Jet Flame In Crossflow Research Articles

Effect of excitation Strouhal number on a backward-inclined jet flame in crossflow

Combustion is often accompanied by environmental pollution due to incomplete reaction. A non-excited flame usually exhibits poor combustion performance with features of diffusion flame such as dual-peak temperature distributions and large combustion product concentrations. Efficient fuel-air mixing has been identified as a key factor in mitigating this issue. The present study investigated the effects of the acoustic excitation Strouhal number on the combustion performance of a backward-inclined jet flame in crossflow. A loudspeaker was used to induce jet pulsations with excitation Strouhal numbers ranging from 0.47 to 1.54 at a pulsation intensity of 0.90. Time-averaged and instantaneous flame images were captured using digital cameras. A customized R-type thermocouple and a commercial gas analyzer were employed to analyze the thermochemical structures. A critical excitation Strouhal number of approximately 0.9, distinguishing two characteristic flame modes: strongly affected flame (SA flame) and weakly affected flame (WA flame), was found. At excitation Strouhal numbers lower than the critical value, the SA flame featured single-peak temperature profiles and low unburned hydrocarbon, carbon monoxide, and nitric oxide emissions. The combustion performance was significantly improved due to the strong mixing effect induced by the acoustic excitation. The WA flame appeared at excitation Strouhal numbers higher than the critical value. It exhibited dual-peak temperature distributions and slightly improved combustion performance. The dynamic behavior and emissions of acoustically excited jet flames offered a fascinating insight into the influence of excitation Strouhal numbers on combustion characteristics. More excitation conditions were expected for future research.

Effects of Damköhler Number on Lean Premixed Jet Flames in Crossflow Using Direct Numerical Simulation

ABSTRACT In the present work, lean premixed jet flames in crossflow were studied using direct numerical simulation to explore the effects of Damköhler numbers () on turbulence and flame characteristics. The configuration is based on an experiment, where a jet of lean premixed ethylene-air mixtures ( = 0.6) was injected into a hot vitiated crossflow. The case with a higher and lower Da is denoted as case Da+ and case Da-, respectively. It was found that the turbulence is weakened around the reaction zone of case . In contrast, the turbulence of case is barely influenced by heat release. The vorticity dynamics are examined with the enstrophy transport equation budget terms, and the viscous dissipation term is the main sink for both cases. The windward and leeward flame branches of case are dominated by auto-ignition and premixed flame propagation respectively, while the auto-ignition and flame propagation modes coexist on the leeward flame branch of case . The scalar gradient magnitude in the leeward branch is generally lower compared with the windward branch so that the flame is overall thicker. The normal strain rate is negative, which tends to enhance the scalar gradient, but it becomes positive on the leeward side of case , which is influenced by the alignment between the scalar gradient and the principal strain rates.

A direct numerical simulation study on the structures and turbulence–flame interactions of a laboratory-scale lean premixed jet flame in cross-flow

In the present work, direct numerical simulation of a laboratory-scale lean premixed reacting jet in cross-flow was performed to explore the flow–flame structures and turbulence–flame interactions. A jet of lean premixed ethylene–air mixtures (equivalence ratio $\phi = 0.6$ ) was injected into a hot vitiated cross-flow. Both non-reacting and reacting cases were simulated. It was found that the reacting jet penetrates deeper in the cross-flow with a weaker shear layer compared with the non-reacting one. The wake of the non-reacting and reacting jet is characterized by vertical vortices and recirculation zones, respectively. As for the flame structure of the reacting case, the reaction intensity varies considerably in different flame zones. The heat release rate on the leeward side is higher than that on the windward side, but lower than that of the corresponding laminar flame. The analysis of the turbulence–flame interactions of the reacting case showed that the large local Damköhler number ( $Da$ ) related to reaction-induced dilatations results in an increased tendency of the scalar gradient to align with the most extensive strain rate, which is more evident in the regions with high heat release rate on the leeward side. Negative dilatation regions with positive tangential strain rate and negative normal strain rate are observed on the windward side. High positive dilatations appear on the flame front of the leeward side. The tangential strain rate is negatively correlated with the normal strain rate and curvature. Regions with a high local $Da$ on the windward side correspond with high positive curvature regions.

