Role of cross-drilled holes in enhanced cooling of ventilated brake discs

As constructive guidance for brake design engineers to improve brake disc cooling, understanding of thermo-fluidic behaviors associated with ventilated brake discs has attracted much attention. In this study, a systematic comparison of the thermo-fluidic characteristics between standard and cross-drilled ventilated brake discs incorporating radial vanes is carried out using numerical simulations. Mechanisms for heat transfer enhancement by the cross-drilled holes are clarified. To validate the numerical model, a series of experiments are also conducted. The gradient of axial pressure is found to drive the cooling air into the ventilated channel through cross-drilled holes. Such a special flow accelerates external boundary layer flow over the rubbing surface, enhancing therefore local heat transfer. Additionally, the through-hole flow removes heat via the surfaces of the holes while air jets issuing from the cross-drilled holes deflect to suction sides of the vanes, improving further local heat transfer. However, blockage by the jets not only reduces the pumping capacity through ventilated channel inlet but also causes a low-momentum wake downstream each jet. Hence, local heat transfer on the inner surface of each rubbing disc is deteriorated. Eventually, relative to the standard brake disc, the cross-drilled brake disc exhibits a 22-27% higher overall Nusselt number within the typical operating range of 200-1000 RPM. The contribution of each mechanism to overall heat transfer enhancement is quantified.