Hydrogen Flame Research Articles

Dynamical Systems Characterisation and Reduced Order Modelling of Thermoacoustics in a Lean Direct Injection (LDI) Hydrogen Combustor

Abstract In this study, the thermoacoustic instabilities of partially premixed hydrogen flames in a lean direct injection (LDI) multi-cluster combustor are characterised using dynamical systems theory. The combustor is operated at a range of bulk velocities (30 - 90 m/s) and equivalence ratios(0.2-0.6), and time-resolved pressure oscillations and integrated OH* chemiluminescence measurements were taken. The thermoacoustic system reveals a variety of dynamical states in pressure such as period-1 Limit Cycle Oscillation (LCO) with a single characteristic frequency, period-2 LCO with two characteristic frequencies, intermittent, quasi-periodic and chaotic states as either bulk velocity or equivalence ratio is varied. At a bulk velocity of 30 m/s, as the equivalence ratio is gradually decreased from 0.6 to 0.2, the dynamical behaviour follows a sequence from an intermittent state to a period-1 LCO, then to a quasi-periodic state, and eventually reaches a chaotic state. As the equivalence ratio is decreased for a bulk velocity of 60 m/s, the pressure oscillations evolve from a period-2 LCO to quasi-periodic state before flame blows off. The emergence of period-2 and quasi-periodic states indicates the presence of strong non-linear interactions among the cavity acoustic modes. The emergence of period-2 and quasi-periodic states indicates strong non-linear interactions among the cavity acoustic modes. These modes and their spatial behaviour are investigated using a Helmholtz solver, showing that hydrogen flames can excite a wide range of cavity acoustic modes

Effect of momentum flow ratio on entrainment of a confined coaxial jet in a co-flow

Iron ore pellet production processes produce large amounts of CO2 emissions when burning fossil fuels. One example is the grate-kiln process, where normally a coal flame is used to indurate pellets. To mitigate the emissions, coal used in rotary kilns can be replaced with for example hydrogen. However, the fuel is mixed with secondary process air, and replacing a solid fuel with a gaseous one changes the mixing characteristics. This demands different means of injecting the fuel. A coaxial jet can be used to control the mixing of fuel and secondary air, as well as the flow field. The aim is to control the mixing of hydrogen and secondary air to achieve a hydrogen flame that is similar to the reference coal flame. This study numerically investigates different coaxial jet configurations. Steady-state simulations of a simplified model of the real kiln are performed using a Reynolds Stress turbulence model. Results show that decreasing the momentum flow ratio between the outer and inner jet, M jet , to a certain value delays the spread of the hydrogen jet and thus gives a longer jet. Further decreasing this ratio gives an even longer jet, but has the side effect of producing recirculation of hydrogen.

Comparative Analysis Of Cycle-To-Cycle Variabilities And Combustion Development In An Optical Spark-Ignition Engine Fueled By Pure Hydrogen And Propane: Insights From Chemiluminescence and PI

Hydrogen, derived from renewable sources and devoid of carbon emissions, is a pivotal energy carrier for the future. Representing a viable substitute for fossil fuels in internal combustion engines (ICEs), contemporary studies advocate using very-lean and ultra-lean hydrogen-air mixtures, with a fuel-air equivalence ratio below 0.5, as a potent strategy to reduce NOx emissions in hydrogen-fueled ICEs. An experimental setup with an optical spark-ignition engine was devised to investigate the primary factors influencing cycle-to-cycle variations in H2ICEs by optimizing mixture homogeneity and focusing the study on flame interaction with in-cylinder aerodynamics. Simultaneous in-cylinder pressure, chemiluminescence, and PIV analyses were performed to characterize and compare the behaviors of ultra-lean hydrogen-air and propane-air flames, assessing flame development and cyclic variation features. Results indicate that the flame development of hydrogen and propane is significantly different. Propane exhibited increased in-cylinder pressure trace variability at lean and rich flammability limits with a high flame development difference between fast and slow cycles. In contrast, hydrogen-air mixtures under various equivalence ratios (from very-lean to ultra-lean) presented much more stable in-cylinder pressures. Furthermore, fast propane flames were mainly advected to the cylinder's symmetrical axes, while fast hydrogen flames started further away from the spark plug. Horizontal and vertical PIV measurements showed a single structure flow field over the studied conditions with mainly intensity variations over the measured cycles. Fast flames were associated with more intense tumble motion. The differences between fast and slow flames for hydrogen are attributed to the early flame development. However, after initial differences in flame development over several crank angles, there are similarities in all cycles, suggesting that the low variability in in-cylinder pressures is mostly due to the robustness of hydrogen flame ignition and its independence from in-cylinder flow small variations at the early development phase.

