Non-Axisymmetric Flow Structure in Vaneless Diffuser of Centrifugal Compressor for Turbochargers

7/28/2019 Non-Axisymmetric Flow Structure in Vaneless Diffuser of Centrifugal Compressor for Turbochargers

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1 Copyright 2012 by ASME

Proceedings of ASME 2012 Gas Turbine India ConferenceDecember 1, 2012, Mumbai, Maharashtra, India

GTIndia2012-9676

NON-AXISYMMETRIC FLOW STRUCTURE IN VANELESS DIFFUSER OFCENTRIFUGAL COMPRESSOR FOR TURBOCHARGERS

LAN Chuanj ie, ZHENG Xinq ian*

State Key Laboratory of Automotive Safety and Energy,Tsinghua University, Beijing 100084, China

Email:[email protected]

TAMAKI HideakiTurbo Machinery and EngineTechnology Department, IHI

Corporation, Yokohama 2358501, J apan

ABSTRACTTurbocharger technology is widely used in internalcombustion engines. With the downsizing of internalcombustion engines and the introduction of strict emissionregulations, there is urgent demand for turbochargers featuringcentrifugal compressors with a wide flow range. The flow in acentrifugal compressor of a turbocharger is non-axisymmetricdue to the inherent asymmetry of the discharge volute. Theasymmetric flow field inside the diffuser has great influence onthe performance of centrifugal compressor. In order to developa flow control method that facilitates a wider flow range ofturbocharger compressors, further understanding of theasymmetric flow structure is very important. The main subjectof this study is to reveal the asymmetrical characteristics of theflow field in the vaneless diffuser of a centrifugal compressorfollowed by a volute. Oil flow visualizations and numericalsimulations were used. The results of the numerical simulationsare consistent with that of the oil flow visualizations near chokeand at designed flow rate. The results show that a dual-zonemode asymmetric flow structure exists near the shroud of thevaneless diffuser at near choke condition. A bifurcation point atthe volute tongue that divides the flow and creates two distinctflow patterns was found. The asymmetry of the flow structurenear the hub was much less significant than that near theshroud. At the design flow rate, asymmetric flow patterns arefound neither near shroud nor near hub. At near surgecondition, the pattern of the oil flow traces near the shroud is

very different from those near choke.

1 INTRODUCTIONEnergy saving and environmental protection are two of the

hottest topics nowadays. Internal combustion engines consume25% of the global energy. Turbocharging can improve anengines power and efficiency. At the same time it can decreasethe emission of CO2 and NOx. To achieve further downsizingand power improvement for diesel engines, the tendency is to

design turbochargers with ever higher pressure-ratio [1, 2]. Thincrease in the pressure ratio causes the flow to be transonicand decreases the stable flow range of centrifugal compressorsExtending stable flow range has thus been a key issue inresearch and development of turbochargers for diesel engine[3, 4].

Compared to diesel engines, gasoline engines do nodemand as high a pressure ratio. Yet gasoline engines exhibit awide speed range. Therefore they need a large stable operationrange when the centrifugal compressor is operated in a lowpressure-ratio area. Extending the centrifugal compressorstable flow range is hence also a key issue for the gasolinengines.

The static pressure around the volute is assumed to becircumferentially constant at the design speed of the centrifugacompressor. However, the static pressure will not remainconstant when the compressor is operated in off-designcondition. When the flow rate is higher than the design flowrate, the pressure will decrease in direction of rotationbeginning from the volute inlet. In that case the volute will acas a nozzle. In contrast, when the flow rate is lower than thdesign flow rate, the pressure will increase in direction orotation and then the volute will act as a diffuser [5]. It has beereported that the asymmetric flow field inside the diffuser andthe volute has great influence on the flow field in the impellerand the diffuser [6, 7].

In 1986, Sideris et al. [5] discovered the asymmetric

distribution of the static pressure in the vaneless diffuser andthe impeller. In their measurements, the flow field became moreasymmetric upstream and the phase difference increased as theflow rate became larger. Sorokes et al. [6] carried out a detailedmeasurement of the asymmetric flow field. They found thathere is asymmetry both at the inlet and the outlet of impeller

The asymmetric inlet conditions mean asymmetric inlet flowangles, thus stall occurs locally, at single passage inlets, beforethe impeller inlet flow stalls as a whole. Hillewaert et al. [8
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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used numerical methods to study the asymmetric flowphenomena. They found the static pressure distorted stronglynear the volute tongue. That caused part of the fluid to flowdirectly into the suction side of the volute tongue. Gu et al., onthe other hand, reported that there is no phase shift of the flowdistortion in the vaneless diffuser and impeller [9]. Theirfindings differed because the instability factor caused by thevolute-impeller interaction can be neglected when the Strouhalnumber is small enough.

