Stagnation Plane Research Articles

Comparison of macroturbulence measured using decomposition of PIV, ADV and LSPIV data

Visualization and length-scale estimates are provided for turbulence in open channel flow over a gravel-bed. Results obtained using the digital particle image velocimetry (PIV) followed by post-processing with proper orthogonal decomposition showed higher and lower velocity regions aligned along a stagnation plane extending from the bed at approximately a 20° angle. Periodicity of second and fourth quadrant events equalled 0.20 s and agreed well with the outer variable defined bursting period. Normalized turbulence length scales were measured after performing the decomposition of the turbulent fields collected with various instrumentation techniques. Acoustic Doppler velocimetry and PIV had turbulence length scales equal to the flow depth, and the good agreement highlights the ability of the former to estimate length scales in streams. Large scale PIV provided length scales equal to 1.5 times the flow depth, but care should be used if directly inferring scales of macroturbulence from the free-surface.

Water Droplets Behavior in Extinguishing the Methane-Air Counterflow Diffusion Flame

The behavior of fine water droplets was investigated in a methane-air counterflow diffusion flame, specifically when the water droplets were added to the inlet air feed and used to extinguish the diffusion flame. The water droplets studied had a relatively broad size distribution (from 1 to 60 ?m), with the number mean diameter of 15 ?m and Sauter mean diameter of 25 ?m. With an increase in the velocity gradient, the flame approaches towards the stagnation plane and flame thickness decreases. The flame properties, such as flame location and flame thickness are insensitive to the water mass loading. Along the stagnation stream line, the velocity decreases towards a local minimum just before the flame front. In contrast, the velocity increases in the flame zone due to the thermal expansion, and then decreases towards the stagnation point. In the decelerating zone, the droplet mass flux decreases on approach to the flame zone due to a non-equilibrium in velocity of large droplets between liquid and gas phases. Furthermore, as the droplet-laden air approaches the flame front, the flow stream begins to diverge in the counterflow field. Since the small droplets move away from the burner axis, the divergence of the air flow acts to reduce the water droplet mass flux along the stagnation streamline. Large droplets moving faster than the air flow “catch up” to slower ones just before the flame front, and the mass flux of the droplets tends to increase, resulting in the droplets accumulating just before the flame zone. Measurements of the velocities of individual droplets show that large droplets move faster than the small droplets which are in equilibrium (in velocity) between liquid and gas phases. Droplet behavior was found to be controlled by the Stokes number. Equilibrium in velocity is re-established just in front of the flame zone. Closer to the flame front, evaporation occurs and the mass flux of the droplets decreases drastically. On the other hand, thermophoresis acts on extremely small droplets to move them along the steep temperature gradient and these droplets are forced from the high to low temperature regions against the convection velocity. The thermophoretic velocity was estimated in the present study to be compared with the measured convection velocity and it was concluded that the thermophoretic effect is negligibly small over all sizes of droplets.

Development of Ultra-Micro Combustor Using Cylindrical Flames

The purpose of this study is to develop an ultra-micro combustor that uses two types of coaxial cylindrical flames, rich premixed flame and diffusion flame. The combustor consists of inner and outer porous tubes, and rich propane-air mixture and air issued, respectively, through the inner tube outwardly and through the outer tube inwardly, forming a cylindrical stagnation plane sandwiched by the inner rich premixed flame and the outer diffusion flame. Petal type flame was also observed in the downstream of the cylindrical flames. Keeping the equivalence ratio φi and flow rate qi of the rich mixture constant, air flow rate qa was varied. The O2 and CO concentrations and temperature of the burnt gas were measured, and heat loss rate ηhl and combustion intensity L were evaluated. The obtained results are described as follows. (1)The relation curve of ηhl with the overall equivalence ratio φall, which is evaluated from the total flow rate of the fuel and the air, has a minimum value. (2)The relation curve of the minimum value of ηhl with L has a minimum value. (3) The CO concentration of the burnt gas increases as qa is increased because of local extinction of the petal type flame. (4)When qa is increased further, petal type flame is also extinguished. After that, the O2 concentration increases and the CO concentration decreases.

