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Copyright 0 1996 byASME
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III IlIllIllillIlIll IIIII
Richard C. Adkins
Arsil Arias a
BREAK
School of Mechanical Engineering
Cranfield University
Cranfield, Bedfordshire, United Kingdom
1 New with Square One consultants Ltd.
4.
Aplartds Closo;ç . idOkeford
Dorset. UnitS KIrlâdorn
ABSTRACT
The =it for using blast baskets when testing large engines with a
high bypass ratio is explained. A theory has been developed to enable
uc drutu cbaiaaaistics of blast baskets to be tarnhS and
the
this has been validated by experimental data. The tlmaical approach
has then bow used in cut to construct a design than for use by
constructors of aigme test facilities aIgirm m.,..uft...4... cn, and engine
isas who have a need to modify their existing tea fmnThn
INTRODUCTION
The edvait of in twtaLiai, large, high bypass ratio fan
engines has required a complete reassessment of test Inme design. In
particular, this reasse ssment was .-w.y, I the exhaust din mid
tof
rof in creased size Dianrtasofthe
new aigineShai jets art in the region of 3 in (119 bd.), warty an
at of rngnitnde 111W than those of of cady n.5ira. Simple
scaling up,'of the existing facilities was but an option an it would
have produced colossal ducts and exhaust ISis and tim costs would
have been prthththvt
The use of high bypass ratio fans has alland the nature of
exhaust particularly after the bet — cam gaws have mired
with the air from the bypass fan. The resulting jet is eow significantly
ocular and at a Iowa velocity than the jets fran previous ntgin
Those .hnr can be used to t sonic of the disadvantage of the
increased thmneta.
Until raitty it was ----- --y to entrain açiass p..nbln of
additional air into the test ccli exhaust system S cut to tSr both
the temperature and flow velocity. These itiam were essential I
cit to pnarU the materials used in the sham. In at to generate
these large air aitrainmts it was necessary to usc exhaust ducting
diametas which were considerably greater than these of the engines.
2 Test Facilities Department
GARLJDA Maintenance Facilities
Jakarta, Indonesia
The new gaivation of 'g!" produce mixed exhaust streams
with temperatures that arc now within the capability of silama
mataials. However, some entrained air is still required to vn,tillitr the
test cell to prevail overheating and rt importantly, to avoid the
problem of vortex ingestion into the engine. The latta reason is
essential as vaIn ingestion can seriously impair the performance and
stability of the — oompressor The pit of vortex ingestion
mainly occurs dim to the use of test cells which art rectangular in
am swim This configuration is prettied for providing
accessibility to the engine and eqthpiumt and b-•-- it simplifies test
CCU construction.. Vortices roll-up from the flat wall sift as air is
drawn into the thwlar section of the — intake. They can be
inhibited by vn,tiilsthon air forming a barrier betw w' the air
supplying the engine and that which follows the cell walls, Fig I. The
wnamt of air that has to be wlia1 for this purpose depends upon
the jr-'-- amflgwatS of the test alL As a ganalisaticm, tim
amuxart of wtiszul air now zcqául is similar in quantity to that
being ingested by the aigim. ltis draw nthrough the cdlbydie
pumping action of the engines exhaust gases an they mix with air
the confines of the exhaust gas d4.m (migmnrtrr). In this
IN
respect, the detuna .cacnbles a huge 'jet pump.' Despite the 1M,rM
ventilation air gccmirnnnit, exhaust gas .l.4...np can still be in the
order, of Sm (19* in.) di....tn and2Om(66ft)long.
The new, smaller pumping task leaves a surplus of kinetic ant'
in the exhaust stream which is advantageous because the SI
aaximztha mc less saStive to n'n'—1 fadas sdi as as winds.
Howeva, that is also a disadvantage the excess in kinetic enagy has
to be extracted be&e S mixed gases can enta the 11am, otherwise
damage would osr to the acoustic linings. A new canpcxiait, the
blast bScet, is used for this tSc
Presented at the International Gas Turbine and Aeroengine Congress & Exhibition
Birmingham, UK - June 10-13,1996
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THE BLAST BASKET
A NEW COMPONENT USED IN TESTING
LARGE, HIGH BYPASS RATIO, FAN ENGINES
marry aerodynamic components of the all. The presnve loss of the
blast basket provides the single most inflomial parameter in
controlling the quantity of ventilation and hence the ambient pressure
inside the test cell.
