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Effects of elevated pressure on thermochemical states of turbulent flame-wall interaction studied by multi-parameter laser diagnostics
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This paper presents a three-dimensional (3D) direct numerical simulation (DNS) study of flame-wall interaction (FWI) and flame-cooling air interaction (FCAI). A preheated, methane/air mixture enters a channel with constant temperature walls, where the top wall is effusion cooled. An imposed vertical hot sheet near the inlet creates two flame branches interacting with the top and bottom walls. The flame is observed to be leaner in the region where it interacts with the effusion cooling jets. In this region, the flame is longer and features reduced CO mass fraction. The fluctuations in the heat release rate (HRR) and CO mass fraction are also relatively small near the top wall. Near the bottom wall, finger-like flame structures are formed due to the interaction of turbulent vortices with the flame surface. These flame structures initially move away from the wall as they propagate further downstream before eventually collapsing at the wall. This leads to the creation of regions of high wall heat flux and CO. While analysis of the CO thermochemical state shows a complex picture near the bottom wall, two-dimensional (2D) manifolds can be identified near the top wall. Therefore, a framework to estimate CO mass fraction due to FCAI based on 1D freely-propagating flame solutions is proposed showing a good agreement with the DNS results.
Flames diluted by combustion products can reduce emissions such as Carbon Monoxide (CO) and Nitrogen Oxides (NOx) in industrial applications. In gas turbines, these flames are confined in a combustor and can interact with relatively cold walls. This interaction can quench the flame, producing incomplete combustion products. In this study, Flame-Wall Interaction (FWI) for methane/air flames diluted by hot combustion products is investigated using direct numerical simulation. A three-Dimensional (3D) turbulent V-flame in a channel with isothermal hot and cold walls is simulated.
It is shown that a main reaction zone in the central region between two walls supported by periodic bulk ignition events changes the position of volumetric reaction zones where CO is formed. The cold wall leads to a longer flame, thereby having disproportionately large contribution to the exhaust CO. Near-wall turbulence-flame interaction creates wrinkled and streaky flame surfaces, and localises the near-wall CO distribution. High mean CO mass fraction develops in the free-stream while a high magnitude of the peak RMS CO mass fraction is present closer to the wall. It is also shown that one-dimensional flame solutions can reasonably describe the changes of CO mass fraction as a function of temperature in the free-stream region and some parts of the near-wall region but not close to the wall. Turbulent mixing and diffusion effects contribute to this deviation. The results highlight the complexities involved in CO modelling for diluted flames and set a benchmark for future work.
The local heat-release rate and the thermo-chemical state of laminar methane and dimethyl ether flames in a side-wall quenching configuration are analyzed. Both, detailed chemistry simulations and reduced chemistry manifolds, namely Flamelet-Generated Manifolds (FGM), Quenching Flamelet-generated Manifolds (QFM) and Reaction-Diffusion Manifolds (REDIM), are compared to experimental data of local heat-release rate imaging of the lab-scale side-wall quenching burner at Technical University of Darmstadt. To enable a direct comparison between the measurements and the numerical simulations, the measurement signals are computed in all numerical approaches. Considering experimental uncertainties, the detailed chemistry simulations show a reasonable agreement with the experimental heat-release rate. The comparison of the FGM, QFM and REDIM with the detailed simulations shows the high prediction quality of the chemistry manifolds. For the first time, the thermo-chemical state during quenching of a dimethyl ether-air flame is examined numerically. Therefore, the carbon monoxide and temperature predictions are analyzed in the vicinity of the wall. The obtained results are consistent with previous studies for methane-air flames and extend these findings to more complex oxygenated fuels. Furthermore, this work presents the first comparison of the QFM and the REDIM in a side-wall quenching burner.
The article describes the development and application of a laser-optical technique for the quantitative measurement of carbon monoxide (CO) in combustion environments of elevated pressures relevant of gas turbine environments. Planar CO measurements can provide important information for the understanding of several aspects of the combustion process, such as localized lean blowout conditions or quenching effects of effusion cooling on the combustor flow. While laser-induced fluorescence is a sensitive non-intrusive measuring technique suitable for planar detection, the CO molecule cannot be excited directly in the visible or near UV. For the present implementation, a two-photon excitation scheme with resulting fluorescence in the visible range is utilized. To qualify the technique, planar measurements in a laminar premixed atmospheric flame are performed in a first step; this flame is later on used for calibration of CO concentrations in a laminar premixed flame at elevated pressure up to 5 bar and in a generic turbulent combustor with a swirl-stabilized flame operated at atmospheric pressure. Pressure effects on spectral structures and the calibration procedure are discussed, as well as potential interference by cross-sensitivity with other species. Based on this optimization strategies are defined for the respective experimental arrangements.
We present a novel approach to still image denoising based on effective filtering in 3D transform domain by combining sliding-window transform processing with block-matching. We process blocks within the image in a sliding manner and utilize the block-matching concept by searching for blocks which are similar to the currently processed one. The matched blocks are stacked together to form a 3D array and due to the similarity between them, the data in the array exhibit high level of correlation. We exploit this correlation by applying a 3D decorrelating unitary transform and effectively attenuate the noise by shrinkage of the transform coefficients. The subsequent inverse 3D transform yields estimates of all matched blocks. After repeating this procedure for all image blocks in sliding manner, the final estimate is computed as weighed average of all overlapping blockestimates. A fast and efficient algorithm implementing the proposed approach is developed. The experimental results show that the proposed method delivers state-of-art denoising performance, both in terms of objective criteria and visual quality.
