Bluff-body Burner Research Articles

Large Eddy Simulation of Ammonia-Hydrogen Non-Premixed Flames Stabilized on a Bluff Body Burner

Abstract The demand for carbon-free fuels such as NH3 and H2 has been growing due to stricter environmental policies on carbon emissions. Since ammonia is easier to store and transport, it is considered an alternative fuel for marine, aviation, and gas turbine industries. However, the narrow flammability, low reactivity, and potential emissions of NOx are important challenges that need to be overcome if ammonia is to be used as a practical fuel for industrial use. Recent works have provided experimental data on the characteristics of NH3 flames under premixed and non-premixed combustion, focusing on flame speed, turbulence-chemistry interaction, and especially the dissociation/cracking of NH3 into H2 and N2. These measurements have encouraged the use of CFD for the simulation and scaling of practical ammonia systems, evaluating the performance of different numerical models for NH3 combustion. In this study, LES is utilized to predict a series of NH3/H2/N2 non-premixed flames stabilized on a bluff body burner. The accuracy of the FGM combustion model combined with LES has been examined, using a commercial CFD solver. The different cracking levels of NH3 into H2 and N2 lead to simulation cases ranging from an H2/N2 flame (i.e., “fully cracked”), to a flame with 50% NH3 (i.e., “half cracked”). The different flame shapes, temperature distributions, and NOx emissions have been simulated, generally matching well experimental data. The developed numerical settings and workflows may be applied to simulating more complex combustors which use NH3 as a fuel.

MILD combustion characteristics of a premixed CH4/air jet flame in hot coflow from a bluff-body burner

This numerical study is designated to comparatively investigate premixed CH4/air jet flames from bluff-body (BB) and non-BB (NBB) burners in the coflow of hot exhaust gas. Specifically, traditional combustion (TC) from BB burner (BB-TC), MILD combustion (MC) from BB burner (BB-MC), and NBB burner (NBB-MC) under varying coflow temperatures (TC) and oxygen concentrations (YO2, C) are considered. The flow field, combustion reactions, temperature rise, heat release, along with reaction zone, and pollutant emissions are thoroughly investigated. The findings underscore the feasibility of achieving MILD combustion using the BB burner. Remarkably, the small recirculation zone generated by the BB enhances entrainment and mixing, leading to superior combustion stability and reduced CO emission in the BB-MC case compared to the conventional NBB-MC case. Moreover, the BB-MC case exhibits a higher peak temperature, reaction rate, and heat release rate than the NBB-MC case, albeit with slightly higher NO emission. When compared to the BB-TC case, NO emission from both the BB-MC and NBB-MC cases are significantly lower, particularly under relatively low TC and YO2, C conditions.

Numerical simulation on combustion characteristics of methane/hydrogen blended fuel for non-premixed conical bluff body burner

Blending hydrogen into methane impacts flame stability, morphology, and pollutant emissions. This study, set against the backdrop of hydrogen-enriched natural gas combustion technology, employs a non-premixed coaxial burner with conical bluff body to create recirculation zones for flame stabilization. Combining EDC and GRI-Mech 3.0, the research fills in the gap of effects of hydrogen blending ratio (0–100%) and equivalence ratio vary (0.4–1.4) within a wide range. Keeping equivalence ratio and fuel volume flow rate as constants, as increasing hydrogen blending ratio, the mean velocity in the chamber decreases, the size of vortex structure located in outer recirculation zones reduces, and the flame length is shortened, illustrating the reaction zone shifts to chamber inlet. Furthermore, when Ф = 1.4, average temperature and temperature uniformity are averagely larger 59.82% and 4.21% than that under Ф = 0.4. The CO2 emission at XH2 = 80% is 54.70% averagely lower than that at XH2 = 0%, but peak concentration of NO increases.