Detailed simulations on the mixed modes of premixed and diffusion combustion in supersonic hydrogen flames

Detailed simulations on the mixed modes of premixed and diffusion combustion in supersonic hydrogen flames

Large eddy simulation of a supersonic lifted hydrogen flame: Impacts of Lewis, turbulent Schmidt and Prandtl numbers

Parametric large eddy simulations (LES) of a supersonic lifted hydrogen flame are reported. The emphases are on two aspects: impacts of (1) Lewis number (Lei of the ith species) and (2) turbulent Schmidt and Prandtl numbers (Sct and Prt) on supersonic turbulent flame and flow structures. Five cases are considered: species-specific Lei, Sct=Prt=1.0 (C0); unity Lei, Sct=Prt=1.0 (C1); species-specific Lei, Sct=0.5, Prt=1.0 (C2); species-specific Lei, Sct=1.0, Prt=0.5 (C3); and species-specific Lei, Sct=Prt=0.5 (C4). Numerical results of instantaneous and/or time-averaged species mole fractions, mixture fraction, heat release rate, flame base location, and mixed modes of premixed and diffusion combustion are compared between cases C0 and C1. Differences in auto-ignition locations and strengths and flame structures and stabilization specify the impacts of Lewis number. They are triggered by different predictions of species mass and thermal diffusions at fuel-coflow and/or coflow-ambient air mixing layers. These differences are rationalized by a scale analysis of mass/thermal diffusion and convection for case C0, which suggests the relatively low but non-negligible former against the latter. Cases C0 and C2–C4 barely see differences in terms of instantaneous and/or time-averaged temperature, velocity, and mixed combustion modes except for further downstream areas where combustion occurs. Both Sct and Prt impose less significant influences than Lewis number, as sub-grid scale mass/thermal diffusion is subordinate to its resolved counterpart according to their scale analysis for case C4.

Numerical simulation study on the interaction between hydrogen flame and particle flame in scramjet

Numerical simulation study on the interaction between hydrogen flame and particle flame in scramjet

Energization test apparatus of HTS coils cooled by liquid hydrogen and manufacture of split-type REBCO external field coil

Abstract Liquid hydrogen (LH2) is a potential coolant of high-temperature superconducting (HTS) devices. However, owing to the high flammability of hydrogen and the risk of hydrogen embrittlement in materials, studies on LH2-cooled superconducting devices are rare. We developed an LH2 test apparatus to analyze the basic characteristics of LH2 as a coolant and evaluate the energizing characteristics of LH2-cooled superconducting wires. A REBCO external field coil was designed and manufactured for conducting cooling stability tests on various HTS coils cooled by LH2. The field coil, comprising eight single pancake coils of 4 mm wide REBCO wires, with inner diameter 106 mm and outer diameter 250 mm was divided into upper and lower sections. The test coil was placed in the central space. A magnetic field perpendicular to the test coil’s wire surface was generated by running currents in opposite directions through the upper and lower sections. Each double pancake coil was securely placed in a stainless-steel housing to withstand repulsive electromagnetic forces. The manufactured field coil was cooled using LH2, energized to 150 A, and successfully generated the intended magnetic field of 1.75 T. No increase in voltage was observed in any of the double pancake coils, and there was no mechanical degradation due to electromagnetic forces. Subsequently, we initiated thermal runaway tests on various HTS coils using the manufactured field coil to assess the cooling stability of LH2-cooled HTS coils. The study will facilitate the development of explosion-proof designs and safety technologies for LH2-cooled superconducting devices and cooling systems.

