Hot Coflow Research Articles

Numerical study of the multi-flame structure in spray combustion

We investigate the multi-flame phenomenon often seen in spray combustion. To do so, simulations are made of two cases from the Delft Spray in Hot Coflow (DSHC) database. These two cases have the same fuel (ethanol) but differ in the type of coflow: room temperature air (case AII) or products of lean premixed combustion (case HII), and they exhibit remarkably different flame structure, respectively with a ‘double flame’ and a ‘single flame’. The two-phase flow was handled with an Eulerian–Lagrangian approach. Large Eddy Simulation (LES) with Flamelet Generated Manifolds (FGM) method was used to account for the turbulence and combustion. Validation was done by using available experimental data as well as information on similar systems reported in literature. Major features of these two cases were correctly reproduced. Simulation results revealed that four spatially separated reaction regions exist in the flame AII and two in the flame HII. These regions are of different types, premixed or non-premixed, and are formed by different species, major fuel (ethanol) or intermediate species, e.g. carbon monoxide. The mechanism underlying the multi-flame structure was investigated and explained in terms of the relative magnitude of different time scales, namely of droplet evaporation, dispersion, convection and reaction. Parametric studies on the effect of spray polydispersity and coflowing air temperature were carried out, demonstrating an even wider range of flame structures. An important observation is that the ‘single flame’ structure usually present in the hot-diluted coflow case (HII), is also generated in a case of room temperature air coflow, provided the injected droplets are small. This is attributed to the fact that representative droplets in all cases with single flame have similar evaporation time scale. Or stated more generally, by matching important time scales, similar flame structure can be created under considerably different conditions.

Large Eddy Simulation of an ethanol spray flame under MILD combustion with the stochastic fields method

A combustion device operating in Moderate or Intense Low-oxygen Dilution (MILD) condition is numerically simulated. The burner consists of a cylindrical hot co-flow generator with an injector installed on the central axis. The hot co-flow is obtained by the lean combustion of Dutch Natural Gas (DNG). The spray is injected through an industrial pressure swirl atomiser using liquid ethanol as fuel. MILD combustion consists in burning the fuel in a high temperature environment so that the temperature gradients are limited and the production of pollutants such as NOx reduced. MILD combustion has been investigated for furnace applications and for various gaseous fuels, recently it has also been explored for spray combustion. The simulation is performed in the context of Large Eddy Simulation (LES) and the transported pdf equation for the scalars is solved using the stochastic fields approach. The validation of the results is based on the comparison with experimental data. The characteristics of the injector are obtained a posteriori by the use of the measurements at downstream location. The simulation correctly reproduces the velocity profiles of the particles within their size classes and the integral particle size distribution which is represented by the Sauter Mean Diameter (SMD). The gas phase temperature and velocity are also in good agreement with the measurements, however, some discrepancies are observed, presumably because of the lack of modelling of primary atomisation. The model employed appears to reliably reproduce the behaviour of the spray combustion system under MILD conditions.

The role of temperature, mixture fraction, and scalar dissipation rate on transient methane injection and auto-ignition in a jet in hot coflow burner

The transient injection and subsequent auto-ignition of a methane jet issuing into a laminar coflow of hot exhaust gas from a lean premixed hydrogen air flame was studied using high-speed planar Rayleigh scattering, yielding two-dimensional measurements of mixture fraction, temperature and scalar dissipation rate with high spatio-temporal resolution. The temporal development of the mixing field between the transient fuel jet and the surrounding coflow prior to the occurrence of auto-ignition was examined at a sampling rate of 10kHz. The impact of the transient jet development on numerical modeling of this test case is discussed. It was found that auto-ignition occurred after the jet transitioned from a transient state into the steady state, thus eliminating the need to model the complete transient fuel injection when the primary focus is on the onset of auto-ignition.Simultaneous high-speed OH* chemiluminescence from two viewing angles was applied to gain 3D-information of the ignition kernel location. This information allowed the selection and analysis of ignition events where the initial kernel formed inside the laser light sheet. Detailed analysis of the dynamics of a single ignition event, as well as statistical analysis of multiple ignition events based on a joint probability density approach, indicated that the ignition kernels occurred at very lean mixture fractions and at locations with low scalar dissipation rates.