Effect of quenching of the oxidation layer in highly turbulent counterflow premixed flames

Three lean-to-stoichiometric premixed flames were studied in the opposed-jet configuration by counterflowing fresh reactants and fully burnt products of combustion at 1850K. The study was performed experimentally under turbulent conditions and computationally under laminar ones. The flame mixtures were selected to have the same unstrained laminar flame speed. The turbulent flames were studied experimentally using simultaneous planar imaging of CO and OH laser-induced fluorescence, which, in combination, yielded a quantity proportional to the forward reaction rate for CO+OH→CO2+H. Since the turbulence characteristics of the feed streams were identical for all flames, the turbulent burning regime was also the same, with a turbulent Reynolds number of 1050 and a Karlovitz number of approximately 5. The oxidation layer of the stoichiometric flame was extinguished, whereas the lean flames exhibited substantial evidence of CO conversion as indicated by the CO+OH reaction rate imaging. To aid the interpretation of the experiments, we numerically investigated the extinction of strained laminar premixed flames with compositions identical to those of the experiments. The calculations corroborated the experimental results, indicating that the stoichiometric flame was the least robust and extinguished at the lowest strain rate. Furthermore, extinction occurred when the flames were very close to the gas stagnation plane and the oxidation layer extended beyond it, towards the burnt product side. The quenching of the oxidation layer is suggested as a possible reason for either local or overall extinction of highly strained premixed flames.

Fractal-Generated Turbulence in Opposed Jet Flows

The opposed jet configuration presents a canonical geometry suitable for the evaluation of calculation methods seeking to reproduce the impact of strain and re-distribution on turbulent transport in reacting and non-reacting flows. The geometry has the advantage of good optical access and, in principle, an absence of complex boundary conditions. Disadvantages include low frequency flow motion at high nozzle separations and comparatively low turbulence levels causing bulk strain to exceed the turbulent contribution at small nozzle separations. In the current work, fractal generated turbulence has been used to increase the turbulent strain and velocity measurements for isothermal flows are reported with an emphasis on the axis, stagnation plane and the distribution of mean and instantaneous strain rates. Energy spectra were also determined. The instrumentation comprised hot-wire anemometry and particle image velocimetry with the flows to both nozzles seeded with 1 \(\upmu\)m silicon oil droplets providing a relaxation time of ≃ 3 \(\upmu\)s. It is shown that fractal grids increase the turbulent Reynolds number range from 48–125 to 109–220 for bulk velocities from 4 to 8 m/s as compared to conventional perforated plate turbulence generators. Low frequency motion of the order 10 Hz could not be completely eliminated and probability density functions were determined for the location of the stagnation plane. Results show that the fluctuation in the position of the stagnation plane is of the order of the integral length scale, which was determined to be 3.1±0.1 mm at the nozzle exits through the use of hot-wire anemometry. Flow statistics close to the fractal plate located upstream of the nozzle exit were also determined using a transparent glass nozzle.

Effect of fine water droplets on extinguishment of diffusion flame stabilized in the forward stagnation region of a porous cylinder

The roles of the physical, thermal and chemical effects of fine water droplets on extinguishment of diffusion flame subjected to high strain are quantified experimentally using a methane counterflow diffusion flame. Water droplets were generated by twin-fluid atomizers and ultrasonic mist generators. The distributions of water droplet diameter were measured by a PDPA and found to be nearly normal, with the number mean diameter ranging typically from 15 to 25 μm and the Sauter mean diameter from 25 to 30 μm. The limit of extinguishment was determined by the velocity gradient and the non-dimensional fuel-ejection velocity. The critical velocity gradient at extinguishment without water droplets was found to be 423 s −1. Introduction of water droplets into air stream resulted in reduction of the velocity gradient at extinguishment due to evaporation which causes the cooling and depletion of oxygen in the flame zone. The critical velocity gradient at extinguishment decreases with the increase of the surface area parameter, Σ, which is expressed by the ratio of mass fraction of water droplets to Sauter mean diameter. By a simple analysis, the lifetime of a droplet was compared with the residence time and it was found that there exists threshold below which droplets can be evaporated within the flame zone. PDPA measurement also showed that the water droplets with diameters larger than the threshold pass through the flame zone towards the stagnation plane. The threshold was found to depend on the velocity gradient. The maximum temperature measured by a thermocouple decreases with the mass loading of water droplets as well as the velocity gradient. However, the maximum temperature at extinguishment is rather independent of mass loading of water droplets and also of velocity gradient and the thermal effect is predominant in the extinguishment of the counterflow diffusion flame.

