Vladimir Vassiliev

Impact of the Inflow Conditions on the Heavy-Duty Gas Turbine Exhaust Diffuser Performance

The flow in exhaust diffusers along with the channel geometry strongly depends on the inflow conditions, including Mach number level, total pressure distribution, flow angle, and turbulence. In the first part of this paper, the impact of these parameters is analyzed using computational fluid dynamics, experimental data from the test rig, and field measurements. A widespread opinion is that the optimal condition for the diffuser is an axial uniform inflow. However, it is shown in this paper that nonuniform pressure distribution compared with a uniform one can lead to better diffuser performance and that a moderate residual swirl can improve the performance as well. In the second part of this paper, the minimization of exhaust losses in heavy-duty gas turbines is discussed and illustrated by two practical examples.

Impact of the Inflow Conditions on the Heavy-Duty Gas Turbine Exhaust Diffusers Performance

The flow in exhaust diffusers along with the channel geometry strongly depends on the inflow conditions, including Mach number level, total pressure distribution, flow angle, and turbulence. In the first part of this paper, the impact of these parameters is analyzed using CFD, experimental data from the test rig and field measurements. A widespread opinion is that the optimal condition for the diffuser is an axial uniform inflow. However, it is shown in this paper that non-uniform pressure distribution as compared to a uniform one can lead to better diffuser performance and that moderate residual swirl can improve the performance as well. In the second part of the paper, the minimization of exhaust losses in heavy-duty gas turbines is discussed and illustrated by two practical examples.Copyright

Refitting of Exhaust Diffuser of Industrial Gas Turbine

This paper describes the aerodynamic optimization of the GT26 exhaust diffuser. The need for optimization was triggered by an upgrade of the compressor, resulting in a higher mass flow and a higher power output. The expansion turbine remained unchanged. However, the increase in mass flow had a significant impact on the Mach number. Secondly, the residual swirl at the turbine outlet, and therefore, the exhaust loss in original diffuser would have increased. The re-optimization of diffuser allowed minimization of the losses and improvement of the overall engine performance.© 2008 ASME

CFD Application in Design of GT Structural Components

Currently CFD is extensively used in turbine and compressor blading design and analysis with numerous examples of CFD applications to be found in literature. At the same time, there are no reports about CFD simulation of secondary flows inside the turbomachine (e.g. cooling air transit, purge, etc). One probable reason is that the amount of air/gas involved in these flows is typically too small to have a direct impact on the engine performance. However, these secondary flows have a big impact on the heat transfer in cavities between structural components, and as a result have an impact on the thermal state, life of structural components, and clearances in the main flowpath. Therefore, the proper prediction of the flow between structural components is also an important part of the design procedure. In spite of the recent progress in computational hardware and software development, CFD simulation of flow between structural components remains still a challenging task due to very complex geometries, 3D turbulent flow structure with separations, reattachments and vortices. In addition to this, to be applied in design practice, the CFD code should satisfy certain important requirements, for instance the ability to automate the calculations in order to execute the analysis within a reasonable time and cost. Only recently has it become possible to satisfy this requirement, and now CFD starts to penetrate into design practices. This paper presents the experience accumulated within ALSTOM during the last few years in CFD simulation of flow between different structural components using commercial and in-house software. The following issues are presented: • Validation of numerical models; • Automation of numerical calculation; • Application examples (the flow in different cavities between casings and in rotor cavity).Copyright

Impact of Turbine Blade Internal Cooling on Aerodynamic Loss

Modern gas turbines operate at high temperature, which exceed the endurance limit of material, and therefore the turbines components have to be cooled by the air taken from the compressor. The cooling providing positive impact on lifetime of GT has negative impact on its performance. Firstly because the cooling air bypasses combustor and its capacity is not fully utilized. This effect is usually accounted in thermodynamic calculations of gas turbine. Secondly the injection of cooling air in the turbine disturbs the main flow, and may lead to increased losses. In addition cooling requirements lead to limitation on the blade shape (e.g. limiting the minimal size of trailing edge) and thereby negatively affect the losses. These effects were already discussed in the literature, but further investigations for better understanding of flow physics and design improvement are still useful.There is also additional impact of cooling - impact of heat transfer on near wall boundary layer and coolant properties. This effect was not sufficiently discussed in the open literature, where quite often the walls are considered as adiabatic.The paper consists of two main parts. In the first part the results of experimental investigations of several linear cascades with and without trailing edge injection are presented and discussed.In the second part the results of detailed numerical investigations of one of these cascades are presented. One set of calculations were done at the test rig conditions for comparison with measured data. These calculations were used for validation of CFD model. The next sets of calculations were done for engine typical conditions, including the simulation of blade material temperature. The calculations were performed for adiabatic wall and for surface with heat transfer, including the impact of heat transfer on coolant injection. This analysis provides split of losses caused by different factors, and offers the opportunities for efficiency and lifetime improvements of real engine designs/upgrades.Copyright

Simulation of Silo Combustor Liners Transient Behavior for Advanced Design Upgrade

Many Alstom heavy-duty gas turbines with a silo combustor are in service and moreover undergoing upgrades for performance augmentation, lifetime extension, and emission reduction. The combustor liners, which are exposed to high gas temperatures, may require design tuning for these upgrades, and therefore reliable simulation of their behavior is of utmost importance.This paper focuses on a case study of transient behavior of the Hot Gas Casing, a transition liner between compressor, turbine and silo combustor. Following a three-dimensional thermal assessment based on computational fluid dynamics, detailed structural analysis is done to identify the drivers behind different types of Hot Gas Casing deformation, observed after upgraded combustor introduction.Thermal Barrier Coating application is proposed to reduce Hot Gas Casing bending. The solution was confirmed analytically and successfully introduced in the field. Good correlation of field findings and finite element prediction was found. It is shown that only a combined effect of thermal deformation, cyclic loading and creep may explain the observed behavior of the part.A detailed design sensitivity study is performed, comparing different approaches to deformation reduction: application of ribs, corrugations and wall thickness increase. The best solution in terms of manufacturability and its impact on part deformation is chosen. It is found that wall corrugation does not provide the desired effect due to the nature of part deformation.Copyright

