Quantitative technique for imaging mixture fraction, temperature, and the hydroxyl radical in turbulent diffusion flames

A technique for obtaining simultaneous quantitative images of the hydroxyl radical, OH, temperature, mixture fraction, and scalar dissipation rates in turbulent diffusion flames is described. Mixture fraction is obtained from images of Rayleigh and fuel Raman scattering. We quantified the OH laser-induced fluorescence (LIF) images using detailed calibration and a correction for quenching and population distribution effects based on the simultaneous mixture fraction and temperature images. This correction was derived from calculations of laminar counterflow diffusion flames for identical fuel mixtures. These laminar flame computations are further used to estimate the errors in the measured OH concentrations. The technique is applied to piloted, nonpremixed flames over a range of jet velocities. The measured mixture fraction, temperature, and OH concentrations are in good agreement with those obtained earlier in similar flames using the single-point Raman/Rayleigh/LIF technique.     

Three-dimensional temperature measurement of combustion flames using a single monochromatic CCD camera

This paper presents an imaging-based instrumentation system for three-dimensional (3-D) temperature measurement of a combustion flame. A combination of image-processing techniques and two-color radiation thermometry is used to first reconstruct band-limited grayscale representations of the flame and then to determine its temperature distribution. The reconstruction process assumes rotational symmetry in the structure of the flame. A series of experiments has been conducted on a laboratory-scale combustion rig to evaluate the performance of the system. The results obtained demonstrate the capability of the system to determine flame temperature on a 3-D basis.     

Quantitative hydroxyl concentration time-series measurements in turbulent nonpremixed flames

Quantitative hydroxyl concentration time-series measurements have been obtained by picosecond time-resolved laser-induced fluorescence in a series of methane-air and hydrogen-argon-air nonpremixed flames. The recovery of a quantitative time series is complicated by the need to account for fluctuations in the fluorescence lifetime. We have recently developed instrumentation that enables the simultaneous measurement of fluorescence signal and lifetime. The present research represents the first application of this technique to turbulent flames. The correction for hydroxyl lifetime fluctuations is shown to be significant for mean concentrations and thus probability density functions but negligible for power spectral densities (PSD's). The hydroxyl PSD's were found to vary slightly with radial and axial location in the flames and to vary significantly with Reynolds number. However, the PSD's in the H(2)-Ar-air flames are nearly identical to those in the CH(4)-air flames.     

Experimental data base for numerical simulations of turbulent diffusion flames

The objective of the presented measurements is to provide an experimental data base for comparison with numerical simulation results of turbulent H₂-air diffusion flames. Additionally, the date base may also be used for a proof of new measurement techniques, when the same flame conditions are applied. The data base contains time and spatial resolved data on all three velocity components, all Reynolds-stress tensor components, temperature, mixture fraction, species concentrations, higher statistical moments of these quantities and probability density functions for three different flames. The data are given as original measurement data in dependence on flame conditions and location in the flame, as absolute and normalized data and as evaluated data, like anisotropy tensor. The measurements are made to improve the understanding of turbulent transport processes under the influence of combustion and to help the effort to couple the turbulence and combustion model. A Laser-Doppler-Velocimeter was used to obtain three velocity components simultaneously. Temperature was measured with spontaneous Raman-Rayleigh spectroscopy and Coherent Anti-Stokes Raman spectroscopy, separately, while species concentrations and mixture fraction are measured with spontaneous Raman-Rayleigh spectroscopy. Measurements are done from nozzle exit into the self-preserving region up to x/d=100 so that the whole flow field including all boundary conditions are quantified for numerical prediction. A mixture of hydrogen and nitrogen with a mole ratio of 1:1 is used as fuel. Reynolds number and Froude number are set at different values. This complete data set is available upon request.     

Two-dimensional imaging of soot volume fraction in laminar diffusion flames

A technique for acquiring two-dimensional soot-volume-fraction measurements in laminar flames has been demonstrated. The technique provides a map of very low noise concentration over a range of wavelengths (250-1100 nm). A noise level of 0.0007 in extinction and a spatial resolution of 30-40 mum for soot concentration were achieved with an arc lamp source that was filtered to provide greater spatial coherence and a CCD detector. The broadband arc lamp source also allowed us to avoid the added noise resulting from speckle with coherent laser sources. Beam steering, due to refractive-index gradients in the flame, was measured and compared with theoretical predictions. The optical arrangement to minimize the effect of beam steering is described. As a result the beam steering had no effect on the soot measurements in the flames examined. Flame-transmission maps obtained with this system in an ethylene/air laminar diffusion flame are presented. Tomographic analysis from use of an Abel inversion of the line-of-sight data to obtain radial profiles of soot concentration is described.     

