Recent approach to refurbishments of small hydro projects based on numerical flow analysis

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1 Recent approach to refurbishments of small hydro projects based on numerical flow analysis by Swiderski Engineering Ottawa, Canada Preamble Computational Fluid Dynamics (CFD) already established its strong presence in the hydropower industry as an engineering tool. Since 1842, when Navier in France first formulated general form of equations, enhanced later by Stokes in England, which in their opinion gave complete description of behavior of the viscous compressible flow, application engineers had to wait many years before a practical solution to it was found. So now we have 3D viscous flow, multiple frame of reference, multi-block, structured or non-structured grids, two cavitation models, transient states simulations, two turbulence models to suit our needs. This presentation is about selected practical applications of Computational Fluid Dynamics (CFD) based on CFX-TASCflow software. Why would older turbines need to be upgraded classical design methods Classical design methods, used in development of turbines in question, had been published and lectured for many years. Until today, we do not have a consistent theory on how to solve a 3D design problem, although we know how to simulate the 3D real water flow with a very good proximity, which very often is within a physical measurements error. As we know today, from sophisticated observations conducted in fluid dynamics ( i.e.: hydro and aero) laboratories and from the CFD results that there are several factors, which are neglected in the 2D theories. Let s attempt to list them: a) Flow around the leading edge air and water big difference b) Existence of 3 rd dimension component of the flow within the blade-to-blade space of a turbine runner c) The upstream influence As the leading edge flow can be resolved with the 2D approach, the flow - and therefore energy and momentum transfer between the analyzed surfaces is, by the 2D theory neglected. Omissions or simplifications of the above listed aspects result in a sometimessignificant discrepancy of the desired and tested turbine performances. The model testing therefore must be a part of a design process. So how efficient could it be, if the construction of a physical model is included, testing numerous modifications? This very costly and time labor consuming process could be implemented by large Page 1 of 10

2 organizations, which are involved in major projects. Thanks to scalability of fluid behavior, the small hydro industry could apply some of hydraulic solutions from the large hydro projects. Experimental verifications The crucial feedback, which had to come from laboratory investigations, regarded general performances of the turbine like output, efficiency, cavitation, as well as some/limited observations of the turbine behavior e.g. stability/pressure pulses It is difficult, even applying sophisticated observations methods, to determine local flow phenomena, which could help to locate any source of the undesired behaviors. The observer associated with the stationary reference system, can make only indirect measurements (i.e. strobe light) and observations, which were difficult to translate to a set of data helpful to make a decision on how to modify the hydraulics of the turbine to reach a goal. Design based on CFD verification Following the assumption that results of CFD analysis represent a real flow, the virtual hydraulic laboratory can be established. This would allow conducting various observations of the flow the observatory can travel into areas, which are physically impossible to reach in the real laboratory and observations can be made in stationary and rotating coordinate systems. With this new laboratory, the design process can proceed as per the classical algorithm (model-testing-observations-corrections), or other methods can be explored, while the entire process can be programmed. The following are major design strategies exercised by the industry: (A) Classical design approach: model CFD analysis observations modifications (B) Newer approach as above, but atomized: model generation CFD analysis decision on shape modification (C) Attempts to solve reverse problem should there be a strict mathematical to the N-S equations, the solution to a problem of finding a shape of flow channel to achieve certain effect would be possible. As there is no such a solution yet, theoretical attempts are dealing with certain simplifications to the NS equations, so they are easier to solve mathematically. However the flow phenomena presented by the mathematical model is getting further from the near-reality description. Page 2 of 10

3 Methodology of an upgrade The following are steps, which must be undertaken in order to successfully complete and upgrade project: 1) Numerical model full geometry of the turbine including - Intake - Spiral casing - Distributor (all stay vanes and wicket gates) - Runner - Draft tube 2) Tuning-up the numerical model - Grid quality: verification and refinement. Based on couple of runs of the flow analysis, the nodes distribution is adjusted according to the velocity/pressure field. - Operating parameters. In the non-dimensional factors, the CFD results must be within a certain range from the field measurements. 3) CFD analysis flow solver 4) Analysis of results - Energy dissipation field (losses). - Pressure gradients estimate possibilities for cavitation - Determination of the flow areas, where the velocity field has highest nonuniformity 5) Strategy for upgrade based on expected cost/benefit ratio - Intake shape - Distributor (wicket gates profile, stay vanes set-up) - Runner design - Draft tube shape intake casing stay ring wicket gates runner draft tube elbow draft tube extension Page 3 of 10

4 Upgrades implemented Spiral Case Kaplan Unit Vertical Kaplan turbine unit, designed in 1970 s, tested in laboratory, was commissioned in early 1980 s. The nominal capacity of this unit is 2400 kw, however the maximum power a turbine has been able to reach was 2250 kw. As the machine operates under the design nominal Net Head, it was proposed to the owner to conduct full CFD-based diagnostics of the turbine unit to locate possible points of improvement. The flow simulation of the entire turbine pointed to two major areas for improvement: 1) Stay ring 2) Runner blades The design team decided that the stay vanes modification must be conducted first, because it could improve runner inflow significantly. The applied solution was not ideal, due to time constrains, however the project is a very good example of the bad spiral case-stay ring assembly design. As it is clearly visible on the presented flow field pictures, each stay vane had to have different position, in order to minimize head lo9sses within the distributor. It is a very common situation for non-proper spiral case design. After the CFD model was tuned-up, the design work progressed and the final new stay vanes position and shape was found to minimize hydraulic losses in the distributor. The predicted increase of the power output (simulation of the entire turbine approx. 1.2 million nodes), due to efficiency and flow increase was 8.2%. The guaranteed increase of 2.0% was enough to justify an expense of stay vanes replacement. Having two identical units, the owner decided to conduct modification on unit 1 and compare production figures of both units. After over 4 months, 8% increase in energy production was recorded, and the customer decided to conduct modification on the second unit. Modification of the stay vanes position resulted in 8% energy production Page 4 of 10