Insights into the flow and scalar structures when shifting from methane to hydrogen turbulent flames using simultaneous PIV – OH PLIF and spontaneous Raman scattering

This study discusses fundamental turbulence-chemistry interactions in a canonical non-premixed bluff body burner fueled with 100% methane or hydrogen. Simultaneous time-resolved PIV&OH-PLIF and 1D Spontaneous Raman Scattering (SRS) have been employed to provide deeper insights into the difference in combustion regimes between CH4 and H2 operations. The analysis of the instantaneous time-resolved PIV and OH-PLIF datasets reveals the presence and absence of local extinctions in methane and hydrogen flames despite the mean flow topology being similar across the test cases. The instantaneous scatter plots of 1D Raman data in the mixture fraction space further quantified the spatial evolution of temperature and major species. Finally, the regime identification scheme is implemented over instantaneous 1D SRS data to identify the different flame/mixture regimes. The change in combustion regime is observed even very close to the burner exit while switching between CH4 and H2, which is attributed to the probability of localized flame extinctions. Overall, this study provides detailed interlinks between flow field aerodynamics and scalar structures in the two different flames whose thermo physical properties are entirely different and form a comprehensive database for cornerstone computational model validation.

The Effect of Ignition Procedure on Flashback of Hydrogen-Enriched Flames

Abstract The impact of different ignition sequences on the ignition dynamics of CH4-H2 flames in a bluff body burner is investigated at atmospheric conditions. Experiments are performed over a wide range of operating conditions covering pure methane injection (PH0) to pure hydrogen injection (PH100). A perforated plate of total porosity σ=0.17 is positioned at the outlet of the combustion chamber to increase the chamber back pressure and trigger transient flashback during ignition. Time-series of pressure, OH* chemiluminescence and OH-PLIF images of the propagating flame branch are recorded simultaneously to characterize the ignition process. Two ignition procedures are investigated. For ignition procedure A, designated as an early ignition procedure, the spark is initiated before fuel injection. For ignition procedure B, designated as a late ignition, the spark is only activated after the fuel injection. The impact of the fuel air mixing on the final stabilization state is investigated by changing the fuel delivery time (dt) before the initial spark. Three different time delays are considered dt = 1, 3, and 5 s. The final state of the flame is found to be highly sensitive to the selected ignition procedure and increasing dt favors the occurrence of flashback. At constant power, the magnitude of the pressure peak is driven by a competition between the fuel mass flowrate at the moment of ignition and the high reactivity of hydrogen, which shifts the flammability limit toward lower equivalence ratios, hence generating a lower reaction rate. For procedure A, the peak of the chamber over pressure shows a nonmonotonic growth for increasing levels of H2 in the fuel blend, while it linearly increases for procedure B. Experiments are then conducted at a fixed injection flow velocity Ub = 5 m s–1 and fixed laminar burning velocity Sl0=0.25 m s–1 by varying the level of hydrogen enrichment. For procedure A, the over pressure amplitude decreases with increasing the hydrogen enrichment leading to a soft ignition for all CH4-H2 blends. Under ignition procedure B, the amplitude of the over pressure reached during ignition is found to be relatively unaffected by the hydrogen concentration, but the flame stabilization mode shows a strong dependence to both the level of H2-enrichment and fuel delivery time dt. OH* as well as OH-PLIF images reveal that the trajectory of the flame leading point changes as dt increases. The different dynamics of the flame leading points is likely to be the cause the different types of stabilization modes observed.

Experiment Study and Industrial Application of Slotted Bluff-Body Burner Applied to Deep Peak Regulation

During the deep peak shaving period, the boiler needs to operate at a lower load, so higher requirements were set for the boiler's stable combustion. In order to better evaluate and compare the stable ignition capacity of bluff-body burners and slotted bluff-body burners, guidance and calculation support for the design of boiler deep peak shaving were needed. This study adopted the reflux heating chain ignition analysis method to study the differences in the steady combustion mechanism between the bluff-body burner and the slotted bluff-body burner. A thermal combustion experiment was conducted in employing the single angle coal powder combustion furnace. The experimental results were compared against the theoretical analysis results. In addition, a study was conducted on the application of slotted bluff-body burners in the deep peak shaving of a 330MW unit power plant boiler. The results showed that as long as the small slot flow can catch fire, the mixed temperature of the reflux fluid of the slotted bluff-body burner will be higher than that of the bluff body burner. This will enhance its steady combustion ability. The combustion test results were found to be consistent with the analysis results, indicating that when the slotted bluff-body burner is used on the 330MW unit, the boiler combustion is in good condition. In this case, it has a stable combustion capacity of 66MW (20% economic continuous rating) without oil injection at low loads. This study revealed the advantages of the slotted bluff-body burner in terms of its steady combustion mechanism compared to traditional bluff-body burners. It verified the feasibility for boiler deep peak shaving in practical applications. This is of great significance for improving the flexibility of coal-fired power generation units, enhancing the flexibility of the power system, and promoting carbon reduction.

