Optical Diagnostics Overview

Plasma Ignition

Thu, 17 Apr 2008 09:04:07 -0400

The principal objective of this program is the development of a novel ignition system for high-speed and -altitude propulsion systems based upon non-equilibrium plasma effects. Generally, ignition systems utilized today function through brute-force electrical heating of the combustion gas mixture.

This is, by far, the most inefficient way to heat and dissociate the gas. The program we are undertaking will investigate the use of non-equilibrium plasmas for the increased production of radical species that are important to flame ignition and propagation. Recent experiments with H2 - N2 plasmas indicate that substantial dissociation can occur at very low plasma energies because of multiquantum vibrational energy-transfer processes. 1, 2 As a result, much lower energies are needed for dissociation; this could translate into smaller, more reliable ignition systems.

We are using a low-pressure parallel-plate pulse discharge to investigate the effect of discharge pulse characteristics on the production of H-atom radicals in a H2 - N2 gas mixture. Two-Photon Allowed Laser-Induced Fluorescence (TALIF) is used to monitor H-atom radical production. 3 Two photons excite the H-atom radicals, present in the discharge, from the ground state to an excited state.
The Balmer alpha emission from the H-atom excited states, at 656 nm, is detected by a photomultiplier tube. The TALIF technique will be used to monitor the production of H-atom radicals as a function of discharge rise- and fall-time and pulse duration. The optimum discharge conditions can be determined for H-atom production.
For further information on this work, please contact Dr. James Williamson at (937) 252-2706.
This work is supported by Dr. B. N. Ganguly (Air Force Research Lab, Wright-Patterson AFB, Ohio).

Plasma Shielding

Thu, 17 Apr 2008 08:51:47 -0400

Shock Remediation

For several years Russian researchers have reported on changes in shockwaves when they propagate through an electrical discharge plasma. The shockwaves were observed to decrease in amplitude, suffer dispersion, and increase in velocity. Various explanations for these observations were advanced such as passage through thermal gradients in the gas or energy coupling with the vibrationally excited gas.
Since this effect, if it can be reproduced on a large scale, may decrease the drag caused by shockwaves in supersonic flight and also improve the combustion process in supersonic combustors, we have started a program to investigate shockwave propagation in low temperature, non-equilibrium discharges. We have verified the effects in small scale experiments.
In addition, we have observed that these effects can not solely be explained by thermal gradients. Also, since we have observed similar results in rare gases, coupling to excited vibrational energy levels cannot provide the sole explanation.
Our experimental setup is shown in Fig.#1. We use four laser beams whose optical deflections by the shock density gradients are detected by four detectors. The time difference between each detector pair gives the local velocity between these detectors. In this way we can measure the local velocity at two positions in the uniform positive column of the discharge. Laser signals for two detectors are shown in Fig.#2 for neutral gas (0 mA) and for a current of 40 mA (nitrogen gas at 30 Torr pressure)1.
Note that the shock front as displayed by the laser deflection signal is greatly reduced in amplitude and also dispersed when the discharge is on. The time difference between the laser pulses decreases when the plasma is on, indicating an increase in velocity. Plotting the velocities at the two laser pair positions versus current (Fig.#3), we notice that the velocity at the position farther along in the plasma has become larger than the velocity closer to the shock origin even at small currents. As the current in the discharge is increased, the downstream velocity continues to increase even more relative to the upstream velocity. This effect cannot be explained by thermal inhomogeneities.
At present there is no detailed theoretical explanation for the effects observed. Our future efforts will be devoted to a better understanding of the physics of the observed shock modification. This is essential before considering future technical applications.
For further information on this work, please contact Dr. Peter Bletzinger at (937) 252-2706.
This work is supported by Dr. B. N. Ganguly (Air Force Research Lab, Wright-Patterson AFB, Ohio).

Thin-Filament Velocimetry

Wed, 16 Apr 2008 15:48:18 -0400

TFV of a Flame

Thin-Filament Velocimetry and Pyrometry
Larry P. Goss, William L. Weaver and Darryl D. Trump  Innovative Scientific Solutions, Inc.
and
James R. Gord  AFRL/PRSC

A novel technique employing a thin ceramic filament has been developed for the simultaneous measurement of temperature and velocity in combusting flowfields. The technique utilizes the optical analog of hot-wire anemometry for velocimetry and blackbody emission for thermometry. The energy flux from a laser is employed to heat a section of 14-m -SiC filament, and the temperature relaxation of the filament is tracked by its graybody emission. Heat-transfer coefficients are measured directly, allowing gas properties to be determined. A modified-King's-Law Nusselt-number correlation was found to yield the best agreement with experimental results. A fully explicit time-dependent model was employed for numerical fitting of the experimental velocity results. Profiles of a premixed propane-air flame were obtained and compared with LDV measurement results.