Physical and Chemical Effects of CO2Addition on CH4/H2Flames on a Jet in Hot Coflow (JHC) Burner

Moderate or intense low-oxygen dilution (MILD) combustion is a promising technology for energy savings and pollutants emission reduction, and it has been experimentally shown that the MILD regime can be achieved more easily when using CO2 as diluent with respect to N2 [Li, P.; Combust. Flame 2013, 160 (5), 933−946]. Since CO2 is different from N2 in both physical and chemical properties, the present study aims at distinguishing the physical and chemical effects of CO2 addition on the establishment of MILD combustion. A jet in hot coflow (JHC) burner firing CH4/H2 blended fuel is numerically modeled coupled with a detailed chemical kinetics mechanism. Following the examination of grid independency and model validation, the differences in combustion temperature, minor and major species formations as well as the CH4 oxidation pathway are compared by, respectively, replacing N2 with CO2 or X (artificial CO2) in low-oxygen coflow. An interesting phenomenon is presented that the chemical effects of CO2 play a comparable role in the suppression of temperature rise to the physical effects. However, with the replacement of N2 by CO2, the contribution of chemical effects to temperature reduction is gradually weakened. Moreover, the chemical effects of CO2 are responsible for the ignition delay as well as the enhanced CO emission when is CO2 replacing N2, which is due to the inhibition of CH4 oxidation through Routes I (CH4 → CH3 → CH2(S)/CH2 → (CH → CH2O →) HCO → CO → CO2) and II (CH4 → CH3 → CH2O → HCO → CO → CO2) together with the enhanced CO2 dissociation by R99 (OH + CO ↔ H + CO2) and R153 (CO2 + CH2(S) ↔ CO + CH2O).

Inverse diffusion flame of CH4–O2 in hot syngas coflow

The structure and combustion mode of inverse diffusion flame of CH4 and O2 in hot syngas coflow are numerically studied to gain a fundamental understanding of the flame in non-catalytic partial oxidation (NC-POX) reformer. The configuration is modified based on the burner system of Cabra et al. [Combust. Flame2005, 143 (4), 491–506] to make the flame representative of that in NC-POX reformer. The Eddy Dissipation Concept (EDC) model with the detailed GRI 3.0 mechanism is used to model the turbulence–reaction interactions. Results of the study show that the flame is stabilized by autoignition with a wide reaction zone located far away from the stoichiometric line. Analyses on combustion mode show that the flame is established in Moderate and Intense Low-oxygen Dilution (MILD) mode. The inverse diffusion flame configuration which ensures a fully dilution of oxygen plays a key role in achieving MILD combustion in fuel rich coflow. The Increase of coflow temperature or decrease of jet velocity within the range of this study can lead to an early autoignition, but doesn't change the combustion mode.

Chemical kinetic effect of hydrogen addition on ethylene jet flames in a hot and diluted coflow