The effect of the dielectric barrier discharge plasma actuator in the control of non-reactive flow in a non-premixed bluff body burner

Active flow control methods based on dielectric barrier discharge plasma actuators can be used to increase the efficiency of combustion systems. In this study, the influence of the location of plasma actuators on the bluff body in a non-premixed burner on the non-reactive flow field of fuel and oxidizer is investigated numerically. Flow field properties and spatial mixing deficiency (SMD) are calculated to evaluate the plasma actuator's influence on the reactants' mixing inside the burner. The results show that the plasma actuator can influence the recirculation areas and are effective in mixing fuel and oxidizer. The presence of the plasma actuator results in the formation of a vortex, which slows down the movement of the flow and improves the mixing between the fuel and airflow streams resulting in more favorable combustion. The results show that at a higher air velocity (4 m/s), the formation of a plasma zone near the air duct strengthens the external circulation zone (ERZ) in such a way that it surrounds the internal recirculation zone and reduces the value of SMD by an average of 7.89%. While activating the actuator also strengthens the ERZ for a lower air velocity (0.3 m/s), this affects the air inflow, and the flow field becomes dominated by the fuel jet flow. When the diameter of the bluff body is increased, both when the plasma actuator is active or inactive, the ERZ is greatly strengthened, and the flow field is more dominated by the airflow.

Precursor Coherent Flow Structures and Droplet Dispersion Relevant for the Stabilization of Swirling Lifted Spray Flame on a Bluff Body Burner

Precursor Coherent Flow Structures and Droplet Dispersion Relevant for the Stabilization of Swirling Lifted Spray Flame on a Bluff Body Burner

The influence of a plasma actuator embedded in a bluff-body on the stabilization of nonpremixed jet flames

This study is to experimentally investigate the influence of an annular plasma actuator embedded in a bluff body burner on flame stabilization, extending the previous work on non-reacting jets to propane jet diffusion flames. The objective of the actuator is to produce a vortex ring in the vicinity of the jet exit, subsequently to alter the local flow velocity, and thus to achieve flame stabilization. Results show that repetitive flame reattachment may occur with the presence of the induced vortex ring. Flame reattachment frequency and attachment time are analyzed with fast Fourier transform (FFT) and a probability density function (PDF), respectively, and both results suggest that the degree of interaction between the flame and the vortex ring has strong dependence on the imposed flow momentum, characterized by the momentum ratio of the vortex ring to the main jet. A flame describing function (FDF), characterizing the dynamic flame response to the induced flow, exhibits a trough where the vortex ring momentum is comparable to the jet momentum. Given the constant vorticity independent of the jet momentum, a greater degree of interaction between the vortex ring and the fuel jet leads to more repetitive flame reattachment, resulting in more steady heat release.

Tomographic imaging using multi-simultaneous measurements (TIMes) of emission and refractive index 3D fields in turbulent flames

The present work combines background-oriented schlieren tomography (BOST) with computed tomography of chemiluminescence (CTC) using the TIMes technique to provide simultaneous three-dimensional fields of refractive index (i.e., density) and CH* chemiluminescence (i.e., flame luminescence) on a single-shot basis. The feasibility of the proposed non-intrusive technique is assessed based on simulations and experiments. Its accuracy with respect to the number of cameras is evaluated using phantoms of a realistic simulated turbulent flame. The technique is experimentally demonstrated to investigate turbulent flames issuing through a double-annular bluff-body burner (Cambridge-Sandia stratified swirl burner) under a variety of operating conditions. The simultaneous measurements are performed using a cost-effective setup consisting of 29 cameras split between BOST and CTC. The reconstructed instantaneous and averaged results reveal the three-dimensional fields of the refractive index (related to the temperature, pressure and species concentration) and CH* chemiluminescence (indicator of the flame front), showing their structure interaction.

