scaling of the two phase flow downstream of a gas turbine combustor swirl cup part i mean quantities

Samuelsen UCI Combustion Laboratory, University of California, Irvine, CA 92717-3550 Scaling of the Two-Phase Flow Downstream of a Gas Turbine Combustor Swirl Cup: Part I— Mean Quantit

H Y Wang

V G McDonell

W A Sowa

G S Samuelsen

UCI Combustion Laboratory, University of California, Irvine, CA 92717-3550

Scaling of the Two-Phase Flow Downstream of a Gas Turbine Combustor Swirl Cup: Part I—

Mean Quantities

A production gas turbine combustor swirl cup and a3x -scale model (both featuring co-axial, counterswirling air streams) are characterized at atmospheric pressure

Such a study provides an opportunity to assess the effect of scale on the behavior

of the continuous phase (gas in the presence of spray) and droplets by comparing the continuous phase velocity, droplet size, and droplet velocity at geometrically analogous positions Spatially resolved velocity measurements of the continuous phase, droplet size, and droplet velocity were acquired downstream of the production and 3 X -scale swirl cups by using two-component phase-Doppler interferometry in the absence of reaction While the continuous phase flow fields scale well at the exit

of the swirl cup, the similarity deviates at downstream locations due to (1) differences

in entrainment, and (2) a flow asymmetry in the case of the production hardware

The droplet velocities scale reasonably well with notable exceptions More significant differences are noted in droplet size, although the presence of the swirl cup assemblies substantially reduces the differences in size that are otherwise produced by the two atomizers when operated independent of the swirl cup

Introduction

Co-axial, counterswirling air streams have been studied for

a variety of applications in combustion and other systems

Most of the studies have been conducted at nonreacting,

single-phase (i.e., gas single-phase in the absence of spray) conditions Some

of these studies (e.g., Habib and Whitelaw, 1980; Vu and

Gouldin, 1982; Gouldin et al., 1983) observe that only

coun-terswirl produces recirculation, while others (e.g., Samimy and

Langenfeld, 1988; Mehta et al., 1989) find that both coswirl

and counterswirl can generate a recirculation zone

One of the practical applications featuring two co-axial

counterswirling streams is the GE SNECMA CFM56 engine

combustor swirl cup (Fig 1) Fuel is injected by a simplex

atomizer mounted in the center of the swirl cup A portion of

the droplets convect directly downstream while the remainder

impinge onto the inner surface of a primary venturi (which

separates the primary swirling air from the secondary swirling

air), form a thin liquid film, and are re-atomized in the shear

field produced between the two counterswirling air streams

A goal of this swirl cup assembly is to produce a uniformly

distributed field of similarly sized fine droplets

Contributed by the International Gas Turbine Institute and presented at the

37th International Gas Turbine and Aeroengine Congress and Exposition,

Co-logne, Germany, June 1-4, 1992 Manuscript received by the International Gas

Turbine Institute February 6, 1992 Paper No 92-GT-207 Associate Technical

Editor: L S Langston

air injected through 8 holes injected through

arte passages

1 Atomizer

o'p- ^ • \ / Continuous-phase zero axial

'„ j^ , i velocity streamline 4.Secondary swirler < ' 5.45°Conical sleeve

6.Mounting plate

Fig 1 Swirl cup assembly

Table 1 Characteristics of the PDI

1 0 0 - I ^ E 1x Atomizer

80 „ °0° o Transmitter

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Fringe spacing (fim)

Waist ((tm)

0.4880 (im line (V or I*02

Fringe spacing (^m)

Waist (/xm)

Receiver

Collection lens (mm)

Focusing iens (mm)

Spatial filter (tun)

