IIncreasing the operational performance of gas turbinesis driving engineers to look beyond conventionalcomputational fluid dynamics (CFD) steady flow

analysis, and tackle the inherent unsteady flowsencountered around turbine blades. Hot spots on theseturbine blades create fatigue weaknesses in the blade,potentially causing blade failure. Until now manufacturershave taken the precautionary route of checking andreplacing blades more frequently than may be reallynecessary. However this means taking the turbine out ofservice and is costly in terms of the reduced blade life,down time and replacement parts, and is a timeconsuming process.

Using FIELDVIEW visualisation software, engineers atQinetiQ, one of Europes leading science and technologyresearch organisations, are, for the first time, able tovisualise the development of hot spots and in so doinggain a better understanding of why and where they occurand how to reduce their occurrence. Understanding andpreventing hot spots will ultimately allow manufacturers tomore accurately predict the life of a blade and thus avoidunnecessarily replacing blades.

In modern gas turbine engines the combustor exit flow hasa non-uniform temperature profile because of the discretenature of the injection of fuel and dilution air, and the wallcooling flows. The affect of this non-uniform temperatureprofile on the aerodynamics and heat transfer rate ofnozzle guide vanes (NGV) and turbine blades is difficult topredict.

As the general trend of advancing turbine performancecontinues, the drive to raise combustor temperaturesremains, so that power to weight ratio and cycle efficiencymay be increased. The maximum sustainable componentmetal temperature during operation of the engine limitscycle temperatures. Rotating components are furthercomplicated by typically experiencing up to 16,000revolutions per minute, and centrifugal blade loadingsequivalent to 50,000 times gravitational acceleration are

now common. In such high stress environments creep mayoccur well below the melting point of the metal, and forengines in which extended service is an important issue,metal temperatures must be minimised. Typical turbineblade metal temperatures are around 1100 K, while thecombustor exit temperature may be as high as 2100 K.

One of the most difficult problems facing the turbinedesigner is that of predicting the heat transfer rates andhence metal temperatures of the components. As themetal temperature affects the life of a component, andthe overall thermodynamic cycle efficiency of the engine,any improvement in cycle efficiency comes not just fromallowing an increase in the turbine entry temperature, butfrom a reduction in [ expensive] cooling flows madepossible by more accurate temperature predictions.

Hot SpotsHot SpotsHot spots are created within the gas due to the discretepositioning of the burners within the combustion chambercombined with the influence of the combustor liningcooling flow. Hot gas hits the blades at a different locationevery time creating an unsteady flow environment. Thesehot spots in the flow pass through the turbine and canhave a large impact on the efficiency of the turbine as wellas component life and temperature distribution.

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Unsteady Flow Unsteady Flow Visualisation TVisualisation Takes the akes the Heat out of Hot SpotsHeat out of Hot SpotsImproved visualisation of complex unsteady flow results is helping engineers at QinetiQ to use CFD to gain a better understanding of important flow features.

Figure 1

The temperature profile of the hotspot can be characterisedby an Overall Temperature Distortion Factor (OTDF) whichindicates the degree of temperature distortion.

In detail, the flow around turbine vanes in a gas turbineengine is highly unsteady. This is partly due to the forcedperiodic oscillations in the pressure field as blade rowsmove relative to each other, and thus occurs at the bladepassing frequency.

The boundary layer separation at the trailing edge of anozzle guide vane can cause localised regions of very highunsteadiness, which are generally rather more persistentin nature than the unsteadiness caused by combustormixing or potential interaction of blade rows. These wakesslowly mix-out as they move downstream, and, because ofthe relative motion of succeeding blade rows, are choppedto form pockets of high unsteadiness in the exit flow of thesucceeding blade row.

The unsteadiness associated with secondary flow vorticesmay be significant for low aspect ratio blade rows. In thetransonic turbine, trailing edge shock formation maycause further unsteadiness, as shocks impinge on thedownstream blade row, although this may be a relativelysmall affect for modest exit Mach numbers. These affectscause unsteadiness in the boundary layer, which cancause quite marked changes in the aerodynamics andheat transfer to a vane surface. A good model is thereforeessential for satisfactory prediction of the aero-thermalbehaviour of a vane.

Flow AnalysisFlow AnalysisConventional steady state CFD cannot accurately modelthe true passage of these flow features through the turbineand an unsteady code is needed. QinetiQ has been usingtheir in-house TRANSCode unsteady CFD code to analysethe flow through a research turbine. To conduct 3Dpredictions of heat transfer at leading edge and regions ofhigh turbulence requires large computational meshes.Using TRANSCode, QinetiQ has achieved goodagreement between measurement and prediction.

To be successful in this analysis researchers must also beable to display and animate the results of the unsteadyanalysis. The complexity of the flowfield requires a highlyvisual, unsteady post-processor to visualise unsteady flowsin 3D analyses to show whats happening to the accuracyQinetiQ required in making meaningful hot spotpredictions.

Success with QinetiQs unsteady analysis has been madepossible by using FIELDVIEW to accurately animate thelarge quantities [several Gigabites of data] resulting fromthe analysis. FIELDVIEW is a powerful CFD post-processorthat enables users to understand results and identifyimportant flow features quickly and easily from bothsteady and unsteady cases.

FIELDVIEWs unsteady visualisation capability alloweddetailed study of the 3D flowfield, enabling changes in thepressure and temperature distributions to be viewedconcurrently. Advanced features such as particle trackingwere invaluable in tracing the path of the fluid fromdifferent inlet regions through the turbine. The unsteadyanalysis coupled with new experimental data has givenQinetiQ a better understanding of the flowfield in theturbine.

QinetiQ also found the post-processor visualisationsoftware useful for producing high quality customer andcolleague presentations. The animation feature allowedthe viewer to follow the flow through the model usingcoloured streamlines or to tour through the modelwatching how the flowfield develops.

From its research so far, QinetiQ is closer to betterpredicting turbine blade life which will ultimately result inthe reduction in unnecessary inspections during runninghours.

In addition, design changes can be recommended toextend blade life by ensuring that the air used for coolingcan be more directed, thus reducing wasted air, which inturn can have a real and positive effect on the efficiencyof the turbine. Another benefit of modelling the unsteadyCFD analysis will be a dramatic reduction inexperimentation required prior to final build - a roughestimate might suggest a fifth of the conventionalexperimentation.

ContactVisit www.acel.co.uk for further information

April 2004

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The Nusselt number gives the ratio of actual heat transferred between

two parallel plates at different temperatures, a moving fluid to the

heat transfer that would occur by conduction.