Inside an automobile engine, there are a camshaft, a crankshaft, a chain sprocket, etc. that transmit driving force. There are many rotating parts installed there. For example, there are gears soaking in engine oil, and bubbles are generated by rotating gears which drag air into engine oil. That raises void fraction of the engine oil. The void fraction is the gas phase fraction per unit volume. In general, high void fraction causes decreasing performance of the engine oil, such as lubrication, cooling and pressure transmission. Although it is required to reduce void fraction and establish a defoaming technique, the processes of bubble generation have not been clarified. One of the reasons is difficulty to measure void fraction and other physical parameters in oil flow using a real engine. Therefore, the influence of rotating speed, number of teeth, and kinematic viscosity of test fluid on bubble generation behavior is not grasped in detail.
In this study, the motivation is to understand the mechanism of bubble generation behavior around a rotating body. Detailed visualization was conducted around a rotating body soaking in fluid. The actual phenomena were reproduced by using silicone oil having the same kinematic viscosity of real engine oil. From the visualization using a high speed video camera, there are some characteristic phenomena such as dragging air, subdividing bubbles, and bubbles coalescence. By using observed images, we measured bubble diameter and bubble number density with image processing. Rotating speed, shapes of the rotating bodies, and kinematic viscosity of the test fluids affected on bubble generation behavior. In particular, as the rotating speed increased, the bubble diameter decreased and the bubble number density increased.