2D temperature measurements in the wake of a heated cylinder using LIF

A technique is described to measure the instantaneous 2D temperature distribution in the wake of a heated cylinder using `laser-induced fluorescence'. Rhodamine B, a fluorescent dye, is used as a temperature indicator. The relation between fluorescence intensity and temperature is determined by means of calibration experiments in the temperature range of 20–35 °C with an accuracy of ±0.1 °C. The temperature distribution behind the heated cylinder is well visible and can be measured with a high spatial resolution. Corrections for variation in laser energy and intensity distribution in the laser sheet have to be made to further improve the accuracy of the measuring method. Received: 3 January 2001/Accepted: 18 May 2001     

Molecular tagging thermometry with adjustable temperature sensitivity

We report an extension to the technique of molecular tagging thermometry which allows for adjustable temperature sensitivity. The temperature dependence of laser-induced phosphorescence of the water-soluble phosphorescent triplex (1-BrNp•Mβ-CD•ROH) is used to conduct temperature measurements in aqueous flows. It is shown that the temperature sensitivity of phosphorescence intensity can be adjusted by changing the time delay between the laser excitation pulse and the start of the phosphorescence emission acquisition. For example, for a phosphorescence integration period of 1 ms, the temperature sensitivity of the measured phosphorescence intensity varies in the range 8.15–18.2% per °C at 25°C as the time delay changes from 1 to7 ms. This temperature sensitivity is much higher than that of most fluorescent dyes used for temperature measurements (e.g. less than about 2% per °C for Rhodamine B). The implementation and application of this new approach are demonstrated by conducting temperature measurements in the wake of a heated cylinder.     

A laser-fluorescence technique for turbulent two-phase flow measurements

A non-intrusive measurement technique has been developed for accurate determination of gas and particle velocities in a turbulent two-phase flow field. The principle of the technique is based on the discrimination between the scattered light from particles and the fluorescence emission from particles coated with a fluorescent dye. A high-powered, argon-ion based, single-channel, on-axis backscatter laser-Doppler velocimetry system was used. The fluorescent dye was Rhodamine 6G. A study of the gas-solid two-phase flow behaviour in the freeboard of a cold gas-fluidized bed was undertaken. The solid phase contained two particle groups: bed material (sand) and fuel particles (wood). Measurements of the axial velocity and turbulence intensity distributions of the gas phase and both particle groups within the solid phase were made along the column centre and across the freeboard. Excellent discrimination of velocities from the two phases and from the two particle groups within the solid phase was achieved.     

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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In situ LIF temperature measurements in aqueous ammonium chloride solution during uni-directional solidification

We present in situ whole-field measurements of the temperature field using laser-induced fluorescence in a study of bottom-chilled uni-directional solidification of aqueous ammonium chloride. We utilize a two-color, two-dye, ratiometric approach to address the significant spatial and temporal variations of laser sheet intensity field due to refractive index variations caused by the evolving concentration and temperature fields. In our work we take advantage of two temperature sensitive fluorescent dyes with opposite temperature sensitivities in order to increase the overall sensitivity and temperature resolution of the measurements. The resulting temperature sensitivity (about 4% K⁻¹) is more than a factor of two higher than the original work of Sakakibara and Adrian (Exp Fluids 26:7–15, 1999) with a sensitivity 1.7% K⁻¹. In situ measurements of the temperature field during solidification are presented, along with temperature characteristics of some of the complex flow features, such as plumes and fingers.     

Optically sliced measurement of velocity and pH distribution in microchannel

A simultaneous measurement technique for the velocity and pH distribution was developed by using a confocal microscope and a 3CCD color camera for investigations of a chemical reacting flow field in a microchannel. Micron-resolution particle image velocimetry and laser induced fluorescence were utilized for the velocity and pH measurement, respectively. The present study employed fluorescent particles with 1 μm diameter and Fluorescein sodium salt whose fluorescent intensity increases with an increase in pH value over the range of pH 5.0–9.0. The advantages of the present system are to separate the fluorescence of particles from that of dye by using the 3CCD color camera and to provide the depth resolution of 5.0 μm by the confocal microscope. The measurement uncertainties of the velocity and pH measurements were estimated to be 5.5 μm/s and pH 0.23, respectively. Two aqueous solutions at different pH values were introduced into a T-shaped microchannel. The mixing process in the junction area was investigated by the present technique, and the effect of the chemical reaction on the pH gradient was discussed by a comparison between the proton concentration profiles obtained from the experimental pH distribution and those calculated from the measured velocity data. For the chemical reacting flow with the buffering action, the profiles from the numerical simulation showed smaller gradients compared with those from the experiments, because the production or extinction of protons was yielded by the chemical reaction. Furthermore, the convection of protons was evaluated from the velocity and pH distribution and compared with the diffusion. It is found that the ratio between the diffusion and convection is an important factor to investigate the mixing process in the microfluidic device with chemical reactions.     

