24th International Colloquium Tribology - January 2024 117 A Study on the Effect of Surface Tension on the Drag Torque of Wet Clutches Nikolaos Rogkas 1* , Vasilios Spitas 1 1 Laboratory of Machine Design and Dynamics, School of Mechanical Engineering, National Technical University of Athens, 9 Iroon Polytechniou, Athens, 15780 Zografou, * nrogkas@mail.ntua.gr 1. Introduction Wet clutch technology is used to transfer power through friction between two rotating components and many advanced power transmission systems incorporate it to ensure high torque capacity and smooth gear shifts [1, 2]. However, wet clutches are characterized by power loss in their disengaged state due to the viscous shear stresses of the lubricant caused by the rotation of the discs, reducing considerably the overall efficiency of the transmission [3]. Therefore, decreasing drag torque is a primary goal of designers of wet clutches. Drag torque depends on the operating conditions [4, 5] (i.e., rotating speed, film thickness, flow rate), the texturing characteristics of the friction surface (i.e., grooves, dimples etc.) [6] and finally the properties of the lubricant [7]. Usually, researchers tend to use the drag torque-speed relation to investigate the effect of certain parameters. Under usual conditions, drag torque increases linearly in the low-speed region, reaching a maximum value, in the order of 1 Nm. However, when the rotating speed reaches a critical value, drag torque drops rapidly due to aeration effects. In wet clutches, aeration is promoted when sub-atmospheric pressure is developed near the outer radius leading to the suction of air and the development of a two-phase air-ATF flow [3, 8, 9]. The calculation of the critical rotating speed and the overall estimation of the drag torque-speed relation for various operating conditions or grooves characteristics has been pursued by researchers over the last decade by developing simplified analytical models [3, 8, 10], by attempting 2D CFD two-phase simulations for flat discs [11, 12], or by conducting single-disc experiments [9, 13]. More recently, the use of comprehensive 3D CFD two-phase simulations has also been attempted, however, the associated simulation cost is considered extensive [9, 13]. Concerning the properties of the lubricant, besides the well-understood effect of viscosity, most of the available studies either neglect the effect of surface tension or consider a fixed value of this parameter which limits the potential to derive generalized conclusions. The scope of this paper is to address this issue conducting experiments considering the case of non-grooved discs. The CFD simulations are performed utilizing the commercial software ANSYS Fluent, whereas the lubrication experiments are conducted in a prototype test rig. Two in-house developed oils of equal viscosity and density but varying surface tension coefficients are compared regarding their drag-torque measurements. Both the CFD and the experimental results indicate that a deviation of surface tension in the order of 15% may lead to a drop in drag torque even up to 90%. Besides measuring the torque, a high-speed video recording is used to identify the flow characteristics of the two cases. The results of this paper highlight the potential to further improve the technology of wet clutches by controlling a so-far unexplored parameter: the surface tension coefficient of the lubricant. 2. 2D CFD model results Two-phase CFD simulations were conducted using the commercial software ANSYS Fluent. The study adopted a 2D axisymmetric analysis including swirl, considering laminar flow conditions. The Volume of Fluid (VOF) method was employed to accurately track the air-ATF Fig. 1 showcases the results of the CFD simulations, presenting a drag torquespeed graph for the two examined fluids, with surface tension coefficients of 30 mN/ m and 35 mN/ m, respectively. The results suggest that as the rotational speed surpasses a critical value, the drag torque experiences a noticeable drop which is associated with the development of a two-phase flow (suction of air from the outer radius). The drop exhibits a similar slope for both cases under examination. However, it is also important to note that the surface tension coefficient significantly influences the drag torque by affecting the critical aeration speed. Specifically, for σ = 30 mN/ m, the critical speed is recorded at 550 rpm, while for σ = 35 mN/ m, the critical speed almost doubles, reaching up to 1000 rpm. Figure 1: Drag torque-speed relation for two lubricants with different surface tension coefficients. 118 24th International Colloquium Tribology - January 2024 A Study on the Effect of Surface Tension on the Drag Torque of Wet Clutches 3. Experimental results The experiments are carried out using a prototype single-disc test rig, which allows for the adjustment of critical parameters such as rotating speed, discs clearance, and inflow rate. The setup involves the flow between a rotating and a stationary (transparent) disc. Flow patterns are monitored using a high-speed camera (up to 2000 fps). Two images from the flow visualization are displayed in Fig. 2, representing 250 rpm (Fig. 2-a) and 700 rpm (Fig. 2-b) respectively, with a consistent film thickness of 350 μm for both cases. Fig. 2 illustrates the impact of rotating speed on aeration, indicating that as the rotating speed escalates from 250 rpm to 700 rpm, the flow transitions from a single-phase to a two-phase air-ATF flow, marked by a sharp interface of the two phases, positioned near the mean radius. Figure 2: Flow characteristics at two different rotating speeds: a. 250 rpm, b. 700 rpm. The formation of the air- ATF interface in a region close to the mean radius may be observed. 4. Conclusions This study examined the effect of surface tension on the drag torque of wet friction clutches considering non-grooved discs. This was achieved by conducting 2D CFD simulations and single-disc experiments. The findings emphasized the significant impact of the surface tension coefficient on the drag torque, thereby encouraging further exploration of this subject. References [1] Rogkas N., Vasilopoulos L., Spitas V. A hybrid transient/ quasi-static model for wet clutch engagement. 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