Improving our knowledge of flame–wall interaction is of relevance to performing near-wall combustion calculations. Quenching distance is to be determined accordingly, as a major parameter of flame quenching. For this purpose, an equation describing the behavior of single-wall flame quenching has been derived from a simplified model of laminar flame–wall interaction. It allows evaluating quenching distance from wall heat flux and mixture properties; a significant advantage of this formula is the absence of any empirical coefficient. To assess its reliability, the results computed with this equation have been compared to experimental data concerning laminar flame–wall interaction. For this purpose, single-wall quenching parameters have been recorded in both head-on and sidewall configurations. Quenching distance and wall heat flux have been measured simultaneously, during the combustion of quiescent methane–air mixtures in a constant-volume vessel. Quenching distance is determined through direct visualization, whereas wall heat flux is processed from the time evolution of wall surface temperature. The equation has been verified over the pressure range 0.05–0.35 MPa in stoichiometric and lean mixtures. It shows good agreement with experimental data at first order, with less than 20% variation.
A turbulent flame–wall interaction (FWI) configuration is studied using three-dimensional direct numerical simulation (DNS) and detailed chemical kinetics. The simulations are used to investigate the effects of the wall turbulent boundary layer (i) on the structure of a hydrogen–air premixed flame, (ii) on its near-wall propagation characteristics and (iii) on the spatial and temporal patterns of the convective wall heat flux. Results show that the local flame thickness and propagation speed vary between the core flow and the boundary layer, resulting in a regime change from flamelet near the channel centreline to a thickened flame at the wall. This finding has strong implications for the modelling of turbulent combustion using Reynolds-averaged Navier–Stokes or large-eddy simulation techniques. Moreover, the DNS results suggest that the near-wall coherent turbulent structures play an important role on the convective wall heat transfer by pushing the hot reactive zone towards the cold solid surface. At the wall, exothermic radical recombination reactions become important, and are responsible for approximately 70% of the overall heat release rate at the wall. Spectral analysis of the convective wall heat flux provides an unambiguous picture of its spatial and temporal patterns, previously unobserved, that is directly related to the spatial and temporal characteristic scalings of the coherent near-wall turbulent structures.
This paper describes a computational approach to edge detection. The success of the approach depends on the definition of a comprehensive set of goals for the computation of edge points. These goals must be precise enough to delimit the desired behavior of the detector while making minimal assumptions about the form of the solution. We define detection and localization criteria for a class of edges, and present mathematical forms for these criteria as functionals on the operator impulse response. A third criterion is then added to ensure that the detector has only one response to a single edge. We use the criteria in numerical optimization to derive detectors for several common image features, including step edges. On specializing the analysis to step edges, we find that there is a natural uncertainty principle between detection and localization performance, which are the two main goals. With this principle we derive a single operator shape which is optimal at any scale. The optimal detector has a simple approximate implementation in which edges are marked at maxima in gradient magnitude of a Gaussian-smoothed image. We extend this simple detector using operators of several widths to cope with different signal-to-noise ratios in the image. We present a general method, called feature synthesis, for the fine-to-coarse integration of information from operators at different scales. Finally we show that step edge detector performance improves considerably as the operator point spread function is extended along the edge.
This work presents a numerical study dedicated to the formation of unburnt hydrocarbon. Two configurations: head-on quenching (HOQ) on a planar wall and in crevices, are considered. It is well known that they contribute for an important part to the sources of hydrocarbon (HC) emission in a combustion chamber. The aim of this work is to use laminar flame simulations (LFS) to understand how the unburnt HC are produced near walls in gasoline engine. A skeletal mechanism (29 species and 48 reactions) mimicking iso-octane combustion is used. In the HOQ configuration, the flame front propagates toward the cold wall where quenching occurs. The numerical procedure and the chemical scheme used in this study are first validated by comparisons with literature results for the 1D case. Several aspects of flame wall quenching such as oxidation of unburnt HC, wall heat flux, quench distances as well as HC families are investigated by varying parameters like wall temperature and equivalence ratio. In a second part, crevices are considered to study the impact of wall imperfections in combustion chambers. Configurations with different geometrical and thermodynamic properties are tested. It leads to a wide range of flame properties and HC production modes. When incomplete combustion occurs, total HC (fuel + HC) concentration can reach very high levels at the wall. When the crevice is not wide enough, the flame cannot propagate and the quantity of HC is smaller than in the case where the flame can propagate (but the fuel is not oxidizing). If the crevice is wide enough for the flame to propagate, HOQ occurs at the bottom of the crevice and HC accumulate in the corners. The computational results obtained in this work demonstrate the ability of LFS to reproduce incomplete combustion mechanisms that are responsible for a major part of HC production in gasoline engines.
The CO formation as a result of the CO2 photodissociation at 230.08 nm was observed by using the two-photon laser-induced fluorescence (LIF) method. The measurements were performed in a propane–air combustion product flow and in mixtures of CO2 and O2. The temperature dependence of the fluorescence signal caused by CO molecules, produced in the photodissociation of CO2 molecules under the action of laser radiation at a wavelength of 230.08 nm, was measured at temperatures ranging from 1300 to 2000 K. It is shown that consideration of CO2 photodissociation under the action of the probing radiation is necessary when one applies the two-photon LIF method for the measurement of small CO concentrations in high-temperature gas mixtures containing CO2. As an example, a correction is given of the CO concentration profiles measured by the LIF method in the combustion product flow around a cooled metallic plate.
