Finite Reaction Research Articles

Numerical investigation on combustion performance optimization with strut/wall combined-staged injection strategy within an axisymmetric variable geometry combustor at Mach 3

Numerical simulations were conducted on the strut/wall multistage injection strategy to optimize the combustion performance at Mach 3. Two kinds of the combustion models were used: The Eddy-Dissipation Model (EDM) was used first to highlight the effects of mixing performance on the combustion performance; then the Finite-Rate/Eddy-Dissipation Model (FR/EDM) was used to modify the influence mechanism of wall injection positions by incorporating with finite-rate reaction effects. The combustion process was divided into three stages to analyze the average heat release rate and the contribution of average total temperature change of each stage to identify the key influencing factor of each stage. The results indicated that under low flight Mach number conditions, the finite-rate reaction effects exerted a more pronounced influence on the combustion. The strut/wall multistage injection strategy was found to be an effective means of enhancing combustion, with the positive feedback between the strut flame and the wall flame being particularly influential. Additionally, apart from the mixing efficiency, the distribution of heat release length of each stage also emerged as a critical factor in determining combustion performance. By adjusting the wall injection positions, the better combustion performance and the bigger working stability of the combustor could be achieved. The study elucidated the regulatory strategy of wall injection positions on combustion performance, offering value insights for optimizing combustion performance under low Mach number conditions.

NOx Formation Mechanism and Emission Prediction in Turbulent Combustion: A Review

The field of nitric oxide (NOx) production combined with turbulent flow is a complex issue of combustion, especially for the different time scales of reactions and flow in numerical simulations. Around this, a series of approach methods, including the empirical formula approach, the computational fluid dynamics (CFD) approach coupling with an infinite rate chemical reaction, the chemical reaction networks (CRNs), and the CFD approach coupling with CRNs, were classified, and we discussed its advantages and applicability. The empirical-formula approach can provide an average range of NOx concentration, and this method can be involved only in special scenarios. However, its simplicity and feasibility still promote practical use, and it is still widely applied in engineering. Moreover, with the help of artificial intelligence, this method was improved in regard to its accuracy. The CFD approach could describe the flow field comprehensively. In compliance with considering NOx formation as finite-rate chemical reactions, the NOx concentration distribution via simulation cannot match well with experimental results due to the restriction caused by the simplification of the combustion reaction. Considering NOx formation as a finite-rate chemical reaction, the CRNs approach was involved in CFD simulation, and the CRNs approach could forecast the NOx concentration distribution in the flow field. This article mainly focuses on the simulation method of nitric oxide (NOx) production in different combustion conditions. This review could help readers understand the details of the NOx formation mechanism and NOx formation prediction approach.

Numerical study of hydrogen ratio effect on detonation initiation characteristics of aviation kerosene with hydrogen addition

Detonation initiation is one of the most important problems in the research of pulse detonation engine (PDE). It is difficult for aviation kerosene to meet the performance requirement of short-distance detonation initiation in PDE. Hydrogen can improve the detonability of fuels. Aviation kerosene with hydrogen addition provides a new method to shorten the detonation initiation distance in PDE. Therefore, hydrogen ratio effect on detonation initiation characteristics of hydrogen-doped aviation kerosene was studied by two-dimensional numerical simulation with the adoption of standard k-ε model, finite rate reaction model and simplified aviation kerosene/hydrogen/air reaction kinetics mechanism. The results indicated that hydrogen-doped aviation kerosene/air mixtures could not be successfully detonated with the minimum hydrogen ratio of 9.73 %, while the detonation was eventually initiated with higher hydrogen ratios, suggesting that the detonability of hydrogen-doped aviation kerosene was improved by increasing hydrogen ratio. The critical hydrogen ratio of 11 % was determined by further research. As the hydrogen ratio increased, the initiation time and distance of the plane detonation wave first increased slowly and then decreased rapidly, while the detonation velocity raised monotonically. The degree of spherical wave decoupling had an influence on the detonation initiation. Specifically, less spherical wave decoupling induced two Mach stems to propagate at the same time, which was conducive to accelerating the initiation of plane detonation.