A three-component LDV system for measurements of higher statistical moments in turbulent diffusion flames

A three-component LDV system was built with the following features: 1. Three colors, 2. Forward scattering, 3. Direction determination, 4. Avalanche photodiodes, 5. Transient recorder, 6. Fast Fourier transform. With this equipment measurements in a free air jet and a turbulent methane nitrogen diffusion flame were made. The following quantities were determined on the axis and on some levels: velocities u_i, spreading rate r_(0.5·uc)/d, centerline velocity-decay u₀/u_c, Reynolds-stress tensor u′_iu′_j and all third order moments u′_iu′_ju′_k. These quantities were normalized and compared with literature data and with Reynolds-stress model calculations. One of the main objectives of these measurements was the supply of exit velocity profiles because the predictions below the similarity region depend crucially on the boundary conditions. In this paper details of the equipment and of the data processing are explained and new results are shown.     

Quantitative temperature measurements in high-pressure flames with multiline NO-LIF thermometry

An accurate temperature measurement technique for steady, high-pressure flames is investigated using excitation wavelength-scanned laser-induced fluorescence (LIF) within the nitric oxide (NO) A-X(0, 0) band, and demonstration experiments are performed in premixed methane/air flames at pressures between 1 and 60 bars with a fuel/air ratio of 0.9. Excitation spectra are simulated with a computational spectral simulation program (LIFSim) and fit to the experimental data to extract gas temperature. The LIF scan range was chosen to provide sensitivity over a wide temperature range and to minimize LIF interference from oxygen. The fitting method is robust against elastic scattering and broadband LIF interference from other species, and yields absolute, calibration-free temperature measurements. Because of loss of structure in the excitation spectra at high pressures, background signal intensities were determined using a NO addition method that simultaneously yields nascent NO concentrations in the postflame gases. In addition, fluorescence emission spectra were also analyzed to quantify the contribution of background signal and to investigate interference in the detection band-width. The NO-LIF temperatures are in good agreement with intrusive single-color pyrometry. The proposed thermometry method could provide a useful tool for studing high-pressure flame chemistry as well as provide a standard to evaluate and validate fast-imaging thermometry techniques for practical diagnostics of high-pressure combustion systems.     

Quantitative rainbow schlieren deflectometry as a temperature diagnostic for nonsooting spherical flames

Numerical analysis and experimental results are presented to define a method for quantitatively measuring the temperature distribution of a spherical diffusion flame using rainbow schlieren deflectometry in microgravity. The method employed illustrates the necessary steps for the preliminary design of a rainbow schlieren system. The largest deflection for the normal gravity flame considered in this paper is 7.4 x 10(-4) rad, which can be accurately measured with 2 m focal-length collimating and decollimating optics. The experimental uncertainty of deflection is less than 5 x 10-(5) rad.     

Use of Rayleigh imaging and ray tracing to correct for beam-steering effects in turbulent flames

Laser Rayleigh imaging has been applied in a number of flow and flame studies to measure concentration or temperature distributions. Rayleigh cross sections are dependent on the index of refraction of the scattering medium. The same index of refraction changes that provide contrast in Rayleigh images can also deflect the illuminating laser sheet. By applying a ray-tracing algorithm to the detected image, it is possible to correct for some of these beam-steering effects and thereby improve the accuracy of the measured field. Additionally, the quantification of the degree of beam steering through the flow provides information on the degradation of spatial resolution in the measurement. Application of the technique in a well-studied laboratory flame is presented, along with analysis of the effects of image noise and spatial resolution on the effectiveness of the algorithm.     

Heat source density and temperature distribution in a laser ruby crystal

The heat source density in a laser ruby is calculated by numerical integration of the pumping and absorption spectra. The heat source density can be represented approximately by the formula q(r) = =q₁ + q₂ I₀ (r/r₀), which can be used to determine the temperature distribution in the crystal for typical pumping and cooling conditions.     

Calculation of the diffusion coefficient in a heated turbulent medium using optimisation techniques

With a view to measuring the temperature fluctuations in a heated turbulent air stream, the latter is traversed by a laser beam perpendicular to the flow. The ultimate aim is to access the temperature fluctuations along the laser’s path starting from the luminous dot projected by the laser on to a position probe placed at the stream output. Our initial task is to demonstrate that by solving an optimal control problem, it is possible to calculate the variable diffusion coefficient along the beam trajectory. The optimal control problem considered here is formulated in terms of a nonlinear system whose dynamics are described by a mathematical model developed with the Einstein-Fokker-Plank-Kolmogorov equation as starting-point, and where the control action is assumed given by the diffusion coefficient. A fresh approach to solving the problem based on the decomposition-co-ordination principle is introduced in order to solve the corresponding optimal control problem, i.e. to calculate the diffusion coefficient which meets required objectives.     