Effect of the composition of the hot product stream in the quasi-steady extinction of strained premixed flames

The extinction of premixed CH 4/O 2/N 2 flames counterflowing against a jet of combustion products in chemical equilibrium was investigated numerically using detailed chemistry and transport mechanisms. Such a problem is of relevance to combustion systems with non-homogeneous air/fuel mixtures or recirculation of the burnt gases. Contrary to similar studies that were focused on heat loss/gain, depending on the degree of non-adiabaticity of the system, the emphasis here was on the yet unexplored role of the composition of counterflowing burnt gases in the extinction of lean-to-stoichiometric premixed flames. For a given temperature of the counterflowing products of combustion, it was found that the decrease of heat release with increase in strain rate could be either monotonic or non-monotonic, depending on the equivalence ratio φ b of the flame feeding the hot combustion product stream. Two distinct extinction modes were observed: an abrupt one, when the hot counterflowing stream consists of either inert gas or equilibrium products of a stoichiometric premixed flame, and a smooth extinction, when there is an excess of oxidizing species in the combustion product stream. In the latter case four burning regimes can be distinguished as the strain rate is progressively increased while the heat release decreases smoothly: an adiabatic propagating flame regime, a non-adiabatic propagating flame regime, the so-called partially-extinguished flame regime, in which the location of the peak of heat release crosses the stagnation plane, and a frozen flow regime. The flame structure was analyzed in detail in the different burning regimes. Abrupt extinction was attributed to the quenching of the oxidation layer with the entire H–OH–O radical pool being comparably reduced. Under conditions of smooth extinction, the behavior is different and the concentration of the H radical decreases the most with increasing strain rate, whereas OH and O remain comparatively abundant in the oxidation layer. As the profile of the heat release rate thickens, the oxidation layer is quenched and the attack of the fuel relies more heavily on the OH radicals.

Thermochemical heat release of laminar stagnation flames of fuel and oxygen

Heat transfer is a complex phenomenon that can involve conduction, convection, radiation, condensation, and boiling. In the case of heat transfer by flames produced by pure oxygen or oxygen enriched air combustion, a mechanism called thermochemical heat release (TCHR) can be held responsible for up to 60% of the total heat transfer rate. In these very hot flames chemical equilibrium is reached before full conversion into products is achieved. TCHR is the result of recombination reactions in the thermal boundary layer. In this paper a method is described for the numerical calculation of the effect of TCHR which can be applied to model TCHR for fuels of an almost arbitrarily complex composition. In this method the flame chemistry is decoupled from the chemistry in the thermal boundary layer. An equilibrium calculation is used to determine the chemical composition after the flame. This mixture is then used as input for the stagnation layer calculations, for which a simple CH 4 mechanism suffices. It is shown under which conditions this method can be applied, the effect of strain rate is studied, and the method is demonstrated by calculating a TCHR multiplication factor for a number of different fuels. A polynomial fit for the TCHR-factor is presented as function of C / H -ratio, equivalence ratio, equivalent temperature of a reference mixture and stagnation plane temperature. The fit gives accurate results for the TCHR contribution to the total heat transfer for most fuels. Finally, the importance of hydrogen recombination chemistry on the TCHR is indicated.