Exhaust Diffuser Characteristics at Off-Design Conditions

In todays electricity market with a strong mix of renewables and traditional energy sources, heavy-duty gas turbines often have to operate at part load with decreased exhaust mass flow. Decreased mass flow leads to reduced Mach number and this factor drives the exhaust loss down. At the same time off-design conditions lead to reduction of diffuser pressure recovery, and this factor drives loss up. The latter is normally stronger, and therefore the losses at GT low load are higher than at base load.Traditionally exhaust diffusers were optimised for base load operation, and their characteristics were analysed in range close to this regime. However with increased part load operation it became important to investigate strong off-design conditions as well. In this work the numerical analysis of diffuser flow at different conditions corresponding to GT base load and different part loads is performed.In the first part of the paper the numerical model and results of calculations are discussed. The calculations are compared with measurements in real engine, and this comparison demonstrates that numerical model provides good predictions not only for design conditions, but for off-design conditions as well.The validated numerical model was then applied to analysis of diffuser geometry impact on the off design conditions, and the second part of the paper describes the results of these calculations. The analysis showed that modification of central body and front part of diffuser have negligible impact on losses at off design conditions, but significantly reduce performance at base load leading to non-optimal redistribution of losses between different regimes. Therefore original diffuser configuration provides the best compromise for wide operational window.Copyright

On Inflow Distortion Decay in the Blade Cascades

Flow distortions in front of turbomachines can lead to additional mechanical and thermal loading of the blades, and may influence the operational stability of the engine and the lifetime of its parts. In current design practice the impact of inflow distortions is accounted by safety margins, which are often conservative. Therefore the knowledge of real limits helps to increase operational flexibility and recommended service intervals.The distortions inside turbomachines have a wide spectrum of sizes and frequencies depending on their origin. Some of them were extensively studied in the recent years, including wakes interaction with blades, hot streaks propagation in turbines, and some other phenomenon. However, there are areas of practical importance, which are not yet covered by available studies. One of this distortion types is investigated in the present work.These are distortions of a size, comparable with the inlet guide vane pitch of a compressor or the nozzle guide vane pitch of a turbine, both steady in the absolute frame of reference. Such distortions in front of a compressor can be caused, for example, by fouling of the intake, or by some structures purposely installed inside the intake. The distortions in front of the expansion turbine can be caused by structures.Main purpose of the study is to provide an understanding of the decay mechanism of distortions considered. For this purpose the numerical and theoretical analysis of inflow distortion propagation inside the set of stationary and rotating blades cascades is performed. These cascades represent the mid-section of a typical multistage compressor and turbine.The analysis showed that the decay mechanism is different for temperature distortions and total pressure distortions, and that the latter one is stronger affected by the blading configuration.Copyright

Impact of the 3D Flow Effects on the Silo Combustor Thermal State

Many Alstom heavy-duty gas turbines with a silo combustor are in service and moreover undergoing upgrades for performance augmentation, lifetime extension, and emission reduction. Several structural parts of the combustor are exposed to high gas temperatures, and therefore their lifetime depends mainly on the metal temperatures, which must be kept within the acceptable limits. This paper describes methodology based on the state of the art methods of 3D CFD and finite element (FE) computations, which are combined into the computational process for the reliable silo combustor thermal analyses.In the first part of the paper the computational model of the silo combustor is discussed. The model comprises CFD models simulating the hot gas path and the cooling air supply system, as well as the FE model of the structural parts. The CFD models predict the gas temperatures and heat transfer coefficients that are used by the FE model for calculating the metal temperatures.In the second part of the paper the computational results are presented and several 3D flow phenomena are analysed in details. One effect is the interaction of dilution jets in swirled cross flow. At different operation conditions, pairing of those jets occurs, which generates the periodic metal temperature distribution, as recorded in the field. This analysis also revealed factors, which influence on the temperature distribution. The combustor simulation delivers an insight into non-homogeneous temperature profiles in front of the turbine behind the transition channel of the silo-combustor. Finally, by adding leakages into the flow model, the interesting example of the non-homogeneous leakage of cold air, which can lead to local increase of material temperature, was simulated. All these simulations led to reliable silo-combustor upgrades.Copyright

On a Numerical Method for the Simulation of Steady Jet Array Interaction With Rotating Components

Experimental and theoretical investigations show that unsteady effects like moving wakes, tip vortices, passing shocks, pulsating injections and other similar structures significantly affect the aerodynamic characteristics of turbines and compressors. They also influence the thermal state and lifetime of components. Therefore it is very important for designers of turbomachines to properly simulate these effects. On the other hand, time-accurate computations are still expensive and require substantial resources in CPU and computer memory. Moreover the elapsed time is high. However in certain cases the numerical model for unsteady calculations can be simplified, allowing proper capture of the unsteadiness impact, but with much less required computing capacity. This makes the approach acceptable for design applications. Such a simplified method, applicable to a simulation of a steady jets array interaction with rotating components, is described in this paper. The advantages and limitations are discussed, and the validation results and application examples are presented.Copyright