Whole field measurement of temperature in water using two-color laser induced fluorescence

 A technique is described that measures the instantaneous three-dimensional temperature distribution in water using two-color laser-induced fluorescence (LIF). Two fluorescent dyes, Rhodamine B and Rhodamine 110, are used as temperature indicators. A laser light sheet scanned across the entire measurement volume excites the fluorescent dye, and an optical system involving a color beam splitter gives the intensity distribution of the individual fluorescent dyes on two separate monochrome CCD cameras. The ratio of these fluorescence intensities at each point of the image is calibrated against the temperature to eliminate the effect of the fluctuation of illuminating light intensity. A stable thermally stratified layer was measured by this system to evaluate the total accuracy of the measurement system. The random error of the measurement was ±1.4 K with 95% confidence. Measurements of thermal convection over a heated horizontal surface show temperature iso-surfaces having typical structures such as plumes, ridges and thermals. Received: 1 October 1997/Accepted: 23 March 1998     

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     

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.     

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     

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.     

Instantaneous density measurement in two-dimensional gas flow by high speed differential interferometry

The aim of this paper is to present an experimental set-up using a Wollaston prism differential interferometer producing up to twenty successive short exposure white light interferograms at a high framing rate. It is shown that, through optical component calibration, the interferograms can be analysed to yield the instantaneous density field. This method has been successfully tested in the two-dimensional unsteady flow generated by the interaction of a mixing layer and a cavity.List of symbols h height of the downstream edge of the cavity - H height of backward facing step - M Mach number - t time - t time interval between two successive frames - N frequency - double-prism median plane - birefringence angle - _p pressure fluctuation - C _p pressure coefficient - biprism abscissa corresponding to any colour - ₀ biprism reference abscissa corresponding to background colour - _y deviation of light rays - R radius of curvature of spherical mirror - L virtual distance from the middle of the test section to the spherical mirror - E optical thickness - E _e optical thickness corresponding to background colour - d _E difference of optical thickness - d _x abscissa difference - gas density - ₀ stagnation gas density - _e gas density of background colour     

Measurement of mixing processes with combined digital particle image velocimetry and planar laser induced fluorescence

A combined digital particle image velocimetry (DPIV) and planar laser induced fluorescence (PLIF) approach was developed to measure both the time mean and turbulent mass transport in mixing processes. The system couples the two well-known techniques to enable synchronized planar measurements of flow velocities and concentrations in a study area. The potential interference effect between the seeding particles for DPIV and the fluorescent dye excitation for PLIF was carefully investigated. The performance of the system was verified with the experimental results of a turbulent round jet discharging into a stagnant environment. Comparison between the measurements obtained in the present study with the large body of existing information on pure jets is satisfactory. The key advantage of the shorter duration required with this approach compared to point-based techniques is highlighted.     

Acetone laser induced fluorescence for low pressure/low temperature flow visualization

Acetone fluorescence provides a useful way to visualize the fluid mixing process within supersonic wind tunnels, some of which operate in the low temperature (240–300 K) and low pressure range (0.1–1 atm). Measurements are presented to quantify the dependence of the acetone laser induced fluorescence (LIF) signal on temperature and pressure in this range. The temperature and pressure sensitivity of the acetone LIF signal resulted in less than an 8% variation over the experimental conditions for a laser excitation wavelength of 266 nm. Condensation of the acetone vapor was identified as a potential problem for this diagnostic technique. Methods to prevent and check for condensation are discussed. Received: 5 October 1998/Accepted: 10 April 1999     

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.     

Passive scalar measurements in a planar mixing layer by PLIF of acetone

 Quantitative passive scalar measurements were performed in an incompressible planar mixing layer at Re _δ up to 10⁴ using planar laser-induced fluorescence of acetone seeded into one side of the layer. Probability density functions compiled from sets of images showed a preferred mixture composition, favoring the high-speed fluid, which extended across the layer. This preferred composition produced non-marching PDFs and an inflection in the average mixture fraction profile. The spatial resolution of the experiment was found to be sufficient to accurately measure the fraction of mixed fluid within the layer. The mixed fluid fraction was found to increase to an asymptotic value of 0.5 by Re _δ  ≈ 5,000, the approximate location of turbulent transition, in contrast to high Schmidt number experiments which show minimal mixing before the transition point. Received: 5 October 1999 / Accepted: 9 February 2001     

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.     

Dust entrainment by means of a planar shock induced vortex over loose dust layers

Conclusions The recent study of Ben-Dor and Rayevsky (1994) regarding the interaction of planar shock waves with high density layers was reconsidered in order to demonstrate a possible dust entrainment mechanism which has not received appropriate attention so far. It was shown that, as a result of the interaction, a large vortex is generated. This vortex could in fact contribute to the entrainment of dust when planar shock waves interact with loose dusty layers.In addition, the effect of viscosity on the proposed dust entrainment mechanism was also considered.     

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.     

Measurement of the temperature distribution within monodisperse combusting droplets in linear streams using two-color laser-induced fluorescence

Two-color laser-induced fluorescence can be use to perform space-averaged flying droplet temperature measurements. In this paper, the possibility to extend this technique to the measurement of the temperature distribution within a moving combusting droplet is considered and demonstrated. This technique may provide new experimental data related to the heat diffusion in liquid fuel droplets injected in high-temperature gas streams, for example, in combustion chambers. The main principles of the technique and the data reduction process are discussed, and a test on combusting a monodisperse ethanol droplets (200 m in diameter) stream is presented.Nomenclature a _i , b _i temperature sensitivity coefficients for i th spectral band - C molecular concentration of fluorescent tracer - D droplet diameter - I ₀ incident laser beam intensity - I _f fluorescence intensity - K _opt optical constant - K _spec spectroscopic constant - V _c collection volume - R _f fluorescence ratio - T absolute temperature - T _i injection temperature - V _i injection velocity - ( x, y , z) spatial coordinates Greek symbols temperature sensitivity coefficient     

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     

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.     

Quantitative measurement of depth–averaged concentration fields in microchannels by means of a fluorescence intensity method

We have inferred and verified an optical measuring technique, based on fluorescence intensity, for the species concentration field in a liquid microchannel flow. In contrast to macroscopic flows, where a light sheet is usually applied, the complete illumination of the measuring volume in the microchannel provides a height-averaged concentration field. The technique was verified with aqueous solutions within a square glass microchannel of 110 μm width. The verification experiments confirm both a good spatial resolution and a good accuracy of even the concentration gradients within a concentration boundary layer.     

Comparison of turbulent diffusion flame measurements of OH by planar fluorescence and saturated fluorescence

Planar laser induced fluorescence imaging (PLIF) is shown to be a quantitative method of measuring average values, rms fluctuations, and probability density functions of OH concentration in laboratory scale, H₂-air diffusion flames. When compared with single-pulse laser saturated fluorescence (LSF) data, PLIF data show agreement (within a factor of two) for average and rms values in laminar, transitional, and turbulent flames. The unknown temperature dependence of the H₂ quenching cross section introduces a factor of two uncertainty in PLIF measurements in rich flame zones. Extensions of PLIF to other molecules and other combustion systems are discussed.A version of this paper was presented at the ASME Winter Annual Meeting of 1984     

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.