Tip leakage aerodynamics over stepped squealer tips in a turbine cascade

Tip gap flow physics and aerodynamic loss generations for two stepped squealer tips of a “Higher Pressure-side rim and Lower Suction-side rim” (HPLS) tip and a “Lower Pressure-side rim and Higher Suction-side rim” (LPHS) tip have been investigated in a turbine cascade. For a fixed tip gap height-to-chord ratio of h/c = 2.0%, oil film flow visualizations are performed on the casing wall as well as on the cavity floor, and three-dimensional flow fields downstream of the cascade are measured with a five-hole probe. For the HPLS tip, the leakage inflow over the pressure-side rim cannot reach the suction-side rim in the upstream region due to the presence of an inlet flow intrusion, and there exists a strong near-wall flow heading toward the trailing edge all over the cavity floor. On the other hand, the LPHS tip has a mid-chord leakage flow penetration into the blade flow passage, and also provides a downstream leakage flow penetration deeper than that for the HPLS tip. Its cavity floor can be divided into a backward flow region and a wide separation bubble. Aerodynamic loss for the HPLS tip, which is nearly identical to that for the cavity squealer tip, is lower than those for the LPHS and plane tips in a considerable degree.     

透平机械叶尖间隙流场研究的进展

叶尖间隙流动对透平机械性能有很大影响。长期以来,叶尖间隙流动机理一直是透平机械领域研究的一个热点,同时也是一个尚未认识清楚的难点。把叶尖间隙内流动的研究进展分成两个部分:一部分是透平叶栅和透平转子内部叶尖间隙流场的研究进展,另一部分是压气机叶栅和压气机转子内部叶尖间隙流场的研究进展。对目前叶尖间隙研究集中的问题,如泄漏涡系结构,泄漏流动模型,泄漏涡旋涡强度的变化,泄漏涡和激波的相互作用等进行了简要的总结。文中还对透平机械叶尖间隙泄漏流动常用的数值计算方法进行了总结。认为今后应进一步对以下问题进行研究,其中包括研究高速透平机械叶尖泄漏涡旋涡强度变化问题,径流式叶轮机械叶尖间隙泄漏流动过程及泄漏涡发生发展规律问题,泄漏涡与激波相互作用产生阻塞区域的大小问题。      

Tip gap height effects on the aerodynamic performance of a cavity squealer tip in a turbine cascade in comparison with plane tip results: part 1—tip gap flow structure

Tip gap height effects on the flow structure over a cavity squealer tip have been investigated in a linear turbine cascade for power generation, in comparison with the corresponding plane tip results. Oil film flow visualizations are conducted on the tip surface and casing wall for tip gap height-to-chord ratios of h/c = 1.0, 2.0, and 3.0%. The squealer tip has a recessed cavity enclosed by a full length squealer with its rim height-to-chord ratio of 5.51%. The results show that most of in-coming fluid entering the tip gap inlet for the cavity squealer tip is entrapped by the suction-side squealer rim, and the cavity fluid is discharged into the blade flow passage over the suction-side squealer rim in the region from the mid-chord to the trailing edge. Regardless of h/c, the cavity squealer tip makes the leakage flow zone narrower than the plane tip, and is superior to the plane tip in reducing the tip leakage mass flow rate. A qualitative flow model describing full flow features over the cavity squealer tip is suggested. In this flow model, the tip gap exit area is classified into four different regions, and the tip gap height effects on the discharge characteristics in each region are discussed in detail.     

Tip gap flow characteristics in a turbine cascade equipped with pressure-side partial squealer rims

Tip gap flow characteristics and aerodynamic loss generations in a turbine cascade equipped with pressure-side partial squealer rims have been investigated with the variation of its rim height-to-span ratio (h_p/s) for a tip gap height of h/s = 1.36%. The results show that the tip gap flow is characterized not only by the incoming leakage flow over the pressure-side squealer rim but also by the upstream flow intrusion behind the rim. The incoming leakage flow tends to decelerate through the divergent tip gap flow channel and can hardly reach the blade suction side upstream of the mid-chord, due to the interaction with the upstream flow intrusion as well as due to the flow deceleration. A tip gap flow model has been proposed for h_p/s = 3.75%, and the effect of h_p/s on the tip surface flow is discussed in detail. With increasing h_p/s, the total-pressure loss coefficient mass-averaged all over the present measurement plane decreases steeply, has a minimum value for h_p/s = 1.88%, and then increases gradually. Its maximum reduction with respect to the plane tip result is evaluated to be 11.6%, which is found not better than that in the cavity squealer tip case.     

