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No 23 Summer 2003 No 23 Summer 2003

by Etienne Parkinson,VA TECH HYDRO Ltd., Switzerland

Thanks to developments in simulationmethods over the last twenty years, theoptimisation of the hydraulic design ofreaction-type water turbines (Francis andKaplan turbines) is now primarily carriedout by means of numerical flowsimulation. In the design process,numerical flow simulation provides theengineer with a rapid and reliable tool toobtain a better understanding of the flowand to quickly examine various designoptions in order to select the optimumconfiguration.

A similar move towards the use ofsimulation methods is now taking place inthe field of Pelton turbines. Until now, theflow in Pelton turbines has not beenanalysed in such detail as the flow inFrancis and Kaplan turbines. One of thereasons for this is that the flow patternsand the hydraulic losses are very difficultto observe and quantify by experiments.This is due to the very complex flowprocesses that occur in Pelton turbines,which include pressure losses, secondaryflows, jets, film flow, free surfaces, sprayformation, ventilation losses, unsteadiness,and complex interaction between thecomponents.

Following a systematic step-by-stepprocess with an optimum use of numericaland experimental developments, using

CFX SHORTENS FAN DESIGN TIMEFROM THREE MONTHS TO ONEby Chris Robinson and Peter Came, PCA Engineers, UK

The Flakt Woods Group, a well-known manufacturerof large industrial fans, had to deliver to a majordefense organization fans to be used in manydifferent applications with very stringent and widelyvarying technical specifications and requirements. Inorder to meet the delivery schedule, they outsourcedthe design of one of the more challenging fans toPCA Engineers, engineering consultants specializingin turbomachinery design.

The critical requirement was that the fanshould have nearly flat operating characteristics, sothat delivery pressure does not rise significantly asthe volume flow falls. Normally, we would havebegun by looking for a similar design that we haddone in the past, but we had never producedanything similar to this one. So we selected C4airfoils as a starting point based on our client’spreference and our previous experience. We usedour in-house program, Vista-AC Blade, to generatethe initial profile curves, which we imported intoCFX-BladeGen to refine the edges of the rotor andstator, set up clearances and evaluate theinteraction with the annulus. Once we hadcompleted an initial design that looked good, weused CFX-TurboGrid to generate a mesh of theblade passages and CFX-TASCflow to analyze theperformance of the initial design. We used the buildcase option in CFX-TASCflow to automaticallygenerate a two-stage geometry and organize themeshes for each stage.

We ran the analysis at five different flow ratesto construct a pressure-volume curve. The resultswere what would be expected from a typical fandesign: as the volume flow fell, the delivery pressurerose substantially. The results didn’t just tell us thatit didn’t meet the requirements; they also provideddetailed information that helped us to understandwhy the initial design didn’t work.

Looking at the flow velocity plots providedinsight into the physics behind the shape of thecharacteristic curve. Normally, in a standard fan therotor and stator are configured so that theirminimum loss points coincide with the normaloperating point of the fan. But this approachinherently creates a pressure-volume curve with arelatively steep slope. We addressed this issue bychanging the design to deliberately stall the statorover the entire flow range of interest, in order tomitigate the rotor’s tendency to deliver increasing

pressure as the flow is reduced. While thistechnique does reduce the efficiency of the fan, thelosses were still completely acceptable to the client.After creating three more iterations to fine tune theperformance of the design, we had achieved thegoal of a nearly flat pressure-volume curve. Thefinal design had 12 blades in the rotor and 17 inthe stator.

When Flakt Woods built and tested the fan,performance was nearly exactly as predicted, andthey were very pleased both with the performanceof the final design and with the speed with whichwe delivered it. The key to achieving these resultswas the use of CFX for turbomachinery design andanalysis. It provided insight into the performance of

our early design, allowing us toquickly proceed to one that met

the client’s specifications.

CFX-5,VA TECH HYDRO(www.vatech-hydro.com) has built upwide ranging expertise in the simulationof unsteady free surfaces as observed inPelton turbines.

With the aid of many specificexperimental developments designed forvalidation of free surface flows conductedwith our research partners and the stronginteractive collaboration with the CFXgroup,VA TECH HYDRO has nowachieved a major breakthrough in the

simulation of unsteady free surfaces.These simulations include, among manyother applications, the validated numericalflow simulation in a rotating bucket,and the correct prediction of theshape of a water jet following abend pipe.

There are still numerousCFD challenges in thesimulation of unsteady Peltonturbine flows. Every step forwardbrings a new insight into the flowpatterns, which is then translated intodesign and analysis to improve the turbineperformance.This is true for new designs

Developments in numericalflow simulation applied toPelton turbines

but also for refurbishment projects whereonly some of the turbine components arereplaced.A valid analysis, both qualitativeand quantitative, of the existingcomponents and their interactions is a keyfor the success of such projects.

CFX-5 Computational FluidDynamics software is helping to makepossible the analysis of Pelton turbines inorder to improve performance.

‘CFX-5 analysis ofPelton turbines ishelping to improvetheir performance.’

Flow patterns ina Pelton runner.

Jet visualisation of a Pelton turbine in hydraulic laboratory.

Vertical 6 jetsPelton turbine.

CFX simulation of flow field in a bifurcation.

Flow simulation ina Pelton runner.

We were able to design the fan in only one month,where it would have taken at least three monthsusing the traditional build and test approach.

‘The key to achieving

these results was

the use of CFX for

turbomachinery

design and analysis.’

Fan characteristic showing rotor-stator mismatching – The rotor isjust below its peak efficiency flow and is beginning to show signs ofincreased leading edge loading. The stator, on the other hand, isshowing fully separated flow along its entire suction edge.

Top to bottom: CFX prediction of fan flow at high,medium and low volume flow.