Understanding the mechanisms of flame quenching and flame-wall interactions in microchannel

The flow resistance and quenching characteristics of hydrogen flame in the microchannel of corrugated plates under a nitrogen environment are investigated in this paper. The analysis reveals that the explosion pressure and flame velocity decrease as the aperture of the microchannel decreases. Adding 24 % nitrogen increased the hydrogen quenching diameter from 0.5 mm to 1.4 mm in the case of the stoichiometric ratio. In light of the impact of nitrogen and microchannel characteristics, a critical flame velocity prediction formula for hydrogen flames has been established. To clarify the flame quenching mechanism, kinetic simulations and heat transfer calculations are conducted. The flow resistance and quenching temperature under different microchannel diameters are obtained. As the microchannel aperture decreases and the nitrogen concentration increases, the quenching temperature of hydrogen rises. Combined with the exothermic reaction and the heat transfer of microchannel, the flame-wall heat transfer in the microchannel is analyzed in detail. A reduction in the diameter of microchannels is beneficial in terms of enhancing the flame-wall interaction, and promote the thermal and free radical quenching of hydrogen flame. The results can be employed to elucidate the mechanism of hydrogen flame quenching and to reduce the thermal runaway hazard of hydrogen.

On the Chemical Effect of Steam Addition to Premixed Hydrogen Flames with Respect to NOx\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ext {NO}_\ ext {x}$$\\end{document} Emissions and Flame Speed

The present work analyses the effect of water vapour addition on NOx\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext {NO}_\ ext {x}}$$\\end{document} emissions of premixed hydrogen flames. In doing so, the adiabatic flame temperature is maintained by increasing the equivalence ratio, or alternatively increasing the unburned gas temperature, for increasing levels of water loading. Thus, it is possible to elucidate the changes in NOx\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext {NO}_\ ext {x}}$$\\end{document} production at constant-temperature conditions when the mixture is diluted with water. A consistent reduction of NOx\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext {NO}_\ ext {x}}$$\\end{document} emissions for increasing water dilution can be observed from 1-D premixed freely propagating flame simulations. Regarding the chemical kinetics effect of water vapour, the relative importance of different third-body reactions is examined by modifying the corresponding water collision efficiencies individually. For the chemical mechanism adopted, three reactions directly affect the nitrogen chemistry and the remaining relevant reactions are important for the flame structure and radicals concentration. The analysis stresses the importance of indirect effects like the formation and consumption of O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ext {O}$$\\end{document} and H\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ext {H}$$\\end{document} radicals in the pre-heat zone, which enhance the subsequent formation of NOx\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext {NO}_\ ext {x}}$$\\end{document} within the flame. The presence of steam can lead to a reduction of approximately 50%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$50\\%$$\\end{document} in NOx\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\ ext {NO}_\ ext {x}}$$\\end{document} emissions under conditions close to stoichiometry and high water loading (10%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10\\%$$\\end{document} by mass), compared to scenarios without water addition. Furthermore, the efficiency of water in third-body reactions significantly contributes to an emission reduction, and half of NO\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\ ext {NO}$$\\end{document} emissions under the same water loading conditions at high equivalence ratio are observed when the third-body reaction efficiency is activated with respect to the case with zero efficiency. This reduction is primarily attributed to effects on radical concentrations. Finally, the chemical effect via the third-body efficiency of water is examined with respect to flame speed. It turns out that the adiabatic flame temperature plays a key role for the relative influence of the chemical kinetics effect of water dilution. A cross-over temperature is found, below which the chemical effect of water reduces the flame speed, whereas the flame speed is increased above it.

