Flat Flame Research Articles

Spatially-Resolved experimental investigations of combustion characteristics in a solid fuel doped methane swirl flame and the influence on the formation of ultrafine particulate matter

The formation of inhalable fine particles (dp≤10 µm) is an undesirable side effect of solid fuel combustion processes. These particles can accumulate in the human respiratory system and thus cause severe lung damage. Therefore, an understanding of the formation of these particles is of crucial importance to avoid or to reduce the amount of fine particulate matter (PM) released into the atmosphere. Until now, most of the studies have been carried out for laminar and early-stage solid fuel combustion under simplified conditions (e.g. flat flame burner with radial almost homogeneous conditions) and only a few studies have been carried out with more complex turbulent flame structures. As the majority of real applications on an industrial scale involves turbulent conditions, research in this area is highly important. For the investigation of particulate matter formation, a swirled methane-assisted pulverized solid fuel combustion test rig with full optical access which allows intrusive and non-intrusive measurements in different heights and radial positions was developed. In order to study the local influences of flame characteristics on the formation of ultrafine particulate matter, results from laser Doppler velocimetry (LDV), suction pyrometer, Fourier transform infrared (FTIR) spectroscopy, and scanning mobility particle sizer (SMPS) spectrometer measurements are presented and discussed. It is shown that the local differences - such as gas temperature, the velocity of the solid fuel particles, and concentrations species - have an influence on the formation of particulate matter. Especially in regions with high local velocities and shear zones, PM coagulates strongly due to the increased turbulence in this area. The data presented in this paper are an important foundation for further studies to validate CFD simulations using the measurements and to further develop existing PM formation models for turbulent cases.

Proper orthogonal decomposition of continuum-dominated emission spectra for simultaneous multi-property measurements

A novel spectrum calibration method is proposed for high-accuracy emission spectroscopy to extract quantitative gas property information from the spontaneous photon emission spectra. This is challenging, particularly in the presence of broadband emission, which smears the characteristic local spectral features that are often used as property indicators in conventional emission spectroscopy. Proper orthogonal decomposition (POD) can reduce the dimension (ND) of the broadband spectrum profile by selecting a number (NM≪ND) of dominant bases that span the gas-property-dependent spectrum space. Each POD-decomposed spectral basis represents critical broadband spectral features that are sensitive to the gas properties of interest. A reduced-order model (ROM) employing the kriging method correlates the entire emission spectrum profile that decomposed by the bases to multiple gas properties. To provide a spectrum database to train the ROM, chemiluminescence was captured from a flat flame with varied diluent gas compositions (N2 and CO2), fuel-oxidizer equivalence ratios, and pressures (1–5 bar). Subsequently, the ROM employing an emission spectrum can predict the gas properties under high-temperature and high-pressure conditions where conventional sensors are inapplicable. The average relative prediction errors obtained using this new analysis method were approximately 3.2% and 1.3% for the pressure and equivalence ratio, respectively.

Experimental and modeling study of NO formation in methyl acetate + air flames

Premixed laminar flat non-stretched flames of methyl acetate + air have been studied at atmospheric pressure. The heat flux method was employed to stabilize the flames over equivalence ratios ɸ = 0.7–1.5 at initial gas temperatures from 298 to 348 K. The laminar burning velocities of methyl acetate + air mixtures have been measured at these conditions and compared with the literature data. Laser-induced fluorescence was used to measure NO concentrations in the post-combustion region of the flames at 298 K. The detailed kinetic mechanism of the authors was extended by the reactions of methyl acetate, mostly relying on the model developed by Ahmed et al. (2019). The mechanism was tested against selected literature data from measurements in a shock tube and jet-stirred reactor. Moreover, it was compared with new measurements of the laminar burning velocity and NO concentrations in methyl acetate flames. In all cases, good agreement between the experimental and modeling results was observed. A comparison with LIF measurements obtained for methane + air flames at the same conditions shows that NO concentrations in methyl acetate flames are lower than those in methane + air flames both in stoichiometric and rich mixtures. These differences were interpreted using sensitivity and rate-of-production analyses of NO formation via different routes.

