Experimental Investigation of High Pressure Ratio Centrifugal Compressor With Axisymmetric and...

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1 Copyright 2012 by ASME

Proceedings of ASME Turbo Expo 2012

GT2012

June 11-15, 2012, Copenhagen, Denmark

GT2012-68219

EXPERIMENTAL INVESTIGATION OF HIGH PRESSURE RATIO CENTRIFUGAL

COMPRESSOR WITH AXISYMMETRIC AND NON-AXISYMMETRICRECIRCULATION DEVICE

Hideaki TamakiTurbo Machinery and Engine Technology Department, IHI Corporation

1, Shin-Nakahara-Cho, Isogo-Ku, Yokohama, 235-8501, J [email protected]

Xinqian Zheng, Yangjun ZhangState Key Laboratory of Automotive Safety and Energy, Tshingua University

Beijing, 100084, China

ABSTRACTCentrifugal compressors used for turbochargers are

required to have a wide operating range. A recirculation device,which consists of a bleed slot, an upstream slot and an annularcavity connecting both slots, is often used with them. Itimproves the incidence angle of the impeller leading edge, i.e.the blade loading of the inducer, at low flow rates due to the

recirculation flow supplied to the compressor inlet. Howeverthe compressor efficiency drops when there is a recirculationflow from the bleed slot to the upstream slot. A one dimensionalanalysis in the first section of this paper showed that thereduction in the compressor efficiency can be lowered bydecreasing the pressure drop or reducing the recirculation flowrate within the recirculation device. This study examined thepossibility of improvement in the compressor efficiency by theuse of a recirculation device with an asymmetric bleed slot.

An impeller of a turbocharger compressor is normallycontained in a volute. Since the geometry of the volute is notaxisymmetric, the impeller is surrounded by an asymmetric flowfield. Hence each impeller passage, which is formed by two

adjacent full blades, is operated at a different operating point.This means that some of passages need the improvement in theblade loading by the recirculation device but others do not.

There is a possibility that this is realized by a recirculationdevice with an asymmetrically-distributed bleed slot, called anon-axisymmetric recirculation device in this paper. If theasymmetric bleed slot shortens the average distance between thebleed slot and upstream slot or reduces the area of the bleedslot, it can reduce the pressure drop or recirculation flow rate

within the recirculation device, and hence can improve thecompressor efficiency.

This study discusses the characteristics of high pressureratio compressors for turbochargers without the recirculationdevice and those with the recirculation device with anaxisymmetric bleed slot. Further, the effects of non-axisymmetric recirculation devices on the compressor

characteristics are experimentally investigated. Two types ofnon-axisymmetric recirculation devices were tested. One hadthe bleed slot of a sine wave pattern. The other had the bleedslot partially channeled in the circumferential direction. Therewere appropriate positions relative to the volute for both non-axisymmetric recirculation devices. The compressor efficiencywith non-axisymmetric recirculation devices was higher thanthat with axisymmetric recirculation devices, and the surge linesof the compressor with non-axisymmetric recirculation deviceswere located at flow rate lower than or equal to those with theaxisymmetric recirculation devices.

NOMENCLATURE

b3 Impeller exit blade height [m]b4 Diffuser height [m]H Axial impeller length between hub side of impeller

exit and shroud side of impeller inlet [m]M Meridional length [m]m Compressor inlet (discharge) mass flow rate [kg/s]mc Choke flow rate [kg/s]md Design mass flow rate [kg/s]mr Recirculation flow rate [kg/s]


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ms Mass flow rate near surge [kg/s]Mu Peripheral Mach number (=U/(RgT1)

0.5)Ps Static pressure [Pa]PT Stagnation pressure [Pa]PTU Stagnation pressure of fluid injected from upstream

slot [Pa]

R Radius [m]RLs Impeller inlet radius of shroud side [m]Rg Gas constant [J /( kgK)]U Impeller speed [m/s]Z Number of blades Ratio of recirculation flow rate to mass flow rateLb Impeller blade angle at inlet radius (from radial

direction) [deg]3b Impeller back-sweep (from radial direction) [deg] Specific heat ratioEm Ideal specific work between impeller leading edge and

bleed slot at mass flow rate of m [J /kg]P Pressure loss in recirculation device [Pa]

Wm Ideal specific work between bleed slot and impellerexit at flow rate of m [J/kg]

1,m Adiabatic impeller efficiency between impellerleading edge and bleed slot at mass flow rate of m

2,m Adiabatic impeller efficiency between bleed slot andimpeller exit at mass flow rate of m

T,m Adiabatic impeller efficiency with recirculation deviceat mass flow rate of m

*T,m Adiabatic impeller efficiency without recirculation

device at mass flow rate of m Circumferential position relative to volute tongue () Total to total pressure ratio of compressor staged at design flow rate (at m/md=1.0)

s near surge flow rate (at ms/md) Density [kg/m3]Subscript

0 Compressor inlet1 Upstream slot (Figure 2)2 Bleed slot (Figure 2)3 Impeller exit4 Diffuser exit

Definition of non-dimensional parameters

Flow coefficient

32

3)2( UR

Q

Pressure coefficient 2

3

1

0 )1(

U

CpT

0 Work coefficient ( )2

3

03

U

TTCp

INTRODUCTIONA high boost pressure is required in order to increase the

specific output power of Diesel engines. Improved thermalefficiency and reduced emissions are also essential forenvironmental conservation. The pressure ratio of compressors

for turbochargers has been increasing to meet these enginerequirements, in particular to downsize automotive engines andto apply Miller timing to Diesel engines.

