Rake impact on turboshaft compressors, a numerical study

Purpose Turboshaft engines usually include one centrifugal compressor due to its high-pressure ratio, stability and compactness. Many designers rely on positive raking to decrease tip gap flow and therefore losses. However recent optimization studies revealed geometries contradicting this canonic view. Hence, this paper aims to investigate how the rake angle alone can influence performance and to which extent. Design/methodology/approach A turboshaft representative impeller was chosen and altered for null and +/−30° rake angles. Menter's shear stress transport model is used for steady computational fluid dynamics simulations, sweeping the nominal speedline at various tip clearances. Backsweep distribution is identical in all cases, isolating rake influence. Findings Pressure ratio was lowered for the both positively and negatively raked blades, but through distinct aerodynamic mechanisms. Although the flow through the tip gap was lower for the positive rake, this is due to lower blade loading. Splitter comparison reveal that these effects are more pronounced in the radial regions. Practical implications Some of the findings may extend beyond turboshaft engines, into turbochargers, home appliances or industrial blowers. However, all extrapolations must consider specific differences between these applications. Turboshaft compressors designers can benefit from this study when setting up their free parameters and penalty functions in the early concept stages. Originality/value Only few similar studies can be found in the literature to date, none similar to turboshaft applications. Also, this impeller is designed to eliminate leading edge shocks and suction side boundary layer separation, which makes it easier to isolate the tip gap flow effects. The authors also provide a framework on which semi-empirical design equations can be further developed to incorporate rake into 1D design tools.

The Physical Mechanism of Tip Leakage Loss Reduction by Means of Passive Injection in Low Mach Number Flows

Tip clearance losses represent a major source of efficiency losses in turbomachinery. A novel method based on passive injection for reducing tip leakage losses in axial turbine cascades has been proposed in 2007 by Willinger and co-workers, and first experimental demonstrations of the potential of this approach were reported recently. However, these experimental tests were limited to linear cascade experiments w6ith stationary endwalls, and they were not able to explain the underlying physical mechanism of the observed loss reduction. In the present contribution, results of detailed experimental and numerical analyses of the tip gap flow, the interaction of the passive tip injection jet with the gap and the main flow, and the effect of the moving endwall are presented. Both numerical and experimental results indicated that the beneficial effect of the passive injection mainly results from a weaker tip gap vortex and not so much from a simple blockage of the tip gap flow itself.

Control of Tip-Clearance Flow in a Low Speed Axial Compressor Rotor With Plasma Actuation

This research investigates different dielectric barrier discharge (DBD) actuator configurations for affecting tip leakage flow and suppressing stall inception. Computational investigations were performed on a low speed rotor with a highly loaded tip region that was responsible for stall-onset. The actuator was mounted on the casing upstream of the rotor leading edge. Plasma injection had a significant impact on the predicted tip-gap flow and improved stall margin. The effect of changing the actuator forcing direction on stall margin was also studied. The reduction in stalling flow was closely correlated with a reduction in loading parameter that quantifies mechanisms responsible for end-wall blockage generation. The actuation reduced end-wall losses by increasing the static pressure of tip-gap flow emerging from blade suction-side. Lastly, an approximate speed scaling developed for the DBD force helped estimate force requirements for stall margin enhancement of transonic rotors.

Control of Tip-Clearance Flow in a Low Speed Axial Compressor Rotor With Plasma Actuation

This research investigates different dielectric barrier discharge (DBD) actuator configurations for affecting tip leakage flow and suppressing stall inception. Computational investigations were performed on a low-speed rotor with a highly loaded tip region that was responsible for stall-onset. The actuator was mounted on the casing upstream of the rotor leading edge. Plasma injection had a significant impact on the predicted tip-gap flow and improved stall margin. The effect of changing the actuator forcing direction on stall margin was also studied. The improvement in stall margin was closely correlated with a reduction in loading parameter that quantifies mechanisms responsible for end-wall blockage generation. The actuation reduced end-wall losses by increasing the static pressure of tip-gap flow emerging from blade suction-side. Lastly, an approximate speed scaling developed for the DBD force helped estimate force requirements for stall enhancement of transonic rotors.