Flame stability and emission characteristics of turbulent LPG IDF in a backstep burner

The stability characteristics and emissions from turbulent LPG inverse diffusion flame (IDF) in a backstep burner are reported in this paper. The blow-off velocity of turbulent LPG IDF is observed to increase monotonically with fuel jet velocity. In contrast to normal diffusion flames (NDF), the flame in the present IDF burner gets blown out without getting lifted-off from the burner surface. The soot free length fraction, SFLF, defined as the ratio of visible premixing length, H_p, to visible flame length, H_f, is used for qualitative estimation of soot reduction in this IDF burner. The SFLF is found to increase with central air jet velocity indicating the occurrence of extended premixing zone in the vicinity of flame base. Interestingly, the soot free length fraction (SFLF) is found to be correlated well with the newly devised parameter, global momentum ratio. The peak value of EINO_X happens to occur closer to stoichiometric overall equivalence ratio.     

Flame length scaling in a non-premixed turbulent diluted hydrogen jet with coaxial air

The effect of fuel composition on flame length was studied in a non-premixed turbulent diluted hydrogen jet with coaxial air. Because coaxial air entrained in a fuel stream enhances the mixing rate of fuel and air, it substantially reduces flame length. The observed flame length was expressed as a function of the ratio of coaxial air to fuel jet velocity and compared with a theoretical prediction based on the velocity ratio. Four cases of fuel mixed by volume were determined: 100% H₂, 80% H₂/20% N₂, 80% H₂/20% CO₂, and 80% H₂/20% CH₄. In addition, fuel jet air velocity and coaxial air velocity were varied in an attached flame region as u_F = 86-309 m/s and u_A = 7-14 m/s. In this study, we derived a scaling correlation for predicting the flame length in a simple jet with coaxial air using the effective jet diameter in a near-field concept. The experimental results showed that the visible flame length was in good relation to the theoretical prediction. The scaling analysis is also valid for diluted hydrogen jet flames with varied fuel composition, which affects flame length by varying the density of the fuel.     

Nitrogen dilution effect on flame stability in a lifted non-premixed turbulent hydrogen jet with coaxial air

The nitrogen dilution effect on flame stability was experimentally investigated in a lifted non-premixed turbulent hydrogen jet with coaxial air. Hydrogen gas was used as the fuel and coaxial air was injected to initiate flame liftoff. Hydrogen was injected into an axisymmetric inner nozzle (d_F = 3.65 mm) and coaxial air jetted from an axisymmetric outer nozzle (d_A = 14.1 mm). The fuel jet and coaxial air velocities were fixed at u_F = 200 m/s and u_A = 16 m/s, while the mole fraction of the nitrogen diluent gas varied from 0.0 to 0.2 with a 0.1 step. For the analysis of the flame structure and the flame stabilization mechanism, the simultaneous measurement of PIV/OH PLIF was performed. The stabilization point was in the region of the flame base with the most upstream region and was defined as the point where the turbulent flame propagation velocity was found to be balanced with the axial component of the local flow velocity. The turbulent flame propagation velocity increased as the nitrogen mixture fraction decreased. The nitrogen dilution makes the flame structure more premixed. That is, the stabilization mechanism shifts from edge flame propagation based mechanism toward premixed flame propagation based mechanism. We concluded that the turbulent flame propagation velocity was expressed as a function of the turbulent intensity and the axial strain rate, even though the mole fraction of the nitrogen diluent varied.     

Experimental studies of bluff-body stabilized LPG diffusion flames

Bluff-body stabilized turbulent jet diffusion flame has received renewed attention in recent years due to its practical applications. An experimental study is carried out to investigate the effect of coaxial air velocity, U_a, and lip-thickness, δ of the bluff-body on the flame stability limits and emission levels. The stability limits of a typical diffusion flame can be characterized in terms of two parameters namely flame lift-off height and blow-off velocity. It is experimentally observed that lift-off height is not linearly dependent on the fuel exit velocity, U_f, as compared to the simple jet. The flame stability is found to be improved for larger lip-thickness bluff-body because of the presence of lower pressure in the wake region behind the bluff-body. Flame length is observed to be dominated by buoyancy and momentum regimes. The transition from buoyancy to momentum regime is found to be extended with increase in lip-thickness. It is also observed that the blow-off limit is also extended further by 10% as compared to simple jet diffusion flames under similar conditions. The emissions data are reported in terms of mass based emission index, EINO_x (g [NO_x]/kg [fuel]) for a wide range of flow conditions. It is concluded that the addition of coaxial air in the larger lip-thickness bluff-body flames causes a marginal reduction in emission levels relative to smaller lip-thickness bluff-body.     