Thermodiffusively unstable laminar hydrogen flame in a sufficiently large 3D computational domain – Part II: NOx formation mechanism and flamelet modeling

In this work, the NOx formation mechanism in three-dimensional (3D) thermodiffusively unstable premixed hydrogen flames is investigated through direct numerical simulations (DNS) and a reaction path analysis based on the DNS dataset. In part I of this study (Wen et al., 2024), the characteristic patterns of the instabilities were studied using the same dataset. Here, first, the effects of the computational setup (2D vs. 3D) and curvature on the flux ratio of nitrogen atom are quantified. In addition, the composition space model (CSM) proposed in previous works is extended to account for the NOx chemical reaction mechanism to predict NOx formation in thermodiffusively unstable premixed hydrogen flames. The flamelet solutions are obtained from the premixed flamelet equations in composition space so that the wide range of curvatures associated with the strongly corrugated flame front can be considered. The performance of the CSM in predicting the NOx species and the important radicals involved in the NOx formation pathways is evaluated through an a priori analysis. To investigate the effects of curvature on the NOx formation pathways and the performance of the composition space model, the positively- and negatively-curved regions in the DNS are studied separately. The results show that different from the 2D simulation, the NNH reaction pathway becomes dominant in the 3D simulation due to the increased range of curvature, which promotes the accumulation of the highly diffusive H radical in the positively-curved regions. The NNH reaction pathway is dominant in the positively-curved regions, while the N2O reaction pathway is more important in the negatively-curved regions. The flamelet model based on the composition space solutions that consider the effects of curvature yields accurate predictions for the radicals that are sensitive to the effects of curvature.Novelty and significance statementThe novelty of the present work includes the following aspects,(i) Analysis of NOx formation mechanism based on the large-scale 3D DNS dataset of a laminar fuel-lean premixed hydrogen flame on a sufficiently large domain;(ii) Quantification of the effects of computational setup (2D and 3D) and curvature on the NOx formation pathways;(iii) Extension of the previous composition space model (CSM) by incorporating the NOx chemical reaction mechanism to consider the effects of curvature on the NOx formation.

Practical approaches for evaluating radiative heat from high pressure hydrogen flame

As the construction of hydrogen stations and other hydrogen-related facilities progresses, it becomes increasingly crucial for safety designs to account for scenarios where leaked hydrogen might ignite. Since a hydrogen flame is usually invisible, detection must rely on radiative heat or temperature. While it is possible to predict accurate radiative heat flux from a high pressure hydrogen jet flame through detailed numerical calculations of hydrogen combustion with its radiation, the objective of this study is to provide a simpler and more accurate prediction method. Previous experimental data of radiative heat flux, Qr from high pressure hydrogen jet flames were compiled, and a simple experimental equation Qr=1463.3(Lo/Lf)−2.32 was derived, where Lf and Lo represent the flame length and the distance perpendicular to the flame axis from the midpoint of the flame length to the observer, respectively. Radiative heat fluxes were computed through detailed numerical simulations using three types of water vapor radiation models: Goody's narrow random band model, the exponential broad band model by Edward and Elsasser's narrow regular band model. The temperature and water vapor partial pressure distributions obtained from numerical simulations were used as input conditions. The results were found to be in good agreement with the experimental equation. Moreover, a simple diagram representing the radiative heat flux from a hydrogen flame was derived using the method of cylinder-like approximation. The results of this study show that if the hydrogen pressure and leakage aperture diameter are given, the radiative heat flux at a certain distance can be predicted with high accuracy. Additionally, the average value of emissivity for the hydrogen diffusion flame was found to be on the order of 0.03.

