Structure Of Diffusion Flames Research Articles

Theoretical analysis of transient counter-flow combustion system fueled by porous biomass particles in a non-premixed configuration under non-adiabatic conditions

The current research aims to implement a reliable analytical model in order to scrutinize the transient behavior of a counter-flow non-premixed combustion system fueled by wet porous biomass particles under non-adiabatic conditions, including several processes such as phase change and dehumidification. A special term is added to the heat equation based on the law of conservation of energy as a means to assess the convective heat loss impacts. In order to analyze the structure of the diffusion flame and predict the time-dependent temperature and mass fractions of reactants distributions, transient forms of energy and mass equations are solved through the asymptotic method. Eventually, the influences of key factors such as equivalence ratio, porosity factor of particles, and reactants Lewis numbers on the flame structure and transient behavior of diffusion flames are elaborately investigated in the non-adiabatic condition, considering convective heat loss and compared to those in the adiabatic condition. The findings reveal that increasing the porosity factor not only improves flame temperature but also leads the flame to be located further from the fuel nozzle. In addition, when the system reaches its steady-state condition, the temperature of the flame formed in the adiabatic condition is about 20 % higher than that formed in the non-adiabatic condition.

CH4/NH3 Flame Structure and Extinction Limit under Flame-Flame Interactions.

Ammonia is considered to play an important role in replacing traditional fossil fuels in future energy systems. In the experimental study, CH4/NH3 flame was lit by applying a double-nozzle burner to gain insight into the structure, and the laminar diffusion flame structure, CH*/OH* intensity maximum, and flame size were analyzed by an ICCD camera. In addition, the extinction limit (lower limit) of the CH4/NH3 flame under different conditions was also studied. The results showed that with the increase of burner pitch, the two diffusion flames showed four states of merged flames, merging flames, inclining separated flames, and independent flames in turn. In the process of flame separation, the continuous pitch between merging flames was short. At this point, higher syngas flow could help increase the continuous pitch to keep merging form. The paper investigated the flame structure and found that the flame size would decrease when the NH3 content in the fuel was high. The flame stability also decreased with an increase of the NH3 content in the fuel. These findings provided experimental proof and a theoretical basis for future studies on the stability of CH4/NH3 co-firing.

Influence of buoyancy on the mixing, flame structure, and production of NO in hydrogen diffusion flames

This study investigates the impact of buoyancy on the mixing and flame structure of hydrogen jet diffusion flames. A validated Computational Fluid Dynamics (CFD) model for hydrogen diffusion flames is developed, employing the k−ω turbulence model and a steady flamelet combustion model. By accounting for the buoyancy term, the model accurately predicts the stoichiometric hydrogen flame length within a mere 0.2% deviation from the experimental value. In contrast, excluding the buoyancy term leads to a prediction divergence of around 5%.Deeper analysis of the mixture fraction field indicates that buoyancy enhances mixing in the flame’s far-field region, facilitating a more rapid dilution of the fuel stream within the air. This, in turn, results in a shorter flame. The scope of the modelling is expanded to encompass buoyancy effects onhydrogen flames with jet Reynolds numbers (ReJ) up to 15,000. While the buoyancy’s influence on flame control weakens as ReJ increases, a noticeable ∼4% variation in stoichiometric flame length persists between scenarios with and without buoyancy.

Detailed numerical simulation and experiments of a steadily burning micron-sized aluminum droplet in hot steam-dominated flows

