Quantitative planar imaging of turbulent buoyant jet mixing

2009 ◽  
Vol 643 ◽  
pp. 59-95 ◽  
Author(s):  
L. K. SU ◽  
D. B. HELMER ◽  
C. J. BROWNELL
Keyword(s):  
Mass Fraction ◽  
Rayleigh Scattering ◽  
Near Field ◽  
Outer Boundary ◽  
Turbulent Transport ◽  
Scalar Dissipation ◽  
Planar Imaging ◽  
Buoyant Jet ◽  
Jet Mixing ◽  
Dissipation Rates

Planar Rayleigh scattering provides quantitative mixing measurements in the developing region of axisymmetric turbulent helium jets issuing into air. The measurements focus on the relatively near field, in which the jets are primarily momentum driven. The imaging parameters are specified to ensure high spatial resolution. The mean jet fluid concentration fields attain self-similarity within the measurement region, though the forms of the mole fraction profiles indicate a reduction in turbulent transport at the jet outer boundary, arising from the reduced jet fluid density. Nevertheless, jet-like scaling pertains for the concentration fields. Mass fraction fluctuations on the jet centreline attain the expected asymptotic value of ≈23% of the centreline mass fraction values. The scalar dissipation rates, however, show an axial decay rate that is slower than theoretical predictions. The two-dimensional extent of the measurements also allows spatial filtering similar to that inherent in large-eddy simulations (LESs). The results confirm that fluctuation levels and scalar dissipation rates determined for the filtered fields are reduced as the effective resolution is reduced, but while the fluctuation profiles for the filtered fields are similar for the different filter sizes, the forms of the scalar dissipation profiles are highly dependent on filter size. These latter results in particular are of a form that will be useful for grid-dependent assessments of LES results.

Imaging of Turbulent Buoyant Jet Mixing

2006 ◽  
Author(s):  
David B. Helmer ◽  
Lester K. Su
Keyword(s):  
Mole Fraction ◽  
Rayleigh Scattering ◽  
Quantitative Imaging ◽  
Mean Field ◽  
Jet Flows ◽  
Buoyant Jet ◽  
Buoyant Jets ◽  
Scaling Properties ◽  
Jet Mixing ◽  
The Mean

This paper presents quantitative imaging measurements of jet fluid mole fraction fields in turbulent buoyant jets of helium issuing into air. The measurements use planar laser Rayleigh scattering. Signal levels are low, necessitating a novel approach to background subtraction in the signal processing. The jet flows considered are classified as momentum-driven, meaning that buoyancy effects are presumed to be confined to the small scales of the flow. We focus here on the near-nozzle, developing region of the jet, which is of particular interest to flows with combustion. The results suggest that buoyancy affects the details of the evolution of the mixing field even while the mean field maintains scaling properties consistent with non-buoyant jets. Specifically, the mean jet fluid mole fraction profiles show a sharper jet/ambient fluid interface relative to non-buoyant jets. The mole fraction fluctuations within the jet are also weaker than those reported in non-buoyant jets. These results will inform ongoing efforts to model the mixing process in flows with density differences, such as combustion systems.

scholarly journals Thermal and Scalar Dissipation Rates of Stretched Cylindrical Diffusion Flame

2013 ◽  
Vol 79 (804) ◽  
pp. 1685-1693 ◽  
Author(s):  
Yosuke SUENAGA ◽  
Hideki YANAOKA ◽  
Michio KITANO ◽  
Daisuke MOMOTORI
Keyword(s):  
Diffusion Flame ◽  
Scalar Dissipation ◽  
Dissipation Rates ◽  
Cylindrical Diffusion

Simulation of Scalar Mixing in Co-Axial Jet Flows Using an LES Method

2005 ◽  
Author(s):  
M. Dianat ◽  
D. Jiang ◽  
Z. Yang ◽  
J. J. McGuirk
Keyword(s):  
Prandtl Number ◽  
Turbulent Transport ◽  
Scalar Fields ◽  
Scale Model ◽  
Jet Flows ◽  
Turbulent Prandtl Number ◽  
Time Resolved ◽  
Jet Mixing ◽  
Cfd Models ◽  
Mixing Problem

