High-speed OH Research Articles

Emissions Performance of Staged Premixed and Diffusion Combustor Concepts for an NH3/Air Flame With and Without Reactant Humidification

Abstract Renewably generated ammonia offers a form of carbon-free chemical energy storage to meet the differences between uncertain supply and fluctuating demand and has the potential to support future energy requirements. The storage/transportation characteristics of NH3 are favorable compared with H2; however, there are combustion research challenges to enhance fuel reactivity while reducing harmful emissions production. The purpose of this work was to evaluate different fuel delivery concepts for a representative gas turbine combustor. An experimental and numerical comparison was made between swirl-stabilized premixed and diffusion NH3–air flames at elevated inlet temperature (473 K). The exhaust NOx and NH3 emissions generated from each concept were quantified to optimize combustor performance. High-speed OH* and NH2* chemiluminescence was employed to characterize the change in flame topology with variation in fuel–air equivalence ratio, and the resultant influence on measured emission concentrations. Chemiluminescence intensities were shown to elucidate changes in sampled exhaust emissions, enabling detailed analysis of intermediate chemistry. A comparison was made between experimental data and kinetic simulations, demonstrating the sensitivity of NOx emissions to premixed fuel–air equivalence ratio. A comparison was also made between exclusive primary airflow, and the staged introduction of secondary air, to quantify the change in NOx production between each configuration and improve fuel burnout. Secondary air loadings were incrementally increased through the combustor. Finally, reactant humidification was employed as a secondary process for NOx reduction, having shown favorable performance with NH3–H2 mixtures, with the efficacy compared for both premixed and diffusion configurations.

Investigation of turbulent premixed methane/air and hydrogen-enriched methane/air flames in a laboratory-scale gas turbine model combustor

Methane and hydrogen-enriched (25 vol% and 50 vol% H2-enriched CH4) methane/air premixed flames were investigated in a gas turbine model combustor under atmospheric conditions. The flame operability ranges were mapped at different Reynold numbers (Re), showing the dependence on Re and H2 concentrations. The effects of equivalence ratio (Φ), Re, and H2 enrichment on flame structure were examined employing OH-PLIF measurement. For CH4/air cases, the flame was stabilized with an M shape; while for H2-enriched cases, the flame transitions to a П shape above a specific Φ. This transition was observed to influence significantly the flashback limits. The flame shape transition is most likely a result of H2 enrichment, occurring due to the increase in flame speed, higher resistance of the flame to the strain rate, and change in the inner recirculation zone. Flow fields of CH4/air flames were compared between low and high Re cases employing high-speed PIV. The flashback events, led by two mechanisms (combustion-induced vortex breakdown, CIVB, and boundary-layer flashback, BLF), were observed and recorded using high-speed OH chemiluminescence imaging. It was found that the CIVB flashback occurred only for CH4 flames with M shape, whereas the BLF occurs for all H2-enriched flames with П shape.

Multiple optical diagnostics on effects of fuel properties on spray flames under oxygen-enriched conditions

Spray flames development in diesel engines is typically diffusion combustion process controlled by mixing rate, which may cause considerable NOx and soot emissions. Breakthroughs in the field of oxygen preparation technology make it possible to apply oxygen-enriched combustion of spray flames, which has the potential to improve the thermal efficiency of engines and reduce pollutant emissions. In this paper, multiple optical diagnostics were applied to investigate spray flames at oxygen-enriched conditions. The experiments were conducted in an optical constant volume vessel, simulating the in-cylinder conditions of diesel engines. High-speed imaging, two-color pyrometry, and OH* chemiluminescence were used to investigate flame structure, soot flame temperature and KL factor, and flame lift-off length, respectively. The main fuel properties were separated by contrasting different fuels, including diesel (D100), 70% diesel + 30% n-heptane (D70H30), 70% diesel + 30% iso-octane (D70O30), 70% diesel + 30% n-butanol (D70B30), all of which were mixed by volume. The oxygen concentration of the ambient gas was in the range of 21%-70%. Experimental results show that the oxygen concentration has significant influence on the shape, structure, temperature, and species distributions of spray flame, while the fuel properties including cetane number, latent heat of vaporization and oxygen content, have more effects on flame ignition and soot formation under relatively lower oxygen concentration. With the increase of oxygen concentration, the flame lift-off length (FLoL) experiences a fast decrease firstly and then goes down slightly until it remains basically unchanged. The overall trend of FLoL under high oxygen concentrations shows that the FLoL of D100 is the shortest while the FLoL of D70B30 is the longest, and FLoL values of D70H30 and D70O30 are almost under the same level. With the increase of oxygen concentration, the effects of fuel properties on the ignition and development of the spray flame are weakened, the natural luminosity of spray flame increases, the length and width of spray flame reduce in size. At higher oxygen concentrations, the differences in KL factors and soot flame area among all test fuels become smaller. Under the same concentration, pixel averaged flame temperature of D100 is slightly higher than that of the other three fuels, while D70B30 is the lowest. The temperature differences among four fuels are no more than 100 K.

