Giulio Mori

Thermoacoustic Analysis of Combustion Instability Through a Distributed Flame Response Function

Modern gas turbines equipped with lean premixed dry low emission combustion systems suffer the problem of thermoacoustic combustion instability. The acoustic characteristics of the combustion chamber and of the burners, as well as the response of the flame to the fluctuations of pressure and equivalence ratio, exert a fundamental influence on the conditions in which the instability may occur. A three dimensional finite element code has been developed in order to solve the Helmholtz equation with a source term that models the heat release fluctuations. The code is able to identify the frequencies at which thermoacoustic instabilities are expected and the growth rate of the pressure oscillations at the onset of instability. The code is able to treat complex geometries such as annular combustion chambers equipped with several burners. The adopted acoustic model is based upon the definition of the Flame Response Function (FRF) to acoustic pressure and velocity fluctuations in the burners.In this paper, data from CFD simulations are used to obtain a distribution of FRF of the κ-τ type as a function of the position within the chamber. The intensity coefficient, κ, is assumed to be proportional to the reaction rate of methane in a two-step mechanism. The time delay τ is estimated on the basis of the trajectories of the fuel particles from the injection point in the burner to the flame front.The paper shows the results obtained from the application of FRF with spatial distributions of both κ and τ. The present paper also shows the comparison between the application of the proposed model for the FRF and the traditional application of the FRF over a concentrated flame in a narrow area at the entrance to the combustion chamber. The distribution of the intensity coefficient and the time delay proves to have an influence, both on the eigenfrequency values and on the growth rates, in several of the examined modes.The proposed method is therefore able to establish a theoretical relation of the characteristics of the flame (depending on the burner geometry and operating conditions) to the onset of the thermoacoustic instability.Copyright

Micro Gas Turbine Combustor Emissions Evaluation Using the Chemical Reactor Modelling Approach

Chemical Reactor Modelling approach has been applied to evaluate exhaust emissions of the newly designed ARI100 (Patent Pending) recuperated micro gas turbine combustor developed by Ansaldo Ricerche SpA. The development of the chemical reactor network has been performed based on CFD reacting flow analysis, obtained with a global 2-step reaction mechanism, applying boundary conditions concerning the combustion chamber at atmospheric pressure, with 100% of thermal load and fuelled with natural gas. The network consists of 11 ideal reactors: 6 perfectly stirred reactors, and 5 plug flow reactors, including also 13 mixers and 12 splitters. Simulations have been conducted using two detailed reaction mechanisms: GRI Mech 3.0 and Miller & Bowman reaction mechanisms. Exhaust emissions have been evaluated at several operating conditions, obtained at different pressure, and considering different fuel gases, as natural gas and a high H2 content SYNGAS fuel. Furthermore, emissions at different thermal loads have been investigated when natural gas at atmospheric pressure is fuelled. Simulation results have been compared with those obtained from combustion experimental campaign. CO and NOx emissions predicted with CRM approach closely match experimental results at representative operating conditions. Ongoing efforts to improve the proposed reactors network should allow extending the range of applicability to those operating conditions whose simulation results are not completely satisfying. Given the small computational effort required, and the accuracy in predicting combustor experimental exhaust emissions, both CO and NOx , the CRM approach turnout to be an efficient way to reasonably evaluate exhaust emissions of a micro gas turbine combustor.Copyright

A QUANTITATIVE COMPARISON BETWEEN A LOW ORDER MODEL AND A 3D FEM CODE FOR THE STUDY OF THERMOACOUSTIC COMBUSTION INSTABILITIES

The study of thermoacoustic combustion instabilities has an important role for safety operation in modern gas turbines equipped with lean premixed dry low emission combustion systems. Gas turbine manufacturers often adopt simulation tools based on low order models for predicting the phenomenon of humming. These simulation codes provide fast responses and good physical insight, but only one-dimensional or two-dimensional simplified schemes can be generally examined. The finite element method can overcome such limitations, because it allows to examine three-dimensional geometries and to search the complex eigenfrequencies of the system. Large Eddy Simulation (LES) techniques are proposed in order to investigate the instability phenomenon, matching pressure fluctuations with turbulent combustion phenomena to study thermoacoustic combustion oscillations, even if they require large numerical resources. The finite element approach solves numerically the Helmholtz equation problem converted in a complex eigenvalue problem in the frequency domain. Complex eigenvalues of the system allow us to identify the complex eigenfrequencies of the combustion system analyzed, so that we can have a valid indication of the frequencies at which thermoacoustic instabilities are expected and of the growth rate of the pressure oscillations at the onset of instability. Through the collaboration among Ansaldo Energia, University of Genoa and Polytechnic University of Bari, a quantitative comparison between a low order model, called LOMTI, and the three-dimensional finite element method has been examined, in order to exploit the advantages of both the methodologies.Copyright

Micro Gas Turbine Combustion Chamber Design and CFD Analysis

In the framework of the non-standard fuel combustion research in micro-small turbomachinery, a newly designed micro gas turbine combustor for a 100-kWe power plant in CHP configuration is under development at the Ansaldo Ricerche facilities. Combustor design starts from a single silo chamber shape with two fuel lines, and is associated with a radial swirler flame stabiliser. Lean premix technique is adopted to control both flame temperature and NOx production. Combustor design process envisages two major steps, i.e. diagnostics-focussed design for methane only and experimentally validated design optimisation with suitable burner adaptation to non-standard fuels. The former step is over, as the first prototype design is ready for experimental testing. Step two is now beginning with a preliminary analysis of the burner adaptation to non-standard fuels. The present paper focuses on the first step of the combustor development. In particular, main design criteria for both burner and liner cooling system development are presented. Besides, design process control invoked both 2D and 3D CFD analysis. Two turbulence models, FLUENT standard k-e model and Reynolds Stress Model (RSM), are refereed and the results compared. Here both a detailed analysis of CFD results and a preliminary analysis of main chemical kinetic phenomena are discussed.Copyright