Tip gap height effects on the aerodynamic performance of a cavity squealer tip in a turbine cascade in comparison with plane tip results: part 2—aerodynamic losses

Tip gap height effects on aerodynamic losses downstream of a cavity squealer tip have been investigated in a linear turbine cascade for power generation, in comparison with plane tip results. Three-dimensional flow fields are measured with a five-hole probe for tip gap height-to-chord ratios of h/c = 0.5, 1.0, 1.5 and 2.0%. The cavity squealer tip has a full length squealer with its rim height-to-chord ratio of 5.51%. For a fixed value of h/c, the tip leakage vortex for the cavity squealer tip is always weaker than that for the plane tip, and the flow field in the passage vortex region for the cavity squealer tip is less influenced by the tip leakage flow than that for the plane tip. For the cavity squealer tip, there is no appreciable change in local aerodynamic loss with h/c in the passage vortex region, but local aerodynamic loss in the tip leakage vortex region increases with h/c. The roles of the cavity squealer tip in reducing aerodynamic loss in comparison with the plane tip case are twofold: (1) the cavity squealer tip decreases the leakage flow discharge in the region from the leading edge to the mid-chord, which leads to an aerodynamic loss reduction in the passage vortex region and (2) it also decreases the leakage flow discharge downstream of the mid-chord, which results in an aerodynamic loss reduction in the tip leakage vortex region.     

Effect of multiple DBD plasma actuators on the tip leakage flow structure and loss of a turbine cascade

In this paper, the effects of multiple dielectric barrier discharge (DBD) plasma actuators on the leakage flow structures and loss conditions have been numerically studied in an axial turbine cascade. Kriging surrogate model is adopted to obtain the optimal cases. The physical mechanism of flow structures inside the gap that control leakage flow is presented, which is obtained by analyzing the flow topology, the evolution of the flow structures and its influence on the secondary velocity and loss conditions in the passage as well. The results show that the induced vortex caused by DBD actuators can change the leakage flow direction inside the tip gap and make the separation bubble break earlier, leading to a new type of the flow pattern. When the actuators are applied, the speed of leakage flow is significantly reduced and the angle between leakage flow and main flow has an obviously diminution, causing the reduction of mixing losses in the passage compared with the Baseline case. Furthermore, the comparison of secondary velocity shows that the tip leakage vortex (TLV) approaches the suction surface, resulting in reduced affected area and weakened loss strength. Plasma actuators can diminish the loss coefficient in both TLV and passage vortex near the casing (PVC) zones. The actuators arranged near the trailing edge mainly affect the strength of TLV, while the actuators in the leading edge area contribute to the loss reduction in the zone of PVC.     

叶片转动对涡轮间隙内部流动影响的数值研究

机匣与叶片的相对转动是影响涡轮叶顶间隙流动的重要冈素之一.对LISA 1.5级轴流涡轮间隙内部流动的数值计算结果表明:叶片转动对涡轮间隙流动有阻塞作用.叶片静止时,由于阻塞作用消失,导致间隙入口速度增大,间隙流鼍增加,并且通过间隙的流体全部卷起形成间隙涡.同时在叶片顶部吸力面侧前缘、中部各形成一个间隙涡,使得间隙流动损失增加.而且转速下降会加剧动叶出口截面气流过偏/偏转不足现象.同时叶片静止时,间隙前部各个弦长截面内静压自间隙入口开始一直呈增加趋势,直到叶片尾缘附近截面,间隙截面内静压才逐渐稳定.     

3D pressure imaging of an aircraft propeller blade-tip flow by phase-locked stereoscopic PIV

The flow field at the tip region of a scaled DHC Beaver aircraft propeller, running at transonic speed, has been investigated by means of a multi-plane stereoscopic particle image velocimetry setup. Velocity fields, phase-locked with the blade rotational motion, are acquired across several planes perpendicular to the blade axis and merged to form a 3D measurement volume. Transonic conditions have been reached at the tip region, with a revolution frequency of 19,800 rpm and a relative free-stream Mach number of 0.73 at the tip. The pressure field and the surface pressure distribution are inferred from the 3D velocity data through integration of the momentum Navier-Stokes equation in differential form, allowing for the simultaneous flow visualization and the aerodynamic loads computation, with respect to a reference frame moving with the blade. The momentum and pressure data are further integrated by means of a contour-approach to yield the aerodynamic sectional force components as well as the blade torsional moment. A steady Reynolds averaged Navier-Stokes numerical simulation of the entire propeller model has been used for comparison to the measurement data.     

