Giulio Ghirardo

Giulio Ghirardo
Doodle

Doctor of Philosophy

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27
Publications
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360
Citations

Publications

Publications (27)
Article
Full-text available
Rotationally symmetric annular combustors are of practical importance because they generically resemble combustion chambers in gas turbines, in which thermoacoustically driven oscillations are a major concern. We focus on azimuthal thermoacoustic oscillations and model the fluctuating heat release rate as being dependent only on the local pressure...
Article
Full-text available
Final journal version here: https://www.cambridge.org/core/journals/journal-of-fluid-mechanics/article/amplitude-statistics-prediction-in-thermoacoustics/B11F37A2D6A3D3921A23EA234DEE999A We discuss the statistics of acoustic pressure of thermoacoustic oscillations, either axial or azimuthal in nature. We derive a model where the describing functio...
Article
Full-text available
Rotationally symmetric systems can exhibit acoustic fluctuations in the azimuthal direction. In experimental works the nature (standing/spinning) of these fluctuations is often described by a set of indicators. These indicators either depend on the chosen frame of reference or are not state space variables for the acoustic field. Conversely, in the...
Conference Paper
Full-text available
Can-annular combustors consist of N distinct cans set up symmetrically around the axis of the gas turbine rotor. Each can is connected to the turbine inlet by means of a transition duct. At the turbine inlet a small gap between the neighbouring transition ducts allows acoustic communication between the individual cans. Thermoacoustic pulsations in...
Article
Full-text available
We consider the acoustic flow field of rotationally symmetric systems, like an annular combustor and the flow in a round duct, in absence of a mean azimuthal flow field. We focus on azimuthal instabilities, which manifest as either spinning (rotating) waves or standing waves, or a linear combination of the two. These instabilities are often excited...
Article
The stability of thermoacoustic systems is often regulated by the time delay between acoustic perturbations and corresponding heat release fluctuations. An accurate estimate of this value is of great importance in applications since even small modifications can introduce significant changes in the system behavior. Different studies show that the no...
Article
In the current study azimuthal forcing of an annular combustor with swirling flames has been performed to present for the first time the Heat Release Rate (HRR) response to all possible pressure fields of the first azimuthal mode up to a finite amplitude limit. The response is first quantified through the conventional Flame Describing Function (FDF...
Article
Full-text available
This paper presents a RANS turbulent combustion model for CH4/H2/air mixtures which includes the effect of heat losses and flame stretch. This approach extends a previous model concept designed for methane/air mixtures and improves the prediction of flame stabilization when hydrogen is added to the fuel. Heat loss and stretch effects are modelled b...
Article
Full-text available
Heavy-duty land-based gas turbines are often designed with can-annular combustors, which consist of a set of identical cans, acoustically connected on the upstream side via the compressor plenum, and, downstream, with a small annular gap located at the transition with the first turbine stage. The modeling of this cross-talk area is crucial to predi...
Conference Paper
The stability of thermoacoustic systems is often regulated by the time delay between acoustic perturbations and corresponding heat release fluctuations. An accurate estimate of this value is of great importance in applications, since even small modifications can introduce significant changes in the system behavior Different studies show that the no...
Conference Paper
Full-text available
This paper presents a RANS turbulent combustion model for CH4/H2/air mixtures which includes the effect of heat losses and flame stretch. This approach extends a previous model concept designed for methane/air mixtures and improves the prediction of flame stabilization when hydrogen is added to the fuel. Heat loss and stretch effects are modelled b...
Conference Paper
Full-text available
Heavy-duty land-based gas turbines are often designed with can-annular combustors, which consist of a set of identical cans, acoustically connected on the upstream side via the compressor plenum, and, downstream, with a small annular gap located at the transition with the first turbine stage. The modeling of this cross-talk area is crucial to predi...
Article
Identification of thermoacoustic systems usually requires low-order modeling, often obtained by means of a Galerkin projection onto a single acoustic mode. The resulting dissipative self-excited oscillator equation describes the dynamic balance between acoustic energy sources (i.e. the flame) and sinks. Previous works studied the case of a nonlinea...
Article