The present paper describes a study that is aimed at establishing and quantifying the benefits of the Large Eddy Simulation (LES) method for predicting scalar turbulent transport in a combustor relevant jet-mixing problem. A non-reacting co-annular jet mixing configuration is considered for which comprehensive experimental data for both velocity and scalar fields have recently been obtained. Detailed comparisons are presented for the development of the axial velocity field in terms of both mean and turbulence intensity. Similarly, the mixing between the jets is examined by comparison with measurements for the mean concentration and the variance of concentration fluctuations. Agreement with these statistically averaged fields is demonstrated to be very good, and a considerable improvement over the standard eddy viscosity RANS approach. Illustrations are presented of the time-resolved information that LES provides such as time histories, and also conserved scalar pdf predictions. The LES results are shown, even using a simple Smagorinsky sub-grid-scale model, to predict correctly lower values of the turbulent Prandtl number (∼ 0.6) in the free shear regions of the flow, as well as higher values (∼ 1.0) in the wall-affected regions. The ability to predict turbulent Prandtl number variations (rather than input these as commonly done in most combustor RANS CFD models) is an important and promising feature of the LES approach for combustor simulation since it is known to be important in determining combustor exit temperature traverse.

On the Inflow Phenomenon in Near Field of Buoyant Jet at Low Froude Number

2012 ◽  
Author(s):  
Atsushi Maeda ◽  
Takayuki Yamagata ◽  
Nobuyuki Fujisawa
Keyword(s):  
Reynolds Number ◽  
Froude Number ◽  
Near Field ◽  
Quantitative Information ◽  
Experimental Result ◽  
Inflow Rate ◽  
Buoyant Jet ◽  
Square Duct ◽  
Wide Range ◽  
Low Froude Number

In the present paper, the inflow phenomenon in the near-field of a buoyant jet issuing from a square duct is studied by using scanning LIF and scanning PIV measurements. The scanning LIF visualization allows an insight into the critical condition of the inflow phenomenon in a wide range of Froude number and Reynolds number. While, the scanning PIV allows the quantitative information on the inflow rate through the duct exit. The experimental result shows that the critical Froude number increases with an increase in Reynolds number in the duct exit up to Reynolds number 2,000, though it is weakened at higher Reynolds number. The examination of the inflow rate indicates that the large magnitude of the inflow rate occurs in the lower Froude number and Reynolds number.

A second-order integral model for buoyant jets with background homogeneous and isotropic turbulence

2019 ◽  
Vol 871 ◽  
pp. 271-304 ◽  
Author(s):  
Adrian C. H. Lai ◽  
Adrian Wing-Keung Law ◽  
E. Eric Adams
Keyword(s):  
Isotropic Turbulence ◽  
Second Order ◽  
Integral Model ◽  
Length Scale ◽  
Buoyant Jet ◽  
Buoyant Jets ◽  
Jet Mixing ◽  
Mixing Characteristics ◽  
Background Turbulence ◽  
Homogeneous And Isotropic Turbulence

Buoyant jets or forced plumes are discharged into a turbulent ambient in many natural and engineering applications. The background turbulence generally affects the mixing characteristics of the buoyant jet, and the extent of the influence depends on the characteristics of both the jet discharge and ambient. Previous studies focused on the experimental investigation of the problem (for pure jets or plumes), but the findings were difficult to generalize because suitable scales for normalization of results were not known. A model to predict the buoyant jet mixing in the presence of background turbulence, which is essential in many applications, is also hitherto not available even for a background of homogeneous and isotropic turbulence (HIT). We carried out experimental and theoretical investigations of a buoyant jet discharging into background HIT. Buoyant jets were designed to be in the range of $1<z/l_{M}<5$, where $l_{M}=M_{o}^{3/4}/F_{o}^{1/2}$ is the momentum length scale, with $z/l_{M}<\sim 1$ and $z/l_{M}>\sim 6$ representing the asymptotic cases of pure jets and plumes, respectively. The background turbulence was generated using a random synthetic jet array, which produced a region of approximately isotropic and homogeneous field of turbulence to be used in the experiments. The velocity scale of the jet was initially much higher, and the length scale smaller, than that of the background turbulence, which is typical in most applications. Comprehensive measurements of the buoyant jet mixing characteristics were performed up to the distance where jet breakup occurred. Based on the experimental findings, a critical length scale $l_{c}$ was identified to be an appropriate normalizing scale. The momentum flux of the buoyant jet in background HIT was found to be conserved only if the second-order turbulence statistics of the jet were accounted for. A general integral jet model including the background HIT was then proposed based on the conservation of mass (using the entrainment assumption), total momentum and buoyancy fluxes, and the decay function of the jet mean momentum downstream. Predictions of jet mixing characteristics from the new model were compared with experimental observation, and found to be generally in agreement with each other.

