Turbodesign-Next Generation Design Software for Pumps

8/9/2019 Turbodesign-Next Generation Design Software for Pumps

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F EATURE • DESIGN SOFTWARE

WORLD PUMPS February 2003 0262 1762/03 © 2003 Elsevier Science Ltd. All rights reserved32

In many mixed flow and centrifugalpumps, the flow field is dominatedby complicated three-dimensionalviscous effects, such as secondaryflows and corner separations. It is onlyrecently and as a result of develop-ments in computational fluid dy-namics (CFD) coupled with vali-dation by experiments that we havebeen able to obtain a better under-standing of the detailed three-dimensional flow field in these typesof pumps. CFD methods (e.g. Walkerand Dawes 1) can predict the flow inthe pump impeller or diffuser andshow the occurrence of unfavourableflow features such as flow separationor secondary flows. However, CFDdoes not show directly what modifi-cation should be made to improvethe flow field.

The main difficulty currently facingpump designers is how to utilize theimportant insights into the pump flowfield provided by CFD in the hydro-dynamic design of pump components.This is mainly due to the fact thatcurrent design practice is based onmaking trial and error changes toexisting geometries, which are nor-mally defined in terms of blade angledistributions. The designer starts froman initial blade shape and thenevaluates the flow field in the impelleror diffuser by the application of CFD.Ideally, it should then be possible tomodify the blade shape if the flow

conditions are not those required.

In practice, however, there aredifficulties in determining the degree

and direction of any modifications.These difficulties are compounded bythe fact that the change of bladeshape at one location can affect theflow at other parts of the blade.Therefore, the conventional designapproach has to rely on empiricismand previous design experience.Furthermore, since there is no directlogical relationship between the inputto the design process (the blade

shape) and the output (the flow field),it is difficult to create an easy-to-usedatabase of design know-how.Therefore, minor changes to thedesign specification (such as flow rateor head) will require another longprocess of iterative modification of the blade geometry.

Inverse design

A more systematic approach to thedesign of pumps is the inverse designapproach in which the blade shape iscomputed for a given distribution of pressure distribution or blade loading.Because the flow field is directly relatedto the pressure or loading distributionthis approach enables the designer touse his insights into the flow hydro-

dynamics, gained through the use of CFD codes, directly for the choice of his design inputs. Inverse designmethods for turbomachinery designhave been available since the 1940s(see Lighthill 2, and Hart and White-head 3). But they have had limitedapplication in pump design due to the

Turbodesign -1 : Next generationdesign software for pumpsUsing the inverse design approach in the design of pumps allows the shape of componentssuch as impeller blades to be computed for a given pressure distribution or blade loading.The latest 3D inverse design codes are a significant advance on earlier methods as they areable to take account of 3D flow effects, which dominate centrifugal and mixed flowpumps. Here, M. Zangeneh and A. Goto outline the basic input data for one such inversedesign code, TURBOdesign -1 . Two examples are then used to illustrate how a designer canuse his fluid dynamics insight to improve the design of pump components. Finally theydiscuss issues related to data export and manufacturing.

Figure 1. The met hod of specification of blad e load ing.

HUBSlope

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fact that early inverse methods weremainly two-dimensional and thereforefailed to take account of 3D floweffects, which dominate centrifugaland mixed flow pumps. In addition,many of the early inverse designmethods in which the pressure distri-

bution was specified on the blade upperand lower surfaces did not enable thedesigner to control the blade thicknessand therefore in many cases the designscould not be manufactured.

