Andrea Silingardi

Multi-Objective Constrained Aero-Mechanical Optimization of an Axial Compressor Transonic Blade

This paper presents a flexible and effective optimization approach to design an axial compressor transonic blade for heavy duty gas turbines. The design goals are to improve design efficiency, choke margin and off-design performance while maintaining mass flow in design point as well as structural integrity. The new blade has to provide a wide operating range and to satisfy tight geometrical constraints.A database of aero-mechanical calculation results is obtained for three operating conditions. A number of 3D flow simulations are performed using a CFD solver with endwall boundary layer simplified model (thin layer) to reduce computational costs. The optimization process adopts a set of artificial neural networks (ANN) trained for each operating condition and a random walking search algorithm to determine the multi-objective Pareto Front. ANN enables speed up of the optimization process and allows high flexibility in choosing criteria for optimum member selection. Random walking algorithm gives a fast and effective method to predict the multi-dimensional Pareto Front.© 2012 ASME

Heavy Duty Gas Turbine Simulation: A Compressor IGV Airfoil Off-Design Characterization

The correct simulation of power plant behavior over a variety of operating conditions has to be extremely detailed in order to provide reliable help to the turbomachinery developers. The latter instance implies for designers and commercial personnel to be equipped with reliable calculation tools (in-house developed or commercial). In particular, Performance Analysis Codes (PACs) allow the designers to analyze different system configurations. To predict off-design behavior, these codes need to be not limited to thermodynamic analysis, but also able to perform a simplified description of each component that require a specific set of correlations. The selection of suitable correlation sets for compressor IGV airfoils could be very difficult. This paper deal with a procedure based on 2D-CFD analysis to provide a reliable evaluation of compressor IGV airfoils deviation and profile loss coefficients in a wide range of operating condition. The analysis were set up on the IGV of the Ansaldo Energia AE94.3A compressor and the developed correlations were successfully implemented in an in-house PAC called ESMS.Copyright

Rotor tip stall from a designer's perspective

The operating range of a compressor is the most critical aspect to consider when dealing with new designs. Two distinct routes to compressor stall are known in literature: modal-stall that occurs near the peak of the overall characteristic and spike-stall that initiates on the negatively-sloped part of the characteristic curve. Modal type stall can be taken into account from the very beginning during the design process. The basic mechanisms that drive the phenomena are essentially 2D in nature and there are well-established and experimentally validated design criteria available. Unfortunately this statement no longer applies when dealing with spike-type stall. This short length-scale instability is related to a three-dimensional breakdown of the flow that appears at rotor tip. Since when firstly observed, much effort has been expended trying to understand the flow features underlying this abrupt stall. Nevertheless, a well-established and validated design criterion is still missing. This paper deals with the topic from a designer’s perspective. A discussion which tries to highlight the design variables affecting the spike-stall is presented in the first part of the paper. Then CFD results on core stages of an heavy-duty axial compressor are discussed. The aim is to derive useful considerations for the designer work and discuss which design actions are more effective to successfully complete a spike-stall safe design.

A Method for Axial Compressor Start-Up Assessment

This paper presents a method to assess compressor mass flow, pressure line and stability, including blow-off lines behavior, during a heavy duty gas turbine (GT) start up. A Company proprietary compressor mean line analysis 1-dimensional code (C1D) has been calibrated on 3D RANS calculations and matched with a 1D Fanno model to simulate the compressor behavior during GT start-up; discharge compressor pressure and secondary air system boundary conditions are provided by filed test data. C1D is based on correlations tuned with available CFD and experimental data; within the present work the code has been validated with experimental data at low mechanical speed too, so that it can be used in such applications where conventional CFD analyses are most likely to fail. In this paper C1D is used to analyze the compressor start-up characteristic from idle to full speed no load operation. Pressure rise along the compressor and blow-off line mass flow are compared and validated throughout a field test campaign. Finally the method developed is applied to an evolutionary compressor in order to analyze how the stage-wise load distribution varies with mechanical speed and blow-off mass flow.Copyright

Gas turbine integrated conceptual design approach

In order to improve performance of heavy-duty gas turbines, in terms of power, efficiency and reliability, accurate calculation tools are required. During conceptual design phase, an effective integration of main GT components design into a single modular simulation tool can significantly reduce design iterations and improve the results. Thanks to an innovative modular-structured program for the simulation of air-cooled gas turbines, the one-dimensional design of compressor and turbine flow paths is used to create a complete gas turbine model including a detailed secondary air system and a simplified heat transfer model. This zero-dimensional heat transfer model is applied to each turbine row in order to calculate the cooling flow required to keep turbine blades and vanes metal temperatures below a prescribed threshold. After a description of the air cooled gas turbine modular model, the integrated design approach adopted by Ansaldo Energia is described. The knowledge of technical risks that the designers have to withstand developing advanced technologies during conceptual engine design is fundamental. The inter-disciplinary influence of some disciplines is analyzed and finally it is shown how Ansaldo Energia approach can track expected performance results and provide recovery plans during the conceptual design phase.© 2015 ASME

