Conference on Modelling Fluid Flow (CMFF’09) The 14th International Conference on Fluid Flow Technologies Budapest, Hungary, September 9-12, 2009
2D LDV MEASUREMENTS OF SWIRLING FLOW IN A SIMPLIFIED DRAFT TUBE Alin I. BOSIOC1, Constantin TANASA2, Sebastian MUNTEAN3, Romeo F. SUSAN-RESIGA4 1
Corresponding Author , PhD Student, Department of Hydraulic Machinery, “Politehnica” University of Timisoara, Bv. Mihai Viteazu 1, 300222, Timisoara, ROMANIA, Phone/Fax: +40256403692 , E-mail:
[email protected] 2 PhD Student, Department of Hydraulic Machinery, “Politehnica” University of Timisoara, E-mail:
[email protected] 3 PhD, Senior Researcher, Romanian Academy – Timisoara Branch, E-mail:
[email protected] 4 PhD, Professsor, Department of Hydraulic Machinery, Politehnica University of Timisoara, E-mail:
[email protected]
ABSTRACT
ui
The swirling flow emerging from a Francis turbine runner is decelerated into the draft tube cone. When the turbine is operated at partial discharge, the decelerated swirling flow downstream the runner becomes highly unstable, with the development of a spiral vortex breakdown, also known as pressing vortex rope in the engineering literature. The flow unsteadiness results in severe pressure fluctuations that hinder the turbine operation or may cause blade cracks. A novel swirling flow control technique has been developed by Susan-Resiga et al, where a water jet is injected from the runner crown downstream into the draft tube cone. The paper presents experimental investigations of the velocity field in a swirling flow with spiral vortex breakdown. In order to investigate the vortex rope 2D measurements with LDV were made. The measurements were performed for the case with vortex rope and different volume flow rates of the jet control. The main goal of the paper is to investigate experimentally the novel jet control method introduced by Resiga et al. [4] in order to mitigate the vortex rope and eliminate the negative effects of his. Keywords: draft tube, jet control, LDV, vortex rope
NOMENCLATURE Qjet
[m3/s]
Qfunct
[m3/s
Rthroat
[m]
N
[-]
volume flow rate from the auxiliary circuit volume flow rate from the main circuit minimum radius between convergent divergent test section number of velocity samples
[m/s]
measured velocity at a given moment Subscripts and Superscripts LDV Laser Doppler Velocimetry
1. INTRODUCTION When hydraulic turbines operate at partial discharge especially runners with fixed blades, downstream of the runner, in draft tube appears a decelerated swirling flow. From decelerated swirling flow result a vortex with a precession motion also called the vortex rope. The vortex rope produces severe pressure fluctuations or damage of the runner blades. In order to investigate the swirling flow with vortex rope precession two methods are used: numerical simulation and experimental investigations. For experimental investigation are used the measurements for velocity profiles and pressure fluctuations. The velocity profiles allow comparing the results with numerical simulation in order to validate the numerical model with the phenomenon. A first experimental investigation with LDV of a swirling flow with vortex rope [1] leading to conclusion that the vortex rope act like solid objects. The vortex rope is unstable and axisymmetric and produces low frequency oscillations. Ciocan et al. [2] performed an experimental investigation as well as the numerical simulation in the test ring of Francis turbine for partial flow rate. The purpose was to analyse for the first time the dynamics and the Computational Fluid Dynamics (CFD) methodology in association with experimental study of the swirling flow phenomena. The analysis of the mean velocity field showed a good agreement and validates the phenomenology of the vortex rope in numerical simulation.
To reduce the swirling flow and control the pressure fluctuations in draft tubes of Francis runners different methods are used such: stabilizer fins, co-axial cylinders or special aerators [3]. All these methods produce other instabilities in draft tubes cones and reduce the cause but not eliminate the effect and drop the efficiency. A new, simple and robust method proposed by Resiga et al. [4], [5] to mitigate the vortex rope is investigated. This method requires a water jet issued from the crown type of the runner. The main advantages of the method are: mitigate the pressure fluctuation and the vortex rope, adjustable according to the operating point and simple and robust for practical implementation [7].
2. EXPERIMENTAL TEST RIG In order to investigate experimentally the swirling flow in conical diffusers with axial water jet control, a new test rig was designed and built, see Figure 1.
Figure 2. Swirl generator from experimental test rig
Figure 1. Closed loop test rig for experimental investigations of swirling flow The main circuit is use to supply the test section. It has a lower reservoir (4 m3) from stainless steel, a main centrifugal pump with variable speed with 0-35 l/s volume flow rate, and two ball valves downstream and upstream of the pump, an electromagnetic flow meter with 0.2% accuracy. The upper reservoir provides a well controlled uniform flow thought ø150 mm pipe to the test section. The auxiliary circuit supplies the water jet control. The auxiliary pump with variable speed supplies the jet control with variable discharge between 0-4 l/s. The jet discharge Qjet is measured with a turbine flow meter by 0.05% accuracy. Figure 3. Swirl generator with test section from plexiglass.