It was this important lack of design data that prompted the
present study which was conducted in the School of Mechanical
Fagineering at Cranfield University.
Blast baskets take the form of a cylindrical colander, located on
the end of the aagmmcc dad and closed at the fiudwst cod, as shown
in Fig. 2. In addition to dissipating energy they assist in turning the
flow upwards imo the cadhma[ stack They can also help to prevent the
generation of low 5equency noise, often a serious problem of early test
cells. The kinetic energy dissipation is achieved by the turbulent
mn®g, bath of die engine jet with the entrained air and of the small
jets created as the flow passes through the holes in the basket wall.
These jets diffuse and fill the void, adernal to the blast basket, with
gases at low velocity that can easily be honed upwards into the
silencer stack In fulfilling this laser task, the blast basket replaces the
cascaded bead that was used for tmaing exhaust gases in many of the
earlier cagme test facilities. These cascades were often pvoae to
mechanical failum
Pe
EANd Dada Sknm
Pt
mi
Ettra ed Ai
\
P1
-
Arpasat
Enyw Homo
Duct
4
8YG BMSd
Figure 3 Mass Flow Distribution Through Blast Basket wall
Figure 2. Test House Exhaust Sys&®
The redrstion in low fioquency now tim be anticipated because
the perforated wall avoids the basket Hem having a clearly defined
acoustic kagdL The so called, `organ pipe i saBancy' is thereby
inhibited Pt must be [toted that owing to thus inherent pressure lass,
blast baskets can only be used when modest quantities of wtrammwt
air are requb4
Akhaagb blast baskets me mechanically s®ple and hence
attractive components for iadu®on in teat cells, there is a dearth of
aerodynamic data on then performance Such data is mumal because
of the compka nacragion betwaw the wgme under test and all the
The objective of this study was to simplify the media= of
pressure losses thro ugli blast basket walls. This was to be achieved
by laovidaig a means for determining an overall coefficient of
diarhmgy of the blast basket as a whale.
The [dloweng analysis has bow simplified by —M3Mg that pIq
How erasts along the complete length of the basket and that the basket
wall can be treated as a succession of mall isolated orifices. A
uniform pressure fled, eadanal to the basket, was also assumed which
neglected die influence of erdernal flows between the basket and
ciduant chamber walls that would occur in reality. Thraugbod the
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F4MI TM Cd Ai Fiiaaiwt
THEORETICAL CONSIDERATIONS
The major component of pressure losses occurs when the ramrod
Sam pass through the holes in the basket wall, forming small lets
which then drrse while creating high levels of turbulence. By using
small holes the diffisim is relatively rapid and any raise geocrattd is
at a @alamLy that can be readily alter by the silenow AwIL
Accurate prediction of this flow through Bee basket wall is
eomplerk As flow escapes through the holes the internal gas velocity
in the basket redtaces and there is a corresponding rise in static
pressure: • Initially, it was also anticipated that internal total pressure
would drop due to bah friction and habulenee These two Imgthwise
internal press m: gradients wwM have omsidQable influence on the
Bow ran through the basket wall This would occur not only from the
variation in pressure drop across the wall but also form the effort that
pressure ratios have apw the local coefficients of discharge of the
fides. This latter effect is not elis ar to that experienced by How
eotaivg combustor liners and which was the subject of a previous
Crrmfrdd surly by Adkins and Guerod (1999). In the light of this
pmvias ciperiwee, the How condition for blast baskets can be
anticipated and is sketched in Fig. 3 where the length of the arrows is
indicative: of relative mass flow.
fro_ —
-
pa
This niabta the calculation of a coefficient of contraction, Cfr.,
t
Cfc = [1- exp(-1) + exP( ,jj!.j)J. K1
whom
Following which the co.11ei..W of discharge, Cd is given as
Cd = dcfc2_I + e
ley
Note. For this particular application it was fixed necessary
— to
cultist the rang of correlations S Cdt
fcvf<005 than Cd = 00889J5,
for f=l.O than CdtO.632
m- drn
Using the assumptive of plug flow, than the reduction in intanal
velocity, dv, occurring while Ut flow passes through the d,.d is
given by'.
