Endwall Region Research Articles

Aerodynamics Performance of Endwall Film Cooling under the Influence of Purge Flow in High Pressure Turbine Cascade

Modern gas turbine requires sophisticated cooling technologies to avoid thermal failure due to the extreme operating environment. Film cooling is one of the most important cooling technologies used for gas turbine hot-section components, particularly for blade aerofoil surfaces and endwall. Previous research has shown that the endwall region is considerably more difficult to cool than the blade aerofoil surfaces because of the existence of complex secondary flow structures such as horse-shoe vortex, cross flow and passage vortex in the blade passage. Therefore, this study focuses on aerodynamics interaction of the cooling air through the upstream slot with the secondary flow field. Experiments carried out using 5-holes Pitot tube have revealed the secondary flow field at blade downstream of linear cascade of high pressure turbine. A baseline condition without any leakage flows was compared with the leakage ejection case. Finally, both cases were validated by simulations from commercial software, ANSYS CFX.

Comparative investigation of unsteady flow interactions in endwall regions of shrouded and unshrouded turbines

The flow in turbomachinery is inherently unsteady, and the endwall losses are major sources of lost efficiency in turbine cascades. Therefore, the investigation of unsteady endwall flow interactions and the consideration of the effects into turbine design are valuable to improve the turbine performance. Comparative investigation into the physical mechanisms of unsteady endwall flow interactions of 1.5-stage shrouded and unshrouded turbines are performed by using a three-dimensional Navier–Stokes viscous solver. Emphasis is placed on how unsteady stator–rotor interactions affect turbine endwall secondary flows, and the feasibility of incorporating the unsteady endwall flow effects in turbine design is also discussed. The results show that unsteady interactions between upstream wake, tip leakage vortex/mixing zone and downstream passage vortex are the main factor affecting turbine endwall secondary flows. Unsteady interactions can reduce the radial vorticity of turbine endwall secondary flows, and the effects of these interactions on the streamwise vorticity of endwall secondary flows depend on upstream wake characteristics. The properly controlled unsteady interactions can reduce the size and intensity of endwall secondary flows, and thus improve the turbine performance. Because of the difference of turbine tip architectures, the periodic fluctuations of the flow in the shrouded turbine have smaller amplitude than those in the unshrouded turbine, and the shrouded turbine is of better unsteady performance than the unshrouded turbine.

Performance improvement of shrouded turbines with the management of casing endwall interaction flows

The flow in turbomachinery is inherently unsteady, and the understanding of the flow interaction in the casing endwall region of shrouded turbine stages is a key requirement in achieving further improvements in performances. Thus, comparative numerical investigations of casing endwall flow interactions of 1.5-stage shrouded turbines with straight and bowed vanes are performed, in an attempt to incorporate flow unsteadiness in the control of casing endwall secondary flows and losses. The investigation focuses on a comparison of the flow interaction mechanisms of casing secondary vortices with upstream straight and bowed wakes, and the influences on the performance of casing endwall region are also discussed and quantified. The results show that the bowed wake at different radial sections passes different distances to reach rotor leading edge, which affects their consequent interactions with following blades. And, this is quite similar to clocking effects but with a limited extent. The interaction with upstream bowed wakes has a beneficial effect on casing endwall flow behavior resulting in weakened secondary flows. The combined unsteady effects on turbine performance depend on the balance of unsteady endwall mixing and endwall secondary flows, and the negative impact of these effects should be alleviated to achieve enhanced performance.

Effects of Unsteady Wakes on the Secondary Flows in the Linear T106 Turbine Cascade

In modern low pressure turbines the efforts to increase aerodynamic blade loading by increasing blade pitch and optimizing midspan performance in order to reduce weight and complexity can produce increased losses in the endwall region. Airfoils of high flow turning and high pressure gradients between the blades generate strong secondary flows which impair the global aerodynamic performance of the blades. In addition, the unsteady incoming wakes take influence on transition phenomena on the blade surfaces and the inlet boundary layer, and consequently affect the development and the evolution of the secondary flows. In this paper, the T106 cascade is used to identify the effect of unsteady wakes on the development of secondary flows in a turbine cascade. Numerical and experimental results are compared at different flux coefficients and Strouhal numbers, the relative differences and similarities are analyzed.

