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.     

Statistical particle tracking velocimetry using molecular and quantum dot tracer particles

We present a statistical approach to particle tracking velocimetry developed to treat the issues associated with nanometer-sized tracer particles such as fluorescent molecules and quantum dots (QDs) along with theory and experimental results. Extremely small tracers pose problems to traditional tracking methods due to high levels of thermal motion, high levels of intensified camera noise, high drop-in/drop-out rates and, in the case of QDs, fluorescence intermittency (“blinking”). The algorithm presented here compensates for these problems in a statistical manner and determines the physical velocity distributions from measured particle displacement distributions by statistically removing randomly distributed, non-physical tracking events. The algorithm is verified with both numerically simulated particle trackings and experiments using 54 nm diameter fluorescent dextran molecules and 6 and 16 nm diameter QDs.     

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.     

Gas-phase velocity field measurements in sprays without particle seeding

A laser-based technique is presented that can be used to measure the instantaneous velocity field of the continuous phase in sprays and aerosols. In contrast to most well established laser-based velocity measurement techniques, this method is independent of particle seeding and Mie scattering. Instead of that it is based on gaseous flow tracers and laser-induced fluorescence (LIF). Inhomogeneous tracer gas distributions, which are created by an incomplete, turbulent mixing process, are exploited for flow tracing. The velocity field can be measured close to the droplets, because frequency-shifted LIF is separated from Mie scattering by optical filters. Validation tests and results from a water spray in air are given. Accuracy and spatial resolution are discussed in detail. Received: 26 April 1999/Accepted: 16 October 1999     

Evaporating and combusting droplet temperature measurements using two-color laser-induced fluorescence

 The paper presents a new technique based on laser-induced fluorescence, allowing droplet temperature measurement of evaporating and combusting droplets to be performed. The liquid spray is seeded with a low concentration of rhodamine B. The fluorescence, induced by the green line of an argon laser, is measured on two separated color bands. It is demonstrated that two color bands can be selected for their strong difference in the temperature sensitivity of the fluorescence quantum yield. The determination of the fluorescence ratio between the fluorescence intensity corresponding to each color band allows the tracer concentration and the droplet size dependences to be eliminated. The technique was applied on a monodisperse spray: the effect of a thermal impulse on the distribution of the droplet temperature is studied and, the temperature of combusting droplets is investigated. Received: 16 June 2000/Accepted: 10 November 2000     

A simple shear cell for the direct visualization of step-stress deformation in soft materials

We introduce a custom-built stress-controlled shear cell coupled to a confocal microscope for direct visualization of constant-stress shear deformation in soft materials. The torque generator is a cylindrical Taylor–Couette system with a Newtonian fluid between a rotating inner bob and a free-to-move outer cup. A spindle/cone assembly is coaxially coupled to the cup and transfers the torque exerted by the fluid to the sample of interest in a cone-and-plate geometry. We demonstrate the performance of our device in both steady-state and transient experiments with different viscoelastic materials. Our apparatus can conduct unidirectional constant-stress experiments as accurately as most commercial rheometers, with the capability to directly visualize the flow field using tracer particles. Further, our step-stress experiments on viscoelastic materials are devoid of creep ringing, which is an advantageous aspect of our torque generation mechanism. We believe that the device presented here could serve as a powerful and cost-effective tool to investigate the microstructural determinants of nonlinear rheology in complex fluids.     

汽油蒸气/空气预混火焰的无拉伸层流燃烧速率

为了研究汽油蒸气/空气的爆炸特性,介绍了汽油蒸气/空气预混火焰的无拉伸层流燃烧速率。通过实验研究了无拉伸层流燃烧速率,与汽油的主要组分异辛烷和正庚烷与空气的预混火焰层流燃烧速率做了对比,发现汽油蒸气/空气的无拉伸层流燃烧速率小于异辛烷和正庚烷与空气的预混火焰无拉伸层流燃烧速率,但无拉伸层流燃烧速率随当量比的变化规律相同,随着当量比增大,无拉伸层流燃烧速率呈先增大、再减小的变化趋势,最大值在当量比为1处取得。     

A New Analytical Formulation for Interwell Finite-Step Tracer Injection Tests in Reservoirs

Analytical models to describe tracer transport in reservoirs commonly set conditions either on the tracer concentration or the tracer flow at the injection border. Here a different formulation based on tracer sources is presented. This approach avoids some of the physical inconsistencies that can be found when setting conditions on the tracer concentration. The case of a tracer injected as a finite-step in an infinite one-dimensional homogeneous reservoir with a uniform flow is considered. The solution is analytically obtained. The results are confronted against the two common boundary cases. The new approach predicts slightly delayed and broader pulses. The tracer breakthrough curves differences can be large for small Peclet numbers. These differences weakly reduce by increasing the injection period. The new approach contains tracer injection elements that can make it suitable to describe real conditions found in reservoir tracer tests.     