INTRODUCTION

 In previous studies by the authors, the graybody emission from a thin -SiC filament was utilized to investigate the temperature profiles of premixed and diffusion flames. [1-6] The -SiC filament is commercially available, has a diameter of 14 m, and displays low thermal conductivity along the filament axis, making it ideal for spatial determination of temperature profiles.
Because of the small size of the filament, it responds quickly to temperature (and velocity) changes in its surroundings; its high emissivity (0.88), which is constant over a wide temperature range, allows quantitative conversion of filament-emission intensity to gas temperature. The technique known as Thin-Filament Pyrometry has been applied in several studies involving non-sooting laminar, [1] turbulent, [2-6] and sooting flames. [7] The thermal properties of the filament have been modeled (steady state) in two earlier studies, [8-9] and a full non-steady, time-dependent heat-transfer model has also been developed. [7] Because of the strong dependence of the filament response on Reynolds number and the thermal properties of the flowfield, velocity as well as temperature information can be obtained from the filament emission.
The advantages of this technique over the use of conventional hot-wire-anemometer probes is that both temperature and velocity can be determined simultaneously and that the technique can be applied to combusting flowfields. [10] The authors previously investigated the velocimetry capabilities of the thin filament in the case of a laminar air jet. [11]
Three separate approaches to obtaining velocities by heating the filament with a laser source were examined: 1) steady state, 2) impulse (square wave), and 3) sinusoidal.   The response of the laser-heated filament was modeled using an explicit time-dependent model; in the present study this model was modified for numerical fitting of experimental decays. The present study represents the first application of the combined thin-filament velocimetry and pyrometry techniques in a combusting environment.
Experimentally, the energy flux of a laser is used to heat a small section of the filament, the emission from which is observed by an InGaAs detector and converted to filament temperature and velocity. In the impulse-flux-heating case (square wave) utilized in this study, the relaxation of the temperature to that of the ambient surroundings after heating by the laser is tracked to determine the gas velocity. The velocities obtained by the Thin-Filament Velocimetry (TFV) technique in a premixed propane-air flame and those obtained by a conventional Laser Doppler Velocimetry (LDV) instrument are compared.

THEORY AND MODEL

(1)

 where k is the thermal conductivity of the filament, T is the filament temperature, T gas is the temperature of the surrounding gases, T ref is a reference temperature, x refers to the distance along the axis of the filament, is the density of the filament, h is the convective heat-transfer coefficient, d is the diameter of the filament, is the Stefan-Boltzmann constant, is the emissivity of the filament, is the laser flux used to heat the filament, and C is the heat capacity of the filament.
The expression presented in Equation 1 assumes that the energy balance of the filament is coupled with the exact transport solutions for the surrounding fluid only through the heat-transfer coefficient. All fluid properties are included in the calculation of h and do not appear explicitly in the filament energy balance. Radiation, absorption, and emission from surrounding gases or soot are neglected because of the additional requirements to model the surrounding fluid and combustion processes. In addition, it is assumed that no temperature gradient exists within the differential section of the filament.
Equation 1 was used as the basis for an explicit, finite-difference numerical calculation of filament temperature as a function of time. The heat-transfer coefficient was calculated using a Nusselt-number correlation and could be varied as a function of position and time to correspond to spatial and time dependent velocity profiles.   Gas-temperature and external-energy-flux terms could also be varied as a function of position and time.
Equation 1 can be rearranged for calculating the heat-transfer coefficient, h , from the temperature of the filament and of the surrounding gas as a function of time and position

(2)

If a correlation is employed for the relationship between Nusselt number and Reynolds number, it is possible to calculate the velocity of a fluid flowing in the direction perpendicular to the filament. Such a correlation is the experimentally obtained modified King's-Law correlation of the Nusselt number for convective flow across a cylinder when the Reynolds number is between 0 and 44, as given by

(4)

Rearranging Equation 4 and solving for temperature yields

(5)

where

(6)

is the convective time constant of the filament for a given gas temperature and velocity. Equation 6 shows that the convective time constant is a function of filament properties (i.e., density, heat capacity, and diameter--all of which are assumed to be constant in this study), and gas properties through the heat-transfer coefficient, h . The convective time constant is a maximum for low temperature/low velocity and a minimum for high temperature/high velocity.