A conjugate axisymmetric model of a high-pressure shock-tube facility

Purpose– An axisymmetric shock-tube model of the high-pressure shock-tube facility at the Texas A&M University has been developed. The shock tube is non-conventional with a non-uniform cross-section and features a driver section with a smaller diameter than the driven section. The paper aims to discuss these issues.Design/methodology/approach– Computations were carried out based on the finite volume approach and the AUSM+ flux-differencing scheme. The adaptive mesh refinement algorithm was applied to the time-dependent flow fields to accurately capture and resolve the shock and contact discontinuities as well as the very fine scales associated with the viscous effects. The incorporation of a conjugate heat transfer model enhanced the credibility of the results.Findings– The shock-tube model is validated with simulation of the bifurcation phenomenon and with experimental data. The model is shown to be capable of accurately simulating the shock and expansion wave propagations and reflections as well as the flow non-uniformities behind the reflected shock wave as a result of reflected shock/boundary layer interaction or bifurcation. The pressure profiles behind the reflected shock wave agree with the experimental results.Originality/value– This paper presents one of the first studies to model the entire flow field history of a non-uniform diameter shock tube with a conjugate heat transfer model beginning from the bursting of the diaphragm while simultaneously resolving the fine features of the reflected shock-boundary layer interaction and the post-shock region near the end-wall, at conditions useful for chemical kinetics experiments. An important discovery from this study is the possible existence of hot spots in the end-wall region that could lead to early non-homogeneous ignition events. More experimental and numerical work is needed to quantify the hot spots.

Study on Impact Property and Fracture Morphology of Injection-molded Optical-grade Polycarbonate

The low-velocity, low-energy impact response of optical-grade polycarbonate (PC) was characterized by the Izod impact testing at ambient temperature. The following factors affecting impact response were investigated: mold temperatures (80, 90, 100, 110, and 120°C) and annealing treatment (120°C for 12 h). The results showed that the annealing treatment remarkably reduced the impact strength. The maximum impact strength was obtained when the mold temperature was 100°C for both unannealed samples and annealed ones. Moreover, the annealing treatment changed the failure mode of specimens from ductile failure to brittle failure, which was confirmed by fracture morphology analysis using scanning electron microscopy (SEM). The ductile failure was attributed to shearing behavior, and the fracture surfaces were rough and irregular with many river-shaped striations. The brittle fracture was caused by a craze failure mechanism. The brittle fracture sections could be divided into three regions: fracture origin, mist region, and end-wall banded region.

The Application of 3D Blading and Non-Axisymmetric Hub Endwall Contouring in a Dual-stage Axial Flow Counter-Rotating Compressor

D blading and non-axisymmetric endwall contouring technologies are effective ways to control secondary flows in cascades. 3D blading can effectively reduce the secondary flows near the blade surface, but it has little impact on the endwall region. Non-axisymmetric endwall contouring is widely used in the control of endwall secondary flows in turbines. However, the use of non-axisymmetric endwall contouring in compressors is rarely. The effect of non- axisymmetric endwall contouring on compressors is not clear. The research object of the report is an axial flow dual-stage counter-rotating compressor. In order to further improve the performance of the compressor and to explore the method of reducing secondary flow loss, the two rotor blade rows and the hub endwall of the secondary rotor row are redesigned by 3D blading and non-axisymmetric endwall contouring technologies based on optimization algorithm. The flow fields of original compressor and redesigned compressor are also compared. After 3D blading optimization, the secondary flows near the suction surface of the rotor blades are obviously reduced and the efficiency of the optimization point increases. Nonetheless, the secondary flows at the hub endwall improve scarcely any. Based on the 3D blading optimization, the second row rotor hub endwall are parameterization designed, and non- axisymmetric endwall contouring is designed by optimization algorithm. After non-axisymmetric endwall contouring optimization, the hub endwall consist of a hill near the rotor pressure surface and a valley near the suction surface. The contouring reduces the pressure gradient at 0~40% axial chord near the hub. And the secondary flow loss is reduced effectively. This paper successfully combines the advantages of both 3D blading and non-axisymmetric endwall contouring technologies; the performance of the counter-rotating compressor is improved

1001 高負荷低圧タービン翼の負荷分布が二次流れに与える影響の調査

This paper details experimental and studies on the secondary flow losses generated in highly loaded LPT (Low-Pressure Turbine) cascades. Secondary flow loss in LPT significantly contributes to its overall loss, especially in increased stage loading and high-lift airfoil cases. Hence it becomes more important to reduce the secondary flow loss. In the present study, we focus on the effects of load distributions on the secondary flow loss in the highly loaded turbine blade as a method of loss reduction. Detailed experiments are carried out in the cascade outlet endwall region using a 5-hole pressure probe.

