Innovations from Venti OeldeComputational Fluid Dynamics for fans and plants

2 The mathematics behind CFD

The set of equations describ-ing the processes of momen-tum, energy and mass transferis known as the Navier-Stokesequations. These partial differ-ential equations, which werederived in the early 19th cen-tury and have no known gen-eral analytical solution, can besolved numerically. For the nu-merical solution of this systemof equations, the differentialtransport equations are firsttransformed into algebraicequations by so-called discret-ization. In the case describedhere, the Finite Volume Meth-od is used for this purpose.This method is generally ap-plied as the basis for simula-tion programs in the Computa-tional Fluid Dynamics sector.In the FM method, a three-dimensional computationalmesh is laid over the flow vol-ume to be investigated. Deter-mination of the distribution of aflow value Φ thus takes placevia discrete calculation points

In the field of turbomachin-ery construction, CFD has,within the space of just afew years, become an indis-pensable tool for the opti-mization and new design ofturbomachines, as well asfor the elimination of fluid-mechanic problems in in-stalled systems. The follow-ing paper demonstrates thebasic knowledge as well asapplied examples of the CFDsimulations of process fans.

Computational Fluid Dynamics for fans and plants

Fan optimization with CFD modelling

1 Introduction

Computational Fluid Dynam-ics (CFD) is a method that isused for the computerized cal-culation of technical flows withstate-of-the-art accuracy. Withthis method, it is possible tomake far more precise state-ments regarding the behav-iour of a technical or naturalflow than could be made onthe basis of classical partly-empirical, test-based ap-proaches. CFD has been em-ployed on an economic scalesince around 1973, when itwas first applied in the aircraftconstruction sector. The un-derlying partial differentialequations according to Na-vier-Stokes, which preciselydescribe the flow characteri-stics of a Newton fluid, havebeen known since the first halfof the 19th century. The cal-culation work required for solv-ing these differential equa-tions is very complex: the flowvolume has to be subdividedinto a number of smaller par-

tial volumes that depend onthe occurring flow gradients.These partial volumes thenform the basis for the equa-tions. For complicated geome-tries, such as fans, such cal-culations are therefore onlyfeasible if they are performedby computer. This particularlyapplies to simulation of tran-sient flow conditions, i.e. con-ditions that vary over time, asa number of related timesteps have to be computed inorder to obtain sensible re-sults.

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vantages of CFD vis à visclassical test bench methodsis the ability to examine in de-tail any desired area withinthe simulated boundaries andthus to clearly identify andsubsequently optimize the zones of rough flow. The fieldof application of the softwarethus stretches from the devel-opment of new productsthrough the optimization ofexisting machine series andup to the elimination of fluid-mechanic problems in in-stalled systems. The CFDuser gains an understandingof flow phenomena that hecan hardly hope to achievethrough practical trials, or thatcan only be obtained at thehighest academic level wherethe limiting factors are thenthe cost and time involved aswell as know-how transfer.

At Venti Oelde the method isnot only used for fan con-struction, but also for systemengineering, for instance inthe optimization of duct routes, stacks, filter plants,

(nodes). In this way, the con-tinuous distribution of Φ is re-presented by the Φ values atdiscrete points. This createsan algebraic equation (finitedifference equation) for everycalculation point representinga single computation volume(cell). This algebraic equationis then used for determiningthe flow values. The differen-tial equations from all the cal-culation points form a systemof linked algebraic equations.This system of equations hasto be solved with a numeric al-gorithm. A number of direct oriterative calculation methodsare available for the solutionof algebraic equation systems.

As the processing power ofavailable computer technologyincreases, the application ofCFD is becoming more andmore interesting for even me-dium-sized and small compa-nies, in spite of the highpurchase and maintenancecosts of the software and thepresent lack of specialistusers. One of the biggest ad-

cyclones or the air handlingsystems of recycling plants. Inaddition to such tasks as aero-dynamic optimization, otherfields of application for thesoftware used at Venti Oeldeare the simulation of mixingprocesses in stacks and heattransfers during drying pro-cesses and to determine theforces imposed on differentstructural geometries by theflow passing through them.These can then be employedas boundary conditions forstructure-mechanical simula-tions in order to determine themaximum occurring stressesor for modal analysis to deter-mine possible resonance ef-fects caused by transient forces. In contrast to classicaltrials, a simulation entails norestrictions with regard to con-struction size, power con-sumption, possibly harmfulconveying media or high tem-peratures. To summarize, itcan be stated that given thecurrent high technological lev-el of turbomachinery construc-tion, it is practically impossible

