Luca Bozzi

Development of Numerical Tools for Stator-Rotor Cavities Calculation in Heavy-Duty Gas Turbines

In high performance heavy-duty engines, turbine inlet temperature is considerably higher than the melting point of the metals used for turbine components e.g. nozzle guide vanes, turbine rotor blades, platforms and discs, etc. Cooling of those components is therefore essential and is achieved by diverting a few percent of the compressed air from extraction points in the compressor and passing it to the turbine through stationary ducts and over rotating shafts and discs. All those elements form the so-called secondary air system of the gas turbine, whose correct design is hence fundamental for safety, reliability and performance of the engine. Secondary air system analysis is generally performed using one dimensional calculation procedures, based correlations both for pressure losses and heat transfer coefficient evaluations. Such calculation approach, usually used in industry, takes advantages in terms of reduced computational resources. Besides, for those elements of air systems where multidimensional flow effects are not negligible and the flow field structure is highly complex, the one-dimensional–correlative modeling needs to be supported by CFD investigations. Among these elements, rotating cavities need a careful modeling in order to correctly estimate discs temperature and the minimum amount of purge air to prevent hot gas ingestion. Ansaldo Energia is facing the investigation of secondary air system of Vx4.3A gas turbine models also by using numerical tools developed by Dipartimento di Energetica “Sergio Stecco” of University of Florence. They include both a one-dimensional cavity solver and a 3D unstructured finite volume code of compressible Navier-Stokes Equation based on open source C++ Open-Foam libraries for continuum mechanics. The first numerical tool has been widely employed in simplified analysis of stator-rotor cavities and is undergoing to be integrated into a in-house lumped-parameters fluid network solver simulating the entire secondary air system. This paper is aimed at discussing some interesting results from numerical tests performed with the above discussed programs on stator-rotor cavities of a V94.3A2 gas turbine. Such numerical analysis was addressed both for better understanding the flow phenomena in the wheel space regions and for testing and verifying the experimental correlations and the calculation procedure implemented in the one-dimensional program. A detailed comparative analysis between the two different codes will be shown, both in adiabatic and heat transfer conditions.Copyright

Heavy Duty Gas Turbine Simulation: Global Performances Estimation and Secondary Air System Modifications

This paper reviews a modular-structured program ESMS (Energy System Modular Simulation) for the simulation of air-cooled gas turbines cycles, including the calculation of the secondary air system. The program has been tested for the Ansaldo Energia gas turbine V94.3A, which is one of the more advanced models in the family Vx4.3A with a rated power of 270 MW. V94.3A cooling system has been modeled with SASAC (Secondary Air System Ansaldo Code), the Ansaldo code used to predict the structure of the flow through the internal air system. The objective of the work was to investigate the tuning of the analytical program on the basis of the data from design and performance codes in use at Ansaldo Energy Gas Turbine Department. The results, both at base load over different ambient conditions and in critical off-design operating points (full-speed-no-load and minimum-load), have been compared with APC (Ansaldo Performance Code) and confirmed by field data. The coupled analysis of cycle and cooling network shows interesting evaluations for components life estimation and reliability during off-design operating conditions.Copyright

Procedure for Calculation of Component Thermal Loads for Running Clearances of Heavy-Duty Gas Turbines

The energy market development in the last decade has been influenced by several driving factors. To meet the strict customers’ requirements (related to low emissions, flexibility and high performances), operate gas turbine plants safely over a variety of off-design operating conditions has been fundamental. Accordingly, accurate evaluation of running clearances in stationary and transient conditions plays a significant role. On the other hand, the study of heat transfer in turbo-machinery is a fundamental activity to study the implications of off-design operating on components’ lifing.Focus of the paper is the analysis of variations of components’ wall temperature and clearances due to the fluctuation of thermal loads acting on gas turbines components during operation.The study is divided into two main sections: heat transfer analysis allows evaluating thermal loads and then a FEM analysis is performed in order to calculate the radial and axial clearances between rotor components and casing.Thermal loads are obtained by a computational method based on heat transfer correlations. Parametric curves have been developed to calculate variations of thermal loads in transient conditions from steady-state data obtained by the correlative computational tool.The two procedures for heat transfer analysis and evaluation of clearances have been validated against experimental data in several operating conditions. In particular, relative and absolute movements of rotor and turbine casing have been measured by means of proxy-meter probes located into the turbine bearing casing.Copyright

