Aero-thermal-mechanical interactions in ultra high-speed micro gas turbines

Abstract

Aero-thermal induced mechanical response of engine components in an ultra high-speed micro gas turbine engine system is assessed. Scaling down gas turbine engines for high performance requirement dictates substantial thermal-induced effects on engine operation due to high temperature gradient relative to that in conventional large gas turbine engines. Experiments indicate that the sustainable operation is limited by mechanical response of shaft-bearing housing system. It is hypothesized that this is due to thermal-induced mechanical deformation of shaft-bearing housing that results in bearing clearance variation that differs from the design intent. An unsteady CFD conjugate heat transfer computation of flow and temperature distribution in the engine system is first implemented; this is followed by determining the corresponding mechanical deformation of engine components based on finite element analysis. The computed result shows that at the beginning of the engine start-up process, radial expansion of the shaft is larger than that of the bearing housing, resulting in a smaller bearing clearance. Toward steady-state operation, a larger bearing clearance is observed. The computed results and experimental observation are in agreement thus confirming the hypothesis. The key controlling non-dimensional parameters characterizing the aerothermal-mechanical interaction and response are identified using a reduced order model that yields thermal-induced mechanical deformation in agreement with the unsteady computations. For geometrically similar engine system, the controlling thermal and structural parameters consist of: (1) shaft fin parameter, (2) housing fin parameter, (3) ratio of heat diffusivity of housing to that of shaft, (4) 3 cooling flow parameters, and (5) ratio of coefficient of thermal expansion of the housing to that of shaft. The non-dimensional parameters serve as a guideline for developing strategies for controlling bearing clearance under the acceptable margin, including selecting shaft and housing materials with appropriate properties as well as tailoring the cooling flow. An approximate scaling rule for thermal-induced shaft-bearing housing clearance variation in engine of various sizing is formulated.