The main part of the test rig is the swirl generator shown in Figure 2. It has provided a swirling flow in divergent section like swirl from the draft tube of Francis turbine. For this condition we have designed and manufactured a swirl generator with two blades that generate at the divergent section inlet a swirling flow similar with the outlet of the Francis turbine runner [6]. The water from auxiliary circuit is collected by a jet supplying ring, see Figure 3. Further, the water passes through the leaned strouts into a central body with ø50 mm diameter. After that, the water is injected at the inlet of the divergent section with a nozzle with diameter of ø30 mm. The pressure level from the test rig can be adjusted in order to investigate the non-cavitating and cavitating vortex rope. In this paper, the measurement on non-cavitating vortex rope is presented. The convergent-divergent test section is manufactured from plexiglass, in order to allow a better visualisation of the phenomenon, as well as to investigate the velocity field using LDV and is show in Figure 3 [8].
3. LDV SYSTEM The experimental data were obtained with Dantec Dynamics LDV with two-components. This is non contact system, which allow to measure two velocity profiles without affecting the measuring volume. The LDV system consists in an argon-ion source with 300 mW power and optical fiber that guides the beams to the flow. The main characteristics of the optical systems are: focal length of the probe 159.6 mm, beam diameter 2.2 mm and the beam spacing 39.2 mm. Two pairs of beams with wavelength of 488 nm and 514.5 nm are generated, and the probe includes a photomultiplier with incorporated amplified. A 3D traversing system is used for probe positioning within 0.01 mm accuracy on each axis. On each axis measurements were made with a step size of 1 mm. In order to reflect the light which is produced by the laser, in the test rig were introduced aluminium particles with 10 µm mean diameter.
3.1. LDV measurements The velocity measurements were performed on three optical windows of the test section, see Figure 5. First window is located in the convergent part, where the swirl generated by the blades is checked. The next two windows are located immediately after the throat, and further downstream into the cone. The windows located on the cone section are used to investigate the velocity profile for the swirling flow with vortex rope, and with variable control jet discharge. On the survey axis 0 were measured 31 points from window to the nozzle. Survey axis 1 contains 113 measurement points with 8.5° angle slope which is the same angle of the conical diffuser. Last survey axis 2 has 140 measurement points and it has the same angle as survey axis 1.
Figure 4.Visualisation of the vortex rope Figure 4 presents the vortex rope in the test rig section generated by the swirl generator with two blades. It has a helical shape and it is located in all length of the cone. Visualisation was made at 0.03 m3/s flow rate; speed of the free runner was 870 rot/min; with cavitations. Figure 5. Survey axis for LDV measurements
In the experimental investigations Qfunct on the main circuit was of 0, 03 m3/s at a value of 80% from the power of the main pump. The speed of the free runner was of 870 rot/min. In Figure 8 is showed the speed variation of the runner at different Qfunct.
vthroat
Q funct
(2)
2 Rthroat
Figure 8 presents the dimensionless velocity profiles in convergent area, in order to investigate the swirl generated by the blades. In order to validate the velocity profiles in the convergent section (on the survey axis 0) were performed four measurements sets at different discharge values Qfunct, see Figure 8. One can observe that the meridian velocity component has positive values due to the accelerated flow from the convergent part. Meanwhile, the tangential velocity component is negative because the sign is opposite to the conventional rotation sign.
Figure 6. Speed of the free runner at different Qfunc. Measurements with LDV were made in each point with 5000 samples or 25 seconds which was set before. Figure 7 presents an example of velocity histogram for one point of measure. Mean velocity was computed with eq. (1): N 1
u i 0
1 ui N
(1)
Where ui is velocity measured for a single sample.
Figure 8. Comparation of velocity profiles in convergent section All graphics of velocity profiles have the points resulted from experimental investigation, and the variation of Random Mean Square Velocity (uRMS) for each point [9]. URMS was calculated with the following equation: N 1
u RMS
1
N u
i
u
2
(3)
i 0
Figure 7. Example of velocity histogram for one point of measure.
4. ANALYSIS OF VELOCITY PROFILES The processed data will be presented in dimensionless values, using the following reference values: - the radius of the test section throat Rthroat= 0.05 m - the mean velocity from the throat computed according to eq. (2)
Figure 9 presents the velocity profile on the survey axis number 1 (near to throat test section, see Figure 5) and 2 (near to the outlet of the test section, see Figure 5) without jet. On the first survey axis, meridian velocity has a weak in the middle of the section which reveals a stagnation region. The same distribution is observed in circumferential velocity profile where the values are negligible in the middle, Figure 10. On the second survey axis the stagnation region has a large zone which is remarked in the meridian velocity profile. In this region, due to the swirling component, the flow is pushed towards the wall. Consequently, the main stream is near to the cone
wall. Moreover, one can observe the central region has a solid body rotation.
uRMS in the central zone because the jet has only an axial component, and the variation of velocity is smaller.
