A11105 43blbfl

NATL INST OF STANDARDS & TECH R.I.C.

All 102436168

Pitta, William M/Chemlcally reacting tur QC100 U56 NO.85-3299 V1986 C.2 NBS-PUB- Chemically Reacting Turbulent

Flow

Reference

NBS

PUBLICATIONS

William M. Pitts Takashi Kashiwagi

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

March 1986

-QC

100

- U 5 6

85-3299

1986

art by:

ce

ientific Research

keseaiCh EKrawtlTtOH CKTQ

NBSIR 85-3299

CHEMICALLY REACTING TURBULENT

FLOW

William M. Pitts Takashi Kashiwagi

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

March 1986

Sponsored in part by:

U.S. Air Force

Office of Scientific Research

U.S. DEPARTMENT OF COMMERCE. Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director

TABLE OF CONTENTS

Page

LIST OF FIGURES iv

Abstract 1

1. OBJECTIVES 2

2. STATUS OF RESEARCH 3

2.1 Introduction 3

2.2 Major Accomplishments and Findings 4

2.2.1 Development of Laser-Induced Rayleigh Light

Scattering as a Concentration Probe 5 2.2.2 Development of a Technique for Simultaneous

Concentration and Velocity Measurement 6

2.2.3 Development of a Digital Line Camera for

Concentration Measurements 8

2.2.4 Response Behavior of Hot-Wires and Films to Flows

of Different Gases 14

2.2.5 Effects of Global Density and Reynolds Number

Variations on Turbulent Mixing Behavior 15

2.2.6 Final Remarks 20

3. PUBLICATIONS FOR FY83, FY84 , AND FY85 20

4. PROFESSIONAL PERSONNEL 21

5. INTERACTIONS FOR FY83, FY84, AND FY85 21

-iii-
LIST OF FIGURES

Page

Figure 1. Examples of simultaneous, time-resolved measurements of

concentration and velocity for a turbulent jet of propane

flowing into air are shown 23

Figure 2. The line camera system designed for line measurement of

concentration is shown 24

Figure 3c The entire experimental system for making line

measurements in the turbulent flow field of an axisymmetric

jet is shown 25

Figure 4, An example of the results of time-resolved line measurements

in a propane/air jet/coflow is shown. A seven level gray

scale represents propane concentration having a maximum

mole fraction of 0.40 26

Figure 5. Correlation of Nusselt number as a function of Reynolds

number for heat transfer from a hot-wire to nine different

gases 27

Figure 6. The experimental system for time-resolved shadowgraph

imaging of variable density flows is shown schematically.

Note that the coflowing gas is passed through the

enclosure surrounding the nozzle 28

Figure 7. A superposition of three shadowgraphs for an axisymmetric,

turbulent jet of helium at Re = 3950 flowing into a slow

coflow of air is shown 29

Figure 8. A superposition of four shadowgraphs for an axisymmetric,

turbulent jet of propane at Re = 3960 flowing into a slow

coflow of air is shown 30

Figure 9. A shadowgraph is reproduced of an SF^ j e t flowing into air

which clearly shows the formation of a fountain by this

strongly negatively buoyant jet 31

Figure 10. Centerline values of Y

q

/Y are plotted as functions of z/rQ

for the six different gas7coflow combinations listed 32

Figure 11. Experimental values of Y /Y are plotted as a function

of (z - z )/r for the s?x different jet/coflow combinations

listed. . 33

Figure 12. Centerline unraixedness values for six different jet/coflow

combinations are plotted as functions of z/r 34

o Figure 13. The experimental results of figure 12 for Y’/Y are replotted as functions of z/r

£ 35

-iv-
Figure 14. Unmixedness values are plotted as functions of z/r

Q for

propane/air turbulent jets at three different Reynolds

numbers 36

-v-

CHEMICALLY REACTING TURBULENT FLOW

William M. Pitts

Takashi Kashiwagi

Abstract

This report summarizes the results of the first three years of a study on

chemically reacting turbulent flow. This project has been jointly sponsored

by the Air Force Office of Scientific Research and the National Bureau of

Standards. The development of new diagnostics for variable density flows are

described. These include Rayleigh light scattering for real-time, spatiallyresolved concentration measurements, combined Rayleigh light scattering and

hot-wire anemometry for simultaneous concentration and velocity measurements,

and the development of a digital line camera which has allowed the concentration measurements to be made along a line. A study of heat transfer from

heated cylinders is discussed which has generated a much improved correlation

of experimental results. Shadowgraph visualization studies and centerline

concentration measurements are described for variable density, axisymmetric

turbulent jets covering jet to coflow density ratios of 0.14 to 37. These

studies have also included a limited investigation of Reynolds number effects.

The observed dependence of the mixing behavior on the density ratio and Re

have led us to make new hypotheses concerning the nature of turbulent mixing.