Low NOX combustion characteristics by hydrogen micro jet flame in cross flow

Low NOX combustion characteristics by hydrogen micro jet flame in cross flow

Flame characteristics of backward-inclined pulsating combusting jet in crossflow

The flame appearances, thermal characteristics, and combustion product concentrations of a backward-inclined pulsating combusting jet in crossflow were experimentally studied. The time-averaged and streak instantaneous flame images were acquired using a still camera and a high-speed camera, respectively. The flame extinguishment processes were captured and analysed on the basis of streak instantaneous images. The flame temperatures and combustion product concentrations were measured using a homemade fine-wire thermocouple and a gas analyser, respectively. Three characteristic flame modes (crossflow dominated, transitional, and jet-dominated) were found when the backward-inclined jet flame in crossflow was subject to acoustic excitation. These characteristic flame modes were observed in different regimes of pulsation intensity and jet backward inclination angle. Discrete flame puffs with a frequency equal to the acoustic excitation frequency appeared in the jet-dominated flame. Periodic flame puffing became unstable when the pulsation intensity was increased to values near the instability limit. When the pulsation intensity was beyond the instability limit, a strong stretching effect caused the recirculation flame, which played the role of a flame ignitor, to almost vanish (misfire). The flame extinguishment then occurred from the area around the lee side of the burner tube tip. The temperature and gas concentration measurement results showed the significant improvement of combustion performance in low inclination angle even at small pulsation intensity in the regime of the crossflow dominated flame. To enhance combustion performance at a large jet backward inclination angle, the pulsation intensity must be large enough to push the flame in the regime of the jet-dominated flame.

A DNS study on the flame structures and flame stabilization mechanism of a laboratory-scale lean premixed jet flame in crossflow

In the present work, direct numerical simulation (DNS) of a laboratory-scale lean premixed reacting jet flame in crossflow was performed to understand the flame structures and the flame stabilization mechanism. In the DNS, an ethylene-air jet with an equivalence ratio of 0.6 was injected into a hot vitiated crossflow. The jet Reynolds number reaches 6161. The DNS results were compared with those of the experiment with a good agreement. It was found that the windward and leeward branches of the flame show significantly different behaviors. The windward flame branch, appearing lifted and discontinuous, is located in the shear layer regions with high temperature, low vorticity and low scalar dissipation rate. The location of the peak heat release rate shifts to a higher mixture fraction with increasing distance from the jet exit. The leeward branch of the flame anchors in the shear layer near the jet exit. The recirculation zone in the wake of the jet facilitates the stabilization of the leeward flame. The chemical explosive mode analysis (CEMA) and species budget analysis were employed to characterize the local combustion mode. Auto-ignition plays a key role in the stabilization of the windward flame where a large range of extinction is also found due to the high strain rate. In contrast, premixed flame propagation is dominant on the leeward side.

An Experimental Study Of Natural Gas Jet-Flames In Crossflow With High Hydrogen Admixture At Elevated Pressure Conditions

A series of high hydrogen-content jet flames in crossflow are investigated experimentally using high-speed (10 kHz) stereoscopic particle image velocimetry, dual-plane laser-induced fluorescence of the hydroxyl radical, and OH* chemiluminescence imaging. The flames were fueled with natural gas enriched to 70-, 80-, 90- and 100% H2, by volume at conditions approaching those found in the mixing duct of a modern gas turbine engine, specifically in close confinement, at 7 bars pressure and with a crossflow preheat of 511 K. The measurements indicate the recirculation zone on the leeward side of the jet plays a critical role in stabilization of the lifted jet flame in crossflow. The measurements show that local heat-release from a flame in this region acts as a self-reinforcing feedback mechanism, strengthening the recirculation zone and increasing local residence time there. This, in turn, creates conditions more favorable to flame stabilization. The measurements also showed how close confinement of the JICF by the flow channel walls affects jet-penetration and growth of the counter-rotating vortex pair.