Sensitivity Analysis to Flutter for Front Stages Compressor Blades

Heavy duty gas turbine front stages compressor blades aero-elastic behavior is deeply analyzed and investigated by means of an uncoupled, non-linear and time-accurate CFD URANS solver. The travelling-wave approach and the energy method have been applied in order to assess the aerodynamic damping (in terms of logarithmic decrement) for each inter blade phase angle (IBPA) and thus to localize the flutter stability region. The work is mainly focused on a sensitivity analysis with respect to blade operating conditions, eigen-mode shapes and frequency in order to improve the understanding of flutter mechanism and to identify the key parameters. Transonic, supercritical and subsonic blades are investigated at different operating conditions with their corresponding eigenmode and eigen-frequency (first and second flexural mode and first torsional). The results show that non-linear effects can be neglected for subsonic blades. Besides, the modal-shape and the shock structure, if any, are identified to play a key role for flutter stability.Copyright

Thermo-Mechanical Rotor Analysis Tuned by Engine Operational Data

Gas turbine engines must withstand severe thermo-mechanical conditions during present-day load operation, characterized by cyclic transients and long dwell times. Indeed engine components are subject to thermal transient conditions, thermo-mechanical strain and stress fields; those are not easily measurable during operation, making calculations hardly confirmable. All these operational factors can lead a turbine component life reduction, finally increasing lifetime costs.The developed approach has been based on several calculations, such as thermal and FEM stress evaluation on the rotor components, tuned or validated by different field measurements carried out by thermocouples in the rotor core and the pre-tightening load variation of tie-rod. Transient disks and tie-rod temperatures (calculated by an in-house Secondary Air System code) have been tuned on experimental data. Thus, for rotor thermo-mechanical analysis more reliable boundary conditions have been provided. Rotor FEM analysis has been finally checked comparing the variation of the tie-bolt tension (calculated by FEM analysis) with the experimental behaviour observed during different operating conditions.Copyright

Minimum Environmental Load Reduction in Heavy Duty Gas Turbine by Bleeding Lines

The power generation and energy market scenarios are requiring the power generation plants to fulfill more flexible operations respect to the recent past. One of the main concerns of plant operators is the lowering of minimum load at which the machines can be exercised while respecting the pollution limits. A strategy to improve minimum turndown capability by reducing the minimum environmental load of heavy duty axial gas turbines is here presented: it is based on the use of the compressor air bleeding lines (blow-off lines).The described technical development activities are based on the numerical modeling of blow-off lines and bleeding compressor sections; these preliminary tasks have been followed by on-field plant testing.The blow-off lines modeling reserves a particular regard, due to the somehow non-usual fluid dynamics involved. A Fanno flow 1D approach has been adopted to properly model the bleeding lines fluid flow whereas full 3D numerical solutions have been developed to get a better insight of the bleeding plenums and of the line sector including the valve. In addition, the gas turbine components off-design behavior and the overall performances are computed by the Ansaldo modular simulation code. Numerical analysis and performed field tests are here presented and results are compared, showing a good agreement, in accord to the simplified model adopted. Additional comparisons with different alternative strategies are finally presented in terms of gas turbine power and excess air variation.The described technique by blow-off lines opening shows to be able to fulfill the required task by incrementing the plant operative flexibility and guaranteeing safe plant operation. The technique drawbacks are a gas turbine slightly lower efficiency and the lower output flue gas temperature, whose relative importance have to evaluate by the plant operators. At present the long term sustainability of the new operative condition is the object of a deeper and longer field testing phase.Copyright

Axial Compressor Degradation Effects on Heavy Duty Gas Turbines Overall Performances

The operation of a gas turbine is the result of the aero-thermodynamic matching of several components which necessarily experience aging and degradation over time. An approach to treat degradation phenomena of the axial compressor is provided, with an insight into the impact they have on compressor operation and on overall GT performances. The analysis is focused on the surface fouling of compressor blades and on rotor tip clearances variation.A modular model is used to simulate the gas turbine operation in design and off-design conditions and the aerodynamic impact of fouling and rotor tip clearances increase is assessed by means of dedicated loss and deviation correlations implemented in the 1D mid-streamline code of the compressor modules.The two different degradation sources are individually considered and besides the overall GT performance parameters, the analysis includes an evaluation of the compressor degradation impact on the secondary air system.Copyright

FANNO DESIGN OF BLOW-OFF LINES IN HEAVY DUTY GAS TURBINE

ABSTRACT In typical heavy duty gas turbines the multistage axial compressor is provided with anti-surge pipelines equipped with on-off valves (blow-off lines), to avoid dangerous flow instabilities during start-ups and shut-downs. Blow-off lines show some very peculiar phenomena and somewhat challenging fluid dynamics, which require a deeper regard. In this paper the blow-off lines in axial gas turbines are analyzed by adopting an adiabatic quasi-unidimensional model of the gas flow through a pipe with a constant cross-sectional area and involving geometrical singularities (Fanno flow). The determination of the Fanno limit, on the basis of the flow equation and the second principle of thermodynamics, shows the existence of a critical pipe length which is a function of the pipe parameters and the initial conditions: for a length greater than this maximum one, the model requires a mass-flow reduction. In addition, in the presence of a regulating valve, so-called multi-choked flow can arise. The semi-analytical model has been implemented and the results have been compared with a three-dimensional CFD analysis and cross-checked with available field data, showing a good agreement. The Fanno model has been applied for the analysis of some of the actual machines in the Ansaldo Energia fleet under different working conditions. The Fanno tool will be part of the design procedure of new machines. In addition it will define related experimental activities.