Collection angle

9.03 223.32 9.31 211.82

629 f/5.7

238

100

30 deg off-axis

9.88 187.17 9.84 177.53

1000 f/9.3

238

100 forward

*U, V, W, and D are axial, radial, tangential velocity, and droplet

diameter, respectively

Due to the complexity of the co-axial, counterswirling air

flows and the lack of adequate advanced diagnostics, few

stud-ies have been conducted on the two-phase behavior in the

presence of such flows Only recently have such flows been

considered in a series of tests conducted at the UCI Combustion

Laboratory (e.g., Wang et al., 1991a, b, c; 1992a)

The studies have been conducted at two scales First, tests

have been completed in a 3 x -scale model Secondly,

meas-urements have been acquired in production hardware (i.e.,

"1 x-scale") While the purpose of the tests was to provide

data in support of spray modeling, the results offer a unique

opportunity as well to study (1) the extent to which the results

scale on the basis of geometric similarity, and (2) the behavior

of droplets at two geometrically similar scales

In the present study, the time-averaged droplet size and

velocity distributions are compared downstream of the

pro-duction hardware and the 3 x-scale model of the swirl cup

The liquid and air mass flow rates in the 1 x test are about

1/9 of those in the 3 x test, following the ratio of the air inlet

area of the practical swirl cup to that of the 3 x -scale model

swirl cup The purpose of this choice was to maintain a constant

velocity from the swirling air outlet and a constant liquid

loading rate (or airtoliquid ratio) for both the 1 x and 3 x

-scale tests

Experiment

Swirl Cup Assembly Hago simplex atomizers, having flow

numbers of 0.65 and 7.30 (based on ratio of flow rate, in

lb/hr, to square root of injection pressure differential, in lb/

in.2), were used in the 1 x - and 3 x -scale tests, respectively

A 6.35 mm polycarbonate honeycomb (101.6 mm thick) was

placed 50 mm above the top of the swirl cup in both cases to

provide a uniform velocity profile at the entrance plane to the

swirlers

Characterization Chamber Two different

characteriza-tion chambers were utilized Although not specifically designed

for these tests, each chamber was similar in design, and that

used for the 3 x -scale test was a 2.7-scale version of that used

for the 1 x -scale tests In both chambers, the test article was

centrally located within a square duct (495 mm x 495 mm for

1 x-scale; 1330 mm x 1330 mm for 3 x-scale) and oriented

downward The test article was attached to a vertical traverse,

which was connected to the chamber The chamber itself was

suspended from an optical table using a two-dimensional

trav-erse, thus giving the test article three degrees of freedom In

each case, the diagnostics were fixed, and the test article was

moved Additional details about each facility are available

elsewhere (3 x-scale: Wang et al., 1990; lx-scale: Wang et

al., 1991c, and McDonell and Samuelsen, 1991)

Instruments A two-component phase-Doppler

interfer-ometer (PDI) (Aerometrics Model 3100-S) was used to measure

the droplet size and velocities An Ar+ laser provided the laser

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beams for the PDI measurements The PDI setup used for both tests is shown in Table 1

Test Condition and Sample Points

Test Condition The inlet area of the swirlers for the

pro-duction swirl cup is 1/9 of the 3 x -scale model To make the air outlet velocities through the 1 x swirlers the same as those

of the 3 X -scale model, the 1 X -scale tests were conducted at

an air flow rate of 0.017 kg/s (30.2 scfm), which is 1/9 of the air flow rate used in the 3 X -scale test

Water was used in both tests To maintain the liquid-to-air ratio the same as a stoichiometric ratio of a kerosene fuel (about 14.78), the liquid flow rate of the production and 3 x -scale swirl cups should be 1.1 g/s and 10.0 g/s, respectively

While the 3 X -scale tests were conducted at 10.0 g/s (Wang et al., 1991a, b), the lx-scale hardware was operated at 0.86 g/s due to flow limitations in the test stand This provided a liquid-to-air loading rate of 5.0 percent rather than 6.5 percent

in the 3 x -scale model test However, the swirl cup flow field

is dominated by the aerodynamics (Wang et al., 1991a, b), and this difference is considered negligible with respect to affecting both the gas-phase flow and droplet dispersion