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to make further improvementswithout the aid of numeric methods, because advance-ment of machine designs isgenerally accompanied by anexponential rise in costs com-pared to benefit.

any modifications in the ge-ometry can be quickly incor-porated into the model by sim-ply entering the new dimen-sion, whereupon the entiremodel and any adjacent zones adapt themselves tothe new dimension.

This brings a significant timesaving when optimizing themodel compared to the other-wise necessary new creationof a different model. In add-

ition, one or more two-dimen-sional sketches are produced.These can be used for cre-ating three-dimensional mod-els, for example by extrusion,rotation or by means of Bool-ean operations like cutting ormerging.

a) Creation

The first step is to create athree-dimensional model ofthe geometry using CAD soft-ware with 3D capability. Itmust be remembered at thisstage that in the case of flowsimulations the 3D geometryof the flow zone is neededand not that of the structure,as is usual in other types ofanalysis. Increasingly, thecreated 3D models are com-pletely parameterized (Figs. 1and 2) in order to ensure that

3 Performance of a CFD simulation

To allow performance of aCFD simulation a number ofwork steps have to be carriedout in the sequence describedbelow. It is presumed for thispurpose that the geometrydata and the boundary condi-tions are known:

Figure 1 Parameterized layout with modification

Figure 2 Adapted 3D models

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Figure 3 Unstructured computational mesh

Figure 4 Block-structured hexahedral mesh

Figure 5 Flow profile

Figure 6 Boundary layer discretization with vector arrows

by the boundary layer, as theflow in this zone has to over-come not only the pressure in-crease but also the wall shearrate. If there is turbulent flow(and nearly all technical flowsare turbulent) the boundarylayer is only a few tenths of amillimetre thick and has a para-bolic curve with respect to thevelocity distribution profile (Fig.5). The computational meshfor this zone must be givensuch a fine resolution that thisparabolic curve is representedwith sufficient accuracy. In anormal view of the wall, this re-quires between 8 and 14 ele-ments in the boundary layerzone (Fig. 6).

b) Discretization

After the required 3D modelshave been designed, a three-dimensional computationalmesh has to be created. Theelements of this mesh form thebasis for the control volumesthat are needed, as alreadymentioned, for the partial dif-ferential equations, so thatthese can be iteratively solvedin matrix form in accordancewith the number of elements.Depending on the employedsoftware, the user is generallyable to create either a simple-to-generate, unstructured com-putational mesh (Fig. 3) or aconsiderably more difficult-to-generate, block-structuredmesh (Fig. 4). As a rule, ablock-structured mesh with acomparable structure resolu-tion produces better results insignificantly shorter processingtime.

For this reason CFD studies atVenti Oelde are, with very fewexceptions made for time rea-sons, all based on block-struc-tured computational meshes.Care has to be taken that theflow dynamic boundary layer isgiven an adequate resolution,because – for instance – flowdisruptions are always caused

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Laminar boundary layer

Turbulent flow

δ1

w

d

k

W maxr

Figure 8 ANSYS CFX Solver

Figure 9 ANSYS CFX Post

Figure 7 ANSYS CFX-Pre

c) Simulation

After the 3D model and sub-sequently the computationalmesh have been created, thesimulation software (ANSYSCFX) is brought into use.

ANSYS CFX consists ofthree modules: ANSYS CFX-Pre (Fig. 7) for definition ofthe boundary conditions ofthe simulation, ANSYS CFXSolver Manager (Fig. 8) foriteratively solving the partialdifferential equations until apredefined convergence cri-terion is reached, and ANSYSCFX Post (Fig. 9) for visualand numeric evaluation of thesimulation results.

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ent coming from the left wasconsiderably higher than that entering the collectingduct from below. As can beclearly seen from Fig. 11, thiscauses a severe constrictionof the air stream entering thecollecting duct from the bot-tom left. As a consequence,there were pressure fluctua-tions in excess of 250 Pa inthis supply duct (Fig. 12), andthese throttled the fan so se-verely that it reacted with sig-nificantly increased vibrationvelocities.