A study on methane–air premixed flames interacting with syngas–air premixed flames

Numerical study on the interaction between methane–air and syngas–air premixed flames is conducted according to equivalence ratio and global strain rate in detailed chemistry. This study targets at understanding of an interacting combustion system as an alternative retrofit concept where one can modify the existing facilities minimally in industrial and power plant burners in order to reduce the emission of carbon dioxide. It is seen that methane premixed flame interacting with syngas premixed flame can be sustained even over the rich flammable limit of single methane premixed flame. The inspection of detailed flame structure such as the distributions of major species and chain carrier radicals, flame separation distance, spatial flow velocity, and spatial distribution of the rate of production and consumption of CH 4, H 2, and CO is also conducted to depict the flame interactions. The importance of global strain rate and thus the flame separation distance in the enhanced burning of methane premixed flame is also stressed through the inspection of the consumption rates of H 2, CO, and CH 4. Anomalous phenomena such as the migration of premixed flame cross the stagnation plane and the nearly constant flame separation distance are also discussed.

An experimental and numerical study of the structure and stability of laminar opposed-jet flows

Experiments, simulations, and numerical bifurcation analysis are used to study the incompressible flow between two opposed tubes with disks mounted at their exits. The experiments in this axisymmetric geometry show that for low and equal Reynolds numbers, Re, at both nozzles, the flow remains symmetric about the plane halfway through the nozzle exits and the stagnation plane is located halfway between the two jets. When Re is increased past a critical value, asymmetric flow fields are obtained even when the momentum fluxes of the two opposed streams are equal. For unequal Re at the jet exits, when the fixed velocity (and the corresponding Reynolds number, Re 1 ) of one stream is low, the stagnation plane location, SPL, changes smoothly with the Re 2 . For high enough Re 1 , a hysteretic jump of SPL is observed. Particle Image Velocimetry and flow visualization demonstrate that within the hysteretic range, the two stable flow fields are anti-symmetric. The experimental setup is also studied with transient incompressible flow simulations using a spectral element solver. It is found that to accurately model the flow, we either need to extend the domain into the nozzles, or impose experimental velocity profiles at the nozzle exits. As in the experiments asymmetric flows are obtained past a critical Re. Finally, bifurcation analysis using a Newton–Picard method shows that the transition from symmetric to asymmetric flows results from the loss of stability of the symmetric flows at a pitchfork bifurcation.

Richtmyer-Meshkov Parameter Study

The growth of an interfacial perturbation after acceleration by a shock wave, known as the Richtmyer-Meshkov instability (RMI), plays an important role in the compression of an ICF target. Experiments studying the RMI are performed in a vertical shock tube by observing the growth of the interface between a pair of gases after acceleration by a planar shock wave. A near 2D, sinusoidal, membraneless interface is created in a shock tube by oscillating rectangular pistons at the stagnation plane between the two gases. The interface is visualized by seeding one of the gases with acetone, smoke, or atomized oil and observing the fluorescence or Mie scattering from a planar laser sheet. The results presented here span a range of Atwood numbers, 0.30<A<0.95, and shock wave strengths, 1.1<M<3. Numerical simulations of the experimental conditions are performed and compared with the experiments using the 2D hydrodynamics code Raptor (LLNL).

Effect of "stagnation plane" in the electric field during the electrospinning process

Recent experiments demonstrated that the essential electrospinning mechanism is a rapidly whipping dielectric liquid jet in an electric field. There are generally two main methods to control the jet, one is to control the solution and the other is to control the flight of the electrospinning jet through manipulation of the electric field. In this paper, a modified electric field distribution, providing a stagnation surface, is discussed in order to know the effect on the electrospinning jet. The results illustrate that if the bending starting point happened below the stagnation point, the jet movement could be random to any direction as no electric force on the jet at the stagnation point and if the bending starting point happened before the jet reached the stagnation point, the jet movement could be bending as the modified jet path. The results also show that the modified electric field distributions could change the jet path and control the fiber morphology.

