Application of rainbow schlieren deflectometry to measure temperature and oxygen concentration in a laminar gas-jet diffusion flame

The quantitative rainbow schlieren deflectometry (RSD) technique was applied to measure temperature and oxygen concentration in an axisymmetric hydrogen gas-jet diffusion flame burning in quiescent air at fuel jet exit Reynolds number of 70. Schlieren measurements were compared with conventional measurements using a thermocouple and a gas-sampling probe. Good agreement between the two measurement techniques was achieved on the fuel-lean side of the flame.     

Measurements of a supersonic turbulent boundary layer by focusing schlieren deflectometry

 Some novel, non-intrusive, high-frequency, localized optical measurements of turbulence in compressible flows are described. The technique is based upon focusing schlieren optics coupled with high-speed quantitative measurement of light intensity fluctuations in the schlieren image. Measurements of density gradient fluctuations confined to a thin slice of the flowfield are thus obtained. The new instrument was used to investigate the structure of a two-dimensional, adiabatic, wind tunnel wall boundary layer at a Mach number of 3. The measurements were compared to data obtained using hot-wire anemometry and good agreement was found between the two. Distributions of broadband convection velocity of large-scale structures through the boundary later were also measured. In marked contrast to earlier results, it is shown here that the convection velocity is essentially identical to the local mean velocity. Further, results obtained using the VITA conditional sampling technique shed new light on the turbulent boundary layer structure. Overall, the data presented herein serve to validate the new measurement technique. Received: 12 February 1997/Accepted: 31 January 1998     

Dilution and mixing in an unsteady jet

 The mixing characteristics of a round, turbulent, unsteady jet were studied experimentally. A gravity-driven flow was created by releasing dyed fluid from a vertical tube into a large water tank. The jet velocity increased from zero to a maximum and then decreased continuously such that each run lasted about  s. The jet dilution was examined by an optical absorption technique that measured the line integral of concentration across the jet diameter. These measurements revealed that the portion of the unsteady jet corresponding to the deceleration phase dilutes more than the steady jet. The molecular scale mixing, as deduced from an acid-base neutralization reaction, corroborated the finding that the jet mixes in a shorter distance than the steady jet. Received: 22 August 1996/Accepted: 4 February 1997     

Mixing in a confined turbulent impinging jet using planar laser-induced fluorescence

 The non-intrusive Planar Laser-Induced Fluorescence (PLIF) technique was applied to the study of the mixing of a turbulent water jet impinging orthogonally onto a flat surface. A procedure for calibrating the system at each pixel of a CCD camera array was first developed and tested. Post-processing of the PLIF data gave quantitative results of good quality. The mixing at the entrance of the deflection zone was also investigated. Average concentration fields in the centre plane of the jet were calculated and compared with Large Eddy Simulations (LES) and also with data from the literature. Probability density functions, space coefficients of correlation and radial concentration fluctuation profiles were calculated to further quantify the spreading of the jet, both in the free and deflection zones. Inside the deflection region, a slight tendency towards intensified mixing at the outer edge of the jet was found. This was attributed to a deceleration of the fluid which resulted in accelerated diffusion. Received: 11 July 1997 / Accepted: 9 January 1998     

Flowfield characterisation in the wake of a low-velocity heated sphere anemometer

Heated sphere anemometers (HSA) are the most widely used instruments for low-velocity measurements in the heating, ventilation and air-conditioning industry. Experiments were conducted to characterise the flowfield around the spherically shaped sensor and upper probe assembly of a HSA. Particle image velocimetry was the main quantitative experimental technique. Measurements of the flowfield around a HSA probe and a 2:1 scaled-up model were performed in a uniform isothermal axisymmetrical jet air flow at Re around 350, based on sensor diameter, for different pitch angle incident flows. Additionally, extensive flow visualisation studies around scaled-up models of the HSA probe were performed. Received: 24 April 2001/Accepted: 16 December 2001     

Experiments with large diameter gravity driven impacting liquid jets

 The phenomenon of a liquid jet released under gravity and falling through or impacting onto another liquid before colliding with an obstructing solid surface has been studied experimentally under isothermal conditions. Usually the jet diameter was sufficiently large to ensure jet coherency until collision. Direct flow visualization was used to study jets released into water pools with no air head space and jets impacting onto water pools after falling through an air head space. It is shown that distances predicting the onset of buoyancy and the entrainment of air using derivations from continuous plunging jets, are not applicable for impacting jets. The morphology of jet debris after collision with the solid surfaces correlates with the wetting properties of the jet liquid on the surface. Received: 28 November 1997 / Accepted: 21 May 1998     

