Hydrogen Combustion Mechanism Research Articles

Parametric Study of Flow and Combustion Characteristic in a Cavitied Scramjet with Multi-Position Injection

This study focuses on the three-dimensional flow and combustion characteristics of a cavitied scramjet engine with multi-position injection. A single-equation large eddy simulation (LES) turbulence model is employed, with a detailed reaction mechanism for hydrogen combustion, as described by Jachimowski. The combustion characteristics of hydrogen in the scramjet combustion chamber are analyzed. Based on the combustion chamber model, the influence of different equivalence ratios, injection timing, injection positions, and injection pressures on the flame formation and propagation process are compared. The results indicate that within a certain range, an increase in the equivalence ratio enhances the combustion intensity and chamber pressure. In the case of multi-position injection, the order of injection from different nozzles has little effect on the final flame stabilization mode and pressure distribution. The opposite-side distribution of nozzles can effectively improve the fuel efficiency and the internal pressure. Furthermore, when the nozzles are closely placed in the opposite-side distribution, the combustion efficiency increases, although this leads to a higher total pressure loss. In scenarios where the fuel injection duration is short, an increase in the injection pressure at the upstream nozzles of the cavity results in a higher local equivalence ratio, as well as reduced fuel mixing and ignition time.

Comparative analysis and optimisation of hydrogen combustion mechanism for laminar burning velocity calculation in combustion engine modelling

Hydrogen stands out as a compelling alternative to fossil fuels for combustion engines. Predictive combustion models are instrumental in developing hydrogen-fuelled engines. A fundamental metric of these models is the laminar burning velocity (LBV), which can be precisely determined through laminar flame propagation simulations. In this context, the selection of an appropriate combustion mechanism is critical. This paper aims to propose the appropriate combustion mechanism for calculating LBV in predictive combustion models of hydrogen-fuelled engines. 15 state-of-the-art combustion mechanisms were applied to reproduce the LBV measurements in engine-like conditions, especially considering the application of lean combustion and water injection. The FFCM 1.0 mechanism was identified from them and further optimised to improve its prediction accuracy at elevated pressures for the lean mixture. The maximum deviation of LBV was reduced from 17.6 % to 8.7 % by this optimisation, in comparison to 10.5 % for its closest competitor mechanism, ELTE (Varga et al., 2015).

Investigating the flow characteristics and thermodynamic performance of curved detonation waves

Oblique detonation waves (ODWs) are induced by oblique shock waves (OSWs) and hold significant research value for air-breathing ramjet engines due to their high thermodynamic efficiency and specific impulse. In this research, detonation is induced using a curved shock wave (CSW) that provides greater flexibility compared to an OSW. Curved detonation waves (CDWs) are classified into two categories based on the curvature of the CSW: positive-curvature curved detonation waves (PC-CDWs) and negative-curvature curved detonation waves (NC-CDWs). We conduct a numerical investigation of the flow characteristics and potential thermodynamic performance in curved detonation waves (CDWs) by solving the two-dimensional multi-species Euler equations with a detailed hydrogen combustion mechanism. In contrast to ODWs, we observe continuous compression and expansion effects in the flow fields of PC-CDWs and NC-CDWs, respectively. Continuous compression shortens the initiation length of the detonation wave and intensifies the compression wave preceding the combustion front, whereas continuous expansion produces the opposite outcome. Changes in the curvature of the CSW result in alterations to the flow characteristics of the CDW. These flow characteristic changes are manifested in their thermodynamic and propulsion-related performance. More specifically, a lower overdrive of the detonation wave corresponds to a reduced loss in total pressure.