This paper numerically investigated the chemical kinetic effect of hydrogen addition, ranging from 0% to 60% (by vol.), on ethylene jet flames in a hot coflow. The Eddy Dissipation Concept model with the San Diego mechanism was used for all the calculations. To validate the present modeling, four flames were predicted under the experimental conditions of Medwell et al. [Combust. Flame 152 (2008) 100–113] and the predictions are found to agree quite well with the measurements. As the hydrogen content is higher, the jet entrainment and the jet velocity decay are enhanced, whilst the local equivalence ratio in the reaction zone is decreased. Moreover, under MILD condition, the hydrogen addition leads to remarkable increases in the mole fractions of H, O and OH radicals in the reaction zone, which then promotes the oxidation of C2H4 significantly. When hydrogen is added, the increasing rate of H mole fraction is greater than that of OH mole fraction. This hence makes the reactions attacked by H strengthened while those attacked by OH weakened. With respect to those at traditional air condition, the higher-carbon path (C2H4 → C2H3 → C2H2 → C2H, HCCO → CH2CO → CO → CO2) of the C2H4 oxidation at MILD condition becomes more important while the lower-carbon path (C2H4 → [C2H5→]CH3 → [S–CH2, T–CH2, CH3O, CH3OH, and CH2OH] → CH2O → HCO → CO → CO2 and C2H4 → [C2H5→]CH3 → [S–CH2 → T–CH2] → CO → CO2) is weakened. Further, under MILD condition at XO2∗ = 3%, the H2 addition weakens the importance of higher-carbon path but enhances that of the lower-carbon path, thus the C2H2 mole fraction is greatly reduced. Considering that C2H2 is an important precursor of soot, the decrease of C2H2 mole fraction and the local equivalence ratio indicate that H2 might have the potential to reduce the soot emission.

Extension of the Eddy Dissipation Concept for turbulence/chemistry interactions to MILD combustion

Over the past 30years, the Eddy Dissipation Concept (EDC) has been widely applied in the industry for the numerical simulations of turbulent combustion problems. The success of the EDC is mainly due to its ability to incorporate detailed chemical mechanisms at an affordable computational cost compared to some other models. Detailed kinetic schemes are necessary in order to capture turbulent flames where there is strong coupling between the turbulence and chemical kinetics. Such flames are found in Moderate and Intense Low-oxygen Dilution (MILD) combustion, where chemical time scales are increased compared with conventional combustion, mainly because of slower reactions (due to the dilution of reactants). Recent modelling studies have highlighted limitations of the standard EDC model when applied to the simulation of MILD systems, noticeably a significant overestimation of temperature levels. Modifications of the model coefficients were proposed to account for the specific features of MILD combustion, i.e. an extension of the reaction region and the reduction of maximum temperatures. The purpose of the present paper is to provide functional expressions showing the dependency of the EDC coefficients on dimensionless flow parameters such as the Reynolds and Damköhler numbers, taking into account the specific features of the MILD combustion regime, where the presence of hot diluent and its influence on the flow and mixing fields impacts on the reaction rate and thermal field. The approach is validated using detailed experimental data from flames stabilized on the Adelaide Jet in Hot Co-flow (JHC) burner at different co-flow compositions (3%, 6% and 9% O2 mass fraction) and fuel-jet Reynolds numbers (5000, 10,000 and 20,000). Results show promising improvement with respect to the standard EDC formulation, especially at diluted conditions and medium to low Reynolds numbers.

A novel plasma heater for auto-ignition studies of turbulent non-premixed flows

In this paper, the development and characterization of a novel test rig for auto-ignition (AI) studies of a fuel jet propagating into a hot turbulent co-flow is reported. The test rig, based on microwave plasma heating, is capable of achieving co-flow temperatures up to 1300 K and velocities up to 40 $$\hbox {ms}^{-1}$$ . Important boundary conditions at nozzle exit such as temperature, species, and velocity field were determined to prove the capabilities and limitations of the test rig. Liftoff height (LOH) measurements of $$\hbox {CH}_4$$ , $$\hbox {C}_2\hbox {H}_4$$ , and $$\hbox {CH}_{4}/\hbox {H}_{2}$$ jets, propagating into a turbulent heated air co-flow, were taken using chemiluminescence imaging. Effects of the temperature and Reynolds number (Re) of co-flow and jet were also studied. Results showed that the flame stabilization mechanism is supported substantially by AI rather than pure flame propagation. While the co-flow temperature dominates the AI process, the Re and temperature of the jet just have a small impact on the LOH.