Role of Buoyancy on Instabilities and Structure of Transitional Gas Jet Diffusion Flames

Transitional jet diffusion flames provide the link between dynamics of laminar and turbulent flames. In this study, instabilities and their interaction with the flow structure are explored in a transitional jet diffusion flame, with focus on isolating buoyancy effects. Experiments are conducted in hydrogen flames with fuel jet Reynolds number of up to 2,200 and average jet velocity of up to 54 m/s. Since the fuel jet is laminar at the injector exit, the transition from laminar to turbulent flame occurs by the hydrodynamic instabilities in the shear layer of fuel jet. The instabilities and the flow structures are visualized and quantified by the rainbow schlieren deflectometry technique coupled with a high-speed imaging system. The schlieren images acquired at 2,000 frames per second allowed exposure time of 23 μs with spatial resolution of 0.4 mm. Results identify a hitherto unknown secondary instability in the flame surface, provide explanation for the observed intermittency in the breakpoint length, show coherent vortical structures downstream of the flame breakpoint, and illustrate gradual breakdown of coherent structures into small-scale random structures in the far field turbulent region.     

An iterative method for temperature gradient measurements in two-dimensional thermal boundary layer flows by speckle photography

An iterative method for measuring two-dimensional refractive index fields induced by convective heat transfer phenomena is presented. Starting from the so-called correction parabola, this reconstruction technique takes into account the local second derivative variation of the refractive index field. The efficiency of the method is analytically defined as a function of non-dimensional parameter and numerically investigated using a ray-tracing code for different classic index profiles and a thermal boundary layer case. Finally, as an implementation test, this algorithm is applied to an impinging jet heat transfer experiment using speckle photography measurement data set. The results show a relative dispersion of 20% compared to the parabola reconstruction when the refractive index gradient becomes more severe.     

Refractive index matching in large-scale stratified experiments

The measurement of dilution in jets and plumes by planar laser-induced fluorescence in large-scale stratified environments with refractive index matching is discussed. The density and refractive index of various salt (NaCl) and ethanol solutions were measured to high precision, and it is shown that linear stratifications with density differences at least up to 20 σ _t with acceptably small refractive index variations can be achieved at reasonable cost. Equations are presented to facilitate the design of experiments with arbitrary density differences. The presence of ethanol significantly increases the laser attenuation, however. The rate of attenuation was measured for various solute concentrations and equations presented to predict the attenuation for arbitrary salt and ethanol concentrations. The rapid attenuation due to ethanol may limit the maximum density differences attainable with long laser traverse distances. Examples are given of discharges into linearly stratified fluids including a jet in a stationary environment, and a buoyant jet in a cross flow. Received: 9 April 1999/Accepted: 8 November 2000     

Color schlieren methods in shock wave research

Schlieren methods are widely known and well established to visualize refractive index variations in transparent media. The use of color allows one to obtain more data and previously inaccessible information from a picture taken with this technique. In general, a hue can be related to a certain strength or a certain direction of a refractive index gradient. While the first case essentially corresponds to the usual black- and-white system the latter correlation cannot be made adequately evident without the use of color. Two color schlieren techniques are presented here, which reach or even exceed the quality and sensitivity range of conventional black- and-white methods. Using a powerful short duration light source these methods are applied to visualize transient flow phenomena in a shock tube.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1990.     

Flow visualization of interactions among large coherent structures in an axisymmetric jet

Interactions between large coherent structures are visualized with both schlieren photography in two air jets and dye photography in a water jet. The density difference needed for the schlieren technique is provided by an electrically heated wire ring surrounding the jet. External forcing with either single axisymmetric, single non-symmetric, combined axisymmetric or combined non-symmetric modes was applied. It was found that forcing the jet with a pair of different spinning modes leads to azimuthal distortions of the mean flow. This observation confirms and explains existing hotwire data. Simultaneous excitation with two axisymmetric modes may produce structures of higher modes or even cause structurally undistinguishable development. Streamwise structures are observed both in the unforced jet and in the axisymmetrically forced jet. They do not seem to be caused by a Görtier instability from the concave curvature of the conventional nozzle, since they were also found in a jet flow from a specially designed nozzle with only convex contraction surface.Supported in part by the National Science Foundation under Grant No. MSM 8900086 and by the Deutsche Forschungsgemeinschaft DFG, Fi178/34-1     