Effects of Hydrogen-Enrichment on Flame-Holding of Natural Gas Jet Flames in Crossflow at Elevated Temperature and Pressure

The effect of hydrogen (mathrm {H}_{mathrm {2}}) enrichment on the flame-holding characteristics of two natural gas jet flames in crossflow is investigated here, experimentally. The flame and flowfield measurements are analyzed using simultaneously acquired high-speed (10 kHz) stereoscopic particle image velocimetry, planar laser-induced fluorescence of the hydroxyl radical, and OH* chemiluminescence. The flames, enriched with 20% and 40% mathrm {H}_{mathrm {2}}, by volume, are studied at conditions typical of the mixing duct of a modern gas turbine engine; specifically in confinement, at 10 bars, and with a crossflow preheat of 530 K. Consistent with previous findings, the 40% mathrm {H}_{mathrm {2}} flame was found to be stabilized on the windward and leeward side of the jet, while the 20% mathrm {H}_{mathrm {2}} flame was stabilized only on the leeward side. Analysis of mean and instantaneous velocity fields showed no major differences in the trajectories and principal compressive strain fields of the two flames. The presence of the windward stabilized flame in the 40% mathrm {H}_{mathrm {2}} case was, however, found to decrease the centerline velocity decay and greatly reduce or eliminate large scale vortices along the windward shear layer. The difference in the flame-holding here was attributed to the difference in the extinction strain rate from the addition of hydrogen, which would impact the local and global extinction of the flame along the high shear windward region of the flame.

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.

Effects of Pulsation Intensities on Flame Characteristics of a Small Backward-Inclined Jet Flame in Crossflow

Abstract The effects of pulsation intensities on the flame characteristics of a 10 deg-backward-inclined jet flame in the crossflow were investigated in a wind tunnel. The jet and the crossflow Reynolds numbers were 1527 and 2165, respectively. The jet-to-crossflow momentum flux ratio was 0.10. A loudspeaker was used to acoustically excite the jet flame. The excitation Strouhal number was 0.73, while the jet pulsation intensities varied from 0 to 1.26. The flame behaviors were studied through photography techniques. The flame temperatures were measured using a fine-wire R-type thermocouple. The combustion-induced emissions were probed by a commercial multi-gas analyzer. The jet flames were categorized into five characteristic modes with increasing pulsation intensities. Mode I was characterized by a yellowish down-washed recirculation flame, a blue neck flame, and a yellow tail flame. Modes II and III featured a split yellow tail flame, a yellowish recirculation flame, and a blue neck flame. Mode IV was characterized by a blue down-washed recirculation flame and neck flame, as well as a split yellow tail flame. Mode V was identified by a single yellow tail flame and the absence of the down-washed recirculation flame. When the jet flames were excited beyond mode I, the combustion-induced pollutants of carbon monoxide and nitric oxide were significantly reduced. However, the excited jet flame in mode V displayed low temperatures in the near-tube region.

Flow and Temperature Characteristics of a 15° Backward-Inclined Jet Flame in Crossflow

The flow and flame characteristics of a 15° backward-inclined jet flame in crossflow were investigated in a wind tunnel. The flow structures, flame behaviors, and temperature fields were measured. The jet-to-crossflow momentum flux ratio was less than 7.0. The flow patterns were investigated using photography and Mie-scattering techniques. Meanwhile, the velocity fields were observed using particle image velocimetry techniques, whereas the flame behaviors were studied using photographic techniques. The flame temperatures were probed using a fine-wire R-type thermocouple. Three flame modes were identified: crossflow dominated flames, which were characterized by a blue flame connected to a down-washed yellow recirculation flame; transitional flames identified by a yellow recirculation flame and an elongated yellow tail flame; and detached jet dominated flames denoted by a blue flame base connected to a yellow tail flame. The effect of the flow characteristics on the combustion performance in different flame regimes is presented and discussed. The upwind shear layer of the bent jet exhibited different coherent structures as the jet-to-crossflow momentum flux ratio increased. The transitional flames and detached jet dominated flames presented a double peak temperature distribution in the symmetry plane at x/d = 60. The time-averaged velocity field of the crossflow dominated flames displayed a standing vortex in the wake region, whereas that of the detached jet dominated flames displayed a jet-wake vortex and a wake region source point.

