TY - BOOK

T1 - Turbine–Diffuser Interaction

AU - Mimic, Dajan

N1 - Doctoral thesis

PY - 2021

Y1 - 2021

N2 - Diffusers increase the power output and cycle efficiency of gas turbines by reducing the back pressure of the turbine, thus, increasing the work extracted from the fluid by the turbine. They are, however, challenging to design. This is due to the inherent predisposition of the flow to separate under the adverse pressure gradients generated by diffusers, hence negating their benefit. This condition of imminent flow separation is aggravated because diffuser designers seek ever-shorter diffusers with correspondingly steeper opening angles and, thus, higher adverse pressure gradients, to reduce frictional losses and costs. This work presents a novel theory of turbine–diffuser interaction. More specifically, this theory addresses the stabilisation of diffuser boundary layers induced by tip-leakage vortices from an upstream rotor. The theory provides a framework to characterise tip-leakage vortices based upon integral stage-design parameters. The stage parameters loading coefficient, flow coefficient, swirl an- gle, and non-dimensional blade-passing frequency have been identified as the determinants for the intensity, orientation, and duty cycle of the tip-leakage vortices. These parameters have been condensed into the stabilisation number as a predictor for the inflow-dependent diffuser performance. Several hypotheses are derived from the theory and subsequently confirmed using partially scale-resolving simulations and experimental data. Additionally, a prediction method for the vortex-induced boundary-layer stabilisation in annular diffusers has been developed. The results of the prediction method are shown to be consistent with the theory presented.

AB - Diffusers increase the power output and cycle efficiency of gas turbines by reducing the back pressure of the turbine, thus, increasing the work extracted from the fluid by the turbine. They are, however, challenging to design. This is due to the inherent predisposition of the flow to separate under the adverse pressure gradients generated by diffusers, hence negating their benefit. This condition of imminent flow separation is aggravated because diffuser designers seek ever-shorter diffusers with correspondingly steeper opening angles and, thus, higher adverse pressure gradients, to reduce frictional losses and costs. This work presents a novel theory of turbine–diffuser interaction. More specifically, this theory addresses the stabilisation of diffuser boundary layers induced by tip-leakage vortices from an upstream rotor. The theory provides a framework to characterise tip-leakage vortices based upon integral stage-design parameters. The stage parameters loading coefficient, flow coefficient, swirl an- gle, and non-dimensional blade-passing frequency have been identified as the determinants for the intensity, orientation, and duty cycle of the tip-leakage vortices. These parameters have been condensed into the stabilisation number as a predictor for the inflow-dependent diffuser performance. Several hypotheses are derived from the theory and subsequently confirmed using partially scale-resolving simulations and experimental data. Additionally, a prediction method for the vortex-induced boundary-layer stabilisation in annular diffusers has been developed. The results of the prediction method are shown to be consistent with the theory presented.

U2 - 10.15488/10777

DO - 10.15488/10777

M3 - Doctoral thesis

CY - Hannover

ER -