Non-premixed Jet Flames Research Articles

Reinforcement Learning for Submodel Assignment in Adaptive Modeling of Turbulent Flames

Reinforcement learning (RL), an unsupervised machine learning approach, is innovatively introduced to turbulent combustion modeling and demonstrated through the automated construction of submodel assignment criteria within the framework of zone-adaptive combustion modeling (AdaCM). In AdaCM, the appropriate combustion submodel—whether the cost-effective species transport model or the advanced transported probability density function (TPDF) method—is adaptively assigned to different regions based on a criterion crucial for performance. The use of RL avoids the extensive manual optimization that involves repetitive calculations and struggles to account for multiple factors. Specifically, RL agents observe local variables as the state and determine the appropriate submodel through a policy. The policy is refined to maximize a reward measuring both accuracy and efficiency through the interaction between RL agents and the AdaCM solver. The methodology is demonstrated for a turbulent non-premixed jet flame, and a sophisticated RL criterion exhibiting a nonlinear and nonmonotonic dependency on the two-dimensional state of mixture fraction and Damköhler number is learned. The AdaCM with the trained criterion provides predictions that are nearly indistinguishable from those obtained using the TPDF method for the whole computational domain, while substantially reducing the computational cost with the speedup of 3.4 and only 22% of cells for TPDF.

Large Eddy Simulation of NO Formation in Non-Premixed Turbulent Jet Flames with Flamelet/Progress Variable Approach

Large Eddy Simulation of NO Formation in Non-Premixed Turbulent Jet Flames with Flamelet/Progress Variable Approach

Lifting of Transversely Forced Turbulent Nonpremixed Coaxial Jet Flames

The lifting behavior of turbulent nonpremixed coaxial jet flames was investigated experimentally as a function of the center jet Reynolds number, annular-to-center jet velocity ratio, acoustic frequency, and acoustic amplitude, all with and without transverse acoustic disturbances acting on the flames situated at a pressure antinode. Global lifting regimes were mapped and consisted of attached flames, periodically lifted flames, and permanently lifted flames. With one exception, all flames were attached in the absence of acoustics and lifted only because of the applied acoustic excitation. The exception occurred at the highest Reynolds number and velocity ratio, where the flames were unconditionally lifted in all cases. Between these two extremes, a range of lifting regimes was observed. Lower applied frequencies and smaller amplitudes were found to generally promote attachment to the burner. Flow visualizations using high-speed Schlieren and OH* chemiluminescence imaging revealed a lateral spreading effect near the jet exit. The lateral spreading was hypothesized to be caused by an axially oriented counterflow collision between the downstream flowing jets and presumed local upstream portion of the acoustic velocity fluctuations. Physical mechanisms were hypothesized based on this observation, which appear to consistently explain most of the observed behavior.

Effects of O2 concentration of O2/CO2 co-flow on the flame stability of non-premixed coaxial jet flame

Greenhouse gas emissions should be reduced to stop the advance of global warming. Oxy-fuel combustion is one of the methods to reduce CO2 emissions. This paper aims to study the stabilization of CO2-diluted oxy-methane non-premixed jet flames and compare this with the results of methane-air flames. The lower the co-flow velocity and the higher the oxygen concentration in the co-flow, the higher the stability. If the flame stability is strong enough to withstand flame shape changes from the over-ventilated flame to the under-ventilated flame, the flammable range without blowout increases significantly under the under-ventilated flame. Otherwise, by decreasing the liftoff flame stability, the over-ventilated flame cannot change to the under-ventilated flame and blows out near the globally stoichiometric condition. The flame stability of a CO2-diluted oxy-fuel flame can differ from that of methane-air flames due to the properties of CO2 and N2. The effective velocity proposed by Guiberti et al. shows a good collapse of the dimensionless liftoff height among the O2/CO2 co-flow, but the dimensionless liftoff height under the air co-flow is not on the same line with the O2/CO2 co-flow. This study suggests a modified dimensionless liftoff height equation considering the fuel and co-flow mixture kinematic viscosity instead of the fuel kinematic viscosity and the ratio of the thermal diffusivity of the mixture. The thermal diffusivity ratio does not affect the result of the air co-flow. Additionally, the new coefficient β and turbulent Schmidt number are suggested because these numbers are factors depending on the burner geometry. The modified effective velocity using new coefficients and the modified dimensionless liftoff height show a good collapse for not only O2/CO2 co-flows but also air co-flow. Also, the modified effective velocity shows the possibility of the critical liftoff velocity prediction.