Visualization of a high-speed,luminous plasma jet using a focusing schlieren technique

The near-field structure of a luminous, high-speed plasma jet was visualized using a stroboscopic focussing schlieren technique. The glow from the jet and the effects of convective currents in the enclosure are overcome by this method thus providing greater details of the near-exit region of the jet.The work was supported by the National Science Foundation under the grant # NSF-DDM-9215846. The authors would like to thank R. Gansert for his help in conducting the experiments.     

Full-field measurements of self-excited oscillations in momentum-dominated helium jets

Flow structure of momentum-dominated helium jets discharged vertically into ambient air was investigated using a high-speed rainbow schlieren deflectometry (RSD) apparatus operated at up to 2,000 Hz. The operating parameters, i.e., Reynolds number and Richardson number were varied independently to examine the self-excited, flow oscillatory behavior over a range of experimental conditions. Measurements revealed highly periodic oscillations in the laminar region at a unique frequency as well as high regularity in the flow transition and initial turbulent regions. The buoyancy was shown to affect the oscillation frequency and the distance from the jet exit to the flow transition plane. Instantaneous helium concentration contours across the field of view revealed changes in the jet flow structure and the evolution of the vortical structures during an oscillation cycle. A cross-correlation technique was applied to track the vortices and to find their convection velocity. Time traces of helium concentration at different axial locations provided detailed information about the oscillating flow.     

Tulip flames: changes in shape of premixed flames propagating in closed tubes

 The experimental results that are the subject of this communication provide high-speed schlieren images of the closed-tube flame shape that has come to be known as the tulip flame. The schlieren images, along with in-chamber pressure records, help demonstrate the effects of chamber length, equivalence ratio, and igniter geometry on formation of the tulip flame. The pressure/time records show distinct features which correlate with flame shape changes during the transition to tulip. The measurements indicate that the basic tulip flame formation is a robust phenomenon that depends on little except the overall geometry of the combustion vessel. Received: 22 April 1997/Accepted: 7 July 1997     

Residence time measurement of an isothermal combustor flow field

Residence times of combustors have commonly been used to help understand NO_x emissions and flame blowout. Both the time mean velocity and turbulence fields are important to the residence time, but determining the residence time via analysis of a measured velocity field is difficult due to the inherent unsteadiness and the three-dimensional nature of a high-Re swirling flow. A more direct approach to measure residence time is reported here that examines the dynamic response of fuel concentration to a sudden cutoff in the fuel injection. Residence time measurement was mainly taken using a time-resolved planar laser-induced fluorescence (PLIF) technique, but a second camera for particle image velocimetry (PIV) was added to check that the step change does not alter the velocity field and the spectral content of the coherent structures. Characteristic timescales evaluated from the measurements are referred to as convection and half-life times: The former describes the time delay from a fuel injector exit reference point to a downstream point of interest, and the latter describes the rate of decay once the effect of the reduced scalar concentration at the injection source has been transported to the point of interest. Residence time is often defined as the time taken for a conserved scalar to reduce to half its initial value after injection is stopped: this equivalent to the sum of the convection time and the half-life values. The technique was applied to a high-swirl fuel injector typical of that found in combustor applications. Two test cases have been studied: with central jet (with-jet) and without central jet (no-jet). It was found that the relatively unstable central recirculation zone of the no-jet case resulted in increased transport of fuel into the central region that is dominated by a precessing vortex core, where long half-life times are also found. Based on this, it was inferred that the no-jet case may be more prone to NO_x production. The technique is described here for a single-phase isothermal flow field, but with consideration, it could be extended to studying reacting flows to provide more insight into important mixing phenomena and relevant timescales.     

Rainbow refractrometry on particles with radial refractive index gradients

The rainbow refractrometry, under its different configurations (classical and global), is an attractive technique to extract information from droplets in evaporation such as diameter and temperature. Recently a new processing strategy has been developed which increases dramatically the size and refractive index measurements accuracy for homogeneous droplets. Nevertheless, for mono component as well as for multicomponent droplets, the presence of temperature and/or of concentration gradients induce the presence of a gradient of refractive index which affects the interpretation of the recorded signals. In this publication, the effect of radial gradient on rainbow measurements with a high accuracy never reached previously is quantified.     