Centrifugal compressors used for turbochargers need tohave a wide operating range. However the above enginerequirement on turbochargers makes it difficult for the

compressor to ensure a wide operating range. There are manycountermeasures to enhance the compressor operating range. Arecirculation device, which consists of a bleed slot, an upstreamslot and an annular cavity connecting both slots, is often usedwith turbocharger compressors. A typical recirculation device isshown in Fig. 1. There are many studies about recirculationdevices. Hunziker et al. [1] adapted a recirculation device to acentrifugal compressor with pressure ratio of 4.2 and succeededin the enhancement of the surge margin without sacrificingcompressor efficiency on a typical engine operating line.Sivagnanasudaram et al. [2] investigated the effect of a bleedslot on flow characteristics in a centrifugal compressor with apressure ratio of 4.5 and proposed a proper width of the bleed

slot. They also showed that suction of separation vortex andover-tip vortex by the bleed slot reduces the formation of stallflow.

Cavity

Bleed Slot

Upstream Slot Cavity

Bleed Slot

Upstream Slot

Fig. 1 Schematic view of conventional recirculation deviceAn impeller of a turbocharger compressor is normally

contained in a volute. Since the geometry of the volute is notaxisymmetric, the impeller is surrounded by a non-axisymmetricflow field [3]. The cross-sectional area of the volute is designedto increase from the tongue towards the discharge section toaccommodate the increasing flow [4]. The variation of staticpressure around the volute depends on the flow rate. There is aflow rate at which the average velocity in the throat section ofthe volute, i.e., at the exit of the volute, is equal to the meanvelocity in other cross sections of the volute. The static pressurearound the volute is nearly uniform at this flow rate. If the flowrate is reduced to less than this flow rate, the flow is decelerated

and the static pressure increases towards the exit of the volute.Fink el al. [5] measured static pressures on a casing of aturbocharger compressor versus time with high-responsepressure transducers and found asymmetric stalling of theimpeller. As the compressor was throttled, local flow reversal atthe inducer was found, first near a circumferential location ofthe volute tongue, and then away from that location. Theasymmetric stalling of the impeller at the circumferentiallocation of the volute tongue was considered to be linked to the


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onset of a deep stall. Near the tongue, a pressure rise around theimpeller was lower relative to the angular positions away fromthe tongue. This asymmetry was still seen in the inducer leadingedge. They also showed that the axial velocity at the inducer tipdropped to zero at the angular position of the volute tongue.Fisher [6] pointed out that a recirculation device reduced the

circumferential pressure distortion at the inducer tip at surge,i.e., it decoupled the inducer from the circumferential pressurefield in the rest of the compressor stage.

Zheng et al. [7] paid attention to the distorted flow field ina compressor caused by an asymmetric geometry of the volute.

They devised a non-axisymmetric recirculation device, therecirculation device whose bleed slot is asymmetrically-distributed around the impeller. It was confirmed that the non-axisymmetric recirculation device had more potential for theenhancement of compressor operating range than theconventional recirculation device whose bleed slot isaxisymmetric around the impeller. Yang et al. [8] tried toexplain the reason why the non-axisymmetric recirculation

device was more effective in enhancing the compressoroperating range than the conventional recirculation device.

They found that the non-axisymmetric recirculation device candistribute the recirculation flow in the circumferential directionon the basis of the distance from the leading edge of theimpeller to the bleed slot. They concluded that a well-designednon-axisymmetric recirculation device could depress theasymmetric flow in the inducer more effectively compared tothe conventional recirculation device, and this contributed topreventing the asymmetric stalling of the impeller.

m m+mr m

mr

Bleed slot : 2Upstream slot : 1

Compressor inlet : 0

Impeller leading edge

(a) Existence of a recirculation flow

m m-mr m

mr

Bleed slot : 2Upstream slot : 1

Compressor inlet : 0

Impeller leading edge

(b) Near choke conditionFig. 2 One-dimensional model of recirculation device

Fig. 2 is an illustration of a one-dimensional model for theimpeller with the conventional recirculation device. Most of the

turbocharger compressors operate at high Mach numbers withsignificant compressible effects. However it is expected that asimple analysis based on incompressible flow would be able toexplain the impact of the recirculation device on the impellerefficiency qualitatively. In the case of incompressible flow, thespecific enthalpy is the ratio of pressure to density.

