USAGE OF PIV IN RADIAL PUMP MODELSVOČ – FST 2011

Adam Scheinherr,Západočeská univerzita v Plzni,

Univerzitní 8, 306 14 Plzeň,Czech Republic

ABSTRACTThis article is focused on PIV (Particle Image Velocimetry) measurement technique that have been used duringexperimental investigation of flow fields in radial pump model. The experimental set-up of the experiment was built inthe school laboratory. The article provides closer view on the experimental model and PIV measuring method used toinvestigate flow phenomena. The results show that PIV method performs well as a measuring method inturbomachinery and so the model of radial pump was successfully designed to challenge with primary tasks in PIVmeasurements like full optical access and minimum reflections from boundaries.

1. INTRODUCTIONRadial machines constitute a large class of devices, which are used to transfer energy either to or from the flowingstream by the dynamic action of one or more moving blade rows. They have found a wide range of applications mainlyas pressure-increasing turbomachines and vary from fans that produce pressure rises equivalent to a few millimeters ofwater to pumps producing heads of many hundreds of meters of water. The term pump is used when referring tomachines that increase the pressure of a flowing liquid. The term fan is used for machines imparting only a smallincrease in pressure to a flowing gas. In this case the pressure rise is usually so small that the gas can be considered asbeing incompressible. In contrary a compressor gives a substantial rise in pressure to a flowing gas.This work is concerned with elementary flow analysis and design of radial (=centrifugal) pump. The essential of thosemachines is employing centrifugal effects for increasing fluid pressure. The complex flow fields within them are three-dimensional, turbulent and inherently unsteady. These complex flow phenomena effect the performance, efficiency andrange of operating conditions of these machines, as well cause an undesired phenomena, such as noise, vibrations, stalland sometimes failure. Understanding of the flow phenomena and its effect on performance is essential for development

Figure 1 : Essential elements of radial pumps.

of more-efficient, quilter and reliable machines. Such insight requires detailed experimental data on the mean andfluctuating components of the flow, pressure and temperature as well as on power forces, torques and vibrations. In thiswork I confine on the velocity-flow measurements.

¨2. RADIAL PUMPSA centrifugal pump consists essentially of a rotating impeller followed by a diffuser. Figure 1 shows the elements of aradial pump. Fluid is drawn through the inlet casing into the eye of the impeller. The function of the impeller is toincrease the energy level of the fluid by whirling it outwards to the diffuser, thereby increasing the angular momentumof the fluid. Both the static pressure and the velocity are increased within the impeller. The purpose of diffuser is toconvert the kinetic energy of the fluid leaving the impeller into pressure energy. Outside the diffuser is volute whosefunction is to collect the flow from diffuser and deliver it to the outlet pipe.For my experiment was the model of radial pump made mainly from transparent plexiglass as can be seen in sketch inFigure 1 to ensure good optical access conditions for PIV measurements. Details of the construction will be discussedlater in this work.

3. MEASURING METHODS IN TURBOMACHINERYFor the flow measurement in turbomachinery is used a wide range of various non-optical and optical measurementtechniques. For measuring the velocity and turbulence there are used for example non-optical measurement methods ashot-wire/film anemometry or optical measurement methods as for example laser Doppler velocimetry (LDV), Dopplerglobal velocimetry (DGV), Laser two focus velocimetry (L2F), and Particle image velocimetry (PIV). Closer view onthe various techniques of measuring methods in turbomachinery, their specifications and applications can be viewed onpages 919 – 957 in [1], in this work I will be confined on usage of PIV measuring technique in radial pump model,which advantage consists mainly in non-intrusive measuring with the chance of phase synchronization with the impellerof the pump.

Figure 2: Experimental arrangement for PIV.

4. PARTICAL IMAGE VELOCIMETRYPIV is optical measurement technique that are typically non-intrusive and can be implemented within the rotatingblades. The primary challenge is to provide appropriate optical access to the flow field within the machine. PIV becamein last years one of the most commonly used technique for measuring velocity and turbulence within turbomachines.This technique rely on measurement of the displacement of the seed particles. The size of the particles has to considerthe parameters of the observed flow and fluid. In applications involving air-flow, where the particles can't be buoyant,they must be kept enough to follow the fluid motion. Consequently, the typical particle size should be around 0.5µm toobtain a tolerable velocity lag. In applications involving liquid, the specific gravity of the particle can be better matchedwith that of the fluid, and significantly larger particles, in order of 5-20µmcan be tolerated, depending on thecharacteristic flow scales.PIV consists of illuminating the flow field seeded with microscopic particles with a thin light sheet, and recording a pairof images, separated by a short time interval. The principle can be seen on Figure 2 taken from Rafael [2]. The velocityis determined by dividing the images into small interrogation areas, and using cross-correlation analysis to measure thedisplacement of particles within each area. This process provides the instantaneous distribution of two in-planecomponents of the velocity.

5. EXPERIMENTAL SET-UP AND PIV MEASUREMENTS a) Radial pump set-up

The radial pump test facility (Figure 3) was designed to enable detailed and complete velocity measurements within theentire diffuser, rotor and volute. This was achieved by plexiglass construction of all stationary walls of the pump andthe plexiglass shroud (=plane attached to the vane ends) of the impeller. In spite of that manufacturing of plexiglassmaterial is much more difficult than iron materials, all of the details of the construction were kept as originallydesigned. The other task was machining and/or heat-bending of all these parts with transparent quality. The keychallenge in application of PIV good optical was achieved, and the another challenge to minimize the reflections fromblade surfaces and impeller wall was managed by black matt finish on those parts. The impeller is driven by a airplane

Figure 3: Radial pump experimental set-up.

