Low Swirl Burner Research Articles

CFD study of hydrogen combustion effects on the heat-up characteristics of steel samples using a low-swirl burner: A comparative analysis with methane

CFD study of hydrogen combustion effects on the heat-up characteristics of steel samples using a low-swirl burner: A comparative analysis with methane

Experimental study of turbulence and flame characteristics on low swirl burner integrated with modified square fractal grids

To leverage the exceptional turbulence merits of the square fractal grid, including bespoke turbulence enhancement and dissipation variation, we investigated a modified square fractal grid(SFG) with a 70% blockage ratio within a low swirl injector framework for gas turbine systems. In the non-reactive flow field of pre-dump regions, we analysed turbulence characteristics such as velocity, intensity, and isotropy across various reduction ratios of bar thickness(RRBT). Results indicate that increasing RRBT enhances turbulence intensity, highlighting the SFG’s bespoke turbulence ability. This contrasts with findings from cross-fractal grid studies, highlighting the SFG’s unique properties. Turbulence dissipation, assessed through integral and Taylor micro scale lengths, revealed that RRBTs shift the dissipation coefficient from Richardson–Kolmogorov equilibrium to a non-equilibrium state, strongly correlating with the Reynolds number based on mesh size. In the reactive flow field, SFG-modulated turbulence resulted in distinct flame shapes, including bowl and W-shapes. The turbulence intensity at RRBT 1.0 notably impacted the reactive flow, with fluctuations 1.7 times higher than at RRBT 0.6. Despite these variations, all SFGs maintained self-similarity, which is crucial for stable low-swirl flame stabilisation. Three distinct turbulence-local displacement turbulent flame speed model—conventional, transitional, and magnified—were integrated into a single model that correlates well with turbulence fluctuations and grid parameters. Furthermore, SFGs significantly reduced NOx emissions across various equivalence ratios. Overall, these findings highlight the pivotal role of fractal grids in shaping both non-reactive and reactive flow fields, emphasising their potential to enhance bespoke turbulence control, reinforce the flame speed, and reduce emissions in low-swirl combustors.

Integrating Flow Field Dynamics and Chemical Atmosphere Predictions for Enhanced Sulfur Corrosion Risk Assessment in Power Boilers.

In this work, we attempt to explain the phenomenon of sulfur corrosion of power boiler water walls under the conditions of large fluctuations in carbon monoxide concentrations. To assess the conditions required for corrosion formation, a criterion based on the chemical and flow field parameters of the flue gas is proposed. The formulated sulfur corrosion criterion is based on the mixture fraction variance and the turbulence time scale. Numerical modeling of coal combustion in a 250 MW power boiler is performed using ANSYS. Two cases of combustion in a boiler are analyzed, with the first simulating the boiler operated using classic high-swirl burners and the second one accounting for boiler operation with modified low-swirl burners. Calculations of pulverized coal combustion are performed using the standard k-ε turbulence model and the combustion described by the mixture fraction. The simulation results reveal that the low-swirl burner is characterized by higher values of the mixture fraction variance and a higher frequency of fluctuation of the velocity field, which is strongly related to an increased corrosion rate. The study outcomes show the validity of using the criterion of the mixture fraction variance and velocity field fluctuations to determine the areas at risk of sulfur corrosion.

Combustion and emission characteristics for various swirler geometries and fuel heating values in a premixed low swirl combustor system

This study investigated combustion and emission characteristics with various swirler parameters (i.e., swirl angle and perforated screen blockage) and fuel heating values (4,000 kcal/Nm3∼9,560 kcal/Nm3, 16.3 MJ/kg∼55.5 MJ/kg in SI unit) using the premixed low swirl combustor system. Flashback limits were evaluated as a flame stability and flame structures were analyzed by acquiring chemiluminescence (OH* and CH*) and OH-Planar Laser Induced Fluorescence (PLIF) signals. NOx and CO emissions were evaluated by calculating emission indices (EINOx and EICO). As a result, the swirl angle did not affect flame stability, whereas screen blockage increased flashback limits. The highest screen blockage (β = 0.8) showed the flashback limit of 5.6 kW, around 1.5 times higher than the lowest screen blockage (β = 0.6). Also, the blockage affected the radical distributed position due to the ratio between swirling and non-swirling flow. The lower heating value fuel showed reduced flashback limits, decreased radical intensities, and lower emissions (EINOx and EICO). The lowest heating value gas (HV 4000) had an EINOx of less than 0.5 g/kg and an EICO of less than 5 g/kg, respectively. In conclusion, these findings would be helpful databases for optimizing heating values to solve environmental pollution issues.