Chemistry reduction for modelling flameless combustion of ethylene

Flameless combustion is a novel combustion technology able to ensure high combustion efficacies with low pollutant emissions thanks to the dilution of reactants, usually achieved through recirculation of combustion products. The technology has been successfully applied in several processes and has been found to be able to handle a large variety of fuels, including low grade fuels, industrial by-products and hydrogen. Further development of this innovative combustion technology would benefit of Computational Fluid Dynamics (CFD) tools; however, modelling flameless combustion is much more challenging than conventional flames, because of the strong coupling between turbulent mixing and chemical kinetics. In particular the chemical kinetics plays a fundamental role, even though there no common opinion on the degree a mechanism can be reduced. Some useful works may be found on flameless burners fed with methane, but there is lack of information on different fuels. The present work describes the numerical modelling of an ethylene jet flame issuing in a hot coflow burner, emulating flameless combustion and fully characterised in literature, with the scope of investigating the potential for chemistry reduction in the context of flameless combustion. A Principle Component/Variable Analysis is used to investigate the most important species in the chemical mechanism and subsequently a dimension reduction technique based on the Rate-controlled constrained-equilibrium (RCEE) principle is applied to the detailed mechanism to be coupled to the CFD code, in order to make simulations more affordable. Results indicated that the use of Principle Component/Variable Analysis leads to a good choice of the variables to be retained as results with the reduced scheme were found to be consistent with those obtained with the full mechanism.

3D DNS of MILD combustion: A detailed analysis of heat loss effects, preferential diffusion, and flame formation mechanisms

Moderate or intense low oxygen dilution (MILD) combustion is a relatively new technology which combines low emissions with high efficiency. As the name suggests, it requires high degrees of dilution in addition to preheating of the reactants using the combustion products. It has been applied to industrial burners for some time, but the complex physical mechanisms are yet to be resolved to extend the application to other areas. In this study, our aim is to analyze MILD combustion conditions to reveal the mentioned physical phenomena and their interactions. To this end, direct numerical simulations (DNS) in the form of autoigniting mixing layers are conducted, and the results are thoroughly analyzed. Conditions used in the simulations are taken from the jet in hot coflow experiments, which are designed to mimic MILD conditions. Detailed chemistry and transport models are employed in the numerical tools to fully understand the interactions of turbulence, molecular diffusion and chemical reactions. In addition, temperature variations due to heat loss effects in the experiments are taken into account. We have found that the heat loss and preferential diffusion effects are crucial in predicting not only the ignition delay, but also the flame structures and heat release rates. In addition, it is found that the flame formation is initiated by autoignition with different ignition delays along the most reactive mixture fraction, instead of a flame propagation following an initial autoignition spot. The findings of this study will broaden the knowledge on MILD combustion, and provide useful insight in developing reduced turbulence and chemistry models in the future.

Response of flame thickness and propagation speed under intense turbulence in spatially developing lean premixed methane–air jet flames

Direct numerical simulations of three-dimensional spatially-developing turbulent Bunsen flames were performed at three different turbulence intensities. The simulations were performed using a reduced methane–air chemical mechanism which was specifically tailored for the lean premixed conditions simulated here. A planar-jet turbulent Bunsen flame configuration was used in which turbulent preheated methane–air mixture at 0.7 equivalence ratio issued through a central jet and was surrounded by a hot laminar coflow of burned products. The turbulence characteristics at the jet inflow were selected such that combustion occured in the thin reaction zones (TRZ) regime. At the lowest turbulence intensity, the conditions fall on the boundary between the TRZ regime and the corrugated flamelet regime, and progressively moved further into the TRZ regime by increasing the turbulent intensity. The data from the three simulations was analyzed to understand the effect of turbulent stirring on the flame structure and thickness. Statistical analysis of the data showed that the thermal preheat layer of the flame was thickened due to the action of turbulence, but the reaction zone was not significantly affected. A global and local analysis of the burning velocity of the flame was performed to compare the different flames. Detailed statistical averages of the flame speed were also obtained to study the spatial dependence of displacement speed and its correlation to strain rate and curvature.