RESULTS

 For evaluation of the TFV technique in cold flow, a series of measurements was made on a 6-mm-diameter nitrogen laminar jet. The velocity measurements were made 2 mm above the jet exit and covered a wide range of flow conditions (0.5 - 25 slpm). Velocities obtained using the TFV technique were compared with those obtained using an LDV instrument. The results are shown in Figure 1. In this figure the velocity obtained with the TFV technique is plotted as a function of that obtained with the LDV instrument under each flow condition. The experimentally observed slope of 1.012 for the resulting line indicates close agreement between the results from the two velocimetry techniques.
For evaluation of the TFV technique under flame conditions, a premixed propane-air flame (0.3 slpm propane, 5.2 slpm air) was chosen for study because of the small change in gas properties which occurs between products and reactants. The choice of gas is important when velocity measurements are to be made since both the kinematic viscosity and the thermal conductivity of the gas affect the observed rate of cooling (or heating) of the filament. A comparison of results obtained with the LDV and TFV instruments for the centerline profile of the flame is shown in Figure 2. The agreement between the results achieved with the two techniques is quite good.
The TFV technique was used for simultaneous spatial profiling of the values of temperature and velocity of the premixed propane-air flame. A 7 x 12 point two-dimensional grid was adopted for studying the flame. Starting at a location 1 mm above the exit of the 10-mm-diameter tube, a 2-mm radial/6-mm axial grid was used to profile the flame. Because of the symmetry of the flame, only one-half of the flame was profiled. Figure 3(a) displays an overlay of the velocity contour and temperature image of the flame. For this figure the acceleration of the velocity field resulting from the high-temperature zones can be clearly seen. Figure 3(b) shows an overlay of the temperature contour and velocity image of the flame. The height of the cold inner cone is evident, along with the flame velocity acceleration with temperature.

Planer Doppler Velocimetry

Wed, 16 Apr 2008 15:23:28 -0400

PDV of Vortices

Planar Doppler Velocimetry (PDV) and the closely related techniques of Doppler Global Velocimetry (DGV), Filtered Planar Velocimetry (FPV), and Filtered Rayleigh Scattering (FRS) are a new class of diagnostic techniques which offer the promise of enabling non-intrusive, instantaneous velocity measurements over an entire plane in a flow field. With PDV, one measures the Doppler shift of light scattered by seed particles in the flow. The Doppler shift ( Dn ) is dependent on the incident light wavelength ( l ), the velocity of the scattering particle (V), and the observation (ô) and incident light (î) directions. This relationship is given as:
Dn = V . (ô - î)/l

Thus, the measured velocity component lies along the bisector of the incident light and observation direction vectors.

If the frequency shifts are accurately measured, the velocity of a particle in the flow field may be determined. The crucial aspect of this technique is the ability to resolve the small frequency shifts associated with the scattered light. For this purpose, an absorption line of molecular iodine is commonly used (see Figure 1). The iodine cell has absorption profiles suitable for use with PDV systems at the frequencies of both an argon-ion laser (514.5 nm) and a frequency-doubled Nd:YAG laser (532 nm). A schematic of the PDV system can be seen in Figure 2.
ISSI conducted tests in the Subsonic Aerodynamics Research Laboratory (SARL) wind tunnel at the Air Vehicles Directorate of the Air Force Research Laboratory (AFRL) in collaboration with Prof. Greg Elliott of Rutgers University and with the support of Dr. Thomas Beutner (AFRL). These tests measured the interaction of a vortex generated by a sharp-leading-edge delta wing with and without twin vertical tails. For details on the facility and model please see the reference. A few sample results are shown below.
Figures 3 and 4 show data taken by the reference and signal cameras, respectively, of the flow over a delta wing in the SARL. These images show the flow field viewed by the cameras on a plane oriented normal to the delta wing at 85.7% of the root chord.
The variation in intensity as a function of velocity is apparent in the signal camera image. The raw data was post-processed and the resulting instantaneous velocity map of the flow field is shown in Figure 5. Due to insufficient seeding, signal levels in some areas of this image were too low to produce acceptable data. Any pixel having a signal level of less than 300 counts after the background image was subtracted was not included in the processed image. The vortex structures over the delta wing are clearly visible in this image.
Figure 6 shows the result of averaging 50 data shots similar to the data shown in Figure 5, giving an aggregate image of the flow field at this location. The averaged data fills in regions where seeding was poor on individual shots. The flow field over the delta wing was steady. A CFD solution for similar flow conditions is available and is shown in Figure 7.
In both figures, a line through the vortex core (shown as a black line in the images) has been drawn, and a plot of the velocity variations across this line is shown below each image. A second black line at the bottom of Figure 6 indicates the position and span of the delta wing surface at this chord location.
Similar data can be seen for the delta wing with tails in Figures 8 and 9 at chord locations of 85.7% and 114.3% root chord. In these cases, the flow is unsteady. Due to the unsteadiness of the flow, the aggregate images are still somewhat noisy.

