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Produced by the NASA Center for Aerospace Information (CASI)
Ingdni4 r Grind

R.M. A^UETR Dr. U. YOGBL

09 5798 RAPPORT - BERICHT

CO 232184

H. J. SCHAM

Study of high-speed combustion

flows by laser velocimetry

Saint-Louis, 26.10.1984

V auteur

Der Verfasser

q, 1, G&

Dr. B. J. SCHIM

Les Directeurs

Die Direktoren

d doNwe 481,1W 

U . it PAM

E^p1. lM

ss

OMw MMit MiMft Emm
3 _ 7 

Presentation given during

a visit to NASA Langley Research Center,

Hampton (Virginia /USA), 5 October 1984

Key-words

Combustion chambers

Laser anemometer

Velocity measurement

Correlation

Experimentation

Measurement procedures

Experimental data

Fluid mechanics

I^ _
Table of contents

Page

Introduction S

Hot free jet facility S

Experimental problems 6

Laser Doppler velocimster 6

Influence of refractive index fluctuations on the measurement accuracy 7

Seeding 9

Signal processing and data acquisition 9

Flow profiles 10

Spectral analysis 11

Space-time correlation 11

Evidence of a coherent structure 12

Size of the coherent structure 13

Convection velocity 13

Conclusions 14

28L - CO 232/84
-S -

Introduction

over the last decade the Franco-German Research Institute ISL at Saint-Louis,

France,has been conducting research on the structure of turbulent free jets and

particularly on the understanding of noise generation in jets.

The understanding of jet noise has been improved considerably since the apparent role of coherent structures in the noise producing process became evident.

Since then, many authors have identified coherent structures in free jets, thereby

using various measurement techniques. At ISL the investigations were concentrated on

analysing the velocity fluctuations by means of laser velocimetry and relating the

results to the acoustic emission of the jet.

It was within the framework of this research program that an experimental

study of high-speed combustion flows has been started. This study was primarily

aimed at demonstrating the feasibility of laser velocimstry in a high-temperature

jet rather than at providing a detailed description of its turbulence structure.

The main objectives were to overcome the particular problems encountered in a

combustion flow and, moreover, to compare the measurement results with those found

previously in an isothermal free jet. In addition to the usual information on an

and fluctuating velocities the measurements should be capable of providing the

spectrum of the velocity fluctuations as well as their space-time correlation.

According to the above, this paper will be deviled into three main ciections :

1 - test facility and particular problems encountered in the high-temperature jet ;

2 - how we, at ISL, tried to overcome the problems ;

3 - some exparimental results to demonstrate the capabilities of the measuring

technique.

Hot free jet facility

The experiments were carried out with the hot free jet facility of ISL. The

centre piece of this facility is a combustion chamber, which is a model of an aeroengine combustor. In this combustion chamber a fuel-air mixture is burnt and the

ISL - CO 232/84

S
resulting gases are expelled through a convergent nozzle. The exit diameter of the

nozzle is 80 sm. Between the combustion chamber and the nozzle a 400 mm long duct

is located such that a good mixing and stabilization of the flow can occur before

it enters the nozzle.

For the measurements described here, the mass flow rate was kept at 0.56 kg/s

resulting in an efflux velocity of about 400 m/s. The exit plane temperature was

about 1200 K and the Mach number 0.6.

Experimental problems

When trying to use a laser Doppler velocimeter for measurements in a combustion

flow, a number of particular problems have to be considered which are usually not

present in isothermal flows. The principal problems are :

- the hostile environment with high noise and high vibration levels,

- a non-isotropy and fluctuating refractive index,

- the need for appropriate seeding.

In addition, particular requirements are to be met by the signal processing

and data acquisition system :

- high speed and high accuracy,

- capability of providing spectral information,

- capability of identifying coherent structures.

Laser Doppler velocimater

The requirements to be met by the laser velocimeter are far beyond the capabilities of commercial systems. It has therefore been necessary to select the most

suitable optical layout and to develop an appropriate signal processing to produce

meaningful results in the rigorous environment of the combustion chamber. For reasons

of improved signal-to-noise ratio, ease of alignment, and relatively high stability

against vibrations, a one-component fringe mode laser velocimeter with forward-scatter

collector optics was used.

A schematic drawing of the optical arrangement is shown in figure 1 : an argonion laser emitting about one watt at the 315 nm wavelength is utilised. The beam

is split into two parallel beams which pass through acousto-optic modulators (Bragg

cells, B). These modulators have separate driving units and allow frequency difference

ISL - CO 232/84
-7-

between the beams of 2, S and 10 MHz to be obtained. The beams are brought to

intersection by lens L 1 . With the aid of mirrors S 1 , S2 and 83 the beams are directed towards the jet flow. The interference fringes in the probe volume are oriented

for the measurement of the axial velocity component.