Effect of temporal increase in equivalence ratio on combustion instability of a lean-premixed swirling hydrogen flame

The effect of temporal increase in the equivalence ratio on the combustion instability of a lean-premixed low-swirl hydrogen jet flame in a low-swirl combustor (LSC) is investigated in detail using a high-fidelity Large-Eddy Simulation (LES). The equivalence ratio is linearly increased from 0.3 to 0.5 over a duration of 0.4 s. The results show that the pressure oscillation amplitude in the combustor increases significantly when the equivalence ratio at the combustor inlet (ERCI) exceeds 0.42, and the maximum pressure amplitude and the combustion instability mode exhibit trends consistent with those in a previous experiment and numerical simulation conducted with the same LSC setup at a fixed equivalence ratio of 0.39. Temporal variations in the equivalence ratio and consequently the temperature inside the combustor cause the drastic amplification of pressure oscillation (when the ERCI exceeds 0.42) whose amplitude is larger than that at the fixed equivalence ratio (= 0.39). Prior to the onset of this exceptionally strong combustion instability, a transient irregular oscillation phenomenon comprising instantaneous changes in the pressure oscillation frequency is observed. While the pressure oscillations in the combustor and in the injector channel are in phase after the onset of strong combustion instability, they are in opposite phases during the occurrence of the irregular oscillation phenomenon prior to the onset of strong combustion instability. This irregular oscillation phenomenon predicted by the LES may play a crucial role in the mechanism of transition from stable combustion to combustion instability.

Plasma assisted combustion of different biogas mixtures in low swirl burner

This study explores the impact of plasma driving frequency on diverse biogas compositions, emphasizing flame behaviour by analysing spatial emissions of OH*, C2*, and CH* from plasma-assisted flames. Experimental investigation performed combusting mixtures with 25 % and 60 % CH4 in CO2 under lean combustion conditions with the varying fuel equivalence ratios from 0.71 to 0.83 and plasma discharge frequencies from 60 to 120 kHz resulting in higher energy density.Obtained results indicate that the plasma assistance notably enhances flame stability under lean fuel conditions with increasing frequency from 80 to 120 kHz. The discharge frequency of 60 kHz indicates insufficient energy to enhance combustion of low calorific value mixtures as the flame lift-off height increases approximately by 0.43–1.5 % for the mixture of 25 vol% CH4 in CO2 and by 0.26–7.7 % for the mixture of 60 vol% CH4 in CO2. Changing the plasma frequency from 80 to 120 kHz, the flame position is shifted closer to the burner nozzle for both mixtures and the highest effect is determined at 120 kHz. The flame lift-off height is reduced by 2.43–16.34 % for the mixture with 25 vol% of CH4 and by 3.57–51.28 % for the mixture with 60 % of CH4. The plasma-assisted combustion leads to improved combustion efficiency as CO emissions are reduced from 1550 ppm to 45 ppm for biogas mixture, but in the case of low calorific value gas, the CO emissions increase linearly with the frequency increase even the flame curvature and stability is improved.

Investigation into the Effect of H2-Enriched Conditions on the Structure and Stability of Flames in a Low-Swirl Combustor Derived from Aero-Engine Design

This study introduces an innovative approach involving the injection of hydrogen into a low-swirl, non-premixed flame, which operates with gaseous fuels derived from an air-blast atomizer designed for aero-engine applications. The aim is to characterize how hydrogen enrichment influences flame structures while maintaining a constant thermal output of 4.6 kW. Using high-speed chemiluminescence imaging, three fueling conditions were compared: the first involved pure methane/air, while the second and third conditions introduced varying levels of hydrogen to an air–methane mixture. The results reveal significant effects of hydrogen enrichment on flame characteristics, including a slightly shorter length and a wider angle attributed to heightened expansion within the Combustion Recirculation Zone. Moreover, the emission of UV light underwent considerable changes, resulting in a shifted luminosity zone and reduced variance. To delve deeper into the underlying mechanisms, the researchers employed Proper Orthogonal Decomposition (POD) and Spectral Proper Orthogonal Decomposition (SPOD) analyses, showing coherent structures and energetic modes within the flames. Hydrogen enrichment led to the development of smaller structures near the nozzle exit, accompanied by longitudinal oscillations and vortex shedding phenomena. These findings contribute to an advanced understanding of hydrogen’s impact on flame characteristics, thereby propelling efforts toward improved flame stability. Additionally, these insights hold significance in the exploration of hydrogen as an alternative energy source with potential environmental benefits.