Routes of formation and destruction of nitrogen oxides in CH4/H2 jet flames in a hot coflow

The formation/destruction mechanisms of nitric oxides (NOx) in CH4/H2 jet flames in a hot coflow are systematically investigated by numerical modeling. All calculations use the Eddy Dissipation Concept (EDC) model coupled with GRI-Mech 2.11. The modeling is validated by the measurements of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]. Dependences of the NOx formation/destruction routes on YO2∗, Tcof∗ and YH2∗ are examined, where YO2∗, Tcof∗ and YH2∗ are the coflow oxygen concentration and temperature, and the hydrogen fraction in the fuel mixture, respectively. The following is obtained.For YO2∗≤3% at Tcof∗=1300K and YH2∗=12%, the N2O-intermediate, prompt and NNH routes are the first (≈50%), second (30%) and third (20%) contributors to the total NO emission; as YO2∗ is increased, the contributions of the three routes all decrease whereas that of the thermal route becomes significant. For YO2∗=3% and YH2∗=12%, the N2O-intermediate route dominates the NO formation (about 90%) at Tcof∗<1000K; as Tcof∗ is increased from 1000 K, the importance of either the prompt, NNH, or thermal routes is enhanced. For YO2∗=3% and Tcof∗=1300K, as YH2∗ is elevated, the contribution of the NNH route to the total NO production increases while those of the N2O-intermediate and prompt routes decrease. Also, in general, the reduction of YO2∗, Tcof∗ or YH2∗, or altogether is found to strengthen the importance of NO2 to the total NOx emission and the NO-reburning.

Reduced NO formation models for CFD simulations of MILD combustion

Two reduced kinetic models, incorporating thermal, N2O, NNH as well as HNO/NO2 intermediate routes, are proposed for the quick evaluation of NO emissions from MILD combustion of H2-enriched fuels through post-processing of Computational Fluid Dynamics simulations. The models were derived from a Rate Of Production Analysis carried out with two different detailed kinetic schemes. The models were tested using data from the Adelaide Jet in Hot Coflow burner fed with CH4/H2 mixture and operated with three different O2 contents. Very satisfactory predictions of in-flame NO measurements were achieved for the three cases, indicating a good applicability of the models across a wide range of MILD combustion conditions. Significant impact of the NNH intermediate path was observed.

Modeling Lifted Jet Flames in a Heated Coflow Using an Optimized Eddy Dissipation Concept Model

Moderate or intense low oxygen dilution (MILD) combustion has been established as a combustion regime with improved thermal efficiency and decreased pollutant emissions, including NOx and soot. MILD combustion has been the subject of numerous experimental studies, and presents a challenge for computational modeling due to the strong turbulence–chemistry coupling within the homogeneous reaction zone. Models of flames in the jet in hot coflow (JHC) burner have typically had limited success using the eddy dissipation concept (EDC) combustion model, which incorporates finite-rate kinetics at low computational expense. A modified EDC model is presented, which successfully simulates an ethylene-nitrogen flame in a 9% O2 coflow. It is found by means of a systematic study in which adjusting the parameters and from the default 0.4082 and 2.1377 to 3.0 and 1.0 gives significantly improved performance of the EDC model under these conditions. This modified EDC model has subsequently been applied to other ethylene- and methane-based fuel jets in a range of coflow oxidant stream conditions. The modified EDC offers results comparable to the more sophisticated, and computationally expensive, transport probability density function (PDF) approach. The optimized EDC models give better agreement with experimental measurements of temperature, hydroxyl (OH), and formaldehyde (CH2O) profiles. The visual boundary of a chosen flame is subsequently defined using a kinetic mechanism for OH* and CH*, showing good agreement with experimental observations. This model also appears more robust to variations in the fuel jet inlet temperature and turbulence intensity than the standard EDC model trialed in previous studies. The sensitivity of the newly modified model to the chemical composition of the heated coflow boundary also demonstrates robustness and qualitative agreement with previous works. The presented modified EDC model offers improved agreement with experimental data profiles than has been achieved previously, and offers a viable alternative to significantly more computationally expensive modeling methods for lifted flames in a heated and vitiated coflow. Finally, the visually lifted flame behavior observed experimentally in this configuration is replicated, a phenomenon that has not been successfully reproduced using the EDC model in the past.

