Pio Astrua

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

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

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