Comparative Analysis of Bearings for Micro-GT: An Innovative Arrangement

Abstract

Microgas turbines are a widespread technology in cogenerative and propulsion applications. Bearings are a key factor in their design and development. The aim of the present research work is the development of the support system for a typical microturbine intended for power generation. To this goal, the present chapter defines the typical requirements of the machine and, afterward, describes the different technologies available to develop the support system of a reliable microturbine. Conventional (rolling element and oil-film) supports and cutting-edge (magnetic, aerodynamic, and aerostatic) bearings are reviewed. Particularly, their suitability to the operating conditions is compared by means of a literature review and elaboration of the relevant data. By analyzing all this information, a new concept for the design of a micro-GT support system is devised. Instead of using a single type of bearing as usual, the new system includes different types in order to take advantage of the best characteristics of each one and, simultaneously, to minimize the effects of the relevant flaws. The innovative support system requires a suitable bearing arrangement, which is compared with the conventional ones. The conceptual design of the innovation is completed by a discussion of its advantages, drawbacks, and prospective improvements.

FEM Design of a Cutting-Edge Support System for Micro-GT

“…Compressor and turbine axial loads are the most significant in a microturbine, since they are 10x greater than the radial ones (shaft weight). For the application in this study, due to high axial loads, durations of rolling-element bearings correlated to fatigue phenomenon are very low compared with the life of a micro-GT unit (60.000-80.000 hours) [12].…”

“…Differently, in case B, both thrusts are directed toward the inner side of the machine and the total thrust direction reverses too ( < 0, > 0). As detailed in [12], for different impeller geometries, which may yield different pressures on the clearances between the casing cover and impeller back shroud (backside pressures), the impeller thrusts can be either external or internal, as in cases A and B, respectively.…”

“…As already explained in detail in [12], the main goal of the novel assembly is to separate the axial load exerted by the impellers into two parts. The first part of the thrust directly loads the main axial bearing, without getting through the shaft.…”

“…In [12], we have presented the conceptual design of a new support system that is capable of removing axial load from the air thrust bearing during mixed lubrication regime, thus eliminating start/stop wear. The axial load is shared by two axial bearings of different types.…”

“…The operation of helical splines is analyzed by means of a nonlinear structural model of the hub-shaft assembly and the finite element method (FEM). Table 1 reports the main data of a 100 kW micro-GT unit designed in previous works [12,16]. It also includes the relevant loads, that is, the turbine and compressor thrusts and , respectively, as well as the vertical load .…”