Visualizing near-surface concentration fluctuations using laser-induced fluorescence

A method for observing near-surface fluctuations in pH caused by a water–air flux of carbon dioxide under conditions of ambient atmospheric carbon dioxide levels is developed and tested. Peaks in fluorescence intensity measured as a function of pH and turbulence are shown to be consistent with predictions from a chemical kinetics model of CO₂ exchange. The square root of the frequency of the pH fluctuations scale linearly with independently measured bulk air–water gas transfer velocities in agreement with surface divergence models for air–water gas transfer. These data indicate that the method proposed here is tracking changes in near-surface CO₂ concentrations. This laser-induced fluorescence method can be used to study the air–water exchange of CO₂ in wind-wave tunnels without the need for elevated CO₂ concentrations in the gas phase.     

Time-average measurement of velocity, density, temperature, and turbulence velocity fluctuations using Rayleigh and Mie scattering

Measurement of time-averaged velocity, density, temperature, and turbulence velocity fluctuations in sparsely seeded gas flows using a non-intrusive, point-wise technique based on Rayleigh and Mie scattering is discussed. A Fabry-Perot interferometer (FPI) is used to spectrally resolve laser light scattered by molecules and particulates in gas flows. The spectral content of the scattered light provides information about velocity, density, and temperature of the gas. A CCD camera is used to record images of the fringes formed by scattered light passing through the interferometer. Models of the spectral components are used in a least squares fitting routine to estimate the parameters from fringe images. Flow measurements are presented for subsonic and supersonic jet flows. The application range for this technique is mostly for high velocity situations (>25 m/s). Velocity, density, temperature, and turbulence velocity fluctuations were determined with accuracies within 5 m/s, 4%, 2%, and 5 m/s, respectively.     

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.     

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     

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.     

Total pressure fluctuations and two-phase flow turbulence in hydraulic jumps

The large-scale turbulence and high air content in a hydraulic jump restrict the application of many traditional flow measurement techniques. This paper presents a physical modelling of hydraulic jump, where the total pressure and air–water flow properties were measured simultaneously with intrusive probes, namely a miniature pressure transducer and a dual-tip phase-detection probe, in the jump roller. The total pressure data were compared to theoretical values calculated based upon void fraction, water depth and flow velocity measured by the phase-detection probe. The successful comparison showed valid pressure measurement results in the turbulent shear region with constant flow direction. The roller region was characterised by hydrostatic pressure distributions, taking into account the void fraction distributions. The total pressure fluctuations were related to both velocity fluctuations in the air–water flow and free-surface dynamics above the roller, though the time scales of these motions differed substantially.     

Instantaneous profiles and turbulence statistics of supersonic free shear layers by Raman excitation plus laser-induced electronic fluorescence (Relief) velocity tagging of oxygen

A new method of flow tagging based on the vibrational excitation of oxygen is applied to both supersonic and high-speed subsonic air flows to generate instantaneous velocity profiles and turbulence statistics across the free shear layer. By simultaneously tagging two lines, both transverse and streamwise velocity correlations are found. Rayleigh scattering can also be imaged, so this flow diagnostic technique has the capability of instantaneously recording density cross sections and velocity profiles. A version of this paper was presented at the 11th Symposium on Turbulence, University of Missouri-Rolla, Oct. 17–19, 1988     

Evaluation of temperature gradients within combusting droplets in linear stream using two colors laser-induced fluorescence

The scope of this paper concerns the heating process of fuel droplets injected in a hot gaseous environment. The two colors laser-induced fluorescence technique allows measuring the temperature distribution within a droplet by scanning the droplet volume by a sufficiently small probe volume compared to the droplet volume itself. The temperature field is reconstructed using two approaches which have been compared. One is based on a geometrical optics model and the other is based on the 3D calculation of the internal excitation field within the droplet, using the generalized Lorenz-Mie theory. Experimental results have been obtained on a combusting monodisperse ethanol droplet stream (diameter around 200 m).     

Production of temperature fluctuations in grid turbulence: Wiskind's experiment revisited

Measurements have been made in nearly-isotropic grid turbulence on which is superimposed a linearly-varying transverse temperature distribution. The mean-square temperature fluctuations, , increase indefinitely with streamwise distance, in accordance with theoretical predictions, and consistent with an excess of production over dissipation some 50% greater than values recorded in previous experiments. This high level of production has the effect of reducing the ratio,r, of the time scales of the fluctuating velocity and temperature fields. The results have been used to estimate the coefficient,C, in Monin's return-to-isotropy model for the slow part of the pressure terms in the temperature-flux equations. An empirical expression by Shih and Lumley is consistent with the results of earlier experiments in whichr 1.5, C 3.0, but not with the present data where r 0.5, C 1.6. Monin's model is improved when it incorporates both time scales.List of symbols C coefficient in Monin model, Eq. (5) - M grid mesh length - m exponent in power law for temperature variance, x ^m - n turbulence-energy decay exponent,q ² x ^-n - p production rate of - p pressure - q ² - R microscale Reynolds number - r time-scale ratiot/t - T mean temperature - U mean velocity - mean-square velocity fluctuations (turbulent energy components) - turbulent temperature flux - x, y, z spatial coordinates - temperature gradient dT/dy - thermal diffusivity - dissipation rate ofq ²/2 - dissipation rate of - Taylor microscale (₂=5q₂/) - temperature microscale - _v temperature-flux correlation coefficient, /v - dimensionless distance from the grid,x/M     