Characterisation of turbulent non-premixed hydrogen-blended flames in a scaled industrial low-swirl burner

The performance of a scaled industrial, non-premixed, low-swirl burner design was experimentally investigated for hydrogen addition to natural gas. Two strategies for introducing hydrogen are considered, namely, conserving (i) heat input and (ii) velocity/volumetric flow of the original fuel. This work characterises the effects on key performance metrics of the burner as hydrogen fraction is increased. Compared with natural gas, the results with hydrogen showed a 33 % reduction in the radiant fraction and up to a 380 % increase in NOx emissions. The lift-off height was reduced by a maximum of 23 % and 51 % for addition of 10 and 30 vol% hydrogen addition, respectively, with 100 % cases becoming completely attached to the burner. The influence of hydrogen-addition strategy and air adjustment was shown to be significant with respect to NOx emissions but less significant than the resulting changes in fuel composition and heat input with respect to flame appearance, stability and radiant heat transfer.

Large-eddy simulation of a lean-premixed hydrogen flame in a low-swirl combustor under combustion instability

Large-eddy simulation (LES) of a lean-premixed hydrogen turbulent jet flame with combustion instability (CI) in a low-swirl combustor (LSC) is performed by employing a dynamically thickened flame model with a detailed chemical reaction model with 9 chemical species and 20 reactions, and the LES validity and the CI characteristics are investigated in detail. The results show that the present LES can accurately reproduce the experimentally observed characteristics of the CI such as intensity, frequency, sporadic decay of pressure oscillations, and a flame–flow interaction inducing the periodic transitions of an inverted conical flame structure and a flat flame structure in the LSC. The sporadic decay of pressure oscillations and the flame–flow interaction are caused by the temporal decoupling of pressure and heat release rate and the periodic outward and inward deflections of the inflow, which is associated with the flow behavior in the upstream injector channel, respectively.

On the effects of fractal geometry on reacting and nonreacting flows in a low-swirl burner: A numerical study with large-eddy simulation

Understanding and optimizing turbulence in combustion systems has a profound effect on combustion efficiency, emission reduction, and safety in applications ranging from industrial burners to propulsion systems in aerospace. This paper contributes to this pivotal field by investigating a real-scale low-swirl combustor utilizing a range of turbulence generation plates characterized by different blockage ratios and numbers of fractal pattern iterations. The work encompasses two states, namely nonreacting and premixed reacting modes, utilizing a large-eddy simulation method equipped with a dynamic Smagorinsky sub-grid model and adaptive mesh refinement for high accuracy. Verifying the robustness of the approach, the simulation aligns closely with experimental data, registering a maximum error of less than 0.9% in the swirl number. The research provides an insightful evaluation of the impact of four different fractal geometries on turbulence intensity, a vital element for attaining more complete combustion. Findings indicate a fractal with a 73% blockage ratio and four iteration levels enhances several key parameters including turbulence intensity, flow residence time, vorticity, and velocity gradient in the nonreacting mode. Conversely, a fractal with a 73% blockage ratio and three iteration levels shows the least progress in the reacting mode. Moreover, the paper delves into a comparative analysis of these two cases in the reacting mode, particularly observing the reaction zone. The appropriate fractal geometry unveiled significantly improves combustion efficiency, evidenced by an increased presence of the OH radical and a decrease in NO emission gas, thus demonstrating potential for wider application in enhancing combustion systems.