Numerical Investigation of Combustion Mechanism with Multi-Position Injection in a Dual-Mode Combustor

To improve the flame propagation, combustion stability, and uniformity of the temperature field, multi-position injection is applied in a dual-mode combustor by controlling heat release in different locations. Using the chemical reaction of the finite rate combustion model and the detailed reaction mechanism of hydrogen combustion as described by Jachimowski, the influence of different multi-position injection patterns in a dual-mode combustor is analyzed. The one-equation Large Eddy Simulation (LES) turbulence model was chosen to define the sublattice turbulent viscous terms in a three-dimensional scramjet model. Based on a combustion chamber, the effect of the injection equivalent ratio (0.35–0.70), the relative position of the nozzle holes, and the injection pressure on the combustion process and flow field characteristics are analyzed with multi-position injection. The combustion efficiency, total pressure recovery coefficients, and pressure distribution under different operation conditions are compared. We observed that the combustion intensity increases and the upstream combustion shock string distance becomes greater with increased equivalent ratios. When the global equivalent ratio of multi-position injection remains unchanged, the arrangement of nozzles with the small injection spacing, i.e., two injection holes arranged face to face on the upper and lower walls, or the setting of multiple injection holes with the same pressure, can effectively increase the stability rate of the combustion flow field. In addition, the combustion efficiency at the outlet and the internal pressure of the combustion chamber in the stable state are also improved, relative to the increased total pressure loss.

Numerical study on the influence mechanism of different types of burst disc on high pressure hydrogen spontaneous combustion in tube

High-pressure transportation and storage are the current main methods for storing hydrogen. Burst discs, which are common in high-pressure storage devices, can cause hydrogen leaks to ignite and create serious accidents when certain conditions are met. To investigate this, this paper studies the effect of different blasting slot shapes on the spontaneous combustion of leaked high-pressure hydrogen. Numerical simulations, using LES, EDC model, Ten-Step Burst Disc Opening Method and 19-Step Detailed Hydrogen Combustion Mechanism, were conducted to analyze the spontaneous combustion of hydrogen with annular grooves, star grooves, cross grooves and grooveless burst discs. The results of the study show that the shape of the groove in the bursting disc will significantly affect the shock wave structure and spontaneous combustion time of leaked high pressure hydrogen in the tube. Among the four types of bursting discs, the order of spontaneous combustion in the tube is annular groove (approximately 90 μs), star groove (approximately 110 μs), cross groove (approximately 120 μs), and grooveless bursting disc (approximately 130 μs). Secondly, the double spherical shock waves produced by annular groove bursting disc collide with each other, resulting in a higher rising speed of pressure and temperature in the tube than with other bursting discs. Finally, the spontaneous combustion time of the leaked hydrogen is proportional to the open area of the bursting disc. When the opening area is the same, the smaller the exit shape coefficient is, the shorter the spontaneous combustion time is. The research results can provide some reference for the design and application of bursting disc in high pressure hydrogen storage equipment.

Numerical simulation of the effect of multiple obstacles inside the tube on the spontaneous ignition of high-pressure hydrogen release

Self-ignition may occur during hydrogen storage and transportation if high-pressure hydrogen is suddenly released into the downstream pipelines, and the presence of obstacles inside the pipeline may affect the ignition mechanism of high-pressure hydrogen. In this work, the effects of multiple obstacles inside the tube on the shock wave propagation and self-ignition during high-pressure hydrogen release are investigated by numerical simulation. The RNG k-ε turbulence model, EDC combustion model, and 19-step detailed hydrogen combustion mechanism are employed. After verifying the reliability of the model with experimental data, the self-ignition process of high-pressure hydrogen release into tubes with obstacles with different locations, spacings, shapes, and blockage ratios is numerically investigated. The results show that obstacles with different locations, spacings, shapes and blockage ratios will generate reflected shock waves with different sizes and propagation trends. The closer the location of obstacles to the burst disk, the smaller the spacing, and the larger the blockage ratio will cause the greater the pressure of the reflected shock wave it produces. Compared with the tubes with rectangular-shaped, semi-circular-shaped and triangular-shaped obstacles, self-ignition is preferred to occur in tube with triangular-shaped obstacles.