Experimental and numerical investigation of the transition from non sooting to sooting premixed n-butane flames, encompassing the nucleation flame conditions

In a small range of rich equivalence ratios, premixed flat flames, so-called nucleation flames, were shown to produce very small-size (2–4 nm) soot particles, which undergo negligible growth with the residence time. In such flames, the contribution of the soot nucleation step with respect to the soot growth process is larger than in standard sooting flames, making nucleation flames ideal target flames to better understand the inception process. In order to scrutinize the transition from non sooting to sooting flames, three premixed atmospheric n-butane/oxygen/nitrogen flames surrounding the nucleation flame conditions were investigated: the flame at ϕ=1.6 is a fuel-rich non-sooting flame, the flame at ϕ=1.75 is a nucleation flame and the flame at ϕ=1.95 is a lightly sooting flame. The soot volume faction (SVF) profiles were previously measured by laser-induced incandescence as well as the soot size distributions. In the present study, mole fractions of stable species up to benzene were measured by gas chromatography while those of naphthalene and pyrene were obtained by jet-cooled laser-induced fluorescence. It is found that acetylene, propyne, benzene, naphthalene and pyrene species show the highest sensitivity with the equivalence ratio, while a two orders of magnitude increase of SVF is observed. The chemical flame structure of these flames was modeled using three kinetic mechanisms of the literature to which a recently developed soot code, based on a sectional approach, was coupled. The chosen model is representative of the current state-of-the-art of models describing all steps of soot formation and growth and fully coupled with the gas phase. The ability of such a model to predict soot volume fraction with reasonable predictions of gas-phase species is validated here based on the new experimental database presented in this work. It also highlights the improvement required on particle inception modeling to correctly predict particle size distributions.

Combined effect of experimental and kinetic uncertainties on NO predictions in low-pressure premixed laminar H2/CH4/CO-air and H2/CH4/CO/C6H6-air flames

Accurate predictions of nitrogen oxides (NOx) are crucial for the design of low-emissions combustion technologies and require a truthful description of chemical kinetics and underlying thermo-chemical field. Nowadays, uncertainty remains in the reaction rates of different pathways of NOx formation. Experiments in simple systems, such as 1D flat premixed laminar flames, are necessary to shed light on the NOx formation process. However, experimental uncertainty cannot be avoided and influences the subsequent NOx formation modelling. In this context, forward propagation of experimental and kinetic uncertainties can be beneficial to assess their impact on the predictions of NOx emissions. In this work, forward Uncertainty Quantification (UQ) is performed via Polynomial Chaos Expansion (PCE), based on a sparse grid, to quantify the influence of the variability of temperature measurements and kinetic parameters on NO predictions in H2/CH4/CO-air 1D premixed laminar flames at low pressure, without and with 1.12% of benzene (C6H6) in the fuel blend. A significant uncertainty affects the prediction of NO for all the investigated flames. The measured temperature, with its uncertainty, is proven to be one of the most sensitive factor affecting the prediction of NO, particularly for the flames operated in lean and stoichiometric conditions. The results demonstrate that an accurate quantification of the uncertainty affecting the temperature measurements is critical for the development of predictive NOx formation models.

Experimental and numerical study on adiabatic laminar burning velocity and overall activation energy of biomass gasified gas

The adiabatic laminar burning velocity (SL,0) and overall activation energy (Ea) of biomass gasified gas (BGG) were investigated experimentally and numerically in this study. Non-adiabatic laminar burning velocity (SL) and heat loss (qsw) were measured by a McKenna burner with a cooling-water system under different equivalence ratios (φ). SL,0 and Ea were calculated by a premixed flame linear model based on the flat flame method. Five detailed chemical mechanisms were tested against the experimental data. The results show that the measurable range of BGG is 0.65 ≤ φ ≤ 1.00. Flat flame of BGG has obvious lower and upper limits. qsw is affected by both SL and φ. The reliability of the premixed flame linear model is high and the correlation index (R2) is greater than 0.99 for all cases. The SL,0 of BGG increases from 17.1 to 32.9 cm/s with the increase of φ from 0.65 to 1.00. Since BGG contains a large amount of dilution components, φ has little effect on Ea. Among the five detailed chemical mechanisms, GRI 3.0 has the highest accuracy for predicting SL,0 when 0.80 ≤ φ ≤ 1.00, but USC-Mech II has the best overall performance. San Diego has the best accuracy for predicting Ea, and the maximum relative error is not more than 5.9%.