A Combustion Kinetic Model Based on the Fast Chemistry Assumption: Application to Non-Sooty and Sooty Laminar Diffusion Flames

ABSTRACT In this article, a combustion kinetic model based on the fast chemistry (FC) assumption is proposed to compute the finite reaction rate of gaseous species in laminar diffusion flames. The algorithm developed for this purpose is numerically robust and relies on the adiabatic flame temperature to limit the fuel consumption rate. While there is no FC model specifically developed for laminar flames, the Eddy Dissipation Model (EDM) correction scheme for laminar regimes is taken as the reference model. The proposed model addresses the shortcomings of the reference model, i.e. the sensitivity to the transport time scale and the cell size and the need to calibrate a numerical constant repeatedly. The new model formulation does not rely on any numerical constant and completely removes the dependency on the cell size and the species transport time scale, as shown for multiple counterflow and axisymmetric laminar diffusion flames. Furthermore, the proposed model provides consistent temperature and species mass fraction predictions across the mentioned flames. Finally, a sooty axisymmetric flame is simulated using the mentioned models to investigate the numerical interaction between combustion and soot chemistry. The results show that the reference model requires recalibration to capture the temperature and soot volume fraction distributions while the proposed model yields reasonably accurate temperature and soot predictions.

Direct numerical simulations of high-enthalpy supersonic turbulent channel flows including finite-rate reactions

Direct numerical simulations of temporally evolving high-enthalpy supersonic turbulent channel flows are conducted at a Mach number of 3.0 and Reynolds number of 4880 under isothermal wall conditions. Air is assumed to behave as a five-species mixture, and chemical non-equilibrium and equilibrium assumptions are adopted to investigate the influence of finite-rate reactions on the turbulent statistics and large-scale structures. The two wall temperatures of 1733.2 and 3500 K are such that the mixture components undergo strong dissociation and recombination reactions along the channel. Investigation shows that the turbulent intensity is weakened and the mean and fluctuating temperatures are smaller when finite-rate reactions are considered. The mean dissociation degree is a quadratic function of the normal position, and its curvature under the chemical non-equilibrium assumption is greater than that under the chemical equilibrium assumption. The fluctuating mass fractions of the generated species seem to decrease slightly in the near-wall region, and their distributions are obviously different from those of the fluctuating velocity and fluctuating temperature. Finite-rate reactions increase the proportion of turbulent kinetic energy production in the skin friction decomposition, especially when the wall temperature is 3500 K. The large-scale structures visualized by the cross correlation between temperature and species mass fraction become stronger in the normal direction. The turbulent Schmidt number and several velocity–temperature correlations, including the recovery enthalpy and strong Reynolds analogy, are insensitive to the chemical reaction rate and wall temperature.

Electro-chemical effects on combustion chemistry in electric field fire suppression

In order to investigate the role played by electro-chemical effects in the combustion of flames under an external electric field, it is necessary to simulate the chain effect brought about by the increase of electron energy during the combustion reaction by modifying the electron energy according to the ethanol combustion mechanism containing ionic chemical reaction. Due to the large amount of computation involved in carrying out numerical simulations using detailed chemical reaction mechanisms, the mechanisms are simplified in this paper in order to save computational resources. Combining the simplified mechanism, a finite rate chemical reaction model is used to simulate the chemical reaction of ions during the transverse electric field fire extinguishing process, and the relationship between the concentration of intermediate components in ethanol air diffusion combustion and temperature is analyzed.

