Numerical and experimental investigation of flow in horizontal axis Pelton
      turbines
Creators: name: Petley, Sean
Publisher: name: Lancaster University
Year: 2019

description: The Pelton turbine is widely considered a mature technology, existing for
      more than a century. However, manufacturers concerns over operating
      efficiency are a key driver in the continued design development. In recent
      decades, the use of Computational Fluid Dynamics (CFD) has become well
      established in the field of hydro turbine design, analysis and
      optimisation. However, the complex flow phenomena within impulse turbines
      have meant that only recently the simulation of the operation of Pelton
      turbine injectors and runners has been possible. Modern advances in
      computer hardware enables larger models to be studied in shorter
      timescales, despite this the application of CFD to the full Pelton turbine
      including the casing is still very demanding and as far as the author is
      aware there are no available studies in the public domain, documenting how
      Eulerian mesh based CFD solvers can be used for this task. 
      
      The main objective of the present thesis is to understand the impact of
      the casing geometry on the performance of a twin jet Pelton turbine. This
      objective has been met by employing advanced CFD models, which has been
      complemented by experimental testing. In parallel these allowed the
      description and detailed understanding of the flow features within a
      horizontal axis Pelton turbine casing and the effects the geometry has on
      this flow and subsequently turbine performance.
      
      The CFD analysis was carried out using the commercial code ANSYS® FLUENT®
      and included a number of numerical and physical assumptions to simplify
      the problems, including the usage of symmetry plane and modelling of only
      six (out of eighteen) consecutive buckets to reduce the size of the
      computational domain. The investigations can be split into two phases. In
      the first phase the CFD model was applied to a ‘naked – case’ and examined
      the impact of the addition of casing inserts such as a jet shroud and bolt
      on baffle and side shroud which direct the flow away from the injectors
      and runner, respectively. The second phase examined the variations of the
      casing width. The CFD models at this stage are only a visual indicative
      tool and cannot quantitatively predict efficiency as a result of
      variations in casing geometry, therefore experimental testing has been
      carried out to complement this work.
      
      Experiments were carried out at the Laboratory of Hydraulic Machines
      within the National Technical University of Athens (NTUA). The first phase
      experimental testing has found that the addition of the inserts have an
      overall effect on efficiency of no more than 0.5%, for single and twin jet
      operation. Furthermore, some inserts and baffles have a positive impact
      and some are detrimental. The experimental testing of the second phase
      revealed that a more considerable reduction of as much as 3% in efficiency
      is experienced when reducing the casing width by 50% and efficiency
      reduces linearly with width. 
      
      The CFD results of the casing provide a visual assessment of the flow in
      the casing and a key outcome of the PhD has been learning how to interpret
      these patterns and then to make decisions on what experiments to carry
      out. As a second case study, further CFD analysis was applied to an
      in-house test rig at Lancaster University designed by the author, and
      based on this work further experimental testing was then carried out. 
      
      A second objective of the present thesis was to examine the influence of
      the injector spear and nozzle angle on the flow behaviour and hydraulic
      efficiency. The 3D CFD study has shown that injectors with noticeably
      steeper angles of 110° &amp; 70° and 150° &amp; 90°, respectively,
      attain a higher efficiency than the industry standard of 80° &amp;
      55°. Moreover, experimental testing of the upper jet at NTUA showed gains
      of ≈1% in efficiency at the best efficiency point of the turbine can be
      achieved, however there appears to be an upper limit beyond which steeper
      designs are no longer optimal. Further insight was provided by carrying
      out additional simulations whereby the 3D velocity profiles obtained from
      the injector simulations are applied as an inlet boundary condition to a
      runner simulation and then examining the impact the jet shape has on the
      runner torque profile during the bucket cycle and the influence this had
      on turbine efficiency. It can be concluded that the secondary velocities,
      which contribute to the development of more significant free-surface
      degradations as the spear and nozzle angles are increased, result in a
      non-optimal jet runner interaction.
      
      Further investigations were carried out in order to compare the absolute
      difference between the numerical runner efficiency and the experimental
      efficiency. In doing so, the various losses that occur during operation of
      the turbine can be appraised and a prediction of casing losses can be
      made. Firstly, the mechanical losses of the test rig are estimated to
      determine the experimental hydraulic efficiency. Following this the
      numerical efficiency of the runner can then be ascertained by considering
      the upstream pipework losses and the aforementioned runner simulations,
      which are combined with the 3D velocity profiles from the injectors. The
      results indicate that out of all of the experimental cases tested, in the
      best case scenario the casing losses can be approximated to be negligible
      and in the worst case scenario ≈3%.