Compressible Brake Fluid Turbulent Flow Simulation and Experimental Verification on Brake Bleeding Performance Improvements of an EPB Caliper

Brake bleeding is the process of removing air bubbles present on hydraulic brake systems from the master cylinder to the calipers of a vehicle, including the brake pipes and hoses. This is very important procedure affecting on brake performance, but still has been a key issue in automobile industry for last decades because reaching best bleeding performance has a limit that there is always remaining air in brake system. In this paper, it is reported on numerical and experimental investigations into the topic of bleeding performance improvements. Compressible brake fluid turbulent flow simulation with two-phase mixture model was performed to investigate the details of the bleeding performance drop during its cycles. The rig test of the hollow cylinder was carried out in order to secure the brake consumption amount curve whose results were used for the criterion of the parametric simulations using Tait equation to estimate the property of the brake fluid with the bulk modulus of 19,535 bar and 0.00016%. It was observed that the experimental curve data from the rig test of the hollow cylinder is divided into two regions with high and low compressibility, and more volume change in the low region below 1 bar is required to gain the same pressure variation due to the compression of the tiny air bubbles. The improved design of the nut-spindle with 6 holes in circumferential direction was drawn for the better removal of the trapped air. The simulation of the improved model showed the manual bleeding performance improvements of 18.9% than the baseline model due to the holes effect on circulation of the trapped air. It was experimentally verified that the air bubbles from their visualization are compressed to form the smaller size bubbles in the process of pressurization and gathered on the topmost side, whereas they are again expanded to form the larger size bubbles in the process of pressure release and spread into the wider space. The bleeding performance for the improved model was also experimentally verified to be effective up to approximately 51% through the caliper performance tester.