The following discussion is possible when a recirculatingflow is present. Stagnation enthalpy at the upstream slot station,1, is the sum of the stagnation enthalpy at the compressor inlet,0, and that of the recirculation flow injected from the upstreamslot.

///)( 01 TUrTTr PmmPPmm +=+ (1)

The stagnation pressure of the fluid injected from theupstream slot PTU, is described by Equation (2).

PPP TTU = 2 (2)

P is the pressure loss in the recirculation device between

the bleed slot and the upstream slot including the mixing loss atthe upstream slot. Using an ideal specific work between the

impeller leading edge and the bleed slot provided by theimpeller at a mass flow rate of mEm, the stagnation pressure atthe bleed slot PT2, can be written as:

rr mmmmTTEPP

+++= ,112 (3)

1,m+mr is the efficiency of the impeller between the impellerleading edge and the bleed slot at the mass flow rate of m+mr.1,m+mrEm is the work transferred from the impeller to the fluidthrough the impeller between the impeller leading edge and thebleed slot accordingly. Using the above equations,

)( ,101 PEPP rr mmmmTT += ++ (4)

where is the recirculation ratio that is defined as mr/m. Thestagnation pressure at the impeller exit PT3 is

mmmmmmT

mmmmmmT

mmTT

WPEP

WEP

WPP

rr

rr

+++=

++=

+=

++

++

,2,10

,2,11

,223

)1(

(5)where W mis the ideal specific work between the bleed slotand the impeller exit provided by the impeller at the mass flowrate of m. 2,m is the efficiency of the impeller between thebleed slot and the impeller exit at a mass flow rate of m.

The impeller efficiency with the recirculation device at amass flow rate of mT,m, is expressed as:

mmm

mmmmmm

mmmr

TT

mT

WE

PWE

WmEmm

PPm

r

rr

r

++

++=

++

=

+

++

+

)1(

/)1(

)(

/)(

,2,1

03

,

(6)Em+mr is smaller than Em because of the increase in the massflow rate due to mr. 1,m+mr is higher than 1,mbecause of theimprovement of incidence. When the product of (m+mr) and


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Em+mr is comparable to the product of m andEm, andP/ isapproximated byEm, Equation (6) becomes,

mm

mmmmm

mTWE

EWE

+

+=

)( 1,2,1,

(6)

where1,m+mr is split into the following two terms.

1,1,1 +=

+ mmm r

1 corresponds to the increase in the impeller efficiency.Near the choking flow rate, a part of the inflow gets into the

upstream slot and comes out of the bleed slot. In this situation,the stagnation enthalpy at the bleed slot satisfies Equation (7).

/)()(

/)(/

0,1

02

PPmEmm

PmmmP

Trmmrmm

TrT

rr++

=

(7)P is the pressure loss in the recirculation device between theupstream slot and the bleed slot including the mixing loss at thebleed slot. Near the choking condition, the pressure drop P

corresponds to the negative pressure at the bleed slot which isproduced by the fluid pumped out by the part of the impellerbetween the upstream slot and the bleed slot. mr is the flow ratepassing through the recirculation device. The stagnationpressure at the impeller exit, PT3 is

mmmmmmT

mmTT

WEPP

WPP

rr++=

+=

,2,10

,223

)1(

(8)where is the flow ratio defined as mr/m.

The impeller efficiency with the recirculation device at amass flow rate of mT,m, is expressed as:

mmm

mmmmmm

mmmr

TT

mT

WE

PWE

WmEmm

PPm

r

rr

r

+

+=

+

=

)1(

/)1(

)(

/)(

,2,1

03

,

(9)Em-mr is larger thanEm due to the decrease in the mass flowrate by mr. 1,m-mr is higher than1,mbecause of the reduction inthe incidence loss. When the product of (m-mr) and Em-mr iscomparable to the product of m and Em, and if P/ isconsidered to be equal to Em, Equation (9) becomes the sameas Equation (6).

mm

mmmmm

mT WE

EWE

+

+=

)( 1,2,1

,

(9)

where1,m-mr is split into following two terms.

1,1,1 +=

mmm r

1 corresponds to the increase in the impeller efficiency.In the case of the impeller without a recirculation device,

the impeller efficiency at a mass flow rate of m *T,m, can beobtained by changing 1,m+mr to 1,m and substituting 0 into mrand in equation (6).

mm

mmmmmT

WE

WE

+

+=

,2,1,

*

(10)

A comparison of (6) and (9) with (10) indicates that theefficiency of the compressor with the recirculation devicebecomes higher with decreasing pressure loss in the

recirculation device which is required to get a certainrecirculation flow ratio of. It also requires the development ofrecirculation devices that can enhance the compressor operatingrange with the recirculation flow ratio being made as small aspossible. Reducing the distance between the upstream slot andthe bleed slot is one of countermeasures for the reduction in thepressure loss or the recirculation flow ratio in the recirculationdevice. If a recirculation device with an asymmetric bleed slot,called non-axisymmetric recirculation device in this paper,could really provide an extra operating range, there is apossibility that non-axisymmetric recirculation devices can bemade which satisfy following two features;

(1) Equal to or wider operating range than that offered by

the recirculation device here with an axisymmetric bleed slot,called conventional recirculation device.