5 Semi-Spiral Case Kaplan Unit Semi-spiral cases are formed out of concrete, which, in belief of many, makes no difference to the turbine performances because velocities are low anyways. It is not for the first time, that we analyzed the distributor inflow conditions, and the results tend to repeat. Quite bad inflow conditions on one side of the runner and very good on the other side is a typical observation one can make analyzing results of the numerical flow simulation. Bad inflow conditions Good inflow conditions Flow lines (streaklines) released at selected points of the turbine inlet. (a) (b) Illustration of the velocity (a) and energy (b) fields in Kaplan turbine distributor. Page 5 of 10

6 Semi-spiral Kaplan unit cavitation diagnostics The erosion on runner blades of this 6-blade Kaplan runner caused the tip of the trailing edge to fall off on all blades. During full power operation the tip of the blade, which eroded away was in overhung position (below sphere-cone transition of the throat ring). CAVITATING ZONE Cavitation is formed just upstream the blade, so by the time bubbles implode; the blade tip enters the area. This zone results as the overall flow at the blade to-blade space in order to eliminate this zone, the blade geometry must change. Streaklines released from the zone of interest upstream and downstream allow tracking down the source of a problem. In this case, the two colliding streams create a very low-pressure zone, which results in cavitation at the site conditions. Local erosion at the blade inlet edge tip Kaplan turbines A very common problem in many existing Kaplan turbines consists in erosion observed at the runner blade tip near the leading edge and on the throat ring inlet. The inflow conditions at this area of the runner blade are complicated and can be well studied via the CFD. During the operation at large blade opening, which related to the full or near-full power point, the blade-throat ring gap increases as much as 5 times the design value. As this happens, the wicket gates are at largest opening and, what is common to any Kaplan turbine design, their tip is hanging into the runner flow passage. Therefore the following influences the local flow conditions in the area: a) Tip of the wicket gate trailing edge (stationary) b) Transition radius of the bottom distributor throat ring c) Tip of the runner blade leading edge (rotating) Page 6 of 10

7 This is the most complicated and most sensitive to local cavitation configuration, known to the Author. The numerical analysis, giving a clear picture of the local flow field, shows well what kind of measures should be undertaken to limit the probability of creation of the cavitational erosion. Area of local instabilities in classical Kaplan turbines causing localized cavitation and energy loss Various view angles - observations of the flow around the tip of the leading edge of Kaplan runner at full load position. Page 7 of 10

8 Upgrades by replacing Francis runner Nowadays, many understand this approach as the most economically sound investment in hydropower. Flexibility of CFD techniques enables to make custom design runner, which will use available water more efficiently, therefore generating more revenue. Following the general methodology presented earlier, two Francis turbines were upgraded last year by replacing runners only. The expected performances stated, based on the flow simulation matched results of the field test. Site 1 Hnet = 50m Generator output guaranteed = 1615 kw Generator output achieved = 1725 kw Output increase: 15% Final verification of the newly designed runner is conducted for the entire turbine. output guaranteed = 5000 kw Generator output achieved = 5200 kw Page 8 of 10

9 Site 2 Hnet = 105m Output before the upgrade = 4500 kw Output after the upgrade (only runner replaced) = 5200 kw (a) (b) Replacement Francis runner static pressure distribution and streaklines on the surface of the runner blade: (a) full power, (b) part load. Site 3 Hnet = 67 m Generator output = 1300 kw Goal of refurbishment: elimination of cavitation and efficiency increase At the manufacturing stage Replacement Francis runner preliminary design for the ongoing project. Design Page 9 of 10

10 Acknowledgment The Author has a duty and privilege to express sincere appreciation to the owners and a crew of Norcan Hydraulic Turbine Inc., the company, which implements, on everyday basis, turbine designs provided based on the CFD. This company completed successfully projects presented in this paper. References 1. Implementations of Computational Fluid Dynamics in a design practice - virtual hydraulic laboratory, HYDROFORUM 2000, Poland, co-author J. Martin. 2. CFX - design tool for small hydro - in autumn newsletter, AEA Technology Engineering Software Ltd. 3. Automated runner blade design optimization process based on CFD verification, Waterpower XII, Salt lake City, 2001, co-authors: J. Martin, R. Norrena. 4. Application of CFD Turbine Design for Small Hydro Elliott Falls, A Case Study, Waterpower XII, Salt lake City, 2001, co-author Bennett K., 5. Solving Small Hydro Problems with Computational Fluid Dynamics (co-authors: K. Bennett, J-X. Huang), Hydro Review magazine, vol. XXI, No 1, March Design optimisation of replacement Francis runner CFD application in an optimization algorithm, J. Swiderski 13th International on Hydropower Plants, Vienna 2004 (to be published). Page 10 of 10