Thermoacoustic combustion instability of propane-oxy-combustion with CO2 dilution: Experimental analysis

In this study, the effect of CO₂ dilution on the thermoacoustic stability of propane-oxyfuel flames is studied in a non-premixed, swirl-stabilized combustor. The results, obtained at a fixed combustor power density (4 MW/m³ bar) and global stoichiometric equivalence ratio (Φ = 1.0), show that the oxy-flame is stable at 0% and low CO₂ concentrations in the oxidizer. A self-amplifying coupling between heat release and pressure fluctuations was observed to occur at the CO₂ concentration of 45%, which matches the point of flame transition from a jet-like to a V-shaped flame resulting from the formation of inner recirculation zone. The observed frequency for both the pressure and heat release oscillations is 465 Hz and the ensuing thermoacoustic instability is believed to have been resulted from vortexes and flame interactions. Subsequent to the coupling of the oscillations at the CO₂ concentration of 45%, their amplitudes grew at 50% to 60% CO₂ dilution levels. The maximum amplitude was observed at 60% CO₂ concentration after which, as CO₂ dilution level increases, the acoustic amplitude and that of its counterpart in the heat release spectrum decreased due to damping (energy dissipation) arising from heat loss and viscous dissipation. An increase in hydrogen concentration in the fuel and a decrease in the combustor power density were observed to lower the acoustic amplitude. Furthermore, a frequency shift is observed with a change in the combustor firing rate, which shows that the mode scales with the flow velocity, and therefore, unlikely to be a natural acoustic mode of the combustor. This study, therefore, reveals thermoacoustic instability in non-premixed oxy-combustion driven by changes in flame dynamics and macrostructures as the CO₂ concentration in the oxidizer mixture varies.     

Rapidly mixed combustion of hydrogen/oxygen diluted by N2 and CO2 in a tubular flame combustor

In this study hydrogen flames have been attempted in a rapidly mixed tubular flame combustor for the first time, in which fuel and oxidizer are individually and tangentially injected into a cylindrical combustor to avoid flame flash back. Three different cases were designed to examine the effects of fuel and oxidizer feeding method, diluent property, oxygen content and equivalence ratio on the characteristics of hydrogen flame, including the flame structure, lean extinction limit, flame stability and temperature. The results show that by enhancing mixing rate through feeding system, the range of equivalence ratio for steady tubular flame can be much expanded for the N₂ diluted mixture, however, at the oxygen content of 0.21 (hydrogen/air) the steady tubular flame is achieved only up to equivalence ratio of 0.5; by decreasing oxygen content, the lean extinction limit slightly increases, and the upper limit for steady tubular flame establishment increases significantly, resulting in an expanded tubular flame range. For CO₂ diluted mixture, the stoichiometric combustion has been achieved within oxygen content of 0.1 and 0.25, for which the burned gas temperature is uniformly distributed inside the flame front; as oxygen content is below 0.21, a steady tubular flame can be obtained from the lean to rich limits; and the lean extinction limit increases from 0.17 to 0.4 as oxygen content decreases from 0.21 to 0.1, resulting in a shrunk tubular flame range. Laminar burning velocity, temperature and Damköhler number are calculated to examine the differences between N₂ and CO₂ diluted combustion as well as the requirement for hydrogen-fueled tubular flame establishment.     

Dual-pump CARS temperature and major species concentration measurements in counter-flow methane flames using narrowband pump and broadband Stokes lasers

Dual-pump coherent anti-Stokes Raman scattering (CARS) is used to measure temperature and species profiles in representative non-premixed and partially-premixed CH₄/O₂/N₂ flames. A new laser system has been developed to generate a tunable single-frequency beam for the second pump beam in the dual-pump N₂-CO₂ CARS process. The second harmonic output (∼532 nm) from an injection-seeded Nd:YAG laser is used as one of the narrowband pump beams. The second single-longitudinal-mode pump beam centered near 561 nm is generated using an injection-seeded optical parametric oscillator, consisting of two non-linear β-BBO crystals, pumped using the third harmonic output (∼355 nm) of the same Nd:YAG laser. A broadband dye laser (BBDL), pumped using the second harmonic output of an unseeded Nd:YAG laser, is employed to produce the Stokes beam centered near 607 nm with full-width-at-half-maximum of ∼250 cm⁻¹. The three beams are focused between two opposing nozzles of a counter-flow burner facility to measure temperature and major species concentrations in a variety of CH₄/O₂/N₂ non-premixed and partially-premixed flames stabilized at a global strain rate of 20 s⁻¹ at atmospheric-pressure. For the non-premixed flames, excellent agreement is observed between the measured profiles of temperature and CO₂/N₂ concentration ratios with those calculated using an opposed-flow flame code with detailed chemistry and molecular transport submodels. For partially-premixed flames, with the rich side premixing level beyond the stable premixed flame limit, the calculations overestimate the distance between the premixed and the non-premixed flamefronts. Consequently, the calculated temperatures near the rich, premixed flame are higher than those measured. Accurate prediction of the distance between the premixed and the non-premixed flames provides an interesting challenge for future computations.     