A Compact Fiber-Coupled NIR/MIR Laser Absorption Instrument for the Simultaneous Measurement of Gas-Phase Temperature and CO, CO2, and H2O Concentration.

A fiber-coupled, compact, remotely operated laser absorption instrument is developed for CO, CO2, and H2O measurements in reactive flows at the elevated temperatures and pressures expected in gas turbine combustor test rigs with target pressures from 1–25 bar and temperatures of up to 2000 K. The optical engineering for solutions of the significant challenges from the ambient acoustic noise (~120 dB) and ambient test rig temperatures (60 °C) are discussed in detail. The sensor delivers wavelength-multiplexed light in a single optical fiber from a set of solid-state lasers ranging from diodes in the near-infrared (~1300 nm) to quantum cascade lasers in the mid-infrared (~4900 nm). Wavelength-multiplexing systems using a single optical fiber have not previously spanned such a wide range of laser wavelengths. Gas temperature is inferred from the ratio of two water vapor transitions. Here, the design of the sensor, the optical engineering required for simultaneous fiber delivery of a wide range of laser wavelengths on a single optical line-of-sight, the engineering required for sensor survival in the harsh ambient environment, and laboratory testing of sensor performance in the exhaust gas of a flat flame burner are presented.

Experimental investigation of synthesis gas production in fuel-rich oxy-fuel methane flames

Combustion processes with pure oxygen (oxy-fuel) instead of air as an oxidant are attractive for e.g. high temperature thermal or thermochemical and gasification processes. The absence of nitrogen as an inert gas leads in such combustion processes to an increase in temperature and species concentrations. The scope of this study is the determination of axial profiles of major educts (CH4, O2) and major combustion products such as H2, H2O, CO and CO2 in flat fuel-rich premixed methane-oxygen flames (2.5 ≤ ϕ <3.0). A Heat-Flux-burner was used to stabilize a quasi-adiabatic one-dimensional flame at a preheating temperature of TP = 300 K and atmospheric pressure. Gas samples were taken at different HAB using a quartz probe and analyzed by a GC/MSD. The influence of the probe on determined species concentrations was investigated using CFD simulations. Additionally, one-dimensional calculations with detailed chemistry were performed using the GRI3.0 and CalTec2.3 mechanism. Their differences in regards to H2, H2O and CO concentrations were thoroughly investigated and could be explained by the detailed C2-chemistry of the CalTech2.3. The results of the analyzed gas samples show a rapid reduction and increase in the flame zone, of the educts, major products and higher hydrocarbons (C2H2, C2H4 and C6H6), respectively. Both mechanisms show a sharper gradient of the synthesis gas production in comparison to the experimental results, nevertheless a tendency towards the CalTech2.3 scheme is observed. In the post flame zone the determined species concentrations of H2 and CO2 correspond to the results of the GRI3.0, whereas in the case of H2O and CO the CalTech2.3 showed better performance. The calculations of higher hydrocarbons are in better agreement with higher equivalence ratio.

