Detailed Reaction Model Research Articles

Influence of inhomogeneous distribution of inflow equivalence ratio on oblique detonation morphology and characteristic

In this paper, numerical simulations using Euler equations coupled detailed chemical reaction model are performed to reveal the influence of inhomogeneous distribution of inflow equivalence ratio (ER) on the morphology and characteristic of oblique detonation in hydrogen/oxygen/argon mixtures. The purpose of this study is to better understand the key parameters’ variation law of oblique detonation flow field under practical flight conditions so as to guide the design of oblique detonation chamber. Within the scope of our simulations, the results show that the oblique detonation wave (ODW) can still be standing under a large ER gradient. The thermodynamic state and characteristic sizes of the flow field reach the maximum value around ER = 0.8. First, the ODW angle and the post-wave temperature/pressure increase with the homogeneous inflow ER. Then, the inhomogeneity of inflow ER is introduced by assuming a lateral linear distribution covering the whole inflow boundary. When the ER increases along the inflow boundary with ER = 0 at the wedge tip, the overall morphology of the ODW presents a concave structure. Inversely, the ODW is convex with ER = 0 at the top outlet. The morphology and characteristic sizes of ODW are determined by the mixture composition in front of the corresponding wave surface. The transition mode of ODW is mainly determined by the ER of the incoming flow in front of the induction region.

Flame Characterization of Cofiring Gaseous and Solid Fuels in Suspensions.

This work characterizes technical scale flames of suspension firing of gaseous and solid fuel mixtures through in-flame measurements with focus on nitrogen oxide (NOx) formation. The aims are to investigate the impacts of substituting a solid fuel with a gaseous fuel on the important mechanisms for NOx formation and to highlight important considerations for burner design. The investigation was performed in a 100 kW test unit that fires mixtures of propane and lignite. The global emissions levels and in-flame compositions were measured. A detailed reaction model was used to interpret the experimental results. The study highlights the importance of the early release of volatile nitrogen to reduce the levels of NOx. The findings indicate that substituting lignite by propane is advantageous in terms of reducing NO emissions, primarily due to the diminished input of fuel-bound nitrogen to the flame. However, this holds true only if the flame temperature of the gaseous fuel does not increase excessively. Finally, introducing a relatively small quantity of solid fuel to a propane flame appears to alter the flame behavior to resembles that of the "solid fuel," with a longer and wider flame. Despite this, carbon monoxide and nitrogen oxide concentrations remain like gas combustion but more dispersed.

Effect of composition gradient on detonation initiation in fuel-rich hydrogen-air mixtures in an obstructed channel

Numerical simulations were performed to study the effect of composition gradient and obstacle arrangement on detonation initiation in fuel-rich hydrogen-air mixtures. A third-order WENO method with adaptive mesh refinement (AMR) was used to solve the unsteady, fully-compressible, reactive Navier-Stokes equations coupled to a calibrated chemical-diffusive model (CDM). An obstructed channel at a blockage ratio of 0.3 filled with non-uniform hydrogen-air mixtures of an average H2 concentration 50 vol% was considered. The results show that unilateral obstacle arrangement is more conducive to flame acceleration (FA) and DDT than bilateral obstacle arrangement for both homogeneous and inhomogeneous mixtures. For inhomogeneous mixtures, the flame accelerates faster, and the detonation onset time is shorter when obstacles are placed on the sidewall with a hydrogen concentration closer to the stoichiometric ratio than the those when obstacles are placed on the other sidewall. However, the placement of unilateral obstacles on either the upper or lower sidewall does not significantly affect the DDT run-up distance. Besides, for fuel-rich inhomogeneous hydrogen-air mixtures, detonation tends to be initiated in the regions of high H2 concentration. Idealized models were introduced to simplify the intricate interactions of the reaction waves and flow fields to better understand the connection between H2 concentration distribution and detonation initiation. The analysis suggests that the minimum shock strength required for detonation initiation decreases with increasing equivalence ratio. This is supported by a kinetics analysis with detailed reaction model, which shows that ignition delay increases with decreasing equivalence ratio when detonation is about to be initiated.

