Research On The Torque Distribution Characteristics Of Dual-Turbine Torque Converter

Dual-turbine hydrodynamic torque converter is a kind of power shunt fraction hydrodynamic torque converters, which is one of the important parts in transmission system. Compared with single hydrodynamic torque converter, dual-turbine torque converter has many advantages as large adjusting range, big starting torque ratio, and wide high-efficiency region, etc, which improves vehicle power performance and fuel economy, and be widely used in construction vehicles such as loaders and bulldozers. In addition, when automatically shifting with the change of external load, dual-turbine torque converter itself is equivalent to two speed steps respectively so as to reduce the gearbox planetary, thus it simplifies the structure and manipulation.With the rapid advance of construction cause in our country during recent years, rapid developments of construction machinery are promoted, and the nine major vehicle sales of which have an annual increase over 11% evenly. According to the prediction of "the Eleventh Five-Year Plan" issued by our construction machinery industry, only the annual demand of earth-moving vehicles will be up to 14,000 by 2010; if taking the export and military use into consideration, the total need for these vehicles will be more, and it will keep increasing at the annual rate of 15% during "the Eleventh Five-Year" period.Based on the fact above, the three-dimensional numerical simulation and analysis of the flow field in the dual-turbine hydrodynamic torque converter is implemented with numerical fluid dynamics techniques in this paper, the flow law of internal flow field in the hydrodynamic torque converter is understood in detail, the torque distribution law between the first turbine and the second turbine is analyzed, and the factors influencing first torque idling loss at high speed is discussed. On this basis, the optimization design of dual-turbine hydrodynamic torque converter is conducted, meanwhile the hydrodynamic transmission experiment is made to verify theoretical calculation, and the design method is set up to improve the designing strategy, performance and efficiency of dual-turbine hydrodynamic torque converter.1. Three-dimensional flow field numerical simulation of the dual-turbine hydrodynamic torque converterDual-turbine hydrodynamic torque converter is multiple impeller turbine machinery with the fluid channel closed, and the internal fluid flow is unsteady three-dimensional viscous flow which is dominated by physical conservation laws as the law of conservation of mass, the law of conservation of momentum, and the energy conservation law. For incompressible fluid, the heat exchange of system is so little that can be ignored. The continuity equations & momentum equations are known as the Navier-Stokes equations in engineering, which is the most basic equation for incompressible fluid.In order to carry out numerical simulation of turbulence, the NS equations are deal with by Reynolds equation method to receive Reynolds time-averaged equations set in calculation. To make the equations set closed, certain assumption is made such as introducing the model of turbulent viscosity. The most used model is k-εequation, and its control equations include the time-averaged N-S equation and the standard k-εturbulent equation.After control equations is made, the YJSW315 dual-turbine torque converter is taken as an example to discussed the CFD three-dimensional numerical simulation process in details in this paper, which includes numerical simulation assumption, geometric modeling, grid modeling, the algorithm and discrete format selection, boundary conditions setting, identification for the initial flow field and formulation of the convergence criteria. In particular, the non-steady-state solver is used to simulate the internal transient flow properties of the twin hydrodynamic turbine torque converter and the sliding-mesh theory is used to set interface between each working wheel, which makes components with different rotary speed be united and calculated. On the base of flow field numerical solution, the characteristic parameters of dual-turbine hydrodynamic torque converter can be predicted through further numerical calculation.2. Analysis on joint work characteristic of the first turbine and the second turbine of the dual-turbine hydrodynamic torque converter.Detailed analysis of the flow field characteristics and its forming reason of the pump wheels, the first turbine, the second turbine and the guiding wheel channel of the dual-turbine hydrodynamic torque converter is implemented based on numerical calculating results of the three-dimensional flow field in the CFD. Some flow characteristics of the internal flow field are got through this process, which provide a basis to the structural optimization design.The torque distribution law of two turbines under the common working condition can be got through analysis: torque of the first turbine decreases gradually with the speed ratio increasing, and close to zero at the conversion point; torque of the second turbine increases with the speed ratio increasing, and reach to the maximum at the conversion point. It needs to specially notice that the torque sustained by the second turbine under the starting condition may be resistance moment, which hinders the improvement of starting torque, and this phenomenon is not hope to appear. It can be seen from the velocity distribution and the stress distribution of the second turbine that the flow direction in the whole flow channel is from pressure surface to suction surface under starting condition, and the maximum velocity is located in the angle between inner loop and pressure surface, larger range of low speed stall appears in the area of inner loop close to suction surface, and the flow condition is bad; it is seen from the stress distribution that high pressure region distributes in the angle between inner loop and suction surface, which is mainly due to the effect of the exit velocity of the first turbine and the inlet blade angle.In a word, the working principle is deeply understood through analyzing the inner flow of dual-turbine hydrodynamic torque converter at the low speed ratio. The torques of pump, the first turbine, the second turbine and stator as well as their corresponding characteristic parameters are gained by analyzing and calculating different impeller combinations using CFD technique of modern 3-D flow field. Based on comparison of the characteristic parameters of the first and the second turbine working together, the distribution rule of the torque characteristic is summarized, which provides the basis for further optimization.3. Analysis of the characteristics of the idling condition and the energy lossAssuming that the torque sustained by the first turbine is zero at the conversion point from working to idling, the idling conversion point may be found by CFD numerical analysis. The idling speed of the first turbine can be measured by the speed sensor in hydrodynamic transmission test. Likewise, the idling torque sustained by the first turbine can also be got in reverse through the analysis on flow field characteristics within the range of high speed by CFD. The actual torque of the first turbine is existent and the direction is the same as the pump. It can prove that there is energy loss to the first turbine during idling. The trend of the energy loss is described by the Rothalpy constant.4. Optimization design of the cascade system and Experimental studyAiming at the conditions as high starting torque, high efficiency, wide high efficiency area, little resistance moment effect caused by the torque of the second turbine as possible, the circulation optimized distribution method is used to adjust the circulation of each working wheel, which makes the resistance moment of the second turbine reduced as small as possible at low speed ratio after adjusted. Through CFD numerical simulation, cascade system with better performances is determined. The result shows that the performance of the optimization dual-turbine hydrodynamic torque converter is improved slightly.The characteristics test of the optimization dual-turbine hydrodynamic torque converter is done, the characteristic curve is got, and the experimental result is compared with the prediction result. It verifies that the true characteristics of the dual-turbine hydrodynamic torque converter can be reflected by the method of CFD prediction.