Flame Front Structure Research Articles

Combustion synthesis of ZnS in microgravity

The self-heating nature of SHS (Self-propagating High-temperature Synthesis) makes it particularly suitable for microgravity processing of materials where weight and power requirements are severely restricted. The absence of convection, hydrostatic pressure, and phase separation permits the combustion front dynamics and solidification processes of SHS to be studied under controlled conditions. This paper describes recent ground-based and microgravity (NASA KC-135 parabolic flight) experiments on SHS processing of ZnS. A novel technique was used for preparing the precursor mixture of Zn+S by mixing the zinc with molten sulfur, which allows the synthesis of a high-density and high-purity product. The flame speed, quenching diameter, and temperature profile in the flame front and crystal structure of the synthesized samples have been determined. Thermocouple measurement of the temperature profile in the flame front indicates that the thermal thickness of the flame is less than 0.3 mm. The average flame speed is of the order of 7 mm/s, and slightly lower values (≈4 mm/s) are observed near the quenching limit. It was found that the flame speed is not stable along the samples with diameters more than 12 mm. The quenching diameter is found to be of the order of 5 mm (in microgravity less than 4 mm). X-ray diffraction data show a wurtzite structure both in ground-based and in-flight synthesized samples, and the lattice's parameters are most similar to the ideal ZnS wurtzite structure in the outer part of samples synthesized in microgravity. The ability to provide containerless SHS processing of molten ZnS in microgravity also has been demonstrated.

Simultaneous 2D single-shot imaging of oh concentrations and temperature fields in an si engine simulator

Two-dimensional (2D) temperature measurements in engines are required for the detailed understanding of the combustion process. Because of the unsteady and turbulent nature of engine combustion, these measurements must be performed on a single-shot basis to obtain cycle-resolved results. Details of the flame front structure are also important to know for the numerical modeling of internal combustion processes. Simultaneous measurements of temperature and hydroxyl radical distributions provide such data sets. The combustion of methane in a fully transparent square piston engine was studied with a combination of 2D laser-induced fluorescence of hydroxyl radicals and 2D Rayleigh scattering. A tunable KrF excimer laser at 248 nm was used for the measurementsThe wavelength of the laser was tuned to excite the P 2 (8) transition in the OH (3,0) band of the A-X system. Spectral filtering of the resulting fluorescence to detect solely the fluorescence from the (3,2) band significantly reduces the effect of collisional quenching imposed to the fluorescence to the predissociating v ′=3 level by vibrational energy transfer (VET) to lower vibrational levels. Using a second camera, which only records the Rayleigh signals after appropriate filtering, allows the simultaneous measurement of temperature fields with a single laser. The analysis of corresponding OH and temperature images allowed the decision that the combustion conditions are in a regime where the flamelet approach for modeling is appropriate. Good agreement with model predictions has been achieved for the peak temperatures.

Geometrical Properties of Turbulent Premixed Flames: Comparison Between Computed and Measured Quantities

Abstract The constant density scalar field equation for a converted unsteady interface is solved numerically under prescribed monochromatic pulsating two-dimensional flow fields, with spatial frequencies (κ), temporal frequencies (ω) and rms amplitudes (w) simulating a Kolmogorov spectrum with parameter β = ω/kw ranging between 0.1 and 100. The effect of multiple velocity scales on the interface, which is considered to be a flame front, is accounted for by applying the renormalization theory on the monochromatic flame responses. Numerical calculations are performed for values of uv′/uL (turbulent intensity to laminar flame speed) up to fifteen. The results are then compared with turbulent flame front structure measurements obtained in an Internal Combustion Engine. The contribution of small scales to the flame surface area increase is shown to be smaller in cases with larger values of uv′/uL. The flame front does not obey a simple fractal-like scaling behavior between the extreme scales of the Kolmogorov ...

Investigation of flame-generated turbulence in premixed flames at low and high burning velocities

Knowledge of the dependence of turbulent flame velocity on such parameters as fuel properties, turbulent fluctuations, and length scales is of central importance for industrial applications such as combustion in motors, gas turbine combustors, and domestic burners. Two unanswered questions concern how aerodynamic turbulence influence the reaction and/or flame speed and how the reaction intensity influences the aerodynamic turbulence values. The reaction velocity has been measured in a stagnation point flame, the turbulence intensities and length scales before and after the flame front being recorded by an LDV system. Laminar burning velocity was varied by using different concentrations of methane-hydrogen-air mixtures. The results show under certain conditions an immense increase in turbulent fluctuations in and after the flame front, measured using conditioned and unconditioned LDV techniques. These measurements indicate that turbulence is produced in the flame front, the extent of turbulence depending on the incoming laminar burning velocity, which was varied over a comparatively wide range. Some results are illustrated by tomographic pictures, which clearly show the increase of turbulence and turbulent flame front structure in dependence on laminar burning velocity.