Effects of Cofiring Coal and Biomass Fuel on the Pulverized Coal Injection Combustion Zone in Blast Furnaces

CO2 emissions are a major contributor to global warming. Biomass combustion is one approach to tackling this issue. Biomass is used with coal combustion in thermal power plants or with blast furnaces (BFs) because it is a carbon-neutral fuel; therefore, biomass provides the advantage of reduced CO2 emissions. To examine the effect of co-firing on pulverized coal injection (PCI) in BFs, two coals of different ranks were blended with the biomass in different proportions, and then their combustion behaviors were examined using a laminar flow reactor (LFR). The PCI combustion primarily functions as a source of heat and CO to supply the upper part of the BF. To create a similar PCI combustion environment, the LFR burner forms a diffusion flat flame with an oxygen concentration of 26% with a flame temperature of ~2000–2250 K at a heating rate of 105 K/s. The combustion characteristics, such as the flame structure, burning coal particle temperature, unburned carbon (UBC), and CO and CO2 emissions were measured to evaluate their effect on PCI combustion. With the increase in the biomass blending ratio, the brightness of the volatile cloud significantly increased, and the particle temperature tended to decrease. The fragmentation phenomenon, which was observed for certain coal samples, decreased with the increase in the biomass blending ratio. In particular, with an increase in the biomass blending ratio, the optimum combustion point occurred, caused by the fragmentation of coal and volatile gas combustion of biomass.

Role of a hot coflow on establishment of MILD combustion of biomass gasified gas

Biomass gasification is a promising clean technology. However, biomass gasified gas (BGG) suffers from problems related to actual utilization. BGG contains approximately 65% dilution components, that cause ignition difficulty, combustion instability, and high pollutant emissions. Within this context, Moderate or intense low-oxygen dilution (MILD) combustion is a clean and efficient combustion mode under dilution conditions, which is naturally suitable for BGG; however, the mechanism of establishing BGG MILD combustion remains unclear. Here, we investigated the MILD combustion of BGG with a hot coflow generated by a non-premixed CH4-air flat flame. Chemiluminescence and OH planar laser-induced fluorescence (PLIF) imaging were performed to characterize flame structures. Further, NOx and CO were detected in flue gas. Results show that CO emission can be reduced by 43–70% and NOx emission by 63–81% under MILD combustion. Decreasing the equivalence ratio (φ) of the hot coflow can reduce NOx emissions and increase CO emissions. The BGG MILD combustion can also reduce NOx from the hot coflow, which supports the hot coflow to enter industrial or civil burner applications. Introduction of the hot coflow causes local self-ignition; therefore, the combustion zone of the BGG flame moves upstream. This effect becomes more significant with an increase in φ. Further, BGG MILD combustion can be established by a coflow with a φ of 1.1; O2 in the hot coflow was no longer necessary to establish the MILD combustion. The limitations of BGG MILD combustion applications can be reduced considerably, and the mixed combustion of natural gas and BGG based on this jet in a hot coflow configuration can easily establish MILD combustion.

Evolution of Droplet Size of Jet-A Surrogate Fuel Measured by High-Speed Interferometric Laser Imaging for Droplet Sizing in High Temperature Region Formed by Methane/Air Flat Flame in Laminar Counterflow Field

Evolution of Droplet Size of Jet-A Surrogate Fuel Measured by High-Speed Interferometric Laser Imaging for Droplet Sizing in High Temperature Region Formed by Methane/Air Flat Flame in Laminar Counterflow Field

Synchronic measurements of temperatures and concentrations of OH, NH, and NO in flames based on broadband ultraviolet absorption spectroscopy