Zheng et al [10] measured the detailed static pressuredistribution in the vaneless diffuser of a high pressure ratiocentrifugal compressor in 2010. They found that the staticpressure distribution exhibited a minimun 90 degreedownstream of the tongue. In contrast, a maximum of thepressure distribution was found there near choke condition.Overall, the research done by Zheng et al [11, 12] shows thatthe asymmetric flow has an important impact on the flowstability of a turbocharger compressor. The relative restrictionof the stable flow range due to the asymmetric flow can be upto 47 percent.

In order to develop a flow control method that facilitates awider flow range of turbocharger compressors, furtherunderstanding of the asymmetric flow structure is veryimportant. To that end, this paper analyzes the flow fieldstructure in a vaneless diffuser of a turbocharger compressor byusing oil visualizations and numerical simulations.

2 RESEARCH METHODS

2.1 Experiment Methods. The outlet diameter of theanalyzed impeller is 62 mm and the rated speed is 120000 rpm.More parameters can be seen in table 1; the compressor map isdepicted in Fig. 1. The three small circles in Fig.1 show theconditions referred to in this paper as near choke condition,design condition and near surge condition.

Table 1 Geometry parameters of compressor

parameters value

Designed rotational speed 120,000 rpm

Inlet diameter 40.9 mm

Exit diameter 62.15 mm

Inlet diameter of diffuser 70 mm

Outlet diameter of diffuser 100 mm

Blade number 6+6

Fig. 1 Compressor performance by experiments

This study used oil visualization to reveal the asymmetricflow field in the vaneless diffuser. A paint consisting of a mixof oil and powder was brushed on the diffuser surfaces. Aftethe air flow blew off the oil, powder stayed on surfacerecording the flow movement [13]. Investigating the oil flowtraces (oil map) visualized by the powder, flow field featurescan be recognized. These include flow direction, separationvortices, and so on. Although oil visualization has been widelyused in wind tunnel experiments, it is hard to conduct because

of the high air flow speed and high temperature. Theexperimental procedure chosen in this research was thefollowing:

(a) Chose suitable oil, powder and other additives. Mixedthem together to obtain a mixture with high viscosityuniformity and low volatility.

(b) Brushed the mixture on the surface of vanelesdiffuser.

(c) Accelerated the compressor to a predeterminedcondition, kept it there for some minutes and thestopped it quickly.

(d) Disassembled the compressor, took a photo of the omap and analyzed it.

In this paper, silicone oil was chosen as the solvent

titanium dioxide as the powder, and oleic acid as the dispersantCompared to other oils, silicone oil can keep a good viscosityin high temperature and high shear stress the environmeninside compressor.

Oil viscosity and mixture ratio,have significantly influenceon the clarity of the oil map. To get a clear oil map, this studyprepared several mixtures with different ratio and observed howeach component affected the properties of the mixture in


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different temperature. Mixtures of the preliminary test can beseen in table 2.

Table 2 Mixtures in preliminary test

Oilviscosity

(cps)

Oil: Powder(volume ratio)

Oleic acid(drop)

Temperature()

500 1:1 3:1 1:0 5~6 2~3 100 20

1000 1:1 3:1 1:0 5~6 2~3 100 20

2000 1:1 3:1 1:0 5~6 2~3 100 20

The preliminary tests revealed that a high oil viscosity canimprove the viscosity of the mixture, without having aninfluence on its uniformity. The viscosity of the mixturebecame higher as the share of titanium dioxide was increased,while its uniformity decreased at the same time. The propertiesof the mixture were sensitive to oleic acid: the mixturesviscosity intensively dropped and its uniformity intensivelyincreased when only little of the oleic acid was added. The

influence of each component on the volatility was opposite tothe viscosity. Table 3 shows the detailed results. In addition, thetemperature has no influence on the results.

