Internal Flow Simulation And Performance Prediction Of A High-speed Small Scale Hydraulic Coupling

A hydraulic coupling is a kind of flexible coupling, and it is composed of the pumpwheel and turbine runner. The rotating mechanical power is transmitted by the workingfluid which circulates between the pump and the turbine. As there is no powerconsumption component such as guide wheel, and the working medium is fluid in ahydraulic coupling, it has many advantages of high energy transmission efficiency, longservice life and good adaptability, etc.Because of its advantages compared to the traditional coupling, the hydrauliccoupling has become the most popular transmission energy-saving technology in theworld and it is widely applied and studied in the field of engineering and scientific. Inthe “National Basic Research Program of China”(973Program), the hydraulic couplingis used to connect the power turbine and the engine crankshaft in the internalcombustion engine where uses the “waste heat recovery technology”. The hydrauliccoupling is playing a key role in the system. For the whole system to be compact andefficiency-optimized, the research emphasis should focus on the miniaturized,high-speed and high-power hydraulic coupling.Firstly, computational fluid dynamics (CFD) technology was used to simulate thehydraulic performances of the YOX150hydraulic coupling. The geometricmodels of the pump wheel and turbine runner were built respectively.Structured hexahedral mesh was adopted to ensure the quality of the mesh, andgrids were refined in the boundary layer on each blade surface to ensure thecompatible y+values with the chosen turbulence model. The flow in a partiallyfilled hydraulic coupling is essentially a gas-liquid two-phase flow, and thetwo-phase simulation will take more time and can be hard to converge. So byconsidering the similarity principle, in this thesis, SST k turbulencemodel was employed for steady simulations of full filled conditions. Byobtaining the characteristic curves of hydraulic coupling with different inputspeeds (3000r/min and7500r/min), the influence caused by speed increasingcould be figured out. Meanwhile, RNG k turbulence model and VOFtwo-phase model were employed for unsteady simulations of partially filled conditions at some specific operating points. By analyzing the two-phase flowfield and figuring the hydraulic losses, the foundation for further modificationof the hydraulic coupling would be laid.At the same time, by building the test bench, low-speed (3000r/min)characteristic test was developed. The test results were in good agreement withthe numerical results, which proved the accuracy of the numerical simulation.It is concluded that the similarity principle of the efficiency of the hydrauliccouplings does not apply in this case due to the relatively high rotating speed and smallgeometric specifications, which means that torque factor of pump does not remainconstant. And flow conditions have some similarity when the input speed is different andspeed ratio is not. Near the blades and walls, some phenomena such as flow separationappear which cause the losses. The two-phase flow transforms to large circulation in thehydraulic coupling with speed ratio decreasing.When input speed is7500r/min, thetransmitted torque of the hydraulic coupling designed by traditional method is only17kW at the rated condition, which is far lower than the international level and cannotmeet the practical needs.Based on the analysis of the simulating results, the original hydraulic coupling wasremodeled. By increasing the number of blades and using the three-dimensional bladesinstead of traditional straight blades, the numerical results showed that the flow fieldwas improved,and the transmitted power was increased significantly.