Detailed numerical simulations are conducted in comparison with experimental results to study the flame structure and burning rate of a steadily burning aluminum droplet in hot steam-dominated environments. The droplet surface temperature, flame temperature, and flame stabilization position are measured along with the droplet burning rate estimated from the droplet size evolution. A numerical model accounting for detailed transport properties and chemical kinetics is presented and applied to unveil the flame structure, species and temperature distributions, and heat/mass transfer between the droplet and the surrounding gas. The numerical results of the temperature, velocity, and species distribution profiles demonstrate that the aluminum vapor flame is of classical diffusion flame structure, where near the droplet, there is a non-negligible amount of AlOAl apart from the main product Al2O3(l). This supports the deposition and formation of an alumina cap on the surface proposed in the literature. The simulation correctly captured the flame temperature and flame stabilization distance for a range of droplet sizes. Net heat flux analysis shows that conduction heat from the flame front accounts for less than 30% of the heat needed in aluminum evaporation, which warrants further quantification on other heat sources. The experimental and numerical results enrich the knowledge of the heat/mass transfer and chemical reactions near the droplet, which helps deepen the understanding of aluminum droplet burning.

Comparative Study of the Structure and Height of CH4 Laminar Diffusion Flames: Effects of Fuel-Side versus Air-Side Dilution.

The strategy for introducing diluents is a critical practical concern in diluted combustion; however, a comprehensive understanding of the effects of fuel-side dilution versus air-side dilution is currently lacking. This numerical investigation systematically studied the effects of dilution strategies on methane coflow diffusion flames, with a focus on the flame structure and flame length. Common additives in practical combustion, specifically H2O and CO2, were introduced to either the fuel or the air streams, with dilution ratios (Z) ranging from 0 to 0.2, and the impacts of four dilution strategies were quantified and ranked. Detailed simulations were conducted using a well-validated two-dimensional (2D) flame code to gain a deep understanding of OH formation, flame attachment, temperature of the burner nozzle, and flame height. Systematic analyses in terms of heat transfer, molecular diffusion, and chemical kinetics were conducted. Results demonstrate that introducing diluents into the air stream exerts a more profound influence on suppressing OH formation compared with fuel-side dilution. Moreover, air-side dilution has a negligible influence on flame attachment, while increasing Z on fuel side significantly inhibits flame attachment, and the latter behavior is attributed to the diminished mass diffusion of CH4 toward the oxidizer side. As the flame attachment weakens, it causes a consequential reduction in heat transfer from the flame base to the burner. Accordingly, the nozzle temperature exhibits a more remarkable decrease with the fuel-side dilution ratio than with the air-side dilution ratio. Simultaneously, a more profound influence of Z on flame length was observed for fuel-side dilution than for air-side dilution, and the underlying mechanisms governing these two distinct dilution strategies were theoretically elucidated.

Effects of n-decane substitution on structure and extinction limits of formic acid diffusion flames

Formic acid (FA) is a low-carbon fuel that can be produced from renewable hydrogen (H2) and carbon dioxide (CO2). However, transitioning completely to low carbon fuels might be lengthy and challenging, so it makes sense to gradually introduce low carbon fuels into the energy sector in conjunction with existing fuels. One practical approach could be blending FA with existing fossil fuels, such as kerosene in aviation. This study investigates the behavior of flames comprising FA and its blends with n-decane. The extinction limits of pure FA and FA/n-decane mixtures were measured in a counterflow non-premixed laminar flame setup at various blending ratios. Additionally, the flame structure was studied by measuring temperature and species distribution using probe-based sampling. The results indicate that replacing FA with n-decane in the FA-N2 flame greatly enhances the flame's reactivity. Blending the fuels also led to a decrease in CO2 production in the flame, while increasing the flame temperature and the concentration of H2 and CO species. Experimental results were modeled using an updated FA and n-decane model. The kinetic model agrees with the experimental trends, but slightly overpredicts H2, CH4 species, and flame temperature. The kinetic model was also used to investigate the kinetic coupling between FA and n-decane in non-premixed laminar flames. Kinetic analyses indicate that HOCO and OCHO intermediates play a crucial role in pure FA flames by reacting with active radicals like H and OH to produce CO2 and CO, while in n-decane blended flames, fuel decomposition proceeds through two other pathways leading to H2 and H2O2.

