M. A. Corticelli

Land mine detection by infrared thermography: reduction of size and duration of the experiments

The effectiveness of infrared thermography applied to the detection of abandoned land mines is not yet acceptable. It can probably be improved, however, by computerized processing of the thermal images. This requires reference data, which must be provided mainly by experiments. A method is presented here, by which the heating and cooling cycles of a soil with a buried land mine can be replicated with reduced size and duration. The reference data acquired in the laboratory can be associated to realistic on-field tests by simply stretching the space and time scales. This will permit to reproduce indoors, quickly and effortlessly, the outdoor conditions of any place where the detection of buried land mines must be performed. In this paper, the general thermal problem is described, and the proposed method is comprehensively explained. The results of computer simulations and some laboratory tests are finally reported for validation.

A FAST CARTESIAN SCHEME FOR UNSTEADY HEAT DIFFUSION ON IRREGULAR DOMAINS

A numerical integration method is presented for the treatment of transient heat conduction problems. A Cartesian formulation is developed that is suitable for the treatment of irregular domains under general boundary conditions. The qualities of the scheme are demonstrated, in terms of both accuracy and computational efficiency, by comparison with analytical and numerical solutions. Results for the basic two-dimensional annular geometry show that the method has nearly second-order accuracy in space and time, at least in simple cases. Finally, a complex multiconnected domain is considered, to test the method performance under more severe conditions, including the presence of multiple length scales. The numerical experiment demonstrates that the numerical scheme is efficient, stable, and convergent.

CFD Analyses of Syngas-Fired Industrial Tiles Kiln Module

Abstract Industrial kilns for ceramic tiles production demand thorough control of the firing parameters to ensure uniform product quality. A given temperature profile must be imposed along the kiln length, while spanwise temperature profile should be as uniform as possible at the tiles level at any location. Due to special needs in emerging markets, interest is growing towards the use of gases produced by gasification processes as an alternative to methane. This requires specific burner design and proper re-calibration of the firing parameters. In the present work, computational fluid dynamics is used to analyse an industrial kiln section for different fuels, nominal burner powers, and burner nozzle diameters. The results are given in terms of temperature and velocity fields in the kiln room, and temperature distributions over the tiles floor. It is shown that a sensible combination of the three parameters investigated can lead to satisfactory results, even with gases having poor heating value.

Experimental and Numerical Analysis of Thermal Interaction Between Two Droplets in Spray Cooling of Heated Surfaces

ABSTRACT Dropwise cooling is a subject of interest for numerous industrial applications, which fosters fundamental research on the related mechanisms. The present work is focused on studying the cooling effect of 2 water droplets gently released onto a heated solid surface. The nominal initial temperature of the substrate was lower than 100 °C, thereby referring to evaporation regime. Heat-transfer phenomena were analyzed by an experimental and numerical approach at the solid/liquid interface and over non-wetted regions, thus evaluating mutual interaction between droplets. Infrared thermography was employed in a facility built to measure surface temperature from below through a fully non-intrusive approach. An infrared-transparent disk served as the substrate; its black-painted upper surface allowed heating and droplet deposition to occur on a blackbody. A numerical code was developed to model heat transfer within all bodies and at all interfaces by the finite-volume discretization method. Numerical results showed very good agreement with experimental temperature profiles and heat-flux distribution was predicted over the whole sampling region. Cooling effect was determined quantitatively together with the extent of the mutual-interaction region, where the influence of 2 sequentially-released droplets was proved higher and longer than that of a single-droplet configuration with the same amount of deposited water.

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.

Accident analysis in LMFBR fuel rods by the fenht code

Abstract This paper describes the fenht code capability related to the safety requirements in nuclear plants. The code solves the non-linear transient heat-transfer problem for the fuel element of a nuclear reactor, in order to simulate any accidental, operational and emergency power transient with arbitrary initial conditions. The temperature distribution in the fuel, gap and cladding is obtained by a finite-element technique based on minimizing the thermal potential with respect to the temperature vector at the nodes of the finite elements. The non-linear differential matricial equation is linearized by an iterative procedure and solved by the Crank—Nicholson method. Also the thermoelastic stresses in the cladding are valued, by the usual Hookes law. The code has been applied to the analysis of two reference accidents (incidental power transients) occurring in a liquid-metal fast-breeder reactor (LMFBR); the results are reported and briefly discussed.