Toluene addition to turbulent H2/natural gas flames in bluff-body burners

A key challenge in the transition towards using hydrogen as an alternative carbon-free fuel is the reduced thermal radiation due to the absence of soot. A novel solution to this may be doping with highly sooting bio-oils. This study investigates the efficacy of toluene as a prevapourised dopant in turbulent pure hydrogen and blended hydrogen/natural gas flames as a means of improving soot loading and radiant heat transfer. All flames are stabilised on bluff-body burners to emulate the recirculation component of many industrial combustors. Total heat flux and illuminance increase non-linearly with toluene concentration for fuel blends and bluff-body diameters. By reducing the bluff-body diameter from 64 mm to 50 mm, a 20/80 (vol%) H2/natural gas mixture produces a more radiative flame than a 10/90H2/natural gas mixture in the smaller bluff-body. Opposed-flow flame simulations of soot precursors indicate that as strain rate increases, although overall soot precursor concentration decreases, a 20 vol% hydrogen mixture will produce more soot than a 10 vol% mixture. This suggests the addition of hydrogen up to 20 vol% may be beneficial for soot production in high strain environments.

Dynamic adjustment of the Eddy Dissipation Concept model for turbulent/combustion interactions in mixed combustion regimes

The Eddy Dissipation Concept (EDC) combustion model, in comparison with some other combustion models, has drawn attention, especially for the Moderate or Intense low oxygen Dilution (MILD) combustion. The original formulation of the EDC combustion model is not developed for the MILD combustion regime, and a revision of the model could be considered. In this study, the effect of the characteristic frequency on the EDC combustion model has been investigated, and some parametric studies on the ratios of length and time scales of the fine structures to the Kolmogorov scales have been performed. Results revealed that finding optimum model constants for all combustion field with a wide range of Damköhler numbers seems to be complicated. The variations in scales ratio of fine structure to the Kolmogorov eddies shows different consequence at low and high Damköhler number combustion regions which leads to a conclusion on difficulties for EDC model global optimization. As a solution, a localized modulation method based on some local variables such as the turbulence Reynolds number and Damköhler number has been developed and applied on the Adelaide Jet in Hot Coflow(JHC) and Delft Jet in Hot Coflow (DJHC), and in addition in supplementary material on Sandia/TUD piloted flame and Sydney bluff-body burners. These burners demonstrate combustion regimes with both low and high Damköhler numbers. Results have shown the better performance of the local approach in the simulation of the burners. The improvement is more perceptible at the sections far from the inlet, where Damköhler number is increased, in comparison with the standard (default) EDC model. The average of deviations from the measurements is decreased by more than 10% and even more by a decrement in oxidizer oxygen level.

Effect of confinement ratio on flame structure and blow-off characteristics of swirl flames

Blow-off of swirl flames as well as its mechanism are fundamental issues and attract continuous attention due to its wide application in modern combustors. This study experimentally investigated the blow-off and flame macrostructure transition behavior of unconfined and confined premixed CH4/air flames stabilized on a swirl and bluff-body burner. PIV and OH-PLIF techniques were used to measure the flow field and instantaneous OH profile, respectively. It was observed that the combustor confinement could significantly broaden the blow-off limits, which was further widened with the increase of confinement ratio (CR) of the confined combustor. The influence of CR on flame stabilization was revealed by analyzing flame structures and flow fields. For the unconfined combustor, when the flames are far away and close to the blow-off, the shear layer and inner recirculation zone (IRZ) play the critical role in stabilizing the flame front, respectively. For the confined combustor, when the flame was far from blow-off, both the shear layer and the outer recirculation zone (ORZ) contributed to the stabilization. With the decrease of equivalence ratio, the stabilization effect of ORZ started to decrease and gradually weakened. When near lean blow-off limit, for CR = 2 (Φ= 0.48), the flame was mainly stabilized by the IRZ, while for CR = 3 (Φ = 0.46), the flame stabilization depended on the upstream IRZ/inner shear layer (ISL) and the downstream IRZ/ISL and ORZ/outer shear layer (OSL). Furthermore, the ISL vortexes would further accelerate the blow-off near the lean limit for confined and unconfined flames. The transition from stable to unstable was extended by increasing the CR in the confined combustor.

