tip leakage flow

One of the factors that can cause a reduction in the efficiency and performance of axial compressors is the tip leakage flow of the compressor blade. In the first, the compressor's performance curve is compared with experimental results obtained under the condition of no air injection, and a statistically significant agreement is observed. The present study investigates the impact of various parameters, including flow rate, diameter, angle, and injection location, on the compressor's performance curve and flow structure, taking into account the injection of air into a passage. The results indicate that the compressor's stall margin and stable range extension are at their maximum values at a specific scale of each of the aforementioned parameters. Any deviation from this scale, either by reducing or increasing the injection parameters, leads to a reduction in the above characteristics. Although the presence of injection leads to an increase in the total pressure ratio in all injection states compared to the state without injection, the adiabatic efficiency at similar mass flow rates exhibits no significant change. The results also indicate that flow injection in the most suitable state increases the stall margin amount by 27% and the stable range extension of the compressor by 192.

This paper aims to understand the effects of circumferential inlet distortion and tip injection on a transonic impeller performance and flow field. For distorted inflow, the impeller is subjected to a stationary 120-degrees circumferential total pressure distortion. Full annulus unsteady three-dimensional analysis has been used to study the inlet distortion and tip injection effects on the impeller performance, stability and flow field. The results show that the circumferential inlet distortion reduces the impeller total pressure ratio and adiabatic efficiency; however, it has no significant impact on the safe operating range. Unlike the inlet distortion, the tip injection considerably increases the operating range. According to the results, the distortion and tip injection effect on the compressor performance is mainly due to changes in tip leakage flow. The inlet distortion has unfavorable influences on the flow field, especially near the impeller tip; however, the tip injection ameliorates the flow field in this region. In both the clean and distorted inflow, the tip injection causes downstream shock transmission, weakening the shock-tip leakage interaction. Hence, stall inception is postponed, and the impeller stability is improved in the presence of the tip injection.

In this paper, the influence of a shallow reversed slot-type casing treatment on the performance of a tip-critical transonic compressor has been numerically investigated. Firstly, the complex flow fields in the rotor tip region are studied in details. It shows the severe blockage induced by suction surface boundary separation triggers compressor stall at 100% design speed, while the blockage due to tip leakage vortex dominates at 80% design speed. Secondly, the mechanism of stability extension is presented at different rotating speeds. The casing treatment alleviates greatly the tip blockage by manipulating the tip leakage flow, accompanied by the redistribution of aerodynamic loading and mass flux. As a result, the casing treatment is more efficient for the blockage induced by tip leakage vortex (at 80% design speed). Further analysis of the pressure field and passage shock distribution demonstrates that the passage shock intensity and its location will affect the effectiveness of casing treatment. Finally, the instability characteristics of compressor with casing treatment are revealed. The numerical results reflect when the mass flow approaching the instability boundary, the stator passage blockage presumably is dominant for triggering the compressor stall.

The impact of varying the tip clearance of each rotor on the performance of a counter-rotating axial compressor has been investigated based on numerical simulations. The main purpose was to investigate the sensitivity to the tip clearance of each of the two individual rotors and the corresponding aerodynamic mechanisms associated with the performance variation in this compressor. The results indicated that both the total pressure ratio and the efficiency decreased as the tip clearance was increased, and the sensitivity curve for peak efficiency for both rotors was found to be an approximately linear negative relationship with increasing tip clearance. The variations of peak efficiency and stability margin of Rotor 2 were more sensitive to changing tip clearance than Rotor 1. An optimum combination of tip gaps existed for this compressor, i.e. 0.5τ for Rotor 1 and 0.25τ for Rotor 2 (where τ represents the nominal tip clearance value). At this optimum configuration, the peak efficiency and stability margin were improved by 0.63% and 29.4%, respectively. The location of the onset of the tip leakage vortex was found to be shifted downstream when the tip clearance increased. The nature of the tip leakage flow for each rotor was found to be influenced by the variation of tip clearance in the other rotor. Rotor 2 showed a more significant impact on Rotor 1. Additionally, varying the combination of tip clearances changed which of the two rotors was the first to stall.