Study on the Inhibition Effect of C6F12O on Charged Particles in the Flame of Transformer Oil Discharge Gas

ABSTRACT The accidental combustion of insulating oil poses a significant risk in the event of transformer failure. The charged particles generated through the combustion ionization reaction exert considerable influence on the combustion speed and stability of the flame. Therefore, the detailed chemical reaction mechanism including neutral matter and charged particle is simplified. The two-dimensional finite rate reaction model is established. The theoretical calculation investigates the suppressive effect of the novel fire extinguishing agent C6F12O on charged particles in the flame resulting from transformer oil decomposition. The analysis encompasses changes in flame temperature, free radicals, and charged particles in the decomposition gas of transformer oil under C6F12O concentrations ranging from 0% to 4%. The findings reveal that the addition of C6F12O exerts a discernible inhibitory effect on the flame temperature of the transformer oil exhaust gas. The large reduction of charged particles in the flame caused by C6F12O is achieved by its consumption of large amounts of O and OH radicals in the flame. It hampers key elementary reactions responsible for charged particle generation. In conclusion, C6F12O demonstrates a pronounced inhibitory effect on charged particles in the flame resulting from transformer oil decomposition.

Computational study of a high-pressure hydrogen storage tank explosion at different heights from the ground

A computational study was carried out to investigate the explosion of a 35-MPa, 72.4-L high-pressure hydrogen storage tank at different heights from the ground. The numerical simulations were carried out using OpenFOAM computational fluid dynamics code. The numerical simulation incorporates a k−ω shear stress transport turbulence model and an eddy dissipation concept combustion model with a one-step finite rate reaction. The prediction performance of the numerical approach was validated based on experimental results obtained from previous studies. The hydrogen tank explosion was investigated at different heights (h; 0.0, 0.2, 0.5, 0.8, and 1.0 m) from the ground and at h = 0.2 m from the ground with θ = 65°). The numerical predictions showed that tank height and position significantly affect the blast overpressure strength and propagation. The reflected pressure magnitude increased as the tank height decreased with respect to ground level. At farther distances, the explosion scenario of h = 0.2 m and θ = 65° showed stronger maximum overpressure and impulse hazards than the other height settings. The difference in maximum overpressure magnitude between the different explosion scenarios was insignificant at 4 m from the tank. Hydrogen gas auto-ignites closer to the ground, due to the compression effect attributed to the Mach stem phenomenon. Our results showed that blast effects in the radial direction are more hazardous than those in the axial direction.

Laminar premixed hydrogen/air flame dynamics with/without sidewall interactions

This paper investigates laminar premixed hydrogen/air flame-wall interactions (FWI) subject to flame stretch (curvature and strain rate). Direct numerical simulations of finite-rate chemical reactions and in-depth molecular diffusion are used to numerically study two-dimensional (2D) V-shaped flames, with one side functioning as an isothermal cold wall that interacts with the flame and the other as a symmetrical center for free-propagating flame. An isometric grid with a grid resolution of 10 μm is used to resolve the FWI zone. It is discovered that the wall at the flame tip experiences increased negative stretching and enhanced local preferential diffusion which reduces the intensity of local combustion. This effect is especially noticeable at low equivalent ratios. Additionally, there is a close relationship between quenching behavior and flame dynamics. The present results show that the Markstein numbers for both the sidewall quenching flame and the freely propagating flame (MaSWQ and MaFF) are negative in lean hydrogen flames and positive in rich hydrogen flames. Cold wall has less impact on Ma. Furthermore, an intersection in the equivalence ratio is observed in the lean-fuel hydrogen/air flame, located at Φ = 0.8–1.0, with a change in the sign of Ma.

Development of a novel subgrid combustion model for buoyant turbulent diffusion flames

A novel approach is presented to incorporate finite rate reactions in modeling of turbulent diffusion flame, with the existence of unresolved flamelets inside the CFD grid cells taken into account. This approach, called SCM (subgrid combustion model), is applied in large eddy simulations of the UMD line burner flames. With the global kinetic parameters fitted for the measurement data and detailed simulations of autoignition in stoichiometric methane-air mixture, as well as for the extinction strain rate in the opposed non-premixed jets, the SCM model is shown to replicate the mean temperature profiles in unsuppressed flame with mean total heat release of 50 kW. For flame suppression via dilution of the oxidizer flow by nitrogen, the lift-off and extinction of non-anchored flame are predicted in agreement with the experimental data. The SCM is designed as a tool to allow for the finite rate kinetics effects in large eddy simulations of buoyant turbulent diffusion flames, with the focus on weakly turbulent regions, in which applicability of conventional combustion models is limited.