Velocities and flame front locations are measured simultaneously in a turbulent, side-wall quenching (SWQ) V-shaped flame during flame-wall interaction (FWI) at 1 and 3 bar by means of particle image velocimetry (PIV) and planar laser-induced fluorescence of the OH radical (OH-PLIF). The turbulent flame brush is characterized based on the spatial distribution of the mean reaction progress variable and a common direct method is used to derive the flame surface density (FSD) from the two-dimensional data by image processing. As the near-wall reaction zone is limited to a smaller region closer to the wall at higher pressure, higher peak values are observed in the FSD at 3 bar. A second definition of the FSD adapted for flames exposed to quenching is utilized similar to previous studies emphasizing the impact of FWI. The influence of the wall on the flame front topology is investigated based on a flame front-conditioned FSD and its variability within the data set. In a last step, an estimate of the mean reaction rate is deduced using an FSD model and evaluated in terms of integral and space-averaged values. A decreasing trend of integral mean reaction rate in regions with increasing flame quenching is observed for both operating conditions, but more pronounced at 3 bar. Space-averaged mean reaction rates, however, increase in the quenching region, as the size of the reaction zone decreases.
In this study, the thermochemical state during turbulent flame-wall interaction of a stoichiometric methane-air flame is investigated using a fully resolved simulation with detailed chemistry. The turbulent side-wall quenching flame shows both head-on quenching and side-wall quenching-like behavior that significantly affects the CO formation in the near-wall region. The detailed insights from the simulation are used to evaluate a recently proposed flame (tip) vortex interaction mechanism identified from experiments on turbulent side-wall quenching. It describes the entrainment of burnt gases into the fresh gas mixture near the flame’s quenching point. The flame behavior and thermochemical states observed in the simulation are similar to the phenomena observed in the experiments. A novel chemistry manifold is presented that accounts for both the effects of flame dilution due to exhaust gas recirculation in the flame vortex interaction area and enthalpy losses to the wall. The manifold is validated in an a-priori analysis using the simulation results as a reference. The incorporation of exhaust gas recirculation effects in the manifold leads to a significantly increased prediction accuracy in the near-wall regions of flame-vortex interactions.
A direct numerical simulation (DNS) with finite rate chemistry was performed to evaluate the main influences on carbon monoxide (CO) emissions in gas turbine combustion. A lean methane/air mixture is burned in fully turbulent jet flames in a domain enclosed by isothermal walls. The formation of CO is found to be affected by the mean strain rate of the turbulent flow, the flame-wall interaction (FWI), and the interactions of the flame with the recirculation zones of the flow. The CO production and consumption in the turbulent flame differ strongly from the reaction rates in a freely propagating flame. In the upstream part of the domain, the mean strain rate of the turbulent flow mainly affects the CO formation, while wall heat loss influences the CO oxidation process towards the end of the domain, where the strain rate decreases. In an optimal estimator analysis, the relevant parameters that dominate the formation and consumption of CO are identified as the local CO mass fraction YCO, the wall heat loss, described by the enthalpy defect Δh, and the mass fraction of the OH radical YOH. The heat loss is particularly influential close to the wall while the effects far from the wall are negligible. Using the local CO mass fraction as parameter describes the late-stage oxidation of CO well in the entire domain. In particular, YCO should not be neglected at the wall. YOH is well suited to describe the processes involved in CO oxidation, as it both parameterizes the turbulent strain and is the main reaction partner for CO oxidation. The combination of YCO and Δh was able to improve the domain-averaged irreducible error by almost half compared to only a progress variable. Adding YOH to the parameter set further reduced the error to 25% of the original error.
A novel, pressurized side-wall quenching (SWQ) burner test rig enabling investigations on flame-wall interactions (FWI) of a fully premixed V-shaped flame at conditions mimicking some characteristics of practical devices, e.g. increased pressures and Reynolds numbers, is introduced within this study. Two operating cases featuring turbulent flow conditions at atmospheric and 3 bar operating pressure are defined and characterized based on velocity fields and flame front positions provided by simultaneous high-speed particle image velocimetry (PIV) and planar laser-induced fluorescence of the OH radical (OH-PLIF). For non-reacting conditions, profiles of mean velocities and mean velocity fluctuations are measured in addition to near-wall flow fields at locations where FWI occurs for chemically reacting conditions. For reacting conditions, near-wall flows are characterized in the vicinity of the quenching zone. Based on measured fluctuations of the streamwise velocity component, the turbulent flame is classified at the transition from the wrinkled to the corrugated flamelet regime. Using the derived flame front positions, the instantaneous flame-wall interaction is classified into SWQ-like, head-on quenching (HOQ)-like and multi-zone quenching events and the transition between these scenarios is identified by means of OH-PLIF image sequences. A statistical analysis of the flame front topology is performed based on the flame brush and deduced frequency distributions. Statistically significant differences between both operating cases are confirmed by chi-squared homogeneity tests.
This study focuses on exploring the thermochemistry in flame-wall interaction (FWI) for fully premixed side-wall quenching of a laminar, atmospheric-pressure dimethyl ether flame at equivalence ratio Φ=0.83 by simultaneous measurement of CO2 and CO mole fractions and gas phase temperature T. The applied laser diagnostics are dual-pump coherent anti-Stokes Raman spectroscopy (DP-CARS) targeting N2 and CO2, laser-induced fluorescence of CO and OH, as well as thermographic phosphor thermometry. The extension to DP-CARS to study FWI processes is the first of its kind, previous studies only provided (CO,T) measurements. The laser diagnostics are benchmarked and calibrated to an adiabatic test case and assessed in accuracy and precision. Subsequently, the approach is used to measure the thermochemistry close to a quenching wall. The nominal flame-to-wall distances from the experiment well match the numerical simulation data with a marginal offset of 20 µm. Conditioning the thermochemical data with respect to the instantaneous quenching point, named quenching-point conditioning, enables a novel tracing of the wall-parallel chemistry evolution across the quenching location. The study provides the first comparison of experimental three-scalar measurements (CO2,CO,T) with two-dimensional (2D) fully-resolved chemistry and transport (FCT) simulations. The validation of numerical simulations can now rely on the three scalars (CO2,CO,T) instead of the two scalars (CO,T) in past studies. The evaluation reveals that this novel three-scalar measurement allows highly sensitive probing of the thermochemical states and is clearly superior to the previously applied two-scalar approach. CO2 is less affected by the quenching wall compared to CO. Differential diffusion effects are experimentally confirmed by comparison to 2D-FCT, with the (CO2,T) state space being more sensitive than (CO,T). As the experimental methodology proved feasible for laminar operation, a transfer to turbulent cases, where the numerical analysis using direct numerical simulations (DNS) including FCT is limited, appears promising.