Numerical and experimental investigation of the turbulent flow through a low‐pressure turbine cascade in the endwall region

AbstractIn the present work, direct numerical simulations (DNS) of the flow through a low‐pressure linear turbine cascade T106 with parallel endwalls were conducted to investigate the effects of unsteady passing wakes of the upstream blade row on the secondary flow in the endwall region of the passage. The impact of the wakes on the secondary flow is discussed by means of the time‐averaged values. Furthermore, the results of DNS are compared with experimental data. (© 2013 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermomechanical Stress Distributions in a Gas Turbine Blade Under the Effect of Cooling Flow Variations

Thermomechanical stresses in gas turbine blades are investigated. Attention is focused on effects caused by varying the cooling airflow that runs through the blade interior, keeping constant a mainstream condition around the blade surface. Stress concentration was predicted numerically under engine real operating conditions. Temperature distributions in the metal blade surface produced by convective boundary conditions were linked with heat conduction within the blade using a conjugate solution. Results of stress concentration in the blade material for reduced cooling flow rate, blocked cooling ducts, and rotation rate were obtained. It is shown that temperature and stress distributions are a strong function of position in blade interior material and surface. Thermomechanical stress concentration was observed in the leading edge, with the endwall region affected by large stress concentration. Stress magnitude increments were found for combined cyclic thermal heating and sustained mechanical loads on specific planes of airfoil span for reduced cooling flow. Also, large stress gradients between leading and trailing regions of the blade were observed. The study reveals that blocking channels increase stresses in the central region of blade transversal cross section.

Vortex control and aerodynamic performance improvement of a highly loaded compressor cascade via inlet boundary layer suction

Effects of inlet boundary layer suction on the vortex structure and cascade loss in a highly loaded compressor cascade were investigated experimentally. Ink-track visualization was undertaken on cascade endwall and the blade surface. Ten traverse planes from upstream to downstream of the cascade in a rectangular wind tunnel were measured by an L-shaped five-hole probe. These tested planes revealed the process of emergence, development and decline of several principal vortices as well as the corresponding additional losses. Details of ink-track visualization displaying the secondary flow behavior of boundary layer upon endwall and blade surface assist to make judgment on vortex evolution. Inspection of the vortex structure revealed that highly loaded compressor was characterized by large-scale vortices in the endwall region. After suction, these vortices are all well organized and under control. Among all of them, passage vortex is most sensitive to the variation of the inlet boundary layer, and its main function is to spread low-energy fluid rather than to produce loss. On the other hand, a wall vortex and a concentrated shedding vortex take place inside and after the cascade, respectively, and engender considerable accompanying loss as they dissipate. The effects of inlet boundary layer suction on them are correspondingly weaker. About one forth of the total loss in the baseline cascade was eliminated when boundary layer suction flow rate reaches 2.5 % of the inlet mass flow. The feasibility of simplifying the suction system is also verified through this work.

Experimental Studies of the Endwall Flow in a Multistage Low-Speed Axial Compressor

Detailed flowfield measurements are made in the end-wall region of a four-stage low-speed large-scare axial compressor test rig. Two ten-hole survey boundary layer probes, one four-hole pneumatic probe, and six dynamic pressure probes each bedded with a high-frequence response Kulite sensor, are designed and manufactured to aid the test. Boundary layer region and mainflow region can be clearly distinguished from the boundary layer measurement results. A few parameters, including boundary layer thickness, displace thickness, momentum loss thickness, energy loss thickness and blockage coefficient are calculated based on the measurements. Results from turbulent boundary layer empirical formula of plate are in good agreement with the mesurements, indicating a way of boundary layer effect estimation. The evolution trajectory of TCV is identified evidently by dynamic pressure measurements. The maximum aerodynamic load point on the blade moves from the trailing to the leading edge as flowrate decreases, which leads to the forward movement of the tip clearance vortex inception point.