(2) Smaller (average) distance between the upstream slotand the bleed slot or smaller area of the bleed slot compared toa conventional recirculation device.

This non-axisymmetric recirculation device will recover theefficiency drop caused by the recirculation flow withoutdeteriorating the operating range which is obtained by theconventional recirculation device.

This paper presents the development of non-axisymmetricrecirculation devices which satisfy the above two features.

Taking the asymmetry of the bleed slot and volute into account,3-D calculations ought to be unsteady. Yang et al. [10] tried to

calculate a compressor with a volute by a steady statecalculation, frozen rotor model. They showed that calculatedcircumferential variation of static pressures near splitter bladeleading edge were significantly different from test resultsparticularly a location where the static pressure became aminimum value, and hence the necessity for unsteadycalculations to understand the effect of the volute on the flowfield. Since unsteady calculations will take a lot of time, thisstudy was mainly done based on experiments. Two types ofasymmetric bleed slots were produced experimentally and thecompressor characteristics with the non-axisymmetricrecirculation devices were compared to those with conventionalrecirculation devices. The first recirculation device wasdesigned to shorten the average distance between the upstream

slot and the bleed slot and the second one was designed tocorrect a defect of the first one.

INVESTIGATED COMPRESSOR AND ITS

CHARACTERISTICSThe specifications and main parameters of the investigated

compressor are listed in Table 1, Table 2, and Table 3. It wasdeveloped by one of the authors. The detailed design of thecompressor has been reported in [9]. The outer radius of the


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impeller is 72.2mm, and the tip clearance is 0.5mm. Figure 3shows two pictures of the compressor. The impeller adoptsdouble splitter blades. Backsweep angles of the full blade andthe two adjoining splitter blades are different from each other.

The compressor was originally designed to be used with vaneddiffuser. However experiments were done with a vaneless

diffuser to exclude the possibility of stall caused by a vaneddiffuser. The original volute designed for the vaned compressorwas also used for the compressor with a vaneless diffuser.

Table 1 Compressor specifications

Mu

1.62 0.108 0.624 5.7

Table 2 Main parameters of impeller

Z 6/6/6 full/1st /2nd splitter

RLs/R3 0.72 b3/R3 0.11Lb 62 /28 shroud/hub

3b 15/16/11 full/1

st

/2

nd

splitterTable 3 Main parameter of vaneless diffuser

b4/b3 R4/R30.75 1.74

1st splitter

2nd splitter

1st splitter

2nd splitter

Fig. 3 Picture of investigated compressor

W

Z

^t

d

D

(a) Pressure ratio of SW and CT

D

D

(b) Efficiency of SW and CTFig. 4 Compressor characteristics of SW and CT

W

Wd

D

Fig. 5 Circumferential variation of static pressuredistribution of SW at 1.14R2

Figure 4 shows the measured compressor characteristicswith and without the original recirculation device. Thecompressor without the recirculation device is called SW in thisstudy. Figure 5 shows the circumferential variation of staticpressure of SW at 1.14R3 at the design peripheral Machnumber. starts from the angular position of the volute tongue.As the volute was originally designed to reduce the volutedischarge velocity, the circumferential variation of staticpressure increases towards the volute discharge section at thedesign flow rate of m/md =1.0 even for the testing with avaneless diffuser. Hence the volute throat is not under-sized forthe vaneless diffuser.

Figure 6 is a meridional view of the compressor with themain dimensions of the original recirculation device called CT.

H/R3 is 0.79. Its bleed slot is axisymmetric around the impeller.The peak efficiency falls by about 1.0 points at the designperipheral Mach number of Mu=1.62 by the use of CT.

Figure 7 shows that of SW and CT at 1.14R3 at the designperipheral Mach number. m/md=0.95 is near surge of SW. nearsurge is the operational point of the smallest flow rate. BothSW and CT show similar distributions. The variation of staticpressure is very large. It has a minimum value just downstreamof the tongue, at a of between 30 and 60.


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,

,

,

,

Fig. 6 Original recirculation device of CT

W

W

d

D

Fig. 7 Circumferential variation of static pressuredistribution of SW and CT at 1.14R2

W

W

d

Fig. 8 Circumferential variation of static pressuredistribution at 0.224H and 0.112H near surge

Figure 8 is the circumferential variation of static pressure ofSW on the shroud at the center of the bleed slot, 0.224H fromthe impeller leading edge, and at 0.112H in the middle of theimpeller leading edge and 0.224H for Mu=1.54 and the designperipheral Mach number of Mu=1.62 . The variation of staticpressure due to the asymmetric static pressure distributiondownstream of the impeller still remains at the inducer tip.