The quenching distance, wall heat flux and pollutant thermochemical states of the laminar premixed biogas-hydrogen impinging flame at ϕ = 0.8, 1.0 and 1.2 were investigated using the numerical simulation with detailed chemical kinetics and transport parameters. With the 10% H2 addition, the biogas composed of 75% CH4 and 25% CO2 was used as fuel in the study. Results show that PeQ ≈5 is obtained for the laminar premixed biogas-hydrogen impinging flame at ϕ = 0.8, 1.0 and 1.2. The smaller PeQ than that of CH4 flame is resulted from the CO2 increasing the flame thickness, as well as the decelerated flame propagation due to the strong stretch effects. The quenching distance of biogas-hydrogen flame is codetermined by intrinsic characteristics of heat transfer process and effects of equivalence ratio and flame stretch on the flame propagation. The wall heat flux of laminar premixed biogas-hydrogen impinging flame is maximum at ϕ = 1.0, followed by ϕ = 1.2 and 0.8. At ϕ = 0.8 and 1.2, the location difference between the quenching point and peak wall heat flux of premixed biogas-hydrogen impinging flame is ascribed to the oxidization of escaped unburned fuel near the wall, and the worse field synergy of temperatue and velocity near the quenching point. Additionally, it is found that the fast NO accumulation near the wall at ϕ = 0.8 is ascribed to the transportation of NO generated by NNH route and the effective prompt NO production, while that at ϕ = 1.2 is resulted from the prompt NO production primarily. In the wall vicinity, the low temperature shift of NO production are resulted from the much faster characteristic time scale of heat transfer, while the NNH route and prompt route play the dominant roles in this low-temperature shifts of NO production at ϕ = 0.8 and 1.2, respectively. This means the increased importance of prompt and NNH routes on the NO emission in the downsized combustor.
Thermochemical states of a turbulent, lean premixed dimethyl ether/air flame were assessed for the first time using simultaneous measurements of gas temperature T, CO2 and CO mole fractions with locations as close as 120 μm above the quenching wall. This is realized by combined dual-pump coherent anti-Stokes Raman spectroscopy (DP-CARS) and two-photon laser-induced fluorescence (LIF) of CO. In addition to thermochemical states, the flow and flame dynamics were measured separately using a combined two component particle image velocimetry (PIV) and planar LIF of the OH radical at high (4 kHz) and low (50 Hz) repetition rates. The data from the independent measurements was linked by the instantaneous flame front topologies, determined by the qualitative OH-LIF in both experiments. The grid-generated turbulence intensity was found to be relatively low in the bulk flow (∼4.5%, streamwise velocity component) with the turbulent flame classified within the regime of wrinkled flamelets (w′/sL between 0.3 and 0.75, for streamwise velocity component). The flame-wall interaction could be assigned to either a side-wall quenching (SWQ)-like (∼50%) or a head-on quenching (HOQ)-like scenario (∼50%), with a transition between these scenarios taking place within a few milliseconds. The thermochemical states depend significantly on whether a SWQ-like or a HOQ-like scenario is present. Here, the thermochemistry of the SWQ-scenario is studied in detail, and three zones, A, B, C, could be distinguished in the state space on the basis of the (CO2,T) correlations. Zone A is characterized by strong wall heat losses and mixing influences, while zone B features less pronounced wall heat losses and mixing processes. In zone C the impacts of turbulence almost completely disappear and conditions comparable to a laminar near-wall flow are observed. The distinguishability of the three zones in the (CO,T) or (CO,CO2) correlations is less clear, which underlines the importance of the additional CO2 measurement in the DP-CARS methodology.
Design of efficient, downsized piston engines requires a thorough understanding of transient near-wall heat losses. Measurements of the spatially and temporally evolving thermal boundary layer are required to facilitate this knowledge. This work takes advantage of hybrid fs/ps rotational coherent anti-Stokes Raman spectroscopy (HRCARS) to measure single-shot, wall-normal gas temperatures, which provide exclusive access to the thermal boundary layer. Phosphor thermometry is used to measure wall temperature. Measurements are performed in a fixed-volume chamber that operates with a transient pressure rise/decay to simulate engine-relevant compression/expansion events. This simplified environment is conducive for fundamental boundary layer and heat transfer studies associated with engine-relevant processes. The thermal boundary layer development and corresponding heat losses are evaluated within two engine-relevant regimes: (1) an unburned-gas regime comprised of gaseous compression and (2) a burned-gas regime, which includes high-temperature compression and expansion processes. The time-history of important boundary layer quantities such as gas / wall temperatures, boundary layer thickness, wall heat flux, and relative energy lost at the wall are evaluated through these regimes. During the mild unburned-gas compression, Tcore increases by 30 K and a thermal boundary layer is initiated with thickness δT ~ 200 μm. Wall heat fluxes remain below 6 kW/m², but corresponds to ~6% energy loss per ms. In the burned-gas regime, Tcore resembles adiabatic flame temperatures, while Twall increases by 16 K. A thermal boundary layer rapidly develops as δT increases from 290 to 730 μm. Energy losses in excess of 25% occur after flame impingement and slowly decay to ~10% at the end of expansion. Measurements also resolve thermal mixing of fresh- and burned gases during expansion, which yield strong temperature reversals in the boundary layer. Findings are compared to canonical environments and demonstrate the transient thermal boundary nature during engine-relevant processes.