Study of the turbulent inverse diffusion flame in recessed backstep and coaxial burners

A comparative study of the turbulent Inverse Diffusion Flame (IDF) in recessed coaxial and backstep burners is carried out, based on visible flame appearance, flame length, flame stability, centerline temperature distribution, centerline oxygen concentration, and NO _x emissions. The backstep burner is observed to produce a compact flame shape with less luminosity at a higher air-fuel velocity ratio, as compared to the coaxial burner. Moreover, slightly better thermal characteristics and marginal reduction in NO _x emissions are provided by the backstep IDF, as compared to the recessed coaxial IDF. Besides this, the centerline oxygen concentration is marginally increased in the backstep IDF due to higher entrainment of ambient air. Interestingly, a lower flame stability limit is seen in the backstep burner than in the coaxial IDF, which can be attributed to its enhanced fuel-air mixing.     

Liftoff height behavior in a non-premixed turbulent hydrogen jet with coaxial air

To understand hydrogen lifted flames, the experimental approximation of liftoff height in non-premixed turbulent conditions was studied. The objectives were to analyze liftoff height behavior and to derive the normalized expression for lifted jet with the effective diameter (d_F,eff). Hydrogen flow velocity varied from 100 m/s to 300 m/s. Coaxial air velocity was regulated from 12 m/s to 20 m/s. For the simultaneous measurement of velocity field and reaction zone, particle image velocimetry using hydroxyl radicals (PIV/OH) planar laser-induced fluorescence (PLIF) techniques with neodymium-droped yttrium–aluminum-gamet (Nd:YAG) lasers and charge-coupled device/intensified charge-coupled device (CCD/ICCD) cameras were used. Liftoff height decreased with increased fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. The stabilization point was defined as the point where local flow velocity is balanced with turbulent flame propagation velocity. On the basis of the far field concept, we could derive the experimental approximation of the liftoff height divided by the effective diameter.     

Spatial Structure of a Reacting Turbulent Swirling Jet Flow with Combustion of a Propane–Air Mixture

Results of an experimental study of the spatial structure of a reacting flow during combustion of a propane–air mixture in a turbulent swirling jet escaping into atmospheric air are presented. The fuel-to-air equivalence ratio is φ = 0.7, and the Reynolds number of the jet is Re = 5 · 10³. The time-averaged spatial distributions of velocity, local density, and concentrations of the main species of the gas mixture are measured in low-swirl and high-swirl flows. In both cases, the flame front is stabilized in the internal mixing layer formed by the axial region of jet retardation, where hot combustion products are concentrated. In a high-swirl flow, the temperature distributions in the cross section y/d = 0.5 show that the region with the maximum temperature of the gas is located at the periphery of the central recirculation zone. Moreover, in the case of a high-swirl flow, there exists a recirculation zone at the axis, and the CO₂ concentration is twice higher than in a low-swirl jet. The opposite situation is observed for O₂.     

Parametric study of combined premixed and non-premixed flame coal burner

A combined gas/air mixture–coal burner was developed to include heat recirculation by utilizing a radiative solid material with premixed flame jets impinging onto the downstream side to preheat the fuel/air jet on the upstream side. Providing the heat recirculation mechanism at different air staging degrees enhanced the destruction rates of the fuel nitrogen oxides. Concentric elliptical premixed gas/air and coal/air jets had a stronger preheating effect and a consequent increased NO_x reduction effectiveness as compared to concentric circular jets, where the inner elliptical jets enlarged the contact diffusion area and entrainment thus increasing the preheating time. The parametric variation in the feeding ports to the coal combustor affected the exhaust emissions, wherein the use of an inclined or shifted injection from the centre-line contributed to the NO_x reduction. Increasing the jet angle in the upstream direction reduced the CO concentrations, while the NO_x emissions varied depending on the degree of staging. The inverse/normal flame configuration was found more effective than the normal flame configuration with respect to NO_x reduction that was enhanced at higher heat input ratios. Utilizing inverse triple flames led to a further NO_x reduction since higher temperatures prevailed in the initial flame region with a five reaction zone structure. Finer particles produced less NO_x, which was further reduced by blending the coal with biomass.     