This paper first characterizes the acoustic field of two annular combustors by means of data from acoustic pressure sensors. In particular the amplitude, orientation, and nature of the acoustic field of azimuthal order n is characterized. The dependence of the pulsation amplitude on the azimuthal location in the chamber is discussed, and a protecti...
Preprint
Full-text available
We consider the acoustic flow field of rotationally symmetric systems, like an annular combustor and the flow in a round duct, in absence of a mean azimuthal flow field. We focus on azimuthal instabilities, which manifest as either spinning (rotating) waves or standing waves, or a linear combination of the two. These instabilities are often excited...
Article
Full-text available
Annular combustors can exhibit azimuthal thermoacoustic instabilities, which can rotate as a spinning wave at the speed of sound in the azimuthal direction, oscillate as a standing wave with pressure nodes fixed in space, or be a linear combination of these. These oscillations happen if a positive feedback loop between acoustics and the response of...
Conference Paper
Full-text available
This paper first characterizes the acoustic field of two annular combustors by means of data from acoustic pressure sensors. In particular the amplitude, orientation, and nature of the acoustic field of azimuthal order n is characterized. The dependence of the pulsation amplitude on the azimuthal location in the chamber is discussed, and a protecti...
Article
Can-annular combustors consist of $N$ distinct cans set up symmetrically around the axis of the gas turbine. Each can is connected to the turbine inlet by means of a transition duct. At the turbine inlet a small gap between the neighbouring transition ducts allows acoustic communication between the cans. Thermoacoustic pulsations in the cans are dr...
Preprint
Full-text available
Azimuthal instabilities occur in rotationally symmetric systems, either as spinning (rotating) waves or standing waves. We make use of a novel ansatz to derive a differential equation characterizing the state of these instabilities in terms of their amplitude, orientation, nature (standing/spinning) and temporal phase. For the first time we show ho...
Article
Can-annular combustors consist of a set of independent cans, connected on the upstream side to the combustor plenum and on the downstream side to the turbine inlet, where a transition duct links the round geometry of each can with the annular segment of the turbine inlet. Each transition duct is open on the sides toward the adjacent transition duct...
Conference Paper
Full-text available
Can-annular combustors consist of a set of independent cans, connected on the upstream side to the combustor plenum, and on the downstream side to the turbine inlet, where a transition duct links the round geometry of each can with the annular segment of the turbine inlet. Each transition duct is open on the sides towards the adjacent transition du...
Article
Full-text available
This paper concerns the influence of the phase of the heat release response on thermoacoustic systems. We focus on one pair of degenerate azimuthal acoustic modes, with frequency ω0. The same results apply for an axial acoustic mode. We show how the value φ0 and the slope −τ of the flame phase at the frequency ω0 affects the boundary of stability,...
Conference Paper
Full-text available
A successful low-order model introduced by Schuermans et al. [19], Noiray et al. [13] studies thermoacoustic instabilities assuming the fluctuating heat release rate q to be in phase with the acoustic pressure p, by neglecting the component of q out of phase with p. In this investigation we remove this hypothesis and consider a model in which, if p...
Conference Paper
Full-text available
Rotationally symmetric annular combustors are of practical importance because they generically resemble combustion chambers in gas turbines and aeroengines, in which thermoacoustically driven oscillations are a major concern. We focus on thermoacoustic oscillations of azimuthal type, neglect the effect of the transverse acoustic velocity in the azi...
Article
Full-text available
The describing function is a powerful tool for characterizing nonlinear dynamical systems in the frequency domain. In some cases, it is the only available description of a nonlinear operator characterizing a certain subcomponent of the system. This paper presents a methodology to provide a state-space realization of one given describing function, i...
Conference Paper
Full-text available
We presented on this poster some on-going work on mapping nonlinear operators from frequency to time domain. This lead to the article "2015 - Giulio Ghirardo , Bernhard Ćosić, Matthew P. Juniper, Jonas P. Moeck - State-space realization of a describing function - Nonlinear Dynamics, doi:10.1007/s11071-015-2134-x"
Article
Full-text available
This theoretical study investigates spinning and standing modes in azimuthally symmetric annular combustion chambers. Bot modes are observed in experiments and simulations, and an existing model predicts that spinning modes are the only stabl state of the system. We extend this model to take into account the effect that the acoustic azimuthal veloc...

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