Numerical study on the effect of carbon particles on flow field characteristics of rotating detonation engine

Rotating detonation engines have high thermal efficiency and thus have been extensively studied. Using solid particles as fuel can reduce costs, and experiments have demonstrated the feasibility of gas-solid two-phase rotating detonation. However, numerical simulation research is required that examines the flow field characteristics of a rotating detonation engine. In order to explore the role of carbon particle fuel in a gas-solid two-phase rotating detonation wave, the weighted essentially non-oscillating scheme and third-order total variation diminishing Runge-Kutta scheme are used to solve the gas-solid unsteady Eulerian–Eulerian equations. Finite rate chemical and surface reaction models are used to simulate the combustion of gaseous substances and carbon particles. The influence of the proportion of carbon particles in the fuel and the diameter of the carbon particles on the rotating detonation flow field characteristics is analyzed. The results indicate that when the size of carbon particles is in the micrometer scale or below, the two-phase rotating detonation waves exhibit double-front or single-front detonation structures, respectively. Adding carbon particles to the fuel enables the rotating detonation engine to achieve a total pressure gain.

Scramjet Engine Flowpath That Improves Specific Impulse Using JP-7 Fuel

A flowpath geometry was computed that improves the specific impulse of a dual-mode scramjet engine in a generic X-51 hypersonic vehicle. Six parameters were varied: inlet contraction ratio, the diameters and numbers of fuel injectors, divergence angle of the combustor wall, nozzle flap deflection angle, and flight Mach number. The maximum specific impulse was 2296 s in the ram mode and 832 s in the scram mode. A reduced-order model simulates the finite-rate chemistry of JP-7 fuel and the unstart limits. Results show that both combustion efficiency and drop to unacceptably low levels when the finite-rate chemical reaction rates are weakened by flame strain-out due to the large air velocities, or when the flame becomes longer than the combustor. Small occurs when the following are too small: the inlet contraction ratio, the inlet compression ratio, the number of fuel injectors, and the diameter of fuel injectors. When these parameters are too large, excessive heat release causes unstart. The operating range was identified between these limits. For JP-7 fuel, it was found that the inlet should raise the pressure to above 5 atm. Results are explained by the interactions between reactions, mixing, and flame strain-out.

Improved Delayed Detached Eddy Simulation of Combustion of Hydrogen Jets in a High-Speed Confined Hot Air Cross Flow

The paper deals with the self-ignition and combustion of hydrogen jets in a high-speed transverse flow of hot vitiated air in a duct. The Improved Delayed Detached Eddy Simulation (IDDES) approach based on the Shear Stress Transport (SST) model is used, which in this paper is applied to a turbulent reacting flow with finite rate chemical reactions. An original Adaptive Implicit Scheme for unsteady simulations of turbulent flows with combustion, which was successfully used in IDDES simulation, is described. The simulation results are compared with the experimental database obtained at the LAERTE experimental workbench of the ONERA—The French Aerospace Laboratory. Comparison of IDDES with experimental results shows a strong sensitivity of the simulation results to the surface roughness and temperature of the duct walls. The results of IDDES modeling are in good agreement with experimental pressure distributions along the wall and with the results of videoregistration of the excited radical chemiluminescence.

Synergy of grain boundary and interface diffusion during intermediate compound formation

ABSTRACT The kinetics of reactive growth of a compound with a columnar structure with frozen bulk diffusion is revisited. The finite rate of redistribution and reaction along moving interfaces between reagents and growing compound is taken into account. Cases of planar and curved inter-phase interfaces are discussed. A new kinetic equation for the phase growth with evolving grain boundaries is suggested. Three cases are considered: (1) frozen grain structure, (2) normal lateral grain growth which does not depend on phase growth and on diffusion fluxes, (3) flux-driven grain growth which is induced by grain-boundary diffusion fluxes across the growing compound layer. The latter model provides the best fit to the experimental data.

