Flow Characteristics at the Pump-Turbine Interface of a Torque Converter at Extreme Speed Ratios

Abstract

The average velocity field at the pump-turbine interface in a scaled version of a truck torque converter was studied. Seven different turbine-to-pump rotational-speed ratios were examined, ranging from near stall (0.065) to overspeed (1.050) so as to determine the effect of the speed ratio on the flow field and on the mass flow rate. Laser velocimetry was used to measure the flow velocity through the pump's exit and the turbine's inlet plane. At the pump's exit, as the speed ratio increases, the high velocities move to the pressure-shell corner and then to both the core-suction and the pressureshell corners. Concentrated velocity gradients are largest at the lowest speed ratio, but areas of velocity gradients are largest near the coupling point. Near the coupling point, the flow field is most nonuniform, which yields a highly periodic flow into the turbine inlet. Above the coupling point, the high velocity remains in the pressure-shell corner but separation is seen to develop at the highest speed ratio. At the turbine's inlet, reverse flow is seen at low speed ratios and is an indicator of flow leakage through the core. Velocity gradients are very large at low speed ratios. As the speed ratio increases to the coupling point, the high velocities remain on the shell side. Above the coupling point, the high-velocity flow migrates from the shell side to the core side. The mass flow rate decreases significantly and nonlinearly with the increase of the speed ratio, but for speed ratios greater than 1.000, the negative slope decreases.Torque converters are commonly used in cars, buses, locomotives, and construction equipment as a mean of smooth torque transmission between the engine and the automatic transmission and to provide torque amplification during start-up conditions. The typical torque converter is a recirculating hydrodynamic, mixed-flow turbomachine (containing both radial and axial flow) with three independent blade cascade elements that govern the internal flow field. These three elements are the pump, which energizes the working fluid and which is connected to the engine; the turbine, which extracts the energy from the flow and translates it into output torque; and the stator which, ideally, redirects the turbine's exit flow back into the pump with a zero flow incidence at a specific design speed ratio. The pump and turbine are rotating at different angular speeds while the stator is either locked or allowed to float freely. Because of the significant curvature of the flow path and the high interaction among these three elements, the internal flow field is highly three-dimensional.The flow field, the torque, and the efficiency curves are normally determined only from the zero speed ratio (turbine has stopped) to the coupling point (turbine torque and pump torque are equal), which typically occurs at a speed ratio of 0.85 to 0.90. Performance characteristics are normally required only in the range from stall to cruise. A torque converter, however, can be operated at high positive speed ratios beyond the cru...

Flow simulation in hydrodynamic torque converter

“…Due to the computer processing power limitation, the turbulent flow model "k-ε" was applied for the calculations of the fluid flow in the turbine. The turbine was selected because of its most complicated shape and a wide range of angular velocity [11].…”