Measurements of laminar burning velocities and flame stability analysis for dissociated methanol–air–diluent mixtures at elevated temperatures and pressures

The laminar burning velocities and Markstein lengths for the dissociated methanol–air–diluent mixtures were measured at different equivalence ratios, initial temperatures and pressures, diluents (N₂ and CO₂) and dilution ratios by using the spherically outward expanding flame. The influences of these parameters on the laminar burning velocity and Markstein length were analyzed. The results show that the laminar burning velocity of dissociated methanol–air mixture increases with an increase in initial temperature and decreases with an increase in initial pressure. The peak laminar burning velocity occurs at equivalence ratio of 1.8. The Markstein length decreases with an increase in initial temperature and initial pressure. Cellular flame structures are presented at early flame propagation stage with the decrease of equivalence ratio or dilution ratio. The transition positions can be observed in the curve of flame propagation speed to stretch rate, indicating the occurrence of cellular structure at flame fronts. Mixture diluents (N₂ and CO₂) will decrease the laminar burning velocities of mixtures and increase the sensitivity of flame front to flame stretch rate. Markstein length increases with an increase in dilution ratio except for very lean mixture (equivalence ratio less than 0.8). CO₂ dilution has a greater impact on laminar flame speed and flame front stability compared to N₂. It is also demonstrated that the normalized unstretched laminar burning velocity is only related to dilution ratio and is not influenced by equivalence ratio.     

CARS thermometry in practical combustors

The feasibility of coherent anti-Stokes Raman spectroscopy (CARS) for remote, spatially and temporally resolved, measurements in practical combustion systems is demonstrated. N₂ CARS temperature measurements are reported for two different liquid-fueled combustors, a swirl burner, and a JT-12 combustor can, operated at atmospheric pressure and situated in a 50-cm diameter tunnel. Crossed-beam phase-matched CARS was employed to ensure good spatial precision. Delicate instrumentation was housed in a control room adjacent to the burner test cell and the CARS signals were piped out employing 60-μ diameter fiber optic guides. Average temperature measurements and demonstrations of single-pulse thermometry were performed.     

Hydrogen addition effects in a confined swirl-stabilized methane-air flame

The effect of hydrogen addition in methane-air premixed flames has been examined from a swirl-stabilized combustor under confined conditions. The effect of hydrogen addition in methane-air flame has been examined over a range of conditions using a laboratory-scale premixed combustor operated at 5.81 kW. Different swirlers have been investigated to identify the role of swirl strength to the incoming mixture. The flame stability was examined for the effect of amount of hydrogen addition, combustion air flow rates and swirl strengths. This was carried out by comparing adiabatic flame temperatures at the lean flame limit. The combustion characteristics of hydrogen-enriched methane flames at constant heat load but different swirl strengths have been examined using particle image velocimetry (PIV), micro-thermocouples and OH chemiluminescence diagnostics that provided information on velocity, thermal field, and combustion generated OH species concentration in the flame, respectively. Gas analyzer was used to obtain NO_x and CO concentration at the combustor exit. The results show that the lean stability limit is extended by hydrogen addition. The stability limit can reduce at higher swirl intensity to the fuel-air mixture operating at lower adiabatic flame temperatures. The addition of hydrogen increases the NO_x emission; however, this effect can be reduced by increasing either the excess air or swirl intensity. The emissions of NO_x and CO from the premixed flame were also compared with a diffusion flame type combustor. The NO_x emissions of hydrogen-enriched methane premixed flame were found to be lower than the corresponding diffusion flame under same operating conditions for the fuel-lean case.     