Multi-scale modeling of a spray coating process in a paddle mixer/coater: the effect of particle size distribution on particle segregation and coating uniformity

The purpose of this paper is to investigate the effect of particle size distribution on the flow dynamics, segregation, and coating uniformity in an industrial paddle mixer/coater. First principles modeling approaches with the discrete element method (DEM) and spray post-processing analysis were applied to model particle motion and coating growth. A monosize seed distribution with a particle size of 10.52mm was used as a baseline DEM simulation. Important spray region variables such as the residence time in the spray region, number of spray region visits, and area exposure percentage were analyzed to characterize the mixing dynamics in the spray. The seed particles were then changed to a log normal size distribution with a relative standard deviation varying from 0.134 to 0.389. Small particles appear preferentially in the spray zone in all polydisperse particle systems, and further analysis on the size dependence of the spray variables shows that small particles have a relatively high frequency of spray zone visits and low shielding. The binary particle DEM simulations with 11.69mm large particles and 9.01mm small particles demonstrate segregation patterns in which large particles collect in the end wall region and small particles collect in the spray region, which explains the observed spray preference for small particles. The results of this study highlight the need to understand the effect of mixer geometry on segregation in the particle flow field in order to predict the coating preference with particle size.

Influence of non-axisymmetric terms on circumferentially averaged method in fan/compressor

The governing equations are derived by circumferentially averaging the three-dimensional (3D) Navier-Stokes equations, which are solved using a time marching finite volume approach. Both Euler throughflow model and Navier-Stokes (N-S) throughflow model are employed to investigate the performance and flow fields of a highly loaded transonic single-stage fan ATS-2 and a four-stage fan. The results are compared with the experimental and three-dimensional computational results. It shows that the throughflow models can provide reasonable performance characteristics and N-S throughflow model gives better predictions in endwall regions. A throughflow computation in which all the non-axisymmetric terms are included has been performed at off-design condition and the radial distributions of the flow field can be well described.

Effect of honeycomb seals on loss characteristics in shroud cavities of an axial turbine

The loss in efficiency due to shroud leakage or tip clearance flow accounts for a substantial part of the overall losses in turbomachinery. It is important to identify the leakage loss characteristics in order to optimize turbomachinery. At present, little information is available in the open literature concerning the effect of honeycomb seals on the loss characteristics in shroud cavities of an axial turbine, despite of the widespread use of the honeycomb seals. Therefore, interaction between rotor labyrinth seal leakage flow with and without honeycomb facings and main flow is investigated to provide the loss characteristics of the mixing process of the re-entering leakage flow into the main flow. The effects of honeycomb seals on the flow in shroud cavities and interaction with the main flow are analyzed. An additional study on the impact of subtle shroud cavity exit geometry is also presented. The investigation results indicate that the honeycomb seal affects the over tip leakage flow and reduces mixing losses when compared to the solid labyrinth seal. The leakage flow interactions with the main flow have considerably changed the flow fields in the endwall regions. The proposed research reveals the effects of honeycomb seals on the loss characteristics in shroud cavities and the impact of subtle shroud cavity exit geometry, and it is helpful for the design optimization of turbomachinery.

Aero-Thermal Performance of Purge Flow in Turbine Cascade Endwall Cooling

The endwall and blade film cooling systems are the typical solution adopted within gas turbines to allow further increase of turbine inlet temperature, avoiding critical material thermal stresses. Due to complex secondary flow field in the blade passage, endwallis more difficult to cool than blade surfaces. In the matter of fact, in endwall film cooling studies, it is necessary to investigate the interaction between coolant air and the secondary flow. In present study, the flow field of high-pressure turbine cascade has been investigated by 5-holes pitot tube to reveal the secondary flows behavior under the influenced of purge flows while the heat transfer measurement was conducted bythermochromic liquid crystal (TLC). Experimental has significantly captured theaerodynamics effect of purge flowat blade downstream close to the endwall region. Furthermore, TLC measurement illustrated that the film cooling effectiveness and heat transfer coefficient contours were strongly influenced by the secondary flow on the endwall.

Secondary Flow Loss Reduction Through Blowing for a High-Lift Front-Loaded Low Pressure Turbine Cascade

Efforts to increase individual blade loading in the low pressure turbine have resulted in blade geometries optimized for midspan performance. Many researchers have shown that increased blade loading and a front-loaded pressure distribution each separately contribute to increased losses in the endwall region. A detailed investigation of the baseline endwall flow of the L2F profile, which is a high-lift front loaded profile, is performed. In-plane velocity vectors and total pressure loss maps are obtained in five planes oriented normal to the blade surface for three Reynolds numbers. A row of pitched and skewed jets are introduced near the endwall on the suction surface of the blade. The flow control method is evaluated for four momentum coefficients at the high Reynolds number, with a maximum reduction of 42% in the area averaged total pressure loss coefficient. The same blade is also fitted with midspan vortex-generator jets and is tested at a Reynolds number of 20,000, resulting in a 21% reduction in the area averaged total pressure loss.