Thermochemical interaction – represented by CO mole fraction and gas phase temperature measurements – between flame and cooling air is investigated in a close-to-reality effusion-cooled single sector model gas turbine combustor. To investigate the influence of effusion cooling air mass flow on the thermochemical state, a parametric study is conducted. Temperature measurements are performed using ro-vibrational N2 coherent anti-Stokes Raman spectroscopy (CARS). CO mole fraction is measured by means of quantitative CO two-photon laser-induced fluorescence (CO-LIF) using a temperature dependent calibration acquired in an adiabatic pressurized laminar flame. Significantly different thermochemical states are observed in the inner and outer shear layer of the swirl stabilized flame. Within the primary zone, increasing cooling air mass flow leads to decreased CO concentrations. Close to the effusion cooled liner, the interaction varies with axial coordinate. In the outer recirculation zone, increased CO mole fractions were measured with increasing cooling air mass flow, indicating occurrence of chemical quenching in the late oxidation branch in the CO-T diagram. Further downstream, processes are dominated by mixing and CO concentrations decrease with the amount of supplied effusion cooling air. To our best knowledge, this is the first time that these effects has been shown experimentally.
Near-wall transient heat transfer and flame–wall interaction (FWI) are topics of great importance in the development of downsized internal combustion (IC) engines and gas turbine technology. In this work we perform measurements using 1D hybrid fs/ps rotational CARS (HRCARS), thermographic phosphors (TGP) and CH* imaging in an optically-accessible chamber designed to study transient near-wall heat transfer processes relevant to IC engine operation. HRCARS provides single-shot gas-phase temperatures (40 µm spatial resolution and up to 3 mm wall-normal distances), while thermographic phosphors measures wall temperature and CH* measures the flame front position. These simultaneous measurements are used to resolve thermal boundary layer (TBL) development and associated gaseous heat loss for three important processes of gas–wall interactions: (1) an unburned-gas polytropic compression process, (2) FWI, and (3) post-flame and gas expansion processes. During a mild polytropic compression process, measurements emphasize that even a relatively small wall heat flux (≤5 kW/m²) yields an appreciable temperature stratification through a developing TBL. During FWI, thermal gradients induced by the flame are resolved within the TBL. Gases closest to the wall (y<0.2 mm) continue to experience thermal loading from polytropic compression until the flame is within ∼1.4 mm from the wall. Immediately afterwards, the wall first senses the flame as the wall temperature begins to increase. During FWI, gas temperatures up to 1150 K impinge on the wall, producing peak wall heat fluxes (620 kW/m²) and the wall temperature increases (ΔTwall=14 K). Gaseous heat loss in the post-flame gas occurs rapidly at the wall, yielding a TBL of colder gases extending from the wall as wall heat flux slowly decreases. HRCARS further captures the rapid cooling of gases in the TBL and core-gas during the mild expansion and exhaust process.
A detailed investigation on flame structures and stabilization mechanisms of confined high momentum jet flames by 1D-laser Raman measurements is presented. The flames were operated with natural gas (NG) at gas turbine relevant conditions in an optically accessible high pressure test rig. The generic burner represents a full scale single nozzle of a high temperature FLOX® gas turbine combustor including a pilot stage. 1D-laser Raman measurements were performed on both an unpiloted and a piloted flame and evaluated on a single shot basis revealing the thermochemical states from unburned inflow conditions to burned hot gas in terms of average and statistical values of the major species concentrations, the mixture fraction and the temperature. The results show a distinct difference in the flame stabilization mechanism between the unpiloted and the piloted case. The former is apparently driven by strong mixing of fresh unburned gas and recirculated hot burned gas that eventually causes autoignition. The piloted flame is stabilized by the pilot stage followed by turbulent flame propagation. The findings help to understand the underlying combustion mechanisms and to further develop gas turbine burners following the FLOX® concept. Together with the connected papers A, B and D, the results form a unique and comprehensive data set for the validation of numerical simulation models.
This study is focused on the characterization of wall heat fluxes and its influence upon CO formation/oxidation within atmospheric flames in a side-wall quenching geometry. The influence of different wall temperatures ranging between 330 K and 670 K is compared for stoichiometric methane and dimethyl ether (DME) flames. Coherent anti-Stokes Raman spectroscopy (CARS) and two-photon laser induced fluorescence (LIF) of the CO molecule are used to determine pointwise gas phase temperatures and CO concentrations. Simultaneously, wall temperatures are measured using one-dimensional phosphor thermometry and flame front positions are identified by planar OH-LIF imaging. Wall heat fluxes are estimated from measured gas and wall temperatures. For increasing wall temperatures, quenching distances decrease significantly and the maximum wall heat fluxes rise in the quenching region. Additionally, thermochemical states are analysed using CO/T scatter plots. Compared to one-dimensional unbounded laminar flame calculations, the CO/T dependencies are altered significantly by the presence of a wall. Very close to the wall, for methane/air flames and to a lesser extent for DME/air flames at y = 100 µm, the CO formation branch is shifted towards lower temperatures. In contrast, in the entire near-wall region the CO oxidation branch is shifted to lower temperatures for both fuels. One-dimensional premixed flame calculations accounting for enthalpy losses indicate that the heat loss to the wall is the most likely cause rather than different chemical reaction pathways. Studying the impact of turbulence, both the CO formation and oxidation branch are shifted to lower temperatures in state space. Additionally, an increasing number of intermediate CO mole fractions is observed filling the state space in between both branches. The analysis of turbulent integral time scale derived from PIV data indicates that this phenomenon is dominated by heat transfer, which is enhanced by turbulence.