4 Applications

4.1 Optimization of the in-take flow of a stack

In this case, after modificationof a pressure-side duct lead-ing to a stack and containingseveral branches (Fig. 10), vi-brations occurred on one ofthe fans, causing an automat-ic safety shutdown. These vi-brations had not been experi-enced prior to the modifica-tion. As it could be presumedthat the vibration problemswere a consequence of non-optimum design of the supplyduct, this was analysed in detail by CFD. The stack is around 70 m high while thehorizontal duct is approx. 3 mhigh, 2 m wide and has a totallength of approx. 25 m. Thementioned vibration problemsaffected the fan upstream ofthe marked duct. It was estab-lished that the fan vibrationswere caused by poorly de-signed cross-sections. Themeasured flow velocity in thehorizontal section of the ductwas 22 m/s, while that in thevertical section marked in Fig.10 was only approx. 2-3 m/s.With approximately compar-able duct cross-sections, theflow momentum (mass flow xvelocity) of the flow compon-

Figure 10 Stack geometry with marked problem zone

Figure 11 Velocity distribution in stack

Figure 12 Pressure pattern at measuring points

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Pressure gradient at measuring points

Pre

ssu

re in

Pa

Time in s

Outlet F3B 1Outlet F3B 2Outlet F3B 3

Figure 16 Pressure gradient with baffle plates

As the described unsatisfac-tory conditions had to be re-medied without significantlyinterrupting operation or al-tering the design, it was ne-cessary to eliminate the prob-lem by the simplest meanspossible. Without altering theduct geometry (Fig. 13), thiswas achieved by installingtwo baffle plates (Fig. 14),which accelerated the flowfrom the two lower supplyducts to the extent that thevelocities of all three gasstreams are identical at thepoint where they meet. Thismeasure succeeded in opti-mizing the velocity distribu-tion (Fig. 15) compared to the

Figure 15 Velocity with optimized geometry

Figure 14 Duct with baffle platesFigure 13 Original geometry

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Pressure gradient at measuring points

Pre

ssu

re in

Pa

Time in s

original situation (Fig. 11), sothat the gas streams meetwith identical velocities andno longer negatively affecteach other. Also, the pres-sure fluctuations measured atthe points shown in Figures21 and 25 were reduced from250 Pa to below 10 Pa. Sub-sequent to the conversionwork, the upstream fan couldbe run up to its rated speedwithout any problem. The vi-bration characteristics wereso greatly improved by theconversion work that meas-urements taken at the fan de-tected no further effects onfan operation.

Outlet F3B 1

Outlet F3B 2

Outlet F3B 3

the first branch the pressuredrop is approx. 260 Pa. Fromthe inlet until the rear branchthe drop even reaches 430 Pabecause of the flow disruptionat the rear of the horizontalduct, as shown in Figure 19.

In Figure 20 the points of par-ticularly high pressure dropare highlighted. The blackcircles indicate unfavourablesituations because this designinvolves transitions that aretoo sharp-edged and thusprevent the flow from follow-ing the contours, resulting inturbulence. This causes pres-sure drops and thus leads toadditional expenses for elec-trical energy. By contrast, thewhite circle indicates a ductend producing an inefficientfluid flow. At the marked rearend of the duct the flow formsa vortex (Fig. 19), which hasseveral negative conse-quences. Firstly, the flow issubjected to pressure fluctu-ations because the vortex, asalready described, wandersbetween the left-hand andright-hand duct walls. Sec-ondly, a portion of the vortexis sucked through into the fan(Fig. 21, view from below intothe duct), so that the fan hasto cope with strongly swirlinginflow air and therefore losesefficiency and may suffer frommechanical vibrations (reso-nance).