Large eddy simulation of turbulent non-premixed oxy-fuel jet flames with different Reynolds numbers

Due to differences in the physical and chemical properties of CO2 and N2, along with a reduction in the momentum ratio between oxidant and fuel streams, the local extinction of oxy-fuel flames is significantly more pronounced compared to conventional air–fuel flames, which poses challenges in the design and operation of oxy-fuel burners. This study further validates the species-weighted flamelet/progress variable (FPV) model proposed in previous work [Jiang et al., 2023], particularly in oxy-fuel flames characterized by highly local extinction. Large eddy simulations were conducted on the Sandia oxy-fuel jet diffusion flame at various Reynolds numbers. The predictions are systematically compared with experimental data, and the influence of Reynolds number on local extinction is thoroughly analyzed. The results demonstrate that the numerical simulation effectively predicts mean temperature, species mass fractions, differential diffusion parameters (ZHC), and local extinction in oxy-fuel jet flames across a wide range of Reynolds numbers. The errors in predicted mean temperature and mass fractions exhibit a slight increase with rising Reynolds numbers, yet remain below 15 %. As the Reynolds number increases from 12,000 to 18,000, the predicted peak ZHC decreases by 30 %, and the beneficial effect of preferential diffusion of H2 weakens, while the adverse effect of CO2 on combustion becomes stronger.

Opposed jet burner and Tsuji burner for representative laminar flamelet with differential molecular diffusion for non-premixed combustion modeling

Modeling differential molecular diffusion in turbulent non-premixed combustion remains challenging for flamelet models. Laminar flamelet is a key building block of flamelet models for turbulent combustion. One significant challenge that has not been adequately addressed is the representativity of laminar flamelet for the characteristics of differential molecular diffusion in turbulent combustion. Laminar flamelet is typically generated based on two conceptual burner configurations, the opposed jet burner and the Tsuji burner. The two burners are commonly viewed to be equivalent for the description of laminar flamelet structures. In this work, a major difference between them is revealed for the first time when they are used to represent differential molecular diffusion. The traditional opposed jet burner yields almost fixed equal diffusion locations defined as the points with equal values of mixture fractions based on different elements in the mixture fraction space. The Tsuji burner, with a slight extension, can produce a continuous variation of the equal diffusion locations in the mixture fraction space. This variation of the equal diffusion locations is shown to be an important feature of non-premixed combustion, as demonstrated in a laminar jet mixing layer, a turbulent jet mixing layer, and a turbulent jet non-premixed flame. Theoretical analysis is conducted based on an idealized opposed jet mixing layer to identify the major cause of the difference between the two burners. It is found that the inlet diffusion caused the major difference in differential molecular diffusion in the two burners. The curvature difference, another main difference between the two burners, however, is not found to cause any major difference in differential molecular diffusion in the two burners. Based on the finding, a modified opposed jet burner with disabled inlet diffusion is introduced and is shown to be able to reproduce the feature of differential molecular diffusion observed in the Tsuji burner. Both burners, after properly generalized with suitable boundary conditions and additional parameters (like the velocity ratio of fuel and oxidizer), are expected to be suitable and equivalent choices for the generation of representative laminar flamelets that can be used eventually in flamelet modeling of differential molecular diffusion in turbulent non-premixed combustion.Novelty and significance statement: (1) A significant difference between the opposed jet burner and the Tsuji burner is revealed for generating representative laminar flamelet with differential molecular diffusion (under specific conditions); (2) The flamelet generated from the Tsuji burner is shown to be representative of differential molecular diffusion found in practically relevant mixing and combustion problems; (3) The treatment of inlet diffusion is identified to be the major cause of the difference observed between the two burners; (4) Modifications are introduced to the opposed jet burner to reconcile the two different burners for producing representative flamelet that can be used in the modeling of differential molecular diffusion in turbulent non-premixed combustion.