Figure 9. Meridian (black) an circumferential (red) velocity profiles on the survey axis 1 (top) and 2 (bottom) without jet.
Figure 10. Meridian (black) an circumferential (red) velocity profiles on the survey axis 1 (top) and 2 (botom) with jet control (Qjet/Qfunct= 5%)
When jet discharge Qjet reaches 5% from the operation discharge Qfunct, on the first survey axis the meridian velocity becomes the same value like the main stream, see Figure 10. That means that the jet fills the weak and consequently the stagnation region disappears. On the second window exists a stagnation region which is observed in meridian velocity, but it is reduced compared to the case without jet control. Additionally, the circumferential component presents a narrow flat region in the middle associated with the influence of the axial jet control. When the jet control has a discharge around 10% from the operating discharge Qfunct, the meridian velocity on the first survey axis has an excess of axial velocity in the middle. The meridian velocity distribution is a jet profile. The circumferential velocity has a small variation of the
In the second survey axis, meridian velocity has a flat area in the central zone which means that exists a stagnation region (see Figure 11), but it is much smaller than in the previous two cases. Moreover, the flat region in the middle of the circumferential velocity component extends, that means the influence of axial jet control is more significantly than in previous case Qjet/Qfunct=5%.
5. CONCLUSIONS This paper has presented experimental investigation with LDV in a straight diffuser with swirling flow. There were analysed three velocity profiles on each survey axes without jet control and with jet control. First window was use to check the flow downstream of the free runner and two windows were use to analyse the flow in the conical
diffuser. In the case without jet a stagnation region is observed in the middle of the area for survey axis 1 and 2.
ACKNOWLEDGEMENTS The present work has been supported by the Romanian National Authority for Scientific Research through the CNCSIS PCE 799 project.
REFERENCES [1] Garg, A.K.and Leibovich. S., 1979, “Spectral characteristics of vortex breakdown flowfield”, AIP, Phys. Fluids, Vol. 22, no. 11, pp. 2053-2064. [2] Ciocan, G.D., Iliescu, M.S., Vu, T.C., Nennemann, B., Avellan, F., 2007, “Experimental Study and Numerical Simulation of the FLINDT Draft Tube Rotating Vortex” ASME Journal of Fluids Engineering, Vol. 129, pp. 146-158. [3] Thiche, R. H., 1981, “Practical Solutions for Draft Tube Instability”, Water power and Dam Construction, Vol. 3(2), pp. 31-37. [4] Susan-Resiga, R., Vu, T. C., Muntean, S., Ciocan, G. D., and Nennemann, B., 2006, “Jet Control of the Draft Tube Vortex Rope in Francis Turbines at Partial Discharge”, Proceedings of the 23rd IAHR Symposium on Hydraulic Machinery and Systems, Yokohama, Japan, pp. 192. [5] Susan-Resiga, R., and Muntean. S., 2008, “Decelerated Swirling Flow Control in Discharge Cone of Francis Turbine”, Proceedings of the 4th International Symposium on Fluid Machinery and Fluid Engineering, Beijing, China, pp. 89-96.
Figure 11. Meridian (black) an circumferential (red) velocity profiles on the survey axis 1 (top) and 2 (botom) with jet control (Qjet/Qfunct=10%) When the water is injected, the stagnation region disappears only in the survey axis 1. On the second window (survey axis 2) the stagnation region is smaller than in the case without jet but still exist. From the analysis of uRMS result a large variation in the middle of the cone without jet and a small variation when the jet is injected because stagnation region disappears and is replaced with water supplied by jet control. In addition validation of the experimental data with results from numerical simulation is necessary. The results will be needed in order to analyse the phenomenon of vortex rope control with a correct water injection.
[6] Susan-Resign, R., Muntean, S., Tanasa, C., and Bosioc. A., 2008, “Hydrodynamic Design and Analysis of a Swirling Flow Generator” Proceedings of the 4th German-Romanian Workshop on Turbomachinery Hydrodynamics, Stuttgart, Germany. [7] Susan-Resiga, R., Ciocan, G. D., Anton, and I., Avellan, F., 2006, “Analysis of the Swirling Flow Downstream a Francis Turbine Runner”, ASME Journal of Fluids Engineering, Vol. 128, pp. 177-189. [8] Bosioc, A., Susan-Resiga, R., and Muntean, S., 2008, “Design and Manufacturing of a Converfent-Divergent Section for Swirling Flow Apparatus”, Proceedings of the 4th German-Romanian Workshop on Turbomachinery Hydrodynamics, Stuttgart, Germany. [9] Dantec Dynamics, 2006, BSA Flow Software, Version 4.10, Installation and User Guide.
