Moderate Or Intense Low-oxygen Dilution Combustion Conditions Research Articles

Combustion stage configurations for intercooled regenerative reheat gas turbine systems

Low-power gas turbines have been widely used for decades as combined heat and power. In the recent years, they received increasing interest with respect to applications such as range extenders in the automotive sector and for alternative fuels use.In this framework, the present work analyzes the combustion stages optimization of an intercooled regenerative reheat gas-turbine system, for a low power gas turbine. In particular, a thorough analysis was carried out to identify the optimal combustion system configuration in order to ensure suitable turbine inlet temperature and global efficiency of the thermodynamic cycle higher than 40%. Starting from the typical intercooled regenerative reheat gas turbine system, several configurations were analyzed. In particular, air bypass systems were introduced as suitable strategy able to tune the combustion chamber working temperatures and turbine inlet ones, according to operating requirements and combustion unit specifications. Furthermore, different bypass ratios were considered, analyzing the effectiveness of air bypass systems in terms of efficient combustion and reduced pollutant emissions (CO and NOx).An innovative cyclonic flow burner, assumed to operate under MILD (Moderate or Intense Low oxygen Dilution) combustion conditions, was considered as main combustion unit and in the reheat process. In this respect, the choice was motivated by the well-established high fuel flexibility and very low pollutants emission typical of such a combustion unit and the MILD process.Analyses were performed considering methane as fuel, which is currently the most used fuel in land-based applications. However, results here reported have general validity and may be directly applied with respect to the use of innovative energy vectors.

Four-step global kinetics mechanism for diluted combustion fueled with kerosene

The aim of the present study was to develop a global kinetics mechanism capable of predicting characteristics of the combustion of kerosene diluted with combustion gases. A four-step model of the Aachen mechanism was developed via modification of a mechanism for lower molecular weight hydrocarbons. A simplified fuel was used to simulate kerosene, the thermodynamic properties of which were estimated based on the surrogate fuel in the Aachen mechanism. Combustion of this model fuel in a perfectly stirred reactor (PSR) operating under high-temperature air combustion (HiTAC) conditions was simulated using inflow mixtures preheated and diluted with nitrogen. In this new mechanism, the activation energy for the fuel consumption reaction and the pre-exponential factor for the hydrogen oxidation reaction were adjusted. The resulting model accurately predicted variations in temperature and major species over time. The four-step mechanism was also validated under conventional flame and moderate or intense low oxygen dilution (MILD) combustion conditions. The PSR simulations with the four-step mechanism accurately predicted variations in temperature and major species under MILD combustion conditions. In addition, this four-step mechanism was confirmed to be superior to other global mechanisms with regard to predicting the mole fractions of various species. Experimental trials were carried out with diluted kerosene flames formed above parallel jet burners composed of a fuel spray nozzle and an oxidizer nozzle, and it was found that NOx emission decreased with increasing distance between nozzles even for spray combustion. These diluted flames were numerically simulated using the eddy dissipation concept model combined with the four-step mechanism. The temperature distributions obtained from the simulations showed good agreement with the experimental data in the downstream region.

Characterisation of hydrogen jet flames under different pressures with varying coflow oxygen concentrations

The elevated temperature of hydrogen combustion increases the formation of thermal NOx. Moderate or intense low oxygen dilution (MILD) combustion is known to reduce NOx emissions and increase thermal efficiency. Pressure is often also used for increasing thermal efficiency. The impact that pressure has on fluid dynamics and chemical kinetics is especially relevant in MILD combustion conditions. Hydrogen jet flames issuing into a hot and vitiated coflow were imaged using OH∗ chemiluminescence at different pressures (1–7 bar) and oxygen levels (3–9% by vol.). Laminar flame simulations complemented the experiments. The observed mean radial OH∗ width increased with increased pressure, but only at O2 content less than 9%, suggesting that pressure has greater influence on kinetics when oxygen is reduced. The integrated OH∗ signal strength remained constant at 3% coflow O2, despite an apparent increase in flame width, suggesting a spatial broadening of the flame with pressure. Numerical results indicate that at 3–6% O2, conditions for MILD combustion of H2 are met across a wide range of strains and pressures, supporting the experimental observations for 3% O2.