A Generalised Series Model for the LES of Premixed and Non-Premixed Turbulent Combustion

In this study, the generality and prediction accuracy of a generalised series model for the large eddy simulation of premixed and non-premixed turbulent combustion is explored. The model is based on the Taylor series expansion of the chemical source term in scalar space and implemented into OpenFOAM. The mathematical model does not depend on combustion regimes and has the correct limiting behaviour. The numerical error sources are also outlined and analysed. The model is first applied to a piloted methane/air non-premixed jet flame (Sandia Flame D). The statistical (time-averaged and RMS) results agree well with the experimental measurements, particularly with regard to the mixture fraction, velocity, temperature, and concentrations of major species CH4, CO2, H2O, and O2. However, the concentrations of the intermediates CO and H2 are over-predicted, due to the limitations of the reduced reaction mechanism employed. Then, a Bunsen-piloted flame is simulated. Most of the statistical properties of both the reactive species and progress variables are well reproduced. The only major discrepancy evident is in the temperature, which is probably attributed to the experimental uncertainties of temperature fields in the pilot stream. These findings demonstrate the model’s generality for both a premixed and non-premixed combustion simulation, as well as the accuracy of prediction of reactive species distribution.

NO emission characteristics of air coflowed non-premixed ammonia jet flame at elevated ambient temperatures and with N2 dilution

Ammonia is promising carbon-free alternative fuel, while the poor combustion performance and severe NO emission hinder its application. In this work, a strategy combined with preheating and dilution was discussed, and NO emission characteristics of air coflowed non-premixed ammonia jet flame were experimentally investigated over 550–700 °C and with N2 dilution, using a novel preheating facility. Results show that the general excess air coefficient is significant for NO emission, but not the unique factor. NO emission monotonically increases with the general excess air coefficient, while the slope decreases. Ambient temperature is also of a remarkable effect, that NO emission decreases by about 100 ppm (15% O2, dry) when the ambient temperature rises from 550 °C to 700 °C. Besides, effects of dilution were considered with three specific strategies. Air coflow dilution is of a negative influence on NO emission, while, NH3 jet dilution promotes NO emission in contrast. The interaction between the two single-dilution strategies is limited, and the effect of double dilution seems to be a simple sum of that of the other two single-dilution strategies. In addition, flame liftoff was observed in experiments and a considerable decline of NO emission was detected with the flame regime transition. According to the experimental results, the combination of elevated ambient temperature and air dilution is evaluated to be a promising low NO emission combustion method of ammonia.

A detailed of analysis of joint soot volume fraction/temperature statistics in non-premixed jet flame: Implication for soot emission turbulence/radiation interaction

The main objective of this study is to provide insights on the modelling of soot emission Turbulence Radiation Interaction (TRI) through a detailed analysis of the joint SVF/temperature statistics in turbulent non-premixed flames. The analysis relies on RANS/Transported Probability Density Function (TPDF) simulation of the Sandia ethylene jet turbulent non-premixed flame. RANS/TPDF predictions reproduce with fidelity the exhaustive set of experimental data, namely the flame structure, the soot and temperature statistics including the soot intermittency, and the radiative outputs. The marginal PDF of SVF exhibits the typical shape of an intermittent process and can be decomposed into a non-sooting mode and a sooting mode, with this latter being reasonably represented by a truncated Gaussian PDF. As expected, the marginal PDF of temperature can be reasonably represented by a clipped Gaussian PDF whereas that of the soot temperature follows a significantly narrower PDF as soot is fully oxidized in the fuel rich side of the flame. Analysis of the joint SVF/temperature PDF shows that both SVF/temperature and SVF/soot temperature first-order cross correlations are positive in the soot formation region and becomes strongly negative in the region of strong soot emission. The TPDF predictions are analysed a priori to design a new joint presumed PDF (PPDF) of SVF and temperature through the Gaussian copula. The Gaussian copula-based PPDF depends on six parameters, namely the five related to the marginal PDFs of SVF and temperature added to the first-order cross-correlation coefficient of SVF and temperature. Mean soot emission term computed with the copula-based PPDF agrees very well with the reference TPDF solution over the entire flame and improves significantly the solutions obtained by neglecting either TRI or the correlation between SVF and temperature. These results indicate that the Gaussian copula-based joint PPDF model is a good candidate to interpret soot emission-related experimental data in turbulent non-premixed flames.

