Mixing in a confined turbulent impinging jet using planar laser-induced fluorescence

 The non-intrusive Planar Laser-Induced Fluorescence (PLIF) technique was applied to the study of the mixing of a turbulent water jet impinging orthogonally onto a flat surface. A procedure for calibrating the system at each pixel of a CCD camera array was first developed and tested. Post-processing of the PLIF data gave quantitative results of good quality. The mixing at the entrance of the deflection zone was also investigated. Average concentration fields in the centre plane of the jet were calculated and compared with Large Eddy Simulations (LES) and also with data from the literature. Probability density functions, space coefficients of correlation and radial concentration fluctuation profiles were calculated to further quantify the spreading of the jet, both in the free and deflection zones. Inside the deflection region, a slight tendency towards intensified mixing at the outer edge of the jet was found. This was attributed to a deceleration of the fluid which resulted in accelerated diffusion. Received: 11 July 1997 / Accepted: 9 January 1998     

Measurements of vaporized and liquid fuel concentration fields in a burning spray jet of acetone using planar laser induced fluorescence

Planar LIF of acetone has been performed in the near development field of a burning spray jet. The main difficulty of such investigations comes from the large range of signal levels provided by the vapor and the size dispersed spray, which cannot be covered by the camera dynamics. The key point of the present work lies in a strong compression of the fluorescence signal dynamics as the UV laser radiation is strongly absorbed by the liquid phase. Preliminary experiments were made in homogeneous vapor and with calibrated drops to quantify the acetone fluorescence signals. Analysis of the histograms of signal level in the spray shows that a cut-off signal level can be used to reject the contribution of the liquid phase. The single shot fluorescence profiles have been processed to restore the fields of fluctuating and mean concentration of acetone vapor in the spray. The liquid concentration field was obtained by extracting the individual drops data from the single shot fluorescence images with a reduced gain of the camera. A statistical correction accounting for the discrete extinctions of the laser by the drops has been used. Analysis of the results shows the influence of the input atomization parameters on the structures of the condensed and vaporized concentration fields.     

Quantitative imaging of concentration by planar laser-induced fluorescence

The planar laser-induced fluorescence (PLIF) technique is attractive for instantaneous and non-intrusive imaging of species concentration in gaseous flows. This paper provides a framework for determining the experimental resolution in PLIF experiments and gives error estimates for concentration measurements in turbulent jet mixing experiments using biacetyl as the molecular tracer. The procedures to correct for experimental artifacts in the PLIF images are outlined. Images of the instantaneous, average, rms, and dissipation of concentration in a turbulent jet are presented.     

Acetone: a tracer for concentration measurements in gaseous flows by planar laser-induced fluorescence

The structure of fully-developed turbulence in a smooth pipe has been studied via wavenumber spectra for various friction velocities, namely, u ,=0.61 and 1.2 m/s (the corresponding Reynolds numbers based on centerline velocity and pipe radius being respectively 134,000 and 268,000) at various distances from the wall, namely y ⁺ = 70, 200,400 and 1,000. For each distance from the wall, correlations of the longitudinal component of turbulence were obtained simultaneously in seven narrow frequency bands by using an automated data acquisition system which jointly varied the longitudinal (x) and transverse (z) separations of two hot-wire probes. The centre frequencies of the bandpass filters used correspond to a range of nondimensional frequencies ⁺ from 0.005 to 0.21. By taking Fourier transforms of these correlations, three-dimensional power spectral density functions and hence wavenumber spectra have been obtained at each y ⁺ with nondimensional frequency ⁺ and nondimensional longitudinal and transverse wavenumbers k _x ⁺ and k _z ⁺ as the independent variables. The data presented in this form show the distribution of turbulence intensity among waves of different size and inclination. The data reported here cover a wave size range of over 100, spanning a range of wave angles from 2° to 84°. The effects of friction velocity and Reynolds number on the distribution of waves, their lifetimes and convection velocities are also discussed.List of symbols A wave strength function - C _x streamwise phase velocity - C _z circumferential phase velocity - f wave intensity function - k resultant wave number = [k _x ² + k _z ² ]^1/2 - k _x , k _z longitudinal (x) and transverse (z) wavenumber respectively - P(k _x ⁺ , k _z ⁺ , ⁺) power spectral density function in u - R radius of pipe - Re Reynolds number (based on centerline velocity and pipe radius) - R _uu (x ⁺, z⁺, ) normalized correlation function in u - R _unu (x ⁺, z⁺,¦⁺¦) normalized filtered correlation function in u, as defined in equation (1) - t time - U mean velocity in the x-direction - u, v, w turbulent velocities in the cartesian x, y and z directions respectively - û, v, turbulent velocities in the wave coordinate x, and directions respectively - u friction velocity - x, y, z cartesian coordinates in the longitudinal (along the pipe axis), normal (to the pipe wall) and transverse (along the circumference of the pipe) directions respectively, as defined in Fig. 1 - wave angle - difference between two quantities - v kinematic viscosity - time delay - circular frequency (radians/s) - + quantity nondimensionalized using u and v - overbar time average A version of this paper was presented at the 12th Symposium on Turbulence, University of Missouri-Rolla, 24–26 September, 1990     