These hypotheses are shown to lead to qualitative predictions for the turbulent mixing which are in agreement with experimental findings.

- 1 -
1 . OBJECTIVES

The primary objective of this research program has been and remains the

improvement of the fundamental understanding of chemically reacting turbulent

flow. Despite intense efforts for several decades, this complicated mixing

process remains poorly characterized. This is unfortunate, since chemically

reacting turbulent flows are of paramount importance in a wide range of

combustion devices and other chemical reactors which have military and industrial uses. An improved understanding of the interactions of fluid dynamics

and heat release is not only necessary to improve device efficiencies and

contribute to the conservation of dwindling fuels and feed-stocks, but also to

allow designs which limit the production and release of undesirable

pollutants.

In order to meet the above objective, an approach has been adopted which

seeks to first generate detailed experimental information concerning the

effects on turbulent mixing of global density variations based on chemical

composition or temperature. Once these effects are fully characterized, the

study is to be extended to reacting flows where local fluctuations of density

occur due to the heat release of chemical reactions. It is our conviction

that it is first necessary to quantify the effects of global density differences before it is possible to understand the more complicated effects of

local heat release. The experimental results which are described in this

report not only serve to improve the qualitative understanding of mixing in

variable density flows, but are also to be used for comparison with workers

who are striving to develop quantitative models of these complex flows.

-2 -
An important secondary goal has been the development of new experimental

techniques having high spatial and temporal resolution for the investigation

of concentration and velocity in variable density flows. These diagnostics

are required in order to gain insights into the temporal behavior of turbulent

mixing processes.

2. STATUS OF THE RESEARCH

2.1 Introduction

This report summarizes findings for the initial three-year period of a

study of chemically reacting turbulent flow which has been jointly sponsored

by the Air Force Office of Scientific Research (AFOSR) under contract numbers

ISSA-83-00012 , ISSA-84-00005, and ISSA-00012 and the National Bureau of

Standards (NBS). Initially, this project was intended to be an integrated

theoretical and experimental effort. To this end, Dr. Howard Baum of the

Center for Fire Research was included as a principal investigator. However,

during the course of the project, all of the parties involved agreed that it

was best for the theoretical efforts of Dr. Baum to be supported by AFOSR as

an independent project. During the past year this change has been initiated

and for this reason, the results to be summarized here are primarily experimental in nature. An active collaboration still exists between Dr. Baum and

his coworkers and the principle investigators for this project.

Detailed descriptions of research findings during the first two years of

this study are available in two previous annual reports, three project renewal

requests, and several archival publications. Only short summaries of this

-3 -
work will be included here. Major emphasis will be placed on the research

accomplishments during the past year.

2.2 Major Accomplishments and Findings

The following is a list of major accomplishments and findings which have

been made during the three year period covered by this report. Short descriptions of each item follow. Recent results on the development of the line

camera and the effects of global density variations on turbulent mixing

behavior will be emphasized.

Diagnostic Development

Development of laser-induced Rayleigh light scattering as a real-time

concentration probe in turbulent binary gas mixtures.

* Combination of laser-induced Rayleigh light scattering and hot-wire

aneraometry to allow real-time, simultaneous measurements of concentration and velocity in binary gas mixtures.

® Design and fabrication of a digital line camera capable of making

real-time concentration measurements along a line located within a

turbulent binary gas mixture.

-4 -
Major Research Accomplishments

o Completion of a study of heat transfer from cylinders to a wide

variety of gases. Results have led to the development of a new

correlation procedure which incorporates the effects of molecular

property variations with temperature.

0 Completion of an investigation of global density effects on turbulent

mixing in axisymmetric jets. Density ratios for jet to surrounding

gases cover the unprecedented range from 0.14 to 37. Shadowgraph

flow visualization and quantitative centerline concentration measurements have provided unique insights into the effects of global

density variations on turbulent mixing as well as a new interpretation of the turbulent mixing process.

2.2.1. Development of Laser-Induced Rayleigh Light Scattering as a Concentration Probe

This work was initiated prior to the period covered by this report and

completed during the early phases of the three-year study. Laser-induced

Rayleigh light scattering was shown to be an extremely useful method for

performing real-time, spatially-resolved measurements in the flow fields of

binary gas mixtures. Such measurements had been very difficult to make in the

past. Measurement rates as high as 5 kHz for observation volumes as small as

0.0003 mm^ were easily obtained. A maximum of 32,768 measurements could be

recorded and stored as a continuous data record.

-5 -
Procedures were developed which allowed the average, rras , skewness, and

kurtosis values to be calculated for the time-resolved concentration measurements. The availability of real-time data allowed the generation of interraittency functions which could be subsequently employed to provide conditionallysampled values of the concentration fluctuation moments. The time-resolved

concentration records were used to generate both correlation functions and

power spectra for the concentration fluctuations in the turbulent flow fields.