* This work is supported by U.S. Air Force Small Business Innovative Research Contract with ISSI.

Laser Doppler Velocimetry

Wed, 16 Apr 2008 15:09:11 -0400

Combined Cars/LDV System
 Coherent Anti-Stokes Raman Spectroscopy (CARS) has evolved over the past decades from a laboratory curiosity into a practical engineering tool for combustion research. The CARS technique has been applied to practical combustion systems, including small-scale burners, combustor cans, turbine engine afterburners, industrial furnaces, internal-combustion engines, magnetohydrodynamic (MHD) and coal-fired facilities, and burning propellants.

This rapid growth in applications of the technique can be attributed to an enhanced understanding of the CARS process and the development of sophisticated spectral-synthesis codes which are necessary for quantitative interpretation of CARS data. While the CARS instrumentation and associated calculations are quite complex in many cases, the technique has survived to become a viable tool because of its unique ability to provide precise point-wise temperature and concentration measurements of major flame species in extremely hostile, particle-laden environments. This flexible technique combined with other diagnostic methods such as Laser Doppler Velocimetry (LDV) has allowed velocity-scalar measurements in turbulent reacting flow fields. From these combined data, correlation coefficients of velocity and temperature can be determined.

To demonstrate the capabilities of the combined CARS-LDV instrument (Fig. 1), a propane jet diffusion flame was chosen for examination. The burner setup used for this flame consisted of a contoured 1 cm fuel jet surrounded by a 24 cm co-annular air jet. The fuel employed was propane at a flow rate of 8.5 standard liters/min. An air jet velocity of 15 cm/sec was employed to reduce room air disturbances. A sheet lighting visualization photograph of this jet diffusion flame is shown in Fig. 2. Superimposed on this photograph are the results of independent CARS measurements.
The interaction between the outside vortices and the flame surface is evident in this photograph. This photo also displays bimodal temperature p.d.f.s in the areas associated with the flame bulge. The temperature p.d.f can be interpreted as the fraction of time during which the flame zone is located in the sample volume. The bimodal behavior is typical of a flame surface which is oscillating as a function of time. The oscillations are evident in the temperature r.m.s data as well. Simultaneous CARS and LDV measurements were made at the 200 mm axial location because of the presence of the flame bulge in this area.
The mean uT, mean vT, and mean uv correlation coefficients are depicted in Fig. 3. The mean uT data indicate a strong correlation between axial velocity and temperature in the flame bulge area associated with the acceleration of the flame front due to buoyancy.
This large positive mean uT correlation is expected in a flow field dominated by buoyancy. The Reynolds shear stress uv displays a similar behavior, indicating a strong interaction between the outer vortices and the flame. The mean vT correlation is lower in magnitude but peaks in the bulge vortex region. Thus, the dominance of the buoyancy effect in relatively low Reynolds number jet diffusion flames can be clearly seen from the visualization, independent temperature p.d.f., and simultaneous data.
For further information on this work, please contact:  Dr. Larry P. Goss (937) 429-4980 x117

Holographic PIV

Wed, 16 Apr 2008 14:58:18 -0400

Holographic Flow Vis

Holographic Flow Visualization and Holographic PIV
Flow visualization has been a major experimental tool in fluid mechanics and combustion for decades. Despite its contributions, current flow visualizations are basically limited to two-dimensional (2-D) representations of 1) a flow structure as seen with the depth of field of the imaging lens employed, 2) a 2-D slice of a structure illuminated by a light sheet, and 3) a 2-D field resulting from integration of a 3-D density field along the path of a laser beam.
Holographic imaging, which has the capability of instantaneous 3-D representation of spatial objects including particle ensembles, holds great promise as a 3-D diagnostic tool--both qualitative and quantitative--for spatially- and temporally- evolving complex flow structures.
ISSI staff in collaboration with Prof. Hui Meng (Kansas State University, Manhattan, KS) developed a unique, flexible Holographic recording and reconstruction system for Air Force Research Lab's combustion research. This system has the ability to provide high quality 3D flow visualizations as well as 3D quantitative velocity information of the flow field. The results of this research work were recognized by The Fluid Dynamics Division of the American Physical Society and The Japanese Journal of Flow Visualization.

Vortex-Flame PIV

Wed, 16 Apr 2008 14:45:06 -0400

Vortex-Flame PIV - Particle Image Velocimetry Vortex-Flame Interactions in Hydrogen Jet Diffusion  Flames: A DPIV and DNS Investigation

Vortex-flame interactions in a hydrogen jet diffusion flame are investigated with two-color Digital Particle Image Velocimetry (DPIV) and the results are compared with Direct Numerical Simulations (DNS). Driven-jet vortex-flame interactions are of particular interest because they are reproducible turbulent-like events that can be comprehensively investigated to gain insight into turbulent combustion.