Lifting of Single and Coaxial Nonpremixed Jet Flames Under Transverse Acoustic Forcing

The dynamics of gaseous reacting turbulent nonpremixed single and coaxial jet flames in unforced and acoustically forced environments was investigated at atmospheric pressure. Two burner configurations were used: one having a single fuel jet and one having a coaxial jet of fuel and oxidizer streams. The flames were exposed to transverse acoustic waves tuned to produce pressure antinode acoustic forcing at various frequencies and amplitudes. The flame responses were recorded using high-speed Schlieren and OH* chemiluminescence imaging. The center jet was set to be the same between the single and the coaxial jets at two different Reynolds numbers. The annular flow of the coaxial jet was set to have the same bulk velocity as the ambient coflow, a configuration here called a coaxial single jet. The configuration was so named because it was expected that a coaxial jet having the same annular bulk velocity as the ambient coflow would produce results similar to that of the single jet in the same coflow. It was found instead that the flame lifting behavior was considerably different, even qualitatively. The flow visualizations revealed differences in large-scale structures that explain the different lifting regimes.

Modeling Edge Flame Propagation for Flame Particle Tracking in the Forced Ignition Process of Turbulent Nonpremixed Jet Flame

ABSTRACT To ensure safe and reliable operation, ignition performance in propulsion and power devices needs to be thoroughly assessed. Reduced order models (ROMs) for ignition have gained increasing attention, given that first-principles models are restricted by immense computational costs that preclude rapid assessment for the design purpose. Currently, flame self-propagation is generally neglected in the ROM descriptions of ignition process and the impact on the flame front dynamics as well as the ignition probability map is not clear for low swirl or jet flames which are of interest for industrial applications. In this study, an empirical model for flame self-propagation is proposed to augment the capability of Lagrangian flame particle tracking method, a ROM model to predict ignition probability based on nonreacting flow field. The model adopts the classic scaling law of turbulent flame speed together with fuel gradient correction and flame self-propagation to mimic the edge flame dynamics during the ignition process. The model performances in terms of flame front dynamics and ignition probability map are quantified in a turbulent nonpremixed methane-air jet flame. Results reveal that only with flame self-propagation, the flame particle tracking can well capture the transient evolution of the leading flame front as observed in experiments. It is found that the predicted ignition probability map agrees well with experimental data with high ignition probability around the vicinity of the stoichiometric region at the downstream axial distance of 6 to 40 times the jet diameter. This study illustrates the importance to account for flame self-propagation to correctly predict ignition probability in jet dominant nonpremixed flames.

LES investigation of soot formation in a turbulent non-premixed jet flame with sectional method and FGM chemistry