Time averaged static pressure variations between twoadjacent full blades were derived from averaging instantaneousstatic pressures which were sampled periodically are shown in

Fig. 9. The operating conditions are near surge at Mu=1.54 andthe design peripheral Mach number of Mu=1.62. Four differentcircumferential positions of=60, 150, 240 and 330 wereselected as installation positions of high frequency pressuretransducers (Kulite XCE-062). The region where the staticpressures were measured was between -0.151H (negative value

corresponds to upstream from the leading edge) and 0.302H.Eight pressure transducers were installed in the axial directionfor each circumferential position. Although the contours are notsmooth because of the limited number of sensors, the staticpressure distribution obtained by the data at =150 clearly hasthe broadest high pressure region for each peripheral Machnumber. The results show the existence of asymmetric staticpressure distribution at the inducer similar to Fig. 8.

WW

&

,

,

(a) Mu=1.54

&

,

,

WW

(b) Design peripheral Mach number of Mu=1.62Fig. 9 Static pressure distribution between full blades

Figure 10 shows the circumferential variation of staticpressure on the shroud at 0.112H from the impeller leadingedge for SW and CT. The operating conditions are near surge atMu=1.54 and the design peripheral Mach number of Mu=1.62.As Fisher [6] pointed out, the recirculation device depresses thecircumferential distortion upstream of the recirculation device.

Figure 10 also shows a comparison of the relation between theflow rate and the circumferentially averaged static pressures at0.112H of SW with that of CT. There are points of intersectionbetween the curves for SW and CT. The flow passing throughthe recirculation device does not exist around this intersection.Near the choke condition, some of the compressor inlet flowbypasses the impeller between the leading edge and the bleedslot. Since the flow passing through the impeller between theleading edge and the bleed slot reduces, the static pressure


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upstream of the bleed slot increases. This corresponds to thedescription thatEm-mr is larger thanEmin the one-dimensionalmodel. At lower flow rates, the recirculation flow is added tothe incoming flow into the compressor. As the flow passingthrough the impeller between the leading edge and the bleedslot increases, the static pressure upstream of bleed slot

decreases as Em+mr

d

,

(a) Bleed slot of ASCT1 and CT

/

h

/

K

(b) Tested compressor casingFig. 11 Non-axisymmetric recirculation device ASCT1

^d

^d

^d

^d

Fig. 12 Circumferential position of ASCT1

Figure 11 also shows a schematic of the tested compressorcasing. The compressor casing consists of an outer casing, aninner casing, and an inlet part. The outer casing includes a partof the impeller shroud, a vaneless diffuser, and a volute. Theinner casing is nearly cylindrical. The inner side of the inner


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casing is the impeller shroud and inlet. The outer side of thatconstitutes the annular cavity with the inner side of the outercasing. The slot which cuts through the inner casing is the bleedslot. One end of the inner casing is connected to the outercasing by bolting. The inlet part is fixed in front of the otherend of the inner casing and the gap between the inlet part and

the end of the inner part forms the upstream slot. Since twelvebolt holes are evenly placed at the outer casing, the angularposition of the bleed slot relative to the volute can be varied insteps of 30.

Four different circumferential positions of the bleed slotrelative to the volute were tested. The downstream position ofthe bleed slot centerline, 0.224H from the impeller leadingedge, of ASCT1-1 is =0, just at the tongue angular position.

The downstream positions of the bleed slot centerline ofASCT1-2, ASCT1-3 and ASCT1-4 are =90, 180 and 270respectively. Figure 12 shows the circumferential bleed slotposition of ASCT1. For a comparison of the compressorcharacteristics of ASCT1 with those of a conventional

recirculation device, a conventional recirculation device calledCT1 was additionally tested (Fig. 13). CT1 has the bleed slotcenterline at 0.112H from the impeller leading edge. Thisposition is at the middle between the leading edge and thedownstream position of the bleed slot centerline of ASCT,0.224H from the impeller leading edge (Fig. 13). Here thewidth of the bleed slot of CT1 is the same as that of CT.

,

Fig. 13 Recirculation device of CT1

Figure 14 shows the test results of ASCT1. The relationshipof the flow rate and pressure ratio of SW is also shown in Fig.14. Only the change in the phase angle of the sine-wave bleedslot varies the surge line, which is the line connecting theoperational points of the smallest flow rates on all theperipheral Mach numbers. These test results imply that theexistence of the asymmetric flow field in the compressor affectsthe inception of surge or stall. This asymmetric flow field willbe created by the volute. The surge line of ASCT1-1 is locatedat the smallest flow rate among the four bleed slot positions.Figure 15 shows the test results of ASCT1-1, CT and CT1.Figure 15 includes the relationship of the flow rate and pressureratio of SW. The surge line of ASCT1-1 is located at thesmallest flow rate. The surge line of CT1 is almost same as thatof CT. The peak efficiency of ASCT1-1 and CT1 is higher thanthat of CT. The peak efficiency of ASCT1-1 was almost the

same as that of SW (not shown in Fig. 15). However themaximum flow rates of ASCT1 and CT1 are less than that ofSW as shown in Fig. 14 and Fig. 15.