Heat Transfer for the Film-Cooled Vane of a 1-1/2 Stage High-Pressure Transonic Turbine—Part I: Experimental Configuration and Data Review With Inlet Temperature Profile Effects

This paper investigates the vane airfoil and inner endwall heat transfer for a full-scale turbine stage operating at design corrected conditions under the influence of different vane inlet temperature profiles and vane cooling flow rates. The turbine stage is a modern 3D design consisting of a cooled high-pressure vane, an un-cooled high-pressure rotor, and a low-pressure vane. Inlet temperature profiles (uniform, radial, and hot streaks) are created by a passive heat exchanger and can be made circumferentially uniform to within ±5% of the bulk average inlet temperature when desired. The high-pressure vane has full cooling coverage on both the airfoil surface and the inner and outer endwalls. Two circuits supply coolant to the vane, and a third circuit supplies coolant to the rotor purge cavity. All of the cooling circuits are independently controlled. Measurements are performed using double-sided heat-flux gauges located at four spans of the vane airfoil surface and throughout the inner endwall region. Analysis of the heat transfer measured for the uncooled downstream blade row has been reported previously. Part I of this paper describes the operating conditions and data reduction techniques utilized in this analysis, including a novel application of a traditional statistical method to assign confidence limits to measurements in the absence of repeat runs. The impact of Stanton number definition is discussed while analyzing inlet temperature profile shape effects. Comparison of the present data (Build 2) to the data obtained for an uncooled vane (Build 1) clearly illustrates the impact of the cooling flow and its relative effects on both the endwall and airfoils. Measurements obtained for the cooled hardware without cooling applied agree well with the solid airfoil for the airfoil pressure surface but not for the suction surface. Differences on the suction surface are due to flow being ingested on the pressure surface and reinjected on the suction surface when coolant is not supplied for Build 2. Part II of the paper continues this discussion by describing the influence of overall cooling level variation and the influence of the vane trailing edge cooling on the vane heat transfer measurements.

Flow Measurements in a First Stage Nozzle Cascade Having Endwall Contouring, Leakage, and Assembly Features

This work supports new gas turbine designs for improved performance by evaluating the use of endwall contouring in a cascade that is representative of a first stage stator passage. Contouring accelerates the flow, reducing the thickness of the endwall inlet boundary layer to the turbine stage and reducing the strength of secondary flows within the passage. The reduction in secondary flows leads to less mixing in the endwall region. This allows for an improved cooling of the endwall and airfoil surfaces with injected and leakage flows. The present paper documents the component misalignment and injected and leakage flow effects on the aerodynamic losses within a passage that has one contoured and one straight endwall. Steps and injected flows within the passage can lead to thicker endwall boundary layers, stronger secondary flows, and possibly additional vortex structures in the passage. The paper compares losses with various steps, gaps, and leakage flows to assess their importance in this contoured passage. In particular, features associated with the combustor-to-turbine transition piece and the slashface on the vane platform are addressed. An n-factorial study is used to quantify the importance of such effects on aerodynamic losses.

Investigation of endwall flow behavior with plasma flow control on a highly loaded compressor cascade

To discover the flow behavior in the endwall region and mechanism of plasma flow control on a highly loaded compressor cascade, distributions of static pressure coefficient, total pressure loss coefficient and streamline pattern were investigated. Results show that cross flow from the pressure surface to neighboring suction surface exists under pitch-wise pressure gradient. The deflected endwall boundary layer flow interacts with the incoming flow, and then both of them leave off the endwall in the form of a span-wise vortex. Effect of angle of attack on static pressure is greater than that of free stream velocity. The distinct variations of total pressure loss with endwall actuations are mainly located within the outer verge of a triangular area with high total pressure loss. Effect of pitch-wise actuation on separated flows is much better than that of stream-wise actuation, and both enhance with the increase of angle of attack and actuation strength. An efficient method for plasma flow control in the endwall region is the increase of actuation strength, such as adjusting discharge voltage or changing plasma power supply.