Numerical simulation of combustion processes

In this paper, we propose a numerical method for solving the problem of the propagation and combustion of a methane jet in an axisymmetric satellite air flow. Within the framework of the modified turbulence model and the Arrhenius law, mathematical and numerical models of the problem of a turbulent axisymmetric methane jet in an infinite cocurrent air flow at a finite reaction rate have been developed. By introducing functions and generalized Schwab-Zel'dovich functions, as well as the stream function, ten differential equations for the conservation of substances are represented by differential equations equivalent to them. The equations of the turbulent boundary layer of a multicomponent gas for an axisymmetric jet are transformed with the transition to dimensionless variables and the introduction of a stream function. The dimensionless equations of the turbulent boundary layer of reacting gases in von Mises coordinates are used for modeling. For the numerical solution of the combustion problem according to the Arrhenius law, an implicit finite-difference scheme was used, which provides the second order of approximation accuracy in longitudinal and transverse coordinates. This made it possible to significantly reduce the calculation time as a result of using a large calculation step for the longitudinal coordinate. In connection with the nonlinearity of the equations of conservation and transfer of substances, an iterative process was organized. Some results of the computational experiment are presented. Comparison of the results of calculating the change in the temperature of the axial flow according to the turbulence models modified by k − ε and Prandtl with experimental data. The adequacy of the results was verified by the implementation of the laws of conservation of mass, momentum and total enthalpy, as well as by comparing the results with experimental data from other authors with the largest 5% deviation. This means that the previously presented algorithm and calculation program can be used for practical purposes. The results obtained with both turbulence models were compared with experimental data. Analyzing the results, one can notice that the k − ε model coincides more qualitatively with the experiment than the Prandtl turbulence model.

Study on the Influencing Factors of Modal Transformation of Continuous Rotating Detonation Wave Propagation

In order to study the propagation mode of rotating detonation wave, the detailed reaction mechanism of hydrogen-oxygen with 9 components and 19 steps is used, and the finite rate chemical reaction model is used for numerical calculation. After ignition, there is a stable collision period in the flow field for a certain period, and the collision extinguishing and new detonation wave generation occur during the mode conversion. By adding nitrogen or argon to the hydrogen-oxygen mixture for dilution, when the dilution ratio increases, the activity of the gas mixture decreases, the height of the combustible gas layer increases, and the temperature of the combustion product after the detonation wave decreases away from the head of the detonation wave, which will reduce the mixture that is burned in advance. There are different degrees of reflected shock at the outlet of the combustion chamber, and the upward detonation wave will cause the height change of the detonation wave head and the instability of the flow.

Finite Rate Reaction Mechanism Adapted for Modelling Pseudo-Equilibrium Pyrolysis of Cellulose

This manuscript is related to a formulation for modelling cellulose pyrolysis with a pseudo-equilibrium approach. The objective is to model the kinetics of the cellulose pyrolysis with a semi-global mechanism obtained from the literature in order to obtain the yield and the rate of formation, mainly that of char. The pseudo-equilibrium approach consists of the assumption that the solid phase devolatilisation can be described kinetically—at a finite rate—thus preserving the competitive characteristic between the production of char and tar, while the gas phase can be described directly by means of chemical equilibrium. The aforementioned approach gives a set of ordinary, linear, and nonlinear differential equations that are solved numerically with a consistent numerical scheme (i.e., the Totally Implicit Euler method). Chemical equilibrium was solved using CANTERA coupled with a code written in MATLAB. The results showed that the scheme preserved the tar-gas competitive characteristic for cellulose pyrolysis. The gas phase was defined as a mixture of CO2, CO, H2O, CH4, H2, and N2, showing a similar composition compared to models from the literature. Finally, the extension of the model to biomass in general is straightforward for including hemicellulose and lignin. The formulation is described in detail throughout the document in order to be replicated and evaluated for other biological components.