Suppression limits of low strain rate non-premixed methane flames

The suppression of low strain rate non-premixed flames was investigated experimentally in a counterflow configuration for laminar flames with minimal conductive heat losses. This was accomplished by varying the velocity ratio of fuel to oxidizer to adjust the flame position such that conductive losses to the burner were reduced and was confirmed by temperature measurements using thermocouples near the reactant ducts. Thin filament pyrometry was used to measure the flame temperature field for a curved diluted methane-air flame near extinction at a global strain rate of 20 s⁻¹. The maximum flame temperature did not change as a function of position along the curved flame surface, suggesting that the local agent concentration required for suppression will not differ significantly along the flame sheet. The concentration of N₂, CO_2, and CF₃Br added to the fuel and the oxidizer streams required to obtain extinction was measured as a function of the global strain rate. In agreement with previous measurements performed under microgravity conditions, limiting non-premixed flame extinction behavior in which the agent concentration obtained a value that insures suppression for all global strain rates was observed. A series of extinction measurements varying the air:fuel velocity ratio showed that the critical N₂ concentration was invariant with this ratio, unless conductive losses were present. In terms of fire safety, the measurements demonstrate the existence of a fundamental limit for suppressant requirements in normal gravity flames, analogous to agent flammability limits in premixed flames. The critical agent volume fraction in the methane fuel stream assuring suppression for all global strain rates was measured to be 0.841 ± 0.01 for N₂, 0.773 ± 0.009 for CO₂, and 0.437 ± 0.005 for CF₃Br. The critical agent volume fraction in the oxidizer stream assuring suppression for all global strain rates was measured as 0.299 ± 0.004 for N₂, 0.187 ± 0.002 for CO₂, and 0.043 ± 0.001 for CF₃Br.     

N2 CARS thermometry and O2 LIF concentration measurements in a flame under electrically induced microbuoyancy

Microgravity combustion is fundamentally characterized by the absence of buoyancy driven flows. To facilitate a large range of diagnostics on microgravity flames, it is useful to create an equivalent microbuoyant condition in an earth-based laboratory. This experiment simulates microbuoyancy using electric fields to balance local convection in a region of the flame. Previous studies used N₂ coherent anti-Stokes Raman spectroscopy (CARS) temperature measurements to show that a region exists in which the temperature profile corresponds to that of a spherically symmetric diffusion profile, as would be expected in a true microgravity environment. The current study utilizes CARS thermometry and laser induced fluorescence (LIF) to examine the temperature and O₂ concentration profiles in a region below the flame. The results show that the electrically balanced flame apparatus produces a spherically symmetric wedge of microbuoyant flame extending 10° from the vertical axis of the capillary flame arrangement; thereby quantifying the extent of the microbuoyant region.     

Numerical investigation of diluent influence on flame extinction limits and emission characteristic of lean-premixed H2-CO (syngas) flames

In recent years, research efforts have been channeled to explore the use of environmentally-friendly clean fuel in lean-premixed combustion so that it is vital to understand fundamental knowledge of combustion and emissions characteristics for an advanced gas turbine combustor design. The current study investigates the extinction limits and emission formations of dry syngas (50% H₂-50% CO), moist syngas (40% H₂-40% CO-20% H₂O), and impure syngas containing 5% CH₄. A counterflow flame configuration was numerically investigated to understand extinction and emission characteristics at the lean-premixed combustion condition by varying dilution levels (N₂, CO₂ and H₂O) at different pressures and syngas compositions. By increasing dilution and varying syngas composition and maintaining a constant strain rate in the flame, numerical simulation showed among diluents considered: CO₂ diluted flame has the same extinction limit in moist syngas as in dry syngas but a higher extinction temperature; H₂O presence in the fuel mixture decreases the extinction limit of N₂ diluted flame but still increases the flame extinction temperature; impure syngas with CH₄ extends the flame extinction limit but has no effect on flame temperature in CO₂ diluted flame; for diluted moist syngas, extinction limit is increased at higher pressure with the larger extinction temperature; for different compositions of syngas, higher CO concentration leads to higher NO emission. This study enables to provide insight into reaction mechanisms involved in flame extinction and emission through the addition of diluents at ambient and high pressure.     

