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Mulu, Berhanu
Alternative names
Publications (10 of 20) Show all publications
Amiri, K., Mulu, B., Cervantes, M. & Raisee, M. (2016). Effects of load variation on a Kaplan turbine runner (ed.). International Journal of Fluid Machinery and Systems, 9(2), 182-193
Open this publication in new window or tab >>Effects of load variation on a Kaplan turbine runner
2016 (English)In: International Journal of Fluid Machinery and Systems, ISSN 1882-9554, E-ISSN 1882-9554, Vol. 9, no 2, p. 182-193Article in journal (Refereed) Published
Abstract [en]

Introduction of intermittent electricity production systems like wind and solar power to electricity market together with the deregulation of electricity markets resulted in numerous start/stops, load variations and off-design operation of water turbines. Hydraulic turbines suffer from the varying loads exerted on their stationary and rotating parts during load variations they are not designed for such operating conditions. Investigations on part load operation of single regulated turbines, i.e., Francis and propeller, proved the formation of a rotating vortex rope (RVR) in the draft tube. The RVR induces pressure pulsations in the axial and rotating direction called plunging and rotating modes, respectively. This results in oscillating forces with two different frequencies on the runner blades, bearings and other rotating parts of the turbine. This study investigates the effect of transient operations on the pressure fluctuations exerted on the runner and mechanism of the RVR formation/mitigation. Draft tube and runner blades of the Porjus U9 model, a Kaplan turbine, were equipped with pressure sensors for this purpose. The model was run in off-cam mode during different load variations. The results showed that the transients between the best efficiency point and the high load occurs in a smooth way. However, during transitions to the part load a RVR forms in the draft tube which induces high level of fluctuations with two frequencies on the runner; plunging and rotating mode. Formation of the RVR during the load rejections coincides with sudden pressure change on the runner while its mitigation occurs in a smooth way.

National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-6114 (URN)10.5293/IJFMS.2016.9.2.182 (DOI)2-s2.0-84989184490 (Scopus ID)4508eea2-7f64-4f96-8605-24d08df390e8 (Local ID)4508eea2-7f64-4f96-8605-24d08df390e8 (Archive number)4508eea2-7f64-4f96-8605-24d08df390e8 (OAI)
Note

Validerad; 2016; Nivå 1; 2016-10-31 (andbra)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-11-24Bibliographically approved
Amiri, K., Mulu, B., Raisee, M. & Cervantes, M. (2016). Experimental study on flow asymmetry after the draft tube bend of a Kaplan turbine (ed.). Paper presented at . Advances and Applications in Fluid Mechanics, 19(2), 441-472
Open this publication in new window or tab >>Experimental study on flow asymmetry after the draft tube bend of a Kaplan turbine
2016 (English)In: Advances and Applications in Fluid Mechanics, ISSN 0973-4686, Vol. 19, no 2, p. 441-472Article in journal (Refereed) Published
Abstract [en]

Flow condition in a Kaplan turbine draft tube is investigated using laser Doppler anemometry (LDA) and particle image velocimetry(PIV). The investigated draft tube is composed of a cone followed by an elbow and a straight diffuser. The three velocity components were measured after the elbow at two different locations across the straight diffuser to quantify the flow asymmetry as well as the secondary flows formed in this region. The velocity profiles at the draft tube inlet are measured using a 2D LDA system allowing estimation of the draft tube inlet swirl. The results are presented at three operating points of the turbine. The flow condition after the draft tube bend was shown to be highly dependent on the vortex structures within the straight draft tube; namely Dean vortices and the swirl leaving the runner. At operating points with high flow rates and low swirl, Dean vortices dominate the upstream swirl; a symmetric but inhomogeneous flow resembling flow after a pipe bend forms within the straight diffuser. At part load operating points with high swirl and low flow rate, the flow after the bend is dominated by the upstream swirl resulting in asymmetric flow after the draft tube bend. The flow asymmetry is shown to be a 2nd order function of the swirl-to-Dean ratio.