Table 3 Influence of each component on properties

Component uniformity viscosity volatility

Viscosity ofsilicon oil

Nocorrelation

Positivecorrelation

Negativecorrelation

Quantity oftitanium dioxide

Negativecorrelation

Positivecorrelation

Negativecorrelation

Quantity of oleicacid

Positivecorrelation

Negativecorrelation

Positivecorrelation

It is very important to choose suitable mixtures forresearched operation conditions. For high speed operationconditions, the mixture with high viscous oil and more titaniumdioxide should be chosen. For the experiment with 120,000rpm, this study prepared several mixtures, composed of siliconoil (1000 cps, 2000 cps), titanium dioxide (different quantities),oleic acid (different quantities), for the mixture selection test.Figure 2 shows the results of the mixture selection test withdifferent mixture compositions when the silicon oils viscosityis kept at 2000 cps. The volume ratio given below the figures isthe volume ratio of the silicon oil to the titanium dioxide. The

final composition of the mixture used at 120,000 rpm was 2000cps silicon oil, silicon oil: titanium dioxide = 2:1 (volumeratio), 2~3 drops oleic acid.

(a) Volume ratio =1:1

(b) Volume ratio =2:1

(c) Volume ratio =3:1

Fig. 2 Influence of component ratio on oil mapsclarity (silicon oil: titanium dioxide)

In order to get clear oil map, the holding time is alsoimportant. If the holding time is too long, air flow will sweep

off the powder. If the holding time is too short, the experimencannot get a clear oil map. The holding time selection test waconducted with the selected mixture as described above. Figure3 shows the influence of the holding time on the clarity of theoil map. Accelerating the compressor only swept away thesurface layer and had no interference on the clarity of the oimap. Three minutes was found to be a suitable holding time.


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(a) 0 minutes

(b)3 minutes

(c) 5 minutes

Fig. 3 Influence of holding time on oil map clarity

2.2 Numerical Methods. Numerical simulation wasemployed to investigate the detailed flow field in thecompressor. The solver applied a CFD code based on a 3-Dsteady compressible finite volume scheme to solve Reynolds-averaged Navier-Stokes equations in conservative formulation.

The central scheme was used for the spatial discretization, the

forth-order Runge-Kutta scheme for the temporal discretizationThe Spalart-Allmaras (S-A) 1-equation model was applied foturbulence closure [14]. The S-A model features numerationaaccuracy for the calculation of the turbulent flow in the viscouboundary layer, separated flows of small or medium scale anfree shear turbulent flow except for jet [15].

All passages of the impeller, the vaneless diffuser and thevolute were modeled in the simulation, as shown in Fig. 4 (a)

The flow field in the impeller was calculated in a rotatingcoordinate system, while the vaneless diffuser and volutes flowfield was in absolute coordinate system. The interface betweeimpeller and volute was modeled as frozen rotor, thats becausefrozen rotor can simulate the asymmetry of flow structure bumixing plane approach cant.

The size of the impeller/diffuser grid was 5,271,120Research showed that grid independence could be insured if thegird number of impeller/diffuser was larger than 350,000 whenS-A model was chosen. The grid satisfied a minimal gridquality defined by measures of orthogonality (especially at thblock boundaries; minimum 26.5), relative grid spacing in

boundary layers (expansion ratio3), and grid skewness (aspecratio 1000).

To mesh the overhung volute, a butterfly mesh was appliedin order to augment orthogonality of the boundary layer cells

Total grid number of the volute was 1,025,025. The gridsatisfied a minimal grid quality as defined by measures oorthogonality (especially at the block boundaries; minimum13), relative grid spacing in boundary layers (expansion ratio5), and grid skewness (aspect ratio 1000). In Fig. 4 (b), amesh configuration at the volute exit is displayed in order toshow the butterfly mesh approach.

The first cell width near wall was set to be 0.003mm, thusy+could be lower than 10 to S-A models requirement.

