Evolution of Cavity Tip Vortices in High-Pressure Turbines

• Main Author: Berglund, Albin
• Format: Others
• Language: English
• Published: Uppsala universitet, Elektricitetslära 2017
• Subjects: CFD; turbomachinery; tip gap; fluid dynamics; cavity; rotor blade; Fluid Mechanics and Acoustics; Strömningsmekanik och akustik
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-329369

This degree project in applied physics studies the tip gap flows over the rotor blades of a high-pressure turbine. The rotor blade used in the study has an improved design that utilizes both a cavity tip and an uneven profiling to reduce turbine loss. The designed rotor blade is shown to admit a 21% lower leakage mass flow rate across the tip gap than a reference rotor blade with a flat tip. By studying the designed rotor blade using transient CFD, the flow field of the tip gap region has been studied through one blade passage. The flow field characteristics of particular interest are the leakage mass flow rate across the tip gap region, which is proportional to turbine loss, and the characteristic vortices that reside within the cavity tip. By using post-processing scripts, the leakage mass flow rate has been calculated for every time step across one blade passage, showing a strong time dependence. The characteristic vortices are found using two different vortex detection algorithms, and their respective vorticity magnitude is shown to depend on the leakage mass flow rate. The simulation shows that the vorticity magnitude is increasing above a threshold of leakage mass flow rate, and that it is decreasing under this threshold. This effect is shown to destabilize the leakage mass flow rate, increasing its amplitude over its period of one blade passage.