Experimental Investigation on Leakage Losses and Heat Transfer in a Non Conventional Labyrinth Seal

Different labyrinth seal configurations are used in modern heavy-duty gas turbine such as see-through stepped or honeycomb seals. The characterization of leakage flow through the seals is one of the main tasks for secondary air system designers as well as the evaluation of increase in temperature due to heat transfer and windage effects. In high temperature turbomachinery applications, knowledge of the heat transfer characteristics of flow leaking through the seals is needed in order to accurately predict seal dimensions and performance as affected by thermal expansion. This paper deals with the influence of clearance on the leakage flow and heat transfer coefficient of a contactless labyrinth seal. A scaled-up planar model of the seal mounted in the inner shrouded vane of the Ansaldo AE94.3A gas turbine has been experimentally investigated. Five clearances were tested using a stationary test rig. The experiments covered a range of Reynolds numbers between 5000 and 40000 and pressure ratios between 1 and 3.3. Local heat transfer coefficients were calculated using a transient technique. It is shown that the clearance/pitch ratio has a significant effect upon both leakage loss and heat transfer coefficient. Hodkinson’s and Vermes’ models are used to fit experimental mass flow rate and pressure drop data. This approach shows a good agreement with experimental data.Copyright

A Simplified Model to Assess the Influence of Load Ramps on the Durability of Hot Gas Path Components in Heavy-Duty Gas Turbines

According to current trends in the energy market, heavy duty gas turbines are increasingly being used to fill gaps in the power energy supply and are less frequently operated in pure steady-state base load conditions. This tendency implies more rapid load ramps and is confirmed by utilities’ requirements for more operational flexibility in order to increase their net revenues. In order to assess the effects of such load variations on temperature gradients withstood by the various components, a series of simple correlations are derived that take in account key operating parameters of gas turbines. To this end, each blade and vane has been schematized as a compound of different portions to which specific values of cooling efficiency and gas temperature were assigned. This results in a simplified model of the engine allowing for the prediction of the temperature gradients on the base material of the critical zones of blades and vanes as a function of different cooling schemes. Method results can subsequently be exploited both to improve thermal design of hot gas path components, as well as to set up material testing campaigns targeting any specific duty cycle.Copyright

Numerical and Experimental Investigation of Secondary Flows and Influence of Air System Design on Heavy-Duty Gas Turbine Performance

High turn-down operating of heavy-duty gas turbines in modern Combined Cycle Plants requires a highly efficient secondary air system to ensure the proper supply of cooling and sealing air. Thus, accurate performance prediction of secondary flows in the complete range of operating conditions is crucial.The paper gives an overview of the secondary air system of Ansaldo F-class AEx4.3A gas turbines. Focus of the work is a procedure to calculate the cooling flows, which allows investigating both the interaction between cooled rows and additional secondary flows (sealing and leakage air) and the influence on gas turbine performance. The procedure is based on a fluid-network solver modelling the engine secondary air system. Parametric curves implemented into the network model give the consumption of cooling air of blades and vanes. Performances of blade cooling systems based on different cooling technology are presented. Variations of secondary air flows in function of load and/or ambient conditions are discussed and justified.The effect of secondary air reduction is investigated in details showing the relationship between the position, along the gas path, of the upgrade and the increasing of engine performance. In particular, a section of the paper describes the application of a consistent and straightforward technique, based on an exergy analysis, to estimate the effect of major modifications to the air system on overall engine performance. A set of models for the different factors of cooling loss is presented and sample calculations are used to illustrate the splitting and magnitude of losses. Field data, referred to AE64.3A gas turbine, are used to calibrate the correlation method and to enhance the structure of the lumped-parameters network models.© 2012 ASME

A Higher Turndown Flexibility on AE64.3A Gas Turbine: Design and Operating Experience