Measurements of concentration per size class in a dense polydispersed jet using planar laser-induced fluorescence and phase Doppler techniques

In dense two-phase flows, it is well known that phase Doppler anemometry is not well suited for the measurement of concentration and mass flux. Laser diagnostics based on fluorescence can provide the dispersed phase concentration but without discrimination between size classes. We present a new method of coupling the two techniques, in order to extract the local value of concentration and flux per size class. The method is applied to an axisymmetric turbulent jet, laden with polydispersed droplets 1–90 μm. Droplet concentration profiles are obtained in the development zone (x/d ₀ < 20) of the dense jet and are used to study droplet dispersion. The results are then introduced into the momentum transport equations to analyze the influence of droplets on the carrier phase. We show that the local decrease of the rate of variation of mean momentum with mass loading is due both to an increase in interfacial transfer rate and to a decrease in turbulent diffusion effects. Received: 20 November 2000 / Accepted: 3 April 2001     

Flow-thermodynamics interactions in decaying anisotropic compressible turbulence with imposed temperature fluctuations

The fundamental nature of the non-linear flow-thermodynamics interactions in a compressible turbulent flow with imposed temperature fluctuations is investigated. Direct numerical simulations (DNS) of decaying anisotropic compressible turbulence (turbulent Mach number 0.06–0.6) with imposed temperature fluctuations are performed to examine: (i) interactions between solenoidal and dilatational kinetic energy; (ii) partition between dilatational kinetic energy and thermodynamic potential energy; and (iii) redistribution of solenoidal and dilatational kinetic energy among the various Reynolds stress components. It is found that solenoidal kinetic energy levels and return-to-isotropy are weakly dependent on Mach number but independent of imposed temperature fluctuations in the parameter range studied. The dilatational kinetic energy generated is proportional to the square of the pressure fluctuations associated with the initial solenoidal and temperature fluctuations and thus a strong function of Mach number and heat release intensity. The energy exchange between dilatational kinetic and potential energy is driven by a strong proclivity toward equipartition. Consequently, the dynamics of pressure-dilatation ( ${\overline{pd}}$ ), which is the mechanism of this energy exchange between dilatational and potential energies, is dictated entirely by the requirement to impose energy equipartition. Based on the results, we provide a physical picture of the solenoidal–dilatational–potential energy interactions and the action of pressure-dilatation. The identification of the fundamental precepts underlying the various interactions is of great utility for turbulence closure model development.     

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     

Dependence of laser-induced fluorescence on gas-dynamic fluctuations with application to measurements in unsteady flows

Acoustic waves produced in a gas-filled cell under conditions of resonance are used to study the relation between laser-induced fluorescence and a known disturbed gas state. The objective of the work is to test the theory and to develop a method for making a point measurement of fluctuating pressure in a compressible flow. Iodine was used as a seed gas in nitrogen and the iodine was excited by the 514.5 rim. ouput of an argon-ion laser. The experiments confirm the theoretical prediction that the fractional change in the fluorescence signal is approximately equal to the fractional change in the pressure, but with an opposite sign, when the laser wavelength is tuned to the iodine absorption line center and the pressure is high. Pressure signals of the order of one percent of the undisturbed pressure (40 to 760 Torr) were measured in the test cell using the fluorescent signal.     

Wall pressure fluctuations in the reattachment region of a supersonic free shear layer

A study was made of the wall pressure fluctuations in the reattachment region of a supersonic free shear layer. The free shear layer was formed by the separation of a Mach 2.9 turbulent boundary layer from a backward facing step. Reattachment occurred on a 20° ramp. By adjusting the position of the ramp, the base pressure at the step was set equal to the freestream pressure, and the free shear layer formed in the absence of any turning. An array of flush-mounted, miniature, high-frequency pressure transducers was used in the vicinity of the reattachment region to make multichannel measurements of the fluctuating wall pressure. Contrary to previous observations of this flow, the reattachment region was found to be highly unsteady, and the pressure fluctuations were found to be large. The overall behavior of the wall pressure loading is similar in scale and magnitude to the unsteadiness of the wall pressure field in compression ramp flows at the same Mach number. Rayleigh scattering was used to visualize the instantaneous shock structure in the streamwise and spanwise direction. Spanwise wrinkles on the order of half the boundary layer thickness were observed on the shock sheet.     

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     

Temperature measurements of binary droplets using three-color laser-induced fluorescence

Evaporation of multicomponent droplets is a critical problem in many engineering applications, for example spray combustion. Knowledge of droplet temperature is a key issue in understanding the highly complex heat and mass-transfer phenomena related to multicomponent droplet evaporation and combustion. In this work, optical diagnosis based on three color-laser-induced fluorescence was developed: the objective was to measure the temperature of binary droplets (ethanol and acetone mixtures), even when the composition varies with time. Demonstration on an overheated droplet stream of acetone–ethanol mixtures is described and the experimental data are compared with results from a numerical simulation based on the discrete-components model.