Experimental and kinetic study of laminar flame characteristics of H2/O2/diluent flame under elevated pressure

Laminar burning velocity, Markstein length, and critical flame radius of an H₂/O₂ flame with different diluents, He, Ar, N₂ and CO₂, were measured under elevated pressure with different diluent concentrations. The effects of pressures, diluents, and dilution and equivalence ratios were studied by comparing calculated and experimental results. The laminar burning velocity showed non-monotonic behavior with pressure when the dilution ratio was low. The reason is the radical pool reduced with increasing pressure and leads to the decrease of overall reaction order from larger than 2 to smaller than 2, and further leads to this non-monotonic phenomenon. A modified empirical equation was presented to capture the relationship between active radicals and laminar burning velocity. Critical radii and Markstein lengths both decrease with initial pressure and increase with equivalence ratio and dilution ratio. The calculated critical radii indicate that the Peclet number and flame thickness control the change of R_cr. It can be found that Le_eff has a significant influence on Peclet number and leads to the decrease of critical flame radii of Ar, N_2, and CO₂ diluted mixture. Interestingly, the CO₂ diluted mixture has the lowest Markstein length under stoichiometric conditions and a high value under fuel-rich conditions, consistent as the flame instability observed on the flame images. The reason is that the Le_eff of CO₂ diluted mixture increased rapidly with the equivalence ratio.     

Response of partially premixed flames to acoustic velocity and equivalence ratio perturbations

This article describes an experimental investigation of the forced response of a swirl-stabilized partially premixed flame when it is subjected to acoustic velocity and equivalence ratio fluctuations. The flame’s response is analyzed using phase-resolved CH^* chemiluminescence images and flame transfer function (FTF) measurements, and compared with the response of a perfectly premixed flame under acoustic perturbations. The nonlinear response of the partially premixed flame is manifested by a partial extinction of the reaction zone, leading to rapid reduction of flame surface area. This nonlinearity, however, is observed only when the phase difference between the acoustic velocity and the equivalence ratio at the combustor inlet is close to zero. The condition, Δφ_Φ^′-V^′≈0°, indicates that reactant mixtures with high equivalence ratio impinge on the flame front with high velocity, inducing large fluctuations of the rate of heat release. It is found that the phase difference between the acoustic velocity and equivalence ratio nonuniformities is a key parameter governing the linear/nonlinear response of a partially premixed flame, and it is a function of modulation frequency, inlet velocity, fuel injection location, and fuel injector impedance. The results presented in this article will provide insight into the response of a partially premixed flame, which has not been well explored to date.     

Temperature and concentration profiles in hydrogennitrous oxide flames

Spontaneous Raman spectroscopy has been used to measure temperature and NO, O₂, and N₂ concentration profiles in lean to stoichiometric premixed laminar hydrogennitrous oxide flames. Relative concentration profiles for OH were also obtained for these flames by use of laser induced fluorescence. The present NO concentration results for the stoichiometric flame agree with previous NO measurements obtained by a different optical technique. Profiling these species for various equivalence ratios provides a further test for flame modelers.     

In-depth numerical analysis on the determination of amount of CO2 recirculation in LNG/O2/CO2 combustion

The determination of proper amount of CO₂ recirculation is one of the critical issues in oxy-fuel combustion technology for the reduction of CO₂ emissions by the capture and sequestration of CO₂ species in flue gas. The objective of this study is to determine the optimum value of O₂ fraction in O₂/CO₂ mixture to obtain similar flame characteristics with LNG–air combustion. To this end, a systematic numerical investigation has been made in order to resolve the physical feature of LNG/O₂/CO₂ combustion. For this, SIMPLEC algorithm is used for the resolution of pressure velocity coupling. And for the Reynolds stresses and turbulent reaction the popular two-equation (k–ε) model by Launder and Spalding and eddy breakup model by Magnussen and Hjertager were incorporated, respectively. The radiative heat transfer is calculated from the volumetric energy loss rate from flame, considering absorption coefficient of H₂O, CO₂ and CO gases. A series of parametric investigation has been made as function of oxidizer type, O₂ fraction and fuel type for the resolution of combustion characteristics such as flame temperature, turbulent mixing and species concentration. Further the increased effect of CO₂ species on the flame temperature is carefully examined by the consideration of change of specific heat and radiation effect. Based on this study, it was observed that the same mass flow rate of CO₂ with N₂ appears as the most adequate value for the amount of CO₂ recirculation for LNG fuel since the lower C_p value for the CO₂ relative to N₂ species at lower temperatures cancels the effect of the higher C_p value at higher temperatures over the range of flame temperatures present in this study. However, for the fuel with high C/H ratio, for example of coal, the reduced amount of CO₂ recirculation is recommended in order to compensate the increased radiation heat loss. In general, the calculation results were physically acceptable and consistent with reported data in literature. Further work is strongly recommended for a large-scale combustor such as coal-fired power plant to figure out important parameters caused by the effect of increased combustor size and the presence of particle phase, etc.     