Previously, temperature and species concentration measurements were made in this flame, but a complete understanding of the vortex-flame interactions could not be gained without additional information concerning the flow field. The vortex structure of the combusting flow could not be reproduced using the hot-wire velocity data from cold flows. When the jet exit velocities from DPIV measurements were used as the driving profile for the DNS code, the resulting computations produced a vortex that matched the experimental vortex. The information obtained from DPIV and DNS provided insight into the complex reacting flows of vortex-flow interactions and aided the development of combustion modeling.

Figures (1) and (2) are the flow visualization images obtained by seeding both the jet and the co-flow. These images display a wide-angle view of the entire burner, co-flow, and flame. Figure 1 corresponds to the steady undriven flame which does not contain inner vortex structure and depicts a classic jet diffusion flame. Although buoyant structure is evident in this image, it is much different than the driven flame of Figure 2 which contains large structures in the flame zone much nearer the nozzle. Figure 2 reveals the inner driven fuel-side vortex, the flame-zone where the seed density is low, and the larger flame zone structures above the fuel jet which are produced both by buoyancy and by the periodically driven vortices. It is also evident that the co-flow is effective in producing laminar flow. Although not clearly visible in Figures 1 and 2, a bulge in the flame zone is observed in the driven flame near the inner vortex. The contrasting images show the dramatic effect of the vortex-flame interaction on the overall flame structure.
Two-color DPIV mages were collected throughout the 20-Hz cycle of the vortex-flame interaction. The complete cycle is shown in Fig 3. Each DPIV image is for a different phase of the vortex. From these images the evolution of the vortex can be observed. As the speaker retracts, the fuel jet slows, drawing the flow radially inward as can be seen at t = 6 and 12 msec. Then as the speaker pulses out, it drives the fuel jet into the slower moving fuel ahead of it. Consequently, the fuel moves radially outward, forcing it into the shear layer and forming the toroidal mushroom-shaped vortex as shown at t = 17 and 21 msec. As the images show, the cycle is then repeated after the vortex has convected downstream (t = 24 and 28 msec).
The centerline jet exit velocity was extracted from these images. The DPIV results showed the radial variation of the exit velocity to be small; thus, a top-hat velocity profile was used in the DNS model for the jet-nozzle boundary condition. The centerline jet exit velocity was employed to characterize the driving pulse and was used as the time-dependent boundary condition in the DNS model of the vortex-flame interaction.
When the DPIV exit velocities for each phase were used as input for the DNS driving profile, the vortex was reproduced computationally. Shown in Fig. 4 are the split experimental and computational images of the driven hydrogen/nitrogen flow for both combusting and cold flow conditions. The left half is the DPIV image, and the right side is an image from the computational model. The DNS vortex is indicated using theoretical particle traces, and the peak flame temperature locations are represented by dots. This image shows the remarkable accuracy of the DNS in modeling the complex vortex-flame interaction.

For further information on this work, please contact:
Fred Schauer at (937) 255-6462 or Sivaram Gogineni at (937) 252-2706.

Automotive Fan PIV

Wed, 16 Apr 2008 14:23:48 -0400

Automotive Fan PIV - Particle Image Velocimetry
 Flow Structure in an Automotive Cooling Fan

The characteristics of the flow in an automotive cooling fan were investigated using Digital Particle Image Velocimetry (DPIV). Instantaneous and time-averaged velocity measurements were made at the leading edge, trailing edge, and suction and pressure sides of the blades by synchronizing the passage of the blades with the laser/camera system.
These measurements revealed steady and unsteady flow features at several operating points and allowed a composite DPIV image around the entire fan blade to be made. This composite image was compared with a panel-code solution, and the main differences were attributed to local viscous effects such as flow separation and wake unsteadiness that are not included in the present panel-code implementation.
Typical flow visualizations and the composite images are shown in the figures. The results are for a 254 mm-radius automotive cooling fan operated at 2200 rpm. The location of the laser-sheet plane was 203 mm from the rotation axis. Dominant flow structures and time-averaged features were captured with high-resolution DPIV.
Flow phenomena investigated were the location of the stagnation region near the leading edge, the flow on the pressure side of the blade, the wake structure at the trailing edge, and the effect of operating point on the flow field. Time-averaged DPIV data computed from 30 images were obtained near the trailing and leading edges.
In a comparison of DPIV and time-averaged panel-code calculations, good agreement was found for the global features of the flow. Blade-passage velocity magnitudes were underpredicted by the panel code as a result of the inability to resolve viscous effects--especially flow separation near the fan hub.
While the panel code has proven to be very useful in low-speed axial-fan design, the DPIV results offer valuable information for further enhancement and development of viscous corrections in the code.