The technique was demonstrated by performing a series of concentration

measurements for a turbulent jet of methane flowing into a slow coflow of

air. The results were consistent with previous measurements available in the

literature. Even though of a preliminary nature, these measurements were

recently chosen as the most reliable available for comparison purposes with

the results of conserved scalar measurements in a hydrogen-air diffusion flame

("Superlayer Contributions to Conserved Scaler PDF's in a H2 Turbulent Jet

Diffusion Flame", M.C. Drake, W. Shyy , and R.W. Pitz, Fifth Symposium on

Turbulent Shear Flows, Ithaca, New York, August 7-9, 1985).

2.2.2. Development of a Technique for Simultaneous Concentration and Velocity

Measurement

There are very few velocity measurement techniques available for variable

density flows. Hot-wire anemometry is complicated by the variations in fluid

heat transfer behavior which occur in turbulent gas mixtures. By combining

the laser-induced Rayleigh method with hot-wire anemometry it was possible to

correct the observed hot-wire response for variations in the molecular

properties of the fluid surrounding the velocity probe and obtain accurate

velocity measurements. Since both the light scattering and anemometer signals

- 6-
were digitally recorded, simultaneous time-resolved records of concentration

and velocity were generated. These records allowed the cross-correlation

coefficient for velocity and concentration fluctuations to be calculated.

Such terms appear in many theoretical treatments of turbulent flow and their

measurement is therefore of great interest.

A system was fabricated in which a hot-wire or hot-film was placed very

near the observation volume for a laser-induced Rayleigh light scattering

concentration measurement. By measuring the response of the probe to known

velocities of different binary gas mixtures in the concentration range of

interest, calibration curves were generated which allowed the flow velocity to

be determined when the hot-wire or hot-film response and surrounding gas

composition were available. In this way, the entire velocity time record was

generated.

The effectiveness of this new measurement technique was demonstrated by

making simultaneous, time-resolved measurements of concentration and velocity

within an axisymmetric turbulent jet of propane flowing into air. The experimental results were shown to be in excellent agreement with the limited number

of measurements which are available in the literature. Figure 1 shows an

example of the simultaneous, time-resolved measurements of concentration and

velocity which were recorded.

-7 -
2.2.3. Development of a Digital Line Camera for Concentration Measurements

The two techniques described above permit real-time measurements of

concentration and velocity in variable density flows having an accuracy and

ease which has only become possible very recently. Even so, it is clear that

a great deal of information concerning the nature of the three-dimensional

flow structures which form the turbulent field cannot be elucidated by point

measurements

.

In the past few years new optical techniques have been developed which

allow both line (along a laser beam) and two-dimensional (sheet lighting)

measurements of scalars in turbulent flow fields. Even though these techniques are in a relatively early stage of development, dramatic demonstrations

of their ability to visualize scalar variations in turbulent flow fields have

been published (e.g., see "Visualization of Turbulent Flame Fronts with Planar

Laser-Induced Fluorescence", G. Kychakoff, R.D. Howe, R.K. Hanson, M.C. Drake,

R.W. Pitz, M. Lapp, and C.M. Penney, Science 224 (1984) 382).

These new flow diagnostics have been made possible by the recent

development of suitable area detectors. In general, the optical processes

which are utilized are too weak to be recorded directly by these detectors and

some means of image intensification has been required to generate the necessary light levels. Pulsed laser sources are usually employed. These provide

excellent time resolution, but are incapable of real-time measurements in

turbulent flows due to their low repetition rates.

-8 -
It became clear to us very early that the extension of these new optical

imaging techniques to real-time Rayleigh light scattering measurements

provided a unique opportunity for investigating mixing in variable density

flows. In FY84, NBS provided supplemental funding which allowed the design

and fabrication of a line camera designed especially for making such measurements. During the past fiscal year this camera has been constructed and its

usefulness demonstrated by recording real-time concentration fluctuations

along a line located within the flow field of a turbulent jet of propane

flowing into air.

The design of this camera presented severe technological challenges in

the areas of image intensification, solid-state array detection, and data

acquisition. Each of the components chosen for the camera in its current

configuration are described briefly below.

The utilization of a solid-state detector capable of operating at kHz or

higher rates requires that the laser-induced Rayleigh light scattering intensity be increased in some way. Most past measurements have utilized generation II image intensifier tubes for this purpose. These devices were

originally developed for night vision use. They provide extremely high light

amplification (values of 10^ are not uncommon), but their maximum brightness

is limited by current saturation which occurs in the microchannel plates which

serve as the active gain media in this type of tube. Calculations showed that

generation II image intensifiers could not provide enough brightness to allow

light intensity measurements at kHz rates with commercially-available , solidstate detectors.

-9 -
Fortunately, similar calculations indicated that an earlier type of image

intensifier (designated generation I) could operate at higher internal current

levels and hence generate brighter outputs than generation II tubes. These

earlier devices function by accelerating and electrostatically focusing

electrons generated by a photocathode which subsequently strike a phosphor.