4.2 Optimization of an induced draught fan

In addition to optimization ofthe fan itself, the upstreamand downstream flow situa-tions are of great importancefor the troublefree operationof the fan and achievementof the maximum possible effi-ciency. If, for instance, the in-coming flow is turbulent orswirling because of poorlydesigned bends or changesin cross-section, this inevit-ably results in a worsening ofthe fan’s operating character-istics. Disturbances in the in-flow and outflow zones haveparticularly serious effects inthe case of fans with espe-cially high efficiency levels > 80 %, as such fans dependon a non-swirling inflow in or-der to achieve their efficiencyfigures. As well as aiming atoptimizing the fan impellerand casing it therefore makesabsolute sense to analysethe inflow and outflow situa-tions. This application is therefore concerned with theoptimization of the induceddraught flow of a double-inletfan. Figure 17 shows the sys-tem arrangement drawing.The 3D model derived fromthis drawing is shown in Fig-ure 18. The very conspicuoussharp-edged transitions are very detrimental to the achievement of smooth flowguidance. This becomesplain in the subsequent evalu-ation of the flow simulation(Fig. 19), which reveals ex-tensive flow disruption in therear area that is also unstable,i.e. it oscillates between theleft-hand and right-hand sidewalls and thereby ob-structs a portion of thebranching-off rear induced draught flow. The pressuredrop of this system is corres-pondingly high, as is illus-trated by the total pressureprofile shown in Figure 20.From the inlet until the end of

Figure 17 System arrangement drawing with induced draught fan

Figure 18 3D model of induced draught

Figure 19 Flow lines

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Pressure loss outlet 1 = 261.365 [Pa] Pressure loss outlet 2 = 431.966 [Pa]

Figure 23 Swirl after optimizing designFigure 22 Total pressure profile after optimizing design

Figure 20 Total pressure profile with original design Figure 21 Strong swirl with original design

Optimization of the geometryinvolved redesigning the criti-cal points highlighted in Fig.20 so that the cross-sectiontransitions were smooth. Thisredesigning succeeded in re-ducing the pressure drop atthe front inflow duct to the fanby a factor of four, from 261 Pato 66 Pa (Fig. 22), while thepressure drop at the rear in-flow duct that had been causedby the poorly-designed hori-zontal duct end (Fig. 20, whitecircle) was even reduced to little more than one sixth theoriginal figure. The severe

swirling of the fan intake air(Fig. 21, red circles) was alsosignificantly reduced (Fig. 23),enabling the fan to achieve itsrated performance figures.

When the flow lines of the original duct design and theoptimized duct design arecompared (Figs. 24 and 25) itis obvious that the flow turbu-lence has been completely eliminated and the flow velocityof max. 70 m/s of the originaldesign has been reduced toapprox. 55 m/s, resulting indecreased swirling and also

in a lower pressure drop. Themean pressure drop saving of275 Pa at a volume flow of500000 m³/h significantly cutsthe power consumption. Theresultant power saving of 49 kW, assuming 24-hour operation at an electricity priceof 10 cent per kWh, leads to anannual electricity cost saving of43000 €.

Related purely to the flowthrough the duct, the pressuredrop coefficient ζ, which is de-cisive for the system pressuredrop, has been halved.

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Pressure loss outlet 1 = 261.365 [Pa] Pressure loss outlet 2 = 431.966 [Pa]

Pressure loss outlet 1 = 65.8793 [Pa] Pressure loss outlet 2 = 76.8992 [Pa]

Pressure loss outlet 1 = 261.365 [Pa] Pressure loss outlet 2 = 431.966 [Pa]

Pressure loss outlet 1 = 65.8793 [Pa] Pressure loss outlet 2 = 76.8992 [Pa]

0 10 20 30 40 50 60 70

Velocity [m s^-1]0 1500 3000 [m]

1750 2250

0 10 20 30 40 50 60 70

Velocity [m s^-1]0 1500 3000 [m]

1750 2250

Figure 25 Flow lines of optimized design

Figure 24 Flow lines of original design

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Pressure loss outlet 1 = 261.365 [Pa] Pressure loss outlet 2 = 431.966 [Pa]

Pressure loss outlet 1 = 65.8793 [ Pa ] Pressure loss outlet 2 = 76.8992 [Pa]

4.3 Optimization of a largedouble-inlet fan DHRV 50

During the introduction phaseof the software, its computa-tion accuracy was tested bycomparing simulation andmeasurement using a test-bench fan of type HRV 63Sand it was found that the simu-lation deviated by less than 1% from the measured values.