Sample Points The measurements were conducted at three

axial locations: Z = 1.75, 2.75, and 3.75 R p (where R p is the radius of the primary venturi exit plane), and along the cen-terline of the swirl cup The origin of the coordinates is at the

center of the primary venturi exit plane R p for the 1 x - and

3 X -scale fixtures is 9.7 mm and 29.2 mm, respectively

Results and Discussion

From the myriad of data collected, selected measurements are presented and specific characteristics are identified that are particularly germane to the behavior observed at both scales

Atomizer Comparison in the Absence of the Swirl Cup The

two atomizers were characterized in the absence of the swirl cup assemblies to provide a baseline against which to compare differences observed in the presence of the swirl cup An ex-ample is shown in Fig 2 While large differences in Z?32 are observed due to the geometric difference in size and the order

of magnitude difference in mass flow, far less difference is observed in the presence of the swirl cup assemblies

Comparison of lx- and 3 x -Scale Swirl Cup Results In

the following, comparisons are presented for radial profiles at

three axial planes (Z = 1.75 R p , 2.75 R p , and 3.75 R p ) In

addition, for the velocity results, a center line profile is included and appears in the top portion of the figure Data are provided for the following representative droplet size groups:

"Small"

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cup While the D i2 in the 3 X -scale case is systematically greater

than in the 1 x case, the differences are reduced as the flow

evolves downstream The larger droplet size distribution for

the 3 x -scale case is attributed to the differences in the initial

droplet size distribution

Additional insight is offered by the data rate results shown

in Fig 4 Compare, for example, the data rate of droplet arrival

for the large and small droplets At the first axial location

(Z = 1.75 R„), the data rate of the small droplets is greater

for the 1 x -scale (Fig 4a), and the data rate for the larger droplets is greater for the 3 x -scale (Fig 4c) Note the re-markable correspondence for the medium-sized droplets (Fig

4b) This is the one large population of droplets that both

scales share in common This data set also reveals the first evidence of the effect of scale on droplet entrainment At the

centerline of the Z = 3.75 R p location, the data rate of the

1 x -scale falls relative to the 3 x -scale These data, along with the mean axial and radial velocity data, reveal a

dispropor-tionate entrainment of medium-sized droplets in the 3 x -scale

case Note also the associated impact on the D i2 results at

Z = 3.75 R p (Fig 3) The twin-peak droplet data rate

distri-butions in both cases suggest that the region downstream of

the primary venturi is highly populated with droplets and thus

high in liquid volume flux

Mean Axial Velocities Figure 5 presents results for the

mean axial velocity At the exit of the swirl cup, the axial

velocity profiles of the continuous phase are identical for both

cases (Fig 5a) Downstream, modest but significant deviations

occur For example, the centerline profile reveals that a slightly

longer recirculation zone exists downstream in the 3 x -scale

test and the Z = 3.75 R p profile shows that the recirculation

zone is wider as well This is attributed to differences in

en-trainment rates between the two cases

The 1 x -scale data at Z = 3.75 R p also reveal an asymmetry

in the flow of the practical hardware The flow field of the

3 x-scale model swirl cup is, in contrast, symmetric due to a

stronger axial momentum

The behavior of the droplets is shown in Figs 5{b-d) The

small droplets for both the 1 X - and 3 x -scale tests mirror the

continuous phase (Fig 5b) with one notable exception At

Z = 1.75 R p , the small droplets in the lx-scale test have

not fully relaxed to the continuous phase velocity This is the

first concrete example of the differences in time between the

two scales for droplets to acclimate The medium-sized

drop-lets in the 3 x-scale test also reflect this (Fig 5c) In

addi-tion, the large droplets in the 3 x -scale case are recirculated at

Z = 3.75 R p , whereas those in the 1 X -scale case are not (Fig

5d) The differences are attributed to the relatively long

dis-tance traveled from the atomizer to the same geometrically analogous points in the 3 x scale test compared to the 1 x -scale test Given that the droplet velocities are similar in each case, the medium-sized droplets have more time to approach the velocity of the continuous phase at the same geometrically analogous point in the 3 X -scale test than in the 1 X -scale test