On the structure of buoyant fires with varying levels of fuel-turbulence

This paper employs a novel burner to study the effects of fuel-generated turbulence on the spatial and temporal structure of buoyant turbulent diffusion flames which are representative of large fires. Fuel-turbulence levels are increased using a perforated plate that issues high-velocity jets, enabling shearing of the fuel stream. The perforated plate may be recessed to control the turbulence level at the jet exit plane. It is shown that the exit plane axial velocity fluctuations can be increased from 0.135 m/s to 1.813 m/s. Varying the levels of fuel-turbulence in the burner allows for the control of key processes defining buoyant fires such as the spatial and temporal flame structure and flame instability modes. These processes are characterised by high-speed simultaneous imaging of planar laser-induced fluorescence of the OH radical (OH-PLIF) and Mie scattering from soot particles. Increasing the fuel-turbulence level deforms the flame, which promotes non-radial lateral entrainment into the flame sheet. This results in a sharp increase in the tilting of the near-field flame sheet along the vertical flame axis. Strong angular entrainment forces are shown to overcome the diffusive and thermal expansive forces at the flame neck, which leads to a strained asymmetric sinuous flame pinch-off instability, followed by separation of the flame base. Sinuous pinch-off instabilities occur at a greater frequency than the symmetric varicose pinch-off instabilities observed for flames with low fuel-turbulence. The asymmetric stretching of the flame neck inhibits the formation of the classical puffing instability formed with an axisymmetric plume that defines classically buoyant flames. Probability density functions calculated for the flame front curvature and flame surface area are shown to monotonically broaden in the near-field region of the flame due to lateral entrainment effects. The transition to buoyancy-driven turbulence also shifts to an increasingly more upstream location. This burner, with its well-defined boundary conditions and novel data, forms a platform for advancing capabilities to model complex fire phenomena including turbulence-buoyancy interactions.

Experimental and theoretical study of combustion and aerothermodynamic effects on the flame structure from wind-driven rectangular fires

This work provides a comprehensive model for global morphology parameters of flame structure of turbulent diffusion flames in wind-driven rectangular burner fires. Five flame morphology parameters were configured by using the image from CCD cameras and CFD calculations, demonstrating for the first time the image corresponds to a mixture fraction. The unique physics-based similarity correlations for the combustion were proposed to predict the morphology parameters of flame structures in terms of a characteristic length scale. The reliability of the current physical model and new correlations was validated by comparing it with other literature data. One important result of this paper is that the source size does not affect the flame morphology if a Froude number based on the source size is greater than a fraction of the ratio of the flame length at no wind to the source size. Subsequently, based on top hat similarity of the flow profiles, a phenomenological integral combustion model including momentum and continuity equations was also established and the results were compared well with the experimental data and the new correlations. Finally, CFD numerical modeling and large-scale experimental data in the literature were employed to verify the applicability of the present model for large-scale pool fire combustion.

Characterization of the oxymethylene ether fuels flame structure for ECN Spray A and Spray D nozzles

Synthetic fuels will play a major role on the reduction of pollutant emissions and carbon footprint of ICE engines. The use of renewable energy and CO2 for its production maps out a promising route to achieve ICE carbon neutrality. Among these fuels, oxymethylene ethers are also interesting for their potential to drastically reduce soot formation. However, a proper characterization of their properties and behaviour is mandatory to reach full implementation in commercial engines. For this reason, this work presents a detailed characterization of the flame structure of two types of oxymethylene ethers (OMEX and OME1) using high-speed chemiluminescence imaging and Planar Laser-Induced Fluorescence (PLIF). Test were performed in a constant-pressure combustion vessel, with the operating conditions and two different injector nozzles from the Engine Combustion Network (Spray A and Spray D). Regions associated with low-temperature chemical reactions are observed thanks to the formaldehyde PLIF while evolution of the high-temperature reactions has been analysed based on hydroxyl excited state (OH*) chemiluminescence and hydroxyl PLIF. On-resonant and off-resonant measurements were performed for OH PLIF with a dye laser in order to remove other radiation sources not linked to OH fluorescence, whilst 355 nm radiation from the third harmonic of a Nd:YAG laser are used to excite CH2O molecules. On the one hand, the results show that the combustion of OME1 with Spray A is characterized by a flame structure very different to that of a diffusion flame. It is characterized by large cool flame region followed by a short high-temperature zone. On the other hand, the combustion of OMEX with both nozzles and OME1 with Spray D show more similarities with a diffusion flame structure. The stoichiometry of the fuel and the equivalence ratio fields achieved strongly affect the structure and latter evolution of the flames.