The first results of an ongoingoptimisation of a large double-inlet fan type DHRV 50B-2000are presented in the following.This is an ongoing projectwhich has the aim of deter-mining the optimization poten-tial of the DHRV 50, particu-larly at high peripheral speeds> 150 m/s. The background tothis is the fact that for someyears now, clients have beenrequesting higher and higherpressure differences togetherwith a high volume flow. Theserequirements can only be ful-filled if the speed and thus theperipheral speed of the fan im-peller are increased. This means that the speeds andpressures occurring in the fanreach and exceed the designlimits normally defined in fanspecifications of Mach 0.3 or100 m/s at a pressure ratio of

Figure 27 Mesh of DHRV 50B-2000 with detail of leading edges of impeller blades

max. 1.3. If these limits are ex-ceeded, which takes placemore and more frequently, it isno longer possible to achievemaximum efficiency ratings bymeans of the classical empiri-cal design methods for fans(acc. to Prof. L. Bommes orDr.-Ing. Bruno Eck). However,due to the substantial increasein energy costs and also be-cause of the energy efficiencydirectives aiming at a reductionin CO2 emissions, it is impera-tive to achieve the highest pos-sible fan efficiency rates. Toenable this, it is either neces-sary to plan and carry out ex-tremely expensive, time-con-suming and labour-intensivetest series or to employ CFDtools, which are the state ofthe art in turbomachinery con-struction. One great advantagewhen using CFD for designingand optimizing fans is that itprovides a detailed analysis ofevery flow zone. Many of theflow zones that are of interestto the design engineer, for in-stance the leading edges ofimpeller blades, either cannotbe represented in sufficient de-tail by conventional measuringtechniques or would demand adisproportionate amount of

work and expenditure. For ex-ample, measuring devices fit-ted onto the rotating impellerwould influence the flow in away that would falsify the measurement results or are socomplex that they can only beapplied in academic studies(light sectioning principle). However, in order to improvethe efficiency rates beyond thecurrent status of > 80 %, it isessential to obtain a well-founded knowledge of flow dynamic processes in the im-peller and fan casing. Due tothe ability of CFD to numeric-

ally describe the behaviour ofa Newton fluid in a physicallycorrect manner, CFD is cur-rently the most effective toolfor achieving further optimiza-tion of turbomachines. Particu-larly in the field of compressorconstruction, which is closelyrelated to that of fan construc-tion, CFD has become an in-dispensable tool for ana-lysing, for example, impellerswith twisted blades, twistedleading edges and raked trailing edges. The projectdescribed below was the firststep in the optimization of a

Figure 26 Geometry DHRV 50B-2000

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Figure 28 Specification of the boundary conditions

Figure 29 Solution of the differential equations

quency of 893 min-1, a pres-sure difference of 8500 Pascaland a mass flow of 145.13 kgs-1 at a process temperatureof 300 °C, the transient cal-culation of the case can bestarted (Fig. 29).

After the specified conver-gence criterion has been at-tained and the simulation hasbeen concluded, the resultsare evaluated. Figure 30depicts the flow lines in theimpeller and the casing.Every flow line represents thetrack of a single fluid particlethrough the entire calculationzone. The colour of the flowline indicates the local vel-ocity of the fluid particle. Withthe aid of this flow line dia-gram the design engineercan identify points with par-ticularly high flow disturb-ances, excessively high vel-ocities or excessively sharpdeflections. The diagram isalso used for checkingwhether, for example, the airflows to the impeller blades or

Fig. 27), in order to representthe earlier described boundarylayer with adequate precision.In the case of the describedlarge double-flow fan of typeDHRV 50B-2000, which hasan impeller with an outer dia-meter of 3.2 m, the total num-ber of computational meshcells in all zones is approx. 8 million elements. If, as in thedescribed case, there are adja-cent stationary and rotating zones, these have to be sepa-rately represented in the modeland separately discretized.This is because the computa-

tional mesh of the impeller ro-tates during a transient simula-tion through a defined anglefor each of the individual com-puting steps (usually by 1-5°depending on the number ofblades), while the inlet boxesand the volute casing remainstationary. When the boundaryconditions are being entered,these zones then have to beconnected by interface (Tran-sient Rotor Stator Interface).

After the boundary conditionshave been specified (Fig. 28),including a rotational fre-

large double-inlet fan of typeDHRV 50B-2000.