Numerical investigation of ammonia/coal co-combustion in a low NOx swirl burner

Co-firing of ammonia (NH3) and coal in boilers is a promising technology to reduce CO2 emissions from power plants. However, NH3/coal co-firing in swirl burners can impact the original flame structures and may lead to serious NOx emission issues due to the high concentration of nitrogen in NH3. In this study, a numerical simulation of a low-NOx swirl burner was carried out to investigate the changes in flame structure, carbon emission, and characteristics of NOx generation. The results show that, when the NH3 co-firing ratio is 10 cal%, the flame retains its original swirl structure, and a significant increase in NO formation is observed in situations with different injection positions. When the co-firing ratio increases to 20 cal% and 30 cal% and NH3 is injected through the central air pipe, the flame changes from a swirl flame to an elongated flame. The injected NH3 is wrapped by the low-temperature primary air, separating the NH3 from the high-temperature zone, and pyrolysis becomes the primary reaction pathway for NH3 consumption rather than oxidation. The NO concentrations measured at the chamber outlet can be reduced to 11.2mg/Nm3, which is lower than that from coal combustion. As the co-firing ratio further increases to 50 cal%, NH3 and air are well mixed and burned, forming a “candle-type” flame. This results in a significant rise in NO emissions.

The impact of turbulence and combustion models on flames and emissions in a low swirl burner

Since natural gas fuel boilers are environmentally friendly and efficient, they are widely used in heating systems today. Improvements are continuing in the combustion efficiency and emissions of these boilers. Due to the high costs and long manufacturing times, experimental studies are progressing slowly. For this reason, studies with CFD simulations are also preferred. For realistic CFD results, it is important to correctly choose the turbulence and combustion models. This study conducted an experimental study to obtain temperature and emission measurements from a low-swirl natural gas boiler. Then, CFD simulations were made under the same conditions using the 3D model prepared for the same boiler. In simulation studies, different turbulence and combustion models were investigated. The turbulence models used in the simulations are Standard k-ε, Realizable k-ε, RNG k-ε, and Reynolds Stress Model (RSM), respectively. Combustion models are Eddy Dissipation (ED), Finite Rate/Eddy Dissipation (FR/ED), Eddy Dissipation Concept (EDC), Non-Premix Combustion (NPC) and Partial-Premix Combustion (PPC). When the results obtained by numerical simulations are compared with the experimental data, it is understood that the Realizable k-ε turbulence model provides better convergence than other turbulence models, while PPC and ED provide better convergence in combustion models. Since the two-step chemical mechanism for natural gas used in EDC and FR/ED models neglects the formation of many intermediate products, higher NOx values were obtained than the experimental for these combustion models. Different results were obtained from the standard k − ε in terms of temperature and CO2, H2O, O2, CO, NOx results than other turbulence models.

Detailed unsteady dynamics of flame-flow interactions during combustion instability and its transition scenario for lean-premixed low-swirl hydrogen turbulent flames

This experimental study elucidates the unsteady dynamics of flame-flow interactions during unique thermoacoustic instability (TI) and the transition mechanism from stable combustion to TI for lean-premixed hydrogen turbulent jet flames in a low-swirl combustor (LSC), where a swirler assembly consists of an unswirled central region (CR) and an annular swirler region (SR). Simultaneous 200-kHz pressure fluctuation p’ measurements and 10-kHz OH* chemiluminescence imaging, as well as 40-kHz stereoscopic particle image velocimetry (SPIV) and two-dimensional PIV measurements for steady-state and transient data acquisitions, respectively, were conducted. The SPIV was performed in multiple planes to explore three-dimensional velocity fields. During TI, periodic flashback was possibly caused by significant axial velocity oscillations, resulting in the local mixture velocity falling below the turbulent flame speed. The large-scale vortex ring generated by the velocity oscillations caused axisymmetric radial velocity Vr oscillations with switching signs during the TI period. Similar to a typical low-swirl flow, the positive Vr away from the combustor axis created diverging flow, whereas unlike the typical flowfield, the negative Vr toward the combustor axis generated converging flow while flattening the axial velocity distributions, which was the signature phenomenon for this TI. Using the transient data and dynamic mode decomposition, variations in delay times between the mixture injection and its convection to a region with positive local Rayleigh indices were investigated. During stable combustion, the mixture jet from the SR predominantly induced thermoacoustic coupling (TC). As the combustion transitioned into the TI, the mixture jet from the CR began to induce TC and, eventually, achieved predominance in inducing TC during fully evolved TI. The transition from the SR jet- into CR jet-dominant TI arising from the dynamic flame-flow interactions resulted from the inherent physical characteristics of hydrogen flames, thereby yielding the larger p’ amplitude compared to typical TIs.

