Planar laser induced fluorescence in aqueous flows

Planar laser-induced fluorescence (PLIF) is a non-intrusive technique for measuring scalar concentrations in fluid flows. A fluorescent dye is used as a scalar proxy, and local fluorescence caused by excitation from a thin laser sheet can be related to dye concentration. This review covers quantitative PLIF in aqueous flows, with discussions of fluorescence theory, experimental methods and equipment, image processing and calibration, and applications of the technique.     

An impedance probe for the measurements of liquid hold-up and mixing time in two/three-phase stirred tank reactors

A new probe based on the measurement of the electrical impedance has been developed to measure liquid hold-up in gas/liquid, solid/liquid and gas/solid/liquid stirred tank reactors. It allows measurements of liquid hold-up and mixing time to be made in stirred tanks. The main advantage of the new probe is that it is absolutely non-intrusive, because it uses the shaft and the baffles' support as electrodes, and that it can be used both for laboratory scale reactors as well as for industrial stirred tank reactors. The relation between impedance and liquid hold-up under loading conditions closely follows the predictions made by means of the Bruggeman model. Therefore, the new probe does not need any calibration, in that the liquid hold-up can be theoretically computed by the measurement of impedance. Received: 1 January 2000/Accepted: 8 March 2001     

Dual emission laser induced fluorescence for direct planar scalar behavior measurements

 In this paper, a new method of measuring scalar behavior in bulk aqueous fluid flows is presented. Using a simple ratiometric scheme, laser induced fluorescence from organic dyes can be normalized so that direct measurements of a scalar in the flow are possible. The technique dual emission laser induced fluorescence (DELIF) relies on normalizing the fluorescence emission intensity of one dye with the fluorescence emission intensity of a second dye. Since each dye fluoresces at a different wavelength, one can optically separate the emission of each dye. This paper contains an overview of the basic ratiometric technique for pH and temperature measurements as well as the spectral properties of nine water soluble dyes. It also covers the three most significant sources of error in DELIF applications. To demonstrate the technique, steady state turbulent jet mixing and temperature fields in a thermal plume were quantified. The accuracy was camera limited at under 3% of the fluorescence ratio which corresponds to 0.1 pH units or 1.8 °C. Received 7 June 1996/Accepted 17 June 1997     

Three-dimensional LDA measurements in the impeller region of a turbulently stirred tank

Three-dimensional, angle-resolved LDA measurements of the turbulent flow field (Re = 2.9 × 10⁴) in the vicinity of a Rushton turbine in a baffled mixing tank have been performed. For this goal, a procedure for accurate beam alignment, based on a submerged micro-mirror system, has been developed. Results on the average flow field as well as on the complete set of Reynolds stresses are presented. The anisotropy of the turbulence has been characterized by the invariants of the anisotropy tensor. The trailing vortex structure, which is characteristic for the flow induced by a Rushton turbine, is demonstrated to be associated with strong, anisotropic turbulent activity. Received: 14 September 1998/Accepted: 22 February 1999     

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.     

Vibrational temperature measurements in a shock layer using laser induced predissociation fluorescence

Single shot spatially and spectrally resolved laser induced predissociation fluorescence measurements in a shock layer around a cylinder in a pulsed supersonic free stream are presented. Fluoresence signals were produced using the tuned output of an argon fluoride excimer laser to excite a mixture of rovibrational transitions in molecular oxygen. The signals produced along a line inside the shock layer were focussed onto a two dimensional detector coupled to a spectrometer, thus allowing spectral and spatial resolution of the fluoresence. In this way, it was possible to detect two fluoresence signals from two different transitions simultaneously, allowing the determination of vibrational temperatures without the need for calibration. To minimize problems associated with low signal to noise ratios, background subtraction and spatial averaging was required.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Modeling of dual emission laser induced fluorescence for slurry thickness measurements in chemical mechanical polishing

Dual emission laser induced fluorescence (DELIF) is a technique for measuring the instantaneous thin fluid film thickness in dynamic systems. Two fluorophores within the system produce laser induced emissions that are filtered and captured by two cameras. The ratio of the images from these cameras is used to cancel the effect of the laser beam profile on the image intensity. The resultant intensity ratio can be calibrated to a fluid film thickness. The utilization of a 2-dye system when applied to Chemical Mechanical Polishing (CMP) is complicated by the fluorescence of the polymeric polishing pad and the light scattering particles in the polishing slurry. We have developed a model of DELIF for CMP with 1-dye employing the polishing pad as the second fluorophore. While scattering particles in the slurry decrease the overall intensity of the individual images, the contrast in the image ratio increases. Using the 1-dye DELIF system to measure thin slurry films, our model results indicate that a cubic calibration may be needed. However, experimental results suggest a linear calibration is achieved for slurry films between 0 and 133???m thick with scattering coefficients as high as 8.66?mm^?1 at a wavelength equal to 410?nm.     

