Tip Leakage Effects Research Articles

The exploitation of CFD legacy for the meridional analysis and design of modern gas and steam turbines

In the last decades, the consolidation of 3D CFD approaches in the industrial design practices has progressively moved throughflow codes from the top of design systems to somewhere in between first development stages and the final aerodynamic optimizations. Despite this trend and the typical limitations of traditional throughflow methods, designers tend to still consider such methods as fundamental tools for drafting a credible aero-design in a short turnaround time. Recently a considerable attention has been devoted to CFDbased throughflow codes as suitable means to widen the range of applicability of these tools while smoothing the predictive gap with successive threedimensional flow analyses.The present paper retraces the development and some applications of a modern and complete CFD-based throughflow solver specifically tuned for multistage axial turbine design. The code solves the axisymmetric Euler equations with an original treatment of tangential blockage and body force. It inherits its numerical scheme from a state-of-the-art CFD solver (TRAF code) and incorporates real gas capabilities, three-dimensional flow features (e.g. secondary flows, tip leakage effects), coolant flow injections, and radial mixing models. Also geometric features of actual blades, like fillets, part-span shrouds, and snubbers, are accounted for by suitable models.The capabilities of the code are demonstrated by discussing a significant range of test cases and industrial applications. They include single stage configurations and entire multistage modules of steam turbines, with flow conditions ranging from subsonic to supersonic. Computational strategies for design and off-design analyses will be presented and discussed. The reliability and accuracy of the method is assessed by comparing throughflow results with 3D CFD calculations and experimental data. A good agreement in terms of overall performance and spanwise distributions is achieved in both design and off-design operating conditions.

An investigation for the turbine blade film cooling performance on the suction side tip region under rotating condition

The suction side film cooling performance of a rotating blade is investigated numerically and experimentally in this paper. Three types of tip structures, including flat tip, double squealer tip and single squealer tip, are chosen to generate different leakage vortexes and study the tip leakage effects. The experiments were established in the rotating turbine film cooling facility of Beihang University. In the experiments, the hole position moves from 5/8 to 7/8 of the total blade height, and the effect of hole position was investigated. At last, the rotating Reynolds number changes from 4036 to 6954 and then to 9623. The rotation effects are also investigated. In the numerical simulation, a k-ε model is used, and the results were validated by comparing them with the experimental data. The numerical simulation results show that the film flow is severely influenced by the passage vortex and the leakage vortex. For the double squealer tip results, the leakage flow attaches well. Moreover, the leakage vortex and the passage vortex decrease from the blade tip to the mid-span. At last, the rotation strengthens the passage vortex and weakens the leakage vortex. As a result, the film deflection increases from 5° to 12° in the experiments. Additionally, the film coverage decreases. Regarding the tip structure effects, the double squealer tip proved to achieve the best film performance, while the single squealer tip can help to resist the film deflection.

Secondary flow and radial mixing modelling for CFD-based Through-Flow methods: an axial turbine application

The paper presents the theoretical bases and an application of a CFD-based Through-Flow model. The code solves the axisymmetric Euler equations and takes into account the effect of tangential blockage and body force. It inherits its numerical scheme from a state-of-the-art CFD solver (TRAF code). Blade body forces are calculated directly from the tangency condition to the meridional flow surface, which is iteratively adapted during the time-marching procedure. Dissipative forces are computed through a realistic distribution of entropy along streamlines. Both secondary flow and tip leakage effects on the meridional flow-field are included through the adoption of a concentrated vortex model, while the corresponding loss contributions are evaluated from correlations. Also, a radial mixing model considering both turbulent diffusion and spanwise convection is implemented.The accuracy of the method is assessed by comparison with CFD calculations and experimental data on the transonic CT3 turbine stage tested in the framework of the TATEF2 European project. A good agreement in terms of overall performance and radial distributions is achieved for both design and off-design operating conditions.

Tip Leakage Effects on Flow Field of a Centrifugal Compressor

Tip Leakage Effects on Flow Field of a Centrifugal Compressor

Effect of Tip Clearance on the Internal Flow and Hydraulic Performance of a Three-Bladed Inducer

The influence of the tip clearance on the internal flow and hydraulic performances of a 3-bladed inducer, designed at ALTA, Pisa, Italy, are investigated both experimentally and numerically. Two inducer configurations with different blade tip clearances, one about equal to the nominal value and the other 2.5 times larger, are considered to analyze tip leakage effects. The 3D numerical model developed in ANSYS CFX to simulate the flow through the inducer with 2 different clearances under different operating conditions is illustrated. The internal flow fields and hydraulic performance predicted by the CFD model under different operating conditions are compared with the corresponding experimental data obtained from the inducer tests. As expected, both experimental and numerical results indicate that higher pressure rise and hydraulic efficiency are obtained from the inducer configuration with the nominal tip clearance.

A Model for Flow Bypass and Tip Leakage in Pin Fin Heat Sinks

A model for the pressure drop and heat transfer behavior of heat sinks with top bypass is presented. In addition to the characteristics of a traditional two-branch bypass model, the physics of tip leakage are taken into consideration. The total flow bypass is analyzed in terms of flow that is completely diverted and flow that enters the heat sink but leaks out. Difference formulations of the momentum and the energy equations were utilized to model the problem in the flow direction. Traditional hydraulic resistance and heat transfer correlations for infinitely long tube bundles were used to close the equations. Tip leakage mechanisms were modeled by introducing momentum equations in the flow normal direction in both the pin side and bypass channel, with ad hoc assumptions about the static pressure distribution in that direction. Although the model is applicable to any kind of heat sink, as a case study, results are presented for in-line square pin fin heat sinks. Results were compared with the predictions from a two-branch bypass model and previous experimental data. It is shown that tip leakage effects are important in setting the overall pressure drop at moderate and high pin spacing, but have only minor influence on heat transfer.