Temperature is an important parameter influencing the combustion reaction path and rate and determining the combustion and energy exchange efficiency. The OH, NH, NO and other species are involved in the key elementary reactions of combustion and determine the generation of NO&lt;i&gt;&lt;sub&gt;x&lt;/sub&gt;&lt;/i&gt; pollutants. Therefore, temperature and concentration measurements of OH, NH, and NO are of great significance for combustion diagnostics and research on reaction or emission mechanisms. In this work, a measurement system with high spatial resolution based on broadband ultraviolet absorption spectroscopy is established to realize simultaneous measurements of the temperature and concentrations of OH, NH, and NO in flames. Low detection limits of these three species are achieved by using the established measurement method. The 1&lt;i&gt;σ&lt;/i&gt; detection limit of NH is 1.8 ppb·m (1560 K), which is realized for the first time in atmospheric-pressure flames using absorption spectroscopy. The 1&lt;i&gt;σ&lt;/i&gt; detection limits of OH and NO are 60 ppb·m (1590 K) and 1 ppm·m (1380 K), respectively, which are obviously better than the existing results obtained by using infrared laser absorption spectroscopy. Then, the distributions of temperatures and concentrations of OH, NO and NH are acquired at various heights in an atmospheric-pressure NH&lt;sub&gt;3&lt;/sub&gt;/CH&lt;sub&gt;4&lt;/sub&gt;/air premixed flat flame with a high spatial resolution of nearly 0.1 mm. The broadband absorption spectra of OH and NH are acquired simultaneously inside the flame front, and the spectra of OH and NO are acquired simultaneously above the flame front. Inside or near the flame front, the temperatures deduced from the spectra of OH, NH, and NO are consistent, verifying the ability of these three species to be used to measure temperature. In addition, OH, NH, and NO are found to be suitable for different regions in combustion. The OH absorption is suitable for the post-combustion region with temperatures higher than 1000 K, the NH absorption can be used to acquire the temperature inside the flame front in complex combustion, and the NO absorption was able to provide the temperature in the region before or outside combustion at lower temperatures. Additionally, the experimental temperature and concentration profiles are in good agreement with the computational fluid dynamics predictions based on the mechanism, exhibiting the accuracy of the simultaneous temperature and concentration measurements by using broadband ultraviolet absorption spectra. Moreover, the differences in temperature and OH concentration between experiments and simulations indicate that the carbon sub-mechanism in the mechanism given by Okafor et al. [Okafor E C, Naito Y, Colson S, Ichikawa A, Kudo T, Hayakawa A, Kobayashi H &lt;ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://doi.org/10.1016/j.combustflame.2017.09.002"&gt;2018 &lt;i&gt;Combust. Flame&lt;/i&gt; &lt;b&gt;187&lt;/b&gt; 185&lt;/ext-link&gt;] should be further improved for more accurate predictions of NH&lt;sub&gt;3&lt;/sub&gt;/CH&lt;sub&gt;4&lt;/sub&gt; combustion.

Rapid change of particle velocity due to volatile gas release during biomass devolatilization

Our earlier study showed significant differences in average particle velocity between simulation and experimental results for devolatilizing biomass particles in an idealised entrained flow reactor [N. Guo et al., Fuel, 2020]. This indicates that the simulations do not accurately describe the physicochemical transformations and fluid dynamic processes during devolatilization. This article investigates the reasons for these discrepancies using time-resolved analyses of the experimental data and complementary modelling work. The experiments were conducted in a downdraft drop-tube furnace with optical access, which uses a fuel-rich flat flame (CH4O2CO2) to heat the particles. Gas flow was characterized using particle image velocimetry, equilibrium calculations and thermocouple measurements. High-speed images of devolatilizing Norway spruce (Picea Abies) particles were captured and analysed using time-resolved particle tracking velocimetry methods. The data were used to estimate the balance of forces and fuel conversion. Thrust and “rocket-like” motions were frequently observed, followed by quick entrainment in the gas flow. Rocketing particles were, on average, smaller, more spherical and converted faster than their non-rocketing counterparts. These differences in conversion behaviour could be captured by a particle-size dependent, 0-D devolatilization model, corrected for non-isothermal effects. The results from this investigation can provide a basis for future modelling and simulation work relevant for pulverized firing technologies.

Simultaneous detection of K, KOH, and KCl in flames and released from biomass using photofragmentation TDLAS

Gaseous potassium (K) species released from biomass during thermochemical conversion pose challenges to reactors and human health. Photofragmentation tunable diode laser absorption spectroscopy (PF-TDLAS) was used for simultaneous, high-dynamic range measurements of atomic K, potassium hydroxide (KOH) and potassium chloride (KCl) in flat flames seeded with KCl salt. An expression for the PF-TDLAS signal is presented and experimentally verified. Axial K species concentration profiles recorded at fuel-air equivalence ratios of 0.8 and 1.3 are compared to 2D axisymmetric reacting flow simulations. An overall good agreement is found, but KOH is over-predicted in simulations of fuel-rich flames at the expense of atomic K. Quantification of K species close to softwood and wheat straw particles converted in the flames is demonstrated.