An LIF line scan system for the measurement of scalar concentration in a continuously operated stirred tank

 An LIF line scan system is presented and used to obtain unobtrusive mean and angle resolved concentration measurements in a continuously operated stirred tank agitated with a single Rushton turbine. The measurements showed that the trailing vortex structures caused regions of elevated concentrations gradients and concentration fluctuations between blade passages. In addition, on the centre line of the tank, whilst macromixing was always complete, micromixing was less complete than in other regions of the tank. Compared to point measurement techniques, the line scan system made data collection more comprehensive and convenient and the determination of a line of data allows the possibility of the correction of beam attenuation and the extension of the technique to a larger scale. Received: 25 July 1997/Accepted: 15 October 1997     

Velocity and concentration fields in settling tank of non-interacting solid particles

Summary In this paper there is a synthetic presentation of the theoretical principles of the phase difference method for the simultaneous measurements of size, velocity and concentration of particles dispersed in fluid, its application to spherical glass particles free falling in water at rest, and the measured velocity and concentration fields of the particles in a settling tank model. Particles with diameter between 75÷150 m were used. The method used seems reliable in obtaining the necessary information to analyze the performance of settling tanks in relation to the influencing factors the particles settling.

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Sommario In questo articolo sono presentati in forma sintetica i principi teorici del metodo della differenza di fase per la misura simultanea di velocità, dimensione e concentrazione di particelle disperse in un fluido, ed un'applicazione di tale metodo al caso di particelle di vetro, di dimensioni fra i 75–150 m, disperse in acqua. Le misure effettuate sono relative alla sedimentazione di particelle in acqua in quiete ed ai campi di velocità e concentrazione delle particelle che si realizzano in un modello fisico di vasca di sedimentazione rettangolare. I risultati ottenuti sembrano confermare l'adeguatezza del metodo impiegato per analizzare l'efficienza delle vasche di sedimentazione in relazione ai fattori che influenzano la sedimentazione di particelle.

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This work was supported by the Italian Ministry of Education.     

A laser induced fluorescence technique for quantifying transient liquid fuel films utilising total internal reflection

This paper describes the development of a laser induced fluorescence (LIF) technique to quantify the thickness and spatial distribution of transient liquid fuel films formed as a result of spray–wall interaction. The LIF technique relies on the principle that upon excitation by laser radiation the intensity of the fluorescent signal from a tracer like 3-pentanone is proportional to the film thickness. A binary solution of 10% (v/v) of 3-pentanone in iso-octane is used as a test fuel with a Nd:YAG laser as the excitation light source (utilising the fourth harmonic at wavelength 266 nm) and an intensified CCD camera is used to record the results as fluorescent images. The propagation of the excitation laser beam through the optical piston is carefully controlled by total internal reflection so that only the fuel film is excited and not the airborne droplets above the film, which had been previously shown to induce significant error. Other known sources of error are also carefully minimised. Calibrated temporally resolved benchmark results of a transient spray from a gasoline direct injector impinging on a flat quartz crown under atmospheric conditions are presented, with observations and discussion of the transient development of the fuel film. The calibrated measurements are consistent with previous studies of this event and demonstrate the applicability of the technique particularly for appraisal of CFD predictions. The potential utilisation of the technique under typical elevated ambient conditions is commented upon.     

An extra-thin light sheet technique used to investigate meniscus shapes by laser induced fluorescence

A light sheet technique is described to accurately (50 m) measure meniscus profiles in film formation problems. The use of a slit to create the thin (0.1 mm) laser sheet makes the technique easy to implement, and allows tunable sheet thickness. The low light intensity obtained through the slit is compensated by the induced fluorescence of the tested fluid, which provides good picture contrast. After video recording through a microscope, the actual meniscus is recovered by image processing and proper calibration. The efficiency of the technique is demonstrated on a coating flow experiment. Due to its good accuracy and ease of use, this technique is expected to provide useful quantitative information about meniscus problems, in particular for the validation of CFD solutions of coating flows.     

CFD simulation of particle suspension in a stirred tank

Particle suspension characteristics are predicted computationally in a stirred tank driven by a Smith turbine. In order to verify the hydrodynamic model and numerical method, the predicted power number and flow pattern are compared with designed values and simulated results from the literature, respectively. The effects of particle density, particle diameter, liquid viscosity and initial solid loading on particle suspension behavior are investigated by using the Eulerian–Eulerian two-fluid model and the standard k–? turbulence model. The results indicate that solid concentration distribution depends on the flow field in the stirred tank. Higher particle density or larger particle size results in less homogenous distribution of solid particles in the tank. Increasing initial solid loading has an adverse impact on the homogeneous suspension of solid particles in a low-viscosity liquid, whilst more uniform particle distribution is found in a high-viscosity liquid.     