Comparison of Observations from a Laboratory Model with Stochastic Theory: Initial Analysis of Hydraulic and Tracer Experiments

Hydraulic and tracer tests were conducted in a flow cell containing a mixture of sediments designed to mimic a two-dimensional, log-normally distributed, second-order stationary, exponentially correlated random conductivity field. With 60 integral scales in the direction of mean flow and 25 integral scales perpendicular to this direction, behavior of flow and transport in the interior of the flow cell can be compared directly with stochastic solutions for flow and transport. Using 144 piezometers and 361 platinum electrodes, the distribution of hydraulic head and the concentrations of an ionic tracer could be monitored in substantial detail. The present discussion presents the details of the experimental equipment. Results and initial analysis of hydraulic measurements and characterization of a two-dimensional tracer plume are also presented. Analysis using first-order hydraulic theory shows that the flow through the medium was consistent with an effective conductivity equal to the geometric mean of the conductivity distribution. Further, the semivariogram of head increments as observed in the experimental results was consistent with the semivariogram predicted by theory. The chemical transport experiments are here compared with the early solutions presented by Dagan (1984, 1987). The observed rate of longitudinal spread of two tracer plumes was slightly less than that predicted using this theory. Further, the spread in the transverse dimension was observed to decline from the initial plume dimensions and then remain constant or increase slightly, but at a rate lower than predicted by the theory. The difference between the hydraulic and transport results is believed to be related to the fact that the hydraulic results were averaged over a very large portion of the flow cell such that ergodic conditions could be assumed. In contrast, the initial geometry of the plume covered only approximately five integral scales in the transverse direction such that the validity of the assumption of ergodic conditions must be questioned in the analysis of results for the chemical transport.     

Flow structure of microbubble-laden turbulent channel flow measured by PIV combined with the shadow image technique

The turbulence structure of a horizontal channel flow with microbubbles is experimentally investigated using combined particle image velocimetry (PIV) in order to clarify the mechanism of drag reduction caused by microbubbles. A new system which simultaneously measures the liquid phase and the dispersed bubbles is proposed, based on a combination of particle tracking velocimetry (PTV), laser-induced fluorescence (LIF) and the shadow image technique (SIT). To accurately obtain the velocity of the liquid phase, tracer particles which overlap with the bubble shadow images are almost entirely eliminated in the post-processing. Finally, the turbulence characteristics of the flow field are presented, including measurements for both phases, and the bubble effect on the turbulence is quantified.     

Simultaneous DPTV/PLIF measurements of a turbulent jet

Simultaneous measurements of instantaneous velocity and concentration fields were performed using digital particle tracking velocimetry and planar laser- induced fluorescence for a turbulent jet at a Reynolds number of 3000. The measurements of mean velocity, turbulent stresses, mean concentration, concentration variance, and turbulent flux of tracer all collapse onto self-similar profiles in the far field of the jet. The measurements showed excellent agreement with previous point velocity and concentration measurements. It is concluded that the system is an effective means of measuring the velocity and concentration distributions and turbulent characteristics. Received: 7 July 1999/Accepted: 9 February 2000     

A laser-induced fluorescence measurement for aqueous fluid flows with improved temperature sensitivity

This paper presents temperature-sensitive laser-induced fluorescence measurements of Fluorescein 27 dissolved in aqueous solutions. We show that Fluorescein 27, dissolved in water and excited by a 532-nm Nd:YAG laser pulse, yields improved temperature sensitivity over traditional organic dyes such as Rhodamine B. The high temperature sensitivity of Fluorescein 27 when excited at 532 nm is due primarily to a temperature-dependent shift of the absorption spectrum to longer wavelengths for increased temperatures. The linearity of the fluorescence signal with respect to the incident laser intensity and dye concentration is reported. In addition, Fluorescein 27 dissolved in an aqueous solution remains photo-stable for >10⁵ laser pulses at both ambient and high temperatures (T > 60°C) when excited with low-irradiance laser pulses. Finally, we demonstrate that using a dual tracer (or ratiometric) technique in which the fluorescence from Fluorescein 27 and another dye (e.g., Rhodamine B or Kiton Red 620) are detected following the 532 nm excitation results in a significantly enhanced temperature sensitivity over a single tracer measurement and previously reported dual tracer methods. Such temperature sensitivity is useful in multi-dimensional temperature imaging and temporally resolved measurements.

