Boundary Layer Transitional Flow in Gas Turbines
The distinguishing features of transition in turbomachinery flows are that they are unsteady on time scales longer than typical eddy turnover times, occur in harsh and highly disturbed environments at awkward Reynolds numbers, and are generally three-dimensional (even in the mean). As a consequence, the flow is a veritable-fluid-dynamical ‘zoo’, characterized by separation, reattachment, transition, relaminarization , retransition, etc., all often occurring in the same flow. But how important is transition research for the turbomachinery industry? GE compressor tests (made by Halstead) showing transition extending over 60% of the blade chord, and estimates of potential improvement inefficiency by several percentage points; considering how widely turbomachines are used in energy conversion and propulsion systems, significant economic and environmental benefits are possible. It is found out that the ‘lack of ability to predict the location of boundary layer transition for components in gas turbine engines is impeding our ability to gain maximum benefit from our design effort. If a complete computational fluid dynamics (CFD) design tool incorporating transition were to be available, it is foreseen airfoil designs with higher blade loading that would reduce part count and improve efficiency. It is estimated that a 1% improvement in the efficiency of a low pressure turbine would result in a saving of $52,000 per year on a typical airliner. Improved transition technology was thus very relevant. So keeping in mind such important issue like transition phenomena in design and operation of gas turbines, it is the aim of this review paper to elucidate the recent research activities in this area.