Unsteady Flamelet modeling study on OMEx-type fuels under Engine Combustion Network Spray A conditions

Poly-Oxymethylene Dimethyl Ethers OMEx are synthetic and potentially-renewable fuels that lead to a notable reduction of the lifecycle CO2 emissions while promoting lower soot emissions than conventional Diesel fuel. In the present contribution, a computational study with a single component OME1 and a multicomponent OMEx fuel has been carried out under reference Spray A conditions from the Engine Combustion Network (ECN), which mimic in-cylinder conditions representative of Diesel engines. For both fuels, three ambient temperature conditions have been swept at constant ambient density. Calculations have been carried out using an Unsteady Flamelet Progress Variable (UFPV) combustion model and detailed chemical mechanisms. For both OMEx-type fuels, low temperature ignition in flamelet configurations start in lean mixtures, which shifts towards the fuel-rich zone and eventually leads to high temperature ignition, similar to typical hydrocarbons. In agreement with corresponding fuel cetane numbers, ignition of OMEx occurs at timings similar to those of n-dodecane, which is the reference fuel for ECN studies, while a delayed ignition is obtained for OME1. However, the actual difference in ignition timing between OMEx and n-dodecane depends on diffusion in the mixture fraction space. Moreover, ignition in spray calculations seems to occur fully on the lean side, especially for OME1, as well as for the low temperature cases. This difference in ignitable mixture range between the canonical flamelet configuration and the spray calculations results from the finite residence time for relevant mixtures in the latter case, compared to an infinite residence time in flamelets. Comparison with experiments show that the modeling approach predicts most combustion metrics for both fuels and temperature values. The combination of ambient temperature and fuel-related reactivity has enabled a transition from a short ignition lifted diffusion flame structure (OMEx at 1000–900 K) towards long ignition cases, where lift-off length may eventually be longer than the maximum length of the stoichiometric surface. This results in a reaction front stabilization at very lean conditions, i.e. a type of lean mixing-controlled flame.

Investigation of the stability, radiation, and structure of laminar coflow diffusion flames of CH4/NH3 mixtures

The stability, radiation, and structure of laminar axisymmetric CH4/NH3/air diffusion flames have been studied using photographic images, spectrally resolved measurements of flame radiation, and the spatial distribution of temperature and major species mole fractions obtained by spontaneous Raman scattering. The fit procedure for the Raman spectra of NH3 includes a hitherto unquantified overtone feature, whose inclusion in the fit significantly improves the NH3 fraction obtained. Nitrogen is used to replace NH3 to separate chemical effects of NH3 addition from those due to dilution. The results show that NH3 addition drastically reduces radiation from carbon-containing species, with progressively increasing strong chemiluminescence from excited NO2 and NH2, indicating a substantial change in flame chemistry. While the Rayleigh/Mie scattering from soot particles is still observed in the Raman spectra at 28% NH3 addition, 46% NH3 in the fuel is seen to suppresses soot formation effectively. The measured axial and radial profiles of temperature and major species indicate a substantial contribution from radial transport from the reaction zone, seriously complicating the relation between composition, mixture fraction, and the corresponding equilibrium temperature and mole fractions.