Their light gains are lower than generation II tubes, but the light amplification required for the application described here is relatively modest and the

ability to generate brighter outputs is the primary consideration. On this

basis, a generation I image intensifier tube was chosen to amplify the light

generated by Rayleigh light scattering.

In choosing an output phosphor for the image intensifier it was necessary

that the decay time be sufficiently short to insure that the tube could

respond to the temporal fluctuations of the input light source. The most

common phosphor utilized in these devices is P-20. This particular phosphor

decays relatively slowly and its lifetime is long enough to limit the time

resolution of an image intensifier operating in the kHz range. For this

reason, the image intensifier used in the camera to be described here was

equipped with a P-47 output phosphor. This blue phosphor has a submicrosecond

lifetime which is sufficiently short to insure that the tube responds fully to

input light intensity fluctuations on a kHz time scale.

There are a wide range of solid-state photodetectors commercially

available. Important considerations in choosing a particular array include

pixel size, read rate, and single read capability. For this study, a linear

- 10-
silicon photodiode array produced by Reticon (RL128S) was chosen.* This array

consists of 128 individual pixels located on 0.025 mm centers which have

heights of 2.5 mm. The active length is therefore 3.2 mm. Reticon provides

standard electronics which are used to drive the array and produce a sampled

and held video signal which is a voltage output proportional to the intensity

of light on a given pixel during the integration time for the read. Pixel

read times can be varied with rates approaching a MEz/pixel possible. By

operating at these high rates, the entire array can be scanned in less than

1 ms.

The detector array and image intensifier are both equipped with fiber

optic faceplates. These allow the two devices to be coupled with a 4:1 reduction fiber optic taper. This taper not only serves to isolate the line

scanner from the high voltage of the image intensifier tube, but also allows a

12.8 mm line image on the intensifier to be imaged onto the 3.2 mm active

length of the detector array. This reduces the effective resolution of the

system to * 0.150 mm which is comparable with that available in past single

point concentration measurements utilizing photomultipliers.

The camera system was completed by incorporating a 1:1 lens which forms

an image of the Rayleigh scattering on the input face of the image intensifier. A schematic of the camera and the placement of the various components

is shown in figure 2. The insert shows that the small end of the fiber optic

taper has been coated with aluminum and then scribed across its center with a

Certain commercial equipment, instruments, or materials are identified in

this paper in order to adequately specify the experimental procedure. Such

identification does not imply recommendation or endorsement by the National

Bureau of Standards, nor does it imply the materials or equipment are necessarily the best available for the purpose.

- 11 -
0.200 mm wide line. This arrangement serves to limit light reaching the

detector to only the 0.200 mm wide region defined by the scribed portion.

In order to make quantitative measurements of scattering intensity it is

necessary to record digitally the sampled and held output of the line camera.

This requires that the camera be interfaced to a data acquisition system which

is capable of digitizing and storing data at rates near a MHz. A Nicolet 1180

signal averaging computer was used. Figure 3 shows the system which was

formed by interfacing the line camera to the Nicolet. The minimum digitization time is 3 us. This limits the scan rate for the entire array to a

maximum value of 2.2 kHz. The available memory is 32,768 words which means

that only 256 array scans can be stored before the data must be transferred to

a hard disk.

As a demonstration of the camera system, the time-resolved Rayleigh light

scattering along a laser beam within the turbulent flow region of an axisymmetric jet of propane into air was recorded. Figure 3 shows that the Ar ion

laser beam and the digital line camera are oriented at 90° to one another.

The configuration of the jet and the slow coflow of surrounding air are also

indicated in this drawing.

By calibrating the scattering from pure air and propane it was possible

to obtain quantitative measurements of spatially- and temporally-resolved

propane concentration in the flow field. Figure 4 shows the results for

measurements recorded along a 12.8 mm line located 31.5 jet radii from the jet

orifice and lying along the jet radius from roughly 2 to 6 nozzle radii. In

this figure, the radial location is given simply by the pixel number. This

- 12 -
reflects the fact that the data have not been corrected for small variations

in image intensifier magnification with position. The readout time for the

array was 1.9 ms. The time scale for the measurements and the flow direction

are indicated on the vertical axes. Propane concentration is represented by a

seven level gray scale where the maximum mole fraction is approximately 0.40.

Some noise is evident in the measurements. This is due to a slight

electronic pickup by the linear detector from the image intensifier. In the

future, this noise will be suppressed by improvements in the hardware or by

digital correction. Despite the noise, the turbulent fluctuations of propane

mole fraction are clearly evident. The results show dramatically the importance of large scale structures in turbulent mixing. Data such as that shown

in fig. 4 can be time-averaged in exactly the same manner as the point

measurements described earlier. In this way, it has been shown that concentration measurements utilizing the line camera are in quantitative agreement

with earlier point measurements in the same flow.