Effect of fuel reactivity on flame properties of a low-swirl burner

The present paper examines the capability of low swirl flows in stabilizing low calorific value fuels. To such an aim, CO2 gas is used to adjust the reactivity of CH4/air mixtures. The results show that increases in CO2 percentage in the fresh mixture result in shrinking the burner stability map by moving the lean and rich blowout limits toward the stoichiometric condition. The effects of CO2 addition on rich blowout limits are more intense than the lean blowout limits. For instance, the equivalence ratio associated with the lean blowout increases by 0.23, when the discharge flow velocity from the burner rises from 1.5 to 5 m/s in the 40 % CO2 diluted case. Under a similar augmentation in the burner discharge velocity, the rich blowout limit reduces by more than 0.5 in the 40 % CO2 diluted case. Unlike the lean blowout, the rich blowout limit responds non-monotonically to the discharge flow velocity from the burner for both undiluted and diluted mixtures. For the 50 % CO2 diluted case, the low swirl burner operates efficiently at low discharge velocities (1.5 to 3.5 m/s), but it blows out at high discharge velocities (3.5 to 9 m/s). Furthermore, unsteady features of low swirl flame near blowout limits are studied in detail. Moreover, strong curvy structures appear at the flame boundary prior to the flame flashback. However, such structures diminish occasionally and specifically as the flame approaches the flashback. Some deterministic features are more obvious before the flame blowout than those before the flame flashback.

Investigation of combustion noise generated by an open lean-premixed H[formula omitted]/air low-swirl flame using the hybrid LES/APE-RF framework

An open lean-premixed hydrogen/air low-swirl (LPHALS) turbulent flame exhibiting a pronounced peak in its combustion noise spectra, is investigated numerically using a hybrid Computational Fluid Dynamics/Computational Aero-Acoustics (CFD/CAA) framework. Under this framework, the reacting flow-field of the flame is computed via Large-Eddy Simulation (LES), while the direct combustion noise it produces is captured by solving the Acoustic Perturbation Equations for Reacting Flows (APE-RF). Flame configuration and simulation conditions correspond to those of an experimental study on an open lean-premixed H2/air flame stabilized using a Low-Swirl Burner (LSB). LES results are validated against experimental data. The CAA simulation is able to predict a pronounced sharp peak in the computed combustion noise spectra, similar to one of the two characteristic peaks observed in the measured combustion noise spectra. Frequency of this spectral peak predicted by the CAA simulation is 840 Hz, which is close to that of the higher frequency secondary spectral peak at 940 Hz measured in the experiment. Upon examining the hybrid LES/APE-RF results, the noise generation mechanism at 840 Hz is found to be the intense local heat release rate fluctuations, caused by strong interaction between the flame and the periodically generated vortical flow structures in the shear layers, downstream of the LSB exit. Additionally, analysis of the spectral content and directivity of the noise generated by different acoustic source terms is performed, in order to investigate their impact on the radiated acoustic field, and hence the characteristics of direct combustion noise produced by the open LPHALS flame.

Combustion performance of low calorific gas enriched by oxygen and ozone

Biomethane produced via anaerobic digestion or thermochemical process is one of alternatives, which could substitute fossil fuel and reduce carbon footprints. However, during biomethane preparation for end-users, the waste gas stream could be formed during biogas upgrade/methanation. As these gases consists of few to 30 vol% of methane in CO2, the waste gas stream should be utilized instead of emission to atmosphere. Aside, to achieve clean and stable combustion of such gases, innovative methods are required as high concentrations of CO2 causes the decreased flame temperature and burning velocity.This study concerns the low calorific gas combustion enhancement by oxygen enriched air and plasma-produced ozone. The ozone-assisted combustion was investigated by burning various composition waste gases (30–15 vol% of CH4 in CO2) in a low swirl burner under different oxygen enrichment levels, from 28 to 80 vol%. It was determined that the combustion enhancement by ozone is related to methane concentration in the mixture and to oxygen enrichment level. Moreover, at lower oxygen enrichment levels, the combustion enhancement by the ozone addition was more pronouncing than at higher oxygen enrichment levels. The flame lift-off was reduced by 0.9–25.9% due to the ozone addition and the flame stability improved, but NOx emissions increased by 2–10 ppm compared to cases without O3 addition. Considering the combustion efficiency in terms of flame lift-off height, NOx and CO emissions, the ozone-assisted combustion of WG25 and WG20 was the most efficient under oxygen enrichment of 40 vol% compared to other cases.