Experimental investigation of soot formation in inverse diffusion flames of CO2 and N2 addition: Isolation of dilution and thermal effects

ABSTRACT This experimental study compares the flame structure, temperature, and soot formation characteristics of normal (NDFs) and inverse diffusion flames (IDFs), evaluates the effect of CO2 addition, and isolates the thermal and dilution effects of CO2 and N2 addition on soot formation in IDFs. A hyperspectral imager using the TR-GSVD algorithm measures IDFs with different CO2 addition (XD) in the oxidant, with N2 addition as the comparison. In contrast to NDFs, IDFs have an opened tip, and the visible flame height and flame width decrease as XD grows. IDFs have a higher peak temperature than NDFs, but their peak soot volume fraction is substantially lower. Compared with N2 addition at the same XD, CO2 addition narrows IDFs and makes the reaction zone visible, and the peak temperature of CO2 addition is 350 K lower than that of N2 addition. The suppression effect of CO2 on soot formation is more potent than N2, with a soot formation rate of 30% that of N2 addition and a soot loading of 20–60%. The activation energy of soot formation reaction with CO2 addition is higher than N2 addition. The isolating results reveal that the thermal effect contributes to the main suppression effect of CO2 addition on soot formation, while the dilution effect of N2 addition is stronger than its thermal effect.

Effect of CO2/H2O addition on laminar diffusion flame structure and soot formation of oxygen-enriched ethylene

Oxygen-enriched combustion technology has an important application prospect, in which Oxy-steam combustion is a new generation of potential carbon capture technology. In this paper, the effects of O2/CO2 and O2/H2O atmosphere on soot formation in laminar diffusion flame were studied. The flame image was taken by a color (CCD) camera in the visible band, the directional emission power in R and G bands was calibrated, and the two-dimensional distribution of temperature and soot volume fraction in the flame was reconstructed. The experimental results are compared with the numerical results, and the numerical results well predict the temperature of oxygen-enriched flame in O2/N2/CO2 atmosphere, but overestimate the soot volume fraction. The laminar diffusion flame of ethylene in two kinds of oxygen-rich atmosphere was numerically studied by using a step-by-step decoupling method to separate the density, transport, thermal and chemical effects of the diluent by introducing four virtual substances. The calculation results show that combustion in O2/H2O and O2/CO2 atmosphere reduces the flame peak temperature, but the effect on the flame centerline temperature distribution is different. The addition of H2O enhanced the concentration of OH through the reverse reaction rate of OH + H2=H + H2O, while the addition of CO2 decreased the concentration of OH.O2/CO2 combustion leads to the increase of flame height and radius, while in O2/H2O atmosphere, the flame height is shortened. Both CO2 and H2O have a good inhibitory effect on soot formation. CO2 in the oxidant affects soot nucleation and surface growth rate mainly by reducing flame temperature and H mole fraction, but not by promoting soot oxidation process. Water vapor also inhibits soot formation by increasing OH concentration through chemical effect.

Aluminum Diethylphosphinate as a Flame Retardant for Polyethylene: Investigation of the Pyrolysis and Combustion Behavior of PE/AlPi-Mixtures

The popularity of organic polymers despite their high flammability forces the introduction of flame retardants (FR) such as metal phosphinates into the combustible material. The thermal behavior of aluminum diethylphosphinate (AlPi) as FR in the widely used polymer ultra-high molecular weight polyethylene (UHMWPE) is investigated here. The study focuses on the effect of the FR on the gas phase activity when a polymer is pyrolyzed or burned. For this purpose, the fast pyrolysis of AlPi was investigated by differential mass-spectrometric thermal analysis (DMSTA). Also, the thermal and chemical structures of diffusion flames of UHMWPE + AlPi specimens were investigated using micro thermocouples and molecular beam mass spectrometry, respectively. Small amounts of AlPi (2.5 wt.%) decrease the gas temperature significantly by a maximum of 155 K related to FR-free polymer flames, indicating a retardancy effect of the additive on the flame. From the results of subsequent limiting oxygen index (LOI) tests, it is obvious that a PE burn-up cannot be achieved in a self-sustained flame when an additive content above 10 wt.% is used as FR. In the mass-spectrometric studies, the phosphorus-containing species produced in the pyrolysis experiments (DMSTA) of the neat AlPi as well as the species which are formed in flames during combustion experiments can be detected. In the flames, the concentration of the phosphorus containing compounds peaks at low heights above the polymer surface which indicate a gas phase activity of AlPi or its pyrolysis products. Besides a charring layer on top of the burning surface could be noticed. The use of AlPi as a FR for UHMWPE shows flame retardant effects in both the condensed and the gas phase.