Flame structure of LPG-air Inverse Diffusion Flame in a backstep burner

The present experimental study characterizes the turbulent LPG Inverse Diffusion Flame (IDF) stabilized in a backstep burner in terms of visible flame length, dual flame structure, centerline temperature distribution, and oxygen concentration. The visible flame length for a fixed fuel jet velocity is found to reduce with increase in air jet velocity. Besides this, the effect of air and fuel jet velocities on visible flame length is interpreted using a new parameter, Global Momentum Ratio (GMR). Interestingly, GMR seems to be correlating well with the visible flame length for the air and fuel velocity ranges considered in the present study. Moreover, the dual flame structure of IDF is identified with the help of CH-chemiluminescence signature. The existence of dual flame structure of IDF is confirmed further with the centerline temperature and oxygen concentration measurements.     

Regimes of Unstable Expansion and Diffusion Combustion of a Hydrocarbon Fuel Jet

Results of an experimental study of hydrodynamics and diffusion combustion of hydrocarbon jets are presented. Various regimes of instability development both in the jet flame proper and inside the source of the fuel jet are considered. The experiments are performed for the case of subsonic gas jet expansion into the air from a long tube 3.2 mm in diameter in the range of Reynolds numbers from 200 to 13 500. The fuel is the propane–butane mixture in experiments with a cold jet (without combustion) and pure propane or propane mixed with an inert dilutant (CO₂ or He) for the jet flame. The mean velocity and velocity fluctuations in the near field of the jet without combustion are measured. Among four possible regimes of cold jet expansion (dissipative, laminar, transitional, and turbulent), three last regimes are investigated. The Hilbert visualization of the reacting flow is performed. The temperature profiles in the near field of the jet are measured by a Pt/Pt–Rh thermocouple. An attached laminar flame is observed in the transitional regime of propane jet expansion from the tube. In the case of combustion of C₃H₈ mixtures with CO₂ or with He in the range of Reynolds numbers from 1900 to 3500, the transitional regime is detected in the lifted flame. Turbulent spots formed in the tube in the transitional regime exert a significant effect on the flame front position: they can either initiate a transition to a turbulent flame or lead to its laminarization.     

Experimental measurements of a gas-solid jet downstream of a fuel-rich/lean burner with a collision-block-type concentrator

The mixing mechanisms in the gas-solid two-phase fuel-rich/lean jet from a burner with a collision-type coal concentrator has been experimentally investigated using a fiber optic measurement system in a continuous-two-phase-flow loop of internal diameter 150 mm. The local solid concentration and particle size distribution were measured to investigate the mixing performance of the parallel fuel-rich and fuel-lean streams, and also the process of development of the fuel-rich/lean jet. The measurement results indicated that the concentration ratio between the fuel-rich and fuel-lean streams changes greatly with the height of the collision block. The fuel-rich and fuel-lean streams do not mix together soon after leaving the burner; this leads to better ignition and combustion characteristics of the pulverized coal compared with an ordinary burner. A local zone of high fuel concentration can be achieved at the fuel-rich side, resulting in staged combustion, to control NO_x emissions. A deviation between the jet direction and the nozzle axis was found, resulting from the velocity difference between the fuel-rich and fuel-lean streams, and a larger deviation in the profile of the solid phase was also observed. Such a deviation of the high-concentration zone is favorable for flame stability and NO_x emission reduction.     

Investigation of flue-gas treatment with O3 injection using NO and NO2 planar laser-induced fluorescence

Direct ozone (O₃) injection is a promising flue-gas treatment technology based on oxidation of NO and Hg into soluble species like NO₂, NO₃, N₂O₅, oxidized mercury, etc. These product gases are then effectively removed from the flue gases with the wet flue gas desulfurization system for SO₂. The kinetics and mixing behaviors of the oxidation process are important phenomena in development of practical applications. In this work, planar laser-induced fluorescence (PLIF) of NO and NO₂ was utilized to investigate the reaction structures between a turbulent O₃ jet (dry air with 2000 ppm O₃) and a laminar co-flow of simulated flue gas (containing 200 ppm NO), prepared in co-axial tubes. The shape of the reaction zone and the NO conversion rate along with the downstream length were determined from the NO-PLIF measurements. About 62% of NO was oxidized at 15d (d, jet orifice diameter) by a 30 m/s O₃ jet with an influence width of about 6d in radius. The NO₂ PLIF results support the conclusions deduced from the NO-PLIF measurements.     