Backward-Inclined Diffusion Jet Flames in Crossflow at Low Jet-to-Crossflow Momentum Flux Ratios

In the design of gas turbine combustors, efforts are engineered toward reducing the combustion pollutant emission levels. The pollutant emissions can be reduced by premixing the fuel and the air prior to ignition. However, the main challenges encountered with premixing are flame flashback and blowout, thus, the preference of diffusion flames. In this study, flame behavior, flow patterns, and thermochemical fields of backward-inclined diffusion jet flames in crossflow at low jet-to-crossflow momentum flux ratio of smaller than 0.04 were studied in a wind tunnel. The backward-inclination angle was varied within 0–50 deg. The flames presented three characteristic modes: crossflow dominated flame (low backward inclination angle) denoted by a large down-washed recirculation flame, transitional flame (mediate backward inclination angle) identified by a recirculation flame and a tail flame, and jet dominated flame (high backward inclination angle) characterized by a blue flame base, a yellow tail flame, and the absence of a recirculation flame. Short flames are detected in the regime of the crossflow dominated flames—an indication of improved fuel–air mixing. The findings suggest that for low exhaust emissions which are vigorously pursued in the aviation and thermal power plant industries, especially during low-load operations, the jet dominated flames are the preferable flames as they generate low unburned hydrocarbon, carbon monoxide, and nitric oxide emissions compared to the other flames.

Characteristics of backward-inclined non-premixed jet flames in crossflow

The characteristics of backward-inclined non-premixed jet flames in a uniform crossflow were studied in a wind tunnel. Time-averaged photography techniques were used to study flame behavior. The flow field was captured by short exposure photography and Mie-scattering techniques. Flame temperatures were probed with a fine-wire R-type thermocouple. In the domain of jet-to-crossflow momentum flux ratio R and backward-inclination angle θ, the flames were categorized into three characteristic modes. The first mode consisted of crossflow dominated flames characterized by a down-wash recirculation flame in the wake of the burner tube. The second mode consisted of transitional flames characterized by a yellowish recirculation flame and a tail flame. The third mode consisted of jet dominated flames characterized by a blue flame base and an absence of the down-wash flame. The down-wash recirculation flames were observed for a backward inclination angle of θ < 40°. The ability of the flames to resist blow off when increasing the jet-to-crossflow momentum flux ratio decreased as θ increased. Coherent vortices were observed on the seeded fuel jet, whose type was dependent on R and θ. In the upstream region, the fuel appeared above the flame. However, in the downstream region, the fuel became engulfed within the flame. For a fixed θ, the Strouhal number of the upwind shear layer vortices on the fuel jet was observed to decrease asymptotically as R increased. In the near-field at x/d ≈ 5, the crossflow dominated flames presented temperature profiles characterized by a broad dual-hump peak profile in the symmetry plane, while in the near-field at x/d ≈ 5, the jet dominated flames presented a single peak profile in the symmetry plane.

Non-Premixed Burner-Attached Jet Flames in Crossflow Pulsed at Resonance Frequency

The flame behavior, flow patterns, and thermochemical structures of a combusting propane gas jet in crossflow excited acoustically at resonance frequency were studied in a wind tunnel. The jet and crossflow Reynolds numbers were 1530 and 1399, respectively. The jet-to-crossflow momentum flux ratio was 0.192. The flame behavior was found to be closely related to the shear-layer vortical flow structures in the near-tube region. Six characteristic flame modes were observed in the domain of jet pulsation intensity . In flame modes 1 and 2 (), the shear-layer vortices were dominant. In flame modes 3–6 (), the flames were dominated by a puffing bent jet. Vortical structures increased in size with increasing . Temperature and combustion product measurements showed significant improvements in mixing and combustion, as well as a reduction of combustion-induced pollutants at the near-tube region of the flame. However, pulsing the jet flame in crossflow at pulsation intensities greater than 0.9 led to a significant increase in the maximum values of carbon monoxide and nitric oxide concentrations at the middle- to far-tube flame regions. Therefore, strongly pulsing a jet flame in crossflow may not necessarily lead to an improved combustion.