Pt
Pt
III
By treating the difflmiai I
4, can be niIn,!M.A frmr
m
as idal the rise in Antic
paflC,
dx
¼
¼
P-vI
flgurc 4 PJn,wit of St (S analysis)
Ana
II In.tb &x , of timbshown
in Fig.4
dig
sket, is
supposed that air cuing with a wn Mach munba, m flow rate,
static pressure and'
fliame velocity). A snail annn of
lit S n flow, Sm, -r thraigh a ring of Ma in the bt
flowing into the tnt clanait If
wall while tim
Ut pataS wail has a porosity factor, r, tic Ut m flow
wap.% over this daiclal lnigth can be nInilTht by — the
dnndswd orifice plate equation what:
Sm= CtLz.Dtiy.42.p.P-p)
(I)
As aplaiitd pzcvóSy, the value 0fIWEN4SII of dir t---p, Cd,
varies considerably with flow awlitie-c• For pwwi jnnp U was
nfrnhtei4 Sag an adaptatiat of the equations — by Adkins and
Guauw (1989) and which me slvwn below.
Ha; 0 is a jnne - pawia and can be aacaaJ in
tannofthc....JMnrethownthfig4 a:
- PP.
PC
D- "n-mSm
FipiSitn (1) and (3) can be
to predic a partial
differential eiqns#m In piapk this wild be integrated over the
length, I, of the complete bScet The bounty wxfiUais -----y
fir this would be — by the rate=d ataS prossurej increasing fran
p6 up to the US pwaac, P, and the mass flow which rediccu fran
me down to mo at the dosed aid. However, owing to the
complications involved in calculating the variable, Cd, the p.aA
Mains that to integrate the aivatSs mnnaiSiy. Site the overall
psnt di-op w the bScd was art wiLuwn quantity this had to
be tat by trial and nra'. This was varied until the predicted
mass Bow at the dosed and of the bt t -..zzero. AttrtM
computer program was written S this pw
This numerical I ne was then used to ealndMe the vahais of
Ut overall jxwt drop, (Pp.), for a range of diait blast baduat
caiflgwaUai flows sit Mach maribat lice q...dititin vat
than used to dam the u.aucn,,...Lt o'aalJ discharge ccifl,
Cd...4 which apply to the canpinc Wag bS* t
Cd_Il =
M
z.D.LwJsp-p.)
(4)
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analysis the flow has been treated as compressible and the mixed gas
streams assumed to have the same properties as air.
A previous analysis described by Anas, (1995) made a foil
allowance for the reduction in total pressure down the basket, requiring
calculation of the internal static pressure gradient using moiniwm
theoiy. During his scale model tests, which followed the analysis, he
discavaal that dnzntirr pressure actually gwna .,. nndct down the
aitire length of th e basket. He also found that lit atalic pressure
with .fidnw dnm until, at the basket aid, it equalled
lit Ertian pressianc, From his nyctiuwasb it can be - . rhh1od
that all the wmaii of flaw aa-gpressure loss
n.maustly — thromb Ut beta, thathy leaving the flow that
runained inside the basket at a constant. total pressum It thatfire
Wows that the static poic recovery can be tress as ideal. This
realisation greatly simplifia the theory which is
ibu1 in the
In this form, Cd.q, is a convenient parameta - fir use when designing
atgine test cells that include a blast basket
Ina later section somc of the mulls &rival Ban this program
we compared to the experimental data taken from the scale model
tests.
SCALE MODEL TESTS
In the interact of canny it was <leaded to ocaduct tests on a
small scale model using air at nem - room tenqxrattues and pressures.
The mold blast baskets had a diameter of 152.4mm (6 inches). Three
baskets sere manufactured fran commercially available, perforated
steel sheets, I .5mm thick, which gave geometric porosity factors of
0.24, 0.37 and 030, me Fig. 5 The closed end was forced Iran a
piston which could be locked into a mnnber of diffaent axial kcations
in at to vary the working length., see Fig 6. The piston had a
conical crovm in crda to be geommically similar to the aid plates
which we used to resist pressure loading in the full Inc baskets. A
bole was drilled through the centre of the plata to amble insertion of
a cylindrical, pitot static tube which could be traversed along the
ceMral axis, Fig 7.