Characterisation of ethylene flames under a range of low-oxygen concentrations

New understanding of turbulent ethylene (C2H4) jet flames, issuing into a range of preheated coflowing oxidisers with reduced oxygen (O2) concentrations are reported. These conditions emulate moderate or intense low-oxygen dilution (MILD) combustion. To resolve previously reported non-monotonic trends, comparisons are made for coflow O2 concentrations of 3%, 4%, 5%, 6%, 9% and 11% and coflow temperatures of 1250K, 1315K and 1385K. Instantaneous and simultaneously planar imaging measurements of temperature, hydroxyl radicals (OH) and formaldehyde (CH2O) were taken at eight downstream locations ranging from 9mm to 75mm. The new data reveal non-monotonic lift-off trends in the OH and CH2O formation heights at all three temperatures. Compared with extant measurements, the higher resolution and greater control of coflow composition provide a detailed exploration of the transitional behaviour that occurs when MILD combustion conditions are achieved. The results experimentally demonstrate non-monotonic variation in lift-off height, and show that the same behaviour is present in both the OH and CH2O formation heights, and occurs at all three temperatures, which is due to the movement of the location of the stoichiometric and most reactive mixture fractions to the coflow side of the jet shear layer. The results also show that increasing the oxidiser temperature does not have a significant effect on the OH number density for a given temperature but does result in a significant decrease in lift-off height and CH2O concentration.

MILD Combustion and Biofuels: A Minireview

Even in the presence of the formidable growth of renewable energy sources, advanced combustion processes have and will continue to have an irreplaceable role as a driving force for energy market. This is indeed motivated by the same huge amount of energy that can be conveniently stored in the form of energy carriers, made available from the renewable sources. The coupling of the appropriate combustion conversion technology with locally available biofuels certainly is among the most effective solutions, now on the table, to face the decarbonization challenge. In this framework, moderate or intense low oxygen dilution (MILD) combustion is among the best candidates to support the transition toward the net zero emission target. Indeed, due to its inherent features, it is highly fuel flexible and efficient, allowing for the utilization of whatever energy carrier can be considered in the decarbonization strategy. It is also characterized by an inherent low or absent pollutant emission. The novelty of this minireview is to highlight the relevance of the kinetics involved in MILD combustion processes with particular focus on biofuels, identifying invariant temperatures, relevant for process stabilization for any energy carrier. A critical evaluation of advantages and drawbacks of the use of raw bioliquids in MILD combustion conditions is reported. Then, challenges, open questions, and future perspectives on the use of biofuel in MILD combustion are discussed.

Identification of combustion mode under MILD conditions using Chemical Explosive Mode Analysis

Direct Numerical Simulations (DNS) data of Moderate or Intense Low-oxygen Dilution (MILD) combustion are analysed to identify the contributions of the autoignition and flame modes. This is performed using an extended Chemical Explosive Mode Analysis (CEMA) which accounts for diffusion effects allowing it to discriminate between deflagration and autoignition. This analysis indicates that in premixed MILD combustion conditions, the main combustion mode is ignition for all dilution and turbulence levels and for the two reactant temperature conditions considered. In non-premixed conditions, the preponderance of the ignition mode was observed to depend on the axial location and mixture fraction stratification. With a large mixture fraction lengthscale, ignition is more preponderant in the early part of the domain while the deflagrative mode increases further downstream. On the other hand, when the mixture fraction lengthscale is small, sequential autoignition is observed. Finally, the various combustion modes are observed to correlate strongly with mixture fraction where lean mixtures are more likely to autoignite while stoichiometric and rich mixtures are more likely to react as deflagrative structures.