Mechanisms of the destabilized Mach reflection of inviscid oblique detonation waves before an expansion corner

The stabilization of oblique detonation waves (ODWs) in an engine combustor is important for the successful applications of oblique detonation engines, and comprehensively understanding the effects of the inviscid reflection of ODWs on their stabilization and the relevant mechanisms is imperative to overall combustor design. In this study, the flow fields of ODW reflections in a space-confined combustor are numerically studied by solving the two-dimensional time-dependent multispecies Euler equations in combination with a detailed hydrogen combustion mechanism. The inviscid Mach reflections of ODWs before an expansion corner are emphasized with different flight Mach numbers, Ma, and different dimensionless reflection locations, ζ ≥ 0 (ζ = 0: the ODW reflects precisely at the expansion corner; ζ > 0: the ODW reflects off the wall before the expansion corner). Two kinds of destabilization phenomena of the inviscid Mach reflection of an ODW induced by different mechanisms are found, namely wave-induced destabilization at large ζ > 0 for moderate (not very low) Ma and inherent destabilization at any ζ > 0 for low Ma. Wave-induced destabilization is attributed to the incompatibility between the pressure ratio across the Mach stem and its relative propagation speed, which is triggered by the action of the secondary reflected shock wave or the transmitted Mach stem on the subsonic zone behind the Mach stem. Inherent destabilization is demonstrated through an in-depth theoretical analysis and is attributed to geometric choking of the flow behind the Mach stem.

Mechanism of spontaneous ignition of high-pressure hydrogen during its release through a tube with local contraction: A numerical study

The tendency of spontaneous ignition of high-pressure hydrogen during its sudden release into a tube is one of the main threats to the safe application of hydrogen energy. A series of investigations have shown that the tube structure is a key factor affecting the spontaneous ignition of high-pressure hydrogen. In this paper, a numerical study is conducted to reveal the mechanism of spontaneous ignition of high-pressure hydrogen inside the tube with local contraction. Large Eddy Simulation, Renormalization Group, Eddy Dissipation Concept, 37-step detailed hydrogen combustion mechanism and 10-step like opening process of burst disk are employed. Three cases with burst pressures of 3.10, 4.90, and 8.45 MPa are simulated to compare against the pervious experimental study. The spontaneous conditions and positions agree well with the experimental results. The numerical results indicate that shock wave reflection takes place at the upstream vertical wall of contraction part. The interacted-shock-affected region is generated at the tube center because of the subsequent shock wave interaction. The forward reflected shock wave couples with normal shock wave and increases the pressure of leading shock wave. The sudden contraction of tube blocks the propagation of hydrogen jet and decreases the speed from supersonic flow to subsonic flow. More flammable mixture is generated inside the contraction part, as a results, the length of the flame is increased. Two mechanisms are proposed finally.

Numerical study on the spontaneous ignition of pressurized hydrogen during its sudden release into the tube with varying lengths and diameters

Fuel cell vehicles (FCV) and other hydrogen systems with pressurized hydrogen has a safety hazard of spontaneous ignition during its sudden release into the tube. Tube parameter is a key factor affecting the spontaneous ignition of pressurized hydrogen. In this paper, a numerical study on the spontaneous ignition of pressurized hydrogen during its sudden release into the tube with varying lengths and diameters is conducted. The models of Large Eddy Simulation (LES), Eddy Dissipation Concept (EDC), Renormalization Group (RNG), 10-step like opening process of burst disk and 18-step detailed hydrogen combustion mechanism are employed. 6 cases are simulated based on the previous experiments. Numerical results show that the possibility of spontaneous ignition of pressurized hydrogen increases inside the longer and thinner tubes, which agrees with the experimental results. The increasing of tube length has little influence on the shock wave formation and propagation inside the tube. However, there exists critical tube lengths for the generation of Mach disk and the normal shock wave: the maximum and minimum distances for the generation of the Mach disk in 10 mm diameter tube are 7.8 and 6.7 mm, respectively. As for the normal shock wave, these critical values are 22.1 and 19.4 mm, respectively. In addition, the formation times and initial positions of Mach disk and normal shock wave are delayed inside the thicker tube. Due to the shock-affected time increases with the increasing of tube length, the temperature could rise to the critical ignition temperature and triggers the spontaneous ignition due to the sufficient tube length even though the less hydrogen/air mixture and the contact surface with lower temperature is produced inside the thicker tube. Finally, a simple time scale analysis is conducted.

