Research On Axial Force Of Variable Speed High-power Hydrodynamic Coupling

Hydrodynamic coupling is a hydrodynamic transmission component with many advantages. Especially variable speed hydrodynamic coupling is a better reliable and economical speed regulator, which is widely used for fans and pumps because of its obvious energy-saving effect. However, greater axial force will be produced during its running, which will accelerates the bearing fatigue and wearing, shortening bearing's life cycle and the coupling's life cycle. In fact, hydrodynamic coupling itself doesn't exist wearing owing to its hydrodynamic transmission, the lifespan and reliability of hydrodynamic coupling is mainly depended on bearing and its sealing. In a word, axial force has a significant impact on hydrodynamic coupling's reliability.Aiming at the axial force calculation of variable speed high-power hydrodynamic coupling, an indispensable part of the special subject"Key Technological Research on Hydrodynamic Variable Speed and Energy Saving of Large Pump and Fan"from the National High Technology Research and Development Plan (863) (2007AA05Z256), in the paper, multi-phase CFD method is effectively made use of to carry out the flow field calculation and analysis of hydrodynamic coupling and reveal its flow law, and axial force exeperimental measuring is carried on. Based on this, the influencing factors to hydrodynamic coupling axial force are discussed so as to find the measures of reducing hydrodynamic coupling axial force. The research method, content and conclusions are as follows:1.Theoretical analysis of axial force in Hydrodynamic couplingHydrodynamic couplings axial force includs the pump axial force and turbine axial force. Both consist of working chamber axial force and secondary chamber axial force. Working chamber axial force can be attributed to two aspects, on the one hand, it is resulted from the static pressure initiated from centrifugal force due to flow in the working chamber, on the other hand, it is produced by the circulated flow with direction variation of velocity and variation of momentum. But, secondary chamber axial force is mainly resulted from static pressure owing to centrifugal force. To accurately calculate the axial force, the key is to get the accurate flow field in both the working chamber and the secondary chamber.It can be seen that from the structural and working characteristics of hydrodynamic coupling that there exists an action and reaction axial force pair between the pump and turbine in the secondary chamber; it is same situation as in the working chamber. Therefore, the resultant force of pump and turbine is an force pair; they are the same in size and opposite in direction. In calculating the axial force, anyone calculation of these two forces will be enough.2.Numerical Calculation of Gas-liquid Two-phase Flow in Variable Speed Hydrodynamic CouplingThe inner flow field calculation of hydrodynamic coupling is the key to decide the axial force, which is unsteady, incompressible and viscous two-phase flow. In solving this complex flow, a multi-phase model Mixture is adopted to describe the gas-liquid continuous flow; for turbulent calculation, a Relizable k-εturbulent model based on Boussinesq eddy viscosity assumption is chosen to characterize the turbulent flow by using Reynolds time-averaged method at the result of shorter calculation time and better calculation results; taking into account the hydrodynamic coupling effect between the pump and turbine, the computational domain of pump and turbine is integrated, while the interface of the pump and turbine is dealt with sliding mesh theory; boundary conditions of blade and wall surfaces are set as non-slip wall condition; the N-S equation based on finite volume method is differentiated by a more stable and accurate second-order upwind scheme; PISO algorithm is used in velocity-pressure hydrodynamic coupling algorithm; the two-phase differentiation equation is solved by variable separation method. Finally, YOCQz465 variable speed hydrodynamic coupling is taken for example to show the complete simulation process. For hydrodynamic coupling secondary chamber flow field calculation, the above method validates. 3.Axial Force and Flow Field Analysis in Variable Speed High-power Hydrodynamic couplingTo identify the value and variation law of hydrodynamic coupling axial force, taking YOCQz465 variable speed hydrodynamic coupling for example, flow field and axial force are respectively calculated at different filling rate and speed ratio, further, the flow field and axial force in working chamber and secondary chamber are analyzed under braking condition, traction condition and rated condition to finally obtain the resultant force of hydrodynamic coupling axial force. The following conclusions can be drawn from the analysis:The axial force of hydrodynamic coupling deceases with the increasing of speed ratio at a constant filling rate; the axial force reaches the maximum at braking condition while minimum at rated condition. Variation law of axial force lies in the contradiction between working chamber pressure and secondary chamber pressure. The pressure in working chamber decrease with the increase of speed ratio because the circulation flow reduces with the increase of speed ratio, which results in decreasing the reaction force due to direction alteration of velocity. However, the pressure in the secondary chamber increases with the rising of speed ratio because of the increment of centrifugal force. To sum up, pressure in working chamber is always greater than that in the secondary chamber, thus, the axial force of working chamber plays a dominant role in determining the direction of axial force, both axial forces of pump and turbine are opposite in direction, tending to disengage the pump and turbine.4.Axial Force Experimental Mmeasuring in Variable Speed High-power Hydrodynamic CouplingTheoretical analysis and calculation results must be verified before application, for this purpose, a series of proof tests are done on YOCQz465 in Dalian Hydrodynamic Machinery Company. An innovative method is proposed for measuring axial force of hydrodynamic coupling by virtue of measuring axial strain of thrust plate. This new method is simple and convenient, doesn't need modification of the original hydrodynamic couplings, has little influence on the structure and properties of hydrodynamic coupling and is suitable for large-scale high-speed hydrodynamic coupling. Analysis of the test results shows that the axial force fluctuation is greater but the axial force average is stable during running, which has a greater impact on its structure and bearings. The comparison of the theoretical results with the experimental ones shows that the calculated values are slightly larger than the experimental ones.The mean error is 8%, and the maximum error is 22%; the direction of axial force is identical. Therefore, the calculation method based on CFD is feasible.5.Influencing Factors on Axial Force in Hydrodynamic CouplingFor the control of greater hydrodynamic coupling axial force, the influencing factors must be found. It is hard to decide the influencing factors, thus, a comparative method is used from the viewpoint of theoretical calculation and flow field analysis by focusing on filling rate, chamber shape, circulation diameter and input speed. The analytical results demonstrates that the greater the filling rate, the larger the axial force, existing approximate direct proportional relationship; chamber shape has a greater impact on the axial force, and the axial force of peach-type chamber is significantly greater than circle-type chamber; circulation diameter increasing brings about rapid increase of axial force, which is proportional to the fourth power of the circulation diameter; the greater input speed will produce greater axial force, being proportional to the square of input speed. Based on the above analysis, measures for decreasing axial force are as follows: the first is to open unloading holes in turbine housing so as to reduce the pressure difference between the working chamber and the secondary chamber; the second is the use of dual-chamber structure which can counterbalance the axial forces.