Spatial and Temporal Characteristics of OH in Turbulent Opposed-Jet Double Flames

Simultaneous high repetition-rate, two-point hydroxyl (OH) time-series measurements with associated PLIF/PIV measurements are employed to investigate spatio–temporal scales and flame-velocity interactions in turbulent opposed jets sustaining methane-air double flames. For a fuel-side equivalence ratio, ϕB = 1.2, a rich premixed flame exists on the fuel side while a diffusion flame exists on the air side of the stagnation plane. The bulk Reynolds number (Re) and strain rate (SR) can be adjusted to generate flames at ϕB = 1.2 with both well separated and completely merged flame fronts. Simultaneous PLIF/PIV measurements highlight distinct spatial OH structures of the premixed and diffusive fronts corresponding to variations in the flow field. The self-propagating tendency of the rich premixed front causes large-scale wrinkling, thereby enhancing the OH contour length by 15% as compared to the diffusive front. Two-point OH time-series measurements are implemented to quantify both spatial and temporal fluctuations via study of radial length and time scales. In general, these integral length and time scales follow similar trends and reach a minimum at the axial location of peak [OH]. In comparison to merged double flames having higher Re and SR, greater OH fluctuations are observed in the rich-premixed front as compared to the diffusive front for a well separated double flame. Because of the developing turbulence, the OH length scales exhibit reduced axial gradients across the reaction zone for higher Re in comparison to lower Re. A stochastic time-series simulation, using a state relationship based on a joint mixture fraction and progress variable, is utilized to extract estimated scalar time scales from those of measured OH. The simulations indicate that the hydroxyl fluctuations in double flames are only twice those of the underlying conserved scalar.

Particle formation in opposed-flow diffusion flames of ethylene: An experimental and numerical study

An experimental and numerical study on particles inception and growth is performed in opposed-flow diffusion flames of ethylene and air characterized by different sooting tendencies. Spectrally resolved UV-visible laser induced fluorescence, laser induced incandescence and laser light scattering measurements are used to characterize different classes of combustion-generated compounds. A detailed kinetic model accounting for both gas-phase and particle formation is used. Comparison between experimental results and numerical predictions gives a qualitative view of the mechanism of particle formation in opposed-flow flames.Particle inception is the result of both chemical growth and coagulation of aromatic compounds. In the region close to the flame front where the temperature is relatively high and radicals are abundant, the particle inception is due to a chemical growth mechanism by which aromatic molecules add aromatic radicals leading to the formation of biphenyl-like structures. The growth process continues as high-molecular mass aromatics are moved away from the flame zone towards the stagnation plane by the addition of acetylene and other aromatics forming particles of increasing sizes. Graphitization of these particles and thermal annealing lead to the formation of soot particles. At relatively lower temperatures, found across the stagnation plane, particles growth still occurs and it is mainly due to a process of physical coagulation of PAHs.The experimental and numerical results obtained in this work demonstrate and explain the sensibility of inception and growth of particles to radical concentration and temperature in opposed-flow flame configurations.

Effect of heat release on turbulence and scalar-turbulence interaction in premixed combustion

Stereoscopic particle image velocimetry and planar laser induced fluorescence measurements of hydroxyl radical are simultaneously applied to measure, respectively, local turbulence intensities and flame front position in premixed ethylene-air flames stabilized on a bluff body. Three different equivalence ratios, 0.55, 0.63, and 0.7, and three different Reynolds numbers, 14 000, 17 000, and 21 000, are considered. Laser measurements were made for five different flame configurations within the ranges above and in the corresponding cold flows. By comparing the measurements of the cold and the corresponding hot flows, the effect of heat release on the turbulence and its interaction with the flame front is studied. All the flames are in the thin reaction zone regime. Typical flow features forming behind the bluff body are observed in the cold flows, whereas in the reacting flows the mean velocities and thus the shape, size, and characteristics of the recirculating eddy behind the bluff body are strongly influenced by the heat release. The strong acceleration across the mean flame and the radial outward shift of the stagnation plane of the recirculating eddy yield negative radial velocities which are absent in the corresponding cold flow cases. The spatial intermittency of the flame front leads to an increase in the turbulent kinetic energy. Although a decrease in the mean and rms values of the strain rate tensor eij components is observed for the reacting case as one would expect, the local flow acceleration across the flame front leads to a substantial increase in the skewness and the kurtosis of the probability density functions (PDFs) of eij components. The turbulence-scalar interaction is studied by analyzing the orientation of the flame front normal with the eigenvectors of eij. The PDFs of this orientation clearly show that the normals have an increased tendency to align with the extensive strain rate, which implies that the scalar gradients are destroyed by the turbulence as the scalar isosurfaces are pulled apart. This result questions the validity of passive scalar turbulence physics commonly used for premixed flame modeling. However, the influence of Lewis number on this alignment behavior is not clear at this time.