Low-order modelling of the light-round ignition transient in a premixed annular combustor

This work presents a series of simulations of the light-round process in a premixed annular combustor, consisting of 18 individual swirl bluff body burners, using improvements to the previously published stochastic low-order model SPINTHIR that mimics flame propagation with a Langevin particle model and continuous new particle generation. The improvements presented include the effect of dilatation and a stochastic component to the quenching criterion, resulting in capturing reasonably well the bending behaviour of the turbulent flame speed with turbulent intensity. The results showed that the light-round mechanism was well reproduced compared to experiments. The various versions of the model captured the trend of light-round time as a function of the mixture's laminar flame speed with different degrees of accuracy. Overall, the results here reported improve our low-order capability to model the ignition transient in complex multi-burner configurations, which can help the design process of gas turbine combustors.

Effects of ammonia addition on combustion characteristics in partially-premixed swirling ammonia/methane/air flame

Ammonia combustion has received intense research interest recently for its potential to reduce CO2 emission. This study aims to investigate the turbulent combustion characteristics in a bluff-body burner for CH4/NH3 mixtures with different ammonia blending ratios (15%, 30%, and 45% by mole fraction) through large eddy simulation and experiments. The simulations are conducted using openFOAM with a low Mach number solver and the partially stirred reactor combustion model with a detailed reaction mechanism. The flow field of one typical case is measured using the particle image velocimetry technique to verify the accuracy of the numerical results.The combustion characteristics are discussed. As the ammonia blending ratio increases, the flame height shortens, the flame color gradually changes from blue to orange, and the intermittent local quenching zone moves upstream, indicating that the combustion is becoming unstable. Meanwhile, the flow fields exhibit similar characteristics though the ammonia concentration varies greatly. The CO and NO emissions are also discussed. The CO emission decreases and the NO emission increases as the ammonia blending ratio increases

Nonlinear combustion instability analysis of a bluff body burner based on the flame describing function

Lean premixed combustion is a common form of combustion organization in power equipment and propulsion systems. In order to understand the dynamic characteristics of lean premixed flame and predict and control its combustion instability, it is necessary to obtain its flame describing function (FDF). Based on the open source CFD toolbox, OpenFOAM, the dynamic K-equation model, and the finite rate Partially Stirred Reactor (PaSR) model were used to perform large eddy simulations (LES) of lean premixed combustion, and the response of the unsteady heat release rate to single-frequency harmonic disturbances was studied. The response of the unsteady heat release rate was characterized by the FDF, and the response of the unsteady heat release rate to the two-frequency harmonic disturbance was studied. The results show that the quantitative heat release rate response and flame dynamics have very proper accuracy. In the single-frequency harmonic disturbance, as the forcing frequency increases, the curling behavior of the flame surface and the instantaneous vortex structure change; the nonlinear kinematics effect is manifested by the entrainment of the vortex. At lower forcing frequencies, the heat release response changes linearly with the increase of forcing amplitude; at intermediate frequencies, the heat release response exhibits obvious nonlinear behavior; at high frequencies, the heat release response to amplitude changes decreases. The introduction of the second harmonic disturbance will significantly reduce the response range of the total heat release rate and make the combustion more stable.

Experimental study on flameless combustion and NO emission with hydrogen‐containing fuels

Flameless combustion is a good alternative for a combustion technology that can simultaneously reduce CO and NOx, which generally have a trade-off relationship with each other. The most important feature of flameless combustion is its ability to control the radical and intense combustion atmosphere and suppress the formation of flame hot spots. Existing flameless combustion technology requires an additional diluent or preheating of reactants; but in this study, flameless combustion was achieved by controlling the flow through a bluff-body burner and inducing internal recirculation of the flue gas. In addition, the characteristics of flameless combustion using various fuels (CH4/CO/H2) were analyzed in detail. Consequently, in the flameless combustion atmosphere, the formation of hot zones was suppressed, and the amount of thermal NO was significantly reduced (>85% NO reduction). In the case of CO, most of the combustion characteristics were almost the same as those of methane, whereas when hydrogen was added, the amount of thermal NO generated was relatively large (<15% NO reduction) due to the influence of the hydrogen fuel in the chemical reaction. The effect of hydrogen on NO generation was found to be mainly a chemical reaction such as NNH and N2O intermediate route, which is especially noticeable in a flameless combustion atmosphere where the formation of a localized high-temperature region is suppressed. In addition, stable complete combustion was achieved even at a very low excess air ratio in a flameless combustion atmosphere. These characteristics of flameless combustion can have positive influence in terms of low pollutant emission and fuel diversification.

