Time-average measurement of velocity, density, temperature, and turbulence velocity fluctuations using Rayleigh and Mie scattering

Measurement of time-averaged velocity, density, temperature, and turbulence velocity fluctuations in sparsely seeded gas flows using a non-intrusive, point-wise technique based on Rayleigh and Mie scattering is discussed. A Fabry-Perot interferometer (FPI) is used to spectrally resolve laser light scattered by molecules and particulates in gas flows. The spectral content of the scattered light provides information about velocity, density, and temperature of the gas. A CCD camera is used to record images of the fringes formed by scattered light passing through the interferometer. Models of the spectral components are used in a least squares fitting routine to estimate the parameters from fringe images. Flow measurements are presented for subsonic and supersonic jet flows. The application range for this technique is mostly for high velocity situations (>25 m/s). Velocity, density, temperature, and turbulence velocity fluctuations were determined with accuracies within 5 m/s, 4%, 2%, and 5 m/s, respectively.     

In-cylinder fuel distribution in a port-injected model engine using Rayleigh scattering

The spatial and temporal variation of the fuel concentration (air/fuel ratio) in a model engine was quantified by laser Rayleigh scattering. Fuel was simulated by gaseous Freon-12 injected at various timings and quantities into the intake port. The results showed that the fuel concentration in the engine cylinder was strongly dependent on the injection timing and duration and that the Rayleigh system was able to identify spatial variations of the order of one air/fuel ratio at realistic mixture strengths.     

Ballistic imaging of the near field in a diesel spray

We have developed an optical technique called ballistic imaging to view breakup of the near-field of an atomizing spray. In this paper, we describe the successful use of a time-gated ballistic imaging instrument to obtain single-shot images of core region breakup in a transient, single hole atomizing diesel fuel spray issuing into one atmosphere. We present a sequence of images taken at the nozzle for various times after start of injection, and a sequence taken at various positions downstream of the nozzle exit at a fixed time. These images contain signatures of periodic behavior, voids, and entrainment processes.     

Direct measurement of mixture fraction in reacting flow using Rayleigh scattering

The mixture fraction variable, , is very useful in describing reaction zone structure in nonpremixed flames. Extinction limits and turbulent mixing are often described as a function of this variable. Experimental evaluation of is critical for improving our understanding of the influence of turbulent mixing on the chemistry process. Heretofore, the evaluation of mixture fraction in combusting flow required multiple simultaneous concentration measurements. In this paper we present a fuel designed to permit measurements of mixture fraction by Rayleigh scattering technique only. A Rayleigh intensity/mixture fraction correspondence has been obtained experimentally in a laminar coflow flame. The influence of strain rate and differential diffusion effects have been investigated using laminar counterflow diffusion flame and shifting equilibrium chemistry models. The results obtained from comparisons between experiments and these models are very encouraging and suggest that the Rayleigh/mixture fraction correspondence established is valid under both the turbulent mixing and laminar strained flamelet combustion regimes.     

Quantitative visualization of compressible turbulent shear flows using condensate-enhanced Rayleigh scattering

This paper describes several flow visualization experiments carried out in Mach 3 and Mach 8 turbulent shear flows. The experimental technique was based on laser scattering from particles of H₂O or CO₂ condensate that form in the wind tunnel nozzle expansion process. The condensate particles vaporize extremely rapidly on entering the relatively hot fluid within a turbulent structure, so that a sharp vaporization interface marks the outer edge of the rotational shear layer fluid. Calculations indicate that the observed thin interface corresponds to a particle size of 10 nm or less, which is consistent with optical measurements, and that particles of this size track the fluid motions well. Further, calculations and experiments show that the freestream concentration of condensate required for flow visualization has only a small effect on the wind tunnel pressure distribution. Statistics based on the image data were compared to corresponding results from probe measurements and agreement was obtained in statistical measures of speed, scale, and orientation of the large-scale structures in the shear layer turbulence. The condensate-enhanced Rayleigh scattering technique is judged to be a useful tool for quantitative studies of shear layer structure, particularly for identifying the instantaneous boundary layer edge and for extracting comparative information on the large-scale structures represented there.     

