چكيده به لاتين
Utilizing highly loaded blades in low pressure turbine (LPT) component of a modern turbofan engine is attractive for designers, due to its beneficial effects on maximizing thrust-to-weight ratio and reduction in fuel consumption. This feature of blade along with Reynolds number lower than critical value in flight with cruise condition, are two fundamental factors which result in flow separation in LPTs. To prevent this undesirable phenomenon, accelerating in transition process can be effectively employed. Clocking mechanism, as a passive control method and a step of design process, can be implemented for properly aligning circumferential positions of blades rows relative to a reference frame for increasing aerodynamic performance of axial turbines. As the lack of test-rig required for experimental investigation, three-dimensional numerical simulations have been exclusively used to scrutinize clocking mechanism in this research work. However, to validate numerical approach used in this investigation, comprehensive experimental and numerical studies of flow in a linear cascade of high-lift turbine blades have been conducted. Also, effects of Reynolds number and free-stream turbulence intensity (FSTI) on steady aerodynamic the the turbine blades are studied. Reynolds number and FSTI were ranged between 1.2-3.52×105 and 1.4-3.7%, respectively. Then, unsteady performance of the blades is investigated under various frequencies of upstream wake flows which are generated by moving a series of rods located upstream the blades. Distributions of pressure and skin friction coefficients, intermittency factor and velocity profiles provided to identify flow separation, commencement of transition and possible re-attachment positions. Results show that increasing in Reynolds number at a constant FSTI, or vice versa, causes the total pressure loss and total drag to decrease and lift force to increase. The reason behind these beneficial effects is strongly related to delaying in flow separation and accelerating in commencement of transition on the blade suction side. Unsteady results show that variable kinematic of a wake flow in a transient interval during its convection through the blade passage, leads to increasing in local velocity on the blade suction surface. This in turn causes the lift force and the outlet dynamic pressure to increase. Steady and unsteady clocking analyzes for the second stator of an LPT are conducted using coupling of SST k-ω turbulence model with γ-(Re) ̃_θt transitional model and Zonal-DES turbulence model respectively. Results show an enhancement of aerodynamic efficiency and output power of the second rotor by 0.35% and 0.34%, respectively at optimum clocking position. This unique condition, characterized by impingement of upstream stage wake flows on leading edge of the second stator blades, leads to increasing in total outlet pressure of the second stator up to 0.099% as a result of increasing in outlet dynamic pressure. Regarding with unsteady results, onset points of bypass transition and transition in separated shear layer are, on average, shifted upstream by 5.48% and 47.83%, respectively under optimum clocking in comparison with unsuited one. Furthermore, optimum clocking results in delaying in reverse transition. Examination of intermittency at various distances from the blade suction surface and transition process at a distance of 5.2 μm above the suction side indicates that the outcomes obtained by precise measurement of transition process parameters on the surface can be attributed to flow layers above it. Clocking effectiveness highly depends on geometric and aerodynamic characteristics of fixed and rotating blade rows. Therefore, an in-house computerized code is developed which combines stream line curvature (SLC) and free vortex (FV) methods for designing a high-efficiency 2-stage aero axial turbine in this investigation. Designed turbine consists of a one-stage high-pressure turbine (HPT) and a one-stage LPT. Slight discrepancies were observed between gas dynamics results of the SLC and those of CFD. Total pressure and temperature at the turbine outlet, obtained from SLC method, differed from those obtained by 3D-CFD technique by 13.06% and 1.88% respectively. Regarding time-averaged results, provided by unsteady three-dimensional simulations based on zonal DES turbulence model, total to total efficiency of the designed turbine under worst condition is about 92.87%. This value increased by 0.12% under optimum clocking of the LPT stator. This increasing in efficiency is due to 20.3% reduction in pressure loss of the LPT stator as a result of its optimum clocking position leading to increasing in efficiency of the second stage and its output power by 0.44% and 0.93%, respectively.