Emissions and wall temperatures for lean prevaporized premixed gas turbine combustor

Experimental studies are carried out for investigating emission and wall temperature for traditional gas turbine combustor converted to lean premixed prevaporized (LPP) combustor. Vortex chamber, air preheating system, flat flame burner and inlet temperature control system are designed. Vortex chamber was maintained at the main air inlet port for controlling secondary air flow rate and wall temperature. Kerosene/air mixture temperature at exit from burner and entering combustion chamber was kept constant at 650 K for all runs. Special considerations were given for measuring NO_X, UHC, CO, local A/F ratio, flame temperature, exhaust gases temperature and wall temperature. For swirl and non swirl cases, secondary air ratio and primary zone air/fuel ratio were varied. The different operating parameters affecting flame temperature through it is affecting on local A/F ratio which is the main parameter for controlling flame temperature, emissions and walls temperatures. Flat flame burner and vortex chamber are useful tools for reducing emission and controlling walls temperatures. The inner liner wall temperatures are more affected by primary zone equivalence ratio while the outer liner wall temperatures are more affected by secondary air flow rate. Semi empirical correlations for NO_X, UHC and CO concentrations, exhaust gases temperature and maximum inner liner wall temperature are carried out. Good agreement between the measured and the calculated results are obtained. The present results are useful for further development of the traditional gas turbine combustor converted to LPP combustor.     

Influence of burner-port geometry in hydrocarbon oxidation and NOx formation mechanisms in methane/air flames

The influence of burner-port geometry in the mechanisms of hydrocarbon oxidation and NO_x formation from a 50 kW industrial-type methane-fired burner was investigated experimentally. Imaging and tomographic reconstruction techniques were used to assess the effects of port geometry upon flame visible length and C₂ chemiluminescence distribution in the recirculation zone. C₂ emission of methane flames depicts that low fuel jet velocities allow very rich conditions at recirculation zone and lead methane oxidation through O₂-scarcity mechanism. Higher velocities imply that methane oxidises via a path including dissociation into free radicals. In-furnace measurements were performed from a refractory-lined vertical furnace. NO_x concentration results revealed that NO formation is closely connected with the dissociation process, suggesting that prompt-NO_x mechanism is more important than hitherto supposed.     

Turbulent transition in impactor jets and its effects on impactor resolution

A review of a number of widely used impactors suggests that the poorer performance often seen in some of the stages is due to the onset of turbulence in the jet at too large values of either the Reynolds number Re, or the nozzle-to-collector distance L. This phenomenon is studied here by measuring the collection efficiency versus Stokes number curves η(S) of a low-pressure thin-plate-orifice impactor as a function of Re and L (measured in units of the orifice diameter d_n). A drastic broadening of the η(S) curve is observed in the vicinity of a critical Reynolds number $\text{Re}{}^1\text{(L/d}{}_{\mathrm{<mtext>n</mtext>}}\text{)}$. $\text{Re}{}^1$ increases at diminishing L/d_n, taking values near 800 and 400 for L/d_n of 3.1 and 4, respectively. No transition is seen at L/d_n=1 or 2, even at Re as high as 2700. This transition in the jet modifies both the high and the low S tails of the η(S) curve. It should be distinguished from a previously studied turbulent transition of the boundary layer near the collector plate, which arises at much larger Reynolds numbers, changes only the low Stokes number tail of the η(S) curves and disappears when using small collector plates. A specialized experimental apparatus is used to provide an initial jet with very low turbulence level, as well as to isolate incipient turbulence effects from other mechanisms leading to broadening of the η(S) curves. The particles are brought very close to the axis via aerodynamic focusing, while particle capture by Brownian diffusion is offset with a repulsive electric field. Free-stream turbulence ahead of the impactor nozzle is eliminated by passing only a small fraction of the flow through the critical orifice and the focusing lenses. The remaining gas required to attain jet Reynolds numbers up to 3700 is introduced laminarly and axisymmetrically as sheath air through an outer porous wall right before the impactor nozzle. At Re in the range of a few tens, a strong increase of the critical Stokes number with increasing L/d_n is observed.     