Effects of jet pulsation intensity on a wake-stabilized non-premixed jet flame in crossflow

The jet pulsation intensity effects induced by acoustic excitation on a non-premixed fuel jet flame in crossflow were experimentally studied in a wind tunnel at the crossflow Reynolds number Rew=1416, jet Reynolds number Rej=1550, jet-to-crossflow momentum flux ratio R=0.192, and excitation Strouhal number Stexc=0.21. The jet pulsation intensity Ipul varied from 0 to 0.9. The flame behaviors, stability mechanism of flame blowoff, flow patterns, and thermochemical fields were measured and discussed. The time-averaged and instantaneous flame appearances were photographed by a high speed camera. The instantaneous flow patterns were obtained by the laser-light-sheet-assisted particle-tracing method. The temperature and concentrations of combustion-induced pollutants (unburned HC, CO, and NO) were measured by a fine-wire R-type thermocouple and a gas analyzer, respectively. Analysis of the measured results showed that the flame and flow behaved differently within four ranges of jet pulsation intensities (slight, medium, strong, and over excitations). The flows presented different characteristic modes of enlarged shear-layer vortices, irregular puffs, up-down oscillating bent jet in different ranges of jet pulsation intensities, and therefore induced different flame behaviors in different ranges of jet pulsation intensities. The primary flow features leading to modifications of the flame behaviors were the upwind shear-layer vortices and the recirculation area around the leeward side of burner tube tip. Appling excitation to the combusting jet in crossflow at Ipul>0.55 could lead to significant improvement in mixing and combustion and reduction of combustion-induced pollutants.

Mixing characteristics and emissions of strongly-forced non-premixed and partially-premixed jet flames in crossflow

The effects of forcing and partial premixing on the mixing characteristics and pollutant emissions of jet flames in crossflow (JFICF) are studied experimentally for 70% methane/30% hydrogen fuel blend and propane. For forced non-premixed JFICF, the jet fuel flow is modulated using an acoustic speaker system, which results in a drastic decrease in flame length and soot luminosity. Simultaneous planar laser Mie scattering and CH * chemiluminescence imaging is used to detail the flame structure and dynamics of the reacting forced non-premixed JFICF, whereas acetone planar laser-induced fluorescence is used to quantify the near-field mixing of the non-reacting case. Forced JFICF are characterized by periodic ejections of high-momentum, deeply penetrating vortical structures, which draw air into the jet nozzle and enhance mixing in the near-field region of the jet. The mixing enhancement effectively partially premixes the jet fluid below the flame base. Flame luminosity images and exhaust gas emissions measurements show that forced non-premixed JFICF exhibit similar characteristics to unforced partially-premixed JFICF. Both strong forcing and partial premixing result in net reductions in NO x , but increases in CO and unburned hydrocarbons. NO x scaling analysis is presented for both forced non-premixed and unforced partially premixed flames. Using flame volume arguments, EI NO x scales with amplitude ratio, but does not scale with partial premixing. The difference in scaling behavior is attributed to differences in flame structure.

Numerical Study of Emission Characteristics of a Jet Flame in Cross-Flow

The authors numerically investigated the effect of changes in the fuel exit velocity and the cross-flow on the length, radiant fraction, and emission indices of pollutant species (NOx and CO), as well as the ratio NO2/NOx, of a high momentum jet flame in cross-flow. This flow configuration is of generic interest, and also practically relevant to flaring operations. The flow field is computed based on the Reynolds-Averaged Navier–Stokes equations incorporating the realizable k−ϵ turbulence closure. The combustion process is modeled using the unsteady Eulerian particle flamelet model based on the mixture fraction approach, and the heat loss by radiation is accounted for using the discrete ordinates method. Comparison of the predicted flame length and the trend of the emission indices with experimental measurements reveal good agreement for the range of jet-to-cross-flow momentum flux ratios investigated, namely 100–800.