Experimental investigation of high-frequency transverse instability in Helmholtz resonator-coupled lean-premixed hydrogen combustor

Recent experimental studies suggest that lean-premixed pure hydrogen combustion plays a central role in generating self-sustained pressure oscillations, preferentially coupled with high-frequency tangential modes in land-based gas turbine combustors. The triggering of such high-frequency combustion dynamics is known to be closely connected with relatively small, flashback-resistant, characteristic nozzle dimensions, and the structural and kinematic properties of the premixed hydrogen flames. To identify important trends in the transverse modal dynamics of hydrogen flames, here we conduct detailed measurements of multislit injector-driven hydrogen flame dynamics. We analyze the measured data using the wave decomposition technique for PDF-based statistical assessments of bimodality and nodal line orientation. Experimentally, the planar-to-transverse mode transformation is revealed to be controlled by the combined influence of flame temperature and thermal power, which ultimately leads to the development of well-defined standing modes at around 4.69 kHz under the most severe test condition. Using analytic modeling and rigorous validation measurements, we also investigate Helmholtz resonator-type passive stabilization and its impact on the integrated system's thermoacoustic states with respect to the change in purge air flowrate. Our results show that the coupled system does not retain many of the initial thermoacoustic properties, but instead undergoes prominent changes in the first tangential modal dynamics, progressively evolving from a large-amplitude bimodal to a low-amplitude unimodal state.

Numerical analysis of turbulent nitrogen-diluted hydrogen flames stabilised by star-shaped bluff bodies

In this paper, we investigate the flow and flame dynamics within a combustion chamber supplied with nitrogen-diluted hydrogen fuel and air as oxidiser. The chamber is equipped with a bluff body that serves as a flame stabiliser. We focus primarily on the influence of the bluff body shape on the formation of large and small-scale vortical structures generated at the bluff body edge. We analyse how the geometry of the bluff body alters these structures and examine the impact of resulting modifications on flame characteristics, including key features such as flame length and radial extent, temperature and species field, and completeness of the combustion process. We consider four bluff body geometries, namely a typical cylindrical shape, a star-shaped wall, and a small triangular sharp and smooth wavy structure. The research employs the Large Eddy Simulation (LES) method, providing deep insight into the complex physics of unsteady flow. In the cylindrical bluff body configuration, the key mechanism driving the fuel-oxidiser mixing process is the periodic occurrence of large toroidal vortices shed at an exit of the oxidiser duct and on the bluff body edge. They exert the flow towards and away from a recirculation zone formed above the bluff body. When its geometry is irregular, the appearance of the large vortices is suppressed, however, the wall irregularities induce small-scale flow phenomena that enhance mixing in the shear layer formed between the recirculation zone and the oxidiser stream. The mixing efficiency is largely dependent on the shape of the bluff body wall, and it turns out that seemingly small changes (sharp vs. smooth small triangular parts), which in non-reactive regimes have only a minor effect on the flow dynamics, significantly change the flame length and fuel consumption rate. Compared to the cylindrical bluff body configuration, the small triangular parts cause an axial shift of the maximum temperature downstream and slow down fuel consumption. Contrary, large triangular elements in the star-shape bluff body clearly influence the flow both in non-reactive and reactive regimes. They cause the formation of local oxidiser streams directed towards the fuel-rich mixture. Consequently, the injected fuel burns at a rate similar to that observed in the case of the cylindrical bluff body, but the combustion process occurs at an average temperature approximately 200 K lower and its fluctuations are reduced. These factors may influence the thermal reduction of NOx. On the other hand, a negative aspect is the highest level of hydrogen in the closest vicinity of the bluff body, which in practical applications may accelerate the embrittlement of its surface.