A study of turbulent mixing in a turbine-agitated tank using a fluorescence technique

Two-dimensional images of (Plane) Laser Induced Fluorescence (PLIF) have been used to study the turbulent mixing process in a model stirred tank. A calibration procedure is presented and discussed in terms of its accuracy. Data from the literature are used for comparison. A pattern-recognition algorithm has been designed to identify and quantitatively describe large-scale structures in the flow. This methodology, called “structural analysis”, is based on a conditional analysis of the PLIF data and requires the definition of an appropriate structure-detector function which is calculated locally. The mathematical tools developed have been used to study the mixing in a Rushton turbine-agitated reactor. Particular attention is paid to two specific regions of the tank; namely the bulk and the impeller stream regions, at two measured power input (0.3 and 0.7 W kg⁻¹). The averaged concentration fields show a common two-dimensional steady circulation pattern. Concentration probability density functions reflect well the instability of the flow in the two regions investigated. The data reveal the non-isotropic distribution of these instabilities around a reference point when the feed port is situated in the bulk region only. In this case, the structural analysis quantitatively shows the presence of a folding of the concentration field. It was found that this phenomenon can last several seconds. Received: 16 June 1998/Accepted: 14 April 1999     

Concentration measurements during mass transfer across liquid-phase boundaries using planar laser induced fluorescence (PLIF)

A non-intrusive optical measuring technique, the planar laser induced fluorescence is described to investigate mass transfer of a fluorescent dye (rhodamine B) across an interface between two partial immiscible liquids (1-butanol and water). The spatial resolution of the measurement technique used for the mass transfer investigations was 31.6 μm. The measured concentration profiles do not correspond with those, which were expected on the basis of usual theories. A solution equilibrium at the phase boundary could not be found. Received: 21 January 1999/Accepted: 4 May 1999     

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     

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     

Comparison of electrical and laser spark emission spectroscopy for fuel concentration measurements

Emission spectra from electrical and laser sparks in flowing methane–air mixtures of various compositions have been collected and analysed. The differences and similarities between the electrical and laser sparks in the context of their emission are discussed. The emission spectra from the laser spark were characterized by a weak continuum, onto which several strong atomic lines and some molecular bands were superimposed, in contrast to the spectra of electrical spark where a strong continuum, few atomic lines and several strong molecular bands were evident, making thus the laser spark spectroscopy a more accurate technique to measure hydrocarbon concentration. For both types of spark, the total intensity of the CN chemiluminescence around 388 nm was found to correlate almost linearly with fuel concentration in methane–air mixtures.     

A crossed beam laser shadow technique for spatially resolved measurements in turbulent flow

Optical methods such as the shadowgraph and schlieren techniques do not directly allow three-dimensional spatial resolution because of the inherent integration along the line of sight. For the particular case of the laser shadow technique, it is attempted to circumvent this restriction by employing a pair of crossed laser beams and cross-correlating the optical signals obtained from the beams. The information obtained is supposed to pertain only to the common volume of both beams. It is theoretically shown that this concept will work if the turbulence spectrum does not decay faster than K ^–5 in the range of wave numbers corresponding to the inverse diameter of the laser beam. Experiments on a round jet confirm that local values of the mean flow velocity can be obtained.List of symbols A electromagnetic amplitude at x = 0 - B ₁₂ correlation of light intensity fluctuations - C ₁₂ normalized correlation (Eq. (1)) - I (I) light intensity (fluctuating part) - k wavenumber of light - K wavenumber of turbulent eddies - L ₀ outer scale of turbulence - n refractive index - R ₀ radius of curvature of the wavefront of the laser beam at x = 0 - u (u ₀) (undisturbed) electromagnetic field of the laser beam - drift velocity of the laser shadow pattern - W spatial weighting function (Eq. (11)) - W ₀ diameter of the laser beam at x = 0 - x, y, z spatial coordinates (Figs. 1, 2) - see Eq. (6) - see Eq. (A3) spectrum of turbulent refractive index fluctuations exponent of _n - = (y, z) - ₁ see Eq. (7) - , spatial coordinates (Fig. 2) - , spatial and temporal delay variables in a correlation function     

Large scale planar laser induced fluorescence in turbulent density-stratified flows

Planar Laser Induced Fluorescence (PLIF) is used to obtain a series of two-dimensional concentration distributions in large scale (order of 5 m) time-varying thermally stratified flows. Density gradients due to turbulent fluctuations within the image area were found to cause reflection and refraction of the laser light sheet, reducing the usual simplicity of the PLIF method. To compensate for these effects, a variable local attenuation coefficient has been introduced to relate the attenuation of the laser sheet due to the concentration of the fluorescent dye and its spatial gradient. A mathematical algorithm for image restoration has been developed and applied to produce two-dimensional surface temperature mappings using fluorescent dye as an indicator. The algorithm has been verified using a set of temperature probes placed in the flow. It is found that this method provides both powerful flow visualization and adequate non-intrusive concentration measurements for large scale investigations of density stratified flows.