A study on the fluid dynamics of domestic Gas burners

Domestic gas burners are investigated experimentally and numerically in order to further understand the fluid dynamics processes that drive the cooking appliance performances. In particular, a numerical simulation tool has been developed in order to predict the onset of two flame instabilities which may deteriorate the performances of the burner: the flame back and flame lift. The numerical model has been firstly validated by comparing the simulated flow field with a data set of experimental measurements. A prediction criterion for the flame back instability has been formulated based on isothermal simulations without involving the combustion modelization. This analysis has been verified by a Design Of Experiments investigation performed on different burner prototype geometries. On the contrary, the formulation of a prediction criterion regarding the flame lift instability has required the use of a combustion model in the numerical code. In this analysis, the structure and aerodynamics of the flame generated by a cooking appliance has thus been characterized by experimental and numerical investigations, in which, by varying the flow inlet conditions, the flame behaviour was studied from a stable reference case toward a complete blow-out.

Essays in Macroeconomics

The dissertation consists of four papers that aim at providing new contributions in the field of macroeconomics, monetary policy and financial stability. The first paper proposes a new Dynamic Stochastic General Equilibrium (DSGE) model with credit frictions and a banking sector to study the pro-cyclicality of credit and the role of different prudential regulatory frameworks in affecting business cycle fluctuations and in restoring macroeconomic and financial stability. The second paper develops a simple DSGE model capable of evaluating the effects of large purchases of treasuries by central banks. This theoretical framework is employed to evaluate the impact on yields and the macroeconomy of large purchases of medium- and long-term government bonds recently implemented in the US and UK. The third paper studies the effects of ECB communications about unconventional monetary policy operations on the perceived sovereign risk of Italy over the last five years. The empirical results are derived from both an event-study analysis and a GARCH model, which uses Italian long-term bond futures to disentangle expected from unexpected policy actions. The fourth paper proposes a DSGE model with an endogenous term structure of interest rates, which is able to replicate the stylized facts regarding the yield curve and the term premium in the US over the period 1987:3-2011:3, without compromising its ability to match macro dynamics.

An equilibrium approach to modelling social interaction

The aim of this work is to put forward a statistical mechanics theory of social interaction, generalizing econometric discrete choice models. After showing the formal equivalence linking econometric multinomial logit models to equilibrium statical mechanics, a multi- population generalization of the Curie-Weiss model for ferromagnets is considered as a starting point in developing a model capable of describing sudden shifts in aggregate human behaviour. Existence of the thermodynamic limit for the model is shown by an asymptotic sub-additivity method and factorization of correlation functions is proved almost everywhere. The exact solution for the model is provided in the thermodynamical limit by nding converging upper and lower bounds for the system's pressure, and the solution is used to prove an analytic result regarding the number of possible equilibrium states of a two-population system. The work stresses the importance of linking regimes predicted by the model to real phenomena, and to this end it proposes two possible procedures to estimate the model's parameters starting from micro-level data. These are applied to three case studies based on census type data: though these studies are found to be ultimately inconclusive on an empirical level, considerations are drawn that encourage further refinements of the chosen modelling approach, to be considered in future work.

Stability, viscous dissipation and local thermal non-equilibrium in fluid saturated porous media

In this thesis, the field of study related to the stability analysis of fluid saturated porous media is investigated. In particular the contribution of the viscous heating to the onset of convective instability in the flow through ducts is analysed. In order to evaluate the contribution of the viscous dissipation, different geometries, different models describing the balance equations and different boundary conditions are used. Moreover, the local thermal non-equilibrium model is used to study the evolution of the temperature differences between the fluid and the solid matrix in a thermal boundary layer problem. On studying the onset of instability, different techniques for eigenvalue problems has been used. Analytical solutions, asymptotic analyses and numerical solutions by means of original and commercial codes are carried out.