-model engine with long shaft that was used not to hide any parts of the measuring flow field. Because of bad reflectionfactor of curved plexiglass walls of the pump and air, the whole pump is positioned in a perpendicular aquarium filledwith water.

b) PIV set-upThis work is confined on the flow-field measurement in the outlet from the impeller and then following flow in thediffuser. The basic set-up for this measurement is presented in Figure 4. The measuring plane (=laser light sheet) ishorizontal and perpendicular with the pump axis. Than the basic condition for the PIV measurement is that laser lightsheet is perpendicular to the high resolution lens axis, respectively the camera optics axis is perpendicular to laser opticsaxis. This could be already observed in Figure 2. The last facility is synchronization to generate the images in the sameblade passages. To ensure this synchronization is on the impeller shroud placed small piece of silver tape which reflectseach round the signal from LED diode to photo-transistor.

Figure 4 : Sketch of the PIV measurement set-up.

The entire experiment set-up is illustrated in Figure 5. Above the radial-pump model we observe the CCD camera andLED diode with photo-transistor. In the plane of the radial pump is placed frequency-doubled Nd:YAG laser which istoday the most commonly used laser in PIV. LED diode and photo-transistor are operated from pulse forming device,that is passing on the pulses to synchronizer in order to synchronize the pulse of the laser and the shutter of the camera.The acquired data (images) are then transferred from digital camera to the computer. These images are after elaboratedin Dantec Dynamic studio software.

Devices used for the experiment:

• Laser: Nd:YAG,New Wave Research, Solo 200 XT PIV

• Camera: Flow Sense, 2048 x 2048 pix

• Oscilloscope: Tektronix TDS 2014 B, 100MHz,1 GS/s

• Synchronization box: Dantec Dynamics, DCPIV, F/W: 1.01

• Blade position sensor: equipment of the school laboratory

• Software: Dynamic Studio 3.14, Dantec Dynamics

Figure 5: Experiment set-up. c) PIV Method

The PIV set-up and technique was described in previous chapters. We already know that to get PIV images we need toilluminate the flow field seeded with particles. The water circuit in this experiment is closed and so there is no necessityfor special seeding device (seeder). Poly-amid 20µm particles are mixed with detergent and the mixture is put into thewater in the pump circuit. Power supply of the compressor is set to constant and through the pulse forming device issend the signal to oscilloscope so we can control the speed of the rotating impeller and so the speed of the flowing fluidalready seeded by the particles. The camera is fixed on the 3Dimensional support so it can be adjusted to get the size ofthe particles in acquired images about 2 to 3 pixels. The volume of the particles seeded to the flow is then controlled tohave about 30 to 50 particles in 32x32 pixels interrogation area. Example of taken image is shown in Figure 6.

Figure 6: Original PIV Image.

Figure 7: Detail on the image pair.Two images are recorded separated by 100µs time interval in this case. This very short time interval is achieved due todouble-frequence laser apparatus. To imagine the speed of the flow we can observe Figure 7. There are shown details ofthe image pair with the seeded flow and the end part of the blade. The thickness of the blade is just 1mm so we canimagine that the flow motion is very short, but it is satisfactory to analyze the pair and measure the displacement of theparticles within each area. In order to obtain more accurate results the hardware maximum of 63 image pairs was madein the same vane passages. The packet of images was then statistically examined to get resulting velocity-vectors. Thedetail part of the resulting velocity-vector field is presented in Figure 8. We cannot see any results in the parts of thepicture where the particles are not illuminated by laser (hidden behind the blades), but the other results seems to be verygood and we can clearly examine the flow sense.

Figure 8: Sample PIV data obtained.

6. CONCLUSION AND RECOMMENDATIONSFirst of all was important to design and construct the model of the radial pump. There were made two prototypes ofthem and the second one was already very successful. With the transparent pump-model was obtained full opticalaccess to the inside of the pump and with the black matt finish of the blades was reached minimum of reflections fromtheir walls that don't affect the final results.

PIV measurements .are performed well. Feasibility study has proved that using the PIV measuring method, can besuccessfully applied in a radial pump and satisfactory results can be achieved. As the matter of fact the measurementtechnique for turbomachinery was successfully managed and can serve as sample for future measurements in this fieldof study.

Next step in this work could be interesting results, to get phase-resolved PIV measurements using for example 4 timesteps per impeller blade passage.

Another task for next experiments is to measure the flow-rate in the pump and inlet and outlet pressure of the pump.And from these data could generate the pump characteristic curve.

Also interesting goal could be comparison with CFD data, for example simulated in Fluent software. There could worksmall cooperation and for example new shape of vanes suggested from numerical simulations could be after testedexperimentally by PIV.

7. ACKNOWLEDGMENTSI would like to thank Dr. Yannick Knapp, who was the supervisor of this work.

8. REFERENCES

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[2] RAFAEL, M., WILLERT, C. E., WERELEY, S. T., KOPENHANS, J.: Particle Image Velocimetry – A practicalguide, Second edition. Springer - Verlag Berlin Heidelberg New York, 2007.

[3] UZOL, O., CHOW, Y. C., KATZ, J., MENEVEAU, C.: Unobstructed particle image velocimetry measurementswithin an axial turbo-pump using liquid and blades with matched refractive indices. Work presented at PIV'01 aspaper 1085, 2002.

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