Analysis of Hot Streak Migration Characteristics in a Transonic Turbine Employing Nonlinear Harmonic Method

The migration characteristics of the hot streak (HS) are critical to analyze for the turbine designers to optimize the blade cooling schemes during design phase. Numerical investigations of the HS migration characteristics in a highly loaded transonic turbine have been conducted employing the Nonlinear Harmonic (NLH) method and unsteady RANS. The NLH method using three harmonics has effectively captured the hot streak characteristics with about 7 times less computationally expensive than the unsteady RANS. Two HS clocking positions relative to the stator leading edge (impinging and non-impinging) have been investigated. The turbine efficiency decreases by increasing HS temperature ratio however; the HS clocking position has no significant impact on the efficiency. The radial distribution of the HS remains unaffected through the stator passage however; the HS core temperature drop for impinging case is relatively higher than non-impinging case. The radial distribution of the HS through the rotor passage shows that the radial amplitude of the HS total temperature distortion decays more in the impinging case. The buoyancy force is found to be mainly responsible for radial migration of the HS towards the rotor hub which is directly proportionate to the HS temperature ratio and inversely proportionate to the square of the flow coefficient. The analysis of unsteady rotor surface temperature fluctuations shows that the rotor pressure side bears higher fluctuations for both clocking positions however the time averaged temperature envelop is compact and a balanced thermal load is achieved between the pressure side and suction side for the impinging case. The instantaneous entropy distribution and the time-space plots of total temperature distribution have shown the phenomenon of preferential heating on the rotor pressure side for non-impinging case. The rotor vortex shedding frequency remains unaffected of the HS clocking position as well as the HS temperature ratio. The NLH method has shown remarkable advantage over unsteady RANS in terms of computational cost with comparable results thus offering an opportunity for the designers to imply unsteady computations during design phase.