Development of a full-field planar Mie scattering technique for evaluating swirling mixers

A full-field planar optical diagnostic technique for studying mixing in swirling flows is described. Results were obtained using this technique to provide planar mixing information by seeding a simulated fuel stream with aluminum oxide particles, then inferring concentration from Mie scattering intensity distributions. This facility and measurement technique are unique for several reasons. First, they allow spatial variations in laser sheet energy to be corrected for on a shot-to-shot basis. Second, they allow experiments to be performed for swirlers with practical fuel and oxidizer flow rates, i.e. on the order of 150 g/s (0.33 lbm/s). Finally, they allow full size swirler models to be evaluated, with the entire exit plane imaged simultaneously. Representative results are presented as false color images of the planar mixing fields. These images allow rapid assessment of the mixing process and its changes with variations in operating conditions or swirler geometry.List of Symbols C seed particle concentration, m^–3 - mean component of seed particle concentration, m^–3 - C fluctuating component of seed particle concentration, m^–3 - C ^* time averaged ratio of rms particle concentration fluctuations to average particle concentration, dimensionless - d _p particle diameter, m - I laser energy after passing through the flow, J/m² - mean laser energy, J/m² - I ₀ laser energy before passing through the flow, J/m² - L _v eddy length scale, m - l laser beam path length, m - U _v eddy velocity scale, m/s - V diode voltage reading after passing through the flow, V - mean diode voltage, V - V ₀ diode voltage reading before passing through the flow, V - absorptivity, m² - _rel relative equivalence ratio, dimensionless - fluid viscosity, Ns - _p particle density, kg/m³ - Stokes number= _p / _f , dimensionless - _f flow time scale, s - _p particle response time, s     

Effect of ambient pressure on penetration of a diesel spray

In the present experimental and theoretical work the propagation of a high-speed fuel spray at distances much longer than the breakup length is studied. The motion of the spray is modeled in two regions: the main region of the steady flow and the front region of the spray. The analysis yields the equation of propagation of the tip of the spray. These theoretical results have been validated against experimental data obtained from a common-rail diesel injection nozzle and from other data available in the literature. The importance of the shock wave propagation at the initial stage of the spray injection is demonstrated.     

Heat transfer between a heated plate and an impinging transient diesel spray

An experimental investigation was performed to determine the heat-transfer distribution in the vicinity of a transient diesel spray impinging on a heated flat plate. The spray prior to impingement was characterised in terms of simultaneous droplet sizes and velocities by phase-Doppler anemometry while during its impingement on the plate, which was heated at temperatures between 150–205°C, the instantaneous surface temperature and associated rates of wall heat transfer were monitored by fast response thermocouples. The parameters examined in this work included the distance between the nozzle and the wall surface, the radial distance from the impingement point, the injection frequency, the injected volume and the pre-impingement wall temperature. The results showed that the wall heat transfer rates are dependent on the spray characteristics prior to impingement; the higher the velocity of arrival of the droplet is, the higher the heat transfer. A correlation was thus developed for the instantaneous and spatially-resolved spray/wall heat transfer based on experimentally-determined Nusselt, Reynolds, Prandtl and Weber numbers over a wide range of test conditions.     

Dynamic measurement of temperature, velocity, and density in hot jets using Rayleigh scattering

A molecular Rayleigh scattering technique is utilized to measure gas temperature, velocity, and density in unseeded gas flows at sampling rates up to 10 kHz, providing fluctuation information up to 5 kHz based on the Nyquist theorem. A high-power continuous-wave laser beam is focused at a point in an air flow field and Rayleigh scattered light is collected and fiber-optically transmitted to a Fabry–Perot interferometer for spectral analysis. Photomultiplier tubes operated in the photon counting mode allow high-frequency sampling of the total signal level and the circular interference pattern to provide dynamic density, temperature, and velocity measurements. Mean and root mean square velocity, temperature, and density, as well as power spectral density calculations, are presented for measurements in a hydrogen-combustor heated jet facility with a 50.8-mm diameter nozzle at NASA John H. Glenn Research Center at Lewis Field. The Rayleigh measurements are compared with particle image velocimetry data and computational fluid dynamics predictions. This technique is aimed at aeronautics research related to identifying noise sources in free jets, as well as applications in supersonic and hypersonic flows where measurement of flow properties, including mass flux, is required in the presence of shocks and ionization occurrence.     

