A Numerical Study And Optimization Of The Flow Characteristics And Heat Transfer In The Internal Cooling Structures Of Turbine Blades

To further improve thermal efficiency and thrust-weight ratio of aero-engines,higher gas temperature at the inlet of the blade cascades is desirable.Correspondingly,high-performance internal cooling techniques have to be developed to prevent blade material from overheating.Given that the introduction of internal channels reduces blade temperature and deteriorates strength/stiffness of the blade concurrently,balancing thermal and mechanical properties of a turbine blade based on proper design is crucial.Focusing on the NASA C3 X stator blade,this thesis first explores numerically the effects of pin-fins on fluid flow and heat transfer in the internal cooling channel,in an effort to improve cooling performance.Based on the validated numerical model,optimizations of internal cooling channels separately with Bezier and circular profiles are carried out.By variation of size and position of these channels,temperature and thermal stress in the blade are reduced concurrently.The main contributions of the present thesis are summarized as follows.First,the effects of relative pitch(L/D)of in-line pin-fin arrays on fluid flow in the internal cooling channel are clarified at various Reynolds numbers.With the increase of relative pitch,vortex shedding downstream the pin-fins shows three distinct patterns.A similar pattern to that of a single circular cylinder is observable at L/D < 1.6.Further,no vortex shedding presents for the upstream pin-fins at 1.6 < L/D < 3.6,whilst vortex shedding takes place for all pin-fins when L/D > 3.6.It is also found that internal channels separately incorporating elliptical and square pin-fins exhibit the lowest and highest pressure drop,respectively.Pressure drop increases with increasing Reynolds number.Subsequently,convective heat transfer in a cooling channel with spherical protrusions and elliptical pin-fins is considered.For a given Reynolds number,numerical results reveal that the introduction of protrusions enhances heat transfer but inevitably leads to evidently higher pressure drop.It is found that optimal thermal performance of the cooling channels is achieved when height-diameter ratio of the protrusions is 0.4,which is superior to that of the cooling channel only with elliptical pin-fins.Finally,thermal-mechanical analysis of the C3 X stator blade is carried out.By comparing profiles of temperature and pressure at mid-span of the blade obtained numerically and experimentally,the most efficient turbulence model,i.e.,the SST k-? model,is found.Based on this model,internal and external thermo-fluidic characteristics of the blade are analyzed.Subsequently,with the average and highest temperature taken as objective functions,the two-dimensional and three-dimensional blades are thermal-mechanically optimized.It is found that the cooling holes with Bezier profile can effectively reduce high temperature regions on main part and trailing edge of the blade but lead to higher thermal stress.On the contrary,the utilization of circular cooling holes is beneficial to reduce thermal stress.However,it is not preferable for effective cooling of high temperature regions due to limitations of its shape.The optimized hole topology can effectively reduce blade temperature while make the stress acceptable.