Experimental Feasibility Study of Radial Injection Cooling of Three-Pad Air Foil Bearings

Abstract

The foil bearing (FB) is one type of hydrodynamic bearing using air or another gas as a lubricant. When FBs are designed, installed, and operated properly, they are a very cost-effective and reliable solution for oil-free turbomachinery. Because there is no mechanical contact between the rotor and its bearings, quiet operation with very low friction is possible once the rotor lifts off the bearings. However, because of the high speed of operation, thermal management is a very important design factor to consider. The most widely accepted cooling method for FBs is axial flow cooling, which uses cooling air or gas passing through heat-exchange channels formed underneath the top foil. The advantage of axial cooling is that no hardware modification is necessary to implement it, because the elastic foundation structures of the FB serve as the heat-exchange channels. Its disadvantage is that an axial temperature gradient exists on the journal shaft and bearing. In this paper, the cooling characteristics of axial cooling are compared with those of multipoint radial injection, which uses high-speed injection of cooling air onto the shaft at multiple locations. Experiments were performed on a three-pad FB 49 mm in diameter and 37.5 mm in length, at speeds of 30,000 rpm and 40,000 rpm. Injection speeds were chosen to be higher than the journal surface speed, but the total cooling air flow rate was matched to that of the axial cooling cases. Experimental results show that radial injection cooling is comparable to axial cooling at 30,000 rpm, in terms of cooling performance. Tests at 40,000 rpm reveal that the axial cooling performance reaches saturation when the pressure drop across the bearing is larger than 1000 Pa, while the cooling performance of radial injection is proportional to the cooling air flow rate and does not become saturated. Overall, multipoint radial injection is better than axial cooling at high rotor speeds.