The receiving optics is conventional, consisting of a collecting lens RA,

a narrow-band filter, and a photomultiplier. The remaining parts of the receiving

optics will be discussed later in connection with the cross-correlation measurements.

The laser and the vibration-sensitive part of the transmission optics are

protected from the noise originating from the combustion system by means of a protective housing. This has been found indispensible since the noise level under operating conditions can exceed 140 dB.

Precise positioning of the probe volume at a desired point in the flow is

provided by a traversing system based on a hydraulic x-y table. Receiving and

transmitting optics including the laser are fixed to a common mount and traversed

as the whole. In order to obtain various downstream station measurements in the

flow field the free jet assembly can be moved along its centre-line. The motion of

the hydraulic table and the free jet assembly is controlled remotely by linear

potentiometers providing an electrical read-out of the probe volume position in the

x, y and z direction.

Influence of refractive index fluctuations on the measurement accuracy

As indicated above, it is of interest to examine the extent to which fluctuations

in the fringe system may affect the measurement accuracy when temperature-dependent

variations of the refractive index occur in the flow. In fact, refractive index

gradients appearing normally to the propagation direction of the laser beams are

capable of causing the two beams to deviate from their initial direction. This may

result in changes in the position of the control volume relative to the flow as

well as in variations of the fringe spacing.

The shift of the measuring point can generally be neglected as compared to the

finite extension of the control volume. Variations of the fringe spacing, however,

can simulate an increased turbulence of the flow and may give rise to erroneous

conclusions concerning the spectrum of the velocity fluctuations.

Since a theoretical treatment of these effects is scarcely possible, an

ISL - CO 232/64
.. s.=,•4 .. 

3

- 8 -

extensive experimental study has been carried out with the objective of determining

the fluctuations in a fringe system which is generated in a hot supersonic flow.

In order to detect the fluctuations seen by an individual scattering particle

when passing through the control volume, a time resolution in the order of 25 nanoseconds was necessary. This resolution was achieved by using a high-speed streak

camera with rotating mirror.

Figure 2 shows a small section of a typical recording of the fringe pattern

the length of this section corresponds to a time interval of approximately 8 Ns

the fringe spacing is 70 microns. The picture shows the wave-like displacement of

the interference fringes as a function of time.

In order to determine quantitatively the influence of these fluctuations on

the accuracy of the LDV measurements, the variation in the fringe spacing was

measured for a certain number of fringes and 100 particle trajectories such as

indicated on the recording by the oblique white lines. These lines correspond to

the space-time curves of particles traversing the control volume with the flow

velocity of about 720 m/s.

As a result of an extensive evaluation of the recordings we conclude that

the-variation coefficient of the fringe spacing, even in the worst case, does not

exceed 2,8 X. According to this result, the measured turbulence intensity can be

corrected for the effects arising from the refractive index fluctuations.

The mode of correction is explained in figure 3 : provided that the velocity

fluctuations and the refractive index fluctuations are not correlated, the corrected

or "true" turbulence intensity 

Tcorr 

is given by the relation seen on the upper left

of the diagram, where T means the measured turbulence intensity and Td the variation

coefficient of the fringe spacing.

The figure shows a plot of the turbulence correction AT and the true turbulence

inte,tisity Tcorr 

as functions of the measured turbulence intensity T.

Considering the experimental situation, i.e. turbulence intensities that are

in excess of 8 X, this correction is seen to have no practical weaning as, even in

the worst case, it is only 0,5 X or, in other words, the turbulence intensity T

drops from 8 X to 7,5 X.

ISL - CO 232/84
r

- 9 -

Seeding

Since the concentration of natural scattering particles was too low to get

sufficient data rates, the flow had to be seeded artificially. This was done in

the total air supply upstream of the combustion chamber using a cyclone particle

generator (figure 4). The seed material was a mixture of titanium dioxide and

fumed silica ("Aerosil") which had proved to be a good seeding agent earlier.

The mean diameter of the titanium dioxide particles was found to be smaller than

0,5 ym. The addition of silica particles has bean deemed necessary in order to

prevent the titanium dioxide particles from agglomerating.

Signal processing and data acquisition

The electrical signal from the photomultiplier was processed with a countertype LDV processor the detailed description of which has been given elsewhere. This

processor, developed at ISL especially for high-speed applications, combines high

accuracy and high sampling rate. It is based on zero crossing detection in individual

bursts and employes end-of-burst detection for data validation. Rejection of high

amplitude signals is used to minimize bias errors due to particle lag.