An optical study on spray and combustion characteristics of ternary hydrogenated catalytic biodiesel/methanol/n-octanol blends; part Ⅰ: Spray morphology, ignition delay, and flame lift-off length

A fundamental study on spray morphology, ignition delay, and flame lift-off length of two ternary hydrogenated catalytic biodiesel (HCB)/methanol/n-octanol blends were carried out by performing visualization tests through high-speed Schlieren and OH* chemiluminescence simultaneously within a quiescent combustion chamber. The two ternary mixtures are 68% by volume HCB, 17% octanol, and 15% methanol, as well as 58% HCB, 17% octanol, and 25% methanol, which are denoted as M15 and M25 respectively. It was found that the mixture stability of HCB/methanol is significantly enhanced using n-octanol as the co-solvent. The pure HCB (M0) was also tested under the same operating conditions as references. All the sprays were injected into the chamber through an injector equipped with a single-hole nozzle. Experimental results show that spray of M0 presents a faster penetration, following by M15 and M25, which is mainly caused by the shorter ignition delay of M0; With the increase of methanol percentage in the blends, the stoichiometric fuel mass fraction increases because of the high oxygen content of methanol, which contributes to a longer ignition delay; There are two factors of the blended fuel properties affecting flame lift-off length, namely stoichiometric fuel mass fraction and cetane number. It was found the cetane number plays a much more important role than that of stoichiometric fuel mass fraction on lift-off length.

The effect of dynamic operating conditions on the thermoacoustic response of hydrogen rich flames in an annular combustor

Self-excited thermoacoustic instabilities in symmetric annular combustion chambers typically give rise to spinning, standing and mixed azimuthal modes which are time-varying in nature and occur in a highly noisy environment due to turbulent combustion. We investigate the effect of linear ramps of increasing and decreasing equivalence ratio on the operating limits and thermoacoustic dynamics, from near lean blow-off to near flashback, in a laboratory scale annular combustor. The combustor features 12 lean-premixed hydrogen–methane flames at a fuel composition of 70% hydrogen and 30% methane by power. Equivalence ratio ramps were conducted for different thermal powers P=4, 6 and 8 kW per burner and three different ramp times tramp=5, 20, 60 s to simulate dynamic operation. It was found that ramping leads to self-excited instabilities that exhibit repeatable modal dynamics which depend on thermal power, ramp direction and duration. Different types of hysteresis were observed between the upward and downward ramps which affected the amplitudes and the stable operating range. The hysteresis phenomena also showed repeatable behaviour in terms of the nature and orientation angle. In one specific case, the simultaneous existence of two spinning modes was observed before the appearance of mode hopping leading to both an increase in frequency and doubling of the amplitude. High-speed OH* chemiluminescence of the flames showed that the mode hopping was accompanied by a change in the flame shape which becomes more compact and distributed.

Controlled detonation initiation in hypersonic flow

Experimental evidence of controlled detonation initiation and propagation in a hypersonic flow of premixed hydrogen-air is presented. This controlled detonation initiation is created in a hypersonic facility capable of producing a Mach 5 flow of hydrogen-air. Flow diagnostics such as high-speed schlieren and OH* chemiluminescence results show that a flame deflagration-to-detonation transition occurs as a combined result of turbulent flame acceleration and shock-focusing. The experimental results define three new distinct regimes in a Mach 5 premixed flow: deflagration-to-detonation transition (DDT), unsteady compressible turbulent flames, and shock-induced combustion. A two-dimensional implicit-LES (ILES) simulation, which solves the compressible, reactive Navier-Stokes equations on an adapting grid is conducted to provide additional insight into the local physical mechanism of detonation transition and propagation.