Detailed measurements of equivalence ratio modulations in premixed flames using laser Rayleigh scattering and absorption spectroscopy

Equivalence ratio non-uniformities may give rise to some of the instabilities observed in modern lean premixed combustion systems. The present work intends to investigate the influence of equivalence ratio perturbations on the dynamics of premixed flames. A burner equipped with a secondary injection system is used to generate equivalence ratio perturbations which are convected by the flow and impinge on a conical flame. Two laser-diagnostics, based on Rayleigh scattering and hydrocarbon infrared absorption, respectively, are employed to give insight into the spatial and temporal evolution of the mixture composition field. Rayleigh scattering images also reveal the flame front dynamics providing an indication on the response of a weakly turbulent flame subject to mixture composition inhomogeneities. Laser light absorption provides a time resolved signal which is used to estimate the equivalence ratio perturbation level. A theoretical model based on the G-equation is used to interpret the experimental data and compare the relative effects of velocity and equivalence ratio perturbations.     

Simultaneous planar measurement of droplet velocity and size with gas phase velocities in a spray by combined ILIDS and PIV techniques

A new approach for simultaneous planar measurement of droplet velocity and size with gas phase velocities is reported, which combines the out-of-focus imaging technique ‘Interferometric Laser Imaging Droplet Sizing’ (ILIDS) for planar simultaneous droplet size and velocity measurements with the in-focus technique ‘Particle Image Velocimetry’ (PIV) for gas velocity measurements in the vicinity of individual droplets. Discrimination between the gas phase seeding and the droplets is achieved in the PIV images by removing the glare points of focused droplet images, using the droplet position obtained through ILIDS processing. Combination of the two optical arrangements can result in a discrepancy in the location of the centre of a droplet, when imaging through ILIDS and PIV techniques, of up to about 1 mm, which may lead to erroneous identification of the glare points from droplets on the PIV images. The magnitude of the discrepancy is a function of position of the droplet’s image on the CCD array and the degree of defocus, but almost independent of droplet size. Specifically, it varies approximately linearly across the image along the direction corresponding to the direction of propagation of the laser sheet for a given defocus setting in ILIDS. The experimental finding is supported by a theoretical analysis, which was based on geometrical optics for a simple optical configuration that replicates the essential features of the optical system. The discrepancy in the location was measured using a monodisperse droplet generator, and this was subtracted from the droplet centres identified in the ILIDS images of a polydisperse spray without ‘seeding’ particles. This reduced the discrepancy between PIV and ILIDS droplet centres from about 1 mm to about 0.1 mm and hence increased the probability of finding the corresponding fringe patterns on the ILIDS image and glare points on the PIV image. In conclusion, it is shown that the proposed combined method can discriminate between droplets and ‘seeding’ particles and is capable of two-phase measurements in polydisperse sprays.     

Planar drop-sizing and liquid volume fraction measurements of airblast spray in cross-flow using SLIPI-based techniques

This paper presents planar measurements of Sauter mean diameter (SMD) and liquid volume fraction distributions of airblast spray injected into cross-flow. The experiments are conducted using a combination of structured laser illumination planar imaging with laser sheet drop-sizing (SLIPI-LSD) and particle/droplet imaging analysis (PDIA) techniques. Effect of gas to liquid mass ratio (GLR) and cross-flow velocity (U_cross) is studied. Planar SMD distribution at low GLR improved with increase in U_cross due to secondary atomization of large droplets. Uniform SMD distribution in a range of 10–20 µm is observed for GLR more than 3. The distribution of liquid volume fraction at low GLR condition shows poor dispersion with most of the liquid concentrated near the injector. The liquid volume fraction distribution improves with increase in GLR and better dispersion is observed for GLR more than 3 and two-phase momentum ratio (q₂) greater than 13.06. Spatial bifurcation in liquid fraction is found for high GLR conditions. The SMD in the range of 10–20  µm and uniform distribution of liquid are observed for GLR more than 3 and q₂ > 25.6.     

Effect of ambient density and orifice diameter on gas entrainment by a single-hole diesel spray

The gas entrainment into a non-vaporizing single-hole Diesel spray submitted to variable density is studied experimentally in order to better understand the effect on mixture formation. Particle Image Velocimetry on fluorescent tracers has been applied to obtain measurement in the flow field surrounding the spray. The “quasi-steady” region of the spray (far from the head vortex) as well as the non-stationary region has been investigated. Significant effects of both ambient density and nozzle diameter on gas entrainment have been pointed out.     