Stresses exerted by a source of diffusion in a case of a non-parabolic diffusion equation

The theory of diffusive stresses based on the diffusion-wave equation with time-fractional derivative of fractional order α is formulated. The non-parabolic diffusion equation is a mathematical model of a wide range of important physical phenomena and can be obtained as a consequence of the non-local constitutive equation for the matter flux vector with the long-tale power time-non-local kernel. Because the considered equation in the case 1 ⩽ α ⩽ 2 interpolates the parabolic equation (α = 1) and the wave equation (α = 2), the proposed theory interpolates a classical theory of diffusive stresses and that without energy dissipation introduced by Green and Naghdi. The stresses caused by a source of diffusion in an unbounded solid are found in one-dimensional and axially symmetric cases (for plane deformation). Numerical results for the concentration and stress distributions are given and illustrated graphically.     

Numerical simulation of turbulent gas flames in tubes

Computational fluid dynamics (CFD) is an emerging technique to predict possible consequences of gas explosion and it is often considered a powerful and accurate tool to obtain detailed results. However, systematic analyses of the reliability of this approach to real-scale industrial configurations are still needed. Furthermore, few experimental data are available for comparison and validation.In this work, a set of well documented experimental data related to the flame acceleration obtained within obstacle-filled tubes filled with flammable gas-air mixtures, has been simulated. In these experiments, terminal steady flame speeds corresponding to different propagation regimes were observed, thus, allowing a clear and prompt characterisation of the numerical results with respect to numerical parameters, as grid definition, geometrical parameters, as blockage ratio and to mixture parameters, as mixture reactivity.The CFD code AutoReagas was used for the simulations. Numerical predictions were compared with available experimental data and some insights into the code accuracy were determined. Computational results are satisfactory for the relatively slower turbulent deflagration regimes and became fair when choking regime is observed, whereas transition to quasi-detonation or Chapman-Jogouet (CJ) were never predicted.     

Electrochemical imaging of diffusion through single nanoscale pores

A combined scanning electrochemical-atomic force microscope (SECM-AFM) has been used to probe the diffusional transport of target electroactive solutes in isolated nanopores of a track-etched membrane. A polycarbonate membrane (100-nm-diam pore size) hydrated with an electrolyte solution, containing a redox-active probe molecule, such as IrCl6(3-) or Fe(phen)3(2+), functions as the model membrane system. The use of a mobile Pt-coated AFM probe enables individual solution-filled pores to be topographically identified. Analysis of the corresponding current images for the diffusion-limited oxidation of the redox mediator indicates that solution is largely confined to pores in the membrane. Moreover, the tip collector current response provides information on diffusion of the mediator through the pore. Force-distance tip approach and retract measurements allow the radius of contact between the electrochemical-AFM tip and solution confined within a pore at the point of pull-off to be estimated.     

Cryogenic refrigeration using a low temperature heat source

A non mechanical gas sorption refrigeration system which could be powered by a low temperature heat source was designed and analysed. The system involved no sealing, no mechanical moving parts and no active control. The system was analysed using a 1 W cooling load at 74 K as an example. It was found that the new system was competitive with a passive radiator and superior to a solid cryogen system.     

Systematic errors in optical-flow velocimetry for turbulent flows and flames

Optical-flow (OF) velocimetry is based on extracting velocity information from two-dimensional scalar images and represents an unseeded alternative to particle-image velocimetry in turbulent flows. The performance of the technique is examined by direct comparison with simultaneous particle-image velocimetry in both an isothermal turbulent flow and a turbulent flame by use of acetone-OH laser-induced fluorescence. Two representative region-based correlation OF algorithms are applied to assess the general accuracy of the technique. Systematic discrepancies between particle-imaging velocimetry and OF velocimetry are identified with increasing distance from the center line, indicating potential limitations of the current OF techniques. Directional errors are present at all radial positions, with differences in excess of 10 degrees being typical. An experimental measurement setup is described that allows the simultaneous measurement of Mie scattering from seed particles and laser-induced fluorescence on the same CCD camera at two distinct times for validation studies.     

Velocity and temperature fluctuations in a turbulent suspension

The effect of the particles of a suspension on the spectrum of velocity and temperature fluctuations is studied on the basis of the equations for the second two-point moments.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 55, No. 1, pp. 26–33, July, 1988.