Hawthorne et al.4, proposed a 3Dinverse design method in which theblade shape is computed for a specifieddistribution of blade circulation (orloading) together with blade thickness.This approach was later developed intoa design method for all types of turbo-machinery by Zangeneh 5. This methodhas been applied extensively to thedesign of centrifugal and mixed flowpumps. It has been applied to thedesign of pump impellers and diffusersleading to important design break-throughs such as suppression of second-ary flows in impellers (see Zangeneh etal.6,7) and corner separation in vaneddiffusers (Goto and Zangeneh 8).Furthermore, these breakthroughs inhydrodynamic design have resulted in

substantial (5–6 percentage points)improvements in pump performance,over the best state-of-the-art efficiencylevels. In addition the method has

been applied to the design of pumpstages with improved suctionperformance (Ashihara and Goto 9,10)as well as the design of very compactpump stages, see Goto et al.11

In 1999, the inverse design method of

Zangeneh5

was developed into acommercial design code, TURBO-design-1, by Advanced DesignTechnology Ltd in the UK. The codehas been licensed widely to pumpmanufacturers and is currently beingused for actual product design by anumber of major pump manufacturers.

Main input data toTURBOdesign -1

The inputs of this design method areas follows:

• meridional shape• loading distribution (distribution

of bound circulation 2 πrVθ)• rotational speed ω, which is zero

for a stationary blade• blade thickness distribution• blade number• stacking condition.In the present design system, theblade loading is specified by giving

the distribution of ∂(rV θ)/∂m, whichis the derivative of angular momen-tum (rV θ) along the meridionaldistance, m. The design parameter is

directly related to the pressure load-ing (p + – p-: the pressure differenceacross the blade) through the follow-ing equation for incompressiblepotential flows:

where B is the blade number, ρ isdensity, and W mbl is the blade-to-blade average of the meridionalcomponent of the relative velocity onthe blade. By controlling ∂(rVθ)/∂m itis possible to control the pressureloading and therefore the overall 3Dpressure field.

In TURBOdesign -1 the loading distri-bution is specified simply by using a‘three-segment’ method in which acombination of two parabolic curvesand an intermediate linear line areused, as shown in Figure 1. Fourdesign parameters (connection pointlocations NC and ND, slope of thelinear line, and the derivative ofrVθ [DRVT]) are used to define thedistribution curve. The ∂(rV θ)/∂mdistribution is specified for each of the shroud and hub surfaces, thenthe rV θ distribution is derived bythe integration of ∂(rVθ)/∂m along

the meridional distance m on thehub and shroud surfaces. The rV θ dis-tributions in the intermediate part of the blade are obtained by a linear

mrV

W B

p p mb l ∂∂=− −+ θρπ

2

Figure 2. Velocityvectors on bladesuction surface:

(a) convention al;(b) TURBOd esig n -1.

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interpolation for rV θ between thehub and the shroud.

Blade designexamplesImpeller design withsuppressed secondaryflowsIt is well known that the secondary flowphenomena in an impeller have impor-tant effects on the efficiency and stab-ility of the impeller. In addition to this,the secondary flow has a dominatinginfluence on the generation of the exitflow non-uniformity (so-called ‘jet-wake’ flow pattern) and affects theperformance and stability of the down-stream diffuser. The secondary flows onblade suction surfaces are important,since the boundary layers are thickeron the suction surfaces than on thepressure surfaces. However, the designprocedure to control secondary flowshas not been established until very re-cently due to the complex three-dim-ensionality of the pressure fields. Zan-geneh et al.6 presented a logical methodbased on 3D inverse design to suppressmeridional secondary flows withincentrifugal and mixed-flow impellers,which is briefly described here.

Figure 2(a) shows the flow pattern in aconventional impeller having a typicalblade angle distribution which conn-ects the inlet and exit blade angles by

smooth monotonous curves. Strongspanwise secondary flows were clearlyobserved, which were generated by thereduced static pressure gradient be-tween the hub and the shroud. Thesesecondary flows move all the low mo-mentum fluids on the suction surface of the blade to the suction/shroud corner

resulting in the formation of thejet/wake flow shown in Figure 3(a).

The secondary flow control by the 3Dinverse design code TURBOdesign -1

is rather straightforward as the press-ure fields can easily be controlled bycontrolling the blade loading para-meter ∂(rVθ)/∂m. Namely, the shroud

was fore-loaded while the hub was aft-loaded to reduce the spanwise pressuregradient between the shroud and thehub in the aft part of the impellersuction surface. Figures 2(b) and 3(b)present the flow field in the inversedesign impeller, where it can be seenthat secondary flows are well suppress-ed and as a result a more uniform exitflow pattern is obtained.