3D-DNS & PIV - Particle Image Velocimetry

Wed, 16 Apr 2008 14:14:05 -0400

3D-DNS and PIV - Particle Image Velocimetry Investigation of Transitional Plane Wall Jet

M. Visbal and D. Gaitonde  Air Vehicles Directorate  Air Force Research Laboratory  Wright-Patterson Air Force Base, Ohio 45433
 S. Gogineni Innovative Scientific Solutions, Inc.  2766 Indian Ripple Road  Dayton, Ohio 45440-3638
The three-dimensional transition of a forced plane wall jet is investigated using direct, high-order numerical simulations and high-resolution experimental measurements. Very good agreement is observed between predictions and experimental data. The computed results are then employed to elucidate the fine-scale breakdown of the forced wall jet.

The transition process begins with the formation of shear-layer and wall vortex pairs, which, as a result of energetic forcing, appear near the nozzle exit and are phase-locked for a short distance downstream. In 2-D calculations these vortex dipoles convect in a structurally stable fashion, provided sufficient forcing amplitude is applied.

By contrast, the 3-D situation displays a rapid spanwise breakdown of the rollers. This loss of coherence of the vortical structures begins near the sidewalls and propagates toward the midspan of the wall jet. The secondary transition process of the jet is dominated by the core dynamics of the spanwise vortices, particularly of the shear-layer rollers.

The computed instantaneous flowfield reveals splitting of the spanwise shear-layer vortices into double-helical branches that are wound about the original vortex axis. The sense of winding is consistent with the observed strong crossflow toward the centerline of the jet.

These observations suggest that the principal mechanisms involved in the fine-scale transition process of this jet are spanwise compression and branching of the primary vortices as well as the "collision" of the crossflow counter currents at the jet midspan. Studies with lower forcing amplitudes as well as with various sidewall conditions suggest that the basic mechanisms remain qualitatively unaltered.

PIV - Wall Jet Animation

Wed, 16 Apr 2008 14:03:41 -0400

Wall Jet Animation
Flow Animation Of A Transitional Plane Wall Jet From Phase Resolved PIV And DNS Data

The transitional process of a plane wall jet (near the jet exit) was animated using phase-resolved vorticity distributions obtained from PIV (experimental) and DNS (numerical) techniques. The Reynolds number, based on the exit maximum velocity, is 2150.

The exit-mean-velocity profile is parabolic. The aspect ratio of the channel is 20. The wall extends to 60 jet widths downstream of the channel exit to ensure minimal influence from the end of the plate. PIV images were recorded by simultaneously seeding the jet flow and the ambient flow with sub-micron size smoke particles and illuminated them with a thin laser light sheet.

These images were processed using Young's fringe method of interrogation to obtain instantaneous velocity distributions and central differencing schemes were used to compute vorticity distributions. Numerically, time-accurate computational results were obtained by solving the two-dimensional, unsteady, Navier-Stokes equations.

Results show that under the influence of external excitation, linear-instability growth is bypassed and a discrete shear-layer vortex is formed immediately at the nozzle exit. The vortex interacts with the boundary-layer vorticity which leads to the formation of another vortex in the inner layer. These two vortices form a vortex couple and convect downstream.

Preliminary results indicate that the unsteady surface-pressure fluctuations induced by the passage of the shear-layer vortex are responsible for the initiation of a local boundary-layer detachment and the later formation of a boundary-layer vortex.

By adding either a no-slip or slip boundary condition in the numerical computation, it was determined that the flow development is insensitive to the imposed wall boundary condition. This seems to suggest that the physical mechanism that leads to the formation of the boundary-layer vortex is an inviscid rotational one.

PIV - Turbine Film Cooling

Wed, 16 Apr 2008 12:50:54 -0400

Turbine Film Cooling
One of the Winners of the 13th Annual Fluid Mechanics Photo Contest
Awarded by the American Physical Society/Division of Fluid Dynamics Irvine, CA 1995

High Free-Stream Turbulence Influence on Turbine Film Cooling Flows
S. Gogineni, R. Rivir*, D. Pestian**, and L. Goss
Double pulsed two-color particle image velocimetry (PIV) images of simulated turbine film cooling flows are shown for a range of film cooling blowing ratios (R = r c U c / r i U i ) of 0.5, 0.7, 1.0, and 1.5. The simulated turbine conditions include the film cooling jet l/d = 3, film jet Reynolds number of 20,000, and free-stream turbulence level of up to 17% among other characteristics.
These images are obtained by seeding the jet flow only with submicron size smoke particles and illuminating the particles with a two-color particle image velocimetry system. These images illustrate how the jet spreads and shear layer grows with two of the problem's parameters, the blowing ratio and the free-stream turbulence level. There is a decrease in film cooling effectiveness and increased heat transfer associated with the increase in turbulence intensity which is currently difficult to predict. The particle image velocimetry images and the reduced PIV data are useful in providing additional physics on mixing and dissipation for improved modeling of these flows.