Flame-wall interactions (FWI) of laminar premixed methane-air flames at atmospheric pressure are studied using various laser diagnostic methods. Velocity fields and flame front locations are measured simultaneously by two-component particle image velocimetry (PIV) and planar laser induced fluorescence (LIF) of the OH-radical. Coherent anti-Stokes Raman spectroscopy (CARS) and two-photon LIF of the CO molecule are used to determine temperatures and CO concentrations. The FWI process is investigated using a generic burner setup with well-defined boundary conditions, where one branch of a V-shaped flame interacts with a water-cooled stainless steel wall, corresponding to a sidewall quenching (SWQ) geometry. FWI is studied for equivalence ratios of ϕ = 0.83, 1.0 and 1.2. The quenching distance of the flames is determined using two different methods. Additionally, the near wall behavior of the flame consumption speed is analyzed and compared with that of a freely propagating laminar flame. Thermochemical properties are analyzed using CO/T-state diagrams. Comparison to one-dimensional laminar flame calculations undisturbed by the presence of a wall highlights the severe impact upon thermochemical states. Comparing characteristic time scales of heat transfer processes to chemical processes indicates that diffusion rather than chemical reaction processes is the reason for these observations.
Ultrabroadband coherent anti-Stokes Raman spectroscopy (CARS) has been developed for one-dimensional imaging of temperature and major species distributions simultaneously in the near-wall region of a methane/air flame supported on a side-wall-quenching (SWQ) burner. Automatic temporal and spatial overlap of the ∼7 fs pump and Stokes pulses is achieved utilizing a two-beam CARS phase-matching scheme, and the crossed ∼75 ps probe beam provides excellent spatial sectioning of the probed location. Concurrent detection of N2, O2, H2, CO, CO2, and CH4 is demonstrated while high-fidelity flame thermometry is assessed from the N2 pure rotational S-branch in a one-dimensional-CARS imaging configuration. A methane/air premixed flame at lean, stoichiometric, and rich conditions (Φ = 0.83, 1.0, and 1.2) and Reynolds number = 5000 is probed as it quenches against a cooled steel side-wall parallel to the flow providing a persistent flame–wall interaction. An imaging resolution of better than 40 µm is achieved across the field-of-view, thus allowing thermochemical states (temperature and major species) of the thermal boundary layer to be resolved to within ∼30 µm of the interface.
WIDECARS measures temperature and mole fractions of most of the major species in ethylene–air flames. One of the issues in implementing this technique is fitting the experimental spectra to theory to obtain flame conditions (temperature, species mole fractions). Individual spectra contain many species resonances, and theory is slow to compute. Libraries of precalculated spectra can be used, but a library of sufficient density for accurate interpolation is large given the many variables. A new fitting algorithm is presented which utilizes a less-dense library, and additional spectra are calculated during fitting to maintain accuracy. The iterative convergence method converts the problem of minimizing fit error, which converges slowly, to a zero finding problem, which converges reliably, rapidly, and accurately to best fit. Various practical fitting issues, such as the effects of dye laser mode noise and variability, phase-matching efficiency, and shifts of the spectrum on the spectrometer are addressed. The technique is demonstrated in the analysis of experimental measurements in an equivalence ratio 2.1 ethylene–air flame above the surface of a McKenna burner. Precision errors because of experimental and fitting effects are discussed. Copyright © 2015 John Wiley & Sons, Ltd.
This review discusses the role of laser diagnostics in combustion science and technology. In its first part, it may guide understanding of advanced diagnostic methods, and is particularly helpful for non-specialized experimentalists. Various challenges for future developments and applications of optical combustion diagnostics are highlighted. In the second part of this review, flame-wall interactions are selected for a more in-depth discussion. Flame-wall interactions are scientifically interesting and are of great importance to any enclosed practical combustion process. Following a description of current understanding, the focus is on using optical diagnostics to probe thermal, fluidic, and chemical properties of head-on and sidewall quenching. The review ends with a discussion of issues and implications for future experimental research and specific diagnostic needs.
This paper reports on simultaneous measurement of temperature and CO concentration in atmospheric methane/air jet flames impinging vertically against a water-cooled stainless-steel wall. Flame–wall interactions are investigated for statistically stationary flames and propagating flames, representing the recognized case of head-on quenching. Instantaneous temperatures are determined using nanosecond coherent anti-Stokes Raman spectroscopy of nitrogen (CARS); CO concentrations are measured using two-photon laser-induced fluorescence (LIF). Statistically stationary flames are investigated in a parametric study for equivalence ratios (0.83 < ϕ < 1.2) and two turbulence intensities. Surface temperatures were measured using phosphor thermometry (TP). Extrapolation of the gas phase to the wall temperature allows estimation of the error in determining the wall position. For transient flame–wall interactions flames are initiated by a laser-spark 27 mm below the wall and propagate against the wall. Head-on flame quenching is studied in these cases for 0.83 < ϕ < 1.0. Quenching distances and maximum wall heat fluxes are derived from the quantitatively measured gas phase temperatures. Conditional statistics are deduced from 200 individual quenching events and are analyzed for distance from the wall. Enthalpy losses of the flame to the wall severely impact the thermo-chemical state, causing significant deviation from stationary conditions. Spatial and temporal profiles of the transient flames are also investigated. The quenching layer is found to be in the range of 0.17–0.32 mm with corresponding dimensionless quenching distances between 0.38 and 0.68. During transient flame quenching the wall heat flux is enhanced by a factor of two and reaches values ranging from 0.24 to 0.48 MW/m2. The normalized quenching heat flux is found to be 0.29 for lean and 0.52 for stoichiometric methane/air flames. These values are in agreement with experimental studies that used very different measurement techniques and with results from direct numerical simulations (DNS) reported in the literature.