Numerical study of flame structure in the mild combustion regime

In this paper, turbulent non-premixed CH4+H2 jet flame issuing into a hot and diluted co-flow air is studied numerically. This flame is under condition of the moderate or intense low-oxygen dilution (MILD) combustion regime and related to published experimental data. The modelling is carried out using the EDC model to describe turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI2.11 full mechanism are used to represent the chemical reactions of H2/methane jet flame. The flame structure for various O2 levels and jet Reynolds numbers are investigated. The results show that the flame entrainment increases by a decrease in O2 concentration at air side or jet Reynolds number. Local extinction is seen in the upstream and close to the fuel injection nozzle at the shear layer. It leads to the higher flame entertainment in MILD regime. The turbulence kinetic energy decay at centre line of jet decreases by an increase in O2 concentration at hot Co-flow. Also, increase in jet Reynolds or O2 level increases the mixing rate and rate of reactions.

Multi-environment probability density function approach for turbulent CH4/H2 flames under the MILD combustion condition

The multi-environment probability density function approach has been applied to simulate turbulent CH4/H2 flames under moderate or intense low-oxygen dilution (MILD) conditions. In order to circumvent excessive computational burden, the direct quadrature method of moments (DQMOM) has been adopted as an alternative approach to solve the transported probability density function (PDF) equation. The multi-environment PDF approach has the form of a conventional Eulerian scheme and retains the desirable property of a particle-based method. In this study, the joint-composition PDF is approximated using the two-environment PDF, expressed via the combination of weights and abscissas on the composition and physical space. Micromixing is represented by the IEM model, and the chemical mechanism is based on detailed chemistry. In terms of the unconditional means and conditional statistics for temperature and mass fractions of species and pollutants, the predicted profiles are in reasonable agreement with experimental data. Numerical results clearly indicate that the two-environment PDF transport model has the capability to realistically predict the effects of oxygen mass fraction on the flame lift-off, auto-ignition, flame structure, and NOx formation characteristics in turbulent CH4/H2 jet flames issuing from a jet in hot coflow (JHC). It was also found that the two-environment PDF approach reproduces the sensitivity of the CO and CO2 peak levels versus the O2 concentration variation, as well as the peak levels of NO conditional fluctuations under the MILD combustion condition. Even if considerable discrepancies in the unconditional means exist mainly due to inconsistent treatment of weight combination and shortcomings of singularity treatment procedure, the present three-environment PDF model well captures the measured high-level conditional temperature and CO fluctuations on the fuel-lean side where the local extinction occurs via mixing of the shrouded cold air and the vitiated flame field.

Characterization of atomization and combustion in moderately dense turbulent spray flames

This paper presents a study of spray jets and flames where the liquid fuel loading in the carrier air is such that there are substantial liquid–liquid interactions close to the exit plane. The burner consists of an air-blast atomizer located within a wide hot co-flow used to stabilize turbulent auto-igniting spray flames. Laser/phase Doppler anemometry, microscopic backlit imaging coupled with advanced image processing, broadband chemiluminescence and OH-PLIF imaging are utilized. A key focus here is on a novel characterization of the spray boundary condition in terms of non-spherical shapes of fluid fragments. Three different classes of shapes, namely: ligaments, droplets, and large, generally irregular objects are examined. Statistics for each of these fragments are computed in a range of sprays and it is found that their size and probability of occurence depends on the initial Weber number and fuel/air mass ratio (F/A). The change in chemiluminescence emission that is measured as a function of the F/A ratio trends in the same manner as the change in the size of the largest non-spherical objects in the spray. In addition, it is found that changes in the growth of reaction zone width occur as estimated from OH images of laser induced fluorescence, and this is also controlled by the F/A ratio. Therefore, the spray structure at the exit plane may partly dictate the downstream flame characteristics; and this could be largely due to the presence of non-spherical fragments and hence the different rates of atomization and vapourization amongst the different sprays. The burner, as well as the classification of the spray introduced here, while complex, can form a platform for the improvement of models for moderately dense reacting sprays.