Rotordynamic Forces Due to Turbine Tip Leakage: Part I—Blade Scale Effects

An experimental and theoretical investigation has been conducted on rotordynamic forces due to nonaxisymmetric turbine tip leakage effects. This paper presents an actuator disk model that describes the flow response to a finite clearance at the rotor tip. The model simplifies the flow field by assuming that the radially uniform flow splits into two streams as it goes through the rotor. The stream associated with the tip clearance, or the underturned stream, induces radially uniform unloading of the rest of the flow, called the bladed stream. Thus, a shear layer forms between the two streams. The fraction of each stream and the strength of shear layer between the two are found as functions of the turbine loading and flow parameters without resorting to empirical correlations. The results show that this model’s efficiency predictions compare favorably with the experimental data and predictions from various correlations. A companion paper builds on this analysis to yield a model of the three-dimensional disturbances around an offset turbine and to predict the subsequent cross forces.

Effects of Tip Leakage on Relative Flow and Reynolds Stress at Midpassage of a Centrifugal Impeller.

Effects of Tip Leakage on Relative Flow and Reynolds Stress at Midpassage of a Centrifugal Impeller.

Study on flow of centrifugal impeller. 3rd Report. The effects of tip clearance on the characteristics and secondary flow of the impeller.

To examine the effects of a secondary flow induced by tip leakage, the velocity and pressure distributions are measured within an unshrouded centrifugal impeller with a large tip clearance. The changes of the characteristics between shrouded and unshrouded impellers are discussed in comparison with the previous reports. In spite of the considerable difference of the tip clearance, the secondary flow pattern is similar to the first report. Though a wake in the shrouded impeller of the second report is found on the suction side, the wake in the unshrouded impeller exists on the midposition of blade-to-blade near the casing because of the effects of tip leakage. The position of the largest blade loading in the unshrouded impeller changes to midpassage from first-half-passage in shrouded impeller. Therefore the absolute circumferential velocity at the exit in the unshrouded impeller is larger than that in the shrouded impeller.

Tip Leakage in a Centrifugal Impeller

The effects of tip leakage have been studied using a 1-m-dia shrouded impeller where a leakage gap is left between the inside of the shroud and the impeller blades. A comparison is made with results for the same impeller where the leakage gap is closed. The static pressure distribution is found to be almost unaltered by the tip leakage, but significant changes in the secondary velocities alter the size and position of the passage wake. Low-momentum fluid from the suction-side boundary layer of the measurement passage and tip leakage fluid from the neighboring passage contribute to the formation of a wake in the suction-side shroud corner region. The inertia of the tip leakage flow then moves this wake to a position close to the center of the shroud at the impeller outlet.

Three-Dimensional Navier-Stokes Simulations of Turbine Rotor-Stator Interaction; Part I1 - Results

Fluid flows within turbomachinery tend to be extremely complex. Understanding such flows is crucial to efforts to improve current turbomachinery designs, and the computational approach can be used to great advantage in this regard. This study presents a finite-difference, unsteady, thin-layer Navier-Stokes solution to the flow within an axial turbine stage. The computational methodology developed for this simulation is presented in Part I of this paper. The calculation includes end-wall and tip-leakage effects. Results in the form of time-averaged surface pressures, pressure amplitudes (corresponding to the pressure fluctuation in time), near-surface velocity vectors, and pressure contours in the passage areas are presented. The numerical results are compared with experimental data wherever possible and the agreement between the two is found to be good.

Three-dimensional Navier-Stokes simulations of turbine rotor-stator interaction. Part I - Methodology

Fluid flows within turbomachinery tend to be extremely complex. Understanding such flows is crucial to efforts to improve current turbomachinery designs, and the computational approach can be used to great advantage in this regard. This study presents a finite-difference, unsteady, thin-layer Navier-Stokes approach to calculating the flow within an axial turbine stage. The relative motion between the stator and rotor airfoils is made possible with the use of patched grids that move relative to each other. The calculation includes end-wall and tip-leakage effects. The numerical methodology is presented in detail in the present paper (Part I). The computed results and comparisons of these results with experimental data are presented in a companion paper (Part II).

Study of Three-Dimensional Viscous Flows in an Axial Compressor Cascade Including Tip Leakage Effects Using a SIMPLE-Based Algorithm

A multisweep space-marching solver based on a modified version of the SIMPLE algorithm was employed to study the three-dimensional flow field through a linear cascade. Three cases were tested: one with moderate loading, one with high loading, and one with high loading and tip clearance. The results of the numerical simulation were compared with available experimental measurements, and the agreement between the two was found satisfactory. The numerical simulation provided insight into several important endwall flow phenomena such as the interaction between the leakage and passage vortices, the interaction between the leakage vortex and the wake, the effect of leakage flow and loading on losses and secondary kinetic energy, the suction side corner separation, and the blowing of this separation by the leakage flow.