Tit air supply was metered, upstream of the model, in a
3041mm (12 inch) pipe according to British Standards 1042.
Following this that was a smooth contraction down to all Si din of
Slat tursdrepobe
Figure 6 Arranganan of Scale Model
1514 mm (6 in.) what sunk passim was measured athree points
around the circumference. This measurement plane was located just
bet= the juncture with the Mkt din and blast basica In dm tests
reported hae, the blast basket calmusted /mealy into the laboratory.
Tests were conducted at different flow rates with the maxinm• Mad'
number into the basket region being limited to 0.40. The total
pressure at basket inlet was daival from
4
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Figure S Blast Baskets la Model Tests
I
a inowkidge of static pressure, mass flow and temperance. This
inlet total pas= was used in ader to determine the overall pressure
drop between inlet and ecit static pressure which, in this a was tbe
laboratory ambient mossne. Traverses of total and sudic presumes
along the basket axis wise conducted fix representative tests only.
Velum of overall coeffmiect of discharge wan calcolatecLusing
mac purpose written =Tata program. Selected ashes of
themetically and experiatentally derived data me presented in lie
following figorts Amt detailed dessiption oldie test rig together
with a wider range of experimental data is to be found in the thesis of
An (1995)
Variation of Static Pressure (kPa)
115
1
110
lb:cm-teal
105
o 6tperimental
Total pressare
100
0
10
20
30
40
50
SO
TO
SO
90
1C0
Percertape Le* Along Basket
(a). Pressures manual ekes the canal axis w4 to theoretical predictions and Taiti Pressure
5
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ri Conical Piston Crown and Traverse
0.7
0.6
y
0.5
0.4
0.2
0.1
0
0
20
40
80
60
100
Percentage Length Along Basket
Fig8(b)
Variation of Local Values of Cd With Axial Position
Figure &
Local Values for One Particular Configuration
Bided leogth/thameter ratio = 1.5:1
inlet ?flash Number = 0.36
Porosity Factor = 0.37
9(a). Porosity Factor = 0.24
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
C.
Theoretic&
o Experimental
15
05
2
Basket LID
9(b). Porosity Factor = 0.37
el
•
•
O
0.7
0.6
0.5
0.4
0.3
02
0.1
0
Theoretical
I
Eaperimental
15
05
Basket LID
6
2
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%. 0.3
•
9(c). Porosity Factor = 0.50
0.6
0.5
(,)
0.4
1 03
7
0.1
0
05
1.5
2
Basket UD
Figure 9. Overall Values of Dicharge Coefficient Covering Range of Experimentation
0.7
.
06
03
02
Basket UD
Figure 10.
Design Chad Delved From Theoretical Considerations
The close proximity of the theoretically derived data to the
experimental data in Rip. 8 and 9 would appear to validate the they
Figure 10, intarled for design purposes, was therfore based
solely upon theoretical values.
7
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orstieis
7he-l
02
A thoiretical method has bear derived which ambled the
caladatim of ovezall discharge coefficient fir blast baskets. At
validation by emerimmtal data the theory has been wed to oinstruct a
design chart covering the practical range of parameters normally used
in test facilities for large, high bypass ratio, fan engineo The dray
8SSZ•1CS that the blast basket exhausts into a uniform presort field
and this assumption may not always be realistir. in the situation
where the space between the basket and its housing is wormed then
sane allowance may then have to benzoic
REFERIEMCES
Adkins, 11-C.
Gueroui, D
Ann, A
8
"An improved orthod for prorate prediction of
mass flow through combustor liner holes ASME-GT-149
"Engine Test Cell Aerodynamics?" M.Sc. 'thesis,
Cranfield University, 1995
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ACKNOWLEDGEMENTS
The arm at indebted to Mr. Dack Brown of the workshop
stiff in the Scheel of Modzazical Engineering, Cranfidd University
who ccostrectoi die test model and to Garoda far their monscrship of
Mr. Anas during his stocks.