Modelling of sound-vortex interaction for the flow through an annular aperture

The acoustic characteristics of bluff-body burners play a critical role in the combustion stability for combustors using this type of burners. The acoustic modelling of an axisymmetric bluff-body burner entails properly capturing the sound-vortex interaction for the flow through the annular aperture of the burner. Such a problem pertaining to annular apertures can also be of relevance to other engineering applications, such as acoustic dampers or turbofan duct systems. The methodology of combining suitable acoustic Green’s functions with a vortex sheet model has been applied extensively in previous theoretical studies of the acoustic response of a short circular orifice with a mean flow passing through it. In this work, the Green’s function and vortex sheet model theory is generalised in order to efficiently predict the acoustic characteristics of thin annular apertures sustaining a mean flow, which effectively emulate the typical axisymmetric bluff-body burner configurations in realistic combustors. This requires the incorporation of multiple Kutta conditions for modelling the vortex shedding and multiple vortex sheets for modelling the interaction of the shed vorticity and the acoustics. A high-resolution compressible Large Eddy Simulation (LES) of a simplified representative geometry is performed for validation; the analytical prediction and numerical findings show very good agreement, and the LES further provides key insights into the speed with which vortical disturbances convect downstream.

Flow, mixing, and flame stabilization in bluff-body burner with decreased central jet velocity

The flame stabilization is a complex problem, especially when reducing the fuel supply, as it involves complicated interactions of turbulence, mixing, and chemistry. In this study, the flamelet progress variable combined with large eddy simulation has been used to simulate a bluff-body non-premixed flame to reveal the mechanisms of flow, mixing, and flame stabilization during the central fuel jet velocity reduction. The two flow patterns of jet dominant and coflow dominant are first analyzed by the concept of persistence of decay similarity of jet. The results show that this concept is informative to interpret whether and where the flow is jet dominant and understand the competition between two flows in detail, not only for flow but also for mixing. The results further show that the jet–coflow interaction, which has a pronounced impact on flame topology, has a minimal impact on flame stabilization for the bluff-body stabilized non-premixed flames over a wide range of fuel jet velocities, because of approximately close ignition delay time and flow convection velocities. In addition, it is observed that a ribbon-like structure of formaldehyde forms upstream of the hydroxyl. This phenomenon is caused by autoignition which is favored by high temperature in the recirculation zone and takes place far upstream of the flame. That would particularly facilitate flame stabilization in bluff-body burners.

Ignition, Propagation, and Stabilization of a Premixed Jet Flame from a Bluff-Body Burner

The warm-up and ignition together in real combustors is a complicated process that may involve either forced spark ignition (SI) or unforced autoignition (AI) or both (SI and AI). This numerical study is to systematically investigate both SI and AI processes of a premixed bluff-body (BB) flame in the coflow of exhaust gas, respectively, at two constant temperatures (300 K, 1500 K) and a rising temperature (300–1500 K). The high-temperature sparking (2500 K) is applied in the recirculation zone (RZ) behind a BB to mimic the actual SI. With this sparking, SI kernel appears immediately downstream of the BB, forming a traditional flame. Moreover, under high-temperature (>minimal self-ignition value) hot coflow, AI occurs in the downstream mixing layer and evolves into an AI flame. Later, the AI-generated flame joins up with the upstream BB flame. Such ignition processes are observed in all the 2500 K sparking cases under the heated-up coflow (with different heating times) or constant-temperature coflow, though AI kernels appear later and develop more slowly in the former. Further analyses show that both traditional BB flame and lift-off AI-generated flame coexist in the final state, which are connected by a thin neck zone around the RZ end. Furthermore, both low temperature (1500 K) sparking and nonsparking cases are investigated. It is found that the low-temperature SI fails with only CH₂O and CO generations and no OH and ignition kernels are produced, while AI still occurs in the downstream mixing layer. Additionally, in all the 1500 K sparking and nonsparking cases, AI kernels firstly arise in the mixing layer and then propagate into the central area, finally forming an invisible lifted “flame” in the regime of MILD combustion (MC).