Flame development characteristics inside a reverse-flow combustor at early stages of combustion

Characteristics of flame development and the influence of reverse-flow configuration on these characteristics few seconds after ignition are investigated experimentally. Simultaneous and high-speed OH∗ chemiluminescence and pressure measurements are performed. Methane is used as the fuel, and the fuel–air equivalence ratio is 0.7. Four experimental conditions pertaining to mean bulk flow velocities of 4.0 and 6.4 m/s along with two igniter rod positions (−5 and 1.5 mm with respect to a flame-holder) are tested. The results suggest the flame development can be categorized into ignition, stabilization, and transition phases. During the ignition phase, compared to closed ducts, the reverse-flow configuration allows for achieving relatively large values of the normalized flame edge velocity prior to reaching the stagnation wall. Once the flame reaches the wall, the pressure rate significantly increases. After this, the flame stabilizes on the flame-holder forming a Bunsen-type flame, which is followed by a long term oscillation in the heat release rate for the majority of the tested conditions. During this period, the pressure, the pressure rate, and the spatially-averaged heat release rate feature a peak at the acoustic frequency of the combustor. The normalized power spectrum densities of the pressure and spatially-averaged heat release rate collapse and follow power-law relations, agreeing with the results reported in the literature pertaining to unconfined flames. This suggests, despite significant influence of the stagnation wall during the ignition and stabilization phases, spectral characteristics of pressure and heat release rate are not influenced by the presence of the wall during the transition phase.

Effect of the plasma location on the deflagration-to-detonation transition of a hydrogen–air flame enhanced by nanosecond repetitively pulsed discharges

This work presents a method for using nanosecond repetitively pulsed (NRP) plasma discharges for accelerating a propagating flame such that the deflagration-to-detonation transition occurs. A strategy is developed for bringing the location of the plasma near the tube wall and, thus, reducing the presence of the electrodes in the combustion tube as well as presenting a configuration in which cooling of the electrodes is viable for practical applications. Time-of-flight measurements were used in combination with energy deposition measurements and high-speed OH*-chemiluminescence imagery to investigate the flame acceleration process. For stoichiometric hydrogen–air flames, successful transition to detonation was achieved by applying a burst of 110 pulses at 100 kHz, with energies as low as 10 mJ per pulse. This was also achieved when plasma discharges were applied in the vicinity of the wall. Two enhancement mechanisms for flame acceleration were identified. The essential role of shock–flame interaction was established as being the main mechanism for flame acceleration when the discharges are located near the wall. This work presents an effective alternative that allows for NRP discharges to be applied near the wall while successfully maintaining a promising success rate for detonation transition.

Investigation of thermoacoustic instability in sequential combustor during first stage lean blow-off

The pressing need to increase the operational and fuel flexibility of gas turbines has recently led to the development of new combustor architectures such as constant-pressure-sequential-combustion systems. In this context, we investigated the thermoacoustic instabilities near the lean blow-off limit of the first stage in a generic sequential combustor. High-speed OH chemiluminescence was employed to visualize the lean-premixed first stage swirled flame and the second stage autoignition flame, and piezo-sensors were used to record the acoustic pressure around the first-stage blow-off condition. The study identified two dominant frequencies in the pressure signals: one at 144Hz when flames in both stages existed in the combustor and the other at 84Hz after the blow-off of the first stage flame. A calculation of the acoustic modes of the sequential combustor, using a Helmholtz solver, revealed that these two frequencies both corresponded to the second eigenmode of the combustion chamber. The sequence of thermoacoustic events associated with the first stage flame blow-off was unraveled for this generic setup, and this analysis could support interpreting the corresponding complex spectral signature in case such an event would occur in a practical system.

Effect of Jet-Jet Angle on Combustion Process of Diesel Spray in an RCEM

<div class="section abstract"><div class="htmlview paragraph">The effects of jet-jet angle on the combustion process were investigated in an optical accessible rapid compression and expansion machine (RCEM) under various injection conditions and intake oxygen concentrations. The RCEM was equipped with an asymmetric six-hole nozzle having jet-jet angles of 30° and 45°. High-speed OH* chemiluminescence imaging and direct photo imaging using the Mie scattering method captured the transient evolution of the spray flame, characterized by lift-off length and liquid length. The RCEM operated at 1200 rpm. The injection timing was -5°ATDC, and the in-cylinder pressure and temperature were 6.1 MPa and 780 K at the injection timing, respectively, which achieved a short ignition delay. The effects of injection pressure, nozzle hole diameter, and oxygen concentration were investigated. The results show that the liquid and lift-off length of the jet-jet angle of 30° were shorter than those of the jet-jet angle of 45°, irrespective of injection conditions and oxygen concentration. Particularly, when the injection rate was high with high injection pressure and a large nozzle hole, the liquid and lift-off length rapidly and simultaneously decreased, but when the injection pressure was low or the nozzle hole diameter was small, the lift-off length decreased first, and then the liquid length decreased.</div></div>