First of all, a 3D model of theinlet box, the impeller and thevolute casing (Fig. 26) has tobe created and then providedwith a computational mesh.During the discretization proc-ess, special attention is paid tothe most uniform possible dis-tribution of the mesh elements,which has to be oriented to theoccurring flow gradients. Forinstance, the computationalmesh has to assure great de-tail on all the walls (detail in

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Figure 33 Pressure distribution in impeller and casing

Figure 32 Vector arrows on casing cut-offFigure 31 Vectors at leading edge of impeller

the casing cut-off (the point atwhich the flow is divided) asplanned.

In addition to the flow lines, adiagram of the flow vectors isa suitable way to visualize thecharacteristics of a technicalflow. The flow vectors indicateboth the magnitude of the velocity and its direction. Thisis of great importance, espe-cially in the case of flow turbu-lences. Figure 31 shows thevectors at the leading edge ofthe impeller. Figure 32 showsthe flow vectors at one of themost critical fluidic zones of

the casing, the casing cut-off,where the flow is divided. It isextremely important that theflow divides at the centre ofthe cut-off, as shown in Fig.32. If this does not happen,the result is loss of fan effi-ciency, vibrations and a sig-nificant increase in soundpressure level.

Figure 33 shows the pressuredistribution in the impeller andcasing. The right-hand imageshows the inlet box. The pres-sure distribution reveals thatapprox. two thirds of the pres-sure increase take place in

Figure 30 Flow line diagram

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the impeller while approx. onethird takes place in the cas-ing, which thus acts as acollecting plenum and diffuser.One of the aims of the on-going optimisation of fan typeDHRV 50B is to achieve com-plete uniformity of the cur-rently not entirely homoge-neous pressure distribution atthe periphery of the casing.

The optimization potential stillexisting in the casing can alsobe seen from the flow linesshown in Figure 34. To enableeasier interpretation of the flowlines than is possible on the

basis of Figure 30, thesewere projected as 2D flow lines onto a plane inserted inthe centre of the casing. Thisclearly shows that the vel-ocities in the area of the cut-off (red circle in Fig. 34) areslightly too high and that thefluid does not flow away infully logarithmic manner(black circle). In order to de-termine the optimization po-tential at these points, differ-ent designs are currentlybeing simulated and compa-red. For instance, the casingwidth is being varied in anumber of increments in order

Figure 36 Raw meal caking

Figure 35 Total pressure volumes

Figure 34 Flow lines in 2D

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to improve the flow and to achieve a saving in materialand thus in machine weight.After this test series has beenconcluded, the logarithmicfunction used for designingthe casing periphery will besubjected to a series ofchecks in order to achieve improved flow guidance. Inaddition to the casing, whichoffers the greatest optimiza-tion potential, the impeller isalso being analysed. Onepossibility that has only re-cently been added to the soft-ware is to represent variablesas volumes in specified ranges.Using this software capability,Figure 35 was generated. Thisdepicts the total pressure in the impeller within a range of -4000 to -6000 Pascal. Theshape assumed by the indi-vidual volumes correspondsvery closely to photos of ma-terial incrustations on impel-lers conveying dust-laden me-dia (Fig. 36). If these areas

are selectively optimized inorder to achieve a more uni-form total pressure distribu-tion, the amount of incrusta-tion will certainly be reduced.

important role in system en-gineering and constructionbecause, besides enablingpurely aerodynamic assess-ment of a flow, it also allowscalculation of mixing proced-ures or heat transfers anddistributions.

In the field of turbomachineryconstruction, CFD has, withinthe space of just a few years,become an indispensabletool for the optimization andnew design of turbomachines,as well as for the eliminationof fluid-mechanic problems ininstalled systems.

Sub

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Exhaust air treatment plants

Dust collection and process air cleaning plants

Ventilating, heating and air conditioning plants

Industrial fans

Surface technology

Recycling and waste processing plants

Ventilatorenfabrik Oelde GmbHP.O. Box 37 09D-59286 OeldePhone: +49 25 22 75 - 0Fax: +49 25 22 75 - 2 50info@venti-oelde.dewww.venti-oelde.de

5 Conclusions and prospects

Within a period of one yearafter the test phase, the CFDmethod had completely estab-lished itself at Venti Oelde. Inaddition to its primary functionas a tool for optimizing andadvancing the design of fansand their inflow and outflowzones, the software plays an