Research On Flow Characteristics And Performance Of Drag-type Hydraulic Rotors

The hydraulic rotor is the key component of the hydraulic-energy utilization system.Although lift-type rotors possess high energy-utilization coefficient,their shortage in the self-startup capability and the operation stability is perceivable.Moreover,the manufacturing of lift-type rotors is not an easy task.In comparison,the drag-type rotor is featured by simple structure,good start-up capability and low maintenance cost.In recent years,it has become a product of interest for the projects of small-scale hydraulic-energy utilization.Thus far,a huge amount of literature has been devoted to the application of the drag-type rotor for wind energy utilization;nevertheless,for the utilization of hydraulic energy,the research of the drag-type rotor has rarely been reported,particularly in the aspect of flow experiment.Flow is significantly related to the energy conversion and operation stability of the rotor.Flow patterns near the rotor serve as a straightforward reference to the assessment of the operation ability of the rotor and the optimal design of the rotor.The emphasis of the dissertation is placed on the flow characteristics of the drag-type rotor.Advanced measurement and numerical simulation techniques are used to obtain flow data associated with the drag-type rotor.The factors influencing the conversion of the kinetic energy of water are analyzed.The relationship between the flow and the operation performance of the rotor is thereby established.Major work and conclusions in the dissertation are listed as follows.(1)Two drag-type rotors,namely Savonius rotor and Bach rotor,were investigated.The experiment models of the two rotors were designed and manufactured using the material of copper alloy.Based on the platform of a water tunnel and using particle image velocimetry(PIV),flows around the two rotors were measured.The light source was deployed above and under the rotor respectively to acquire an entire flow field.The effects of the upstream velocity and the rotor setting angle were considered.Based on the experimentally obtained flow velocity distributions and flow patterns,a comparison was enabled between the two rotors.It is demonstrated that vortices of various scales arise downstream of the rotor.For different rotor setting angles,imparity is remarkable in terms of the vortex profile and the position of the vortex core.Immediately downstream of the rotor,large-scale vortices are predominant.As the upstream velocity increases,the vortices deform and the wake flow changes its shape as well.The interaction between large-scale vortices gives rise to smaller vortices,aggravating the dissipation of flow energy.Similarity is indicated between the flow patterns downstream and upstream of the two rotors,respectively.The flow field around the Bach rotor is relatively stable as the operation condition is varied.(2)Computational fluid dynamics(CFD)technique was used to simulate flows near the two rotors respectively and three-dimensional steady simulation was performed as a preliminary step.The dimensions of the geometric model are identical with those of the experiment model.Moreover,boundary conditions for the numerical simulation are in accordance with the experiment conditions.The grid independence examination was conducted and then the validity of the numerical scheme was confirmed with reference to the experiment results.With steady simulation,distributions of velocity and static pressure were obtained,replenishing flow data associated with the rotors considered.With the non-transparent experiment models,characteristics of the flow near the rotor cannot be fully described using the PIV system.In this context,CFD results are used to fill such a gap.The application of CFD enables the construction of velocity distributions over the entire flow field.Furthermore,static pressure distributions over the advancing and returning blades are obtained thereby.With static pressure distributions,key factors influencing the rotor rotation are explained.The effects of the rotor setting angle on the torque performance of the rotor are substantiated.The capability of the two rotors in generating torque is evaluated.(3)Unsteady flow simulation was performed for the two rotors considered.Therefore,transient flow parameter distributions,variations of output torque and power coefficient with the rotation of the rotor were obtained.Fluctuations of output torque and power coefficient in one full rotation cycle were illustrated quantitatively for the two rotors.A correlation analysis was made between fluctuations of the rotor operation parameters and flow characteristics.The capability of absorbing the kinetic energy of water was assessed and the two rotors were compared in terms of rotor performance under the conditions resembling practical cases.The practical operation of the two rotors is understood via unsteady velocity and static pressure distributions.Periodic variation of the output torque arises as the rotor rotates,as is shared by the two rotors.Within a full rotation cycle,the output torque of the two rotors fluctuates obviously.In comparison,the output torque of the Bach rotor is large and the range of the rotor setting angle associated with negative torque is narrow.The power coefficient of the Bach rotor is relatively high and the maximum power coefficient is higher than that of the Savonius rotor by 23.8%.A joint application of flow measurement and numerical simulation is implemented in the dissertation to investigate the drag-type rotor that operates in water.Both flow characteristics and rotor performance are investigated for the Savonius rotor and the Bach rotor as they absorb the kinetic energy of water.Primary factors influencing the performance of the two drag-type rotors are analyzed.The relationship between flows and rotor performance is explained.The study deepens the understanding of the flow principles of the drag-type rotor and provides a helpful reference to the optimal design of the drag-type hydraulic rotor.