Effects of rotor tip blade loading variation on compressor stage performance

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 117-119).Changes in loss generation associated with altering the rotor tip loading of an embedded compressor stage is assessed. Steady and unsteady three-dimensional computations, complemented by control volume analyses, for varying rotor tip loading distributions provided results for determining if aft-loading rotor tip would yield a stage performance benefit in terms of a reduction in loss generation. Aft-loading rotor blade tip yields a relatively less-mixed-out tip leakage flow at the rotor exit and a reduction in overall tip leakage mass flow hence a lower loss generation; however, the attendant changes in tip flow angle distribution are such that there is an overall increase in the flow angle mismatch between tip flow and main flow leading to higher loss generation. The latter outweighs the former so that rotor passage loss from aft-loading rotor tip is marginally higher unless a constraint is imposed on tip flow angle distribution so that associated induced loss is negligible; a potential strategy for achieving this is proposed. In the course of assessing the benefit from unsteady tip leakage flow recovery in the downstream stator, it was determined that tip clearance flow is inherently unsteady with a time-scale distinctly different from the blade passing time. The disparity between the two timescales: (i) defines the periodicity of the unsteady rotor-stator flow, which is an integral multiple of blade passing time; and (ii) causes tip leakage vortex to enter the downstream stator at specific pitchwise locations for different blade passing cycles, which is a tip leakage flow phasing effect. Because of an inadequate grid resolution defining the unsteady interaction of tip flow with downstream stator, the benefit from unsteady tip flow recovery is the lower bound of its actual benefit. A revised design hypothesis is thus as follows: "rotor should be tip-aft-loaded and hub-fore-loaded while stator should be hub-aft-loaded and tip-fore-loaded with tip/hub leakage flow angle distribution such that it results in no additional loss". For the compressor stage being assessed here, an estimated 0.15% enhancement in stage efficiency is possible from aft-loading rotor tip only.by Aniwat Tiralap.S.M

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

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2020Supervised by Choon Sooi Tan. Cataloged from the PDF version of thesis.Includes bibliographical references (pages 123-125).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.by Aniwat Tiralap.Ph. D.Ph. D. Massachusetts Institute of Technology, Department of Mechanical Engineerin