Simulation of Combustion Flow of Methane Gas in a Premixed Low-Swirl Burner using a Partially Premixed Combustion Model

Simulation of Combustion Flow of Methane Gas in a Premixed Low-Swirl Burner using a Partially Premixed Combustion Model

Research on the flame structure characteristics and NOx pathways of low swirl combustion

Low swirl combustion (LSC) technology has the advantage of ultralow NOx emissions, which is of great significance to the development of low-emission gas turbine engines in the future. To investigate the flow field and flame structure characteristics of LSC, a test rig of low swirl burner was designed and developed. Particle image velocimetry measurement results show that the location and size of the recirculation zone are different, and the flow field shows typical “W”- and “U”-shaped distributions under various swirling flow conditions. The self-luminous results of LSC show that there are three flame modes including attached flame, “W”-shaped flame, and “U”-shaped flame. To deeply understand NOx generation pathways, a chemical reactor network model was developed based on experiments and computational fluid dynamics simulations, and the effects of premixed gas components on NOx pathways were calculated by using Chemkin software. It was verified that the NOx production of the CH4 mixture mixed with H2, N2, and CO2 was mainly formed by the thermal NO pathway in the recirculation zone. The increase of H2 promotes the generation of NNH-type NOx in the main flame zone and inhibits prompt NOx. The addition of N2 and CO2 greatly promotes the generation of prompt NOx and at the same time inhibits NNH-type NOx. In addition, there is little prompt NOx formation in the post-flame zone.

Combustion of waste gas in a low-swirl burner under syngas and oxygen enrichment

A biomethane production from biogas could result in an emission of waste gas to the atmosphere. As these gases consist of methane up to 15 vol%, they should be utilized onsite to reduce the impact of climate change. However, due to low calorific value, the combustion process of these gases could be complicated or even impossible inconvenient gas burners. For this reason, this work focuses on an investigation to clarify a complex of measures affecting the flammability limits and effectiveness of burning waste gas in a designed low swirl burner with different swirlers. The swirlers were made with vanes angles of 37°, 45°, and 53°. Critical regions of the flame structure were experimentally examined by means of chemiluminescence to determine the intensity of OH* emissions under syngas and oxygen enrichment. It revealed that the most appropriate combustion conditions taking into account the stable flame and lowest CO emissions were achieved using the swirler with a vanes angle of 45°. Moreover, under the syngas enrichment (by the addition from 0 to 25 vol% of syngas), it is possible to achieve more stable combustion conditions, taking into account the low CO and NOx emissions, when oxygen-enriched air is additionally supplied. However, the oxygen enrichment levels must be selected according to the lift-off height threshold to avoid flashbacks due to the declining trend in the velocity of the flow.

The effect of swirl burner design configuration on combustion and emission characteristics of lean pre-vaporized premixed flames

The present paper experimentally provides a quantitative comparison between the aerodynamic flow field at non-reaction conditions (cold flow) and the combustion and emission characteristics (hot conditions) of two swirl burners having the same swirl number of 0.55 but with two different configurations. The first burner features an annular outer swirler (swirl angle of 40°) and multiple central circular jets, leading to “low swirl combustion”, whereas the second burner has a blocked central passage and an outer swirler (swirl angle of 35°) resulting in “high swirl combustion”; featuring a central recirculation zone. The cold flow study is conducted using a calibrated five-hole probe while the combustion and emission characteristics are obtained via measurements of flame temperatures and the dry volumetric analysis of CO and NOx emissions. Each burner is coaxially mounted to a vertical cylindrical combustor; inside which a lean (equivalence ratio: ϕ = 0.75), pre-vaporized (preheated to a temperature of 523 K) partially premixed mixture of Jet A-1 fuel and air are admitted. The former low swirl combustion burner gives a bulk stable and uniform reaction zone (W-shape) close to the burner exit with a very slight lifting; confirming the concept of “stable lifting” while reducing the pressure drop across the burner exit and eliminating hot spots in the reaction zone. The high swirl combustion burner exhibits a strong central recirculation zone (better flame stability) that is surrounded by an outer V-shape reaction zone having comparatively higher peak levels of temperature, CO and NOx that would lead to greater liability of higher exhaust emission.