This study proposes a detailed soot modeling framework for large-eddy simulation (LES) to accurately predict soot formation and particle size distributions (PSD) in turbulent reacting flows. The framework incorporates Flamelet Generated Manifold (FGM) chemistry and a soot model based on the discrete sectional method (DSM) to predict both qualitative and quantitative sooting behavior while keeping the computational cost affordable. Two elementary modeling strategies are considered in the LES formalism for describing soot formation rates. These strategies rely on an a-priori tabulation of soot formation rates and their run-time computation. The LES formalism is applied to the simulations of a well-characterized, non-premixed, turbulent jet flame. A comparative analysis of strategies employed for filtered soot source term treatment is conducted to investigate their impact on the prediction of soot quantities and the evolution of soot PSDs. The LES results for the gas phase and soot phase are compared against the available experimental data. A good prediction of soot evolution is achieved with the two methodologies. The tabulation of soot formation rates leads to a significant reduction in computational cost compared to the model based on their explicit runtime computation. The LES results reveal that the modeling of filtered soot source terms has a significant impact on the quantitative prediction of soot formation. The possible reasons for the observed differences in the soot prediction are discussed. The run-time computation-based model provides a more consistent treatment of the non-linear interactions between the gas and soot phases in soot source terms compared to the tabulated soot chemistry approach. On the other hand, the tabulated soot chemistry model is an interesting and efficient modeling approach for predicting soot formation in turbulent conditions. Overall, both approaches have their strengths and limitations, and the choice of approach may depend on the specific needs of the application.

Experimental investigation on the development characteristics of explosive overpressure induced by high-pressure hydrogen jet flame

Accidental release of high-pressure hydrogen can induce the self-ignition, and the spontaneously combusting flame can eventually induce a non-premixed jet flame or even explosion. In this study, the explosion overpressure characteristic of hydrogen jet flame outside the pipeline was investigated. Experiments were conducted on high-pressure hydrogen release pipelines with a diameter of 10 mm and pipeline lengths of 750, 1450 and 2200 mm, respectively, under different hydrogen release pressure. The impact of different initial conditions on the peak value of high-pressure hydrogen jet explosion was systematically investigated. The external pressure signals and flame images were completely collected. The results show that the external explosion overpressure decreased with the increase of axial distance away from the nozzle. When the length of the pipeline increased, the peak value of the overpressure was also gradually decreased. Meanwhile, as the high-pressure hydrogen burst pressure increased, the overpressure rose first and then decreased. With the increase of release pressure, when the hydrogen pressure exceeded 8 MPa, the hydrogen flow in the tube formed a blocked flow, so the increase of external explosion overpressure became slow. This topic can provide a reference for the safety development of the high-pressure hydrogen storage system.

A computational investigation of swirl-number and Damköhler-number effects on non-premixed laminar swirling jet flames

Axisymmetric numerical simulations are used to assess the swirl-induced stabilization of low-Mach-number non-premixed jet flames at a moderate Reynolds number (Re=200). Using a one-step model chemistry describing methane-air partially premixed combustion, we carry out a parametric investigation of the coupling between vortex breakdown and laminar flame liftoff/blowoff in a concentric jet configuration involving a central non-swirling methane jet surrounded by a swirling annular air jet issuing from a pipe with radius RA′ rotating with angular speed Ω′. The analysis considers order-unity values of the two relevant controlling parameters, namely, the Damköhler number DN, defined as the square of the ratio of the stoichiometric methane-air flame-propagation velocity to the mean air-jet velocity UA′, and the swirl number S=Ω′RA′/UA′. As the Damköhler number DN is decreased the attached edge flame lifts off from the injector rim. The resulting lifted triple flame migrates downstream on further decreasing DN until a critical blowoff value DN,b is reached. Results for fixed S=1 exhibit lower values DN,b than the corresponding simulations with fixed S=0. For a fixed Damköhler number, it is found that increasing S results in increased entrainment and reduced liftoff heights. At a critical value SB* of the swirl number, equal to SB*=1.2 for DN=0.35, a recirculation zone abruptly forms upstream of the lifted triple flame, enhancing the mixing and facilitating flame stabilization closer to the injector.

The Effects of Cracking Ratio on Ammonia/Air Non-Premixed Flames under High-Pressure Conditions Using Large Eddy Simulations