Formation of stabilized oblique detonation waves in a combustor

Initiation and stabilization of oblique detonation waves (ODWs) are important to the successful application of oblique detonation engines (ODEs), which, however, have been rarely studied under realistic combustor conditions. In this study, the flow structures, stabilization characteristics and potential thrust performance (under different combustor's geometries with different ODW reflection locations) in a typical hydrogen-fueled ODE combustor are numerically studied by solving the two-dimensional multi-species Reynolds-averaged conservation equations with a detailed hydrogen combustion mechanism. Results suggest that all the detonation waves/shock waves can be stabilized in the space-confined combustor, and the boundary layer separation induced by the ODW-boundary layer interaction is found crucial to determining the types of combustion mode in the combustor. Except for the expected ODW-induced combustion, fast combustion induced by a stabilized overdriven normal detonation wave (NDW) may exist in the combustor simultaneously (even up to a large extent, >73.7%). It is demonstrated that the stabilization of the overdriven NDW in the combustor can be attributed to the formation of an effective aerodynamic convergent-divergent nozzle that quickly accelerates the subsonic flow behind the NDW to supersonic, preventing downstream disturbances from propagating upstream. Benefiting from the chemical equilibrium shift caused by the expansion effect of the flow, more heat is released to compensate for the compression loss and the simulated thrust performance is shown not deteriorate significantly even with a large percentage of NDW-induced combustion existing in the ODE combustor. This work would be beneficial to the future developments of the ODEs.

Numerical study on the mechanism of spontaneous ignition of high-pressure hydrogen in the L-shaped tube

There have been reports of ignition of high-pressure hydrogen during its sudden release for unexplained reasons and ignition mechanism still need to be further investigated. In this paper, mechanism of spontaneous ignition of high-pressure hydrogen during its sudden release into the L-shaped tube is investigated. LES, EDC model, 10-step like opening process of burst disk and 18-step detailed hydrogen combustion mechanism are employed. Three cases with burst pressures of 2.16, 6.21, and 9.10 MPa are simulated. It is found that shock wave is strongly reflected after it hits the tube corner wall, forming a reflected-shock-affected region and an energy conversion region with higher temperature, greater pressure and lower velocity. Afterwards, the reflected shock wave moving forward is reflected several times by the tube wall until it disappears and oblique shock is generated. After the hydrogen/air mixture enters the corner, it extends downstream along inner wall and separated from the main hydrogen/air mixture. The reflected shock wave moving backward interacts with the expansion waves and increases the temperature and pressure again, but spontaneous ignition cannot be initiated. Three mechanisms of spontaneous ignition of high-pressure hydrogen in the L-shaped tube are proposed eventually. The results reproduce the experimental spontaneous ignition conditions and positions, indicating that the numerical models can be applied as a tool for hydrogen safety engineering.

Evaluation of different models for turbulent combustion of hydrogen-air mixtures. Large Eddy Simulation of a LOVA sequence with hydrogen deflagration in ITER Vacuum Vessel

• LES with TFM model was validated vs H2 experiments. • Validated model was used to simulate ITER VV LOVA sequence with H2 combustion. • In LOVA sequence burning speed increased 1–3.5 times due to flame acceleration. • Maximum average VV pressure was 4–5.5 times to the initial one. The modelling of premixed combustion in the presence of a turbulent field in three-dimensional (3-D) confined scenarios was studied in this work, and applied to hydrogen combustion within ITER vacuum vessel (VV). Two different combustion approaches were tested with Large Eddy Simulation: a Flamelet Progress Variable (LES-FPV) and a Thickened Flame Model (LES-TFM). For the case of LES-TFM modelling, Dynamic Adaptive Chemistry (DAC) with a detailed kinetic mechanism for hydrogen combustion and in-situ adaptive tabulation (ISAT) methods were employed. Moreover, an adaptive meshing technique was used with the aim of tracking the flame front to ensure an adequate spatial resolution in this region. Experimental validation was performed to assess the ability of the different studied approaches to predict the flame burning speed, flame acceleration, and pressure evolution for lean H 2 -Air volume percent mixtures from 16 to 28 % propagating within a turbulent field. Results revealed that both approaches led to accurate predictions in terms of flame burning speed. When considering DAC and ISAT methods with detailed chemistry, LES-TFM model was found to be a cost-efficient solution. This model was used to analyse two loss of vacuum accident (LOVA) sequences within ITER VV. Results showed that turbulence might increase the flame burning speed by a factor of up to 3.5 for the case of big breaches (0.15 m 2 ) but it would not affect in case of breaches of 0.02 m 2 . Besides, results showed that autoignition with 2 kg of H 2 within the VV at 13.35 kPa might degenerate in detonation with average wall pressure levels ∼70 kPa.