Numerical study of methane and air combustion inside a small tube with an axial temperature gradient at the wall

A numerical study on CH₄ and air premixed combustion inside a small tube with a temperature gradient at the wall was undertaken to investigate the effects of inlet velocity, equivalence ratio and combustor size on combustion characteristics. The simulation results show that the inlet velocity has a significant influence on the reaction zone, and the flame front shifts downstream as the inlet velocity increases. The results also show that, the inlet velocity has no obvious effects on the flame temperature. The highest flame temperature is obtained if the equivalence ratio is set to 1. It is disclosed that the combustor size strongly influences the combustion characteristics. The smaller the combustor size is, the more difficult it is to maintain the steady combustion. The smallest combustor size that the stable flame can be sustained is determined mainly by the wall temperature of the micro-combustor under the given conditions. The higher the wall temperature is, the smaller the smallest combustor size. Therefore increasing wall temperature is an effective way to realize flame stabilization for a given combustor size.     

Effects of different preheated CO2/O2 jet in cross-flow on combustion instability and emissions in a lean-premixed combustor

To reveal the suppression mechanism of thermoacoustic instability flames under CO₂/O₂ jet in cross flow. Experiments on the effects of different preheated CO₂/O₂ jet in cross-flow (JICF) on combustion instability and NO_x emissions in a lean-premixed combustor were conducted in a model gas turbine combustor. Two variables of the JICF were investigated—the flow rate and the temperature. Results indicate that combustion instability and NOx emissions could be suppressed when the JICF flow rate increases from 1 to 5 L/min. The average pressure amplitude decreases from 18.6 Pa to 1.6 Pa, and the average NOx emission decreases from 26.4 ppm to 12.1 ppm. But the average pressures amplitude and NOx emissions increase as the JICF temperature grows up. The sound pressure and the flame heat release rate exhibits different mode-shifting characteristics. The oscillation frequency of the sound pressure almost unchanged under JICF injection. However, the oscillation frequency of the heat release rate jumps from 95 Hz to 275 Hz under different JICF temperatures. As the CO₂/O₂ JICF flow rate arrived 3 L/min, the oscillation frequency of flame heat release rate jumps from 85 Hz to 265 Hz. The color of the flame fronts and roots were changed by the JICF injection. The average length of flame under CO₂/O₂ JICF cases is shorter than the N₂/O₂ JICF cases. There are three different modes of flames when the CO₂/O₂ JICF flow rate varies, and two different modes of flames when the CO₂/O₂ JICF temperature varies. This article explored the joint effects of different CO₂/O₂ or N₂/O₂ JICF on combustion instability and NOx emissions, which could be instructive to the designing of safely and clean combustors in industrial gas turbines.     

Adiabatic burning velocity of H2–O2 mixtures diluted with CO2/N2/Ar

Global warming due to CO₂ emissions has led to the projection of hydrogen as an important fuel for future. A lot of research has been going on to design combustion appliances for hydrogen as fuel. This has necessitated fundamental research on combustion characteristics of hydrogen fuel. In this work, a combination of experiments and computational simulations was employed to study the effects of diluents (CO₂, N₂, and Ar) on the laminar burning velocity of premixed hydrogen/oxygen flames using the heat flux method. The experiments were conducted to measure laminar burning velocity for a range of equivalence ratios at atmospheric pressure and temperature (300 K) with reactant mixtures containing varying concentrations of CO₂, N₂, and Ar as diluents. Measured burning velocities were compared with computed results obtained from one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics and good agreement was obtained. The effectiveness of diluents in reduction of laminar burning velocity for a given diluent concentration is in the increasing order of argon, nitrogen, carbon dioxide. This may be due to increased capabilities either to quench the reaction zone by increased specific heat or due to reduced transport rates. The lean and stoichiometric H₂/O₂/CO₂ flames with 65% CO₂ dilution exhibited cellular flame structures. Detailed three-dimensional simulation was performed to understand lean H₂/O₂/CO₂ cellular flame structure and cell count from computed flame matched well with the experimental cellular flame.     