Numerical and experimental investigations of geometrical parameters on GH2/GO2 injector

To study the influence of injector geometrical parameters on combustor flow fields, detailed numerical and experimental investigations of a single-element injector with gaseous hydrogen and gaseous oxygen (GH2/GO2) propellants were performed at atmospheric pressure. The flow field characteristics of three different injector forms: shear-coaxial, swirl-coaxial and dual-swirl were discussed. The simulations of the combustion process with the use of steady Reynolds-averaged Navier-Stokes (RANS) techniques on a base of the ANSYS software were conducted. The Realizable k-ε turbulence model and eddy-dissipation concept combustion model were employed taking into account reactions of chemical kinetics. Results from the simulations demonstrated the detailed flame structures of the combustion flow fields. Subsequently, an advanced, non-intrusive optical diagnostics approach was adopted, including high-speed imaging and OH* chemiluminescence. The results show that the swirl flow injector approach elicits a distinctly different ignition process, in which the heat load from the swirl flow results in substantial damage to the chamber wall. Finally, with the help of advanced imaging processing method, it verified the validation of the numerical simulated model and acquired a specific linear relationship between the optical data and numerical calculations.

On the bi-stable nature of turbulent premixed bluff-body stabilized flames at elevated pressure and near lean blow-off

This study considers turbulent premixed bluff-body stabilized flames at elevated pressures. Specifically, the lean blow-off (LBO) limit of such flames is determined for a range of bulk velocities (5 ≤ U ≤ 50 m/s) and operating pressures up to 3 bar. Two key observations emerge from this stability assessment. The first is that considering elevated pressure leads to two stability regimes: one at atmospheric conditions and those with elevated pressure and U ≳ 20 m/s (regime-a), and another at elevated pressures with U ≲ 20 m/s (regime-b). The second observation is that within these regimes, LBO limits are insensitive to pressure. Flames in regime-a (S-flames) are found to be more stable than those in regime-b (U-flames). Advanced image-based diagnostics were employed to understand reasons for this difference in stability. Flow field measurements indicate that U-flames are associated with an outer recirculation zone (ORZ) that formed as pressure increased but receded from the burner as U surpassed ∼ 20 m/s. PLIF images of CH2O and OH demonstrated that the ORZ interacts with U-flames such that their downstream regions are prevented from collapsing to the inner recirculation zone (IRZ). Furthermore, analysis of the OH-PLIF images indicate that U-flames possess larger turbulent consumption rates, helping them form large IRZs and rendering them more susceptible to influence from the ORZ. Results of high-speed OH* imaging demonstrate that LBO events differ between U- and S-flames. Namely, while S-flames collapse to their IRZs during LBO, U-flames lift off from the burner, depleting their anchoring regions of reactions and hot products. Losing back-support in this region is what ultimately reduces the stability of U-flames. Finally, the reason U-flames lift off from the burner during LBO is elucidated by joint flow-flame measurements. Specifically, the anchoring regions of U-flames reside in regions of large axial velocity, which likely stems from their enhanced burning rates.

Simultaneous rainbow schlieren deflectometry and OH* chemiluminescence imaging of a diesel spray flame in constant pressure flow rig

The physical and chemical phenomena that take place during fuel injection, entrainment and fuel-air mixing, cool-flame and ignition reaction, and combustion in diesel sprays still require extensive study. Global parameters such as liquid and vapor jet penetration lengths and spreading rates render useful yet still limited information. Understanding of the temporal evolution of the spray as it progresses through various steps is needed to develop advanced clean combustion modes and high-fidelity predictive models with sufficient accuracy. In this study, high-speed rainbow schlieren deflectometry (RSD) and OH* chemiluminescence are used to simultaneously image fuel-air mixing, cool-flame reactions, ignition, flame propagation and stabilization, and combustion in a transient diesel-like flame. A constant pressure flow rig (CPFR) is used to conduct multiple injections in quick succession to obtain a statistically relevant dataset. n-heptane was injected at nominal supply pressure of 1000 bar from a single-hole diesel injector into ambient at pressure of 30 bar and temperature of 800 K. About 500 injections were performed and analyzed to reveal structural features of non-reacting and reacting regions of the spray, quantify jet penetration and spreading rates, and study cool-flame behavior, ignition, flame propagation and stabilization at lift-off length, and combustion at upstream and downstream locations.