On efficient modelling of radical production in cavitation assisted reactors

Process intensification by cavitation is gaining widespread attention due to the benefits that the intense bubble collapse conditions can provide, yet, several knowledge gaps exist in the modelling of such systems. This work studies the numerical prediction of single bubble dynamics and the various approaches that can be employed to estimate the changes in the chemical composition of cavitating bubbles. Specific emphasis is placed on the prediction of the radical production rates during bubble collapse and the computational performance, with the aim of coupling the single bubble dynamics to flow models for reactor hydrodynamics. The results reveal that the choice of chemical reaction approach has virtually no effect on the bubble dynamics, whereas the predicted radical production rates can differ substantially. It is found that evaluating the radical production only on temperature peaks, an approach commonly followed in literature, may result in the most erroneous estimations (on average 12.8 times larger than those of the full kinetic model), while a simplified kinetic model yields more accurate predictions (2.3 times larger) at the expense of increased computational times. Continuous evaluation of the bubble content by assuming equilibrium when the bubble temperature is above a certain threshold (≈1500K) is shown to be capable of predicting total radical production values close to those estimated by solving the kinetics of a detailed reaction model (19.8% difference), as well as requiring only 22.2% more computational costs compared to simulations without chemical reaction modelling. Such an equilibrium approach is therefore recommended for future studies aiming to couple flow simulations with single bubble dynamics to accurately predict radical production rates in cavitation devices, involving numerous bubbles following different flow trajectories. Furthermore, an algebraic expression that successfully approximates the full kinetic simulation results is proposed as a function of the initial nucleus size and the time integral of the liquid pressure when it is under vapor pressure. Such a model can be applied in modelling efforts that do not require local instantaneous radical concentrations, and paves the way for efficient closure modelling of radical production in CFD simulations of hydrodynamic reactors.

Development of a surrogate artificial neural network for microkinetic modeling: case study with methanol synthesis

Speeding up microkinetic modeling by a surrogate artificial neural network.

A NUMERICAL STUDY ON THE REDUCTION OF GREENHOUSE GASEOUS COMPONENT (CO2) DUE TO THE ADDITION OF H2 IN THE FUEL STREAM OF THE COUNTERFLOW CH4/AIR DIFFUSION FLAME

In this study, a series of 1-D and steady-state numerical simulations have been performed for the prediction of the effect of the addition of H2 on the characteristics of a non-sooting counterflow CH4/Air diffusion flame using detailed chemical reaction model, which is composed of 325 elementary chemical reactions and 53 chemical species. Under the steady-state assumption, a set of one-dimensional transport equations of mass, momentum, species, and energy along with the equation of state has been solved numerically at the atmospheric conditions over the counterflow configuration by exploiting an efficient numerical code, OPPDIF (a Fortran Program for Computing Opposed-Flow Diffusion Flames). The grid adaption technique has been used to achieve better convergence as well as to ensure the maximum accuracy of the simulated results. It is found that the flame temperature is increased due to the addition of H2 with CH4, which is injected into the fuel stream. The elevation in the temperature is caused by the augmentation of the integrated heat release rate of the elementary reactions supported by the active radicals (H, O, and OH), which are generated by the higher reactivity of H2. Besides, it is found that the mole fractions of H2O are increased as the percentage of H2 in the loading fuel (CH4) is increased and also, it is identified that the chain propagating reaction, OH + H2 => H2O + H is dominating one which produces highest amount of H2O. Furthermore, it is noticed that the indirect greenhouse gas or precursor, CO is reduced when H2 is added to CH4. Consequently, the mole fraction of the principle greenhouse gas, CO2 is decreased significantly when the fuel, CH4 percentage is modified by the higher percentage of H2. The sensitivity analysis of elementary reactions reveals the fact that the chemical reaction: OH + CO => H + CO2 is a dominating reaction in producing a lower amount of CO2 when the volume fraction of H2 is increased in the fuel (CH4) stream. In the presence of 75 % H2 in CH4, the pressure-dependent reaction, O + CO (+M) => CO2 (+M) appears as another chemical route that also generates greenhouse gas, CO2 but its contribution is negligibly small.