WZ

^d

^d

^d

^d

^t

D

(a) Pressure ratio of ASCT2 and SW

D

D

(b) Efficiency of ASCT2Fig.14 Compressor characteristics of ASCT1

Figure 16 shows static pressure distributions near chokeconditions for SW, CT and CT1 along the impeller shroudwhich were calculated with CFD by changing static pressures atthe exit boundaries. Calculations were steady 3-D using aRANS in-house code developed by IHI [11]. Impeller andvaneless diffuser passages were modeled with one periodicpitch of the impeller. These were discretized by H-grids. TheChakravarthy-Osher TVD scheme was used to discretize theconvective term and the Spalart-Allmaras model for turbulenceclosure. The volute was not modeled in these calculations. Thenumber of grid points for these configurations were about4,500,000 to 5,200,000. Twenty-one (21) grid points werelocated between the blade tip and the casing to represent the tip

clearance. The conventional recirculation device was defined inthe rotating system of the impeller and modeled with the sameperiodicity as an impeller blade passage. The mean value of y+was 3.0 for all the three cases. mc in Fig. 16 is a choke flowrate for each calculation. As the pressure at the exit boundary,i.e. the back pressure of the impeller, is decreased up to acertain value, choking occurs at the throat of the impeller. Forall back pressures below this value, the flow upstream of thethroat is not affected by the back pressure because the pressure


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information from downstream of the throat cannot travelupstream. Hence the throat is roughly found as the closestposition from the impeller leading edge where the staticpressure is unchanged for different back pressures close tochoke condition. The approximate throat position near theshroud is indicated by arrows in Fig. 16. Since the bleed slot of

CT1 is upstream of the throat, the fluid injected from the bleedslot becomes mixed with the flow passing through the impeller.

This mixing process increases the entropy upstream of thethroat, and a blockage at the throat increases accordingly. As thebleed slot of CT is near the throat, the increase in the blockageat the throat due to the fluid injected from the bleed will bemuch smaller than that of CT1. Figure 16 includes thecircumferentially averaged entropy distributions of CT1 andCT. The position of the exit boundary for the calculation of CT1was the same as that of CT. The static pressure at the exitboundary for the calculation of CT1 was equal to that of CT.

The entropy distributions indicate that CT1 has a wider highentropy region than CT near the throat which is roughly located

just downstream of the bleed slot of CT. The reason why themaximum flow rate of CT1 is smaller than that of SW is theincrease of blockage at the throat due to the fluid injected fromthe bleed slot.

WZ

^d

d

d

^t

D

(a) Pressure ratio of ASCT1-1, CT1, CT and SW

D

D

(b) Efficiency of ASCT1 and CT

D

D

(c) Efficiency of CT1 and CTFig. 15 Compressor characteristics of ASCT1-1 and CT1

W

Wd

D D

D

W

Wd

D

D

^t

(a) Static pressure distribution SW

W

Wd

D D

D

W

d

(b) Static pressure distribution CT


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W^

Wd

D

D

D

W

d

(c) Static pressure distribution CT1

>

dd

(d) Circumferentially averaged entropy distributionFig. 16 Static pressure distribution along impeller shroud

and entropy distribution near bleed slot

W

Z

^d^

^d

^t

D

Fig. 17 Compressor characteristics of ASCT1-1 with sealedupstream slot, SW and ASCT1-1

Since most of the bleed slot of ASCT1 is located upstreamof the bleed slot of CT, the reason for the reduction in themaximum flow rate by the use of ASCT1 is probably the sameas that of CT1. To confirm this, ASCT1-1 whose upstream slotis sealed was tested. Figure 17 shows the relations between the

flow rate and pressure ratio of ASCT1-1 with the sealedupstream slot, SW and ASCT1-1. The maximum flow rate ofASCT1-1 with the sealed upstream slot is almost the same asthat of SW. Hence the increase of blockage at the throat due tothe fluid injected from the bleed slot of ASCT1 makes themaximum flow rate smaller than that of SW.

COMPRESSOR CHARACTERISTICS WITH SECOND

NON-AXISYMMETRIC RECIRCULATION DEVICEThe defect in the first recirculation device, ASCT1, is that

the bleed slot increases the blockage at the throat near chokeand reduces the maximum flow rate. Another type of non-axisymmetric recirculation device was designed to overcomethe above defect. The centerline of a new bleed slot was fixed at0.224H from the impeller leading edge, same as that of thebleed slot of CT, to avoid the reduction in the maximum flowrate. However the bleed slot of the new recirculation devicepartially exits within a certain circumferential range. Two innercasings with different new bleed slots were manufactured for

experiments. One is ASCT2 whose bleed slot exists over a 90range of the circumference and the other is ASCT3 whose bleedslot exists over a 60 range of the circumference as shown inFig 18. The bleed slot width of the new bleed slots is the sameas that of CT. The position where the recirculation device or thesuction of low energy fluid is required was found outexperimentally.