Burning characteristics of grape marc under mild combustion conditions

In this experimental study, the burning characteristics of grape marc, as a biomass fuel, under MILD (Moderate or Intense Low-oxygen Dilution) combustion conditions are explored. Specifically, the impact of grape marc particle size on flame structure, stability and pollutant emissions, under MILD combustion, is investigated. Three different particle sizes of the grape marc are considered, in the range of 150–250 μm, 250–355 μm and 355–500 μm. The particles are injected through a central insulated jet into a hot and vitiated coflow inside a vertical MILD combustion furnace. All other operating conditions remain the same. CO2 is employed as a carrier gas and the central jet bulk Reynolds number is set to the value of Rejet = 13,040. The vitiated coflow had ~6% oxygen molar fraction and a flow bulk temperature of about Tco-flow = 1330 K. The heat input from the grape marc fuel is kept constant at 10 kW, corresponding to a mass flow rate of 1.63 kg/h. Stable MILD combustion conditions have been successfully achieved for all cases. The temperature gradient along the vertical and the radial direction of the furnace is limited to below 100 K for all the cases. The variation of particle size does not significantly impact on the furnace temperature and exhaust emissions. Albeit, the case of largest particle size exhibits a higher temperature by ~40 K as compared to the other two cases, and the highest NO production is identified for the middle particle size's case showing peak value of approximately 478 ppm at the exhaust at 3% excess O2. No remarkable NO-reburning process is observed for any cases, which points towards the intense volatile combustion of grape marc under MILD combustion conditions.

Effect of co-flow oxygen concentration on the MILD combustion of pulverised coal

This paper reports the impact of surrounding oxygen (O2) concentrations on the burning characteristics of pulverised brown coal under MILD (Moderate or Intense Low-oxygen Dilution) combustion conditions. A combined experimental and computational study is conducted to probe the volatiles' release and reactions from the micro sized coal particles and its effect on the production of CO, CO2 and NOx in a vertical furnace. Pulverised high volatile Victorian brown coal was introduced into the furnace utilising CO2 as a carrier gas through a central jet with a constant bulk jet Reynolds number Rejet = 20,000, and two co-flow oxygen concentration of 5.9% and 8.9%. For all the cases investigated, stable MILD combustion was achieved, featuring a uniform temperature distribution within the furnace. It was found that the co-flow O2 concentration significantly influences the CO emission. The measured CO emission for the 5.9% co-flow O2 concentration case is approximately eight times higher than that of the 8.9% co-flow O2 concentration case. In addition, the CFD analysis showed that an increase in the local O2 concentration leads to an increase in the volatiles release and reaction rates. Also, up to 63% more NO is produced for the 5.9% co-flow O2 concentration case in comparison with the 8.9% O2 case through the fuel-NO route.

A Refined Global Reaction Mechanism for Gently Preheated MILD Combustion of Methane

Moderate or intense low-oxygen dilution (MILD) combustion is a promising technology for simultaneously reducing NOx emission and improving thermal efficiency. To apply MILD combustion in industrial production, computational fluid dynamics (CFD) modeling plays an efficient role in design and optimization. To reduce the computational time while maintaining the predictive accuracy, a valid global reaction mechanism (GRM) is necessary. In this paper, an accurate and robust GRM was proposed for methane under MILD combustion conditions, especially without the condition of highly preheated air. The adequacy of the proposed GRM was first compared with experimental data from a bench-scale MILD combustion furnace, where ambient-temperature air was used. Subsequently, experimental data from an industrial-scale MILD combustion furnace, where air was slightly preheated to 130 °C, was employed to further validate its capability for practical application. As compared to the previous GRMs, the present proposed GRM exhibi...

Ignition features of methane and ethylene fuel-blends in hot and diluted coflows

Turbulent flames in the moderate or intense low oxygen dilution (MILD) combustion regime have previously exhibited less susceptibility to lift-off than conventional autoignitive flames in a jet-in-hot-coflow (JHC) burner. This has been demonstrated through laser-based diagnostics and examination of CH* chemiluminescence. New experimental observations are presented of turbulent flames of natural gas, ethylene and blends of the two fuels, in coflows with temperatures from 1250–1385K and oxygen concentrations from 3–11% (vol./vol.). Zero- and one-dimensional simulations, as well as turbulent flame modelling, are used to explain the trends seen experimentally with different coflows and fuels. Numerical simulations using simplified batch reactors and opposed-flow flames demonstrate that blending of methane and ethylene fuels is most significant near 1100K. Near this temperature, pure ethylene exhibits a transition between high and low temperature ignition pathways. Further analyses show that a 1:1 methane/ethylene blend behaves more like ethylene near MILD combustion conditions, and more like methane in conventional autoignition conditions. Two-dimensional modelling results of the turbulent flames are then discussed and explained in the context of the simplified reactor results. The flames confined by the lean flammability-limit in the coflow and high strain-rates in jet shear layer, in agreement with previous work using a semi-empirical jet model. The two-dimensional modelling is additionally able to qualitatively replicate the trends in lift-off height, with normalised heat release rate profiles reproducing, and serving to explain, the effects seen in experimental campaigns.