Temperature profile measurements were performed on laminar flame fronts propagating head-on towards a spherical obstacle. The profiles were measured at several points in time during approach in order to study the time-dependent quenching process intensively. The measurements were executed in confined combustions of premixed CH4/air and CH3OH/air at stoichiometry and in a pressure range of 0.5<p≤1 bar. The optical temperature measuring technique is based on the spark induced fluorescence (SIF) already successfully used at freely propagating flame fronts. As a result, head-on quenched flame fronts do not stop ahead of the obstacle in a distance equal to the flame front thickness, but touch the surface very closely. The front thickness at p=1 bar is in the range of 0.5 mm. According to existing theories, in this region the flame temperature should fall to wall temperature, but at the nearest distance where temperature measurements could be executed, at d≤70 μm, temperatures T>1000 K were determined. Heat-transfer coefficients of the transition gas phase-wall could be evaluated from the temperature gradient near the wall. The values lie around α≈50 W/m2K. Since the flame velocity tends toward the burning velocity Su as it approaches the wall, because the velocity of the unburnt gas goes to zero, measurements of Su were possible as well as measurements of the flame front thickness, as long as the front was still unquenched. The measured values are in agreement with literature data.
Raman-scattering measurements have been performed to investigate the effect of flame/wall interactions on unburned hydrocarbon emissions. An impervious cold (isothermal) flat wall was placed perpendicular to a laminar premixed propane/air flame, stabilized on a heat-sink-type one-dimensional-flame burner. A conventional spontaneous-Raman-scattering apparatus was utilized to obtain high-spatial-resolution data in the vicinity of the wall-burner interface. Spatial scans were obtained by translating the burner assembly with micrometer screws. Nitrogen number densities were determined by measuring Q-branch Raman spectra and by applying computer-generated corrections. The resultant densities were converted to temperatures by utilizing the isobaric nature of the flame. 4 refs.
A parametric study of the structure of the side wall quench layer has been performed. An atmospheric pressure premixed hydrocarbonair flame was stabilized on a porous sintered bronze disk and flat plate was located perpendicular to the undisturbed flame. Spontaneous Raman spectroscopy was used to measure temperature and hydrocarbon number densities, with emphasis on the near wall fields. A propane flame with a cooled copper side wall at fuel lean stoichiometric ratio φ = 0.87 was studied as a nominal case. Various combinations of stoichiometry (φ = 0.69, 0.87, 1.0), fuel species (propane, butane, ethylene), and wall conditions (cooled copper, platinum, and Teflon and heated cast iron) were studied.
Quenching of laminar premixed iso-octane flames at cold walls is studied using detailed kinetics. Previous investigations of flame quenching used low-molecular-weight fuels such as methane, methanol, and acetylene. For these fuels postquench oxidation of hydrocarbons is very fast and the amount of intermediate hydrocarbons in the quench layer is low compared to the amount of unreacted fuel. However, this does not hold true for more complex, higher-molecular-weight fuels which exhibit different characteristics, leading to higher levels of intermediate hydrocarbons in the quench layer than for unreacted fuel. Oxidation is considerably slower, resulting in very high levels of unburned hydrocarbons in comparison to the simple, low-molecular-weight fuels. In this study calculations are performed with iso-octane for pressures of 1, 5, 10, and 20 atm, initial temperatures of 300, 400, and 500 K, and equivalence ratios of 0.9, 1.0, and 1.1. The oxidation of intermediate hydrocarbons predominantly controls the overall evolution of unburned hydrocarbons. Thus, the use of global chemistry appears to be inadequate to describe quenching of more complex fuels. The influence of the Soret effect which is often neglected in flame studies is investigated in terms of postquench oxidation. A short mechanism for iso-octane applied previously to flame propagation was found to be inadequate to describe the hydrocarbon evolution after quenching. Especially for low pressures, agreement is not satisfactory. It is shown that by adding a small number of species and reactions to the reduced mechanism, results are improved, leading to better agreement between the detailed and the short mechanism in its extended version.
Laminar flame quenching at the cold wall of a combustion chamber has been studied, using a numerical model to describe the reactive flow. The model combines an unsteady treatment of the fluid mechanics and a detailed chemical kinetic reaction mechanism. Fuels considered included both methane and methanol. The one-dimensional case of flame propagation perpendicular to the wall was studied. Two reference cases are described in detail for flame quenching at 10 atmospheres pressure and a wall temperature of 300/sup 0/K with stoichiometric mixtures of methane-air and methanol-air. In each case a conventional laminar flame propagates toward the wall, approaching to within a distance determined by the thermal flame thickness. Chemical kinetic factors, particularly differences between the temperature dependence of radical recombination reactions and conventional chain branching and chain propagation reactions, are shown to be responsible for quenching the flame near the wall. The flame stagnates, but fuel remaining near the wall diffuses out of the boundary region and is rapidly oxidized away from the wall. Subsequent model calculations demonstrate the effects of variations in pressure, fuel-air equivalence ratio, wall temperature, and type of fuel. Computed results from these methane and methanol flame quenching models indicate that the total unburned hydrocarbon content is considerably smaller than is commonly beleived and that thermal wall quenching may not be the major source for hydrocarbon emissions from internal combustion engines at near-stoichiometric conditions.