Heat transfer along the route to chaos of a swaying thermal plume

Detailed analyses have been recently reported on the low order dynamics of a thermal plume arising from a horizontal cylindrical heat source concentric to an air-filled isothermally cooled square enclosure, together with those of the related flow structures, in the limit of the 2D approximation. In particular, within the range of 0 < Ra < 3Racr, with Racr corresponding to the loss of stability of the stationary buoyant plume, the entire evolution from a periodic limit cycle (P1) to the birth of chaos through a period-doubling cascade has been fully explored. With this respect, special attention has been given to the window of quasiperiodic dynamics onto a T2-torus that is observed to separate the monoperiodic dynamics from the biperiodic dynamics onto a P1 and a P2-limit cycle, respectively. The results of these analyses hint at the bimodal nature of the overall dynamics, in general, and of the subharmonic cascade, in particular, which are still under investigation. Although relevant on a dynamical perspective, a with a main reflection on the laminar-turbulent transition, the observed oscillations appear to be characterised by comparable amplitudes and to be determined by similar evolutions of the flow pattern evolutions, so that their role on the overall heat transfer rate is expected to be marginal. Within this frame, the present study aims at reporting the influence played by the observed dynamics of the thermal plume and of the flow structures on the global heat transfer rate. In particular, the aim is the assessment of the correlation between the Rayleigh number and the average Nusselt number on the cylinder surface, as well as the effect on the latter of the observed series of bifurcations.

CFD analysis of a syngas-fired burner for ceramic industrial roller kiln

Kiln burners for industrial tile production are usually fuelled by methane gas. However, the interest towards the use of coal or synthesis gases is rapidly increasing, mainly due to the opening of important markets in developing countries. The widely variable chemical composition of these fuels demands the gas burner to be adapted on case-by-case basis, since the firing parameters are strictly fixed, to guarantee the required temperature distribution within the kiln. In this context, computational fluid dynamics analysis represents a very convenient alternative to the traditional design based on experiments. In this article three-dimensional numerical predictions are presented for a syngas-fired burner. Three different fuels, two burner layouts and two burner nominal power are considered. Temperature, velocity and oxygen mass fraction distributions are discussed, and general design lines for low lower heating value gas burners are extracted.

Experimental and Numerical Analysis of Droplet Cooling

Dropwise cooling represents a major subject of interest for both academic and industrial researches. The present work is focused on investigating the thermal transient occurring as two water droplets are gently released (We < 30) onto a heated solid surface. This latter has been kept at initial temperature lower than 373.15 K to analyze the single-phase-evaporation regime. To the purpose, both an experimental and a numerical approach have conveniently been employed. Infrared thermography has been used to evaluate the temperature trend at the solid-liquid interface: an experimental facility has been built to carry out measurements from below, thus realizing a fully non-intrusive approach. A transparent-crystal disk has been inserted to serve as the solid substrate; its upper surface has been painted by a black coating, thus providing a black-body surface as the solid-liquid interface. The infrared thermocamera has been placed below and perpendicular to that surface; temperature has been thereby measured, being emissivity a known parameter. A numerical code has been developed to predict the involved physical phenomena: temperature trend, evaporation time and evaporated flux result from discretizing the three-dimensional energy-diffusion equation by the finite-volume method. Moreover, the model is based on structured non-uniform mesh to adapt to the occurring temperature gradients. Very good agreement is shown between experimental and numerical outcomes in terms of thermal transient and recovery.Copyright

Application of Recurrence Analysis to the period doubling cascade of a confined buoyant flow

Recurrence Analysis (RA) is a promising and flexible tool to identify the behaviour of nonlinear dynamical systems. The potentialities of such a technique are explored in the present work, for the study of transitions to chaos of buoyant flow in enclosures. The case of a hot cylindrical source centred in a square enclosure, is considered here, for which an extensive database of results has been collected in recent years. For a specific value of the system aspect ratio, a sequence of period doublings has been identified, leading to the onset of chaos. RA is applied here to analyse the different flow regimes along the route to chaos. The qualitative visual identification of patterns and the statistics given by the quantitative analysis suggest that this kind of tool is well suited to the study of transitional flows in thermo-fluid dynamics.