Near-wall imaging using toluene-based planar laser-induced fluorescence in shock tube flow

A quantitative thermometry technique, based on planar laser-induced fluorescence (PLIF), was applied to image temperature fields immediately next to walls in shock tube flows. Two types of near-wall flows were considered: the side wall thermal boundary layer behind an incident shock wave, and the end wall thermal layer behind a reflected shock wave. These thin layers are imaged with high spatial resolution (15μm/pixel) in conjunction with fused silica walls and near-UV bandpass filters to accurately measure fluorescence signal levels with minimal interferences from scatter and reflection at the wall surface. Nitrogen, hydrogen or argon gas were premixed with 1–12% toluene, the LIF tracer, and tested under various shock flow conditions. The measured pressures and temperatures ranged between 0.01 and 0.8 bar and 293 and 600 K, respectively. Temperature field measurements were found to be in good agreement with theoretical values calculated using 2-D laminar boundary layer and 1-D heat diffusion equations, respectively. In addition, PLIF images were taken at various time delays behind incident and reflected shock waves to observe the development of the side wall and end wall layers, respectively. The demonstrated diagnostic strategy can be used to accurately measure temperature to about 60 μm from the wall.     

Simultaneous imaging of temperature and mole fraction using acetone planar laser-induced fluorescence

Imaging of concentration with acetone PLIF has become popular in mixing investigations. More recently, studies of the temperature dependences of acetone fluorescence have enabled quantitative imaging of temperature using single- or dual-wavelength excitation strategies. We present here the first demonstration of simultaneous imaging of temperature and mole fraction with acetone PLIF. Laser excitation is at 248 and 308 nm; the resulting fluorescence images are captured by an interline transfer CCD camera capable of acquiring two frames with a separation in time of as little as 500 ns. In addition to adding temperature imaging capability, this dual-wavelength approach enables mole fraction to be accurately measured in non-isothermal flows. Tests in a heated turbulent jet demonstrate the ability to record instantaneous mole fraction and temperature structure. The expected correspondence of the temperature and concentration fields is observed, and mean values of these quantities derived from image averaging show the expected radial and centerline profiles as the jet becomes fully developed. Received: 13 January 1999/Accepted 10 February 2000     

Measurements of mean and fluctuating temperature in an underexpanded jet using electrostrictive laser-induced gratings

Instantaneous temperature measurements were obtained in an underexpanded jet using electrostrictive laser-induced gratings. Evaluation of the technique under static, low-pressure conditions provided a baseline uncertainty or precision for single-shot temperature measurements of 4.4% of the local mean temperature, which represents the minimum detectable temperature fluctuation. The underexpanded jet was operated at a nozzle pressure ratio of 2.39 and a fully expanded jet Mach number of 1.19. Data were acquired along the centerline and over two radial traverses through the shear layer. Mean temperature data agree well with expectations, describing the shock-cell structure and the compressible shear layer. The growth in shear-layer width with downstream distance can be identified in the mean and fluctuating temperature measurements. Temperature fluctuations are near the baseline detection limit in the jet core and surrounding ambient air, and reach a maximum in the shear layer. The temperature fluctuation measurements compare well with previous computational and experimental work, confirming the application of the technique to a turbulent, supersonic flow.     

