Three-Dimensional Analysis of Multi-Phase Flow between Rotating Disks with Grooved Friction Surfaces

Wet Clutch;Drag Torque;Engagement;CFD;Mechanical Engineering;Engineering;Mechanical Engineering

Wang, PengchuanSchultz, William W ;

University of Michigan

关键词: Wet Clutch;    Drag Torque;    Engagement;    CFD;    Mechanical Engineering;    Engineering;    Mechanical Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/145799/pechwang_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

A multi-physics model is developed for predicting the dynamic behavior of a wet clutch in an automatic transmission system. The prediction and control of clutch dynamics are of major significance in fuel efficiency. Therefore, the optimal operational characteristics are sought to reduce the viscous drag and improve the shift quality. The model simulates clutch operations under open and engagement conditions. Various CFD models are reported in the literature, but they are often limited to the analyses of SAE #2 test data, which do not reflect an actual clutch control strategy. The present work overcomes previous limitations by constructing comprehensive computational fluid dynamics (CFD) models, accounting for detailed design geometries.The model consists of various modules depending on the status of wet clutches, which are linked by the dynamic behavior of multiple rotating disks. For open clutches, a multiphase CFD model is developed to simulate wet clutch behavior, accounting for detailed design geometry. The model employs the volume of fluid (VOF) method to capture the interface between the automatic transmission fluid (ATF) and air entrained in the gap between clutch plates. Model setup and simulation parameters, including initial conditions, boundary conditions, and relaxation factors are evaluated in terms of convergence rates. A statistical model is developed to account for the chaotic axial movement of plates in a clutch pack. The model can predict the difference in the drag torque with or without fixed clutch plates.The engagement process is modeled using the open clutch solution as an initial condition. A squeeze-film flow model is developed based on an iterative method to reflect the real control strategy. The iterative scheme allows use of the actuator piston force as an input to the computation of the ATF film change. Given the external force responsible for plate movement, the squeeze velocity is calculated by trial-and-error until the external force is balanced by internal fluid stresses. The results are validated using analytical solutions under various boundary conditions. The model captures the flow in micro-channels created by the grooves on the friction material surface. The model also explicitly accounts for the effects of the plate mass on the squeeze film process. The flow through the porous friction material is simulated using Darcy’s law. The influence of surface roughness and mechanical contact are simulated based on the real contact area. The heat generated at the interface between friction and separator plates is computed through both conduction and convention processes. Finally, these models are combined with the baseline squeeze-film flow model to create an integrated clutch engagement model.A series of experiments for calibration and validation of the model were performed at two testing facilities in Japan. Extensive visualization tests were performed to investigate the formation of bubbles in the interface, and validate the volume fraction computed by the model. Convergence of the open clutch model is demonstrated, capturing the peak drag location as a function of rotating speed, until the phase fraction drops to a small value. The simulation results from the statistical model compare satisfactorily with experimental data. An advanced engagement bench test is conducted using in-vehicle clutch slip and pressure profiles for specifically replicating the torque and inertia phases of shifting. It is shown that the CFD model together with the advanced clutch bench testing provide a valuable insight into in-vehicle clutch engagement behaviors.