In its current configuration the sensitivity of the line camera is such

that only the favorable case of a propane jet into air can be investigated at

kHz data rates. In order to extend these measurements to other gas pairs, it

is necessary to increase the light sensitivity of the camera. An analysis of

the optical efficiencies of the various components of the camera has been

completed. Design modifications are now being generated which should significantly improve the camera sensitivity and allow a wide range of gas pairs to

be investigated at high data rates.

- 13-
2.2.4. Response Behavior of Hot-Wires and Films to Flows of Different Gases

During our efforts to develop hot-wire anemometry as a velocity probe in

variable density flows it became apparent that the heat transfer laws available in the literature were not sufficient to predict the heat loss behavior

of a heated cylinder in different pure gases, much less mixtures of two or

more gases. Since such a correlation for heat transfer would greatly simplify

the tedious calibrations required in order to utilize the hot-wire technique

in variable density flows, a study of the responses of hot-wires and films to

varying flow velocities of different gases was carried out. Responses were

experimentally determined for flows of ten gases (air, N 2 , He, CH4 , C 3Hg, CC> 2

>

Ar , CF^j CFgBr, and SFg) having a wide range of molecular weights and

properties. Mixtures of these gases were also studied.

Data of this type are generally correlated by plotting the

nondimensionalized variables of Nusselt number (Nu) as a function of the

Reynolds number (Re) based on the cylinder diameter. In order to improve the

correlation of the results for pure gases, corrections were applied to the

data for heat losses by conduction at the probe ends, Knudsen number effects

due to the small size of the probes, and the lack of total thermal

"accommodation” of the available energy by gas molecules striking the heated

surface of the probe. Even after these corrections, it was necessary to

consider variations in molecular properties with temperature among the different gases. This last correction allowed the experimental results to be correlated to within the accuracy to which the molecular properties are known.

Figure 5 shows the resulting correlation. (KB ) Xanc* (^A^x are P araraeters

which account for the variations in molecular properties among the different

-14-
and

1 355

gases and are given explicitly as (K

fi ) x = [ ( Vm

/ V

oo) x

/

(

vm

/ v«) air ]

< kA>x ’ (Ac > air - [KW^mU0 ' 222 ^^! for gas x ’ 1,16 subscrl P ts ”

and ® refer to dynamic (u) or kinematic (v) viscosities evaluated at the

average of the probe surface and ambient temperatures or ambient temperatures,

respectively.

The correlation shown in fig. 5 is a significant improvement over others

available in the literature. This result demonstrates the importance of

utilizing a number of gases having wide variations in molecular weights and

properties when investigating heat transfer processes.

When the results of calibrations in mixtures of gases were considered,

systematic differences were found between the data and the correlations determined using the procedure for pure gases. We attribute these variations to

the importance of thermal diffusion to heat transfer in gas mixtures. This

conclusion is still a subject of active experimental and theoretical effort in

our group.

2.2.5. Effects of Global Density and Reynolds Number Variations on Turbulent

Mixing Behavior

During the first two years of this project an extensive series of

measurements of centerline concentration behavior were made for He/air,

CH^/air, C3Hg/C02» CgHg/air, SFg/air, and SFg/He jet/coflow gas pair combinations. Most measurements were made with Re a 4000 (based on tube diameter),

but measurements for the CgHg/air and SFg/air cases were extended to higher Re

in order to elucidate the importance of Re on the mixing behaviors of the

jets. Coflow velocities were low enough to insure that the confined jets

could be well approximated as free jets.

- 15-
In FY85 a simple pulsed shadowgraph system was developed which allowed

these variable density flows to be visualized. Figure 6 shows the schematic

for the experimental system and figs. 7-9 show examples of the shadowgraphs

which were observed. The He/air and C 3 Hg/air flows shown in figs. 7 and 8 ,

respectively, were both recorded at the same Re. The different appearances of

the two shadowgraphs are qualitative indications of global density effects on

turbulent mixing behavior (see ahead). Figure 9 shows the downstream region

of a SFg/air flow at relatively low Re. This jet is extremely negatively

buoyant, and, as the shadowgraph clearly shows, forms a fountain at relatively

short downstream distances.

The centerline concentration measurements were extended during the past

year to include CF4 jets at Re of 3960 and 7920 flowing into slow coflows of

air. Quantitative data are now available for seven different gas pairs

covering a jet/ coflow density range of 0.14 to 37. This is the most extensive

density range ever investigated in a single flow system. If these density

variations were due to temperature differences referenced to room temperature,

the range would be 8 to 2100 K. This wide range has allowed us to identify

several effects of global density differences which have not been previously

reported. These results along with those for the Re effects study led us to

generate new hypotheses concerning the development and mixing behavior of

turbulent flows (see ahead). Experimental results have also been compared

with the predictions of several different similarity analyses.