Numerical analysis of the bistable propagation of lean premixed hydrogen flames

Premixed hydrogen-air flames that propagate in slender channels have been recently found to be bistable in the limit of fuel-lean mixtures and non-negligible heat losses. Specifically, two stable configurations conformed by either a circular or a double-cell flame front arise for the same combination of controlling parameters (fuel mixture, heat transfer). Nevertheless, this multiplicity of solutions lacks a satisfactory explanation that predicts the formation of either one or another structure given a particular ignition transient. In this work, the analysis of a set of numerical simulations is offered to unveil the underlying physics controlling the bistable behavior. The formulation employed here proves that simplified one-step Arrhenius reaction and quasi-two-dimensional equations suffice to reproduce the complex dynamics, rendering three-dimensional effects and detailed chemical kinetics as second-order contributions that may be important, however, to provide exact quantitative results. Specifically, the initial temperature profiles and subsequent expansion of the flow field prescribe the growth of the flame front leading to different curvatures and sizes of the kernel that control the evolution into one of the canonical structures. Moreover, the achievement of double-cell flames require splitting and reorientation processes prior to pairing of isolated flames, which call for specific initialization conditions that enable these transient dynamics.

Effect of temporal increase in equivalence ratio on combustion instability of a lean-premixed swirling hydrogen flame

The effect of temporal increase in the equivalence ratio on the combustion instability of a lean-premixed low-swirl hydrogen jet flame in a low-swirl combustor (LSC) is investigated in detail using a high-fidelity Large-Eddy Simulation (LES). The equivalence ratio is linearly increased from 0.3 to 0.5 over a duration of 0.4 s. The results show that the pressure oscillation amplitude in the combustor increases significantly when the equivalence ratio at the combustor inlet (ERCI) exceeds 0.42, and the maximum pressure amplitude and the combustion instability mode exhibit trends consistent with those in a previous experiment and numerical simulation conducted with the same LSC setup at a fixed equivalence ratio of 0.39. Temporal variations in the equivalence ratio and consequently the temperature inside the combustor cause the drastic amplification of pressure oscillation (when the ERCI exceeds 0.42) whose amplitude is larger than that at the fixed equivalence ratio (= 0.39). Prior to the onset of this exceptionally strong combustion instability, a transient irregular oscillation phenomenon comprising instantaneous changes in the pressure oscillation frequency is observed. While the pressure oscillations in the combustor and in the injector channel are in phase after the onset of strong combustion instability, they are in opposite phases during the occurrence of the irregular oscillation phenomenon prior to the onset of strong combustion instability. This irregular oscillation phenomenon predicted by the LES may play a crucial role in the mechanism of transition from stable combustion to combustion instability.

New laminar flame speed correlation for lean mixtures of hydrogen combustion with water addition under high pressure conditions

Hydrogen may become a substitute for liquid fossil fuels, contributing to greenhouse gas emissions reductions in internal combustion engines. Numerical simulations play a critical role in the advancement of these engines, with laminar flame speed being the main input. Experimental data of hydrogen flame speed at elevated pressures are scarce, due to the instability of the flames. Nonetheless, stable hydrogen flames can be predicted using chemical kinetics models. Moreover, the injection of water into the hydrogen fuelled engine could offer several benefits to engine combustion and emission performance, as it modulates the laminar flame speed within the combustion chamber and this effect has not been completely understood. Currently, no correlation exists to predict the laminar flame speed of hydrogen-air combustion with water addition under lean mixture engine operating conditions. In this study, we have extended the newly developed laminar flame speed correlation of hydrogen-air combustion to account for the effects of water addition under engine relevant conditions by using chemical kinetic laminar flame speed values. The laminar flame speed correlation was derived for pressures from 10 to 70 bar, temperatures from 400 to 800 K, equivalence ratios from 0.35 to 1 and water addition by mole from 0 to 20%. The hydrogen laminar flame speed correlation was expressed using polynomial forms with reduced order and number of terms with optimized values of coefficients. Additionally, a new exponential term was proposed to the power term α of the laminar flame speed correlation to capture the coupled effects of pressure and temperature on laminar flame speeds under engine-relevant lean burn water-diluted operating conditions.

