This paper deals with flow investigations using endoscopes in a single-injector reduced scale Pelton turbine performed with a CMOS high-speed camera. Both onboard and external visualizations techniques of the flow in a bucket are presented. The flow observations evidence the unsteadiness of the successive steps of jet/bucket interaction, free surface flow development and evolution throughout the bucket duty cycle

Avellan, F.

Farhat, M.

Perrig, A.

This paper deals with flow investigations using endoscopes in a single-injector reduced scale Pelton turbine performed with a CMOS high-speed camera. Both onboard and external visualizations techniques of the flow in a bucket are presented. The flow observations evidence the unsteadiness of the successive steps of jet/bucket interaction, free surface flow development and evolution throughout the bucket duty cycle.LM

Infoscience - École polytechnique fédérale de Lausanne

Proceedings of 2007 Joint ASME/JSME5th Joint ASME/JSME Fluids Engineering ConferenceJuly 30-August 2 , 2007, San Diego, California, USAFEDSM2007-37628HIGH SPEED FLOW VISUALISATION OF AN IMPINGING JET ON A PELTONTURBINE BUCKETAlexandre Perrig, Mohamed Farhat, Franc¸ois Avellan∗Laboratory for Hydraulic MachinesEcole Polytechnique Fe´de´rale de LausanneAv. de Cour 33bis1007 Lausanne, SwitzerlandEmail: francois.avellan@epfl.chABSTRACTThis paper deals with flow investigations using endoscopesin a single-injector reduced scale Pelton turbine performed witha CMOS high-speed camera. Both onboard and external visual-izations techniques of the flow in a bucket are presented. The flowobservations evidence the unsteadiness of the successive steps ofjet/bucket interaction, free surface flow development and evolu-tion throughout the bucket duty cycle.NOMENCLATUREB2 Bucket width [m]D1 Jet pitch circle diameter [m]Db Runner tip diameter [m]H Test head [m]Q Discharge [m3s−1]zo Number of injectors [−]zb Number of buckets [−]INTRODUCTIONThe flow in Pelton turbines has not been analyzed so far withsuch details as the flow in the reaction turbines because the flowpatterns and the hydraulic losses are very difficult to observe andquantify: the flow processes include jets, spray formation, free∗Address all correspondence to this author.surfaces and film flows, secondary flows, and complex interac-tion between the components. A large population of droplets ofvarious sizes stemming from the water sheets break-up, the jetdispersion, and the water sheets collision with the casing wallsprevail around the runner, making the visibility poor. Moreover,features of interest are hidden between the buckets.As a consequence few experimental visualizations of the flow inPelton turbines were performed so far, and most of them in sim-plified configurations. Guilbaud et al. [1] investigated the flowin a simplified 2D fixed bucket while Lowy [2] investigated par-ticularly the interaction process of the jet by the bucket cutoutin a fixed bucket. Bachman et al. [3] performed flow visual-izations around a rotating Pelton runner with conventional lensesprotected from the splashes by tubes and lighted by stroboscopes.To enable a close observation of the unsteadiness of the bucketflow, a innovative visualization technique using medical endo-scopes bound to a high-speed CMOS camera has been devel-oped. The results obtained from 2 approaches are presented: (i)visualizations of the relative flow in a bucket with an onboardendoscope and (ii) external visualizations of the absolute flow.The water film thickness in the bucket is determined by an imageprocessing technique based on the refractive properties of air andwater.VISUALIZATION TECHNIQUESThe experimental investigations are performed for a single-injector reduced scale Pelton turbine model. The specific en-1 Copyright c© 2007 by ASMEFigure 1. FLOW AROUND A PELTON RUNNERFigure 2. ENDOSCOPE ASSEMBLYergy is limited to 200 JKg−1 (20 m head) to reduce the rotationalspeed and thus the centrifugal load on the distal lens of the on-board endoscope.ONBOARD SETUPA miniature endoscope, see Fig.2, is located at the root ofthe bucket to be instrumented, with the cone of vision cover-ing half of the bucket. No water is to be expected there duringnormal operation, as Pelton’s modification to Knight’s originalimpulse turbine was precisely to avoid water flowing towards theflange [4]. Moreover, the mounting is non-intrusive and the fieldof observation is not obstructed by the splitter, while the radialacceleration prevents water droplets to stick on the distal lens ofthe endoscope. The CMOS camera head is kept outside the rotat-ing parts of the test rig. The observation frames are transmittedvia a 90◦ optical deflector aligned with the runner rotation axis,Table 1. EXPERIMENTAL CONDITIONSshutter frame rate flash resolutionµs f ps ms pixels× pixels6 6000 6 512×512and protruding outside the test rig window. Therefore no datamust be transferred through slip rings and no electronic compo-nent undergoes rotational forces. The flashlights are installed inwaterproof housings that are mounted as close as achievable tothe runner. An optical encoder sends a TTL signal via an ad-justable delay line unit set to trig the camera and the flash unitssynchronously at the desired runner angular position. Even if thelight source and camera capabilities allow measuring a completebucket duty cycle, i.e. more than 90◦, the non-moving assemblyof the lighting system claims for a division of the duty cycle inoverlapping subcycles to maintain adequate lighting conditions.Table 1 summarizes the experimental conditions.IMAGE PROCESSINGThe images captured by the onboard system have the draw-back that the field of view rotates as the endoscope and the op-tical deflector rotate with the turbine rotor. A semi-automatedprocess is set up to transform the raw images output by the cam-era into a sequence of images with a steady point of view.EXTERNAL SETUPThe onboard observations are completed by a series of vi-sualizations carried on in the absolute frame of reference usinga large endoscope. The endoscope ocular is bound to the sameCMOS camera mounted on a light tripod. The distal lens of theendoscope is fitted with a pneumatic wiping system.RESULTSThe flow sequence of a bucket duty cycle is divided in 6steps [3], [5]. The description of the flow sequences is performedaccording to Fig.4 and Fig.5.