Experimental Study on Hydrodynamic Instability Characteristics of N2-Diluted n-C4H10/Air Flat Flames

Experimental Study on Hydrodynamic Instability Characteristics of N2-Diluted n-C4H10/Air Flat Flames

Modeling laser-induced incandescence of Diesel soot\u2014Implementation of an advanced parameterization procedure applied to a refined LII model accounting for shielding effect and multiple scattering within aggregates for {varvec{alpha}}_{{varvec{T}}} and {varvec{E}}left( {varvec{m}} right) assessment

An extensive set of LII signals measured in a Diesel spray flame has been simulated using a refined LII model built upon a comprehensive version of soot heat- and mass-balance equations. This latter includes terms standing for saturation of linear, single- and multi-photon absorption processes, cooling by sublimation, conduction, radiation and thermionic emission in addition to mechanisms depicting soot oxidation and annealing, non-thermal photodesorption of carbon clusters as well as corrective factors allowing considering shielding effect and multiple scattering (MS) within aggregates. A complete parameterization of the so-proposed model has been achieved by means of an advanced optimization procedure coupling design of experiments with a genetic algorithm-based solver. Doing so, the values of different factors involved in absorption and sublimation terms have been assessed for a 1064-nm laser excitation wavelength including the multi-photon absorption cross section for C2 photodesorption and the saturation coefficients for linear- and multi-photon absorption, among others. This parameterized model has then been demonstrated to effectively reproduce signals measured in different combustion media including a CH4/O2/N2 premixed flat flame and a diffusion ethylene flame. As a result of the data derived from the analysis of the Diesel flame, a thermal accommodation coefficient value of 0.49 has been assessed against 0.34 when neglecting the shielding effect. In addition, values of the soot absorption function (Eleft( m right)) comprised between 0.18 and 0.31 have been inferred depending on the particle maturation stage. On the other hand, Eleft( m right) 24% higher on average have been estimated when neglecting MS thus illustrating the importance of aggregate characteristics on soot properties derived through LII modeling. Eventually, the Eleft( m right) evolution observed herein has been compared with results issued from studies conducted with varied hydrocarbons which led to highlight the crucial role played by the soot maturity level over the nature of the burnt fuel as far as optical properties are concerned.

Gas-carrying enhances the combustion temperature of the biomass particles

As a clean and renewable energy, high-temperature combustion of biomass is the key to its application in large-scale industrial production. In this study, four different particle size ranges of sawdust i. e, 150–180, 181–250, 251–380 and 381–830 μm were employed to investigate the relationship between particle size and gas-carrying capacity and its influence on combustion temperature. The morphology and structure of the micropores of the sawdust were determined by ESEM and BET methods. The relationship between particle size and gas-carrying capacity was determined by a dissolved oxygen meter, the combustion experiment was conducted on a Hencken flat flame burner coupled with a high-speed camera. The results showed that smallest particle range (150–180 μm) had the largest pore volume (0.49 cm3/g). Smaller particles had a better gas-carrying performance, particles with a size of 150–180 μm could increase the dissolved oxygen (DO) content in water up to 7.84 mg/L. The gas-carrying capacity of sawdust particles had a huge promotion effect on the combustion temperature, the maximum combustion temperature of particles with a size of 150–180 μm was 1296 °C. This work provided a theoretical basis for the high-temperature combustion of biomass and makes it possible for its application in high-temperature industries.