In order to reduce the adverse effect of the tip leakage flow of cantilever stator on compressor performance, the impact of the axial position of endwall streamwise suction slot on tip leakage flow was numerically studied. The study on the overall performance of the compressor and the details of the flow field near the stator end region with and without suction showed that all suction schemes could weaken the tip leakage flow intensity to a certain extent, and the flow control effect was gradually enhanced with the increase of the suction flow rate. In the case of small suction flow rate, for example, 0.5%, the short slot schemes can improve the overall efficiency of the compressor by about 0.5%, which is more advantageous than the long slot scheme, and the overall efficiency improvement of the latter is about 0.3%. The advantage of the long slot scheme in flow control is reflected in the case of large suction flow rate, that is, 1.0%, which may improve the overall efficiency of the compressor by about 0.96%. The axial position of suction slot has a significant influence on flow control effect of the tip leakage flow. Compared with the downstream suction, which only modified the flow field by reducing the blocking effect generated by tip flow vortex, the upstream suction could better control the tip leakage flow by restraining the development of the initial stage of the leakage vortex. Besides, the endwall suction scheme with a full chord length slot has the greatest impact on the passage vortex, its effect on modifying the flow field near the end zone was determined by the combinatorial action of the enhancement of the passage vortex and the attenuation of the leakage vortex.

To control secondary flow effects and enhance the aerodynamic performance of the compressor, the flow control effects of the flow suction at the endwall with different circumferential positions and at the blade tip were numerically investigated in the cantilever stator of an axial single-stage transonic compressor. The main purpose was to gain a better understanding of the application of boundary layer suction and the associated control mechanisms in the cantilever stator. The studies show that the optimal position of the endwall suction slot should be located up the stator blade, in terms of the leakage flow structures and the blade tip unloading effect. In addition, the flow control effects of the suction at the blade tip on leakage flow upstream is better than that of the endwall flow suction with the same structure. Further, the studies of compressor aerodynamic performance curves illustrate that the efficiency and pressure ratio is increased by 0.34% and 1.09% at the peak efficiency point, and are increased by 0.39% and 0.14% at the near stall point, respectively.

Characteristics of rotor blade tip clearance flow in axial compressors can significantly affect their performance and stable operation. It may also increase blade vibrations and cause detrimental noises. Therefore, this paper is contributed to investigate tip leakage flow in a low speed isolated axial compressor rotor blades row. Simulations are carried out on near-stall condition, which is valuable of being studied in detail. In turbomachines, flows are non-isotropic and highly three-dimensional. The reason arises from the complicated structure of bounded walls, tip leakage flows, secondary flows, swirl effects, streamlines curvatures and pressure gradients along different directions. As a result, accurate studies on tip leakage flow would be accompanied by many challenges such as adopting suitable turbulence models. So, investigations are carried out numerically utilizing two well-known turbulence models of k-ε and k-ω-SST, separately. It is shown that the k-ε model yields poor results in comparison to the k-ω-SST model. To realize reasons for this discrepancy, turbulence parameters such as turbulent kinetic energy, dissipation and eddy viscosity terms at the tip clearance region were surveyed in detail. It is found out that estimation for eddy viscosity term is too high in the k-ε model due to excessive growth of turbulent kinetic energy, time scale, and lack of effective damping coefficient. This leads to dissipation of vortical structure of flow and wrong estimation of flow field at the rotor tip clearance region. Nevertheless, k-ω-SST turbulence model provides results consistent with reality.

Tip leakage loss introduces major part of losses of the rotor in axial gas turbines. Therefore, the rotor blade tip has a considerable effect on rotor efficiency. To understand the flow physics of the rotor tip leakage, we solve the flow field for different tip platforms (passive flow control) and by considering coolant tip injection (active flow control). Various blade tip configurations such as squealers and extensions on both pressure and suction sides, partial PS-squealer and flat tip with various tip clearances are generated. The computational domains are generated using unstructured prism layers for boundary layer resolution and unstructured, tetrahedral mesh for main flow. By using a finite volume CFD solver capable of solving RANS equations in an unstructured domain, the transonic compressible flow in the domain is solved. To capture the turbulent field in blade tip, shear stress transport (SST) k-ω model is employed. By using mixing plane approach, it is possible to couple outlet boundary of stator and inlet boundary of rotor and investigate the stator-rotor interaction in the rotor flow field and its consequence tip leakage flow. To investigate the combined effects of active and passive flow control measures in blade tip region, we simulate baseline geometry with and without tip coolant to show the effects of geometrical features of the rotor tip as well as the effect of tip coolant mass flow rate. Taking into account various rotor tip configurations and their tip leakage losses, it is possible to propose an optimum configuration.