Coherent Anti-Stokes Raman

Wed, 16 Apr 2008 12:24:16 -0400

Combined Cars/LDV System

Coherent Anti-Stokes Raman Spectroscopy (CARS) has evolved over the past decades from a laboratory curiosity into a practical engineering tool for combustion research. The CARS technique has been applied to practical combustion systems, including small-scale burners, combustor cans, turbine engine afterburners, industrial furnaces, internal-combustion engines, magnetohydrodynamic (MHD) and coal-fired facilities, and burning propellants.

This rapid growth in applications of the technique can be attributed to an enhanced understanding of the CARS process and the development of sophisticated spectral-synthesis codes which are necessary for quantitative interpretation of CARS data. While the CARS instrumentation and associated calculations are quite complex in many cases, the technique has survived to become a viable tool because of its unique ability to provide precise point-wise temperature and concentration measurements of major flame species in extremely hostile, particle-laden environments.

This flexible technique combined with other diagnostic methods such as Laser Doppler Velocimetry (LDV) has allowed velocity-scalar measurements in turbulent reacting flow fields. From these combined data, correlation coefficients of velocity and temperature can be determined.

To demonstrate the capabilities of the combined CARS-LDV instrument (Fig. 1), a propane jet diffusion flame was chosen for examination. The burner setup used for this flame consisted of a contoured 1 cm fuel jet surrounded by a 24 cm co-annular air jet. The fuel employed was propane at a flow rate of 8.5 standard liters/min. An air jet velocity of 15 cm/sec was employed to reduce room air disturbances. A sheet lighting visualization photograph of this jet diffusion flame is shown in Fig. 2. Superimposed on this photograph are the results of independent CARS measurements.
The interaction between the outside vortices and the flame surface is evident in this photograph. This photo also displays bimodal temperature p.d.f.s in the areas associated with the flame bulge.
The temperature p.d.f can be interpreted as the fraction of time during which the flame zone is located in the sample volume. The bimodal behavior is typical of a flame surface which is oscillating as a function of time. The oscillations are evident in the temperature r.m.s data as well. Simultaneous CARS and LDV measurements were made at the 200 mm axial location because of the presence of the flame bulge in this area.
The mean uT, mean vT, and mean uv correlation coefficients are depicted in Fig. 3. The mean uT data indicate a strong correlation between axial velocity and temperature in the flame bulge area associated with the acceleration of the flame front due to buoyancy. This large positive mean uT correlation is expected in a flow field dominated by buoyancy.
The Reynolds shear stress uv displays a similar behavior, indicating a strong interaction between the outer vortices and the flame. The mean vT correlation is lower in magnitude but peaks in the bulge vortex region. Thus, the dominance of the buoyancy effect in relatively low Reynolds number jet diffusion flames can be clearly seen from the visualization, independent temperature p.d.f., and simultaneous data.
For further information on this work, please contact:  Dr. Larry P. Goss (937) 429-4980 x117

Laser Induced Fluorescence

Wed, 16 Apr 2008 12:09:56 -0400

Vortex-Flame Interactions in a Counterflow Diffusion Flame
The dynamic interaction between a laminar flame and a vortex is examined using an opposed-jet burner facility designed by Rolon and co-workers (1995, 1996). OH-PLIF (planar laser-induced fluorescence) and PIV techniques were implemented to understand these interactions. The PLIF images of OH shown in Fig. 1 correspond to a vortex-flame interaction in which extinction of the OH layer is absent. Initially, the vortex creates a small dent in the flame, and this dent grows. Eventually, the flame nearly surrounds the advancing vortex as it approaches the upper nozzle. In the later interaction stages, the OH PLIF signal level is observed to increase by up to a factor of five over the levels observed without a vortex. The increased signal level is indicated by the yellow colors in the images. This change in OH signal level is thought to indicate enhanced burning.
Figure 2 shows the sequence of images before and after extinction of the OH layer. Extinction of the OH layer takes place in an annular pattern around the sides of the vortex, leaving a burning layer at its leading edge. After extinction, the isolated island of flame burns away, and the vortex travels upward toward the other nozzle. The flame follows the vortex, traveling up the stem. As the flame overtakes the vortex, it wraps up and turns in on itself.
For further information on this work, please contact:  Dr. Gregory J. Fiechtner (937) 255-6980