Italian power generation market is living today a period of substantial changes due to the liberalization process, climate issues, natural gas price fluctuation and the uncertain future of nuclear and coal. In this framework, many gas turbine power plants, originally designed to operate mainly at base load, feel the necessity to be flexibly and profitably operated into the dispatch and ancillary energy service market. In particular, many operators ask for the possibility to operate their gas turbines intermittently, frequently cycling and quickly ramping up and down to satisfy energy demand. Such using drafts new trade off between profitability and maintenance cost. From this point of view it’s not unusual to shut down the engine when the power demand is low if the unit cannot be cost effectively parked at a suitable low load and then quickly ramped up to base load when the power demand is higher. The main barrier against lowering the minimum load of the gas turbines is the increase of the CO emission. When the engine operates close to its turndown load the compressor airflow is such that the heat released by the flame cannot properly support the conversion of CO into CO2 . In such a condition, the power plant will not comply with the environmental legislation and must be operated at a higher load or, worse, shut down. An operating strategy has been devised to face up such problem. It is based on the adjustment of compressor IGV (Inlet Guide Vanes) and the optimisation of cooling air consumption in order to keep the proper amount of combustion air close to the turndown load. This paper shows the feasibility check, the installation and final field tests of the low load turndown upgrade on a AE64.3A gas turbine which allowed to operate the unit in a more cost effective way even when the power demand is low.Copyright

Heavy-Duty Gas Turbines Axial Thrust Calculation in Different Operating Conditions

Several types of forces give a contribution to the axial thrust of gas turbines shafts: flow-path forces (due to blades, endwalls and shrouds of compressor and turbine rows), forces acting on the surfaces of rotor-stator cavities, disks forces (due to the different pressure levels in the rotating cavities inside the rotor), etc. As a rule, the estimation of the rotor thrust needs the handling of a large amount of output data, resulting from different codes. This paper presents a calculation tool to estimate the rotor axial thrust from the results of compressor, turbine and secondary air system calculations. Applications to heavy-duty gas turbines of different classes and sizes (namely two models of AEx4.3A F-class family, AE64.3A and AE94.3A, and the AE94.2 E-class gas turbines) are presented. On the basis of calculation results, in base load and part load operating conditions, guidelines to determine the rules of variation of axial bearing thrust and the relating scatter band are given. Pressure transducers were installed on the bearing pads of different gas turbines, in order to provide experimental data for the calibration of the calculation procedure. Comparison of experimental data with numerical results proves that the proposed calculation tool properly evaluates gas turbines rotor thrust and the axial bearing load.Copyright

1D Tool for Stator-Rotor Cavities Integrated Into a Fluid Network Solver of Heavy-Duty Gas Turbine Secondary Air System

In order to improve performance of heavy-duty gas turbines, in terms of efficiency and reliability, accurate calculation tools are required to simulate the SAS (Secondary Air System) and estimate the minimum amount of cooling and sealing air to ensure the integrity of hot gas path components. A critical component of this system is the cavity formed between coaxial rotating and stationary discs, that needs a sealing flow to prevent the hot gas ingestion. This paper gives a general overview of a 1D tool for the analysis of stator-rotor cavities and its integration into an “in-house” developed fluid network solver to analyse the behaviour of the secondary air system over different operating conditions. The 1D cavity solver calculates swirl, pressure and temperature profiles along the cavity radius. Thanks to its integration into the SAS code, the cavity solver allows estimation of sealing air flows, taking into account directly of the interaction between inner and outer extraction lines of blades and vanes. This procedure has been applied to the AE94.3A secondary air system and the results are presented in terms of sealing flows variation for the cavities of second and third vane on gas turbine load and ambient conditions. In some different load conditions, calculated secondary air flows are compared to experimental data coming from the AE94.3A Ansaldo fleet.Copyright

An Efficient Procedure for the Analysis of Heavy Duty Gas Turbine Secondary Flows in Different Operating Conditions

Combined cycle and partial load operating of modern heavy-duty gas turbines require highly efficient secondary air systems to supply both cooling and sealing air. Accurate performance predictions are then a fundamental demand over a wide range of operability. The paper describes the development of an efficient procedure for the investigation of gas turbine secondary flows, based on an in-house made fluid network solver, written in Matlab® environment. Fast network generation and debugging are achieved thanks to Simulink® graphical interface and modular structure, allowing predictions of the whole secondary air system. A crucial aspect of such an analysis is the calculation of blade and vane cooling flows, taking into account the interaction between inner and outer extraction lines. The problem is closed thanks to ad-hoc calculated transfer functions: cooling system performances and flow functions are solved in a pre-processing phase and results correlated to influencing parameters using Response Surface Methodology (RSM) and Design of Experiments (DOE) techniques. The procedure has been proved on the secondary air system of the AE94.3A2 Ansaldo Energia gas turbine. Flow functions for the cooling system of the first stage blade, calculated by RSM and DOE techniques, are presented. Flow functions based calculation of film cooling, tip cooling and trailing edge cooling air flows is described in details.Copyright