gasoline; HCCI; Emission reduction

The role of energy transfer and competing bimolecular reactions in characterizing the unimolecular dissociations of allylic radicals

Resonantly stabilized radicals (RSR's) such as allylic radicals typically have bond energies exceeding 45 kcal/mol and are therefore reasonably long-lived up to quite high temperatures. Consequently, kinetics of the unimolecular dissociations and bimolecular reactions of these long-lived RSR's assume significance in high temperature chemically reacting systems such as in flames. Our recent analyses (J. Cho et al., Proc. Comb. Inst., 2022) of the uncertainty propagated by the energy transfer parameters for 1-methylallyl (1MA) dissociation to the laminar flame speed, indicates an intricate coupling between the kinetics of 1MA dissociation (chain propagation) and its reaction with H-atoms (chain-termination). This work extends such analyses to other C3 – C5 allylic radicals: allyl, 2-methylallyl, 1,1-dimethylallyl, 1,2-dimethylallyl, and 1,3-dimethylallyl. Theoretical kinetics for the C3 – C4 allylic radicals were taken from prior literature studies (J.A. Miller et al., J. Phys. Chem., 2008; J. Cho et al., Proc. Comb. Inst., 2022; R.S. Tranter et al., Proc. Comb. Inst., 2017). Potential energy surfaces were newly computed for the dissociations of the larger C5 allylic radicals and the reactions of these radicals with H-atoms. These PES's were then used to calculate rate constants for the relevant unimolecular and bimolecular reactions. Energy transfer has a crucial role to play in these pressure-dependent reactions and therefore the present kinetics calculations relied on a-priori predictions of collisional energy transfer parameters to best characterize the branching between chain propagating unimolecular dissociations of these allylic radicals and chain terminating bimolecular reactions of these RSR's with H-atoms. Prompt dissociations are now established to be a universal feature of free radical dissociations. Here we have also characterized the propensity for prompt dissociations in these more-stable allylic RSR's and their role in combustion simulations.

CFD analysis of karanja oil methyl ester (KOME) blend B20 and hydrogen using ANSYS-Fluent

Combustion and emission characteristics are plays an important role in diesel engines, which have higher combustion efficiency meets the stringent emission norms. Combustion is a very complex phenomenon, which cannot be analyzed easily with analytical method or different visualization technique. Computational Fluid Dynamics(CFD) has becoming useful tool in understanding the fluid dynamics of IC Engines for design purposes. In the current study, CFD code is used to perform 3D simulation of mixture formation, combustion and emissions of K20 and K20 with hydrogen fuels simulated by applying the ANSYS-FLUENT non-premixed k-ε RNG model with combustion chamber moving mesh. The parameters like Pressure, Temperature, Parcel diameter, CO, CO2 and NOx generated inside the combustion chamber is compared with K0. The variation in the combustion behavior is find when the fuel is used likeK0, K20 and K20 with hydrogen. Finally, the emission results of simulation are validated with experimental results. The variations of CFD and experimental results ofemissions are find acceptable.