Investigation Of Classical Design Theory And Unsteady Flow Characteristics Of A Low-Head Axial Flow Turbine

The design for the runner blades and major geometries of axial flow hydro turbines is well-known and proposed by many proponents in the 20th century after Austrian engineer Viktor Kaplan discovered this turbine while working in the Czech city of Brno between 190333.Later Mirloslav Nechleba firmed the principles of design for a wide range of operating conditions（known as specific speeds）followed in this thesis.The design runner for near zero head conditions for a speed range beyond ns-1000 brought the shape beyond that proposed by Nechleba.However,it was necessary to adhere to the condition.Nechleba provided the classical case of the two dimensionless velocity constants,us-the specific peripheral velocity of the runner at the largest diameter and cs-the specific absolute flow velocity through the runner,which were related to the specific speed-ns（horsepower units）.Here the constants abruptly end at ns-900.Nevertheless,the design runner needs to be dimensioned for higher than ns-900.The unknown territory of specific speeds leads us to build an approach with the interface of hugely popular,well-firmed design approach horizontal axis wind turbines（HAWT）.The common aspect of Kaplan and HAWT is the procedure for aerofoil design based on classical lift theory.However,the primary deviation from the two concerns the definition of the shape parameter or specific speed（ns）.The early designers thought that specific speed would be redundant.The head（energy term）is nothing but the kinetic energy encompassed by the absolute flow velocity and hence classified all HAWTs（even some vertical axis machines）in terms of tip speed ratio（l）defined as the peripheral velocity at the rotor tip to the incoming wind velocity,which is nothing but us（defined by Nechleba）.Based on the above case scenario,designing low-head AFT suitable for the near-zero head application becomes imperative.However,starting from the known zone to the unknown territory is crucial.Firstly,a low-head AFT with a specific speed ns-728 within the design parameters provided by Nechleba was designed using the free vortex theory.Further numerical and experimental unsteady flow field studies were conducted on the AFT to reveal the effect of significant system design conditions like axial gap and blade number on the AFT.The design approach of starting from the known zone to the unknown is to reveal the optimal design configurations suitable for the design of the near-zero head,high specific speed AFT.Following the study,the main ideas and innovations are as follows:1.The computational domain was built and gridded with high-quality hexahedral mesh grids.The numerical model was well-validated with experimental results to demonstrate its suitability for hydraulic flow simulations.Numerical investigations were conducted to compare the turbine’s performance and unsteady pressure fluctuations at different blade numbers.Under various operating circumstances,experimental research was conducted in a four-quadrant hydrant test rig to ascertain the hydraulic performance and unsteady pressure pulsations of the AFT with a rotating speed of 1450 r/min.The head,power,and efficiency characteristic curves have been appropriately evaluated and aligned with the AFT’s design requirements.To further evaluate and validate the pressure variations inside the AFT’s flow channels,pressure sensors were added at the intake pipe（P1）,guiding vane（P2）,and outlet pipe（P3）.2.An experimental study on the effect of axial gap on the performance and unsteady pressure pulsation was conducted to measure dynamic pressure pulsation under various operating flow conditions and axial gaps（6）of 0.05D（base gap）,0.09D,0.14D,and 0.18D.Based on the experimental result,a 28%and 2.8%increase in turbine efficiency was recorded when the ’б’ was increased from 0.05D to 0.09D at 0.8Q_d and 1.0Q_d,respectively.Further analysis of the frequency spectra reveals different unsteadiness in the flow structures due to changes in‘б,’ which resulted in various excitation signals.A decrease in the ’б’ leads to increased pressure pulsation intensity.Consequently,AFT with ’б’ of 0.09D was recommended since it provides maximum efficiency with fewer pulses compared with ’б’=0.05D,where the vibrations are at peak value.3.The impact of the system design of the inlet pipe（right-angled and in-line）and guide vane number on unsteady velocity（v_f）and pressure fluctuation（Cp）was carried out on the AFT in order to reveal the conditions that are suitable for its operational