Effects of CO2 and N2 dilution on the characteristics and NOX emission of H2/CH4/CO/air partially premixed flame

For the combustion of the mixture of blast furnace gas, natural gas, and coke oven gas in industrial burners, how to improve combustion efficiency and reduce pollutant emission are of significance. To accomplish this, an industrial partially premixed burner with a combustion diagnostic system is used to experimentally reveal the characteristics and NO_X emission of H₂/CH₄/CO/air flame under CO₂, N₂, and CO₂/N₂ (replacing half of N₂ with CO₂) dilution. NO_X emission and flame length, temperature profile, along with CO, CH₄, and CO₂ concentration profiles are analyzed with the three diluents in the fuel stream under different dilution rates (0–32% by volume). Experimental results show that for lean H₂/CH₄/CO combustion, greater proportions of CO₂ in the diluent affect flame characteristics in various ways. These effects include longer flame length, lower highest flame temperature, the highest flame temperature being located farther away from the nozzle, and the highest CO₂ concentration being located nearer the nozzle. Furthermore, results of CO, CH₄, and CO₂ concentrations indicate that chemical reactions in the flame are significantly affected by CO₂ owing to the series reaction CH₄?CH₃→CO?CO₂. Finally, increasing diluents or the ratio of CO₂ in diluents has the benefit of reducing NO_X emission.     

Experimental study on oxy-fuel combustion of heavy oil

Oxy-fuel combustion of heavy oil can be applied to oil field steam injection boilers, allowing the utilization of both heavy oil and CO₂ resources. The present study investigated the oxy-fuel combustion characteristics of heavy oil under different conditions, including the flame, temperature, and pollutant emission characteristics. The results showed that heavy oil combustion was stable at O₂ concentrations of 29%, as the O₂ concentration was increased, the flame began to brighten gradually, becoming shorter and thicker, while the temperature gradient became higher and the high temperature zone moved closer to the burner exit. The overall temperature and the combustion rates in O₂/CO₂ atmospheres were below those seen in O₂/N₂ atmospheres. The volume of NO emitted in the flue gas was almost unaffected by the change in O₂ concentrations in atmospheres containing high concentrations of CO₂, but it increased rapidly with increasing O₂ concentration in O₂/N₂ atmospheres.     

Experimental and numerical investigation of stability and emissions of hydrogen-assisted oxy-methane flames in a multi-hole model gas-turbine burner

This paper reports experimental and numerical study of stability and combustion characteristics of premixed oxy-methane flames with hydrogen-enrichment (CH₄–H₂/O₂–CO₂ flames) in a model multi-hole burner for clean energy production in gas turbines. The combustor lean blow-out (LBO) limit was presented on an equivalence ratio (Ø) - hydrogen fraction (HF: volumetric fraction of H₂ in a mixture of H₂+CH₄) map spanning over Ø-values of 0.1–1 and HF-values of 0–70% at fixed hole jet velocity and oxygen fraction (OF: volumetric fraction of O₂ in a mixture of O₂+CO₂) of 5.2 m/s and 30%, respectively. The condition of the combustion chamber is assumed to be depicted by the corrugated premixed flame regime. The premixed turbulent flame was modeled using the reaction progress variable flame front topology approach with the Large Eddy Simulation (LES) technique. The recorded combustor stability maps showed great resistance of the micromixer burner technology to flashback, recommending its use for stable gas turbine operation. The results show that H₂-enrichment widens the combustor operability limits (higher turndown ratio) by extending the LBO from Ø = 0.45 at HF = 0% down to Ø = 0.15 at HF = 70% with a slight reduction in the heat release factor by 0.1. The high reactivity and higher flame speed of H₂ ensures the sustenance of flame at lower equivalence ratios. At high equivalence ratios, H₂ addition enhances the reaction rates and makes both the primary and secondary reaction zones shorter and more intense. Increasing HF leads to increase in the Damköhler number (Da) and decrease in both the Karlovitz number (Ka) and flame thickness. The CO emission at the combustor outlet reduced significantly from 241 ppm at HF = 0% to 33.1 ppm at HF = 10%, then it increased back to 364 ppm at HF = 50%.     

Combustion characteristics of a lean premixed LPG–air combustor

Fuel/air mixing effects in a premixer have been examined to investigate the combustion characteristics, such as the emission of NO_x and CO, under simulated lean premixed gas turbine combustor conditions at normal and elevated pressures of up to 3.5 bar with air preheat temperature of 450 K. The results obtained have been compared with a diffusion flame type gas turbine combustor for emission characteristics. The results show that the NO_x emission is profoundly affected by the mixing between fuel and air in the combustor. NO_x emission is lowered by supplying uniform fuel/air gas mixture to the combustor and the NO_x emission reduces with decrease in residence time of the hot gases in the combustor. The NO_x emission level of the lean premixed combustor is a strong function of equivalence ratio and the dependency is smaller for a traditional diffusion flame combustor under the examined experimental conditions. Furthermore, the recirculation flow, affected by dome angle of combustor, reduces the high temperature reaction zone or hot spot in the combustor, thus reducing the NO_x emission levels.