Due to space limitations, the experimental results, analyses, and

conclusions will only be briefly discussed here. A full manuscript has been

prepared and is undergoing NBS internal review. It should be available soon

- 16-
as an NBS internal report and will also be submitted to an archival journal

for publication.

Figure 10 shows the centerline concentration behaviors for six different

gas pairs plotted as Y^/Y^ versus the nondimensionalized downstream distance,

z/rQ

. Y represents the time-averaged mass fraction of jet gas and the subscripts o and m refer to values at the nozzle exit and along the jet centerline, respectively. Clearly, the rate at which the average centerline concen

tration decreases is dependent on the density ratio of the jet to surrounding

gases. Straight lines represent linear least squares curve fits of the data

and show that the virtual origins (z value where Y /Y passes through zero)

o m

also vary systematically with the density ratio.

Approximate similarity analyses available in the literature have

indicated that data for these variable density flows should fall on straight

lines having identical slopes when plotted versus z/r , where r , the

£ £

1/2 effective radius,’ is equal n to r p / P 1 • P and are the densities of

O O 00 O

00

the jet and surrounding gases. This expression is based on conservation of

momentum arguments.

Figure 11 shows the experimental data of fig. 10 plotted as a function

of (z - z 0 )/ r

e

» where z Q are the virtual origins for the different sets of

data. It is clear that the effective radius concept does give an improved

correlation of experimental results. Systematic variations with density rati

remain and are attributed to a breakdown in the similarity conditions at the

near field positions for which the measurements have been made.

- 17 -
RMS values of the centerline concentration, Y', normalized by the local

131 average concentration are shown plotted as a function of z/rQ

in fig. 12.

Y'/Y values are often referred to as concentration intensities or unmixedm m

ness. Figure 12 shows that the unmixedness values for the various jets seem

to approach a common asymptote of (Y'/Y 1 “ 0.23, but that the downstream

distance required for a given flow to attain this value depends strongly on

the density ratio. Negatively buoyant jets require longer distances than

positively buoyant flows.

To our knowledge, this is the first observation of such density effects

on the centerline growth of unmixedness in turbulent axi symmetric jets. A

remarkable collapse of the unmixedness values occurs when the results are

plotted as functions of z/r

£

. Figure 13 shows the results. This is dramatic

evidence that the correct turbulence axial length scale for these flows is

indeed r £ and not rQ

»

A very curious Re number dependence has been observed in this study.

Plots of Y^/Y^ versus z/rQ are found to have slopes which are independent of

and virtual origins which are displaced downstream as the Re is increased. On

the other hand, as fig. 14 shows, the approach of centerline unmixedness to

its asymptotic value is slowed dramatically at higher Re. This finding is in

conflict with the generally accepted view that higher Re are required in order

to observe fully-developed turbulent behavior.

On the basis of the qualitative differences observed in the shadowgraphs

and the quantitative centerline concentration measurements, two hypotheses

have been formulated which lead to qualitative predictions for turbulent flow

- 18 -
behavior which agree with the experimental observations. These hypotheses

are

1. The time-averaged mixing behavior as well as values of unmixedness

are determined primarily by large scale structures; and

2. A finite time and downstream distance are required for the entire

length scale structure of the turbulent flow to come into a pseudoequilibrium. The downstream distance required increases as the

turbulent eddy structure extends to smaller scales.

These assumptions lead to predictions of the variations observed in

centerline values of average concentration and unmixedness for jets having

different jet to coflow density ratios. All that is required is that the

large scale structures in the turbulent eddy size distribution scale as r

£ instead of r

Q

in the axial direction. Evidence that this is indeed the case

can be seen in the shadowgraphs for the different jets where the visualized

structures of the jets having higher jet to coflow density ratios appear to be

elongated along the axial direction (compare figs. 7 and 8). These arguments

suggest that variations in the centerline development of jet fluid unmixedness

with Re are due to the wider range of eddy sizes which are present in the

higher Re flows with the result that longer downstream distances are required

for the eddies to come into a pseudoequilibrium. Note that these hypotheses

might well explain variations in unmixedness development which have been

widely discussed in the literature.

-19-
The ideas discussed above must be considered as speculative at the

present time. However, they do provide a framework for understanding the

results of the current study. Further work is clearly required to test,

validate, and quantify the predictions.

2.2.6 Final Remarks

During the initial three years of this project significant progress has

been made on the development of new diagnostics for variable density flow

fields as well as the generation of the data necessary to characterize and

ultimately predict the effects of global density variations on turbulent

mixing. It is expected that these techniques and the resulting measurements

will contribute significantly to the overall goal of this work - namely,

improving the fundamental understanding of chemically reacting turbulent flow.

3.

PUBLICATIONS FOR FY83, FY84, AND FY85

1. "The Application of Laser-Induced Rayleigh Light Scattering to the Study

of Turbulent Mixing", W.M. Pitts and T. Kashiwagi, National Bureau of

Standards Internal Report (NBSIR 83-2641), January 1983.