A block diagram of the data acquisition system is shown in figure 5. The signal

processing equipment is composed of the photomultiplier tube with preamplifier, a

computer-controlled frequency filter, the fore-mentioned LDA processor, a 1 Mz

clock, and a digital multiplexer. Tc attain maximum sampling rates, the data are

recorded using direct memory access to a minicomputer. The minicomputer not only

provides data acquisition and reduction but also controls the frequency filter bank

consisting of 8 low-pass and 8 high-pass filters. The filter bank is tuned automatically according to the instantaaeous signal frequency. Low-pass and high-pass

cut-off frequencies are thereby set to the center frequency plus and minus three

times the standard deviation, respectively.

In order to incorporate time information in the signal processing, a 1 MHz

clock has been added to the data acquisition system. Time information is recorded

with each valid sample and a pair of velocity and arrival tine is transmitted to

the minicomputer. This allows the time between successive validated signals to

be measured and both particle and time averages of the velocity to be calculated.

Moreover, time information is needed for the determination of the turbulence spectrum.

To obtain the turbulence spectral information, in a first step the autocorrelation

ISL - CO 232/84
function of the velocity fluctuations is established. Then, a Fourier transform of

this outocorrelation function is performed providing an estimate of the turbulence

Paver spectrum.

The entire data acquisition system was checked out carefully with simulated

electronic signals as well as in the course of extensive investigations of cold

free jets. In the velocity range considered here, the rms error of the individual

velocity value was found to be substantially less than 0,5 2. In addition to this

high accuracy, the novel data acquisition system offers high sampling rates. The

minimum time interval between successive samples isas low as 10 ps according to

the maximum data rate of the direct memory access. Only the high data rate along

with appropriate seeding of the flow allows turbulence spectra to be measured

within reasonable lengths of time.

Flow profiles

The experimental results that will be discussed here are the radial distribution

of mean velocity and turbulence intensity measured at various downstream locations

in the flow as well as the turbulence spectra.

Radial surveys of the mean flow velocity and the corresponding fluctuations

were made at 10 axial locations ranging from 0.25 to 7 nozzle diameters (D) downstream

of the nozzle exit. As an example of the results, the transverse scans at 0.5D, 1.5D,

3D and 6D are shown in figure 6. In this diagram 'v is the mean velocity and Cr is the

rms value of the velocity fluctuations, both given in m/s. x and z denominate the

radial distance from the jet centre-line and the downstream location from the nozzle

exit, respectively. Each of the radial profiles is the result of about 25 point

measurements with 2000 individual realizations. This diagram illustrates the variation

of the flow field with increasing distance from the nozzle.

The radial profiles exhibit a strong similarity when plotted in appropriate

coordinates. This is demonstrated in figure 7 : here, the relative velocities are

plotted versus the lateral jet coordinate centered at the nozzle radius and divided

by the axial distance s. The diagram shows a very good collaps of the data for eight

different locations in the jet, which is an indication of the causistancy of the

measurements.

ISL - CO 232/64
 11 

Spectral analysis

As mentioned before, not only mean velocities and turbulence intensities but

also the spectrum of the velocity fluctuations in the hot jet could be determined

owing to the capabilities of the novel data acquisition system. The objective of

the spectral analysis was to find out if, similar to the results in isothermal jets,

also in the hot jet the flow velocity fluctuates at a preferred frequency. Much

could be learned about the noise generation if such preferred frequencies could be

identified and correlated with the noise radiation of the jet.

Figure 8 shows the spectrum measured in the exhaust jet. The power spectral

density is given in units of (m /9) 2/Rz. Indeed, this spectrum shows a preferred

frequency of the fluctuations at about 1.5 kHz.

The spectral density has been averaged over several spectra where each individual

spectrum was established from 3000 velocity samples. The frequency resolution is

61 Rx.

Space-time correlation

In the study of coherent structures it is of particular interest to measure

the correlation that exists between the velocity fluctuations at two neighbouring

points A and B in the flow. The temporal relationship between these fluctuations

is given by the cross-correlation function, defined as

+T

RAB(t) lim v'A(t)vB(t+t)dt

T+

-T

wherev'(t) is the velocity fluctuation at measuring point A and v8(t+t) stands for

the time-delayed fluctuation at point B.

The cross-correlation results often are *=pressed in terse of the correlation

coefficient

A (t)

PAS") .

A B

where uA and *I

3

the variances of the fluctuations at point A and point I.

respectively.