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Absolute intensity measurements of impurity emissions in a shock tunnel and their consequences for laser-induced fluorescence experiments

Absolute intensity measurements of impurity emissions in a shock tunnel nozzle flow are presented. The impurity emission intensities were measured with a photomultiplier and optical multichannel analyser and calibrated against an intensity standard. The various metallic contaminants were identified and their intensities measured in the spectral regions 290 to 330 nm and 375 to 385 nm. A comparison with calculated fluorescence intensities for predissociated laser-induced fluorescence signals is made. It is found that the emission background is negligible for most fluorescence experiments.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.     

Interpretation of internal tracer experiments and local sojourn time distributions

Tracer experiments are a valuable tool in the study of transport phenomena. Most of the theoretical work about tracers has been concerned either with systems having clearly defined inlet and outlet or with recirculating flows. In this article the methods are generalized to more complex situations and an attempt is made to provide a uniform theoretical treatment for local tracer experiments performed at several sites inside the flow systems. Local purging rate and mixing rate are defined and their measurements and applications are discussed. Several sojourn time distribution are discussed, especially that of local remaining life which might be useful in the study of local processes in large flow systems. The results should be of interest in a wide variety of areas in which tracer experiments are used, such as chemical reactor design, physiology, hydrology and the study of dispersion processes in the atmosphere and in oceans.     

Translational diffusion of rubrene and tetracene in polyisobutylene

The translational diffusion coefficients of rubrene and tetracene in amorphous polyisobutylene (PIB) were measured using the holographic fluorescence recovery after photobleaching technique. Over the temperature range from 400 to 235 K (T _g =205 K), tracer diffusion coefficients from 10^–7 to 10^–14 cm²/s were observed. These diffusion coefficients have essentially the same temperature dependence as the rotational correlation times for these two probes in PIB. Both of these observables have a slightly stronger temperature dependence than does the viscosity. These results contrast strongly with the results of similar experiments on polystyrene and polysulfone. These results are consistent with the hypothesis that local segmental dynamics are more spatially homogeneous in PIB than in polystyrene and polysulfone.Dedicated to Prof. John D. Ferry on the occasion of his 85th birthday.     

Tracer Dispersion in Rough Open Cracks

Tracer dispersion is studied in an open crack where the two rough crack faces have been translated with respect to each other. The different dispersion regimes encountered in rough-wall Hele-Shaw cell are first introduced, and the geometric dispersion regime in the case of self-affine crack surfaces is treated in detail through perturbation analysis. It is shown that a line of tracer is progressively wrinkled into a self-affine curve with an exponent equal to that of the crack surface. This leads to a global dispersion coefficient which depends on the distance from the tracer inlet, but which is still proportional to the mean advection velocity. Besides, the tracer front is subjected to a local dispersion (as could be revealed by point measurements or echo experiments) very different from the global one. The expression of this anomalous local dispersion coefficient is also obtained.     

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.     

The Sensitivity of Tracer Concentration to Nonuniform Permeability and Porosity

Concentration of tracer at a producing well is a function of time and of the flow field that transports the tracer from one well to another. Small localized changes in the permeability and porosity fields result in small changes in the observed concentration history. Calculation of the effect of changes in the petrophysical parameter fields on observed concentration is an integral part of most approaches to solving the inverse problem, i.e. calculation of permeability and porosity fields that are consistent with observed tracer concentrations. This paper does not address the inverse problem directly, only the forward problem, but the results are presented as semi-analytic formulas which are intended to provide physical insight into the limitations of the inverse procedure. In particular, for a simple example of dipole flow between two wells in a nearly homogeneous infinite 2D porous medium, the width of the region of significant influence is shown to scale as d where is the dispersivity and d is half the interwell distance. Also, for fixed injection and production rates the influence on concentration of variation in porosity and in log-permeability are shown to be similar in magnitude.