2. "The Application of Laser-Induced Rayleigh Light Scattering to the Study

of Turbulent Mixing", W.M. Pitts and T. Kashiwagi, J. Fluid Mech. 141

(1984) 391-429.

3. "A New Diagnostic for Simultaneous, Time-Resolved Measurement of

Concentration and Velocity in Simple Turbulent Flow Systems", W.M. Pitts,

B.J. McCaffrey, and T. Kashiwagi, Proceedings of the Fourth Symposium on Turbulent Shear Flows, Karlsruhe, West Germany, September 12-14, 1983.

4. "Response Behavior of Hot-Wires and Films to Flows of Different Gases",

W.M. Pitts and B.J. McCaffrey, National Bureau of Standards Internal

Report (NBSIR 85-3203), July 1985.

5. "Response Behavior of Hot-Wires and Films to Flows of Different Gases”,

W.M. Pitts and B.J. McCaffrey, submitted to J. Fluid Mech. (1985).

-20-
6. “Effects of Global Density and Reynolds Number Variations on Mixing in

Turbulent, Axi symmetric Jeets", W.M. Pitts, to be published as National

Bureau of Standards Internal Report (1985).

7. "Effects of Global Density and Reynolds Number Variations on Mixing in

Turbulent, Axisymmetric Jets", W.M. Pitts, to be submitted to J. Fluid

Mech. (1985).

4. PROFESSIONAL PERSONNEL

In addition to the principle investigators. Dr. Bernard McCaffrey of the

Center for Fire Research has worked on the aneraometry calibrations in

different gases.

5. INTERACTIONS FOR FY83, FY84, AND FY85

1. "Concentration Measurements in Simple Turbulent Flow Systems Using

Rayleigh Light Scattering", W.M. Pitts and T. Kashiwagi, presented by

W.M. Pitts at the 1982 Fall Meeting of the Western States Section of the

Combustion Institute, Livermore, CA, October 11-12, 1982.

2. "Chemically Reacting Turbulent Flow", W.M. Pitts, T. Kashiwagi, and

H.R. Baum, presented by W.M. Pitts at the 1982 AFOSR Contractors Meeting

on Airbreathing Combustion Dynamics Research, Clearwater Beach, FL,

November 1-4, 1982.

3. "A New Diagnostic Technique for Simultaneous, Time-Resolved Measurement of

Concentration and Velocity in Simple Turbulent Flow System", W.M. Pitts,

B.J. McCaffrey and T. Kashiwagi, presented by W.M. Pitts at the 1982 Fall

Technical Meeting of the Eastern States Section of the Combustion

Institute, Atlantic City, NJ, December 14-16, 1982.

4. "A New Diagnostic Technique for Simultaneous, Time-Resolved Measurement of

Concentration and Velocity in Simple Turbulent Flow Systems", W.M. Pitts,

B.J. McCaffrey, and T. Kashiwagi, presented by W.M. Pitts at the Fourth

Symposium on Turbulent Shear Flows, Karlsruhe, West Germany,

September 12-14, 1983.

5. "Chemically Reacting Turbulent Flow", W.M. Pitts, T. Kashiwagi, and

H.R. Baum, presented by T. Kashiwagi at the 1983 AFOSR

Contractors/Grantees Meeting on Airbreathing Combustion Dynamics Research,

Scottsdale, AR, September 19-23, 1983.

-21 -
6 . "The Application of Laser-Induced Rayleigh Light Scattering to the Study

of Turbulent Mixing", presented by W.M. Pitts to the Nuclear Engineering

Seminar of the Department of Chemical and Nuclear Engineering, University

of Maryland, College Park, MD, October 30, 1984.

7. "Response Behavior of Hot-Wires and Films in Different Gases", W.M. Pitts

and B.J. McCaffrey, presented by W.M. Pitts at the 37th Annual Meeting of

the American Physical Society, Division of Fluid Mechanics, Providence,

RI, November 18-20, 1984.

8. “Chemically Reacting Turbulent Flow", W.M. Pitts and T. Kashiwagi,

presented by W.M. Pitts at the AFOSR/ONR Contractors Meeting in

Combustion, Pasadena, CA, July 23-25, 1985.

9. “Development of an Intensified Digital Line Camera System for Real-Time

Line Measurements in Turbulent Flows”, presented by W.M. Pitts at the 1985

Fall Technical Meeting of the Eastern Section of the Combustion Institute,

Philadelphia, PA, November 4-6, 1985.

-22-
U

(m/s)

E

(volts)

Xc

3

h

8

Figure 1. Examples of simultaneous, time-resolved measurements of concentration

and velocity for a turbulent jet of propane flowing into air are shown

.