Assessment of Traditional and Flamelets Models for Micro Turbine Combustion Chamber Optimisation

Different approaches for numerical simulation of premixed combustion are considered, in order to assess their usefulness as design tools for micro gas turbine systems. In particular, a flamelet concept routine by N. Peters has been developed taking into account both mixture fraction Z and G function as scalar flame locators, thus allowing computation of complex fully or partial premixed flame structure. The model can be used also in the thin reaction regime. Scalar transport equations for G, Z and their variance are added to the standard Navier Stokes and turbulence set of equation, in order to track the flame position. However, no chemical term appears explicitly in such equations, since the chemical effects are taken into account via pre-computed locally one-dimensional flamelet solutions. Here, the deep interaction between chemical and turbulence has been introduced through flamelets library built in non equilibrium conditions using CHEMKIN modules. The results of this model are compared the data obtained with a standard EBU model and different reaction mechanisms. Models validation has been carried out through experimental data coming from Aachen University for an axisymmetric Bunsen flame; finally, the code was applied to the analysis of a newly designed micro gas turbine combustor.Copyright

Integrated experimental and numerical approach for fuel-air mixing prediction in a heavy-duty gas turbine LP burner

An integrated experimental-numerical procedure has been developed for fuel-air mixing prediction in a heavy-duty gas turbine burner. Optical measurements of the degree of mixing have been performed in a full-scale test rig operating with cold flow. Experimental data have been utilized to validate a CFD RANS numerical model. In fact, it is recognized that the turbulence behavior of jets in swirling air-flow stream is not accurately described by standard k-e turbulence models; therefore advanced turbulence models have been assessed by means of experimental data. The degree of mixing between simulated fuel and air streams has been evaluated at the burner exit section by means of a planar Mie scattering technique. The experimental apparatus consists of a pulsed Nd:YAG laser and a high resolution CCD video camera connected to a frame grabber. The acquired instantaneous images have been processed through specific procedures that also take into account the laser beam spatial nonuniformity. A second-order discretization scheme with a RSM turbulence model gives the best accordance with the experimental data. Such CFD model will be part of a more general method addressed to numerical prediction of turbulent combustion flames in LP technology.

Design and Operating Experience on the Recent Application of the Fuel Staging Technology for AE94.3A Gas Turbine

The paper describes the experience made on the first, recent application of the fuel staging technology on AE94.3A gas turbine engine. The experience started in the frame of combustion upgrade projects by the beginning of 2011 when some conceptual ideas rose.The most effective solution was found in the application of radial fuel staging through the modification of an existing, dual fuel, diagonal swirler. CFD calculation was carried out in order to evaluate numerically the differences that the introduction of the radial fuel staging would have brought in comparison with the standard, reference configuration. After that one promising solution was selected and applied to a prototype, the test phase started. The experimental test was initially performed at the engine conditions on a high pressure test rig. The rig architecture was designed to be as representative as possible of the GT annular combustion chamber. The rig was equipped with the most advanced instrumentation in order to monitor and store all the main parameters of the burner during the test. High pressure tests showed a good agreement with the CFD calculation and an effective capability of the prototype to stage the fuel by generating a “co-pilot” flame. The word “co-pilot” was coined for the behaviour of the resulting flame to act as a second pilot in supporting and stabilizing the main premix flame. The on site validation was carried out on Enel La Casella GT1 power plant, downstream the upgrade of the gas turbine with the last recent AE94.3A model. The validation phase was performed according to two separate sessions: the first one using the standard, reference burner solution and the second one with the radial staged configuration.The advantage of the fuel staging technique was twofold. The first one was concerning the minimum environmental load: the contribution of the fuel staging was approximately 7 MW. The second one was concerning the global performance of the whole combined cycle power plant at the base load. It was possible to increase turbine inlet temperature of about 30 °C and the GT power output of about +7% in comparison with the standard, reference burner solution.Copyright

Integrated Experimental and Numerical Approach for Fuel-Air Mixing Prediction in a Heavy-Duty Gas Turbine LP Burner

An integrated experimental-numerical procedure has been developed for fuel-air mixing prediction in a heavy-duty gas turbine burner. Optical measurements of the degree of mixing have been performed in a full-scale test rig operating with cold flow. Experimental data have been utilized to validate a CFD RANS numerical model. In fact, it is recognized that the turbulence behavior of jets in swirling air-flow stream is not accurately described by standard k-e turbulence models; therefore advanced turbulence models have been assessed by means of experimental data.The degree of mixing between simulated fuel and air streams has been evaluated at the burner exit section by means of a planar Mie scattering technique. The experimental apparatus consists of a pulsed Nd:YAG laser and a high resolution CCD video camera connected to a frame grabber. The acquired instantaneous images have been processed through specific procedures that also take into account the laser beam spatial non-uniformity.A second order discretization scheme with a RSM turbulence model gives the best accordance with the experimental data. Such CFD model will be part of a more general method addressed to numerical prediction of turbulent combustion flames in LP technology.Copyright