scholarly journals Optimization ofs-Polarization Sensitivity in Apertureless Near-Field Optical Microscopy

2012 ◽  
Vol 2012 ◽  
pp. 1-6 ◽  
Author(s):  
Yuika Saito ◽  
Yoshiro Ohashi ◽  
Prabhat Verma
Keyword(s):  
Spatial Resolution ◽  
Rayleigh Scattering ◽  
Incident Light ◽  
Near Field ◽  
Polarized Light ◽  
Polarization Sensitivity ◽  
Experimental Setup ◽  
Experimental Conditions ◽  
General Belief ◽  
Polarization Configuration

It is a general belief in apertureless near-field microscopy that the so-calledp-polarization configuration, where the incident light is polarized parallel to the axis of the probe, is advantageous to its counterpart, thes-polarization configuration, where the incident light is polarized perpendicular to the probe axis. While this is true for most samples under common near-field experimental conditions, there are samples which respond better to thes-polarization configuration due to their orientations. Indeed, there have been several reports that have discussed such samples. This leads us to an important requirement that the near-field experimental setup should be equipped with proper sensitivity for measurements withs-polarization configuration. This requires not only creation of effective s-polarized illumination at the near-field probe, but also proper enhancement of s-polarized light by the probe. In this paper, we have examined thes-polarization enhancement sensitivity of near-field probes by measuring and evaluating the near-field Rayleigh scattering images constructed by a variety of probes. We found that thes-polarization enhancement sensitivity strongly depends on the sharpness of the apex of near-field probes. We have discussed the efficient value of probe sharpness by considering a balance between the enhancement and the spatial resolution, both of which are essential requirements of apertureless near-field microscopy.

Three-dimensional direct numerical simulation of a turbulent lifted hydrogen jet flame in heated coflow: a chemical explosive mode analysis

2010 ◽  
Vol 652 ◽  
pp. 45-64 ◽  
Author(s):  
T. F. LU ◽  
C. S. YOO ◽  
J. H. CHEN ◽  
C. K. LAW
Keyword(s):  
Numerical Simulation ◽  
Direct Numerical Simulation ◽  
Near Field ◽  
Three Dimensional ◽  
Mode Analysis ◽  
Scalar Dissipation ◽  
Complex Flows ◽  
Jet Flame ◽  
Auto Ignition ◽  
Grid Points

A chemical explosive mode analysis (CEMA) was developed as a new diagnostic to identify flame and ignition structure in complex flows. CEMA was then used to analyse the near-field structure of the stabilization region of a turbulent lifted hydrogen–air slot jet flame in a heated air coflow computed with three-dimensional direct numerical simulation. The simulation was performed with a detailed hydrogen–air mechanism and mixture-averaged transport properties at a jet Reynolds number of 11000 with over 900 million grid points. Explosive chemical modes and their characteristic time scales, as well as the species involved, were identified from the Jacobian matrix of the chemical source terms for species and temperature. An explosion index was defined for explosive modes, indicating the contribution of species and temperature in the explosion process. Radical and thermal runaway can consequently be distinguished. CEMA of the lifted flame shows the existence of two premixed flame fronts, which are difficult to detect with conventional methods. The upstream fork preceding the two flame fronts thereby identifies the stabilization point. A Damköhler number was defined based on the time scale of the chemical explosive mode and the local instantaneous scalar dissipation rate to highlight the role of auto-ignition in affecting the stabilization points in the lifted jet flame.