Velocity field analysis of the high density,high pressure diesel spray

In this study, particle image velocimetry (PIV) measurements have been performed extensively on a non-reactive dense diesel spray injected from a single orifice injector, under various injection pressure and steady ambient conditions, in a constant flow chamber. Details of PIV setup for diesel spray measurement without additional seeding are explained first. The measured velocity profiles are compared to those obtained from other similar measurements performed in a different institution, as well as those obtained from a 1D spray model simulation, presenting in both cases a good level of agreement. In addition, the velocity fields under various injection pressures and ambient densities show the dominant effects of these parameters on the behavior of diesel spray. The self-similarity of the transverse cut profiles of axial velocity is evaluated, showing that the measurements are in agreement with the hypothesis of self-similar velocity profiles. Finally, the effect of injection pressure and ambient density on the velocity fluctuations is presented and analyzed as well. While the experimental results presented here could help to understand the complex diesel fuel–air mixing process during injection, they also provide additional spray velocity data for future computational model validation, following the main idea of the Engine Combustion Network.     

Optical study on thermal radiation energy of diesel spray combustion in a shock tube

A tailored interface shock tube was used to measure the thermal energy radiated from diesel-spray combustion. Experiments were performed in a steel shock tube with a seven m long low-pressure section filled with air and a six m long high-pressure section. Pre-compressed fuel was injected through a throttling nozzle into air behind a reflected shock wave. Monochromatic emissive powers and emissive powers of the whole IR-wavelengths were followed with IR-detectors set along the central axis of the tube. Time-dependent-radii, where soot particles radiate, were also determined. Results were : (1) the tailored interface shock tube could be applied to a study of diesel-spray combustion. (2) thermal radiation energy could be described well from the ignition delay of the fuel spray.PACS: 47.40.Nm, 47.70.Mc     

Simultaneous measurement of temperature and fuel mole fraction using acetone planar induced fluorescence and Rayleigh scattering in stratified flames

The use of acetone as a tracer for planar laser induced of fluorescence (PLIF) measurements is very popular both for mixing investigations and for premixed or partially premixed combustion systems when evaluating the local mixture fraction (or equivalence ratio) in the fresh gases. The local structure of a flame front can be investigated by using Rayleigh scattering, and this technique has been quite frequently used in combustion. We present here an application of simultaneous imaging of temperature and fuel mole fraction with both acetone PLIF and Rayleigh scattering techniques. The strong influence of temperature on fluorescence signals can be corrected if the local temperature is known. Simultaneously, the contribution of the acetone Rayleigh cross-section can be evaluated through the local value of acetone mole fraction. An iterative process enables the fuel mole fraction (in the limit of the preheat zone) and temperature fields to be obtained in a reactive configuration. The technique is limited by the maximum temperature that can be corrected and by the tracer specificities. Tests in laminar homogeneous stabilized flames and in stratified stabilized flames demonstrate the ability to record the instantaneous flame structure and fuel mole fraction field. Finally, the paper presents correlations of the local flame thickness with the local methane mole fraction, which underline the strong influence of large scales of the equivalence ratio on the local flame structure.     

Concentration fields in a confined two-gas mixture and engine in-cylinder flow: laser tomography measurement by Mie scattering

Gaseous mixture concentration characterisation by Mie scatter laser tomography has been widely used, except when experimental conditions turn out to be difficult especially in confined flow. This paper describes a complete formulation of the image processing required for a confined flow including effects from wall reflection, quality of the seeding, thermodynamic flow characteristics and instantaneous laser profile disparity. Two applications are described; the first one on a free and confined jet (turbulent, isothermal, isobaric flow) which shows the potential of the technique and the processing needed to reconstitute the mean concentration field, thus demonstrating the reliability of the processing. Second, an in-cylinder engine concentration measurement is put forward. Normal running and a stratified concept are given as examples. The results show the excellent potential of the technique to help engine developers. Thus, this paper presents the problems connected with measurement of concentration fields by laser tomography and Mie scattering from a theoretical and experimental approach. Received: 12 November 1999 / Accepted: 29 January 2001