Selection of appropriate constraints for dimension reduction in MILD combustion simulations via RCCE

Modeling MILD (Moderate or Intense Low-oxygen Dilution) combustion is well known to be computationally demanding due to the need of an accurate description of the chemistry, because the low Damköhler number. The present work investigates the potentials of Rate-Controlled Constrained Equilibrium for the dimension reduction of kinetic schemes for a burner fed with a mixture of ethylene and air emulating MILD combustion conditions. A method based on the Computational Singular Perturbation theory is proposed for the identification of the species to be retained in the reduced representation of the chemistry. A significant reduction was achieved by using only 10 species (out of 34 species in the full kinetic scheme) and 3 elements; the results from the reduced model were in very good agreement with those of the full kinetic scheme. The sensitivity of predictions to the turbulence and combustion models was also preliminary assessed, indicating the need of modifying the residence time constant of the Eddy Dissipation Concept for MILD combustion conditions.

Numerical investigation of the ignition and annihilation of CH4/N2/O2 mixtures under MILD operative conditions with detailed chemistry

The identification of controlling processes on the micro-scale level is critical in the elaboration of effective models with particular regard to MILD (moderate or intense low-oxygen dilution) combustion modelling. The objective of this study is to determine relevant features and controlling mechanisms in the ignition, interference and annihilation of diffusion layers in MILD combustion conditions. The two interfaces between the mixing layers are sufficiently close to cause interference phenomena. The inner region between the two interfaces is assumed to be air to mimic the engulfed air in a fuel jet. The temporal evolution has been analysed in dependence on the initial oxidant layer width and temperature; evidencing possible interactions between the two mixing layers. Interactions can be categorised according to the ignition and annihilation stages. Concerning the ignition, the minimum ignition delay for interfering double diffusive layers is significantly shorter than the corresponding isolated one. However, the ignition exhibits the same most reactive mixture fraction, and the ignition delay dependence on the oxidant layer width behaves similar to a stratified charge condition for the same most reactive mixture fraction. In the ignition region, the scalar dissipation rate is reduced due to expansion toward fuel sides due to heat release. The annihilation time delay scales with the initial layer thickness according to the canonical diffusion equation and does not significantly depend on the initial oxidant temperature. Oxygen depletion during annihilation limits oxidation in the rich side mixture, which produces a final oxidation level near equilibrium values.

Burning characteristics of Victorian brown coal under MILD combustion conditions

In this work a vertical furnace is used to investigate the MILD (Moderate or Intense Low-oxygen Dilution) combustion characteristics of pulverized Victorian brown coal. This paper reports the effect of turbulence on the volatiles’ release and reactions under vitiated co-flow conditions as well as the impact on the pollutants’ formation and emission. Loy-Yang brown coal from the Latrobe Valley, Victoria, Australia, with particle sizes in the range of 53–125µm, is injected into the furnace using CO2 as a carrier gas through an insulated and water-cooled central jet. The bulk jet Reynolds number was varied from Rejet = 5527 to Rejet = 20,000. The furnace walls as well as co-flow temperature and local oxygen concentrations are controlled by a secondary swirling burner using non-premixed natural gas combustion. The co-flow in the furnace was operated with an O2 concentration of 5.9% (db by volume). Detailed measurements of in-furnace temperatures and chemical species are presented and discussed, together with visual observations and CH chemiluminescence (CH*) imaging at the bottom, middle and top parts of the furnace. The CH* signal intensity is found to be significantly lower at the top part of the furnace which is an indication of slow rate of the heterogeneous combustion of char particles. The largest amount of CO concentrations are measured for the highest jet velocity (i.e., Rjet = 20,000) case which implies that with increasing turbulence there is a better mixing and a broad devolatilization zone is formed which produces more CO. The measured NO emission for any case was less than 125 ppmv (db at 3% O2) which provides evidence to the potential benefits of MILD combustion application to Victorian brown coal towards reducing NO emission. Complementary CFD model helped in shedding light on the flow field, turbulence intensity, volatiles’ release rate, combustion of volatile matters, and overall carbon consumption inside the furnace for the three cases. It was found that increasing the jet Reynolds number increases the volatiles release rates and decrease the rate of overall carbon consumption.