ISL - CO 232/84
It has been shown earlier that laser Doppler vslocimetry, when combined

with digital correlation methods, can provide new and useful information about

the turbulence structure of jets. Thus, coherent structures in an isothermal jet

could be identified and characterised by their site and convection speed.

For the cross-correlation studies the conventional optical set-up had to be

modified as shown in figure 9. As usual, two laser bears are brought to intersect

in the flow region. The scattered light originating from the probe volume A is

collected in near forward direction by the receiving optics. 8o far, the optical

system is quite conventional. The cross-over of the two beam is then reproduced

further downstream by means of a spherical mirror. The second measurement point

(a) is also observed in forward-scatter direction. Thus, the cross-talk between

the two probe volumes is ttngligible even at vanishing sep a ration distances. The

separation between A and 8 can be varied continuously from zero to approximately

120 m.

The signals were processed using the burst-counter processor described above.

The majQr units of the entire data acquisition system are shown in figure 10.

The data acquisition system is composed of an analog multiplexer, the forementioned signal processor, a 1 Mtz clock, a digital multiplexer, and a minicomputer.

The velocity data from the two probe volumes appear alternately at the output of

tht signal processor. Together with each velocity sample the elapse time between

successive signals is recorded. The time information is needed for the correlation

analysis.

In order to attain zaxinm sampling rates, direct memory access to the minicomputer is used. The actual processor data rate during this study was typically

15 000 per second. At this data rate a total sampling time of 200 us was sufficient

to establish a reliable cross - correlogram.

bvici ence of a coherent structure

in a first #arias of spacs-time correlation measurements the two probe volumes

were: located on the jet centre-line, one was hold fixed at 3 diametars from the

nozzle exit, the second was moved along the jet axis.

ISL — CO 232/84
- 13 -

Typical correlogrems obtained in this way are shown in figure 11. In these

diagrams the cross-correlation coefficient p is plotted as a function of the delay

time T. Correlograms are shown for two different streamwise separations At between

the measurement points. The sinusoidal appearance of the correlograms reveals the

presence of a coherent structure which means that a certain phase relationship of

the velocity fluctuations is maintained over a considerably long time.

The period of oscillation of the correlation curves gives a measure of the

preferred frequency at which the flow velocity is fluctuating. The present measurements

yielding a preferred Stroubal number of 0.3 are in good agreement with a previous

spectral analysis.

Size of the coherent structure

Figure 12 illustrates how the correlation curves vary with the saparatiun

distance between the probe volumes. In this diagram we have summarised a series of

correlograms. It shows clearly that, as the separation As is increased, the correlation curves and their peaks move to greater time delay. At the same time, the peak

correlation coefficient decreases.

The relatively slow decay of the peak correlation with increasing separati4m

reveals a considerably long lifetime of the coherent pattern. According to figure 12

the lifetime must be in *=sea of 400 us.

The spatial extent of the coherent structure can also be deduced from the decay

of the peak correlation. This is illustrated in figure 13 : here, the peak crosscorrelation has been plotted over the streamwise separation between the probe

volumes A and B (the correlation coefficients have been normalized to unity at

zero separation). If one defines the coherence length of a structure as the separation distance after which the peak correlation drops below the fraction i/s,

the streamvise coherence length is found to exceed 1.5 nozzle diamsers. This result

is in good agreement with similar measurements in an isothermal jet.

Convection velocity

In addition to the size of the or--uctursa the velocity at which they are convected downstream can also be obtained from the cross-correlation measurements.

The convection velocity is given by the ratio of the spacing As and the time delay

M - CO 232/84
to reach maximum correlation i vc • as TU. /

Pigare 14 ohms the centre-line distribution of the convection velocity. The, crosses

are the values obtained from the•correlogram; they are plotted against the stresswise distance from the nozzle. The convection velocity rises with the distance s

and then approaches the mean flow velocity which has been plotted for comparison

(broken curve).

Again, a reasonable agreement with the measurements in an isothermal jet is

found. This is demonstrated in figure 15. Here, the convection speed has been nondimensionalized by the local mean flow velocity V. The crosses are the measured

values for the hot jet. the dashed line represents the measurements for the cold

jet. The slopes of the straight lines are slightly different, but this difference

vanishes if the distance z is normalized by the potential core length of the

respective jet.

conclusions

The present experiments have shown that the particular problems encountered

in a combustion flow can be overcome-if a carefully designed optical set-up as

well as an appropriate signal processing and data acquisition system is used. In

this way, laser Doppler velocimetry can provide new and useful information about

coherent structures in hot free jets. The measurement results are shown to be in

good agreement with previous measurements in an isothermal jet.

ISL - CO 232/84
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ISL - CO 232/84
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