National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-2569 (URN)10.17654/FM019020441 (DOI)2-s2.0-84962361497 (Scopus ID)030d4f61-5f1e-4b85-8c97-ec689d62cc5f (Local ID)030d4f61-5f1e-4b85-8c97-ec689d62cc5f (Archive number)030d4f61-5f1e-4b85-8c97-ec689d62cc5f (OAI)
Note
Validerad; 2016; Nivå 1; 20160331 (kavami)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
Amiri, K., Cervantes, M. & Mulu, B. (2015). Experimental investigation of the hydraulic loads on the runner of a Kaplan turbine model and the corresponding prototype (ed.). Paper presented at . Journal of Hydraulic Research, 53(4), 452-465
Open this publication in new window or tab >>Experimental investigation of the hydraulic loads on the runner of a Kaplan turbine model and the corresponding prototype
2015 (English)In: Journal of Hydraulic Research, ISSN 0022-1686, E-ISSN 1814-2079, Vol. 53, no 4, p. 452-465Article in journal (Refereed) Published
Abstract [en]

This paper reports on unsteady pressure measurements on the runner blades of a Kaplan turbine model as well as torque and radial load bearing measurements on the corresponding prototype at several operating points to investigate the sources of periodic loads exerted on the runner when operating at the best efficiency point and off design. Pressure measurements on the model runner blades indicated that the spiral casing delivers a poorly conditioned flow to the guide vanes close to the lip-entrance junction, resulting in flow separation on the guide vanes. The asymmetric flow delivered to the runner induces large oscillations with respect to the guide vane passing frequency, runner frequency and its harmonics to the runner blades. The torque measurements on the prototype also revealed an asymmetric flow at the distributor outlet. The bearing radial load measurements performed on the prototype support the torque measurement results. The asymmetric hydraulic loads on the runner result in shaft wobbling, and the oscillatory forces exerted on the blades are transferred to the main shaft and bearings. Another source of oscillating forces exerted on the runner blades is the rotating vortex rope (RVR) formation that occurs at part-load operation of the turbine and induces pressure fluctuations at two sub-synchronous frequencies to the runner.Keywords: Bearing load; blade pressure measurement; flow-induced vibration; Kaplan turbine; prototype measurement; spiral casingperformance

National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-11729 (URN)10.1080/00221686.2015.1040085 (DOI)000362881700004 ()2-s2.0-84945474369 (Scopus ID)abbcdca7-14c9-474e-b4b9-58d90c6f5c3c (Local ID)abbcdca7-14c9-474e-b4b9-58d90c6f5c3c (Archive number)abbcdca7-14c9-474e-b4b9-58d90c6f5c3c (OAI)
Note
Validerad; 2015; Nivå 2; 20150604 (kavami)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
Mulu, B., Cervantes, M., Devals, C., Vu, T. & Gibault, F. (2015). Simulation-based investigation of unsteady flow in near-hub region of a Kaplan Turbine with experimental comparison (ed.). Paper presented at . Engineering Applications of Computational Fluid Mechanics, 9(1), 139-156
Open this publication in new window or tab >>Simulation-based investigation of unsteady flow in near-hub region of a Kaplan Turbine with experimental comparison
Show others...
2015 (English)In: Engineering Applications of Computational Fluid Mechanics, ISSN 1994-2060, E-ISSN 1997-003X, Vol. 9, no 1, p. 139-156Article in journal (Refereed) Published
Abstract [en]

his paper presents a detailed comparison of steady and unsteady turbulent flow simulation results in the U9 Kaplan turbine draft tube with experimental velocity and pressure measurements. The computational flow domain includes the guide vanes, the runner and the draft tube. A number of turbulence models were studied, including the standard k-eps, RNG k-eps, SST and SST-SAS models. Prediction of the flow behavior in the conical section of the draft tube directly below the runner cone is very sensitive to the prediction of the separation point on the runner cone. The results demonstrate a significant increase in precision of the flow modeling in the runner cone region by using unsteady flow simulations compare to stage simulation. The prediction of the flow in the runner cone region, however, remains delicate, and no turbulence model could accurately predict the complex phenomena observed experimentally.

National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-5834 (URN)10.1080/19942060.2015.1004816 (DOI)000361560800010 ()2-s2.0-84945917696 (Scopus ID)40555954-697e-4add-b420-b29b2fc416c3 (Local ID)40555954-697e-4add-b420-b29b2fc416c3 (Archive number)40555954-697e-4add-b420-b29b2fc416c3 (OAI)
Note
Validerad; 2015; Nivå 2; 20150314 (cervante)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
Amiri, K., Mulu, B., Raisee, M. & Cervantes, M. (2014). Load variation effects on the pressure fluctuations exerted on a Kaplan turbine runner (ed.). In: (Ed.), A-M Giroux (Ed.), 27th IAHR Symposium on Hydraulic Machinery and Systems, IAHR 2014: Montreal, Canada, 22- 26 September 2014. Paper presented at IAHR Symposium on Hydraulic Machinery and Systems : 22/09/2014 - 26/09/2014. : IOP Publishing Ltd, Article ID 32005.
Open this publication in new window or tab >>Load variation effects on the pressure fluctuations exerted on a Kaplan turbine runner
2014 (English)In: 27th IAHR Symposium on Hydraulic Machinery and Systems, IAHR 2014: Montreal, Canada, 22- 26 September 2014 / [ed] A-M Giroux, IOP Publishing Ltd , 2014, article id 32005Conference paper, Published paper (Refereed)
Abstract [en]