stability in order to ensure its safety and performance.The frequency-domain history for the rightangled inlet reveals an increase in the amplitude of Cp with a rising flow rate from the part load（0.8Q_d）through design flow（1.0Q_d）to overload（1.3Q_d）.However,when the straight inlet pipe was used,the pressure pulsation amplitude reduced across all monitoring points compared with the values of Cp recorded by the right-angled inlet.The changes in the inlet pipe have no significant effect on the harmonics recorded in the turbine,but its effect on the distribution of v_f is significant since it influences the velocity distribution in the guide vane flow passage.The model with the right-angled inlet recorded higher v_f in the flow passage of the guide vane,while the model with a straight entry accounted for the least v_f.4.Unsteady flow simulations were performed to examine the flow behavior caused by runner blade number on velocity and pressure variation to quantify the unsteadiness in the AFT at 0.8Q_d,1.0Q_d,and 1.3Q_d.The strength of pressure and velocity changes inside the AFT under various blade numbers were shown by the internal flow study under various flow circumstances.The unsteady flow fields in the AFT were significantly influenced by blade number and flow rate.Across all blade numbers,the runner leading edge area had the most unsteady flow patterns in the AFT.AFT noise and vibration are caused mainly by pressure fluctuations,primarily caused by the exchanged flow times between the runner and guide vane,according to the significant frequency of harmonic excitations in the flow domains of the runner,guide vane,and the inlet and outlet pipes.Furthermore,the spectral analysis shows that the radial force frequency is close to the blade passing frequency and also increases radially outward since peak values were recorded in this region.Minimal radial force amplitudes were recorded when z=3 across all flow conditions,making this configuration suitable for smooth and reliable operation.The unstable pressure and force pulses that affect the vibration and noise produced in the turbine are caused by the flow exchange between the guide vane and the runner.5.The results from the entropy production analysis revealed that high entropy losses are generated in the runner flow passage compared with the guide vane and the outlet pipe passage of all three runner blade numbers.The entropy produced in the runner domain is mainly caused by flow separation,wake characteristics,and velocity due to vortices.Furthermore,the losses visualized at the blade leading edge and the suction side are higher than the blade trailing edge and the pressure side.The results further show that energy loss increases with a decrease in blade number and an increase in flow rate.Peak turbulence dissipation of 581 W/m³ and 39.11 W.m^-3 were recorded at the leading edge and flow cascade when z=2 at the design flow rate.In contrast,the least dissipation values were recorded for z=4.6.The design of runners for high specific speed Kaplan turbine poses several challenges since design constants like specific peripheral velocity（u_s）and specific absolute flow velocity（c_s）through the runner,related to specific speed（n_s）,are beyond existing standardized limits.The specific velocities（u_s and c_s）required to design a 2m,5 kW Kaplan turbine suitably with a flow rate of about 300～400 1/s falls within the KapanHAWT（Horizontal Axial Wind Turbine）interface.HAWT design methods and computational fluid dynamics（CFD）optimization was employed to obtain the required us and cs after three failed runner designs（ver.01,ver.02,and ver.03）.The results show that ver.03 came closest to the 5-kW requirement with 4.3 kW at 2 m.Two optimization approaches are referred to as ver.03.1 and ver.03.2 were considered.Firstly,the application of mild curvatures with us and cs of 3.09 and 0.66,respectively（ver.0.3.1）,proved successful with 7～10%and 500 W increase in both efficiency and power,achieving 4.94 k W at 2 m.The second approach（ver.03.2）of reducing both cs and us to 0.59 and 2.67 shows a drop in power and efficiency,failing to meet design requirements.Based on the results,reducing cs and us below 0.6～0.67 and 3～4,respectively,is detrimental to the optimal performance of high-specific speed Kaplan turbines.In summary,the main objective of the dissertation is to investigate the classical Kaplan design theory and unsteady flow field of a low head-high specific speed AFT.The proposed design has reduced uncertainty by providing new design constants（u_s and c_s）and an approach for high specific speed-low head AFT.