Large-eddy simulation of a lean-premixed hydrogen flame in a low-swirl combustor under combustion instability

Large-eddy simulation (LES) of a lean-premixed hydrogen turbulent jet flame with combustion instability (CI) in a low-swirl combustor (LSC) is performed by employing a dynamically thickened flame model with a detailed chemical reaction model with 9 chemical species and 20 reactions, and the LES validity and the CI characteristics are investigated in detail. The results show that the present LES can accurately reproduce the experimentally observed characteristics of the CI such as intensity, frequency, sporadic decay of pressure oscillations, and a flame–flow interaction inducing the periodic transitions of an inverted conical flame structure and a flat flame structure in the LSC. The sporadic decay of pressure oscillations and the flame–flow interaction are caused by the temporal decoupling of pressure and heat release rate and the periodic outward and inward deflections of the inflow, which is associated with the flow behavior in the upstream injector channel, respectively.

On the mechanism of initiation steam cracking of C6 hydrocarbons by hyperbranched poly(amidoamine) (PAMAM) initiator

Initiators with low bond dissociation energy can initiate the cracking of hydrocarbons, but the initiation effect on hydrocarbons with different molecule structures remains ambiguous. In this study, the steam cracking of three C6 hydrocarbons, namely n-hexane, iso-hexane and methyl-cyclopentane (MCP), initiated by hyperbranched poly(amidoamine) (PAMAM) were investigated experimentally in a stainless-steel tubular reactor, and the detailed kinetic reaction models and reaction process simulations were conducted by Reaction Mechanism Generator and Chemkin-pro software packages. The results confirm that the addition of PAMAM can significantly promote the cracking conversion and reduce the initial cracking temperature of hydrocarbons because the decomposition of PAMAM at relatively low temperature produces high concentration of active NH2 and CH3 radicals, which significantly increases the rate of hydrogen abstraction. Importantly, the promotion efficiency of the initiator can be highly dependent on the cracking behaviour of hydrocarbons. The initiator will show higher promotion efficiency for hydrocarbon that is more prone to undergo sustained hydrogen abstraction after initiation. Therefore, the promotion efficiency of PAMAM follows the order of n-hexane > iso-hexane > MCP. This study is helpful for a better understanding of the initiation mechanism and will guide the design and screening of effective initiators for naphtha with different compositions.

Determining the global activation energy of methane–air premixed flames

In this work, the determination of the apparent activation energy of a global chemical reaction mechanism of the methane–air flames is revisited. The one-step formulation allows to derive the theoretical background for the method to measure the activation energy within the burner stabilised flame setup. The validity of this approach is demonstrated by using the numerical simulations with the detailed reaction model and direct thin filament pyrometry measurements of the temperature distribution in flame. The combination of numerical and experimental approaches allows us to find the activation energy for various mixture compositions. The prediction of numerical simulations and measured values of the activation energy is found to be in good agreement with each other and the data known from the literature. It is demonstrated that two critical phenomena need to be taken into account to obtain a reliable estimate of the activation energy: the flame blow-off and the onset of the diffusive-thermal instabilities. The effect of these critical events on the accuracy of the measurements is discussed as well as prospects of further investigation.

Numerical investigation on the initiation of oblique detonation waves in stoichiometric methane–air mixtures

The study investigates the oblique detonation characteristics in a stoichiometric methane–air mixture employing reactive Euler equations coupled with a detailed chemical reaction model. The analysis evaluates the initiation variation for both the inflow Mach number and the wedge angle. The results show that an increase in the Mach number from M0 = 9 to M0 = 12 changes the oblique shock-to-detonation transition from an abrupt to a smooth type. Furthermore, the distinction between high and low Mach numbers is explored through a comparison of the induction length along the wave surface. Additionally, the transition is affected by changes in wedge angle elevation under high Mach numbers, while a large wedge angle generates a second detonation wave surface due to a strong reflection shock downstream under low Mach numbers. Finally, the initiation length is quantitatively compared across varying inflow Mach numbers and wedge angles, which can benefit future investigations of a methane-fueled oblique detonation engine.