Numerical study of the Effect of CO2 on the NH3/CH4 Counterflow Diffusion Flame in O2/CO2/N2 Atmosphere

Ammonia, as a carbon-neutral fuel, draws people attentions recently. NH3/CH4 blends is considered as a kind of fuel. A numerical simulation of the effects of CO2 dilution on the combustion characteristics and NO emission of NH3/CH4 counterflow diffusion flame was conducted in this study. Diffusion flame structure, the influence of CO2 radiation characteristics on temperature and NO emission characteristics were studies at normal temperature and pressure. The dilution and radiation of CO2 reduce the flame temperature significantly. NO concentration decreased with the CO2 mole fraction increase effectively. The study extends the basic combustion characteristics of NH3 containing fuel.

Experimental Investigation of Stability and Structure of Vertical LPG Inverse Diffusion Flames Issuing from an Elliptic Burner

The present paper experimentally examines the influence of the progressive variations of the central air jet velocity in a concentric circular-elliptical inverse diffusion flame (IDF) on the visual and thermal structure and stability of the developed flames. All experiments are conducted at a fixed fuel flow rate (liquefied petroleum gas) throughput that emerges from the annular elliptic passage having an aspect ratio of 2:1. The visual images are aided via a digital camera and shadowgraphs, while the thermal structure (axial and radial temperature profiles) is acquired using a bare fine wire (125 μm) thermocouple; rendering radiation loss insignificant. The visual images and shadowgraphs clearly indicate the existence of four regimes: (i) an annular partially premixed region at the burner rim, (ii) an inner central premixed blue flame, (iii) an outer luminous yellowish post combustion zone and (iv) the flame tip buoyant zone. The progressive increases of the central average air velocity (𝑉𝑎 = 7.4 m/s up to 31.3 m/s) result in shortening the visible flame length and narrowing the flame width. These are coupled with changing the flame appearance from a yellowish diffusion flame with a sooty core regime to an intense central premixed flame surrounded by a soot ring to blue flames exhibiting intense central radiation regime associated with soot oxidation. At extremely high air velocity > 21 m/s, locals flame extinction and re-ignition occurs at the boundaries of the ellipse minor axis and further increase of 𝑉𝑎 causes complete extinction of the main flame. These findings are very much supported by the mean gas temperature measurements that indicate steep temperature rise associated with the formation of a central premixed combustion within the flame core which is followed by the diffusion mode of combustion. A plateau of the axial temperature profiles is observed in the transition zone between the two regions whereby the rise in temperature due to soot oxidation is balanced by the radiation loss.

An experimental multiparameter investigation on the thermochemical structures of benchmark ethylene and propane counterflow diffusion flames and implications to their numerical modeling

Counterflow diffusion flame is a canonical non-premixed flame configuration that is suitable for studying complex combustion chemistries such as soot formation. An important objective of such studies is to build predictive kinetic models. For validation of these models, complete and accurate experimental characterization of the flames are essential. Existing counterflow flame- based soot studies focused primarily on the determination of soot volume fraction, while simultaneous measurements of temperature, species concentrations and in particular flow fields were less common. Here we performed an experimental multiparameter investigation for a comprehensive characterization of the most important physical and chemical properties of four benchmark ethylene and propane counterflow flames, with the intention to provide data with sufficient details for the setup of corresponding numerical simulations and the evaluation of the resulted numerical data. Particle image velocimetry, tunable diode laser absorption spectrometry, microprobe sampling with gas chromatography, laser light extinction, laser induced incandescence are the techniques used respectively to measure flow fields, temperature, gas-phase species, and soot. Special attention was given to the effects of radial profiles of nozzle exit velocity on the quasi-one-dimensional counterflow simulation, whose results were compared against both the experiments and two-dimensional simulation. Probe effects on speciation were also quantitatively evaluated by comparing CO2 mole fraction as measured with the intrusive probe sampling as well as an independent optical technique. Our specially chosen ethylene and propane flames offered a direct experimental evidence that a flame producing higher concentration of benzene can have a significantly lower sooting tendency. It is hoped that the present comprehensive dataset can serve as a useful validation target for future high-fidelity gas-phase and soot models. It is also our intention that we provide sufficient details regarding the experimental apparatus and associated measurement techniques so that interested readers can reproduce our experimental data.