Pulverized coal combustion computational modeling approach: A review

CFD (Computational fluid dynamics) is a tool, through which, the entire combustion process for pulverized coal combustion (PCC) can be predicted. The solid fuel combustion modeling is not as simple as gaseous or liquid fuel combustion modeling. In the solid fuel combustion process, various physical transformations and chemical reactions occur simultaneously. Therefore in the computational modeling approach for PCC, basic physical and chemical processes such as; turbulence, radiation, devolatilization, gas-phase combustion, char reaction and soot formation/reduction are integrated to obtain the desired results. However, in order to reach the desired accuracy, different computational models should be chosen carefully. In this review, various common models that are widely used in PCC simulation are discussed scientifically along with their modeling aspects. The basic advantages and disadvantages of these models are also discussed in this study. Moreover, different models preferred by various authors for pulverized coal combustion simulation process are also summarized. From the review, it has been observed that the eddy viscosity based two-equation turbulent models and Eulerian-Lagrangian multiphase approach are mostly preferred due to better computational economy. Two competing rate devolatilization model from various finite rate models and FG-DVC devolatilization model from various phenomenological models are mostly used for PCC. Diffusion kinetic control and Intrinsic char combustion models are mostly chosen to predict the char reaction. Finite reaction rate models are preferred when the computational economy is a concern, whereas phenomenological models provide results with better accuracy because in phenomenological models realistic inputs are given as input for simulation whereas, in finite rate models, finite rate constants are mostly empirical. Most of the gas phase models are given almost equal importance. Discrete ordinate (DO) is mostly used for PCC due to its ability of volumetric radiation calculation and the ability to evaluate radiation effect on the particle phase.

Dynamics of premixed hydrogen/air flames in unsteady flow

Fully resolved numerical simulations with finite rate chemical reactions and detailed molecular diffusion have been conducted for a series of laminar premixed hydrogen–air flames under atmospheric condition. The objective of the work is to study the influence of unsteadiness of the flow on the local and global flame dynamics. Two equivalence ratios with Φ=0.5 and 4 are considered, leading to a negative and a positive Markstein number Ma0 at steady-state condition. The flames are excited with oscillating inflows at pre-defined frequencies f to assess the effect of unsteady flame stretch on flame dynamics. The Damköhler number, defined by the ratio of the inverse frequency of the oscillations and flame transit time, is used to characterize the interactions between the flow and the chemical reactions based on their time scales. For both lean and rich flame conditions, the local flame speed Sl is less sensitive to the flame stretch in an unsteady flow, which results in a reduced magnitude of the Markstein number |Ma¯|. In addition, |Ma¯| is smallest when the time scale of the flow approaches the intrinsic time scale of the flame (Da≈1). The global consumption speed St, computed from integration of the fuel burning rate over the whole computational domain, yields a phase delay and a damped oscillation with respect to the unsteady inflow. The phase delay increases with f or decreasing Da, whereas the reverse trend has been found for the oscillation amplitude of St. The flame is not able to follow the unsteady flow or adjust its flame surface at high excitation frequencies with Da <1, and vice versa in the low frequency range with Da≫1. An efficiency factor E has been introduced to model the damped response of the flame due to flow unsteadiness, which reproduces the asymptotic behavior of E→0 at Da≪1 and E→1 at Da≫1. The simulation results reveal that the fluctuation time scale plays a significant role in elucidating the effect of flame–flow interaction, which should be considered for turbulent combustion modeling.

Experimental and numerical characterization of hydrogen jet fires

Compressed hydrogen gas is considered the most convenient and robust technological solution for long-term storage. However, several safety concerns are still under investigation. This work presents an experimental and numerical characterization of the jet flame produced after the accidental release from a high-pressure tank containing pure hydrogen at pressures ranging from 90 to 450 bar and release diameters ranging from 1 to 5 mm. Results are expressed in terms of temperature history and flame length. The complete set of measurements has been reported in the supplementary materials. Both integral and discrete models were employed. Besides, the computational fluid dynamic was integrated with finite reaction rate and accurate thermodynamic properties (from the ab initio approach) and showed excellent agreement with experimental data.