Over-tip leakage flow and loss in a turbine cascade equipped with suction-side partial squealers

Over-tip leakage flow and loss in a turbine cascade equipped with suction-side partial squealer rims have been investigated for the squealer rim height-to-span ratios (h_st/s) of 0.94%, 1.88%. 3.75%, and 5.63% in the case of a tip gap height-to-span ratio of h/s = 1.36%. The casing wall and tip surface visualizations for h_st/s = 3.75% show that most of the incoming tip leakage flow tends to accelerate through a convergent (nozzle-like) tip gap flow channel and penetrates into the neighboring blade flow passage even upstream of the mid-chord in the form of a wall jet, whereas the rest of it is entrapped by the suction-side squealer rim, flows backward, and is separated from the tip surface along a backward flow separation line. Therefore, the tip surface can be divided into a separation bubble and a backward flow area by the backward flow separation line. A qualitative tip gap flow model for the suction-side squealer tip is suggested in this study. For the present suction-side squealer tip, the total pressure loss coefficient mass-averaged throughout the present measurement plane decreases consistently with increasing h_st/s and is higher than that for the cavity squealer tip or the pressure-side squealer tip regardless of h_st/s.     

A Thickened-Hole Model for Large Eddy Simulations over Multiperforated Liners

Flow instabilities such as Rotating Stall and Surge limit the operating range of centrifugal compressors at low mass-flow rates. Employing compressible Large Eddy Simulations (LES), their generation mechanisms are exposed. Toward low mass-flow rate operating conditions, flow reversal over the blade tips (generated by the back pressure) causes an inflection point of the inlet flow profile. There, a shear-layer induces vortical structures circulating at the compressor inlet. Traces of these flow structures are observed until far downstream in the radial diffuser. The tip leakage flow exhibits angular momentum imparted by the impeller, which deteriorates the incidence angles at the blade tips through an over imposed swirling component to the incoming flow. We show that the impeller is incapable to maintain constant efficiency at surge operating conditions due to the extreme alteration of the incidence angle. This induces unsteady flow momentum transfer downstream, which is reflected as compression wave at the compressor outlet traveling toward the impeller. There, the pressure oscillations govern the tip leakage flow and hence, the incidence angles at the impeller. When these individual self-exited processes occurs in-phase, a surge limit-cycle establishes.     

A review of turbomachinery tip gap effects : Part 1: Cascades

This two part review covers experiments examining the effects of blade tip gaps encountered in turbomachines and the methods by which the synthesised data are currently used in turbomachine design and analysis. Data gained since the 1930's are subdivided for convenience into cascade (Part 1) and rotating machinery¹ (Part 2) data, with a further subdivision into diffusing, or compressor type flows and accelerating, or turbine type flows. The overall trend is that an increasing tip gap, whose effect can reach over most or all of the blade height, reduces turbomachine performance. There is some evidence among the compressor and compressor cascade data that an optimum gap exists when the opposing effects of secondary flows and tip leakage with rotor/wall relative movement tend to balance. Turbine data are, in general, more regular than the body of compressor data, possibly because of the enhanced effect of, usually, undefined boundary layers in diffusing flow in the latter. Comment is made in Part 2 on the predictive and design models reported in the literature     

A review of turbomachinery tip gap effects: Part 2: Rotating machinery

This two part review covers experiments examining the effects of blade tip gaps encountered in turbomachines and the methods by which the synthesised data are currentl used in turbomachine design and analysis. Data gained since the 1930's are subdivided for convenience into cascade (Part 1) and rotating machinery (Part 2) data, with a further subdivision into diffusing, or compressor type flows and accelerating, or turbine type flows. The overall trend is that an increasing tip gap, whose effect can reach over most or all of the blade height, reduces turbomachine performance. There is some evidence among the compressor and compressor cascade data that an optimum gap exists when the opposing effects of secondary flows and tip leakage with rotor/wall relative movement tend to balance. Turbine data are, in general, more regular than the body of compressor data, possibly because of the enhanced effect of, usually, undefined boundary layers in diffusing flow in the latter. Comment is made in Part 2 on the predictive and design models reported in the liturature     