A multiple-inlet mixture fraction model for nonpremixed combustion

The conserved scalar or mixture fraction approach was originally derived for two-feed systems with one oxidizer stream and one fuel stream. Under the assumptions of unity Lewis number for all chemical species and sufficiently large values of the Damköhler number, it is known to provide a widely used and very efficient basis for the calculation of turbulent diffusion flames, which may be handled through the single knowledge of the mixture fraction statistics. The mixture fraction variable is also central to steady laminar flamelet representations, which provide a common building block to a variety of modelling proposals for turbulent combustion in nonpremixed and partially premixed situations. However, as soon as dilution by a third inlet stream comes into play, the corresponding formalisms, which rely on the mixture fraction concept, should be revisited. This restriction occurs, for instance, when an additional stream of oxygen-enriched air is introduced with the purpose of either achieving high temperature levels or favoring flame stabilization. Such a situation clearly exceeds the scope of the standard conserved scalar framework. The present study is aimed at extending the two-inlet mixture fraction basis to the consideration of such multiple-inlet injections. For this purpose, we introduce the notion of a fictive (or notional) injector, whose characteristics are reconstructed from the transport of either (i) inlet tracers or (ii) associated compositions of each inlet. The performance of the proposed approach is illustrated through its application to moderate and intense low-oxygen dilution (MILD) combustion of a jet in a hot coflow (JHC) burner. Comparisons performed between measurements and numerical simulations confirm that the proposed framework is able to render the third inlet influence, as well as the effects induced by variations of oxygen concentration in the hot coflowing stream. The proposed approach thus offers a simple way to extend the use of a wide variety of available turbulent combustion closures, which are based on the mixture fraction, to practical conditions featuring more than two feeding streams.

The role of precursors on the stabilisation of jet flames issuing into a hot environment

This paper seeks to address unusual flame stabilisation behaviour observed in experimental jet flames which issue into a hot coflow. It has been observed that increasing the temperature and/or oxygen concentration in the coflow can lead to an increase in flame liftoff height. The paper isolates the role of chemistry, and in particular flame intermediates, on the observed phenomenon with a view to better understand how the behaviour changes over a range of conditions. A descriptive theory for this behaviour is proposed, which is based on the well-established theory that a build-up of radicals and intermediate species is responsible for autoignition of these flames. This paper systematically examines the role of these precursors with a view to better understanding of the chemical kinetics and to assess if the observed behaviour is chemistry-dominated. To this end, laminar flame calculations and ignition delay curves are presented, and the findings are validated with experiments. The results indicate that chemical effects alone are insufficient to fully explain the observations, but the calculations support the general trends noted in the experiments and highlight the importance and relative effects of some key precursors. In particular, the production and consumption of formaldehyde in a low oxygen environment supports the unusual flame behaviour observed experimentally.

Experimental and numerical studies of the effects of hydrogen addition on the structure of a laminar methane–nitrogen jet in hot coflow under MILD conditions

Abstract In this work we investigate the effects of hydrogen addition on the flame structure of MILD combustion both experimentally and numerically using a laminar-jet-in-hot-coflow (LJHC) geometry. The addition of hydrogen appreciably decreases the flame height (∼25%), however only modestly affects the maximal flame temperature and the thickness of combustion zone. The NO distribution is dominated by mixing of the NO formed in the coflow with the reaction products of the diluted fuel, with negligible NO formation from the fuel in all flames studied. The numerical data are in reasonably good agreement with the measurements.