Statistics and Dynamics of Intermittent Boundary Layer Flashback in Swirl Flames

Flame behavior at conditions approaching boundary layer flashback was studied in fuel-lean premixed swirl flames with a central bluff-body flame holder. High-speed chemiluminescence images were collected at over 450 different combinations of fuel composition (hydrogen/methane blends), equivalence ratio, flow rate, and reactant temperature. A selected group of conditions was further examined using simultaneous high-speed stereoscopic particle image velocimetry and OH planar laser induced fluorescence. Over a range of conditions between stable burning in the combustor and total flashback, the flame would intermittently propagate through the bluff-body boundary layer into the reactant feed tube for a period of time before retreating back to the combustion chamber. Statistical characteristics of the flame dynamics, such as the depth and duration of the flashback events, showed consistent relationships across all combinations of operating parameters. Hence, tracking these statistics provides a potential means of anticipating an upcoming flashback event. The number of transient flashback events per second showed particular promise as an early warning sign due its rather gradual change with flame propagation depth. Laser diagnostics revealed local reductions in axial velocity ahead of the tip of the flame protrusions. The strength of these reductions increased as the flame moved farther upstream; however, no total flow reversal was observed.

Pyrometric imaging of soot processes in a pilot ignited direct injected natural gas engine

This work assesses the soot formation and oxidation processes in a high-pressure direct injection natural gas engine using simultaneous high-speed two-colour pyrometry and OH* chemiluminescence imaging. A parametric investigation considers the effects of fuel injection pressure, injection duration and relative pilot and natural gas injection timings. A typical combustion event consists of natural gas being ignited by diesel pilot combustion products, followed by partially premixed combustion of natural gas near the piston bowl wall. Soot is formed as natural gas is injected into this reaction zone. A toroidal soot cloud forms and grows towards the centre of the piston bowl. For the range of operating conditions tested, the peak apparent heat release rate and the onset of detectable soot were correlated, as were the timing of the peak soot fraction and end of natural gas injection. The latter indicates that the soot formation and transition to a net oxidation process are strongly influenced by the injection process, similar to diesel engines. Changes in the relative timing of the diesel pilot and natural gas injections influenced natural gas premixing times, with increased premixing leading to a higher peak apparent heat release rate and lower peak soot fraction. The injection pressure affected the peak soot fraction though enhanced oxidation was expected to ultimately reduce the engine-out soot for higher injection pressures. Based on this parametric investigation, an existing conceptual model of pilot ignited direct injected natural gas combustion is extended to also describe the soot formation and oxidation processes.

Effect of Pressure on Hydrogen Enriched Natural Gas Jet Flames in Crossflow

The effect of pressure on hydrogen (H2) enriched natural gas jet flames in crossflow is experimentally investigated here. Simultaneously acquired high speed OH* chemiluminescence, OH planar laser induced fluorescence (PLIF), and stereoscopic particle image velocimetry are used to study flames at conditions typical of a gas turbine premixer, i.e. at elevated pressure, preheated crossflow, and in confinement. Two different H2 enrichment levels (40% and 20%, by volume) and pressures (10 bar and 15 bar) were studied here.Flames at the higher H2 enrichment level were found to be stabilized on the windward side, while the flames at the lower H2 enrichment were found to be stabilized only on the leeward side. Increased H2 enrichment was also associated with greater sootiness in the measured region. Jet centerline trajectories showed greater penetration for the higher H2 enrichment flames, which is in agreement with existing theories on the effect of heat release from a flame on crossflow entrainment. There were no significant changes observed in the mean OH* chemiluminescence, OH-PLIF, velocity fields, and velocity fluctuation fields with changes in pressure.

Experimental characterization of flame/flow dynamics during transition between stable and thermo-acoustically unstable conditions in a gas turbine model combustor

This paper presents an experimental study of the flame/flow dynamics in a single-element, swirl-stabilized, lean premixed pre-vaporized gas turbine model combustor under spontaneously intermittent transition between stable and thermo-acoustically unstable conditions. Three regimes, corresponding to stable, transitional and thermo-acoustically unstable were considered. High-speed OH⁎ chemiluminescence (CL) imaging and planar particle image velocimetry (PIV) measurements revealed that the transition process was associated with periodic interactions between vortices and inner shear layers (ISLs), leading to large-scale back and forth propagation and reaction intensity fluctuations of the flame. Proper Orthogonal Decomposition (POD) analysis indicated that the transition featured the presence of precessing vortex core (PVC), which contributed to the helical motion of the flame. Additionally, it was also shown that the flame appeared in outer recirculation (ORZ) zone most frequently during stable state as compared to both transitional and unstable cases. Such phenomenon further suggested that the flame in the ORZ could only serve as a necessary but insufficient condition for the transition from stable to thermo-acoustically unstable states, depending on the geometry and thermal state of the combustor.