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     

Determination of density and concentration from fluorescent images of a gas flow

A fluorescence image analysis procedure to determine the distribution of species concentration and density in a gas flow is proposed. The fluorescent emission is due to the excitation of atoms/molecules of a gas that is intercepted by an electron sheet. The intensity of the fluorescent light is proportional to the local number density of the gas. When the gas flow is a mixture of different species, this proportionality can be used to extract the contribution associated with the species from the spectral superposition acquired by a digital camera. In particular, the fact is exploited such that the ratio between a pair of color intensities takes different values for different gases and that different linear superpositions of different color intensities yield a ratio that varies with the species concentration. This leads to a method that simultaneously reveals species concentrations and mass density of the mixture. For the proper working of a continuous electron gun in a gas, the procedure can be applied to gas flow where the pressure is below the thresholds of 200∼300 Pa and the number density is no greater than 10²³ m⁻³. To maintain the constancy of the emission coefficients, the temperature variation in the flow should be inside the range 75–900 K (above the temperature where the probability to meet disequilibrium phenomena due to rarefaction is low, below the temperature where visible thermal emission is present). The overall accuracy of the measurement method is approximately 10%. The uncertainty can vary locally in the range from 5 to 15% for the concentration and from 5 to 20% for the density depending on the local signal-to-noise ratio. The procedure is applied to two under-expanded sonic jets discharged into a different gas ambient—Helium into Argon and Argon into Helium—to measure the concentration and density distribution along the jet axis and across it. A comparison with experimental and numerical results obtained by other authors when observing under-expanded jets at different Mach numbers is made with the density distribution along the axis of the jet. This density distribution appears to be self-similar.     

Simultaneous velocity field measurements in two-phase flows for turbulent mixing of sprays by means of two-phase PIV

This paper describes a novel derivative of the PIV method for measuring the velocity fields of droplets and gas phases simultaneously using fluorescence images rather than Mie scattering images. Two-phase PIV allows the simultaneous and independent velocity field measurement of the liquid phase droplets and ambient gas in the case of two-phase flow mixing. For phase discrimination, each phase is labelled by a different fluorescent dye: the gas phase is seeded with small liquid droplets, tagged by an efficient fluorescent dye while the droplets of the liquid phases are tagged by a different fluorescent dye. For each phase, the wavelength shift of fluorescence is used to separate fluorescence from Mie scattering and to distinguish between the fluorescence of each phase. With the use of two cross-correlation PIV cameras and adequate optical filters, we obtain two double frame images, one for each phase. Thus standard PIV or PTV algorithms are used to obtain the simultaneous and independent velocity fields of the two phases. Because the two-phase PIV technique relies on the ability to produce two simultaneous and independent images of the two phases, the choice of the labelling dyes and of the associated optical filter sets is relevant for the image acquisition. Thus a spectroscopic study has been carried out to choose the optimal fluorescent dyes and the associated optical filters. The method has been evaluated in a simple two-phase flow: droplets of 30–40 μm diameter, produced by an ultrasonic nozzle are injected into a gas coflow seeded with small particles. Some initial results have been obtained which demonstrate the potential of the method.     

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     

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     

Simultaneous,ensemble-averaged measurement of near-wall temperature and velocity in steady micro-flows using single quantum dot tracking

We present results from a series of experiments demonstrating the use of single quantum dots (QDs) as simultaneous temperature and velocity probes at the micro-scale. The fluorescence intensity of QDs varies predictably with temperature due to changes in quantum efficiency. We use total internal reflection fluorescence microscopy to study the region within 200 nm of a fluid-solid interface. A two-color, time-averaged temperature sensing technique based on the ensemble intensity changes of single QDs as compared to a reference dye (rhodamine 110) is presented. Many single QD intensity measurements are used to build intensity distributions which can be mapped to fluid temperature. Simultaneously, we track the motion of individual QDs, building a distribution of particle displacements, where the mean displacement yields the local fluid velocity. We also show that the width of the displacement distribution (or the diffusion coefficient) captures the scaling of the temperature to viscosity ratio, which may allow for independent viscosity measurement.     

Planar laser induced fluorescence technique for measurements of concentration fields in continuous stirred tank reactors

 A new non-intrusive method based on laser sheet visualization and image processing has been developed to measure the instantaneous concentration fields of a non-reacting fluorescent dye in a continuous stirred tank reactor. The method consists of measuring the fluorescence intensity of a tracer excited by a thin planar laser sheet and in transforming it into an instantaneous concentration field of tracer by a calibration procedure. This allows the characterization of mixing in a plane defined as the cross section of the flow by the laser sheet. Flow visualization images have been recorded on video tape and subsequently digitized. The relationship between the intensity of the fluorescent light and the grey level of the images has been established. The first result is the instantaneous field of dye concentration. A contacting parameter between the fluids coming from the two inlet sources, and emphasizing the average state of the mixing, has been defined and its field has been determined. The field of temporal variance, which characterizes the segregation of the investigated zone, has also been computed. Received: 15 December 1995/Accepted: 28 April 1996     

Spectroscopic properties of a perfluorinated ketone for PLIF applications

This work identifies the fluorescence characteristics of a perfluorinated ketone, 2-trifluoromethyl-1,1,1,2,4,4,5,5,5-nonafluoro-3-pentanone, further referred to as fluoroketone. This compound is suitable for use with the third harmonic of an Nd:YAG laser for quantitative concentration measurements, as it exhibits strong emission even for relatively low excitation and has a near-linear response of fluorescence intensity with concentration. This makes it suitable for a broad range of fluorescence applications. The absorption cross-section of 3.81 × 10⁻¹⁹ cm² was found to be constant for a temperature range of 293–441 K and a pressure range of 1–18 atm. A calibration line has been generated that relates the concentration of gaseous and liquid fluoroketone with its absorption coefficient.     