Introduction of intermittent electricity production systems like wind power and solar systems to electricity market together with the consumption-based electricity production resulted in numerous start/stops, load variations and off-design operation of water turbines. The hydropower systems suffer from the varying loads exerted on the stationary and rotating parts of the turbines during load variations which they are not designed for. On the other hand, investigations on part load operation of single regulated turbines, i.e., Francis and propeller, proved the formation of rotating vortex rope (RVR) in the draft tube. The RVR induces oscillating flow both in plunging and rotating modes which results in oscillating force with two different frequencies on the runner blades, bearings and other rotating parts of the turbine. The purpose of this study is to investigate the effect of transient operations on the pressure fluctuations on the runner and mechanism of the RVR formation/mitigation. Draft tube and runner blades of the Porjus U9 model, a Kaplan turbine, were equipped with pressure sensors. The model was run in off-cam mode during different load variation conditions to check the runner performance under unsteady condition. The results showed that the transients between the best efficiency point and the high load happens in a smooth way while transitions to/from the part load, where rotating vortex rope (RVR) forms in the draft tube induces high level of fluctuations with two frequencies on the runner; plunging and rotating mode of the RVR.

Place, publisher, year, edition, pages
IOP Publishing Ltd, 2014
Series
I O P Conference Series: Earth and Environmental Science, ISSN 1755-1307 ; 22
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-40572 (URN)10.1088/1755-1315/22/3/032005 (DOI)000347441900063 ()84919632882 (Scopus ID)fc08b6b8-5395-4286-9136-f2022232b363 (Local ID)fc08b6b8-5395-4286-9136-f2022232b363 (Archive number)fc08b6b8-5395-4286-9136-f2022232b363 (OAI)
Conference
IAHR Symposium on Hydraulic Machinery and Systems : 22/09/2014 - 26/09/2014
Note
Validerad; 2015; Nivå 1; 20150107 (andbra)Available from: 2016-10-03 Created: 2016-10-03 Last updated: 2018-07-10Bibliographically approved
Sundström, J., Mulu, B. & Cervantes, M. (2014). Wall friction and velocity measurements in multiple frequencies pulsating flow (ed.). Paper presented at International Conference on Experimental Fluid Mechanics : 12/08/2014 - 15/08/2014. Paper presented at International Conference on Experimental Fluid Mechanics : 12/08/2014 - 15/08/2014.
Open this publication in new window or tab >>Wall friction and velocity measurements in multiple frequencies pulsating flow
2014 (English)Conference paper, Oral presentation only (Refereed)
Abstract [en]

Turbulent pulsating flow in a 100 mm diameter pipe has been studied experimentally at a Reynolds number of 14,500. The work covers four different flow conditions; steady, pulsating flow at oscillation frequencies 0.08 and 0.4 Hz, and pulsating flow with simultaneous imposition of both frequencies. The amplitudes of the pulsations were 10 and 7.5% of the bulk flow at 0.08 and 0.4 Hz, respectively. Laser Doppler anemometry, hot-film and pressure measurements show that the mean values of velocity, wall shear stress and pressure gradient are unaffected by the imposed pulsations. In agreement with previous studies of pulsating flow, the phase averaged pressure gradient leads both the velocity and wall shear stress when a single pulsation is imposed on the mean flow. These phase leads remain virtually unchanged when the two frequencies are imposed simultaneously. The amplitude responses of the velocity, wall shear stress and pressure gradient in the combined pulsating flow is shown to be superpositions of the amplitudes from the cases of separate pulsations. There are no signs of non-linear interactions between the harmonics.