New insight into the oxidation of propene at elevated pressure

Comprehensive experimental and kinetic studies of propene (C3H6) oxidation were conducted by using a jet-stirred reactor at 12.0 atm within 725–1020 K under both fuel-lean (Φ = 0.5) and fuel-rich (Φ = 3.0) conditions. Molecular species concentration profiles were obtained by using online gas chromatography and gas chromatography-mass spectrometry, including CO, CO2, CH4, C2H4, C2H6, A-C3H4, C3H8, 1,3-C4H6, 1-C4H8, nC4H10, CH3CHO, C2H3CHO, CH3COCH3, C3H6O1-2 CH3CH2CHO, and C6H6. Sixteen products and intermediates including light hydrocarbons, oxygenated and aromatic species were identified and quantified. Based on Aramco Mech 3.0 and previous works, the rate coefficients of some key elementary reactions were updated, and a detailed kinetic reaction model was developed with reasonable predictions involving 587 and 3037 reactions. Rate-of-production analysis indicates the four main pathways for high-pressure oxidation of C3H6, namely hydrogen atom abstraction reactions, OH addition reactions, H atom addition reactions, and the formation of C3H6O1-2. Sensitivity analysis reveals that H2O2(+M) <=> 2OH(+M) and the H-abstraction of C3H6 by OH radicals are the most promoting and inhibiting reactions for C3H6 consumption, respectively. The benzene formation during C3H6 oxidation was analyzed, mainly generated by the C1 + C5 channel and consumed by OH addition. It was found that increasing pressure can decrease the initial reaction temperature and increase the conversion of C3H6. The importance of addition and hydrogen abstraction reactions in the oxidation of C3H6 was identified by analyzing the pressure effect. To confirm its validity, the proposed kinetic model could be used to the previous literature data, including JSR oxidation at around 1 atm and ignition delay times for C3H6. These experimental and modeling efforts provide a basis for gaining a more comprehensive insight into the oxidation of propene.

Spherically expanding flame simulations using Cantera coupled to an unsteady Lagrangian formulation

A Lagrangian-based one-dimensional approach has been applied, using Cantera libraries, to study the dynamics of spherically expanding flames. Detailed reaction models have been employed to examine spherical flame propagation in hydrogen-air mixtures. In the first part, our approach was validated against laminar flame speed and Markstein number data from the literature. It was shown that the laminar flame speed was predicted within 5% on average but that significant discrepancies were observed for the Markstein number, especially for rich mixtures. This suggests that even application of detailed reaction models can lead to inaccurate prediction of the stability of rich flames, which can be detrimental to the prediction of flame acceleration and the deflagration to detonation transition process. In the second part, a detailed analysis of the thermo-chemical dynamics along the path of Lagrangian particles propagating in stretched flames was performed. For mixtures with negative Markstein lengths, it was found that at high stretch rates, the mixture entering the reaction-dominated period is less lean with respect to the initial mixture than at low stretch rates. This induces faster rates of chemical heat release and active radical production which results in higher flame propagation speed. Opposite effects were observed for mixtures with positive Markstein lengths, for which slower flame propagation was observed at high stretch rates compared to low stretch rates.

On the critical initiation of planar detonation in Noble-Abel and van der Waals gas

The critical decay rate theory (C.A. Eckett, J.J. Quirk, J.E. Shepherd, Journal of Fluid Mechanics 421 (2000) 147–183) on detonation initiation in unsteady flow was extended from ideal to Noble-Abel and van der Waals gas, in which the finite molecular volume effect (or repulsion force) and the inter-molecular attraction effect (or attraction force) were included. Considering the reactive Euler equations, the reaction zone temperature-gradient equation was established independently of the equation of state and wave geometry, and then solved asymptotically by assuming large activation energy, planar wave front and a one-step irreversible reaction model. A critical decay time was derived as the critical condition for detonation initiation. It was found that the inter-molecular attraction effect makes initiation more difficult by increasing the critical decay time, whereas the finite molecular volume effect promotes initiation. The real gas effects are enhanced when increasing the reduced activation energy and/or decreasing the heat capacity ratio of perfect gas. The applicability and generality of the asymptotic solutions were evaluated by comparison with quasi-unsteady simulation employing detailed reaction models for hydrogen-air and ethylene-air mixtures, and with the further simplified, fully analytical solutions based on strong-shock assumption. The critical conditions of the asymptotic solutions are qualitatively consistent with those obtained with the detailed mechanisms. The asymptotic solutions can also be well reproduced by the simple analytical solutions in most conditions. This study thus establishes a simple, yet reliable, method to evaluate the real gas effect on detonation initiation in high-pressure conditions.