Development and characterization of swirl-stabilized diffusion mesoscale burner array

A diffusion mesoscale burner array with integrated fuel manifold and injection holes has been developed for compact and efficient propulsion and power systems. Each burner element is equipped with its own fuel injection holes built into its swirl-inducing geometry to improve flame interaction and reduce flame length. The diffusion mesoscale burner array provides comparable performance to a premixed mesoscale burner array under similar operating conditions despite fuel unpremixedness. Overall, diffusion flames on the mesoscale burner array exhibit 7.8% lower global equivalence ratios at lean blow-off conditions compared to premixed flames. Imaging of two primary combustion intermediates, OH and CH2O, is carried out using planar laser induced fluorescence to visualize the post- and pre-combustion zones of diffusion and premixed flames. Simultaneous 10 kHz high-speed OH fluorescence images and pressure measurements are used to study flame response and stability under external acoustic perturbation at a frequency of 200 Hz. OH fluorescence images show higher heat release and flame front fluctuations for diffusion flames. Rayleigh index reveals similar thermo-acoustic stability characteristics between diffusion and premixed flames. However, diffusion mesoscale flames exhibit large flame surface variation under external acoustic perturbation by spectral proper orthogonal decomposition analysis.

Optical study on spray combustion characteristics of PODE/diesel blends in different ambient conditions

Polyoxymethylene dimethyl ethers (PODE) have been reported as a promising alternative fuel for the potential to reduce soot, CO and HC emissions of the diesel engine and improve the engine performance of gasoline compression ignition and dual-fuel engines. However, the fundamental data of spray and combustion characteristics of fuels with PODE are still deficient. In this study, the ignition delay, flame lift-off length (LOL) and soot distribution of diesel/PODE blends with PODE volume fraction of 0–100% were studied using high-speed OH chemiluminescence and planar laser-induced incandescence under different ambient temperature and oxygen concentration conditions. Results show that the ignition delay decreases and the flame LOL increases with the increase in PODE content. The increase in LOL is more distinct in lower ambient temperature conditions, whereas the decrease in ignition delay is relatively larger in higher temperature and lower oxygen concentration conditions. The total soot amount in the tested region decreases more than 50% and 95% when PODE volume fraction reaches 20% and 50%, respectively. When the PODE fraction is larger than 50%, nearly no soot can be observed in all the tested cases. The influence of PODE on soot reduction is more significant in higher ambient oxygen concentration and lower ambient temperature conditions. The soot distribution profile of the soot shows that the addition of PODE can not only reduce the soot formation but also accelerate the soot oxidation.

Characterization of Additive Layer Manufacturing Swirl Burner Surface Roughness and Its Effects on Flame Stability Using High-Speed Diagnostics

Abstract In this study, two Inconel 625 swirl nozzle inserts with identical bulk geometry were constructed via additive layer manufacturing (ALM) for use in a generic gas turbine swirl burner. Further postprocessing by grit blasting of one swirl nozzle insert results in a quantifiable change to the surface roughness characteristics when compared with the unprocessed ALM swirl nozzle insert or a third nozzle insert which has been manufactured using traditional machining methods. An evaluation of the influence of variable surface roughness effects from these swirl nozzle inserts is therefore performed under preheated isothermal and combustion conditions for premixed methane-air flames at thermal power of 25 kW. High-speed velocimetry at the swirler exit under isothermal conditions gives evidence of the change in near-wall boundary layer thickness and turbulent fluctuations resulting from the change in nozzle surface roughness. Under atmospheric combustion conditions, this influence is further quantified using a combination of dynamic pressure, high-speed OH* chemiluminescence, and exhaust gas emissions measurements to evaluate the flame stabilization mechanisms at the lean blowoff and rich stability limits. Notable differences in flame stabilization are evident as the surface roughness is varied, and changes in rich stability limit were investigated in relation to changes in the near-wall turbulence intensity. Results show that precise control of in-process or postprocess surface roughness of wetted surfaces can positively influence burner stability limits and NOx emissions and must, therefore, be carefully considered in the ALM burner design process as well as computational fluid dynamics (CFD) models.