(1) Approach of the tip to the jet. Beside appearing rippled,the jet surface exhibits perturbations and a series of convectedimpact craters. The convected craters are caused by the impactof large drops (' 3− 5mm) stemming from condensation onthe test rig walls or from the bounce of splashing water on theinjector body.(2) Initial feeding process. Once the tip and the cutout lipsbackside are in contact with the jet upper surface, the jet startsto separate in 2 branches, i.e. the upper one, that flows in thebucket inner surface, and the lower one that continues to feed2 Copyright c© 2007 by ASME6Duty CycleBAJet AxisIIIIIIIV123 45DbD1Figure 3. DUTY CYCLE DEFINITIONI IIIVIIIFigure 4. BUCKET ONBOARD FILM SEQUENCEthe next bucket. The image sequence show an outburst of thejet, and the appearance of a splay crown of water threads onthe upper side. The jet front and the splay crown fly over thebucket and impinges on it later in the cycle. In fact, the cutoutlips appears to act like a spillway [6]. Immediately after theimpact, the onset of foam is clearly visible. At this point thejet is already partially cut by the next bucket. Thereafter, theflow evolves into the expected sheet flow moving first slightlytowards the root and then laterally towards the outer edge.(3) Entire separation of the jet. At this point the impingingmain flow feeds the bucket mainly normally to the splitter.The separated portion of the jet remains attached to the bucketbacksides far in the duty cycle, as predicted by Lowy [2] andobserved by Bachmann [3]. In addition, it seems to be deflectedagainst the bucket backside [5]. The deformations it displaysmatch well the predictions made by Lowy [2]: the cross-sectionof the jet is crescent-shaped. The cross-section consists of2 different flows: the core practically filled with liquid andthe spray, which consists of numerous single separated waterfilaments.(4) Last stage of inflow. The last threads and droplets ofthe distorted and disintegrated end of the jet enter the bucketpartially on the bottom surface and partially on the lateral edgesnext to the cutout. Some of the water escapes the bucket directlythrough the cutout in a radial direction.(5) Last stage of outflow. The water sheet presents a corrugatedsurface at the end as it not fed anymore. The outflow sheetremains fully developed late in the duty cycle, and becomesthinner and thinner until it breaks up, at first near the root andthen towards the front.(6) Series of droplets. The outflow never comes to an end. Aline of droplets is visible along the entire runner periphery. Thedroplets are released from the outer edge tips, on the externalside of the cutout, and from the splitter tip.CONCLUSIONThe flow observation techniques presented in this paper pro-vide a good insight of the successive steps of the bucket dutycycle of a single-injector Pelton runner from the relative frameVIVIVIIIIIIFigure 5. JET SURFACE PERTURBATIONS3 Copyright c© 2007 by ASMEof reference. The unsteadiness of the free surface flow in thebucket is put into evidence. More details can be found in [7].ACKNOWLEDGMENTThe authors would like to thank the staff members of EPFL-Laboratory for Hydraulic Machines in Lausanne and of VATECHHydro SA in Vevey for their dedication and enthusiastic support.This work is financially supported by the swiss promotion agencyfor innovation, KTI-CTI, Grant No. 6139.2, and VATECHHydroSA.REFERENCES[1] Guibauld, M., Houdeline, J. B., and Philibert, R., 1992.“Study of the flow in the various sections of a pelton tur-bine”. Proceedings of the 16th IAHR Symposium, 14-18September, pp. 819–831.[2] Lowy, R., 1944. “Efficieny analysis of pelton wheels”.Transactions of the ASME, 66, pp. 527–538.[3] Bachman, P., Schrer, C., Staubli, T., and Vullioud, G., 1990.“Experimental flow studies on a 1-jet model pelton turbine”.Proceedings of the 15th IAHR Symposium, 11-14 September,pp. 1–13.[4] Doble, W. A., 1899. “The tangential water-wheel”. Trans-actions of the American Institute of Mining Engineers, Cali-fornia Meetting, 29, pp. 852–894.[5] Perrig, A., Avellan, F., Kueny, J. L., Farhat, M., andE.Parkinson, 2006. “Flow in a pelton turbine bucket: Nu-merical and experimental investigations”. Transactions ofthe ASME: Journal of Fluids Engineering, 128(4), pp. 350–358.[6] Chanson, H., 1996. “Air bubble entrainment in turbulent wa-ter jets discharging into the atmosphere”. Transactions of theAustralian Civil/Structural Engineering, CE39(1), pp. 39–48.[7] Perrig, A., 2007. “Hydrodynamics of the free surface flowin pelton turbine buckets”. PhD thesis, E´cole PolytechniqueFe´de´rale de Lausanne.4 Copyright c© 2007 by ASME

High Speed Flow Visualisation of an Impinging Jet on a Pelton Turbine Bucket

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High Speed Flow Visualisation of an Impinging Jet on a Pelton Turbine Bucket

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