We report measurements of the temperature- and species-dependent cross sections for the quenching of fluorescence from the B 1Σ+(v = 0) state of CO. Cross sections were measured for gas temperatures ranging from 293 K to 1031 K for quenching by H2, N2, O2, CO, H2O, CO2, CH4, He, Ne, Ar, Kr, and Xe. The CO B 1Σ+(v = 0) state was populated via two-photon excitation (B 1Σ+←←X 1Σ+), and the B 1Σ+→A 1Π fluorescence was collected. Quenching cross sections were determined from the dependence of the fluorescence-decay rate on quencher-gas pressure. The temperature dependence of the cross sections is well described by a power law for all but the two weakest quenchers, He and Ne. © 2002 American Institute of Physics.
Flame propagation and extinction near a solid surface have been investigated. The flow configuration chosen was an impinging reactant jet and the flame was stabilised around the stagnation point. An interesting phenomenon of alternative combustion modes under exactly the same cold flow condition has been observed, namely the so called “disc-like” and “ring-like” flames. The two flames can be established in a controlled manner dependent on how the flame was ignited. The alternative combustion modes demonstrate the complex nature of flame propagation in the vicinity of a solid surface. Besides global observation and measurement of flame propagation and extinction, a high speed laser sheet tomography technique has also been employed to investigate further into the disc-like flame case studied. Detailed local flame extinction (identified by the reactant touching the water cooled surface) has been resolved by the high speed laser sheet tomography technique. The time resolved images also indicate that local flame extinction will not necessary lead to global flame extinction and the extinguished part can be reignited as long as there is enough hot product from which flame propagation can occur. Time resolved global flame extinction data has also been obtained by the same technique.
Experimental investigations of the head-on quenching of a laminar methane flame have produced conflicting statements about the magnitude of the wall heat flux during quenching and its trend with respect to wall temperature. The current theoretical formulations fail to predict the correct behavior. We have been studying the head-on quenching of a laminar, stoichiometric methane flame at atmospheric pressure in a range of wall temperatures between 300 K and 600 K using numerical simulation. To this end we solved the fully compressible, one-dimensional Navier-Stokes equations with detailed mechanisms for kinetics and diffusion (including cross-transport effects, i.e., Soret and Dufour effect). Four different chemical schemes (two where only the C1 path is included and two that contain also the C2 path) were used in order to minimize uncertainties resulting from different descriptions of the chemical kinetics and to investigate the influence of the C2 chemistry. The wall is considered as chemically inert. Points of interest were the variation of the wall heat flux with wall temperature, as well as the time evaluation of species mass fractions, net heat release rates per species, and detailed reaction rates during quenching at wall temperatures of 300 K and 600 K.
A detailed understanding of transport phenomena and reactions in near-wall boundary layers of combustion chambers is essential
for further reducing pollutant emissions and improving thermal efficiencies of internal combustion engines. In a model experiment,
the potential of laser-induced fluorescence (LIF) was investigated for measurements inside the boundary layer connected to
flame-wall interaction at atmospheric pressure. Temperature and species distributions were measured in the quenching boundary
layer formed close to a cooled metal surface located parallel to the flow of a premixed methane/air flat flame. Multi-line
NO-LIF thermometry provided gas-phase temperature distributions. In addition, flame species OH, CH2O and CO were monitored by single-photon (OH, CH2O) and two-photon (CO) excitation LIF, respectively. The temperature dependence of the OH-LIF signal intensities was corrected
for using the measured gas-phase temperature distributions. The spatial line-pair resolution of the imaging system was 22μm
determined by imaging microscopic line pairs printed on a resolution target. The experimental results show the expected flame
quenching behavior in the boundary layer and they reveal the potential and limitations of the applied diagnostics techniques.
Limitations in spatial resolution are attributed to refraction of fluorescence radiation propagating through steep temperature
gradients in the boundary layer. For the present experimental arrangements, the applied diagnostics techniques are applicable
as close to the wall as 200μm with measurement precision then exceeding the 15–25% limit for species detection, with estimates
of double this value for the case of H2CO due to the unknown effect of the Boltzmann fraction corrections not included in the data evaluation process. Temperature
measurements are believed to be accurate within 50K in the near-wall zone, which amounts to roughly 10% at the lower temperatures
encountered in this region of the flames.
A 1D laser Raman system for the simultaneous measurement of the major species concentrations, mixture fraction, and temperature in gas turbine-like flames is presented. The adaptation of the measuring technique to the test rig and the particular challenges of the measurements are described. The gas turbine model combustor was operated with natural gas and preheated air at pressures of 2 and 10 bar. The measurements characterized the mixing and reaction progress and revealed strong effects of turbulence-chemistry interactions with large deviations from chemical equilibrium. The single-shot data obtained along the imaged line of 7 mm allowed the investigation of spatial correlations. Together with the results from further measurements with different methods, the Raman data form an experimental data base that is used for the validation of LES simulations.
A novel design of “laser” is described which produces a modeless output giving a continuous spectral distribution and a continuously variable bandwidth. Efficiencies and bandwidths are obtained comparable to convetional laser pumped dye laser. The device is suitable for studies of laser bandwidth effects in resonant interactions and nonlinear processes using high power pulsed lasers.