Tip gap height effects on flow structure and heat/mass transfer over plane tip of a high-turning turbine rotor blade

The effects of tip gap height-to-chord ratio, h/c, on the flow structure and heat/mass transfer over the plane tip surface of a large-scale high-turning turbine rotor blade have been investigated for h/c = 1.0%, 2.0%, 3.0% and 4.0%. For near-wall tip gap flow visualizations, a high-resolution oil film method is employed, and the naphthalene sublimation technique is used for local heat/mass transfer rate measurements. From the tip surface visualizations, a pair of vortices named “tip gap vortices” is identified in the leading edge region within the tip gap. The overall tip gap flow is characterized not only by the tip gap vortices but also by the flow separation/re-attachment process along the pressure-side tip edge. Within the separation bubble, there exist complicated near-wall flows moving toward a mid-chord flow converging area. With increasing h/c, the tip gap vortices, the flow separation/re-attachment, and the converging flows within the separation bubble tend to be intensified. In general, higher thermal load is found along the loci of the tip gap vortices and along the re-attachment line, while lower thermal load is observed behind the tip gap vortex system and near the mid-chord flow converging area. Heat/mass transfer characteristics with the variation of h/c are discussed in detail in conjunction with the tip gap flow features. Based on the flow visualizations and heat/mass transfer data, new realistic tip gap flow models have been proposed for h/c = 1.0 and 4.0%.     

Design optimization of axial flow hydraulic turbine runner: Part I—an improved Q3D inverse method

With the aim of constructing a comprehensive design optimization procedure of axial flow hydraulic turbine, an improved quasi‐three‐dimensional inverse method has been proposed from the viewpoint of system and a set of rotational flow governing equations as well as a blade geometry design equation has been derived. The computation domain is firstly taken from the inlet of guide vane to the far outlet of runner blade in the inverse method and flows in different regions are solved simultaneously. So the influence of wicket gate parameters on the runner blade design can be considered and the difficulty to define the flow condition at the runner blade inlet is surmounted. As a pre‐computation of initial blade design on S2m surface is newly adopted, the iteration of S1 and S2m surfaces has been reduced greatly and the convergence of inverse computation has been improved. The present model has been applied to the inverse computation of a Kaplan turbine runner. Experimental results and the direct flow analysis have proved the validation of inverse computation. Numerical investigations show that a proper enlargement of guide vane distribution diameter is advantageous to improve the performance of axial hydraulic turbine runner. Copyright © 2002 John Wiley & Sons, Ltd.     

Drift-flux model for upward two-phase cross-flow in horizontal tube bundles

In relation to void fraction prediction of cross-flow in horizontal tube bundle of shell-tube heat exchangers, a drift-flux correlation has been developed to meet the demand on the study of two-phase flow gas and liquid velocities, two-phase pressure drop, heat transfer, flow patterns and flow induced vibrations in the shell side. Two critical parameters such as distribution parameter and drift velocity have been modeled. The distribution parameter is obtained by constant asymptotic values and taking into account the differences in channel geometry. The drift velocity is modelled depending on the density ratio and the non-dimensional viscosity number. The relationship between the channel averaged and gap mass velocity has been discussed in order to obtain the superficial gas and liquid velocities in the drift-flux correlation. The newly developed drift-flux correlation agrees well with cross-flow experimental databases of air-water, R-11 and R-113 in parallel triangular, normal square and normal triangular arrays with the mean absolute error of 1.06% and the standard deviation of 4.47%. In comparison with other existing correlations, the newly developed drift-flux correlation is superior to other studies due to the improved accuracy. In order to extend the applicability of the newly developed drift-flux correlation to void fraction of unity, an interpolation scheme has been developed. The newly developed drift-flux correlation is able to calculate the void fraction of cross-flow over a full range with different sub-channel configurations in shell-tube type heat exchangers.     