Numerical Study of Pulverized Coal MILD Combustion in a Self-Recuperative Furnace

A numerical study of pulverized coal combustion under Moderate or Intense Low oxygen Dilution (MILD) combustion conditions is presented in a parallel jet self-recuperative MILD combustion furnace. The Reynolds-Averaged Navier–Stokes equations, in a three-dimensional axisymmetric furnace domain, were solved using the Eddy Dissipation Concept model for the turbulence–chemistry interaction. The main aim of this study is to understand the influence of devolatilization models on the prediction accuracy of pulverized coal combustion under MILD combustion conditions. In particular, three devolatilization models are analyzed: a conventional single-rate model, a two-competing-rates model, and an advanced chemical percolation devolatilization (CPD) model based upon structural networks of the coal with a global kinetics mechanism. In addition, a new simplified numerical model is developed for Australian black and/or hard coal and optimized for the MILD combustion conditions. The modeling cases are validated with the experimental results of MILD combustion of high volatile Australian brown coal from Kingston using N2 as a carrier gas and low volatile Australian black coal from the Bowen basin using CO2 as a carrier gas in a MILD combustion furnace. From the comparison, the advanced CPD devolatilization model with a three-step global kinetic mechanism gives, as expected, the best agreement with the experimental measurements for the Kingston brown coal case, while all the models tested provide similar results for the Bowen basin black coal case. The heterogeneous reactions on the char burnout rate for pulverized coal combustion under MILD combustion conditions are presented and discussed. The characteristics of three NO formation mechanisms, namely, thermal-NO, prompt-NO, and fuel-NO, together with the NO reduction mechanism, named NO-reburning, are evaluated using the postprocessing modeling approach. The results reveal that the sum of contributions from the thermal-NO and prompt-NO routes only accounts for less than 3.8% to the total NO emissions, while the fuel-NO route dominates the NO emissions under MILD combustion conditions. It is also found that the total NO emission is reduced by up to 47% for the brown coal case and up to 39% for the black coal case via the reburning mechanisms.

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.

Kinetic and fluid dynamic modeling of ethylene jet flames in diluted and heated oxidant stream combustion conditions

This paper reports the results of a numerical investigation of several turbulent nonpremixed ethylene jet flames, either undiluted or diluted with hydrogen, air or nitrogen, which have been experimentally studied using a jet in hot coflow (JHC) burner. The fuel jet issues into a hot and highly diluted coflow at two O2 levels (3% or 9%) and a temperature of 1100 K. These conditions emulate those of moderate or intense low oxygen dilution (MILD) combustion. The attention is mainly focused on assessing the performance of different models for the turbulence–chemistry interaction. The model predictions are compared to experimental measurements. The Eddy dissipation concept (EDC), flamelet and PDF transport models were used in combination with a two-equation turbulence model (k–ε) and two different kinetic mechanisms (GRI-Mech 3.0 and POLIMI). The effect of the oxygen concentration in the coflow is well captured by all models. A modified version of the EDC model, recently proposed for the modeling of methane MILD combustion conditions, is also used. This model significantly improves the predictions of the EDC in the case of the ethylene flames studied in this paper. The agreement between measurements and predictions of the flamelet model is generally not as good as the PDF transport and modified EDC models, whose predictions are in good agreement with the measurements, and are improved especially for the apparent liftoff heights and the peak flame temperature. The effect of the fuel composition on the CH2O formation is also discussed, showing generally good agreement.