PROPANE MOLE FRACTION

HOT-FILM OUTPUT

CALCULATED VELOCITY

- 23-
EVALUATION CIRCUIT

RET I CON LINE SCANNER

Figure 2. The line camera system designed for line measurement of

is shown

concentration
Figure 3. The entire experimental system for making line measurements in the

turbulent flow field of an axisvmmetric jet is shown.

- 25 -

Figure 4. An example of the results of time-resolved line measurements in

a propane/air jet/coflow is shown. A seven level gray scale

represents propane concentration having a maximum mole fraction

of 0.40.

Pixel #

Increasing r-*

127

- 26 -

Flow

direction

Figure 5. Correlation of Nusselt number as a function of Reynolds number for

heat transfer from a hot-wire to nine different gases.

-27 -
Figure 6. The experimental system for time-resolved shadowgraph imaging of

variable density flows is shown schematically. Note that the

coflowing gas is passed through the enclosure surrounding the

nozzle.

- 28-
Figure 7. A superposition of three shadowgraphs for an axi symmetric , turbulent

jet of helium at Re = 3950 flowing into a slow coflow of air is

shown

.

He/air, Re = 3950

40

30

zAo

20

10

0

-15 -10 -50 5 10 15

r/r0

-29 -

Figure 8

z

A superposition of four shadowgraphs for an axisymmetric

,

jet of propane at Re = 3960 flowing into a slow coflow of

shown.

C3 H8/air, Re = 3960

/r,

10 15

r/r«

turbulent

air is

- 30-

Figure

SF6/air, Re = 3950

45

Z/r0 35

25

-15 -10 -5 0 5 10 15

r/r0

. A shadowgraph is reproduced of an SF

6

jet flowing into air which

clearly shows the formation of a fountain by this strongly negatively

buoyant jet.

- 31-

z/r0

Figure 10. Centerline values of Y^/Y^ are plotted as functions of z/r^ f° r

the six different gas/coflow combinations listed.

- 32 -

Figure 11. Experimental values of Y^/Y^ are plotted as a function of (z-Z

Q

)/r_

for the six different jet/coflow combinations listed.

- 33-
Figure 12. Centerline unmixedness values for six different jet/coflow combinations are plotted as functions of z/r . o

- 34 -
Figure 13. The experimental results of figure 12 for

Y ' /Y m

m functions of z/r .

are replotted as
Figure 14. Unmixedness values are plotted as functions of z/r^ for propane/air

turbulent jets at three different Reynolds numbers.

- 36 -
HgS.IMA iwcv. a-«c)

(J.». DIPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR-85/3299

2. Performing Organ. Report No. 3. Publication Date March 19 86

4. TITLE ANO SUBTITLE

Chemically Reacting Turbulent Flow. Annual Progress Report for October 1, 1984

to September 30, 1985 and Final Technical Report for the Three Year Period from

October 1, 1982 to September 30, 1985

5. AUTHOR(S)

William M. Pitts, Takashi Kashiwagi

i. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s)

national bureau of standards

DEPARTMENT OF COMMERCE

Gaithersburg, MD 20899

7. Contract/Grant No.

AmsRr-Ts.sA-nnni ?

8 . Type of Report & Period Covered

Final 10/1/82-9/30/85

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City, State, ZIP)

Air Force Office of Scientific Research

Bolling AFB

Building 410

Uatjhingfnn r FIT 70777

10.

SUPPLEMENTARY NOTES

I I Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

This report summarizes the results of the first three years of a study on chemically reacting turbulent flow. This project has been jointly sponsored

by the Air Force Office of Scientific Research and the National Bureau of

Standards. The development of new diagnostics for variable density flows

are described. These include Rayleigh light scattering for real-time,

spatially-resolved concentration measurements, combined Rayleigh light

scattering and hot-wire anemometry for simultaneous concentration and

velocitv measurements, and the development of a digital line camera which

has allowed the concentration measurements to be made along a line. A study

of heat transfer from heated cylinders is discussed which has generated a much improved correlation of experimental results. Shadowgraph visualization studies and centerline concentration measurements are described for

variable density, axisymmetric turbulent jets covering jet to coflow density

ratios of 0.14 to 37. These studies have also included a limited investigation of Reynolds number effects. The observed dependence of the mixing

behavior on the density ratio and Re have led us to make new hypotheses

concerning the nature of turbulent mixing. These hypotheses are shown to

lead to qualitative predictions for the turbulent mixing which are in agreement with experimental findings.

14 KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semi colon s Concentration fluctuations; concentration measurement; density effects; digital

line camera; flow visualization; hot wire anemometry; Rayleigh light scattering;

Reynolds number; velocity measurements

13. AVAILABILITY

!~x] Unlimited

CD For Official Distribution. Do Not Release to NTIS

CD From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

20402. H Order From National Technical Information Service (NTIS), Springfield. VA. 22161

14. NO. OF

PRINTED PAGES

37

15. Price

$9.95

USCOMM-OC «0«3-p*0