Total temperature and total pressure together with thevelocity direction were imposed as inlet boundary conditionsstatic pressure was set as outlet boundary condition. No-slipand impermeability conditions were imposed on the solid walls

(a) Computational grid of all the passages of the impeller withdiffuser and volute


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(a)Butterfly meshing approach in volute cross-section

Fig. 4 Computational mesh in detail

3 RESULTS AND ANALYSIS

3.1 Near Choke. Figure 5 shows the results of the oil

visualizations near choke on shroud and hub. On shroud, theflow near the shroud of the vaneless diffuser is divided in two,a state which will be called dual-zone mode flow in thispaper. The position of the volute inlet is defined as 0 (360) of and the angle of in a circumferential direction increases

towards the volute outlet. The positions from =0 to 360areshown in Fig. 5 at every 90. The flow is separated into twoparts and forms a bifurcation near the tongue, at =45. Theradial velocity at the inlet of the vaneless diffuser at from 45to 180 is much smaller than the circumferential velocity,which results in a large flow angle. In these areas, the fluidtravels along a long flow path in the diffuser and enters thevolute at =90 to 225. In contrast, the fluid at the range

from 180 to 360 has a smaller flow angle at the inlet of thevaneless diffuser than the above region. The length of the flowpath to the volute inlet here is much shorter. On hub, althoughthe oil map shows an asymmetric flow pattern, the asymmetryis much weaker than. that on the shroud surface.

(a) Oil map on diffuser shroud near choke

(b) Oil map on diffuser hub near choke

Fig. 5 Oil map near choke at 120,000 rpm

Figure 6 shows the results of the numerical simulations. Ireveals the evolution of the flow in the vaneless diffuser fromthe shroud to the hub. 100% span and 0% span correspond tothe shroud and the hub of the vaneless diffuser, respectively

The color contours indicate the static pressure, while theblack lines show the streamlines. The pressure reaches itshighest valueat =90. Therefore, the pressure rises at from 0to 90 and falls in the range from 90 to 360. The most part othe volute acts as the nozzle except an area just downstream othe tongue. The asymmetric distribution of pressure results inasymmetric flow field near the shroud. The flow patternobtained by the numerical simulations agree with the oil mapqualitatively. The dual-zone mode and the bifurcation can bconfirmed in the results of the numerical simulations. Theffect of the asymmetric pressure distribution on the streamlinedecreases progressively from the shroud to the hub. It is hard tosee obvious asymmetry in the streamlines from 0% span to50% span.

The difference in the streamlines between hub and shroudis due to the jet-wake flow pattern at the outlet of impelle[16]. The radial velocity in the jet is much larger than in thwake. On the other hand, the circumferential velocity in thewake is much larger than that in the jet. The wake and the jemostly occupy the shroud and hub sides of the diffuserrespectively. The fluid near shroud at from 45 to 180 isturned inward to balance the adverse pressure gradient from thcentrifugal force generated by the curvature of the streamline

The fluid follows a long path in the diffuser, and some fluid ieven turned backwards. The adverse pressure gradient becomeweak at from 180 to 360 due to the nozzle effect caused bythe volute, so the curvature of the streamline increases. Hencthe fluid flows into volute rapidly. The fluid near hub is alsoturned to inward at from 45 to 180. However the fluid nearthe hub belongs to the jet and has a much larger radial velocityand the flow angle at the inlet of the vaneless diffuser is muchsmaller than near the shroud. The change of length of the flow


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path near hub is thus much smaller than that at the shroud. Thiscan be understood easily by differentiating the logarithmicspiral with respect to the flow angle at the diffuser inlet. Thereason why there is not obvious asymmetry in the streamlinesfrom 0% span to 50% span is the small impeller discharge flowangle.

(a) 90% span (b)75% span

(c) 50% span (d) 10% span

Fig. 6 Numerical results for the static pressuredistr ibution and st reamlines near choke at 120,000rpm (from shroud to hub)

3.2 Design Condition. At the design condition, anaxisymmetric flow structure in the diffuser is to be expectedsince the asymmetric pressure distribution largely disappears involute. Yet it is very hard to find the design point of the volute

where the asymmetric pressure distribution disappears, both inexperiment and simulation because the impellers designcondition deviates from the design point of the volute. Figure 7shows the oil map at design condition. There is still someasymmetry near the shroud. Near the hub, the asymmetric flowpattern disappears.

