The Influence of Grooves on the Fully Wetted and Aerated Flow Between Open Clutch Plates

In a disengaged or open clutch mode, one plate rotates while the other is stationary. This speed difference between the two plates (on the order of 1000 rpm) and the small clearance between them (on the order of $100μm$⁠) results in large velocity gradients. Transmission fluid is passed between clutches since during reengagement; this oil provides lubrication and carries heat away. However, during the disengaged mode the shearing of this oil as it passes between the plates results in viscous drag that wastes power. Introduction of air between the two plates during the disengaged mode, referred to as aeration, is the most significant way of reducing this friction drag due to low viscosity of air compared with oil. Open clutch drag reduction is enhanced by providing grooves on one of the plates since they are known to promote aeration. Yet, a continuous supply of lubrication oil is necessary, even during disengagement. This study examines the underlying processes responsible for the oil flow between grooved disks and possible aeration through a combination of experiments and numerical computations. A two-dimensional model of the three-dimensional, single-phase flow between a stationary and a rotating clutch plate is presented, which is capable of describing pressures and shear stress distribution for plates with radial grooved geometries. The computational fluid dynamics code FLUENT^® is used to examine the single-phase and aerated flows between the plates. These results are compared with accompanying experimental observations. We also examine new groove designs to study their efficiency in promoting aeration. Finally, we propose reasons for grooves promoting aeration in clutches.