Linking reaction mechanisms and quantum chemistry: An ontological approach

In this paper, a linked-data framework for connecting species in chemical kinetic reaction mechanisms with quantum calculations is presented. A mechanism can be constructed from thermodynamic, reaction rate, and transport data that has been obtained either experimentally, computationally, or by a combination of both. This process in practice requires multiple sources of data, which raises, inter alia, species naming and data inconsistency issues. A linked data-centric knowledge-graph approach is taken in this work to address these challenges. In order to implement this approach, two existing ontologies, namely OntoKin, for representing chemical kinetic reaction mechanisms, and OntoCompChem, for representing quantum chemistry calculations, are extended. In addition, a new ontology, which we call OntoSpecies, is developed for uniquely representing chemical species. The framework also includes agents to populate and link knowledge-bases created through the instantiation of these ontologies. In addition, the developed knowledge-graph and agents naturally form a part of the J-Park Simulator (JPS) – an Industry 4.0 platform which combines linked data and an eco-system of autonomous agents for cross-domain applications. The functionality of the framework is demonstrated via a use-case based on a hydrogen combustion mechanism.

Effects of hydrogen addition on the combustion characteristics of diesel fuel jets under ultra-high injection pressures

Many applications use hydrogen addition and high-pressure fuel injection technology to improve combustion performance. In this study, spray atomization and combustion characteristics of a diesel fuel jet, under the injection pressure of 350 MPa, injecting into a constant volume combustion vessel filled with air-hydrogen mixture at the diesel engine relevant condition are investigated by simulation method. A simplified mechanism of the n-heptane (C7H16) oxidation chemistry mechanism consisting of 26 reactions and 25 species integrated with the Kéromnès-2013 hydrogen combustion mechanism and EDC combustion model are utilized to predict the diesel fuel spray auto-ignition and combustion. The ambient gas is the mixture of air and hydrogen range in volume fraction from 0% to 10%. The ambient temperature and pressure is set to 1000 K and 3.5 MPa, respectively. The results indicate that as the hydrogen volume fraction is 2%, the minimum overall droplet SMD (Sauter Mean Diameter) is approximately 0.95 μm, which is obviously smaller than that of the case with the conventional high injection pressure. In cases that H2 v/v% larger than 4%, the maximum gaseous temperature increased significantly up to 2700 K. There are two peaks in the temperature growth rate curves as the hydrogen fraction of 8% and 10%. The high temperature at the outer edge of the spray is clearly seen due to its high value when the hydrogen fraction is larger than 4%. The hot reaction layer is the main location of CO formation. The H, OH radicals are formed at the edge of the spray where the temperature is high. The hydrogen species obviously promotes the oxidation and combustion of diesel fuel.

A one-step reduced mechanism for near-limit hydrogen combustion with general stoichiometry

A one-step reduced mechanism for hydrogen combustion, previously developed for fuel-lean flames, is extended to apply for all equivalence ratios under near-limit conditions by taking into consideration two additional recombination steps that are important under fuel-rich conditions. It is found that the crossover temperature that appears in the cutoff factor is smaller under fuel-rich conditions. Besides improving insights, the results can be beneficial in speeding computations.

Nvestigation of Single-Step Reaction Mechanism for Hydrogen Combustion in Severe Accidents of Nuclear Power Plants

Changing the pre-exponential factor and the activation energy coefficient, the single-step reaction mechanism for hydrogen combustion was established, and the calculation and analysis of hydrogen combustion have been conducted by this mechanism. The results from the calculation and the experiment were compared, and the calculations of hydrogen combustion were processed in different hydrogen concentration by the single-step reaction mechanism. The results showed that the calculations and experimental flame propagation were conformable, and the modified single-step reaction mechanism for hydrogen combustion can be used to calculate and analyze the hydrogen combustion in nuclear power plants.