(a) Oil map on shroud of diffuser on design condition

(b) Oil map on hub of diffuser near on design condition

Fig. 7 Oil map on design condi tion at 120,000 rpm

Figure 8 shows the results of the numerical simulations odesign condition. The results of the numerical simulations agreewell with the oil map. The static pressure is slightly asymmetricin the diffuser. But a high pressure region exists locally near thtongue. The incomingfluid from the diffuser inlet at =270 to360 passes through this area. When it passes through this highpressure region, the fluid is turned inwards and takes a longeflow path. The flow field near hub is almost symmetric, as inthe oil map.

(a)90%span (b)75%span

(c)50%span (d)10%span

Fig. 8 Numerical results of static pressure distributionand streamline on design condit ion at 120000 rpm(from shroud to hub)

3.3 Near Surge. When the operating condition approachesurge, the oil map on the shroud shows a very differenasymmetric flow structure from that near choke. Figure 9shows the oil map. Although the flow rate decreases, tangentiaangles of the oil flow traces at the diffuser exit near surge aresmaller than those at the design condition. On shroud surface a


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shown in Fig. 9 (a), a part of fluid at diffuser inlet in =0 to45 flows into volute in = 45 to 90. Another fluid atdiffuser inlet in =0 to 45 flows into the volute in =90 to180 together with the fluid at diffuser inlet in =45 to 90.Because the mixture was not swept off between the inlet andthe middle of the diffuser near the hub, it is hard to see the oilflow traces in this region, as shown in Fig. 9 (b). However afew oil traces which flow counterclockwise from inner to outerradius can be found beyond this region, as shown in Fig. 9 (c).

The impeller rotational direction is clockwise in Fig. 9 (c). Thisimplies the possibility of the reverse flow from the diffuser exit(volute inlet). As the reverse flow will accelerate on its wayinwards with decreasing radius, the inward flow might be ableto remove some of the mixture in the downstream part of thediffuser. This reverse flow might increase the flow passingthrough the shroud side and reduce the flow angle near shroud.

(a) Oil map on shroud of diffuser near surge

(b) Oil map on hub of diffuser near surge

(c) Oil map on hub of diffuser near surge

Fig. 9 Oil map near surge at 120000 rpm

Figure 10 shows the corresponding results of the numericasimulations. The streamlines obtained by the numericasimulations are quite different from those revealed by the oivisualization. Although the impeller rotational direction i

counterclockwise in Fig. 10, the streamlines of 90% span iclockwise. Hence the reverse flow occurs at 90% span. Thstreamline of 75% span shows the recirculation flow in thdiffuser. The volute acts as a diffuser near surge. The staticpressure at =180 to 360 is much higher than that at =0to180. Some fluids at 90% span in =180 to 360 are pushedback from the diffuser exit to diffuser inlet. The symptom of threverse flow near hub cannot be seen in Fig. 10. The failure tosimulate the reverse flow near hub might be a cause of thedifference between the oil map and the numerical result. Anunsteady numerical simulation is probably necessary at thicondition.

(a)90% span (b)75% span

(c)50% span (d)10% span

Fig. 10 Numerical results of static pressuredistr ibution and streamlines near surge at 120000rpm (from shroud to hub)


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4 CONCLUSIONS AND REMARKSThis study used oil visualizations and numerical

simulations to investigate the asymmetric flow structure in thevaneless diffuser of a centrifugal compressor with a volute. Thefollowing conclusions can be derived:

(1)The method of oil flow visualization was carefullyconstructed. The results of the oil flow visualizationscould be explained successfully by the numericalsimulations, except at near surge condition. Anunsteady numerical simulation is probably necessaryat near surge condition.

(2) At near choke condition, a dual-zone asymmetricflow structure exists near the shroud. A bifurcationoccurs near the volutes tongue. The asymmetry of thestreamline is weaker near the hub than near the shroud.

This is caused by the difference of the flow angle atthe diffuser inlet between shroud and hub.

(3) At designed condition, the flow in the vanelessdiffuser is almost symmetric because the staticpressure distribution of the volute is more

axisymmetric as the operating condition approachesthe volutes design condition.

(4) At near surge condition, the pattern of the oil map nearthe shroud is very different compared to that nearchoke. Although a clear oil map could not be obtainednear the hub, a few oil traces were found that suggestreverse flow from the volute.

ACKNOWLEDGMENTSThis research was supported by the National Natural

Science Foundation of China (Grant No. 51176087).

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Non-Axisymmetric Flow Structure in Vaneless Diffuser of Centrifugal Compressor for Turbochargers

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