Ammonia is a promising carbon-free fuel. However, one of the main challenges for ammonia combustion is the high level of NO emissions. In this study, simulations were conducted for ammonia/air laminar counterflow flames and turbulent non-premixed jet flames in the KAUST high-pressure combustion duct (HPCD) at a pressure of 5 bar, with two ammonia cracking ratios of 14% and 28%. The influence of ammonia cracking ratio on the flame structure and NO formation mechanism were examined. The laminar counterflow flame results showed that HNO is one of the most critical species related to NO formation and NO is mainly generated through the path of NH2→NH→HNO→NO. For the turbulent flames, the flamelet/progress variable (FPV) approach was employed in the context of large eddy simulations (LES) for high-fidelity simulations. The simulation results were compared with the measured data with promising agreements, which proves the accuracy of the FPV method for the present flames. It was shown that with increasing cracking ratio, not only the flame reactivity is enhanced, but also the generation of NO is increased. The correlation between NO and HNO is weaker when compared to that between NO and radicals such as O, H and OH in the entire flame. Through the distribution of NO source terms, it was found that the NO source term has a higher absolute value in the upstream region and the absolute value rapidly decreases with increasing streamwise distance. The total NO source term is positive in the fuel-lean zone and shows negative values in the fuel-rich zone.

Turbulence effects on the formation and growth of nano-particles in three-dimensional premixed and non-premixed flames

This study examines the impact of turbulence on particle-forming flames via three-dimensional direct numerical simulations. The simulations employ the finite rate chemistry approach to simulate both premixed and non-premixed methane-air turbulent planar jet flames. These flames are doped with titanium tetraisopropoxide (TTIP) to form titanium dioxide TiO2 nanoparticles. The sectional model is employed to solve the population balance equation governing particle dynamics. Through these simulations, a number of the Batchelor scales pertaining to the smaller nanoparticle structures are effectively captured. The analysis conducted on these simulations is to identify and quantify the respective influences of diffusion, coagulation, and inception on variations in particle concentration across different regions within the flame. Several regions of the computational domain with different turbulence intensities are analyzed. Results show that the physical mechanisms that contribute to particle growth are not negligible on the particle concentrations. The impact of normal aN and tangential aT strain rates, which govern the thickness of particle-loaded zones affected by turbulence effects, is also evaluated. Conditional mean values of the strain rates upon the particle number concentration fields and the PDFs of aN and aT confirm that on the iso-surfaces of the particle fields the compressive and stretching effects are predominant. The aforementioned information is used to gain a deeper understanding of the influence of turbulence on premixed and non-premixed particle-forming flames, which will help us to develop transferable models for the simulation of nanoparticle synthesis.

Overall lift-off characteristics of CH4/H2 non-premixed jet flames in laminar/turbulent transition

The effects of adding hydrogen to methane were investigated through the transition between laminar and turbulent regimes based on the characteristics of non-premixed lifted flames. Lift-off heights can show complex trends depending on the fuel tube diameter, jet velocity, and hydrogen ratio. In the laminar regime, the lift-off heights were smaller than the laminar mixing core and decreased with hydrogen addition. In the turbulent regime, the lift-off heights could be much smaller or larger than the turbulent core when the jet velocities were smaller or larger than specific criteria, respectively. Such transitional phenomena were discussed based on the theories related to flow development, premixed flame, and edge flame structures. These included the mixing layer, quenching distance, flow re-direction, Damköhler number, turbulent flame speed, etc. Meanwhile, existing relationships between the lift-off heights and the flow velocities showed significant deviations when the hydrogen ratio was large. An improved relationship between the lift-off heights and the fuel jet velocities was proposed, considering the influence of the virtual origin and the flammable mixing layer. This relationship will apply to various fuel compositions such as propane, methane, hydrogen, and their mixtures. In addition, the blow-off mechanism was explained based on the turbulent burning velocity. Finally, the overall stabilization modes and mechanisms of the lifted flames were explained.