Simultaneous concentration and velocity measurements using combined laser-induced fluorescence and laser Doppler velocimetry: Application to turbulent transport

This paper describes the implementation of an optical technique, allowing to perform concentration and velocity measurements simultaneously and at the same point. This method is based on the coupling of laser-induced fluorescence of rhodamine B, applied to the determination of local concentration, and laser Doppler velocimetry. The method developed provides an accurate measurement of the concentration-velocity cross-correlation. The latter is a parameter linked to the eddy diffusivity tensor of a passive contaminant. This method was tested with a turbulent submerged free jet and it allowed the determination of the mean field of concentration and velocity, the concentration-velocity cross-correlation, and the local eddy diffusivity.List of symbols C molar concentration - c fluctuating part of the concentration - mean value of the concentration - concentration-velocity cross-correlation - D molecular diffusivity - (D _eddy) _ij eddy diffusivity tensor - I _abs absorbed intensity - I _e local incident intensity - K _opt optical constant - N number of samples - r _c half-width radius for the concentration profile - r _v half-width radius for the velocity profile - S _f fluorescence signal - Sc Schmidt number - V _c collection volume - U velocity - U _e flow velocity in the channel - U _i injection velocity Greek symbols kinetic energy dissipation rate - ₁ molar extinction coefficient for the laser radiation (in m²mol^–1l^–1) - ₂ molar extinction coefficient for the fluorescence signal (in m²mol^–1l^–1) - quantum yield - _c Batchelor scale - _k Kolmogorov scale - v kinematic viscosity - normalized values     

Shock tunnel flow visualization using planar laser-induced fluorescence imaging of NO and OH

Temporal sequences of planar laser-induced fluorescence (PLIF) images of several high-speed, transient flowfields created in a reflection-type shock tunnel facility were acquired. In each case, the test gas contained either nitric oxide or the hydroxyl radical, the fluorescent species. The processes of shock reflection from an endwall with a converging nozzle and of underexpanded free jet formation were examined. A comparison was also made between PLIF imaging and shadow photography. The investigation demonstrated some of the capabilities of PLIF imaging diagnostics in complex, transient, hypersonic flowfields, including those with combustion.Nomenclature A spontaneous emission rate - A _las cross sectional area of laser sheet - B laser absorption rate - C _opt constant dependent on optical arrangement, collection efficiency, etc. - D nozzle throat diameter - E _p laser pulse energy - f _J Boltzmann fraction of absorbing state - g spectral convolution of laser and absorption lineshapes - k Boltzmann constant - M _s incident shock Mach number - N noise, root-mean-square signal fluctuation - P static pressure - P ₁ initial pressure of test gas in shock tube - P _a free jet ambient pressure - P _s stagnation pressure - Q electronic quenching rate of excited state - S PLIF signal - t time between shock reflection and image acquisition - T static temperature - T _s stagnation temperature - _a mole fraction of absorbing species     

Simultaneous temperature and 2D velocity measurements in a turbulent heated jet using combined laser-induced fluorescence and LDA

 This paper presents an efficient technique for the characterization of thermal transport properties in turbulent flows. The method is based on the temperature dependence of fluorescence, induced by laser radiation, of an organic dye. The laser-induced fluorescence technique is combined with 2D laser Doppler anemometry, in order to measure in the same sample volume simultaneously and instantaneously the temperature and velocity. The technique is demonstrated on a turbulent heated round jet: the mean and fluctuating dynamic and thermal fields are investigated, and the temperature-velocity cross-correlations are determined in order to characterize the turbulent diffusivity and the turbulent Prandtl number. Received: 30 October 1997/Accepted: 14 July 1998     

Fuel-oil concentration in a gas turbine burner measured with laser-induced fluorescence

We applied the technique of two-dimensional laser-induced fluorescence to measure relative local concentrations of the evaporated fuel oil in a premixed gas turbine burner. The dependence of the fuel-oil fluorescence in the burner on the air inlet temperature, on the total fuel mass flow, and on the residence time of the fuel was investigated. A Mie scattering experiment was performed in order to observe non-evaporated fuel oil. We conclude that the fuel-oil concentrations can be determined from the fluorescence intensity within an error of about 25% in regions of complete evaporation of the fuel oil. Received: 29 March 1999/Accepted: 5 January 2000     