Hydroxyl space-time correlation measurements in partially premixed turbulent opposed-jet flames

A high repetition-rate, two-point, time-resolved, laser-induced fluorescence technique is used to perform simultaneous two-point OH time-series measurements in a series of turbulent opposed-jet partially premixed flames with varying fuel-side equivalence ratio and bulk Reynolds number. Time scales of OH in these flames have previously been reported; however, the extension to two-point detection permits measurements of new spatial and temporal statistics previously unavailable in such flames. In particular, the simultaneous OH time series are used here to compute spatial and temporal autocorrelation functions. Filtered OH length scales (lr,OH), corresponding to radial OH fluctuations in turbulent stagnation flames, are obtained from the spatial autocorrelation function, including their variation across the stagnation plane. In general, maximum OH fluctuations occur at the stagnation plane, thus minimizing the OH integral length scale at the axial location of peak OH. For all flames of this study, trends in OH length scale follow those of axial time scale (τI,OH). For flames with constant Re, lr,OH decreases with less partial premixing. However, this change in integral length scale appears to be more significant for flames at lower Re in comparison to those at higher Re. Similar to OH integral time scales, for flames with the same fuel composition, lr,OH decreases with increasing Re. Moreover, fuel-lean mixtures appear to be more sensitive to Re variations as compared to fuel-rich mixtures.

Experimental investigation of the morphology and stability of diffusion and edge flames in an opposed jet burner

The stability of methane/air and hydrogen/air flames in an axisymmetric counterflow burner was investigated experimentally for different burner geometries, degrees of fuel dilution, and combinations of flow velocities. Both planar diffusion flames and edge flames were observed, and the transitions between these flame types were studied. The experimental results confirmed previously published numerical predictions on diluted hydrogen/air flames: the existence of two distinct stable flame types; the possibility of switching between the two flame types by perturbing the flames, e.g., by suitably changing a flow velocity; and the strong hysteresis for the transition from one flame type to the other. Flame stability diagrams were compiled which delineate the range of fuel and air flow velocities for which the planar diffusion flame and the toroidal edge flame are stable. The lower boundary curve for the edge flame stability exhibits a characteristic minimum at a well-defined value of the fuel velocity. For fuel velocities lower than this value, the transition between the edge and the diffusion structure is reversible, and the flames exhibit bistable behavior. For higher fuel velocities, the decrease of air velocity leads to the extinction of the edge flame. An investigation of both the cold and the reactive flow field identified bistable behavior for the flow field as well. Except for very low flow rates, the stagnation plane stabilizes in two positions, close to either of the two nozzles. Detailed numerical simulations of hydrogen flames capture the essentials of this behavior. The observed flame extinction results from the interaction of the flame dynamics with the dynamics of the flow field.

NO formation in counterflow partially premixed flames

An experimental and computational study of NO formation in low-strain-rate partially premixed methane counterflow flames is reported. For progressive fuel-side partial premixing the peak NO concentration increased and the NO distribution along the stagnation streamline broadened. New temperature-dependent emissivity data for a SiO 2-coated Pt thermocouple was used to estimate the radiation correction for the thermocouple, thus improving the accuracy of the reported flame temperature. Flame structure computations with GRIMech 3.00 showed good agreement between measured and computed concentration distributions of NO and OH radical. With progressive partial premixing the contribution of the thermal NO pathway to NO formation increases. The emission index of NO (EINO) first increased and then decreased, reaching its peak value for the level of partial premixing that corresponds to location of the nonpremixed reaction zone at the stagnation plane. The observation of a maximum in EINO at a level of partial premixing corresponding to the nonpremixed reaction zone at the stagnation plane seems to be a consistent feature of low (<20 s −1)-strain-rate counterflow flames.