Post-detonation fireball modeling: Validation of freeze out approximations

A numerical simulation study was performed to examine the post-detonation reaction processes produced by the detonation of a 12 mm diameter hemispherical pentaerythritol tetranitrate (PETN) explosive charge. The simulations used a finite rate detailed chemical reaction model consisting of 59 species and 368 reactions to capture post-detonation reaction processes including air dissociation from Mach 19+ shock waves that initially break out of the PETN charge, reactions within the detonation products during expansion, and afterburning when the detonation products mix with the shock heated air. The multi-species and thermodynamically complete Becker–Kistiakowsky–Wilson real-gas equation of state is used for the gaseous phase to allow for the mixing of reactive species. A recent simplified reactive burn model is used to propagate the detonation through the charge and allow for detailed post-detonation reaction processes. The computed blast, shock structures, and mole fractions of species within the detonation products agree well with experimental measurements. A comparison of the simulation results to equilibrium calculations indicates that the assumption of a local equilibrium is fairly accurate until the detonation products rapidly cool to temperatures in the range of 1500–1900 K by expansion waves. Below this range, the computed results show mole fractions that are nearly chemically frozen within the detonation products for a significant portion of expansion. These results are consistent with the freeze out approximation used in the blast modeling community.

Criticality for Oblique Detonation Waves Induced by a Finite Wedge in a Hydrogen–Air Mixture

Two-dimensional oblique detonation waves (ODWs) induced by finite wedges in a stoichiometric hydrogen–air mixture have been investigated numerically based on reactive Euler equations with a detailed chemical reaction model. The main zone affected by the expansion wave emanating from the turning point of a wedge is the flowfield downstream of the intersection point of the oblique shock wave (OSW) and the expansion wave. The ODW would be reduced to Chapman–Jouguet (CJ) detonation or decoupled combustion downstream. Three combustion regimes, detonation, decoupled combustion, and no ignition, were observed successively, as the wedge length decreases. It is found that the location of the intersection point is a key parameter for the detonation initiation. When the intersection point is located upstream of the ODW transition point, the expansion wave may quench ODW. Then, the critical wedge length is obtained by theoretical analysis of wave structures and the initiation criterion of ODWs for finite wedges is proposed. When the wedge length is greater than the critical wedge length, ODWs can be initiated. On the contrary, the initiation of ODWs do not occur. For wedge lengths small enough, no ignition occurs. Previously proposed criteria that use the induction length are also examined and compared with the present critical wedge length criterion in this study, and the latter is proven to achieve better results.

Thermal decomposition kinetics, reaction mechanism, and hazard evaluation of the hydroxylamine sulfate solution doping Fe3+

To study the stability of hydroxylamine sulfate (HAS) when mixed with metal ions (especially Fe3+), the effect of Fe3+ doping on the thermal decomposition of HAS was studied by differential scanning calorimeter. The results indicate that HAS decomposes at 365–376 K and average thermal effect of P1 and P2 are approximately 632.6 J/g and 66.9 J/g, respectively. Fe3+ increases thermal effect of P1 and P2, but it decreases the initial decomposition temperature (To) of P1, and delays the To of P2. The density functional theory calculations results reveal a detailed 8-step reaction model of HAS decomposition with the formation of NH3OH+ as the key step. Notably, Fe3+ affects the thermal decomposition process of HAS by forming a coordination compound Fe(NH2OH)63+ with NH2OH. In addition, model-free method prediction shows that Fe3+ reduces the kinetic parameters and the thermal hazard parameters.

Experimental and kinetic modeling studies of N-decane cracking initiated by nitromethane at supercritical pressure

The chemical heat sink of endothermic hydrocarbon fuel has been used to solve the thermal protection problem caused by the high temperature of the scramjet combustion chamber. The fuel is usually in a supercritical state in the cooling channel, so we studied the cracking of n-decane initiated by nitromethane under supercritical pressure in an electrically heated flow reactor. Based on the experimental results, the published low-pressure cracking model and the latest theoretical results, a detailed kinetic reaction model under supercritical conditions of n-decane cracking initiated by nitromethane, including 271 species and 1863 reactions, was established. Through the simulation of the experimental results and the experimental results in the literature, it was proven that this model had good simulation accuracy. The formation rate of n-decane was analyzed to obtain the consumption path of n-decane under supercritical pressure, especially the generation process of C3 above alkanes. The influence of nitromethane on n-decane consumption and product distribution and the cause of its range of action were also analyzed, which provided a theoretical and model basis for the engineering application of fuel.

