Alberto Fichera

Laminar natural convection in a vertical isothermal channel with symmetric surface-mounted rectangular ribs

Abstract In this paper, a numerical investigation of laminar natural convective flows in a vertical isothermal channel with two rectangular ribs, symmetrically located on each wall, is carried out. The governing elliptic equations are solved in a two-dimensional domain using a control volume method and the SIMPLER algorithm for the velocity–pressure coupling is employed. Special emphasis is given in the systematic analysis to detail the effects of the location of the isothermal ribs on the flow structure and isotherms pattern. The profiles of the local Nusselt number are presented for three different locations of the obstructions, near the inlet, at the center and near the outlet of the channel. The influence of the rib conductivity is also considered, the ribs being either perfectly conducting or adiabatic. The variations in the mean Nusselt number and inlet flow rate versus the channel Rayleigh number are investigated. Finally, for centered ribs extra computations are performed for various sizes (width and length) of the ribs and for two aspect ratios to demonstrated the significant effects on the flow and heat transfer characteristics.

Experimental analysis of thermo-acoustic combustion instability

Thermo-acoustic instabilities are dynamic phenomena that represent a major threat for most modern combustion systems. Many studies, mainly undertaking a linear analysis of experimental data, have been carried out to provide a deeper understanding of the underlying phenomena. However, linear analysis may lead to an oversimplified view of the problem, which involves many complex non-linear interactions, and a more detailed non-linear analysis may be necessary. This paper presents both linear and non-linear analyses of experimental measurements observed in a methane-fuelled laboratory combustor. The linear analysis aims to verify the existence of thermo-acoustic instabilities and consists of the elaboration of both power-spectral density distribution and Rayleigh Index of the experimental time series. Non-linear analysis aims to investigate dynamic behaviours by means of the deterministic chaos theory. Results of the analyses show that combustion instabilities occur in all the experimental operating conditions. Moreover, the existence of a chaotic source in the combustion system under study is demonstrated.

Neural prediction of combustion instability

Abstract Combustion instabilities consist in self–exited oscillations in combustion chambers and can cause structure degradations. When combustion instabilities occur, the combustion process is characterised by the coupling of various non-linear phenomena, which can lead to the formation of either a limit cycle or chaos. The intrinsic highly non-linear behaviour, especially when chaos arises, poses a major problem for the formulation of good predictions and the design of reliable control systems. Due to the relevant number of degree of freedom and to the non-linear coupling of different phenomena, the mathematical modeling of combustion instabilities is computationally heavy and may produce an unsatisfactory correspondence between simulated and experimental data. Analogous problems arise also from the uncertainty for the parameters of the process, such as the local velocity of the flame front (and, hence, the reaction rate and the exhausted temperatures), and their unpredictable variations. In fact, note that most of the chemical and thermo-physical variables both strongly depend and influence the instantaneous displacement of the flame front, which is positioned on the unstationary eddies external surface. In order to obtain a reliable model for thermo-acoustic combustion instabilities, a different approach was chosen in this paper. The black-box identification of an experimental combustion chamber was obtained by means of a generalized NARMAX model. The model was implemented by training a Multilayer Perceptron artificial neural network with input-output experimental data. The main advantages of the proposed approach consisted in the natural ability of neural networks in modeling nonlinear dynamics in a fast and simple way and in the possibility to address the process to be modeled as an input-output black box, with little or no mathematical information on the system. Satisfactory agreement between simulated and experimental data was found and results show that the model successfully predicted the temporal evolution of thermo-acoustic combustion instabilities.

Clustering of chaotic dynamics of a lean gas-turbine combustor

This work deals with the dynamic behaviour of a lean premixed gas turbine combustor. The study aims to achieve a classification of experimental burner dynamic behaviour and is based on the geometrical properties of the attractors of the system variables. Several experiments were performed varying the flame stoichiometric ratio [lambda] and the pilot fuel percentage PFP. The dynamics of the experimental time series of the flame front heat release were described by using vectors collecting information on the topological distribution of the attractors. Therefore, unsupervised Kohonen associative memories were trained to create clusters of operating conditions characterised by similar dynamical behaviours. Kohonen associative memories were able to divide the experimental operating conditions into different clusters according to the different values of the flame stoichiometric ratio. The results of the clustering underline the possibility of being able to define an algorithm for combustion-instability pattern recognition that takes into account the highly non-linear effects which govern combustion processes.

Stability maps for rectangular circulation loops

This study aims to define a methodology for the construction of stability maps of rectangular natural circulation loop. The interest for these maps derives from their ability to present in synthetic form the results of stability analysis. In fact, the proposed methodology is based on the linear stability analysis around equilibrium points of a high order nonlinear mathematical model describing the dynamical behaviour of the non-dimensional flow velocity and temperature inside the loop. Imposed heat flux at the loop walls has been assumed at the boundaries; in particular, it has been considered that the fluid is heated and cooled during its passage in the horizontal sections, respectively, at bottom and top of the loop, whereas the vertical legs are supposed adiabatic. Hence, the stability analysis was performed considering the effect of the variation of the modified Grashof number, Grm, for a wide range of loop geometrical configurations, assuming various aspect ratios (ratio of the vertical to the horizontal length of the tube) and inner tube diameters.