MILD combustion in diffusion-controlled regimes of Hot Diluted Fuel

Reactive structures have been characterized in a steady laminar, unidimensional mixing layer on a dense grid of parameters in moderate or intense low-oxygen dilution (MILD) combustion conditions with hot and diluted fuel. The structures have been studied in terms of temperature and heat release profiles in a mixture fraction space for various ranges of stretch rates and for two reference pressures (1 and 10bar) using a standard code and standard kinetic scheme.In the analysis of reactive structure three synthetic characteristics have been pointed out in previous works as discriminative for the occurrence of different combustion regimes. Such characteristics are the thickness of the oxidation structures, the presence/absence of a pyrolysis region and the correlation/no correlation of the regions of maximum heat release with those at which the mixture is stoichiometric. Following the same criteria, maps of regimes have been built up on a inlet fuel temperature – temperature increase plane for fixed stretch rates and different pressures.It has been pointed out that in diffusion controlled regimes of Hot Diluted Fuel, analysed in this paper, MILD combustion conditions are characterized by flame thickening and pyrolysis depression, which are also typical of flameless combustion. In addition, the region of maximum heat release has a generally high level of correlation with the stoichiometric regions. Thus, MILD combustion regime differs from MILD combustion regime found in other diffusion controlled regimes where the region of maximum heat release is generally not correlated with the stoichiometric regions. In the case presented in this paper, only a second solution, corresponding to a very low conversion occurring in a very wide stretch-rate range, shows no correlation between heat release and stoichiometric mixture fraction. This behavior has been attributed to the oxidative pyrolysis of methane.At atmospheric pressure, the flameless characteristics extend to a low level of preheating, provided that the fuel is diluted, with a consequently low level of temperature increase, thus confirming that MILD and flameless regimes are not coincident in all possible feeding conditions.In general, the results obtained in these Hot-Fuel-Diluted-Fuel conditions are consistent with and extend those reported in the literature for Hot-Oxidant-Diluted-Fuel, Hot-Oxidant and Diluted-Fuel conditions, supporting the assertion that these inlet parameters are a suitable choice for the definition of MILD combustion.

Reaction Zone Weakening Effects under Hot and Diluted Oxidant Stream Conditions

This paper reports computational results, to complement experimental observations, on the turbulence–chemistry interaction of nonpremixed jet flames issuing into a heated and highly diluted oxidant stream. It is found experimentally that large-scale vortices and flame stretch can lead to spatial thinning and a decrease in OH concentration. This reduction in OH is described as a weakening of the reaction zone. Accompanying reaction zone weakening is also an increase in H2CO levels. The reduction in reaction rates is most noticeable at low oxidant stream O2 levels. The heated and low oxygen oxidant conditions typify those of moderate or intense low oxygen dilution (MILD) combustion. The computational results indicate that the effects of the low oxygen levels of MILD combustion leads to both a reduction in reaction rates and an increase in transport of O2 across the reaction zone. The relationship between the reaction rate and level of O2 permeation suggests that a form of partial premixing can occur under MILD combustion conditions. This partial premixing leads to the formation of flame intermediates, which contribute to the stabilization of the flames. The permeation effects are most pronounced at high strain rates, which are commonly encountered in practical MILD combustors.

Simultaneous imaging of OH, formaldehyde, and temperature of turbulent nonpremixed jet flames in a heated and diluted coflow

This paper reports measurements in turbulent nonpremixed CH 4/H 2 jet flames issuing into a heated and highly diluted coflow. These conditions emulate those of moderate or intense low-oxygen-dilution (MILD) combustion. The spatial distribution of the hydroxyl radical (OH), formaldehyde (H 2CO), and temperature, imaged using planar laser-induced fluorescence and Rayleigh scattering laser diagnostic techniques, are measured and presented. Comparisons are made between three jet Reynolds numbers and two coflow O 2 levels. Measurements are taken at two downstream locations. The burner used in this work facilitates the additional study on the effects of the entrainment of surrounding air on the flame structure at downstream locations. Reducing the coflow oxygen level is shown to lead to a suppression of OH as a result of the reduced temperatures in the reaction zone. Decreasing the oxygen level of the coflow also results in a broadening of the OH distribution. At downstream locations, the surrounding air mixes with the jet and coflow. The subsequent drop in the temperature of the oxidant stream can lead to a rupture of the OH layer. Localized extinction allows premixing of the fuel with the surrounding air. The result is an increase in the reaction rate, highlighting the need for homogeneous mixing to maintain MILD combustion conditions.