Velocity fields of fuel lean premixed turbulent opposed jet flames

Velocity measurements in the turbulent reacting flow created by an opposed nozzle configuration are reported with emphasis on the axis and stagnation plane. The instrumentation comprised particle image velocimetry (PIV) with Al 2O 3 particles and silicon oil droplets added upstream of both nozzles and with the laser sheet passing through the axis between the nozzles. Results are reported for a set of fuel lean (0.7 ⩽ ϕ ⩽ 0.9) turbulent premixed counterflow CH 4, C 2H 4 and C 3H 8 flames covering a range of Reynolds numbers (4900 ⩽ Re ⩽ 13,700). The study extends past work featuring single point measurements to include comparisons of mean and fluctuating velocity components along the stagnation point streamline and stagnation plane. The study permits a more detailed assessment of flame structures for a comparatively wide range of conditions and experimental results are further compared with computations using eddy viscosity and comprehensive second moment closures for velocity and scalar fields. The effects of closure approximations and boundary conditions were explored and it is shown that a primary sensitivity is associated with the prescribed initial length scale of turbulence. The study also shows that modifications, based on the ratio of the turbulent to mean strain timescale, do not have a significant impact in the current flow. However, sensitivities to closure approximations relating to mean and fluctuating pressure gradient terms are confirmed. Comparisons with data obtained for the corresponding isothermal flows show that the recorded levels of anisotropy are strongly affected by heat release.

An experimental and numerical investigation of n-heptane/air counterflow partially premixed flames and emission of NO x and PAH species

An experimental and numerical investigation of counterflow prevaporized partially premixed n-heptane flames is reported. The major objective is to provide well-resolved experimental data regarding the detailed structure and emission characteristics of these flames, including profiles of C 1–C 6, and aromatic species (benzene and toluene) that play an important role in soot formation. n-Heptane is considered a surrogate for liquid hydrocarbon fuels used in many propulsion and power generation systems. A counterflow geometry is employed, since it provides a nearly one-dimensional flat flame that facilitates both detailed measurements and simulations using comprehensive chemistry and transport models. The measurements are compared with predictions using a detailed n-heptane oxidation mechanism that includes the chemistry of NO x and PAH formation. The reaction mechanism was synergistically improved using pathway analysis and measured benzene profiles and then used to characterize the effects of partial premixing and strain rate on the flame structure and the production of NO x and soot precursors. Measurements and predictions exhibit excellent agreement for temperature and major species profiles (N 2, O 2, n-C 7H 16, CO 2, CO, H 2), and reasonably good agreement for intermediate (CH 4, C 2H 4, C 2H 2, C 3H x ) and higher hydrocarbon species (C 4H 8, C 4H 6, C 4H 4, C 4H 2, C 5H 10, C 6H 12) and aromatic species (toluene and benzene). Both the measurements and predictions also indicate the existence of two partially premixed regimes; a double flame regime for ϕ < 5.0 , characterized by spatially separated rich premixed and nonpremixed reaction zones, and a merged flame regime for ϕ > 5.0 . The NO x and soot precursor emissions exhibit strong dependence on partial premixing and strain rate in the first regime and relatively weak dependence in the second regime. At higher levels of partial premixing, NO x emission is increased due to increased residence time and higher peak temperature. In contrast, the emissions of acetylene and PAH species are reduced by partial premixing because their peak locations move away from the stagnation plane, resulting in lower residence time, and the increased amount of oxygen in the system drives the reactions to the oxidation pathways. The effects of partial premixing and strain rate on the production of PAH species become progressively stronger as the number of aromatic rings increases.