The effects of unburned-gas temperature and pressure on the unstable behavior of cellular-flame fronts generated by intrinsic instability in hydrogen-air lean premixed flames under adiabatic and non-adiabatic conditions: numerical simulation based on the detailed chemical reaction model

ABSTRACT The effects of unburned-gas temperature and pressure on the unstable behavior of cellular-flame fronts in hydrogen-air lean premixed flames are elucidated by numerical simulations. The detailed chemical reaction mechanism with seventeen reversible reactions of eight reactive species and a diluent is adopted for hydrogen-air combustion and heat loss is taken into account. The burning velocity of a planar flame increases as the unburned-gas temperature increases, and it decreases as the unburned-gas pressure and heat loss increase. In the dispersion relation, the growth rate increases and the unstable range widens when the unburned-gas temperature increases. The former decreases and the later narrows when the heat loss becomes larger. The normalized growth rates become large under the non-adiabatic conditions. The normalized burning velocity of a cellular flame is large when the pressure is high and heat loss is large, and it decreases as the temperature increases at high pressure. This shows that the high unburned-gas temperature at high pressure weakens the instability in hydrogen-air lean premixed flames. With increasing the domain size, the burning velocity of a cellular flame increases, and the cellular-flame front becomes more unbalance. This is an aspect of the long-wavelength disturbance to the unstable behavior of cellular-flame front.

Development of a Coal Combustion Model Compliant with the CHEMKIN Format for Use in Co-Firing Analysis with Hydrogen and Low-Carbon Fuels

We developed a simplified coal combustion model that is used in combination with detailed gas reaction models. This model aimed to develop co-firing systems for decarbonized fuels such as coal and hydrogen. The reaction mechanism was described in the CHEMKIN format. We aimed to develop a coal combustion model that is easy to handle even for experts in gas combustion. Based on the experimental results, we modified the following reaction models. (1) Determining the rate constant of NOx reduction by char in the presence of oxygen. (2) Determining the gasification reaction rate constants, especially the rate constant of the steam gasification reaction, from staged combustion experiments. (3) Adoption of a chemical structural model of char based on elemental analysis results. Char was composed of carbon; C, nitrogen; N, and hydrocarbons; CnHm. As a result of the improvement, we achieved the NOx prediction error halved. Improving the calculation accuracy of the coal burnout was most important for improving the accuracy of the gas reactions. If there was an error in the calculated coal burnout due to an error in the gasification reaction, and so forth, the amount of C, H, O, and N species in the coal released into the gas phase would change. These changed the burning conditions, for example, stoichiometric ratio, in the gas phase. As a result, the concentration of radicals such as H, O, and OH changed, causing a decrease in NOx prediction accuracy. We obtained outline of the combustion performances when decarbonized fuel such as hydrogen and coal were co-combusted.

Analysis of the effect of pyrolytic coking on the flow and heat transfer performance of n-decane in cooling channels at supercritical pressure

• Coking deposition of n -decane on the inner surface of the cooling channel was simulated. • This model is capable of making reasonably accurate predictions in pyrolytic coking. • The effects of pyrolytic coking on flow, heat transfer, and pyrolysis performance were investigated. The formation and deposition of coke in cooling channels is the main factor that limits the application of regenerative cooling. To study the effects of coking deposition on regenerative cooling performance under heavy-cracking conditions, we established a two-dimensional axisymmetric numerical model based on the dynamic mesh model. A detailed pyrolytic chemical reaction model, which contains 16 species and 26 step reactions, and a modified coking deposition kinetic model are incorporated in the numerical model. The numerical model is validated by comparing the temperature, mass fraction, and coking thickness between simulation and experimental results. The transient numerical simulation of n -decane flow, heat transfer, pyrolysis, and pyrolytic coking within 20min was carried out. The results reveal that coking deposition has a significant effect on the flow and heat transfer performance of the cooling channel, while having minimal effect on the pyrolytic performance. The conductive heat transfer resistance will be greater than the convection when the coking layer thickness is more than 110 µm. The research methodology adopted in this study can be used to estimate and manage coke deposition in the regenerative cooling systems, and the quantitative results obtained can provide guidance for the design of cooling channels under heavy-cracking conditions.