Note the relatively analogous mean axial velocities at the first axial location for both cases

Mean Radial Velocities Figure 6(a) compares the mean

radial velocities of the continuous phase Positive values on

the + Y side and negative values on the - Y side indicate

velocities away from the centerline

Near the swirl cup, the velocities scale remarkably well Note

in particular the evolution of the 3 x -scale mean radial

veloc-ities downstream At Z = 3.75 R p , for example, away from

the centerline the flow is radially outward, whereas close to the centerline the flow is toward the centerline Downstream

of the Z = 1.75 R p location, the asymmetry in the production hardware is clearly evident Both the centerline and radial profiles reveal a nonzero centerline velocity due to a mismatch

of the aerodynamic and geometric centerlines

As with the mean axial velocity, the radial velocities of the

droplets are well scaled at the upstream location Z = 1.75 R p

Downstream, the mean radial velocities for the small (Fig 6b)

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Mean Tangential Velocities The continuous phase mean

tangential velocities are presented in Fig 7(a) Looking

down-stream from the swirl cup, positive values on the + A'side and

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ro-tation At Z = 1.75 7?,,, the results appear "noisy." In fact,

the data reflect a subtle behavior and, at Z = 2.75 Rp, the

results are similar Specifically, the radial location where the

peak tangential velocities occur is the same and the decay of

the profiles is similar Unlike the mean axial and radial

ve-locities, the tangential velocities of both scales are not precisely

velocity distributions display differences in magnitude and trend not observed in either the axial or radial velocity

The mean tangential velocity distribution of the small

drop-lets is presented in Fig 1(b) The data for the small and

me-dium-sized droplets in the 1 x -scale test reveal a twin-peak distribution on either side of the centerline, with one inside the recirculation zone and the other outside of it To under-stand the peaks, the sources from which droplets emanate to this point must be identified: (1) droplets recirculating while swirling counterclockwise, (2) droplets produced from the edge

of the venturi (dominated by the counterclockwise-swirling secondary air), and (3) droplets injected directly from the at-omizer, which are dominated by the clockwise rotating primary air The relative contribution of these three sources results in strong bimodal velocity distributions (Wang et al., 1991b)

What is different here is how the scale affects the sign of the peak The major question is whether droplets directly injected from the atomizer with clockwise rotation can overcome the negative pressure gradient of the recirculation zone and pen-etrate to this point

The small droplets (Fig lb) show two positive peaks at matched at Z = 1.75 Rp The droplet behavior provides the Z = 1.75 Rp Outside the recirculation zone, the

counterclock-explanation

The droplet mean tangential velocities, presented in Figs

l(b-d), provide especially interesting insights with respect to

the effect of scale In particular, the droplet mean tangential

wise rotating secondary air is dominant, and inside the recir-culation zone the counterclockwise rotating cirrecir-culation overpowers the clockwise rotating droplets emanated from the atomizer directly Note that the 3 x -scale data are significantly