Experimental study of buoyancy-induced instability in the DME and LPG jet diffusion flame

An experimental study is conducted on the buoyant jet diffusion flames to explore the interaction between the fuel jet and the outer vortex and their impact on the flame structure. Dimethyl ether (DME) and liquefied petroleum gas (LPG) diffusion flames are investigated. A laminar diffusion burner setup is used to study the flame dynamics of three different tube diameters (6, 8, and 10 mm) over a wide range of Reynolds numbers (250–2000). Flame flicker frequencies are analyzed using chemiluminescence signals and image processing techniques, and high-speed Schlieren images visualized flame dynamics and structure. The spatiotemporal structure of jet diffusion flames is analyzed using the dynamic mode decomposition (DMD) tool. Analysis of the Schlieren images revealed that the outer vortex structure interacts with the fuel jet, which eventually leads to convective feeding to the outer vortical structure. The outcome is the radial expansion of the leading vortex, which allows the trailing vortex to merge and produce a subharmonic frequency. However, all test conditions, regardless of the presence of subharmonic frequency in the buoyant diffusion flame, follow the established relationship between Strouhal-Froude numbers. The dominant mode of DMD captured the spatial features of the outer vortex at associated temporal frequencies. The large-scale coherent structure is observed in the region of subharmonic oscillation due to the merging of successive vortices. This process occurs at the alternate flame bulge formed near the tube exit.

Combustion Dynamics Simulation of a 30-Injector Rocket Engine

ABSTRACT A computational study of the nonlinear rocket-engine combustion instability is presented for an engine with 30 coaxial methane-oxygen injector ports, a choked nozzle, and a combustion chamber. Three combustion chamber lengths of 18 cm, 24 cm, and 30 cm are simulated with differing instability modes depending on length. Computations using a three-dimensional unsteady k − ω shear-stress transport delayed detached-eddy simulation method provide detailed time-resolved information about the combustion instability. Our flame-index analysis explains why the premixed-flame structures dominate over diffusion flame-structures in the driving mechanism for the instability. This causes the engine to become more unstable as the mixture becomes more fuel rich. Spontaneous longitudinal-mode and tangential-mode instabilities are observed. The oscillation amplitude of the two instability modes vary in alternating fashion during the simulation time. The alternating behavior of the two instability modes can be modified by pulsing the injector mass-flux on the mixture ratio at a particular frequency.

Visualization of hydrogen jet evolution and combustion under simulated direct-injection compression-ignition engine conditions

The evolution and combustion of H2 jets were investigated in an optically-accessible constant-volume chamber under simulated direct-injection (DI) compression-ignition (CI) engine conditions. The parameters varied include injection pressure (84–140 bar) and ambient temperature (1000–1140 K). A detailed characterization of the injector system and the ensuing jet penetration process is reported first. High-speed schlieren imaging, OH∗ chemiluminescence imaging and pressure trace measurements were subsequently used to investigate the auto-ignition and combustion of the H2 jets. The results show that the ignition delay of H2 jets under such conditions is sensitive to ambient temperature variations, but not to injection pressure. Optical imaging reveals that the combustion of H2 jets mostly initiated from a localized kernel, before spreading to engulf the whole jet volume downstream of ignition location. The imaging also indicates that after ignition, the flame recesses back towards the nozzle and appears to attach to the nozzle to form a diffusion flame structure.