Measurements of temperature,density, pressure,and their fluctuations in supersonic turbulence using laser-induced fluorescence

A laser-induced fluorescence (LIF) method has been developed that provides simultaneous measurements of temperature, density, and their fluctuations owing to turbulence in unheated compressible flows. Pressure and its fluctuations are also deduced using the equation of state. Fluorescence is induced in nitric oxide that has been seeded into a nitrogen flow in concentrations of 100 ppm. Measurements are obtained from each laser pulse, with a spatial resolution of 1 mm and a temporal resolution of 125 ns. The method was applied to a supersonic, turbulent, boundary-layer flow with a free-stream Mach number of 2. For stream conditions in the range from 150–300 K and 0.3–1 atm, temperature is measured with an uncertainty of approximately 1% rms, while density and pressure uncertainties are approximately 2% rms.     

Quantifying macro-mixing and micro-mixing in a static mixer using two-tracer laser-induced fluorescence

An experimental procedure has been developed to quantify mixing at large scales (flow-induced) and at small scales (induced by molecular diffusion). It relies on the simultaneous imaging of two different fluorescent tracers using planar laser-induced fluorescence (PLIF). In order to quantify micro-mixing, a suitable neutralization reaction involving the fluorescent tracer uranine has been identified. Using PLIF, uranine is measured simultaneously with another fluorescent tracer, pyridine 2, employed to characterize macro-mixing. Since both tracers are quite inexpensive, this procedure allows an in-depth characterization of mixing properties even in large installations, by measuring the concentration fields of the involved tracers in a non-intrusive manner. This measurement procedure has been applied to a static mixer segment with geometrical features and dimensions similar to that found in practical applications. Laminar inflow conditions are employed. The flow and mixing analysis obtained by post-processing the measurement results is detailed in the present article.     

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.     

Particle concentration and particle size measurements in a particle laden turbulent free jet

Particle concentration and particle size distribution curves have been measured for particle-laden jets of silica gel powder for different loading ratios and air velocities using a Laser Diffraction Method (LDM) and a tomography data transform technique. It was found that the mean particle size at the outer edge of the jet decreases with increasing gas velocity, and that the jet widens with decreasing particle concentration and increasing gas velocity.     

Influence of pressure, temperature and gas phase composition on biacetyl laser-induced fluorescence

The purpose of this work is to get fundamental knowledge and to understand experimentally the fluorescence phenomenon usually used in laser-based imaging diagnostics. A complete review of thermodynamic effects (pressure, temperature, oxygen and tracer concentration) on biacetyl fluorescence is presented for a wide range of temperature and pressure. Biacetyl fluorescence increases with pressure and then levels off at high pressure when the relaxation is complete. The influence of the temperature is explained by a competition between the intersystem crossing and fluorescence. The effect of oxygen fluorescence quenching is important at high pressure and is discussed.     

Measurements of the vaporization dynamics in the development zone of a burning spray by planar laser induced fluorescence and Raman scattering

A linear measurement technique based on simultaneous planar imaging of laser induced dye fluorescence and Raman scattering in the liquid phase is reported. Calibrations in a stream of monosized droplets doped with weak concentrations of rhodamin show that the intensities on the droplet images are proportional to the actual droplet volume for Raman scattering and to the initial volume of the droplet for fluorescence, as the mass of dissolved dye does not vaporize. Thus, the mass fraction of liquid fuel that has vaporized before the probing event can be derived from these simultaneous measurements. Experiments are performed in the early development of a burning spray to derive cumulative information on the vaporization dynamics in terms of mass fraction or evaporation constant. Size distribution from conjoined phase-Doppler measurements are also used to derive the rate of droplet consumption along the axis of the burning spray.     

Measurements of the velocity of high-concentration zones in a turbulent jet

A pH sensitive dye technique has been used to highlight the zones of greatest scalar concentration in a turbulent jet as color regions. Photographs taken with a motion picture camera showed circular zones of the width of the jet moving downstream. These expand downstream and lose color until they are only small patches and wisps of red. A scheme has been developed whereby the Lagrangian velocity of the most concentrated zones can be measured. These move regularly at constant or steadily decreasing speed along rays originating in the source. The speed is about twice that of the mean Eulerian velocity component. Received: 6 September 1996/Accepted: 7 January 1997