Turbulent Jet Mixing Enhancement and Control Using Self-Excited Nozzles

2007 ◽  
Vol 129 (7) ◽  
pp. 842-851 ◽  
Author(s):  
Uri Vandsburger ◽  
Yiqing Yuan
Keyword(s):  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Nozzle Exit ◽  
Large Scale ◽  
Rectangular Cross Section ◽  
Near Field ◽  
Excitation Frequency ◽  
Mixing Enhancement ◽  
Mixing Rate ◽  
Jet Mixing

A new self-excited jet methodology was developed for the mixing enhancement of jet fluid with its surrounding, quiescent, stagnant, or coflowing fluid. The nozzles, of a square or rectangular cross section, featured two flexible side walls that could go into aerodynamically-induced vibration. The mixing of nozzle fluid was measured using planar laser-induced fluorescence (PLIF) from acetone seeded into the nozzle fluid. Overall, the self-excited jet showed enhanced mixing with the ambient fluid; for example, at 390Hz excitation a mixing rate enhancement of 400% at x∕D=4 and 200% at x∕D=20 over the unexcited jet. The mixing rate was sensitive to the excitation frequency, increasing by 60% with the frequency changing from 200 to 390Hz (corresponding to a Strouhal number from 0.052 to 0.1). It was also observed that the mixing rate increased with the coflow velocity. To explain the observed mixing enhancement, the flow field was studied in detail using four-element hot wire probes. This led to the observation of two pairs of counter rotating large-scale streamwise vortices as the dominant structures in the excited flow. Shedding right from the nozzle exit, these inviscid vortices provided a rapid transport of the momentum and mass between the jet and the surrounding fluid at a length scale comparable to half-nozzle diameter. Moreover, the excited jet gained as much as six times the turbulent kinetic energy at the nozzle exit over the unexcited jet. Most of the turbulent kinetic energy is concentrated within five diameters from the nozzle exit, distributed across the entire jet width, explaining the increased mixing in the near field.

scholarly journals Scalar dissipation rates in non-conservative transport systems

2013 ◽  
Vol 149 ◽  
pp. 46-60 ◽  
Author(s):  
Nicholas B. Engdahl ◽  
Timothy R. Ginn ◽  
Graham E. Fogg
Keyword(s):  
Transport Systems ◽  
Scalar Dissipation ◽  
Dissipation Rates

Topology and Physical Interpretation of Turbulence Model Behavior on an Array of Film Cooling Jets

2019 ◽  
Author(s):  
Justin Hodges ◽  
Jayanta S. Kapat
Keyword(s):  
Turbulence Model ◽  
Film Cooling ◽  
Turbulent Viscosity ◽  
Near Field ◽  
Turbulent Transport ◽  
Reynolds Stresses ◽  
Data Set ◽  
Cooling Effectiveness ◽  
Film Cooling Effectiveness ◽  
Array Configuration

Abstract This study strives to provide a critical review and evaluation of influential constituent components in the two-equation k-ε turbulence model class, which are often employed in large full-fidelity gas turbine simulations in industry. All conjectures made, regarding the turbulence model behavior, will be compared to in-house experimental film effectiveness data from a novel film cooling array configuration. This experimental data set is comprised of high resolution adiabatic film cooling effectiveness measurements throughout two film cooling arrays comprised of diffused film holes of modern shape (streamwise inclination of α = 30°, expansion of Φ1,2,3 = 10°, AR = 3.4, and β = 0° relative to the crossflow direction). The difference between each film cooling array is only a staggered verses inline pattern for the film holes, whereby each has a uniform spacing of X/D = P/D = 8. Local coverage, laterally averaged film cooling effectiveness, and superposition analysis was quantified over a variety of testing conditions (M = 0.5, 0.75, 1.0, 1.5, 2.0, 2.5 at DR = 0.9). First, rendered quantifications of the anisotropic nature of the turbulence are shown throughout the near-field injection region, by leveraging the Reynolds stresses to form anisotropic-invariant maps. Next, results from k-ε models using a linear, a cubic, and a quadratic constitutive relation are compared. Furthermore, effective and conservative scaling of the production and destruction terms in the turbulent transport equations was performed, within reasonable bounds, and the resulting impact on the adiabatic film effectiveness was quantified. This scaling encompasses the Cε2 coefficient, as well as the Durbin realizibility coefficient used in the turbulent viscosity definition. Finally, various formulations of the turbulent Prandtl number were compared, with the resulting adiabatic film effectiveness observed.

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