A light attenuation technique for void fraction measurement of microbubbles

 A non-intrusive technique to measure the two-dimensional distribution of line averaged void fraction in a two-phase flow is discussed. A CCD camera is used to measure the attenuation of light as it passes through a bubbly flow, and this attenuation is related to the bubble concentration. The technique is appropriate for microbubbly flows where the bubble size is much smaller than the area imaged by a single pixel and where there are many bubbles attenuating light within each pixel. The measurement system is calibrated by using a two-dimensional line source microbubble plume as a reference. Revised: 30 March 2000/Accepted: 14 April 2000     

On the use of fluorescent dyes for concentration measurements in water flows

An experiment was performed to evaluate the characteristics of various fluorescent dyes used as tracers for concentration measurements in water flows, by laser induced fluorescence. Three common fluorescent dyes (fluorescein, rhodamine B and rhodamine 6G) were used, to select the most suitable fluorescent dye and identify its range of linear response. The results showed that, in terms of the stability of the solution, fluorescein is inferior to either rhodamine B or rhodamine 6G and that for concentrations of rhodamine B less than 0.08 mg/1 the response of fluorescent to the incident light is linear.     

Criteria for measuring mixing performance in a vessel with a rising free surface

A PLIF technique is described which enables the measurement of mixing performance in a 0.06 m square section vessel as the vessel is filled using a vertical dip pipe. The liquids used were Newtonian aqueous solutions of glycerol (μ = 3–12 mPa s); the values of Reynolds number of the entry jet ranged from 343 to 3,735. The vessel was initially charged with a small volume of fluorescent tracer, and a vertical plane passing through the vertical axis of the vessel was illuminated using a laser sheet. The fluorescence emitted by the dye was detected using a CCD camera and a calibration routine was developed to relate the measured fluorescence to the local dye concentration. The technique is shown to be capable of determining the mixing mechanism via visual inspection whilst log-variance (quantitative) analysis was employed to determine the degree of mixedness as a function of the Reynolds number and energy input per unit mass.     

Instantaneous and simultaneous planar velocity field measurements of two phases for turbulent mixing of high pressure sprays

This paper describes the tests of accuracy and the first application of a combined planar visualization technique. Its goal is two-phase flow discrimination, i.e. simultaneous measurements of velocity of droplets and ambient gas in the case of two-phase flow mixing, at the same location and with possible conditioning by “apparent diameter” (AD) of the droplets. It combines the mature techniques of particle image velocimetry (PIV), planar Mie scattering diffusion (PMSD), planar laser-induced fluorescence (PLIF), and it necessitates two synchronized cross-correlation systems, digital image treatment and analysis. This technique was developed with the objective of better describing the mixing between liquid and gaseous phases as in the case of high-pressure spray atomization in quiescent ambient gas. The basic principle of separation is to seed the ambient gas with micrometer particles and to tag the liquid with fluorescent dye. We use digital image treatment and analysis to discriminate between the phases. We use two cross-correlation PIV systems in order to obtain the velocity field of the droplets and gas simultaneously and separately at the same location. The digital image processing for separating the phases involves geometric measurement of droplet shapes. This leads to measurement of droplet parameters close to their real diameter, which could be used for analysis of actual mixing. A synchronized system composed of two CCD cameras is used for image recording, and two Nd:YAG lasers are used for generating pulsed light sheets at times t and t + δt. Tests were performed to check for different sources of errors. The combined technique was applied to measurements in high-pressure spray flow atomizing in a quiescent ambient gas, and first results are presented.     

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.     

New insight into two-color LIF thermometry applied to temperature measurements of droplets

When laser-induced fluorescence of droplets is used for measurements such as droplet temperature, a new dependence of the droplet size on the spectral distribution of fluorescence has been highlighted. The two-color laser-induced fluorescence technique applied to droplet temperature measurement requires a single fluorescent tracer and two spectral bands of detection for which the temperature sensitivity is different. Generally, the ratio of the intensities measured on each of the spectral bands of detection is assumed to be only temperature dependent. However, droplet dependence on diameter is also likely to influence the intensities ratio. This study provides some illustrations of the phenomenon, first on sprays with different mean statistical diameters and secondly on single droplets, for two temperature-sensitive fluorescent tracers in their solvents: sulforhodamine B dissolved in water and pyrromethene 597-8C9 dissolved in n-decane.