LES investigation of a swirl stabilized technically premixed hydrogen flame with FGM and TFM models

Climate change due to carbon emissions is one of the aspects that must be kept under control nowadays. In this fashion, the employment of hydrogen as fuel provides a fundamental solution for both power generation and transportation since lean premixed combustions allow to reset the carbon emissions and reduce the pollutant ones. Indeed, the higher reactivity of hydrogen permits to operate with a very low equivalence ratio which can drastically limit the NOx production with respect to the common fossil fuels. On the other hand, lean hydrogen mixtures are characterized by peculiar aspects that make their study through CFD simulations not straightforward. The present work aims to analyze through two different numerical approaches a technically premixed hydrogen flame experimentally investigated at the Technische Universität Berlin (TUB). In particular, the main differences between the Flamelet Generated Manifold and the Thickened Flame Model strategy are deeply analyzed as well as the effects of the thermal boundary conditions on the flame stabilization mechanism. The comparison with the experimental data shows the treatment employed drastically influences the accuracy of the obtained outcomes.

Influence of slit asymmetry on blow-off and flashback in methane/hydrogen laminar premixed burners

One way to decarbonize the heat production sector is to gradually replace natural gas with hydrogen. The design of laminar premixed burners capable of operating with both methane and hydrogen is however challenging as these fuels have drastically different burning properties, which narrows the operating range. While methane flames face limitations in stabilization due to blow-off at high power, hydrogen flames tend to be susceptible to flashback at low power. This study investigates the effects of slits symmetry breaking on the blow-off of methane flames and flashback of hydrogen flames through two-dimensional direct numerical simulations of canonical asymmetrical slit configurations, revealing uneven interactions between the main openings and smaller auxiliary slits. The equations governing the reactive flow dynamics are coupled to a heat transfer solver in the solid phase to elucidate the thermal and hydrodynamic mechanisms determining the operability limits. Viscous dissipation in the small auxiliary slits together with a substantial preheating of the fresh gases by heat redistribution through the solid phase is found to govern flame stabilization in asymmetrical geometries, improving blow-off resistance of methane-air flames. Then, flashback of hydrogen-air flame is found to be driven by the competition between (i) the mass flow rate distribution between the main and auxiliary slits, (ii) preheating of the gas through the auxiliary slits and (iii) the ability of the main and auxiliary slits to quench the flame. The interplay of these phenomena gives rise to complex behaviors, wherein asymmetrical configurations could exhibit significantly enhanced resistance to flashback compared to symmetrical geometries. This conclusion, verified for different burner thicknesses and slit spacing, may be used to guide the design of fuel-flexible laminar burners.Novelty and significance statement1. Analyzing the effect of slits symmetry breaking on blow-off of methane flames and flashback of hydrogen flames for multi-perforated laminar burners.2. Defining a strategy for wall heat flux redistribution to assess the asymmetrical interaction between the main and auxiliary flames through the flame-holder.3. Demonstrating and explaining the positive impact of symmetry breaking on the blow-off resistance of methane flames.4. Highlighting the substantial and non-monotonous impact of symmetry breaking on hydrogen flame flashback.5. Identifying the interplay of thermal and hydrodynamic mechanisms in asymmetrical slits leading to improvement of flashback resistance for hydrogen-air flames compared to symmetrical configurations.

A detailed kinetic submechanism for OH[formula omitted] chemiluminescence in hydrogen combustion revisited. Part 2