RMS - Structure of Coaxial Jets

Wed, 16 Apr 2008 11:34:49 -0400

Coaxial jets are an integral part of many engineering systems where mixing of streams of different fluids is required. They are used to provide the mixing between fuel and oxider in combustors of propulsion systems and power producing gas turbine systems as well as waste combustion and incineration systems. A properly designed jet will efficiently mix the air and fuel while providing the best overall combustion parameters. Single non-circular jets have been shown to have better mixing characteristics than axi-symmetric counterparts. Therefore, combinations of such jets into coaxial configurations are promising.
ISSI staff conducted experiments on circular and non-circular coaxial jets and implemented reactive Mie scattering and two-color digital PIV techniques. This work was performed in collaboration with Prof. D. Nikitopoulos and Prof. E. Gutmark of Louisiana State University, Baton Rouge, LA.

RMS - Jet In Cross Flow

Wed, 16 Apr 2008 11:22:54 -0400

Reactive Mie Scattering flow visualization technique provides information concerning the mixing and interaction between two fluids. An important attribute of this technique is that at any instant, both the present and the past reaction products are visualized, while non-reaction parts of the flow remain invisible.

In this technique, sub-micron size TiO 2 particles are generated from a chemical reaction between TiCl 4 and water vapor. These particles are formed as a result of mixing at the molecular level when the two fluids come into contact.

Because the two species are affected by co-flowing streams, the rate of formation of the TiO 2 particles is dependent on, and clearly an indication of, the degree of mixedness. The presence of these particles (and, thus, of reaction products) in planar cross sections of the flow can be easily detected by light scattered from a laser sheet (e.g., Nd:YAG laser).

This technique was successfully implemented to a jet in a cross flow problem and evaluated the effect of forcing on the mixing of jet and cross flow fluids. The jet is issued from a square hole having an equivalent diameter of 1.72 cm.
The jet-to-cross-flow dynamic head is 1.0. The Reynolds numbers for the jet and the cross flow are 700 and 8775, respectively. The square jet is manipulated by four piezoelectric actuators-one mounted along each side of the jet conduit near the jet-exit plane.
The pair of actuators having tip displacements in the streamwise direction is referred to as streamwise, and the pair having tip displacements in the spanwise direction is called spanwise. Images shown are for unforced, streamwise, and spanwise forcing conditions.
In these images, the bright regions correspond to the mixed region and the dark regions to the unmixed regions. For details on these images please see the reference.

References:
1. S. Gogineni, L. Goss, and M. Roquemore "Manipulation of Jet in a Cross Flow," Journal of Experimental Thermal and Fluid Science (in press, 1998).

RMS - Flame Vortex Interactions

Wed, 16 Apr 2008 10:50:43 -0400

Flame vortex interactions in a driven fusion flame.

K. Y. Hsu, L.D. Chen*, V. R. Katta, L. P. Goss, D.D. Trump and W.M. Roquemore**
Flame-vortex interactions in a jet diffusion flame are studied in a controlled experiment. [1] The vortices are periodically generated in a methane jet diffusion flame at the frequency of 30 Hz.
The phase-locked reactive-Mie-scattering technique [2] is used to visualize the interactions between the vortices and the flame.
The second harmonic output (532 nm) of a pulsed Nd:YAG laser is expanded to a thin laser sheet vertically across the centerline of the jet.
The scattering particles, TiO 2 , are formed as a result of the reaction of combustion product H 2 O and TiCl 4 vapor added to the fuel.
The sequential images (Figs. 1 and 2), with 3 msec separation, show the complex dynamics of the interactions between generated vortices and the flame. The large vertical structure containing fuel pushes the flame surface outward in the radial direction.
When the flame is stretched, the local flame extinction occurs as is evident by the disappearance of the blue flame.
To illustrate the evolution of flame-vortex interactions, an image consisting of eight consecutive phase angles (time interval of 1 msec) is shown in Fig. 3.

*University of Iowa
**Air Force Research Laboratory

References:
1. K.Y. Hsu, L.D. Chen, V.R. Katta, L.P. Goss, and W.M. Roquemore, "Experimental and numerical investigations of the vortex-flame interactions in a driven jet diffusion flame," 31st Aerospace Meeting and Exhibit, AIAA Paper No. 93-0455, 1933.
2. L.D. Chen and W.M. Roquemore, "Visualization of jet flames," Combust. Flame. 66, 81 (1986).
This work appears in the Gallery of Fluid Motion, Phys. Fluids A, Vol. 5, No. 9, September 1993, p. S4.
This work was supported by the Air Force Office of Scientific Research.