Numerical assessment on a hybrid axial throughflow cooling scheme applied to the thermal management of bump-type gas foil bearings

Axial throughflow cooling is a practical thermal management solution for gas foil bearings (GFBs) with ultrahigh rotational speeds and small bearing clearances. The present study conducted a numerical investigation to assess a hybrid axial-throughflow cooling design (both inner cooling flow passing through the hollow shaft and outer cooling flow passing through the rotor-stator gap) for a specific radial bump-type gas foil bearing using the fluid-solid coupled modelling methodology. First, the individual effects of each cooling flow (i.e., the outer cooling mode only and the inner cooling mode only) on the GFB thermal behaviours are directly compared under a fixed rotational speed of omega = 1 x 105 rpm with a preset eccentricity ratio of epsilon = 0.9. The outer cooling mode exhibited superior cooling efficiency compared with the inner cooling mode with the same cooling air usage, albeit at the cost of a significantly greater flow pressure drop. Second, the conjugate roles of the hybrid cooling flows on the GFB thermal behaviours are related to the total cooling air mass flow rate of mtotal = 10 kg/h. Nine flow distribution relationships were comparatively studied between the two cooling flows under the same preset static bearing load of F = 31 N, wherein the percentage of the outer cooling flow (mouter/mtotal) varied from 10 % to 90 %. The heat removal pathway of the outer cooling flow was found to be dominant. The outer cooling flow exhibited a heat removal proportion close to 60 % when its distribution percentage was 30 %. The results of a comprehensive performance evaluation that considered the peak temperature reduction and cooling air pressure drop suggest a favourable distribution percentage of the outer cooling flow in the range of 30-40 %.