Combustion of residual steel gases: laminar flame analysis and turbulent flamelet modeling

In recovery combustion systems operating in the steel industry, energy is provided by boilers burning residual gases of blast furnace and coke oven. To help understand combustion of this particular type of fuels, a numerical study is conducted where the major chemical properties of steel gas flames are collected. The chemical composition of representative fuel and oxidizer steel gas is varied over a large range in calculations using detailed chemistry and complex transport properties. The chemical equilibrium compositions, premixed flame speeds and diffusion flame extinction strain rates are determined. The advantages and shortcomings of the use of vitiated air emerge, and its introduction into the boiler appears as an interesting alternative to reduce NO_x emission. The detailed information obtained with laminar flame calculations is also introduced in flamelet turbulent combustion modeling. Reynolds Averaged Navier Stokes (RANS) simulations of a test case burner are performed and some comparisons between numerical predictions and experimental results are presented.     

Influence of outer secondary-air vane angle on combustion characteristics and NOx emissions of a down-fired pulverized-coal 300 MWe utility boiler

Industrial experiments were performed on a down-fired pulverized-coal 300 MWe utility boiler with swirl burners. Gas temperature, concentrations of gas components (O₂, CO, CO₂ and NO_x) in the burning region and carbon content in the fly ash were measured with outer secondary-air vane angles of 25°, 32.5° and 50°. Results indicate that with increasing vane angle, NO_x emission and boiler efficiency decrease. Overall evaluation boiler efficiency and NO_x emission, the vane angle of 32.5° is optimum. Using an IFA300 constant-temperature anemometer system, cold air experiments on a quarter-scaled burner model were also carried out to investigate the influence of various outer secondary-air vane angles on the flow characteristics in the burner nozzle region. No central recirculation zone appeared for vane angles of 25° and 32.5°. Most of the pulverized-coal was ignited in the external recirculation zone. For vane angles of 45° and 55°, a central recirculation zone could be observed, and air flow rigidity and axial velocities decreased rapidly.     

Investigation of soot nanoparticles during combustion of liquid hydrocarbons with injection of a superheated steam jet into the reaction zone

The characteristics of soot particles formed during combustion of liquid hydrocarbons in a laboratory model of an original burner with injection of a superheated steam jet into the reaction zone are experimentally studied. The concentration and size distribution of soot particles formed in the burner flame are measured by a diffusion aerosol spectrometer. It is shown that the majority of the primary particles have sizes ranging from 20 to 60 nm. The particle concentration in the external flame rapidly decreases with distance from the burner exit from 10⁸ to 5 · 10⁶ cm^?3. The images obtained by transmission electron microscopy demonstrate a chain-branched (fractallike) structure of aggregates. The primary particles composing these aggregates have a union-like structure with the interplane distance between the layers smaller than 1 nm. Compact aggregates with sizes up to 500 nm are observed in cooled combustion products. The content of soot in combustion products is 35 mg/m³, and the mean particle mass is 7 · 10^?12 mg. Results obtained in the combustion modes with injection of a superheated steam jet and with injection of an air jet are compared.     

Average structural analysis of the extractable material of young soot gathered in an ethylene inverse diffusion flame

Conventional analytical methods such as ¹H NMR, vapor pressure osmometry (VPO) and elemental analysis were used to characterize the soot precursor material represented by the chloroform extractable fractions of the young soot gathered at different heights of an ethylene inverse diffusion flame in terms of average structural parameters. The results indicate that the soot soluble fraction obtained at a 6 mm height has a relatively large molecular weight and has long aliphatic chains which later disappear with an increase in height above the burner base, especially in the region where the temperature is high (1200 K). This behavior is also accompanied by an increase in the aromaticity (f_a) of the samples.     

Study of the influence of vane angle on flow, gas species, temperature, and char burnout in a 200 MWe lignite-fired boiler

We measured various operational parameters of a 200-MW_e, wall-fired, lignite utility boiler under various outer secondary air vane angles. The parameters measured were gas temperature, gas species concentrations, char burnout, and component release rates (C, H and N). Cold air experiments of a single burner were conducted in the laboratory. A double swirl flow pulverized-coal burner has a single ring recirculation zone that forms in the secondary air region in the burner. By decreasing vane angles, maximum values of radial velocity, tangential velocity and turbulence intensity all increase. Moreover, swirl intensity of air flow and recirculation zone size increase. Concomitantly, in the central region of the burner, decreasing the vane angles of outer secondary air increases gas temperatures, CO concentrations, char burnout and component release rates of C, H, and N, while O₂ and NO_x concentrations decrease, and an early ignition of pulverized-coal occurs. Meanwhile, in the secondary air region of the burner, conditions are similar except that NO_x mean concentrations are reversed showing instead an increase. In the side wall region, gas temperatures increase, O₂ and NO_x concentrations decrease, but CO concentrations vary only slightly.