Numerical Simulation of Turbulent Flame Structure with Non-unity Lewis Number

Abstract A numerical simulation of the response of a premixed flame to various randomly defined two-dimensional flow fields is performed in order to obtain an insight into Lewis number effects in turbulent premixed combustion. A one-step reaction with a large, but finite, activation energy in the limit of zero heat release is assumed.The numerical simulation uses vortex dynamics to describe the velocity field, while the scalar field is computed on a mesh.The flow field integral length scale is typically ten times the flame thickness, and the r.m.s. velocity fluctuation is of the order of the flame velocity. Ensemble averages are taken over a total number of 30 realization for Le of 1/2 and 37 realizations for Le of 2, where the Lewis number is Le = λ/(ρcpD). Since the density is kept constant, there is no feedback from the reacting scalar field to the velocity field. For the Le of 1/2 case, there is a clear manifestation of the thermo-diffusive instability mechanism leading to a cellular flame front structure, while for Le of 2, the thermo-diffusive effect reduces the hydrodynamic disturbances of the front.The scalar variable that best characterizes the flame dynamics is the excess enthalpy, which we define with respect to an undisturbed planar flame. The numerical results are analyzed statistically. The probability density functions of the progress variable and the excess enthalpy are interpreted on the basis of the observed flame response. The dissipation terms and the reaction rate in the onedimensional balance equations for the mean scalars and their variances are also given. Flame stretch enhances differential diffusion as shown by a significant correlation between strain-rate and excess enthalpy.

Visualization of Turbulent Flame Fronts with Planar Laser-Induced Fluorescence

This report concerns the quantitative time-resolved visualization of reaction zones in laminar, transitional, and turbulent nonpremixed flames. Two-dimensional OH molecular concentrations were measured with planar laser-induced fluorescence excited by a sheet of light (formed from a single tunable ultraviolet laser pulse) and detected with a two-dimensional, image-intensified photodiode array camera. From the resulting data details of instantaneous flame front structures (including positions, shapes, and widths) were obtained.

Low-temperature zone in a hydrocarbon-flame front VII. Structure of the flame front in hexane with aerosol fuel injection

The flame front structure when hexane vapor is introduced into an O/sub 2/+ Ar gas mixture, and when hexane is introduced as an aerosol into the same gas mixture, is studied. The schemes of the equipment used to produce the mixtures specify thermostatic device, overflow device, burner, and sprayer. The flow system is explained. The flame temperature is measured with platinum thermocouples. Concentration profiles for the flames are derived. Differences in the temperature profiles in the initial part of the flames is explained. Differences in initial temperature do not influence the width of the low-temperature reaction zone.

The effect of the flame front structure on flame propagation in premixed gases

It is well known that the surface of a premixed turbulent flame changes its appearance very quickly and is made of a series of numerous different curvatures. For this reason a so-called “turbulent burning velocity” has been defined as “the ratiobetween the mean volumetric flow rate of the cold mixture and a mean surface around which the burning surface fluctuates”. Final, the turbulent burning velocity depends on the laminar burning velocity as well as on, the way in which the wave is folded. Therefore, the turbulent burning velocity does not indicate anything about the laminar burning velocity in a turbulent flame. In this work experimental data of a pneumatically excited, non-steady laminar flame frontof a methane-air mixture showed that an increasing disturbance of the wave induces an increasing surface, combined with a decreasing laminar burning velocity. It has also been found out that flame stretching, as first described by Karlovitz [2] has been the most important factor of influence on the laminar burning velocity apart from temperature and diffusion effects. Under these aspects further investigations were made in curved stationary premixed flames. They have delivered data on the interaction between flame stretching and laminar burning velocities.