The application of thin-film technology to measure turbine-vane heat transfer and effectiveness in a film-cooled, engine-simulated environment

Thin-film technology has been used to measure the heat transfer coefficient and cooling effectiveness over heavily film cooled nozzle guide vanes (NGVs). The measurements were performed in a transonic annular cascade which has a wide operating range and simulates the flow in the gas turbine jet engine. Engine-representative Mach and Reynolds numbers were employed and the upstream free-stream turbulence intensity was 13%. The aerodynamic and thermodynamic characteristics of the coolant flow (momentum flux and density ratio between the coolant and mainstream) have been modelled to represent engine conditions by using a foreign gas mixture of SF₆ and Argon. Engine-level values of heat transfer coefficient and cooling effectiveness have been obtained by correcting for the different molecular (thermal) properties of the gases used in the engine-simulated experiments to those which exist in the true engine environment. This paper presents the best combined heat transfer coefficient and effectiveness data currently available for a fully cooled, three-dimensional NGVs at engine conditions.     

Exit blade geometry and part-load performance of small axial flow propeller turbines: An experimental investigation

A detailed experimental investigation of the effects of exit blade geometry on the part-load performance of low-head, axial flow propeller turbines is presented. Even as these turbines find important applications in small-scale energy generation using micro-hydro, the relationship between the layout of blade profile, geometry and turbine performance continues to be poorly characterized.The experimental results presented here help understand the relationship between exit tip angle, discharge through the turbine, shaft power, and efficiency. The modification was implemented on two different propeller runners and it was found that the power and efficiency gains from decreasing the exit tip angle could be explained by a theoretical model presented here based on classical theory of turbomachines. In particular, the focus is on the behaviour of internal parameters like the runner loss coefficient, relative flow angle at exit, mean axial flow velocity and net tangential flow velocity.The study concluded that the effects of exit tip modification were significant. The introspective discussion on the theoretical model’s limitation and test facility suggests wider and continued experimentation pertaining to the internal parameters like inlet vortex profile and exit swirl profile. It also recommends thorough validation of the model and its improvement so that it can be made capable for accurate characterization of blade geometric effects.     

Heat transfer and interfacial drag in countercurrent steam-water stratified flow

Local condensation heat transfer coefficients and interfacial shear stresses have been measured for countercurrent stratified flow of steam and subcooled water in rectangular channels over a wide range of inclination angles (4–87°) at two aspect ratios. Dimensionless correlations for the interfacial friction factor have been developed that show that it is a function of the liquid Reynolds number only. Empirical correlations of the heat transfer coefficient, based upon the bulk flow properties, have also been set up for the whole body of data encompassing the different inclination angles and aspect ratios. These indicate that the Froude number as a dimensionless gas velocity is a better correlating parameter than the gas Reynolds number. As an alternative approach, a simple dimensionless relationship for the beat transfer coefficient was obtained by analogy between heat and momentum transfer through the interface. Finally, a turbulence-centered model has been modified by using measured interfacial parameters for the turbulent velocity and length scales, resulting in good agreement with the data.     

Flow control on a 3D backward facing ramp by pulsed jets

This paper presents an experimental study of flow separation control over a 3D backward facing ramp by means of pulsed jets. Such geometry has been selected to reproduce flow phenomena of interest for the automotive industry. The base flow has been characterised using PIV and pressure measurements. The results show that the classical notchback topology is correctly reproduced. A control system based on magnetic valves has been used to produce the pulsed jets whose properties have been characterised by hot wire anemometry. In order to shed some light on the role of the different parameters affecting the suppression of the slant recirculation area, a parametric study has been carried out by varying the frequency and the momentum coefficient of the jets for several Reynolds numbers.     

A novel geometry for rheological characterization of viscoelastic materials

A novel geometry for generating a viscometric flow presents advantages of both cone and plate and parallel plate geometries, regarding uniform shear field and adjustable range of measurement. Kinematics and dynamics of the generated flow have been described mathematically utilizing an orthogonal curvilinear coordinate system based on the shapes of the shearing surfaces which are similar to the surface that generates the flow. Simple equations that allow the calculation of quantities of experimental interest in the rheological characterization of liquid materials, namely, shear rate, shear stress and two normal stress differences, have also been derived.The geometry, called pseudosphere, was tested with two types of fluids (Newtonian and pseudoplastic). Results show that the geometry can be used with low viscosity liquids (Newtonian liquids) by only adjusting the gapH. The behavior of pseudoplastic fluids for both low and moderately high viscosity could also be studied with this geometry. Very reproducible results were obtained when compared with those obtained with cone and plate geometry. Regions of lower shear rate could be studied using only the pseudosphere geometry.