In the case of ASCT2, the inner casing was rotated in stepsof 90 as well as ASCT1. The slot of ASCT2-1 starts at =-34and ends at =56. The slots of ASCT2-2, ASCT2-3 andASCT2-4 start at =56, 146 and 236 respectively as shownin Fig. 18.

^d

^d

^d

^d

(a) Circumferential position of ASCT2

^d

^d

(b) Circumferential position of ASCT3Fig. 18 New type of non-axisymmetric recirculation device


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W

Z

^d

^d

^d

^d

^t

D


D

D

(b) Efficiency of ASCT2Fig. 19 Compressor characteristics of ASCT2

Figure 19 shows the test results of ASCT2. Figure 19 alsoillustrates the relation between the flow rate and the pressureratio of SW. The shift in the angular position of the bleed slotrelative to the volute tongue varies the compressorcharacteristics remarkably. The surge line of ASCT2-1 islocated at the smallest flow rate and the efficiency of ASCT2-1is the highest in the four cases. Figure 20 shows comparisons ofthe compressor characteristics of ASCT2-1 with those of SWand CT. The maximum flow rate of ASCT2-1 is equal to that ofSW and the surge line of ASCT2-1 is located at a slightlysmaller flow rate than that of CT. The efficiency of ASCT2-1 ishigher than or equal to that of SW. Figure 19 demonstrates thatthe surge line of ASCT2-2 is located at a larger flow rate thanthat of SW and that of ASCT2-3 occupies almost the sameposition of that of SW. The positions of the surge line ofASCT2-1, ASCT2-2 and ASCT2-3 indicate that the investigatedcompressor requires the bleed slot at of between -34 and 56but does not require at of between 56 and 236 forenhancement of its operating range. This confirms that eachblade passage is operated at a different operating point in thecircumferential direction.

WZ

^d

d

^t

D

(a) Pressure ratio of ASCT2-1, CT and SW

D

D

(b) Efficiency of ASCT2-1 and CT

D

D

(c) Efficiency of ASCT2-1 and SWFig. 20 Compressor characteristics of ASCT2-1

In the case of ASCT3, two different inner casing positionswere tested. The bleed slot of ASCT3-1 starts at=-4 and endsat=56. The bleed slot of ASCT3-2 starts at=26 and ends at=86 as shown in Fig. 18. Figure 21 shows the test results. The

test result of SW is included in Fig. 21. The surge line ofASCT3-1 is located at a smaller flow rate than that of ASCT3-2and the efficiency of ASCT3-1 is higher than that of ASCT3-2.Only the rotation of the bleed slot by 30 generates considerablevariations in the compressor characteristics. Figure 22 showsthe comparisons of ASCT3-1 with CT and ASCT2-1. Table 4lists the operating range, OPR, for SW, CT, ASCT3-1 andASCT2-1. The definition of OPR in Table 4 is


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1=s

d

d

s

m

mOPR

where d and s are total to total pressure ratio at design andnear surge flow rate respectively. ms is the flow rate near surge.Since an increase in the pressure ratio tends to move the surge

line towards the smaller flow rate, an OPR which includes theeffect of the pressure ratio was used here. The bleed slot of 60provides 1.7% less operating range than CT. However it couldrecover the efficiency drop caused by CT as shown in Fig. 21.

W

Z

^d

^d

^t

D


D

D

(b) Efficiency of ASCT3 and SWFig. 21 Compressor characteristics of ASCT3

Figure 23 shows the circumferential variation of staticpressure at 1.14R2 near surge for CT, ASCT2-1 and ASCT3-1 atthe design peripheral Mach number of Mu=1.62. Although thestatic pressure of CT is lower than that of ASCT2-1 andASCT3-1 because of the lower efficiency of CT compared toASCT2-1 and ASCT3-1, the circumferential variation of staticpressure of CT seems to be similar to that of ASCT2-1 andASCT3-1. Clear improvement of the static pressure distributiondownstream of the impeller could not be found by the use of thenon-axisymmetric recirculation devices.

To summarize the test results of two types of non-axisymmetric recirculation devices, a way to design therecirculation device without deteriorating compressor efficiencyor to improve compressor efficiency with the conventionalrecirculation device can be proposed. The position of bleed slotshould be near the impeller throat to avoid the reduction in the

maximum flow rate. This might contribute to suppress theexcessive recirculation flow, and hence avoid the deteriorationin the compressor efficiency. The bleed slot covering a certainrange in a circumferential direction, 60 to 90 in the testedcompressor, is enough to obtain the operating range theconventional recirculation device can provide. This willsuppress the drop in the efficiency caused by the use of therecirculation device.