In this series of two papers, we present a novel physically sound kinetic submechanism aimed at modeling the formation and decay of chemiluminescent (electronically excited) OH∗ molecules in the ignition and combustion of hydrogen and hydrogen-based mixtures over a wide range of temperatures and pressures. The present Part describes the construction of this detailed reaction submechanism based on the groundwork and findings of the preceding paper. In so doing, we relied both on critical evaluation of known and at times conflicting experimental and theoretical data on the elementary reaction kinetics of OH∗ and on our quantum chemical calculations and rate constant estimates for the key processes. The kinetic model under development was validated against the representative dataset of the observed OH(A2Σ+→X2Π) chemiluminescent emission accompanying the high-temperature (post-shock) oxidation of various H2-containing mixtures. To improve its performance against the collected OH∗ emission data, the rate constants of some reactive and quenching processes, to which the OH∗ kinetics is the most sensitive and for which there is a particular uncertainty, were jointly varied within the limits imposed by theoretical considerations and/or the scatter in the available experimental evidence. Thus, by refining the rate constants of these elementary processes, we obtained a reasonable reaction submechanism for OH∗ that describes the known experimental data better than previous models.Novelty and significance statement Although ultraviolet OH∗ chemiluminescence as an optical signature of the combustion process and the underlying chemistry is known to provide unique capabilities for combustion diagnostics, the development of the detailed reaction mechanisms intended for quantitative interpretation of the OH∗ emission measurements has now almost stagnated. Indeed, existing kinetic models contain a discouragingly small number of elementary processes and, apparently, do not account for the full variety of possible reaction pathways of OH∗ formation and depletion. Therefore, in this serial work, we have intended to revise the current ideas about the kinetics of OH∗ production and consumption during ignition and combustion and focused first on the OH∗ kinetics in a reacting H2/O2/Ar/N2 mixture. The principal outcome of this Part of the series is a new physically based kinetic submechanism aimed at modeling the formation and decay of electronically excited OH∗ molecules in the ignition and combustion of hydrogen and hydrogen-containing mixtures. Its incorporation into any conventional kinetic model of hydrogen oxidation enables quantitative interpretation of the OH(A2Σ+−X2Π) chemiluminescent emission observed experimentally in flames and shock-heated reacting flows with an accuracy higher than that of previous analogous mechanisms. No less importantly, the rate constant approximations for a number of individual elementary processes with OH∗, recommended here based on the critical review of literature data and on our quantum chemical calculations, are also of independent interest for the kinetics of electronically excited OH∗ particles. Finally, the OH∗ reaction subset we suggested for hydrogen flames can form the basis of more complex OH∗ chemiluminescence modeling reaction mechanisms for other fuels and fuel compositions.

Radiating biofuel-blended turbulent nonpremixed hydrogen flames on a coaxial spray burner

The low radiant intensity and luminosity of hydrogen flames can be enhanced by the addition of a small portion of sooting biofuels. To achieve higher effectiveness, the impact of blending turbulent nonpremixed hydrogen flames with liquid biofuels, by gas-assist atomisation, is investigated and compared with the introduction methods of prevapourisation and ultrasonic spray. The flame appearance, luminosity, radiant fraction, centreline temperature, and the near-field spray characteristics of four biofuel surrogates (eucalyptol, D-limonene, guaiacol, and anisole) blended into hydrogen flames are measured experimentally. Radiating biofuel/hydrogen flames are achieved on a coaxial needle spray burner by the addition of 0.1–0.3 mol% biofuel surrogates. Compared with the unblended hydrogen flame, the luminosity and radiant fraction are enhanced by 30%–500% and 2%–15%, respectively, with the addition of biofuel surrogates. The results show that adding the biofuel surrogates by gas-assist atomisation is more effective than prevapourisation and ultrasonic atomisation in luminosity and radiant fraction enhancement. It is found that the local fuel-rich conditions, which are beneficial for soot formation, are further facilitated by the larger droplets and spray objects generated by gas-assist atomisation. Of the additives tested, anisole is the most effective for luminosity and radiant fraction enhancement of a hydrogen flame while exhibiting the largest flame temperature drop due to the enthalpy of vapourisation and the radiative loss from the promoted soot formation. The viscosity and surface tension greatly influence the spray characteristics which in turn impacts the flame characteristics. Guaiacol, the representative of lignin, appears to have the lowest effectiveness in radiant fraction enhancement due to the presence of a hydroxy group, a higher bond dissociation enthalpy, and a coarser spray ascribed to higher viscosity and surface tension.