Yet another kinetic mechanism for hydrogen combustion

Recent suggestion by Burke and Klippenstein (2017) that chemically termolecular reactions H + O2 + R may significantly affect kinetic pathways under common combustion situations requires careful analysis, since, if included in contemporary kinetic mechanisms, these reactions affect global reactivity and calculated burning velocities of laminar premixed flames. In the view of their impact, a detailed kinetic scheme for hydrogen combustion was revisited to elucidate how to counterbalance enhanced chain termination caused by chemically termolecular reactions in attempt to keep or improve model performance. First, recent experimental and theoretical kinetic studies of hydrogen reactions were analyzed. In the new mechanism four reactions were introduced and three rate constants were updated. These changes, however, significantly reduce calculated burning velocities of H2 + air flames as compared to experimental data and earlier model predictions with the major impact from chemically termolecular reactions. It was then found that implementation of the new theoretical transport database developed by Jasper et al. (2014) significantly improves the performance of the updated kinetic model. The new kinetic mechanism for hydrogen combustion which includes updated kinetics and new transport properties was found in good agreement with the consistent dataset of the burning velocity measurements for hydrogen flames obtained using the heat flux method at atmospheric pressure for which the behavior of the previous model of the author was not satisfactory.

Application of thermodynamic analysis in reducing detailed hydrogen combustion mechanism

Application of thermodynamic analysis in reducing detailed hydrogen combustion mechanism

Mechanisms performance and pressure dependence of hydrogen/air burner-stabilized flames

The kinetic mechanism of hydrogen combustion is the most investigated combustion system. This is due to extreme importance of the mechanism for combustion processes, i.e. it is present as a sub-mechanism in all mechanisms for hydrocarbon combustion systems. Therefore, detailed aspects of hydrogen flames are still under active investigations, e.g. under elevated pressure, under conditions of different heat losses intensities and local equivalence ratios etc. For this purpose, the burner stabilized flame configuration is an efficient tool to study different aspects of chemical kinetics by varying the stand-off distance, pressure, temperature of the burner and mixture compositions. In the present work, a flat porous plug burner flame configuration is revisited. A hydrogen/air combustion system is considered with detailed molecular transport including thermo-diffusion and with 8 different chemical reaction mechanisms. Detailed numerical investigations are performed to single out the role of chemical kinetics on the loss of stability and on the dynamics of the flame oscillations. As a main outcome, it was found/demonstrated that the results of critical values, e.g. critical mass flow rate, weighted frequency of oscillations and blow-off velocity, with increasing the pressure scatter almost randomly. Thus, these parameters can be considered as independent and can be used to improve and to validate the mechanisms of chemical kinetics for the unsteady dynamics.

Uncertainty quantification of a newly optimized methanol and formaldehyde combustion mechanism

A detailed reaction mechanism for methanol combustion that is capable of describing ignition, flame propagation and species concentration profiles with high accuracy has been developed. Starting from a modified version of the methanol combustion mechanism of Li et al. (2007) and adopting the H2/CO base chemistry from the joint optimized hydrogen and syngas combustion mechanism of Varga et al. (2016), an optimization of 57 Arrhenius parameters of 17 important elementary reactions was performed, using several thousand indirect measurement data points, as well as direct and theoretical determinations of reaction rate coefficients as optimization targets. The final optimized mechanism was compared to 18 reaction mechanisms published in recent years, with respect to their accuracy in reproducing the available indirect experimental data for methanol and formaldehyde combustion. The utilized indirect measurement data, in total 24,900 data points in 265 datasets, include measurements of ignition delay times, laminar burning velocities and species profiles captured using a variety of experimental techniques. In addition to new best fit values for all rate parameters, the covariance matrix of the optimized parameters, which provides a quantitative description of the temperature-dependent ranges of uncertainty for the optimized rate coefficients, was calculated. These posterior uncertainty limits are much narrower than the prior limits in the temperature range for which experimental data are available. The uncertainty of the self-reaction of HȮ2 radicals and important H-atom abstraction reactions from the methanol molecule are discussed in detail.