Numerical study on high-temperature diluted air combustion for the turbulent jet flame in crossflow using an unsteady flamelet model

The turbulent jet flame in a crossflow with highly preheated diluted air has been numerically investigated. The Favre-averaged Navier–Stokes equations are solved by a finite volume method of SIMPLE type that incorporates the flamelet concept coupled with the standard k– ε turbulence model. The NO formation is estimated by using the Eulerian particle transport equations in a postprocessing mode. For methane and propane with various conditions of inlet air temperature and oxygen concentration, the three-dimensional characteristics of the flame are successfully captured. The jet-flame trajectory is in remarkably good agreement with the existing cold-flow correlations. When the oxygen concentration is high, the maximum flame temperature becomes high and the two fuels show quite different characteristics in the downstream region. On the other hand, for low oxygen concentrations, the temperature difference between the two fuels is relatively small and remains fairly constant throughout the combustion chamber. The propane gives a higher NO formation compared to the methane especially when the oxygen concentration is high. A higher temperature, longer residence time of the combustion gases may be responsible for the higher thermal NO formation.

Pilot Flame Effects on Gas Jet Flames in Crossflow

Flame structure, which includes the radiation power and temperature profiles, soot concentration profiles, and pollutants emissions (NO, NO x , CO, and CO 2 ) of propane and propylene gas diffusion flames with and without ahydrogen pilot flame in crossflow is presented. All of the profiles were obtained at a distance of 50% of the total flame length from the burner. The purpose was to investigate the effects of a pilot flame on the structure of flames in crossflow. The fuel was injected vertically into a horizontal airstream through a burner. The flames at nonpiloted conditions were partially or completely detached form the burner. For the piloted flames, hydrogen was added to completely attach the flames to the burner. Experiments were conducted at two different crossflow speeds, 2.68 m/s and 3.58 m/s. The piloted flames, in general, had lower emissions of NO, NO x , and CO 2 . The temperature profiles in all flames showed a peak near the flame center. The pilot flame did not significantly influence the flame temperature profiles at the crossflow speed of 2.68 m/s. However, at 3.58 m/s, the overall flame temperature for both fuels was lower in the piloted flames. The nonpiloted detached flame showed higher concentrations of NO x , CO 2 , and O 2 , but had a lower soot concentration. All of the piloted flames appeared to be longer and narrower compared to detached flames. Piloted flames also produced higher radiation, produced more smoke, and appeared more luminous than detached flames.

Pressure Effects on Geometry and Structure of Gas Jet Flames in Crossflow

Flame structure and geometry of natural gas andpropanediffusion  ames in cross ow are presented to delineate the effects of buoyancyand ambientpressure. The fuel was injected horizontally into a verticallymovingairstream. The buoyancy acted along the direction of the cross ow. The combustion-chamber pressure was varied from 1 to 1.45 bar and the cross ow to jet velocity ratio from 0.1 to 1.45. Quantitative rainbow schlieren de ectometry was used to visualize the  ame and to measure the geometrical parameters. The centerline  ame trajectory showed three distinct regions, and the dimensionless coordinate of the jet centerline in the jet direction varied with 0.45, 0.25, 0 power of the coordinate in the cross ow direction and i 0.5 power of the Froude number based on the jet velocity in these regions. The  ame trajectories did not change signiŽ cantly with pressure. Flame chord length varied with 0.45 power of the cross ow/jet velocity ratio. Flame length and soot formation characteristics changed notably with pressure. Propane  ames were more susceptible than natural gas  ames to the ambient pressure. The  ame luminosity caused by soot in the propane  ame changed only slightly with velocity ratio. However, the natural gas  ame became nonsooty at high velocity ratios.