Numerical investigation and assessment of flamelet-based models for the prediction of pulverized solid fuel homogeneous ignition and combustion

The homogeneous ignition and volatile combustion of pulverized solid fuel in single-particle and particle group configurations were studied numerically in a laminar flat flame burner. Simulations with increasing particle streams were performed to investigate the influence of the interactions in particle groups on homogeneous ignition and combustion. An extensive set of simulations are conducted considering models with different levels of detail for both the gas-phase and solid fuel chemistry. The reference simulations employ the chemical percolation devolatilization model coupled with a detailed chemistry model for gas-phase reactions. The particle-fluid interactions were modeled with a fully coupled Eulerian-Lagrangian framework. Increased ignition delay times for higher particle streams were successfully validated against available experimental measurements. Furthermore, the transition from single-particle ignition to a conically shaped volatile flame with suppressed reactions near the flame base in particle group combustion was observed in both experiments and simulations. The subsequent detailed investigations revealed that the increased heat transfer to particles and, therefore, lower gas temperature for higher particle number densities together with the local oxygen depletion are the primary reasons for this transition. Based on the reference simulation, different simplified model combinations were assessed. The systematic model reduction investigation started with assessing the fixed volatile composition as a required assumption for flamelet models. Finally, the effects of gas-phase chemistry and different simple devolatilization models on ignition and combustion chemistry were studied. Overall, all model combinations provide reasonable predictions of volatile combustion with minor local deficits in the studied conditions.

Experimental and theoretical studies on thermoacoustic limit cycle oscillation in a simplified solid rocket motor using flat flame burner

Combustion instability is a common phenomenon in solid rocket motors. Coupling between pressure and heat release rate produces pressure oscillation, which seriously affects the operation and safety of the motors. It is necessary to investigate this mechanism in order to prevent or at least suppress pressure oscillations. A segmented combustor is designed, and a movable flat flame burner is used as the heating source. A stability map is plotted by moving the flat flame burner along the combustor and changing the equivalence ratio. The experimental results are predicted by employing the low-order thermoacoustic network model. When the heat source position is fixed, it is shown that the combustion changes from stable to very unstable and then returns to a stable state, with the increase of equivalence ratio from 0.9 to 1.5. The pressure oscillation is the strongest in the rich combustion region (equivalence ratio is 1.2–1.3), with the amplitude of about 122 dB. The pressure oscillation is strongest and the amplitude is about 125 dB, when the heat source is close to the nozzle exit. The calculation results by thermoacoustic network model agree well with the experimental results. The stability map and limit cycle characteristics of thermoacoustic oscillation can be accurately estimated. It indicates that this experimental method and theoretical model can be used to capture the limit cycle behavior, and provide a new scheme for predictions of thermoacoustic oscillation in solid rocket motors.

Quantitative 2D thermometry in turbulent sooting non-premixed flames using filtered Rayleigh scattering.

This paper demonstrates the application of a thermometry method in turbulent sooting non-premixed flames using filtered Rayleigh scattering (FRS). Fuel tailoring is used to establish a specific C2H2-based fuel mixture such that temperature can be determined accurately by a single FRS measurement over the entirety of mixture fraction space, or equivalently, for all relevant thermo-chemical states. Evaluation is performed in a hierarchy of flows to establish measurement precision and accuracy. Initial assessments in a series of heated fuel mixtures; non-sooting, near-adiabatic flat flames; and laminar non-premixed sooting flames show accuracy of the approach over a full range of expected temperatures and high single-shot measurement precision (e.g., 65<SNR<80) for temperatures between 1900 and 2200 K. Subsequently, 2D temperature measurements are evaluated in a turbulent non-premixed sooting flame. For the current fuel mixture, the local mixture-averaged Rayleigh scattering cross section is nearly constant over all composition space, and thus traditional laser Rayleigh scattering (LRS) is used as a measurement standard in the absence of soot. The results show excellent agreement between the proposed FRS thermometry approach and LRS in non-sooting regions. In the presence of soot, the proposed FRS-based approach shows no signs of interference. In addition, 2D temperature imaging shows high SNR (60-74) over all temperature conditions. Thus, we believe the proposed methodology successfully provides a high-resolution 2D temperature method under turbulent sooting flame conditions.