INVESTIGATIONS ON PELTON TURBINE JETS AND DEFLECTORS

Abstract

Hydropower remains the leading global renewable energy source, making up around 15% of the total 30% of electricity from renewable or low-carbon sources. It is anticipated to play a key role in the essential endeavors of decarbonization, making a positive contribution to achieving a net-zero power system and bolstering system flexibility. Despite being the most established renewable technology, hydropower encounters various challenges encompassing the imperative of ensuring sustainability and climate resilience, tackling the aging infrastructure and the necessity for new investments, as well as adapting operation and maintenance (O&M) practices to align with modern power system requirements. The Pelton turbine is one of the most efficient and mature technologies and is useful in hydropower because of the wide range of applicability at low flow rates and high heads, proving high efficiency at part load operation. Despite its popularity, significant efforts continue to improve these turbines to remain competitive in a market dominated by growing concerns about power generation efficiency and mechanical reliability. In the last twenty years, significant progress has been made in experimental methods and computational fluid dynamics (CFD) based numerical tools. These advancements have facilitated the visualization of complex flows within Pelton turbines, streamlining the process of modifying and improving turbine designs. Pelton turbine design is challenging compared to reaction turbines as it mainly relies on expertise and extensive experimentation with scaled-down models. The complexity arises as the Pelton turbine involves four distinct flow regimes: (i) confined steady-state flows in upstream pipes and the distributor, high flow acceleration due to curvature in the nozzle, (ii) free jets emerged out from the injectors and entrainment of the surrounding air into the jet, (iii) transient free-surface flows in the buckets, and (iv) two-phase dispersed flows in the casing. Each regime is characterized by different velocity magnitudes, dominated by different forces, and the flow regimes can be characterized by Reynolds number, Froude number, and Weber number. Formation of droplets at buckets due to cut in the jet by the splitter ridge which leads to cavitation.Control in the Pelton turbine is achieved by adjusting nozzle flow with a movable needle. In emergencies, deflectors redirect the jet operated by the servomotor and governor systems; therefore, its control may be called dual control. Two deflector options include the push-out jet for complete shutdown and the cut-in jet for partial control. The needle requires an external force to move or keep in balance as it faces hydraulic force from high water pressure inside the injector. Furthermore, diverting the high-momentum jet with a deflector requires sufficient torque to overcome reactional torque, initiate deflector motion, and keep balance during partial jet cutting. The force and torque characteristics are required for the reliable design and safe operation of the control system. The efficiency of the Pelton turbine is closely related to jet quality as high-quality jets result in higher efficiency and bucket shape, which is responsible for efficient torque generation. Efficient conversion of the hydraulic energy to kinetic energy depends on the injector design and distributor, leading to uniform velocity distribution and a good quality jet. Non-uniform distributions lead to energy losses. The quality of the emerging jet from the nozzle depends on surface deformation, jet deviation, and jet dispersion. Careful description and investigations are required to understand the turbulence fluctuations, secondary flow, and vortex stretching in the jet flow that leads to perturbation and energy loss. In regulation of the Pelton turbine, the time response of the needle valve cannot be faster in most hydropower plants due to water hammer issues. This challenge of fast response can be achieved by diverting the jet using a jet deflector. A general guideline is maintaining a power gradient (ramp rate) of less than 3% per second. Implementing cut-in jet deflector-based power control in Pelton turbine units ensures frequency stability and fast response to grid requirements. OBJECTIVES Based on the research gaps identified, the following objectives are planned: 1) To design and develop the test rig for the Pelton injector integrated with the jet deflector for the experimental measurement and evaluation of associated uncertainty analysis. 2) To measure and carry out analytical calculations of the jet deflector and cut-in deflector torque at the different angular positions with the variation of the needle stroke for characterization.3) To measure needle force using force transducer and validation with the analytical calculation based on the nozzle needle curve functions for control of Pelton injector. 4) To carry out the CFD analysis with RANS turbulence models and comparison with the DES model for Pelton jet velocity distribution. Also, to measure the jet velocity by LDV. 5) To carry out a CFD analysis of the cut-in deflector to visualize the quality of the existing jet using and control of the Pelton turbine driven by the cut-in jet deflector. In order to achieve the objectives, research work was carried out using the following methods: i. Fabrication of experimental testing rig for Pelton turbine injector model integrated with jet deflector for testing. ii. Installation of the force torque transducer was carried out, and in situ calibration of measuring instruments i.e., measuring tank load cells, torque transducer, single-axis force transducer, and pressure transmitter, was performed using calibrated instruments from the National Accreditation Board for Testing and Calibration Laboratories (NABL) accredited laboratories. iii. Uncertainty analysis for discharge, net head, and deflector torque was performed as per JCGM:100 (2008) and ISO 4185(1980) under different operating conditions. iv. The jet deflector and cut-in jet deflector torque were measured in the test rig, and their characterization was done under different operating conditions. v. Analytical calculations were established based on the impulse-momentum equation for selecting the range of applicability of force and torque transducer. vi. During the partial cutting of the jet, the visualization of the uncut/remaining jet and diverted flow estimation at different angular positions of the cut-in jet deflector were performed using CFD. The flow domain, consisting of the cut-in deflector, the injector, and the casing, was created in the Ansys DesignModeller. vii. The needle force has been obtained for the opening and closing trend of the nozzle by using the single-axis force transducer incorporating the friction effects. viii. For the CFD analysis, the hydraulic domain of the injector was created, and the hexahedral mesh was developed using ICEM-CFD. ix. The two-phase flow simulations of the injector and jet flow were carried out using the volume of fraction (VOF) model with the standard free surface model Reynold’s Average Navier-Stokes (RANS) equation and Detached eddy simulation (DES) model were utilized. x. LDV was set up for the measurement of the jet velocity and turbulence properties of the jet. Point velocity measurements were performed at the different axial jet distances up to x/ Do = 2. xi. The partial regulation of the Pelton turbine was performed numerically by transient CFD simulation by implementing a cut-in jet deflector in the hydraulic domain of the three buckets runner segment. The simulations were performed for needle stroke to nozzle diameter ratios 𝑠 𝑠Ƞ𝑚𝑎𝑥 ⁄ = 0.77, 1, 1.1, and 1.38, and for three angular positions of cut-in deflector δ = 3°, 6°, and 9°. A Pelton turbine injector test rig has been established at the Hydraulic Turbine Research and Development Laboratory, Indian Institute of Technology Roorkee, with a physical injector with a deflector and an associated control mechanism made available by one leading Turbine manufacturer in India. The selected model is a Pelton injector with a nozzle diameter of 36.3 mm and a nozzle-needle angle of 90˚-50˚, based on an external servomotor. The flow’s Reynolds number (𝑅𝑒 = 𝜌𝑣𝑗𝑑𝑗 /𝜇) is 2 × 106, and the maximum specific hydraulic energy at the injector is 684.19 J/kg, determined by the available head and flow rate in the test setup. These hydraulic parameters meet the minimum criteria for model testing according to IEC 60193:2019.