Determining the optimal configuration of a heat exchanger (with a two-phase refrigerant) using exergoeconomics

Abstract In this paper, the exergoeconomic theory is applied to a heat exchanger for optimisation purposes. The investigation was referred to a tube-in-tube condenser with the single-phase fluid to be heated flowing in the inner annulus and the two-phase refrigerant flowing in the external annulus. First, the irreversibility due to heat transfer across the stream-to-stream temperature-difference and to frictional pressure-drops is calculated as a function of two design variables: the inner-tubes diameter and the saturation temperature of the refrigerant, on which the heat-exchange area directly depends. Then, a cost function is introduced, defined as the sum of two contributions: the amortisation cost of the condenser under study and the operating cost of the conventional electric-driven heat-pump in which this component will have to work. The latter contribution is directly related to the overall exergy destruction rate in the plant, whereas the amortisation cost mainly depends on the heat-exchange area. So, design optimisation of the device can be performed by minimising this cost function with respect to the selected design variables. The so-called structural approach (Coefficient of Structural Bond) is used in the optimisation, in order to relate the local irreversibility in the condenser to the overall exergy destruction rate in the heat-pump plant. A numerical example is discussed, in which, for a commercial heat-exchanger, the design improvements needed to obtain a cost-optimal configuration are investigated. The results show that significant improvements can be obtained with respect to devices based on conventional values of the design parameters.

Optimization of a liquefaction plant using genetic algorithms

This paper presents an optimization methodology for liquefaction/refrigeration systems in the cryogenic field. The Figure of Merit has been chosen as the evaluation index, and genetic algorithms as evaluation criteria. This methodology has been applied to an existing helium liquefaction system that works according to a Collins cycle. The results show the possibility of adjusting some of the thermal-pressure variables for the system in order to improve the Figure of Merit.

Nonlinear analysis of a rectangular natural circulation loop

Abstract The aim of this paper is to present a novel approach to the analysis of the dynamical evolution of a rectangular natural circulation loop heated from below. During an experimental phase nonlinear temperature oscillations were detected at different location and for various unstable operating conditions of the loop. The experimental time series were intially filtered using wavelet packet approach. Afterwards, Singular Value Decomposition (SVD) was applied, so to reduce the filtered signals to their dominant features, and the calculation of the Average Mutual Information (AMI) function was performed. Both these approaches were adopted in order both to take into account the intrinsic nonlinear behavior of detected temperatures and to obtain a simplified phase space representation of the dynamics of the system. Such a representation was used to determine the System Dimension, whose knowledge represents the first step in the analysis of any dynamical system. Finally, phase space representations of various operating conditions show the presence of dynamical behaviors characterized by different values of the system dimension, pointing out the possible existence of chaos in the system in study.

NONLINEAR DYNAMICS OF AIR-WATER MIXTURES IN VERTICAL PIPES: EXPERIMENTAL TRENDS

This study reports the results of the characterization of an air-water two-phase experimental apparatus and the preliminary analyses of the experimental time series. The test section of the apparatus consists of a vertical pipe equipped with an impedance void fraction sensor. The carrying frequency of the impedance sensor has been chosen in order to operate it as a resistive sensor. The calibration of the sensor has been performed through comparison of the instantaneous two-phase mixture conductivity signal and the local actual dimension of the bubble as estimated from high resolution photograph. The calibration curve allows, therefore, reliable estimation of the void fraction time series. A preliminary analysis of the time series has been performed both in time and frequency domains, evaluating also the time series autocorrelation. These analyses have pointed out the inadequacy of linear tools for the characterization of two-phase flow dynamics, which are nonetheless characterized by strong recurrence an...

Appearance of quasiperiodicity within a period doubling route to chaos of a swaying thermal plume

The birth, evolution and disappearance of quasiperiodic dynamics in buoyancy-driven flow arising from an enclosed horizontal cylinder are analysed here, by numerical means, in the limit of the 2D approximation. The governing equations are solved on orthogonal Cartesian grids, giving special treatment to the internal, non-aligned boundaries. Thanks to the adoption of a high level of refinement of the Rayleigh number range, quasiperiodicity was observed to emerge from a periodic limit cycle (P1), and to turn into its omologous orbit with doubled period (P2), eventually evolving into a classical period-doubling route to chaos, for further increases of the Rayleigh number. The present study gives a deeper insight to what appears to be an imperfect period doubling bifurcation through a quasiperiodic T2-torus. The approach used is based on the classical tools for time series analysis. The distribution of the power spectral densities is used to search for and characterise the existence of relations between the frequencies of the P1, T2 and P2 dynamics. The topology of the orbits, as well as their evolution within the quasiperiodic window, are analysed with the aid of phase space representation and Poincare maps.