Color schlieren deflectometry study of jet mixing: effect of buoyancy and perforation

Mixing of jets is crucial for optimal performance of many industrial applications and there is a need to optimize both nozzle geometry and flow conditions. The present study reports the influence of buoyancy and perforation on mixing between a jet and its environment. Optical techniques are ideal for the study of jet mixing due to their non-intrusive and inertia free properties. The present study gives an account of mixing between helium jet and the ambient fluid using a combination of color schlieren deflectometry and radial tomographic mathematics. Four different perforation sizes have been used and the experiments are performed for Reynolds numbers 21–676 and Richardson numbers 3.27–0.0015. Color schlieren images show distinct influence of perforation and flow conditions (Richardson number). Oxygen concentration and jet width quantify effectiveness of jet mixing. Buoyancy plays an important role in mixing at high Richardson number. Perforation improves jet mixing i.e. there is about 120% increase in jet width and the size of perforation plays an important role.     

Time-resolved velocity and concentration measurements in variable-viscosity turbulent jet flow

This paper addresses the ability to reliably measure the fluctuating velocity field in variable-viscosity flows (herein, a propane–air mixture), using hot-wire anemometry. Because the latter is sensitive to both velocity and concentration fluctuations, the instantaneous concentration field also needs to be inferred experimentally. To overcome this difficulty, we show that the hot-wire response becomes insensitive to the concentration of the field, when a small amount of neon is added to the air. In this way, velocity measurements can be made independently of the concentration field. Although not necessary to velocity measurements, Rayleigh light-scattering technique is also used to infer the local (fluctuating) concentration, and, therefore, the viscosity of the fluid. Velocity and concentration measurements are performed in a turbulent propane jet discharging into an air–neon co-flow, for which the density and viscosity ratios are 1.52 and 1/5.5, respectively. The Reynolds number (based on injection diameter and velocity) is 15400. These measurements are first validated: the axial decay of the mean velocity and concentration, as well as the lateral mean and RMS profiles of velocity and concentration, is in full agreement with the existing literature. The variable-viscosity flow along the axis of the round jet is then characterized and compared with a turbulent air jet discharging into still air, for which the Reynolds number (based on injection diameter and velocity) is 5400. Both flows have the same initial jet momentum. As mixing with the viscous co-flow is enhanced with increasing downstream position, the viscosity of the fluid increases rapidly for the case of the propane jet. In comparison with the air jet, the propane jet exhibits: (1) a lower local Reynolds number based on the Taylor microscale (by a factor of four); (2) a reduced range of scales present in the flow; (3) the isotropic form of the mean energy dissipation rate is first more enhanced and then drastically diminishes and (4) a progressively increasing local Schmidt number (from 1.36 to 7.5) for increasing downstream positions. Therefore, the scalar spectra exhibit an increasingly prominent Batchelor regime with a ~ k ^?1 scaling law. The experimental technique developed herein provides a reliable method for the study of variable-viscosity flows.     

Dual hologram shearing interference technique for wind tunnel flow fields testing

 A novel optical diagnostic technique, dual hologram shearing interferometry, for measuring density gradients of different phase objects is proposed and demonstrated. The lateral shearing is achieved by using a phase grating. A holographic interferometer has been developed and designed on the base of a single pass Z type conventional schlieren device. The interferometer’s scheme is insensitive to acoustical disturbances, similarly to the conventional schlieren layout, and is capable of recording holograms with a continuous wave laser during the wind tunnel run. The features of the technique make it tolerant to both the temporal coherence of the laser light source and to the relatively low, schlieren quality optical windows of the wind tunnel’s test section. The obtained reconstructed lateral shearing interferograms with a large region of overlap have high contrast and may have an arbitrary orientation and/or spacing of the background interference fringes. It is believed that the proposed approach will become a useful tool for visualization and accurate mapping of the density gradients of gas dynamic flow fields, in wind and shock tunnels, where acoustic noise problems may dramatically affect reference beam holographic schemes. Received: 9 January 1997 / Accepted: 12 April 1997