Flamelet/transported PDF simulations of ethylene/air jet turbulent non-premixed flame using a three-equation PAH-based soot production model

This article reports flamelet/transported PDF (TPDF) simulations of the well-documented ethylene/air turbulent non-premixed jet flame investigated experimentally at Sandia. The transported PDF equation is solved with the Stochastic Eulerian Field method. The soot production is modelled by a validated three equation PAH-based soot model that predicts the mean soot aggregate properties at low computational time and includes a detailed description of the soot production processes. Gas and soot radiation is modelled using the rank-correlated full-spectrum k model. The turbulence/chemistry/soot production/radiation interactions are taken into account by means of the PDF method. Simulations are run by considering or not soot differential mixing. Based on recent conclusions drawn from Direct Numerical Simulations (Zhou et al., Proc. Combsut. Inst. 38 (2021) 2731–2739), soot differential mixing is modelled by neglecting soot mixing owing to sufficiently large mixing timescales. When soot differential mixing is considered, model predictions reproduce reasonably well the exhaustive set of experimental data, including flame structure, soot statistics and radiative outputs without adjusting parameters. In particular, the predictions demonstrate for the first time the capability of RANS/TPDF models to capture the soot intermittency. On the other hand, neglecting the soot differential mixing produces notable reductions in mean and fluctuating soot volume fraction and soot intermittency. Scatter plot analysis shows that the effects of soot differential mixing are more pronounced in regions of the mixture fraction space where soot surface growth and soot oxidation dominate the soot production, affecting these processes in a non-negligible manner. In an opposite way, soot nucleation and PAH condensation are much less significantly affected. Model results show also that disregarding soot differential mixing reduces the mean soot emission as well the soot emission turbulence/radiation interaction.

An experimental study and analysis of lift-off length in inclined nonpremixed turbulent jet flames

The lifted flame behavior of inclined turbulent jets, considering the relative angle between the fuel jet momentum and flame buoyancy was investigated experimentally by varying the inclination angle of nonpremixed fuel jets. Variations of lift-off length from the flame base to the nozzle exit was quantified experimentally with nozzles of various diameters (2, 3, and 5 mm) and inclination angles (range of −90° to 90°). The data was analyzed based on the experimental finding of upstream preheating effect depending on inclination angles. Major findings are as follows: (1) The lift-off length (h) increases linearly with the increase in initial fuel jet velocity (ue) at a fixed inclination angle. The proportionality slope κ of the linear relationship between h versus ue decreases appreciably with jet inclination angle for the negatively inclined flames; while for the positively inclined flames, the lift-off length decreases relatively weakly. (2) Physical analysis on the flow characteristics of inclined jets was conducted, and the preheating effect was proposed based on the combustion behaviors, especially for the negatively inclined jet flames. The preheating temperatures of unburned fuel/air mixtures at the flame base and nozzle exit were experimentally quantified, revealing that the negatively inclination angle can have a significant influence on the preheating temperatures. (3) Based on the proposed preheating mechanism, a physical model accounting for the effect of jet inclination angle was developed to quantify the lift-off length of inclined jet flames. The proposed model successfully represented lift-off lengths at all the experimental conditions with various inclination angles and nozzle diameters. The present findings provide new data set and a reasonable physical model for lifted flame behavior of inclined turbulent jet flames, revealing the effect of the relative angle between fuel jet momentum and flame buoyancy.

Effect of hydrogen addition on combustion and thermal characteristics of impinging non-premixed jet flames for various heating value gases

This study experimentally investigates the effect of hydrogen addition on combustion and thermal characteristics of impinging non-premixed jet flames for low-heating values gases (LHVGs). We evaluate the flame morphology and stability using a concentric non-premixed combustor with an impingement plate. OH radicals are visualized using the OH* chemiluminescence and OH-planar laser-induced fluorescence (OH-PLIF) system. Emission characteristics are investigated by calculating CO and NOx emission indices. The results show that the flame stability region narrows as the heating value decreases but expands as hydrogen has been added. The low-OH radical intensity of LHVGs increases with the hydrogen addition. EICO and EINOx decrease with the reduction of heating values. EICO rapidly declines near the lifted flame limit due to the premixing of fuel and air downstream of the flame region. The effect of the hydrogen addition on EINOx is insignificant and shows very low emissions. The heat transfer rate into cooling water indicates a linear tendency with thermal power regardless of the fuel type. These findings show that LHVGs can be employed in existing-impinging flame systems so long as they remain within flame stability regions. Furthermore, hydrogen addition positively affects the expansion of flame stability, enhancing the utility of LHVGs.