I just received a question from a consulting company asking for our help: “What is the effect of the gap between the rotor blades and the casing on the performance of the machine?” To answer this question you need to have the right tools and the right experience. At SoftInWay we have both and this is why our customer are satisfied by the speed and quality of our services.
To go back to the question, blade tip losses represent a major efficiency penalty in a turbine rotor. These losses are presently controlled by maintaining close tolerances on tip clearances. Tip leakage resulting by gaps between the blade tip and the casing can account for about 1/3 of the total losses in a turbine stage. The reason is mainly the offloading of the tip since the leaking fluid is not exerting a force on the blade, as well as the generation of complicated flow further downstream due to the leakage vortex.
Although we can achieve improved efficiency by decreasing the size of the gap, some clearance is always essential to allow for thermal expansion and avoid contact between the blades and the casing. Various methods of reducing leakage by changing the tip geometry have been used in practice. Squealer tips, winglets and shrouded airfoils are some of them. However, the last two must be cooled and this will eventually add extra weight on the machine, which is not desirable. In addition to conventional labyrinth seals, brush seals can be used for significant reduction of leakages. The AxSTREAM platform is a unique engineering tool that allows the users to design, analyze and optimize any kind of turbomachinery taking into account leakages as well. Leakages are estimated based on seal geometry provided, extractions, injections, recirculation, and cooling flows. Tip blowing is another way of reducing leakage and it could utilize cooling air. High-pressure turbine airfoils are typically cooled, and film cooling is often employed on the blade tips. Cooling air might be directed to counter the leakage flow. Many studies have considered tip heat transfer, but fewer have investigated the aerodynamic effects of tip film cooling. To study the effect of cooling flows and the design of secondary systems to the overall performance of a machine we need a robust tool that can be linked to all the design aspects (efficiency, weight, life span estimation etc.)
At SoftInWay we have developed AxSTREAM NET, a 1D system modelling solver based on finite volume method that uses a thermal-fluid network approach to simulate secondary flows and heat transfer at steady and unsteady (transient) conditions. The non-necessity for 3D geometry allows for its use before finalizing the blades geometry while keeping good accuracy results in a very short computation time. This helps considerably reduce the iteration time required to optimize aerodynamic cooling losses and machine performances. Find more about AxSTREAM NET at http://www.softinway.com/software-applications/cooling-flows-secondary-systems/ or ask one of our engineers to demonstrate live the capabilities of the tool by contacting us at Sales@Softinway.com
Reference Volino RJ. Control of Tip Leakage in a High-Pressure Turbine Cascade Using Tip Blowing. ASME. J. Turbomach. 2017;139(6):061008-061008-12. doi:10.1115/1.4035509.  Booth, T. C. , Dodge, P. R. , and Hepworth, H. K. , 1982, “ Rotor-Tip Leakage—Part I: Basic Methodology,” ASME J. Eng. Power, 104(1), pp. 154–161