National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-30512 (URN)45b6e094-2dfa-4cbc-88d5-5b28aaaa677b (Local ID)45b6e094-2dfa-4cbc-88d5-5b28aaaa677b (Archive number)45b6e094-2dfa-4cbc-88d5-5b28aaaa677b (OAI)
Conference
International Conference on Experimental Fluid Mechanics : 12/08/2014 - 15/08/2014
Note
Godkänd; 2014; 20140912 (sunjoe)Available from: 2016-09-30 Created: 2016-09-30 Last updated: 2017-11-25Bibliographically approved
Amiri, K., Cervantes, M., Mulu, B. & Raisee, M. (2013). Experimental study of fluid structure interaction in a Kaplan turbine runner blade (ed.). Paper presented at IAHR International Workshop on Cavitation and Dynamic Problems in Hydraulic Machinery : 08/09/2013 - 11/09/2013. Paper presented at IAHR International Workshop on Cavitation and Dynamic Problems in Hydraulic Machinery : 08/09/2013 - 11/09/2013.
Open this publication in new window or tab >>Experimental study of fluid structure interaction in a Kaplan turbine runner blade
2013 (English)Conference paper, Oral presentation only (Refereed)
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-27666 (URN)12e751be-833a-4be9-a453-5068b6a5a8f1 (Local ID)12e751be-833a-4be9-a453-5068b6a5a8f1 (Archive number)12e751be-833a-4be9-a453-5068b6a5a8f1 (OAI)
Conference
IAHR International Workshop on Cavitation and Dynamic Problems in Hydraulic Machinery : 08/09/2013 - 11/09/2013
Note
Godkänd; 2013; 20131014 (kavami)Available from: 2016-09-30 Created: 2016-09-30 Last updated: 2017-11-25Bibliographically approved
Mulu, B. (2012). An experimental and numerical investigation of a Kaplan turbine model (ed.). (Doctoral dissertation). Paper presented at . Luleå: Luleå tekniska universitet
Open this publication in new window or tab >>An experimental and numerical investigation of a Kaplan turbine model
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Hydropower is a renewable, reliable and highly efficient source of energy. Hydropower has the ability to run as a base load and to adjust load rapidly. This makes hydropower suitable for coupling with other renewable energy sources to stabilize frequency fluctuations. This ability has been used increasingly over the last decade due to the deregulation of the electricity markets and the introduction of other renewable energy sources, such as wind power. These changes have involved a substantial increase in the load variations and frequent start-stops. Such operating conditions may lead to unnecessary stresses and losses in turbines. Throughout the world, hydropower is the largest renewable source of energy. Currently, there is a need for refurbishment of old hydropower plants because most of them have reached the end of their design period. Efficiency may be improved by upgrading these older turbines. During the design or refurbishment phase of turbines, model testing and computational fluid dynamics (CFD) are the main tools available to predict, test and verify the performance as well as to investigate the flow characteristics.The use of CFD in the design and refurbishment process is becoming increasingly popular due to its flexibility, detailed flow description and cost effectiveness compared to model testing, which has been used over the last century of turbines development. However, issues still must be resolved due to the combined flow physics involved in hydropower machines, such as flow turbulence, separation, vortices, unsteadiness, swirl flow, strong adverse pressure gradients, convoluted geometry and numerical artifacts. Therefore, experimental data in such complicated systems are required to validate the numerical simulations and develop more accurate models.This thesis presents an experimental and numerical investigation performed on a reaction type axial water turbine. The investigation was performed on a model known as the Porjus U9. It is a geometrically similar model of the prototype turbine produced on a 1:3.1 scale. The main objectives were to characterize the flow phenomena in this modern Kaplan turbine model, to build a data bank for the validation of the CFD tools and to study the scale-up between the model and prototype, because the corresponding prototype is available for similar experiments. The investigation was performed at three different operating points: part load, best efficiency point (BEP) and high load. The technique used to investigate the flow was laser Doppler anemometry. The investigation was performed with time- and phase-averaged velocity measurements in several sections of the turbine where different periodic and non-periodic flow phenomena were captured. Some engineering quantities were also calculated to describe the turbine characteristics, such as the pressure recovery factor and the swirl number. At off-design operations, vortex breakdown was present.The numerical analysis of the model is also presented, where several RANS turbulence models were tested. The aim was to evaluate the capability of the turbulence models to predict the flow physics in the water turbines at the BEP. Validation was made with the experimental results in order to extend the range of confidence in the CFD results.

