Ph.D. received on: 12/2/1999

E-mail: poncia@elet.polimi.it

Tutor: Prof. S. Bittanti, Politecnico di Milano
 
 

 ___________________________________________________________________________________________________________

A Study on Thermoacoustic Instability Phenomena
in Combustion Chambers for Active Control   ___________________________________________________________________________________________________________

Advisor:

Prof. S. Bittanti, Politecnico di Milano

Co-Advisors:

 Prof. F.E.C. Culick, California Institute of Technology
 Prof. A. De Marco, Dipartimento di Elettronica e Informazione, Politecnico di Milano

Summary:

Combustion systems based on premixed flames are widespread used in gas turbine plants in order to achieve a low pollutants - high efficiency operation.

In particular situations, unwanted and insidious phenomena, known as thermoacoustic or combustion instabilities, may arise. These phenomena are due to the interaction between the flow field and the heat release mechanisms, giving rise to oscillations of the fluid-thermodynamic variables. The side effect, consequence of this unstable behaviour, is the unwanted and unexpected growth of the noise level, and in extreme situations the plant breakdown.

For this reason, the interest in developing new models and control strategies for the suppression of combustion instability had rapidly grown in the last ten years. The most promising techniques are based on the elimination of the unsteady behaviour by means of a feedback control system imposing the modulation of a suitable control variable. This strategy belong to the field of active control, in opposition to passive control strategies, based on the modification of the
chamber structure.

The Politecnico di Milano and the Italian electricity board ENEL S.p.A. launched a project for the design of active control strategies on gas turbine plants. This thesis has been developed in this cadre, and aims to contribute to the comprehension, analysis and description of combustion instabilities. Modelling and experimental activity are the two considered subjects.

The thesis is structured as follows:

After a short introduction (Chapter 1), a definition of the concept of combustion instabilities is provided, focusing on the problem of active control is given in Chapter 2. The same chapter deals with the characteristics of a novel nonlinear model for combustion instabilities. In Chapter 3 is devoted to the development of the part of the model concerning the gasdynamics, whereas the modelling of the flame dynamics is the subject of Chapter 4. In Chapter 5, the interpretation of the feedback interaction between gasdynamics and flame leads to the model completion. The ability of the model to interpret the behaviour observed in real plants is verified through a set of simulations. Chapter 6 deals with the experimental campaign carried out on a laboratory device characterised by thermoacoustic instability behaviour.
 
 
 

MINORS

A Methodology for the Design of Distributed Web Systems
 

Advisor: Prof. B. Pernici, Dipartimento di Elettronica e Informazione, Politecnico di Milano

The growing diffusion of World Wide Web applications and the increasing complexity of information systems and their interconnection to WWW applications make nowadays the design of such systems a difficult task.
To produce large size web environments, systematic design methodologies and conceptual and design models, based on a
graphical representation, have been developed. We present a methodology for the design of web systems in which a large number of servers are distributed over a wide geographical area. This context obliges us to consider not only the organisation of the information, but also its allocation to servers.
The starting point is RMM (Relationship Management Methodology), developed for hypermedia environments.
Since we consider a distributed environment, this methodology has been extended through the development of adequate tools and strategies. Particular attention has been devoted to the problem of data allocation. The allocation strategy
is based on the fragmentation of the information and on the study of the access dynamics. An example, based on an Intranet system,  has been considered.
 

Modeling and Identification of Current Transformers

Advisor: Prof. M. Santomauro, Dipartimento di Elettronica e Informazione, Politecnico di Milano

Current transformers (CT) are widely used as current sensors in industrial relays switches, when an automatic device for fault detection in electrical circuits, in particular short circuit detection, is designed. These devices allow to perform the detection ability with simple and cheap technologies, but they are characterised by highly non-linear behaviour. Here, the primary current characterised by high intensity enters the primary circuit, and it is transformed into secondary current, with lower intensity. A significant deformation of the secondary current occur when, in short circuit conditions, saturation and hysteresis occur, so that the CT measure become completely unreliable.

The purpose of this research activity is to develop a model and an identification algorithm of current transformers with the primary objective to reconstruct the current at the primary winding, given the measurement of the current at secondary. This calls for a suitable representation of the non-linear characteristics, i.e. saturation effect at high currents, and of the hysteresis behaviour.
 

 _______________________________________