W

Z

^d

^d

d

D

(a) Pressure ratio of ASCT3-1, ASCT2-1 and CT

D

D

(b) Efficiency of ASCT3-1, ASCT2-1 and CTFig. 22 Compressor characteristics of ASCT3-1

Table 4 Compressor operating range

SW CT ASCT2-1 ASCT3-1

OPR (%) 6.6 13.6 13.9 11.9


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W

Wd

D

Fig. 23 Circumferential variation of static pressuredistribution of CT, ASCT2-1and ASCT3-1 at 1.14R2

CONCLUSIONS(1) Shifting the angular position of the non-axisymmetric bleed

slot relative to the volute tongue caused a considerablechange in the compressor characteristics. These resultsconfirm that each blade passage is operated at a differentoperating point in the circumferential direction, and hencethe existence of asymmetric flow field in the impeller. Italso presents indirect evidence of the existence of theasymmetric flow field in the centrifugal compressor affectsthe inception of surge or stall. Since the centrifugalcompressor does not have any asymmetric elements exceptthe volute, this asymmetric flow field is created by thevolute.

(2) Taking the asymmetry of the bleed slot and volute intoaccount, 3-D unsteady calculation is inevitable. I t isconsidered that designing non-axisymmetric recirculation

devices with CFD would take a lot of time. From thepractical point of view, the experimental method byrotating bleed slots is an effective way of designing non-axisymmetric recirculation devices.

(3) The non-axisymmetric bleed slot, which varies its locationbetween the impeller leading edge and original bleed slotposition along the sine wave pattern, could shift the surgeline towards smaller flow rates. However the increase in thethroat blockage due to the injected fluid from the bleed slotmade its maximum flow rate smaller than the maximumflow rate of the compressor without the recirculationdevice.

(4) The recirculation device whose bleed slot was partially

manufactured in a circumferential direction showedpromising test results. The recirculation device with thebleed slot covering 90 in the circumferential directioncould achieve slightly wider operating range than theoriginal recirculation device without deteriorating thecompressor efficiency and without reducing the maximumflow rate which the compressor the recirculation device canachieve.

(5) This study proposed a way to design the recirculationdevice without deteriorating compressor efficiency. Thereis a proper position of bleed slot and a proper range ofbleed slot in a circumferential direction enhancing thecompressor operating range without sacrificing compressorefficiency and other characteristics.

REFERENCES[1] Hunziker, R., Dickmann H.-P., Emmrich, R., 2001,Numerical and Experimental Investigation of a CentrifugalCompressor with an Inducer Casing Bleed System,Proceedings of Institution of Mechanical Engineers, Vol. 215Part A.[2] Sivagnanasundaram, S., Spence, S., Early, J ., Nikpour, B.,2010, An Investigation of Compressor Map WidthEnhancement and the Inducer Flow Field Using VariousConfigurations of Shroud Bleed Slot, ASME GT2010-22154[3] Cumpsty, N. A., 1989, COMPRESSOR AERODYNAMICS,Longman Scientific & Technical, UK, pp.303-309

[4] Whitfield, A., Baines, N. C., 1990, Design of radialturbomachines, Longman Scientific & Technical, UK, pp.131-134[5] Fink, D.A., Cumpsty, N.A., Greitzer, E.M., 1992, SurgeDynamics in a Free-Spool Centrifugal Compressor System,"ASME J . Turbomachinery, Vol. 114, ,No.2, pp. 321-332.[6] Fisher, F. B., 1988, Application of Map WidthEnhancement Devices to Turbocharger Compressors Stages,SAE paper No. 880794[7] Zheng, X., Zhang, J., Bamba, T., Tamaki, H., Yang., M.,2010, Stability Improvement by High-Pressure-Ratio

Turbocharger Centrifugal Compressor by Asymmetric FlowControlPart II: Non-Axisymmetric Self-Recirculation-

Casing-Treatment, ASME Paper GT2010-22582[8] Yang, M., Martinez-Botas, R., Zhang, Y., Zheng, X.,Tamaki, H., Bamba, T., Li, Z., 2011, Investigation of Self-Recycling-Casing-Treatment (SRCT) Influence on Stability ofHigh Pressure Ratio Centrifugal Compressor With a Volute,ASME GT2011-45065[9] Tamaki, H., Unno, M., Kawakubo, T., Hirata, Y., 2009,Aerodynamic Design to Increase Pressure Ratio of CentrifugalCompressors for Turbochargers, ASME GT2009-59160[10] Yang., M., Zheng, X., Zhang, J., Bamba, T., Tamaki, H.,Huenteler, J ., Li, Z., 2010, Stability Improvement by High-Pressure-Ratio Turbocharger Centrifugal Compressor byAsymmetric Flow ControlPart I: Non-Axisymmetric Flow inCentrifugal Compressor, ASME Paper GT2010-22581

[11] Tamaki, H., 2011, Effect of Recirculation Device withCounter Swirl Vane on Performance of High Pressure RatioCentrifugal Compressor, ASME Paper GT2011-45360

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Experimental Investigation of High Pressure Ratio Centrifugal Compressor With Axisymmetric and...

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