As can be seen in Figure 4, the bladeangle distribution between these twodesigns is very different, and it can beconfirmed that the conventional des-ign practice of using smooth bladeangle distributions does not necessarilyguarantee good flow fields. This factclearly demonstrates the importance of

carrying out the blade design based onthe hydrodynamic design parameter(TURBOdesign -1) and not the geo-metric design parameter (conventionaldesign).

Diffuser design withsuppressed corner stall

The hub surface of a vaned bowl diff-user can be highly loaded when theouter diameter of the diffuser is made

compact. In such situations, the optimi-zation of blade shape is extremely im-portant to avoid a large-scale flow sepa-ration along the corner region betweenthe diffuser blade suction surface andthe hub surface. In 1998, Goto andZangeneh 8 presented a design pro-cedure for diffuser blades using the 3Dinverse design code TURBOdesign -1.

Figure 3. Predictedvelocity contou rs atimpeller trailing ed ge:(a) convent ional;(b) TURBOd esig n -1.

Figure 4. Compa rison o f bla de a ngle o f convention al a nd TURBOdesign -1 impellers.

a)

b)


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Figure 5 presents the results of CFDprediction of stage flows for a conven-tional diffuser pump stage with low spe-cific speed (280[m, rpm, m 3/min]) andthe multicolour oil-film flow patternwithin the vaned bowl diffuser part.Due to the spanwise pressure gradienton the diffuser blade suction surface at

the inlet, the spanwise secondary flowswere generated towards the hub surf-ace. At the same time, on the hub sur-face, the secondary flows towards the

blade suction surface were generateddue to the blade-to-blade pressuregradient. Because of these two types of secondary flows, the low momentumfluids were accumulated in the hub/suction surface corner region. The ad-verse pressure gradient is also high inthis region and as a result a large-scalecorner flow separation occurred around

the middle part of the hub surface.

In order to suppress the formation of the corner separation by using

TURBOdesign -1, a fore-loaded distri-bution was used at the hub. This type of loading reduces the adverse pressuregradient in the middle part of the hubsurface. At the same time by using aft-loading at the shroud wall, spanwisesecondary flows were enhanced at theinlet part of the diffuser blade suctionsurface, and the accumulation of low

momentum fluids in the corner regionwas prevented. This combination of loading resulted in successful suppress-ion of the corner separation (Figure 6).

Figure 6. Flow fieldin the conventional

vaned bowldiffuser: (a) CFDpredictions; and

(b) oil flow visualization.

6a)

6b)

Figure 5. Flow field in th e TURBOdesign -1 vaned b ow l diffuser: (a)CFD predictions; an d (b) oil flow visualiza tion .

5a)

5b)


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The stage designed by TURBOdesign -1

was manufactured (Figure 7) and itsperformance was compared to thatof the conventional stage in the sametest stand. The conventional stagewas designed by application of CFDand had an efficiency level thatwas at the top end of state-of-the-art efficiencies for this specific speed.As can be seen, the TURBOdesign -1

stage shows a 5% improvement in

performance over the conventionalstage (Figure 8).

Program interfaceand data transfer

The program features a user-friendlygraphical user interface. The interfaceenables the transfer of the geometry totypical CFD software such as Turbo-grid for CFX-TASCflow, PLOT3Dfor CD-ADAPCO’s STAR-CD, and

G/Turbo for FLUENT. In addition thecode enables the direct generation of IGES files for CAD software and STLfile format for rapid prototyping.

Concluding remarks

TURBOdesign -1 enables the designerto control the flow field in the impellerand diffuser by careful control of 3Dpressure fields, through the specifiedblade loading distribution. Thus, it ispossible to achieve an innovative

design for pump blades having, forexample, high efficiency, high suc-tion performance and very compactmachine size. Once optimum inputdata based on solid physical back-ground are found for the TURBO-deisgn-1, it is possible to apply theseresults to similar designs or at least givea good baseline design to start anoptimization process. This feature isespecially useful for systematic seriesdevelopment of pumps covering a wideflow coefficient (or specific speed)range. The design guidelines or designexpertise thus obtained are expectedto be more universal and operator-independent, and are easily transferredto the next generation.