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2012. p. 278
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-26337 (URN)dca695e5-69a5-4468-9917-9a6021544b12 (Local ID)978-91-7439-418-4 (ISBN)dca695e5-69a5-4468-9917-9a6021544b12 (Archive number)dca695e5-69a5-4468-9917-9a6021544b12 (OAI)
Note
Godkänd; 2012; 20120319 (bermul); DISPUTATION Ämnesområde: Strömningslära/Fluid Mechanics Opponent: PhD Sebastian Muntean, Romanian Academy-Timisoara, Center for Fundamental and Advanced Technical Research (CFATR), Hydrodynamics and Cavitation Laboratory, Timisoara, Romania, Ordförande: Professor Michel Cervantes, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Torsdag den 26 april 2012, kl 09.30 Plats: E231, Luleå tekniska universitetAvailable from: 2016-09-30 Created: 2016-09-30 Last updated: 2017-11-24Bibliographically approved
Mulu, B., Jonsson, P. & Cervantes, M. (2012). Experimental investigation of a Kaplan draft tube: Part I: best efficiency point (ed.). Paper presented at . Applied Energy, 93, 695-706
Open this publication in new window or tab >>Experimental investigation of a Kaplan draft tube: Part I: best efficiency point
2012 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 93, p. 695-706Article in journal (Refereed) Published
Abstract [en]

Hydropower, originally designed as a base electrical power, is now used to balance grid fluctuations that are primarily produced by market deregulation and the introduction of other renewable energy resources. New turbine designs must account for such constraints while also achieving high efficiency. Computational fluid dynamics, now an integrated tool in the hydraulic industry, requires accurate and detailed experimental data for validation purposes.The present work presents the investigation of a modern Kaplan turbine model combined with laser Doppler anemometry and flush-mounted pressure sensors, with a focus on the draft tube at the best turbine efficiency. Mean and phase-resolved quantities are presented for the velocity and pressure at several locations. The results demonstrate the strong influence of the swirl leaving the runner for a well-functioning draft tube as well as the negative impact of the draft tube cone. The blade-hub clearance is also found to have an impact on the flow beneath the runner cone.

National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-11946 (URN)10.1016/j.apenergy.2012.01.004 (DOI)000302836500081 ()2-s2.0-84857992810 (Scopus ID)afe0edfc-9ec7-4a82-9bf0-d2e0ecf5af79 (Local ID)afe0edfc-9ec7-4a82-9bf0-d2e0ecf5af79 (Archive number)afe0edfc-9ec7-4a82-9bf0-d2e0ecf5af79 (OAI)
Note
Validerad; 2012; 20120203 (cervante)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
Jonsson, P., Mulu, B. & Cervantes, M. (2012). Experimental investigation of a Kaplan draft tube: Part II: Off-design conditions (ed.). Paper presented at . Applied Energy, 94(2), 71-83
Open this publication in new window or tab >>Experimental investigation of a Kaplan draft tube: Part II: Off-design conditions
2012 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 94, no 2, p. 71-83Article in journal (Refereed) Published
Abstract [en]

Off-design conditions of hydropower turbines are becoming more frequent with the deregulation of electricity markets and the introduction of renewable energy resources. Originally, turbines were not built to operate under such conditions. It is evident that there is a need to develop turbines that can operate under off-design conditions while attaining high efficiency. This may be achieved with computational fluid dynamics (CFD). However, the complexity of Kaplan turbine flows is challenging to treat using CFD. Therefore, detailed experimental investigations are necessary to validate and develop CFD. This paper presents an investigation of a modern design Kaplan turbine model. The measurements were performed in the draft tube with laser Doppler anemometry and flush-mounted pressure sensors, with a focus on the part load and high load operation of the turbine. Mean and phase-averaged quantities are presented for the velocity and pressure along several sections. A contra-rotating flow region was observed under high load operation. Under part load operation, a rotating vortex rope (RVR) develops due to vortex breakdown. The presence of the RVR significantly reduces the draft tube performance.

National Category
Fluid Mechanics and Acoustics
Research subject
Fluid Mechanics
Identifiers
urn:nbn:se:ltu:diva-4047 (URN)10.1016/j.apenergy.2012.01.032 (DOI)000302842800009 ()2-s2.0-84856793224 (Scopus ID)1e88c379-6163-4c4e-8284-8ce3af218eb5 (Local ID)1e88c379-6163-4c4e-8284-8ce3af218eb5 (Archive number)1e88c379-6163-4c4e-8284-8ce3af218eb5 (OAI)
Note
Validerad; 2012; 20120213 (cervante)Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2018-07-10Bibliographically approved
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