Design Optimization of Blade Tip in Subsonic and Transonic Turbine Stages - Part II: Flow Physics and Augmented Aerothermal Integral Objective Function

In Part I, a companion paper of the two-part article, a subsonic tur-bine stage and a transonic one conditioned at the same Reynolds number, flow coefficient, loading coefficient and reaction, but two different exit Mach numbers are designed to provide a direct contrast between a high-subsonic and a transonic flow conditioning for rotor blade squealer tips. In the present paper as Part II, further analyses are carried out to address the main issues of interest arising from Part I: firstly, to identify the driving flow physical mechanisms for the contrasting aerodynamic efficiency sensitivities of the two stages; and secondly to seek a more suitable heat transfer objective function for the tip aero-thermal design optimization, given the seemingly strong conflicts among those conventionally adopted heat transfer objective functions. Two counter-rotating tip vortical structures, the pressure side vortex (PSV) and the casing-driven cavity vortex (CCV), are shown to impact the aero-performance differently between the two stages. For the subsonic stage, the leakage flow is strongly affected by a stronger residual PSV at the squealer cavity exit. For the transonic stage however, the tip choking in limiting the OTL mass flow and favorable pressure gradient in a transonic flow over a separation bubble led to a much stronger and more persistent CCV and thus lower aerodynamic effectiveness of squealer tip for the transonic stage.