Another important feature of this des-ign method is that it will enable theoptimization of one of the design para-meters while keeping other parametersthe same. The effects of adopting adifferent meridional geometry, forexample, can be evaluated indepen-dently while keeping the blade loadingdistribution and other design para-meters the same. All these featurescontribute to achieve a breakthroughdesign and shorten the whole design/

optimization cycle.

Currently work is in progress at ADTto develop a full 3D viscous inversedesign package and hybrid designsystem with automatic optimizationalgorithm. A splitter design version of TURBOdesign -1 is already availableon commercial release. ■

References

(1) Walker, P.J. and Dawes, W.N.,‘The Extension and Applicationof Three-Dimensional Time-Marching Analysis to Incom-pressible Turbomachinery Flows’,

ASME Journal of Turbomachinery,(1990), Vol. 112, pp. 385-390.

(2) Lighthill, J.M., ‘A new methodof two-dimensional aerodynamicdesign’, ARC R&M, (1945), 2104.

(3) Hart, M. and Whitehead, D.S.,‘A design method for 2D cascadesof turbomachinery blades’, Int. J.

Numerical Methods in Fluids,(1987), Vol. 7, pp. 1363-1381.

(4) Hawthorne, W.R., Tan, C.S.,Wang, C. and McCune, J.E.,

‘Theory of blade design for largedeflections: Part II - Annularcascades’, Trans of ASME, J. Eng.Gas Tur. Power , (1984), Vol. 106,pp. 354-365.

(5) Zangeneh, M,. ‘A compressiblethree-dimensional design methodfor radial and mixed flow turbo-machinery blades’, Int. J. Numeri-cal Methods in Fluids, (1991),Vol. 13, pp.599-624.

(6) Zangeneh, M., Goto, A. and Har-ada, H., ‘On the design criteriafor suppression of secondary flowsin centrifugal and mixed flowim-pellers’, Trans ASME, J. of Turbomachinery, (1998), Vol. 120,pp. 723-35.

(7) Zangeneh, M., Goto, A. and Hara-

da, H., ‘On the role of three-dim-ensional inverse design methodsin turbomachinery shape opti-mization’, Proc. Instn Mech. Engrs ,(1999), Vol. 213 Part C, pp. 27-42.

(8) Goto, A. and Zangeneh, M.,‘Hydrodynamic design of pumpdiffuser using inverse designmethod and CFD’, (1998), ASMEpaper No. FEDSM98-4854. AlsoTrans ASME, J. Fluids Eng.,(2002), Vol. 124, pp. 319-328.

(9) Ashihara, K. and Goto, A., ‘Impro-vements of Pump Suction Perfor-mance using 3D Inverse DesignMethod’, 3rd ASME/ JSME JointFluids Engineering Conference,(1999), ASME paper No. FED-SM99-6846.

(10)Ashihara, K. and Goto, A., ‘Studyon pump impeller with splitterblades designed by 3D inversedesign method’, (1999), ASMEpaper No. FEDSM2000-1103.

(11) Goto, A., Ashihara, K., Sakurai, T.and Saito, S., ‘Compact design of diffuser pumps using three-dim-ensional inverse design method’,3rd ASME/JSME Joint FluidsEngineering Conference, (1999),ASME paper No. FEDSM99-6847.

C ONTACT

M. Zangeneh and A. GotoAdvanced Design Techno logy Ltd ,Monticello House,45 Russell Sq ua re,LondonWC1B 4JP,

UK.Tel: + 44-20-7907-4710Fa x: + 44-20-7907-4711E-mail: [email protected] .uk

Figu re 7. ThemanufacturedTURBOde sign -1 stage.

Figure 8.Comparison ofmeasuredperformance o fTURBOde sign -1and conventionalpump stages.

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Turbodesign-Next Generation Design Software for Pumps