less precise than the mean axial and radial velocities at Z =

1.75 R p Clearly, scale and the associated differences in

en-trainment, and droplet residence time have a major impact

The behavior of the medium-sized droplets is especially

in-sightful The 3 x-scale data at Z = 1.75 R p mirror the small

droplet data The 1 x -scale data, however, reveal a strong

counterclockwise rotation in the recirculation zone, indicating

that insufficient droplet residence time is available for these

sized droplets to accelerate to the locally counterclockwise

swirling flow

Large droplets penetrate farther in the axial direction, as

shown in Fig 1(d) In this case, only two of the above sources

contribute because very few large droplets are recirculated As

a result, the measurements consist of clockwise rotating

drop-lets, which arrive directly from the atomizer and

counterclock-wise droplets arriving from the venturi The behavior of the

large droplets is similar for both cases because, compared to

the small- and medium-sized droplets, their relaxation time is

longer whereas their residence time is relatively shorter

In both cases, with increasing axial distance downstream,

the twin-peak distribution transitions into a single-peak

dis-tribution because the flow is dominated by the

counterclock-wise swirling secondary air flow

Conclusions

The behavior of the continuous phase and droplets in the

flow field downstream of a production and a 3 x -scale model engine combustor swirl cup has been studied Conclusions drawn from the study are as follows:

9 The continuous phase scales well at the exit of the swirl cup in the present cases because the low liquid loading ratio has little affect on the flow Farther downstream, scaling is difficult to evaluate because of differences in entrainment and the presence of asymmetries, especially with the production hardware

• The droplets'reflect the continuous-phase flow field to

a varying degree depending upon droplet size or droplet re-laxation time scale The different behavior of droplets of dif-ferent size can be explained successfully in terms of droplet relaxation time scale and the sources of droplets The different behavior of droplets of same size in both cases can be inter-preted by different droplet residence time reaching the same geometrically analogous point and the origins of these droplets

• In spite of the differences in the atomizer characteristics, the behavior of the droplet size and velocity is similar in both the production (1 x -scale) and 3 x -scale swirl cups, indicating that, for this particular hardware and operating condition, the continuous phase dominates the flow field and droplet dis-persion

9 The droplet D 32 in the 3 x -scale is generally greater than that in the 1 x -scale This is due, in part, to differences in the initial droplet size distribution produced by the two atomizers

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- 4 0 X/Rp

Fig 7(a) Continuous phase

E

£

o

3

Ld

>

_ l

<

LJ

O

15

10

5

0

- 5 -10 -15

15

10

5

0

- 5 -10 -15

1 1 - 2 0 urn, 1x

0 1 1 - 2 0 a m , 3x

X 1&&

Z = 1.75 Rp

jeSifflB-1

1 1 1

Z = 2.75 Rp

X/Rp

Fig 7(b) Small droplets Fig 7 Comparison of mean tangential velocity

Nevertheless, the presence of the swirl cup tends to reduce

these differences and provides more uniform and smaller

drop-let size distribution than using atomizer alone The extent to

which the aerodynamic flow and droplet formation from the

primary venturi contribute to differences observed cannot be determined at this point

9 The shear layer has a high droplet population and thus

a high liquid volume flux for both 1 x - and 3 x -scale tests

E

o

3

UJ

>

_ J

<

1 5T 5 3 0 - 4 0Mm , 1x

1 0 " 3 0 - 4 0 /tm, 3x

- 5

•10

•15

— * m > o

Z = 1.75 Rp

r\^

@^

? 1 5

S 5

- 5

- 1 0

- 1 5

I

1

a w * %

^ * 6 8 o

1 1 1

Z = 2.75 Rp

1 1 1

X/Rp Fig 7(c) Medium-sized droplets

15

10

5

0

- 5

- 1 0

-15

15

10

5

0

- 5 -10 -15

• 7 4 - 8 8 urn, 1x

0 7 4 - 8 8 ^ m , 3x

o a

o

1 1 1

Z = 1.75 Rp

o

m

1 1 '

^*s& «J L

Z = 2.75 Rp

X/Rp Fig 7(d) Large droplets Fig 7 (continued) Although the time-averaged information of the continuous

phase and the droplets is essential to the understanding of the

gas-phase flow field and droplet dispersion, the information

on the corresponding velocity rms and pdf is necessary for further insight into the gas-phase/droplet interaction, which will be discussed in the second part of this series study (Wang

et al., 1992b)

Acknowledgments

The authors acknowledge financial support from GE Air-craft Engines, and the assistance of S W Lee with plotting part of the data and H D Crum with assembling the swirl cup hardware

References

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