A Microturbine Rotor-Bearing System Analysis

Abstract

A microturbine of 12-pound thrust was developed for the Unmanned Aerial Vehicle (UAV) applications. Recent tests of the microturbines reveal problems associated with rear ball bearing integrity after extended run times. The microturbine rotor design originally calls for a critical speed margin of at least 15∼20% to prevent excessive vibrations. However, the microturbine was using an existing turbocharger rotor component with unknown margins. Therefore, the purpose of this paper is to perform both theoretical and experimental analyses of the dynamic characteristics of the 12-pound thrust microturbine rotor-bearing system. This rotor-bearing system analyses will start with a finite element (FEM) rotor-bearing system dynamic model, then using modal testing and dynamic engine test to verify the analysis results including critical speed map and bearing stiffness. In this paper, the rotor-bearing system dynamic model will be established under an inertia coordinate system. Through finite element method, this model can be used to predict natural frequencies, critical speed map, and bearing stiffness. Also, under free-free condition, a modal testing will be performed, and its results are used to compare with the FEM model. Then the gyroscopic moment effects are included in the FEM model to calculate the critical speed map. Finally the critical speed map is used to compare with the results of the dynamic experiments of the 12-pound thrust microturbine engine and the bearing stiffness is estimated through an optimization approach. Examination of the microturbine engine and recent product developments indicate that thrust performance and engine life goals can be improved to upgrade the present design. With the rotor-bearing system analysis, the goal of increasing the current engine life and improved performance is sought as a practical goal for the microturbine design.