Velocity-temperature correlation in premixed flame

The velocity and the temperature were measured simultaneously with a laser Doppler anemometer and a compensated thermocouple, respectively, in the turbulent premixed flame, and their cross-correlation was calculated with a microcomputer. A seed deposition effect of the thermocouple on the cross-correlation coefficient was avoided by compensating for the temperature signal with a somewhat larger time constant than the theoretical value. The effect on the cross-correlation of separating of the measuring control volume of LDA and the thermocouple placed on the downstream side was also investigated. Though a small separation of the thermocouple downstream from the measuring control volume had little effect, the shift of radial direction had a severe effect. The axial velocity-temperature correlation gave large negative values in the whole region where the gradient of mean temperaturealong the axial direction δ/δ x was positive. This fact corresponds to the normal turbulent heat transfer and can be explained by a mechanism whereby temperature fluctuation is induced by velocity fluctuation. However, the radial velocity-temperature correlation showed positive values in the vicinity of the flame front where the radial temperature gradient η/δ r was still positive. This corresponds to an abnormal turbulent heat transport phenomenon. To clarify this phenomenon, the probability density functions and the macroscale of the velocity and the temperature fluctuations were also measured. The bimodal nature of the PDF appeared at the flame front in all fluctuating components, which suggests that the flame front structure of the flame studied was approximated by the wrinkled flame model. Abnormal turbulent heat transfer can be explained by the wrinkled flame front structure or the fluctuation of the heat generated at the measuring point, and cannot be neglected in the modeling of the combustion.

Experimental Investigation of the Gas Phase in a Chemical Vapor Deposition Reaction: Application to the Silane‐Ammonia Reaction Leading to Silicon Nitride Deposits

The investigation of the gas phase composition of a CVD system was carried out applying a method derived from flame front structure research and implying a point‐to‐point analysis of concentrations and temperature profiles. A finely drawn nozzle‐shaped probe is used to draw stable species samples out of the gas phase. They are subsequently compressed into a VPC analyzer. A fine silica‐coated thermocouple is used to draw the temperature profiles. The method was applied to the co‐pyrolysis of silane and ammonia forming silicon nitride on a vitreous sintered silica substrate heated at 950°, 1000°, and 1050°C at different values of the ratio of silane to ammonia. The results obtained localize the space, in the gas phase, and relatively to the substrate, where the solid phase starts condensing as the initial reactants are consumed. The method is reproducible and reliable. If properly developed it should lead to interesting applications in the interpretation of CVD aerothermochemistry and chemical kinetics.

Propagation rate and limits of existence of a turbulent flame

In a theoretical study of turbulent burning it is usually assumed that the average rate of the chemical reaction (heat release) is determined only by the average temperature. Ya. B. Zel'dovich [1] and later T. Karman [2] noted the necessity of taking into account the effect of temperature pulsations on the reaction rate. A quantitative estimate of this effect on the reaction rate constant is given in [3]. A critical analysis of various approaches to the theoretical study of turbulent flames is given in the reviews [4, 5]. In the present article, it is shown that, taking the pulsation component of the temperature and concentration into account, the average rate of the chemical reaction depends on the gradient of the mean temperature and the scale of the turbulent pulsations. The case, where a first-order reaction takes place in the flame is studied in detail. Existence and uniqueness theorems which determine the limits of the propagation of flames are proven. Quantitative rules for the propagation rate, limit, and structure of a turbulent flame front are analyzed with respect to the results of a numerical calculation of a series of variants. Dimensional interpolation equations are presented for the total propagation rate of a flame.

Flame front structures of lean diborane-air and diborane-hydrocarbon-air mixtures

Summary Three types of flame structures have been identified in the diborane-air-hydrocarbon system (Figure 8): (1) Lean diborane-air flames have one luminous reaction zone in which all of the boron is oxidized to boric oxide while the hydrogen is only partially converted to water. The hydrogen oxidation is completed in a diffuse zone downstream from the luminous reaction zone. (2) Addition of hydrocarbons (CH4, C3H8) to lean diborane-air flames leads to considerable cracking and carbon monoxide formation in the luminous diborane oxidation reaction zone. Further cracking and CO formation takes place in a diffuse zone downstream from this reaction zone. (3) For hotter diborane-air mixtures or if an igniting spark is used, a second reaction zone is formed which can couple with the diborane oxidation zone if the flow conditions and hydrocarbon concentrations are suitable. All of the hydrocarbon oxidation reactions are complete downstream from the second reaction zone. The second reaction zone may be lifted away from the diborane zone if the diborane or hydrocarbon concentrations are decreased appropriately.

Interpretation of the sample taken by a probe in a laminar concentration gradient

Current results in the study of laminar flame front structure are being obtained by gas sampling with fine quartz probes. Exclusive of questions of quenching, catalysis, aerodynamic disturbance etc., this technique has raised problems of the significance to be attached to samples withdrawn from regions of large concentration gradients. In the present paper a theoretical study is described in which the situation is approximated by a point sink located in a two-dimensional flow field containing a concentration gradient of a binary mixture. The appropriate diffusion equation is solved by the relaxation technique. The results indicate that the sample entering the sink has a composition which differs only very slightly from that existing at the same point in the unperturbed (no sink) case. A simple experiment is also reported which provides some support for this result, as well as proving conclusively that a probe does not withdraw a sample measuring the total flux (due to diffusion and convection) of each species through a cross section.