Massimo Corcione

Effects of the thermal boundary conditions at the sidewalls upon natural convection in rectangular enclosures heated from below and cooled from above

Steady laminar natural convection in air-filled, 2-D rectangular enclosures heated from below and cooled from above is studied numerically for a wide variety of thermal boundary conditions at the sidewalls. A specifically developed numerical model based on the SIMPLER algorithm is used for the solution of the mass, momentum and energy transfer governing equations. Simulations are performed for several values of both the width-to-height aspect ratio of the enclosure in the range between 0.66 and 8, and the Rayleigh number based on the cavity height in the range between 103 and 106, whose influence upon the flow patterns, the temperature distributions and the heat transfer rates are analyzed and discussed. Comparisons among the different thermal configurations considered are reported. In particular, with reference to the typical configuration wherein the sidewalls are adiabatic, it is found that the heat transfer effectiveness of the bottom wall increases (or slightly decreases) as each adiabatic sidewall is replaced by a cooled (or a heated) sidewall. An opposite behaviour is observed for the top wall. The heat transfer rate results obtained are expressed through dimensionless correlation-equations.

Optimal design of Outdoor Lighting Systems by Genetic Algorithms

The use of genetic algorithms for the optimal design of lighting systems is illustrated and discussed. The lighting of an outdoor tennis court and of a football field are considered as case-studies. The main aims of the paper are: 1) the evaluation of the performance of the genetic algorithm strategies in comparison with more traditional optimization procedures e.g., Montecarlo and gradient methods, through the study of a relatively simple lighting installation, an outdoor tennis court, whose optimal design solution is calculated in a relatively short time by the analysis of all the possible design alternatives; and 2) the evaluation of the performance of the genetic algorithm strategies applied to the design of a larger and more complex lighting system i.e., that for an outdoor football field, for which a comparison between the results obtained through the optimization procedure and those relevant to an existing lighting installation is examined and discussed. Considerations on both the computational times required and the calculation accuracy obtained are also reported.

A Semi-Empirical Model for Predicting the Effective Dynamic Viscosity of Nanoparticle Suspensions

A semi-empirical model for predicting the effective dynamic viscosity of nanofluids, based on a single set of experimental data available in the literature for silica nanoparticles suspended into ethanol, is presented and discussed. The equation, which includes the overall effects of the friction at the solid–liquid interface, the nanoparticle colloidal interactions, and the formation of aggregates, expresses the ratio between the effective dynamic viscosity of the suspension and the dynamic viscosity of the pure base liquid as a function of the nanoparticle size and concentration. The predicted results are in rather good agreement with a wide variety of data relative to nanofluids consisting of several combinations of solid and liquid phases, extracted from different sources. The accuracy and ease of application of the proposed equation make it interesting from the engineering point of view, for both numerical simulation purposes and thermal design tasks.

Natural Convection Heat and Momentum Transfer in Rectangular Enclosures Heated at the Lower Portion of the Sidewalls and the Bottom Wall and Cooled at the Remaining Upper Portion of the Sidewalls and the Top Wall

Laminar natural convection inside air-filled, rectangular enclosures heated from below and cooled from above, with the lower portions of both sidewalls maintained at the temperature of the bottom wall, and the remaining upper portions of the sidewalls maintained at the temperature of the top wall, is studied numerically. A control volume formulation of the finite-difference method is used for the solution of the mass, momentum, and energy transfer governing equations. Simulations are performed for height-to-width aspect ratios of the cavity from 1 to 5, Rayleigh numbers based on the height of the cavity from 5 × 102 to 5 × 106, and values of the heated fractions of both sidewalls from 0 to 1. It is found that when the heated portions of the two sidewalls are different in length, a steady-state solution is reached, with a basic three-cell flow pattern. In contrast, when the heated fractions of the sidewalls are the same, the asymptotic solution may be either stationary, with a flow field consisting of two pairs of superimposed roll cells, or periodic, with a flow pattern consisting of a primary cell and two secondary cells that pulsate about the center of the enclosure. Dimensionless heat transfer correlating equations are proposed.

Pumping Energy Saving Using Nanoparticle Suspensions as Heat Transfer Fluids

The pumping power diminution consequent to the use of nanoparticle suspensions as heat transfer fluids is analyzed theoretically assuming that nanofluids behave like single-phase fluids. In this hypothesis, all the heat transfer and friction factor correlations originally developed for single-phase flows can be used also for nanoparticle suspensions, provided that the thermophysical properties appearing in them are the nanofluid effective properties calculated at the reference temperature. In this regard, two empirical equations, based on a wide variety of experimental data reported in the literature, are used for the evaluation of the nanofluid effective thermal conductivity and dynamic viscosity. Conversely, the other effective properties are computed by the traditional mixing theory. Both laminar and turbulent flow regimes are investigated, using the operating conditions, the nanoparticle diameter, and the solid–liquid combination as control parameters. The fundamental result obtained is the existence of an optimal particle loading for minimum cost of operation at constant heat transfer rate. A set of empirical dimensional algebraic equations is proposed to determine the optimal particle loading of water-based nanofluids.

A method for predicting non-uniform steady sound fields within spaces bounded by diffusive surfaces

Abstract A method for predicting non-uniform steady sound fields within spaces bounded by diffusive surfaces is presented. The definitions of the basic physical quantities employed as well as a detailed description of the methodology application are reported. An experimental verification of the model is conducted through measurements of the sound field due to a point source within rectangular ducts of different lengths. The comparison between theoretical predictions and experimental data shows a meaningful good agreement, thus proving the reliability and effectiveness of the method developed.

Correlating equations for laminar free convection from misaligned horizontal cylinders in interacting flow fields

Steady laminar free convection in air from a pair of misaligned, parallel horizontal cylinders, i.e., a pair of parallel cylinders with their axes set in a plane inclined with respect to the gravity vector, is studied numerically. A specifically developed computer code based on the SIMPLE-C algorithm is used for the solution of the dimensionless mass, momentum, and energy transfer governing equations. Results are presented for different values of the center-to-center cylinder spacing from 1.4 up to 10 diameters, the tilting angle of the two-cylinder array from 0 deg to 90 deg, and the Rayleigh number based on the cylinder diameter in the range between 10 3 and 10 7 . It is found that the heat transfer rates at both cylinder surfaces may in principle be traced back to the combined contributions of the so-called plume effect and chimney effect, which are the mutual interactions occurring in the vertical and horizontal alignments, respectively. In addition, at any misalignment angle, an optimum spacing between the cylinders for the maximum heat transfer rate, which decreases with increasing the Rayleigh number, does exist. Heat transfer dimensionless correlating equations are proposed for any individual cylinder and for the pair of cylinders as a whole.

Laminar free convection from a vertical plate with uniform surface heat flux in chemically reacting systems

Abstract Two-dimensional steady laminar free convection from a vertical plate with uniform surface heat flux rate is studied in a gas where a reversible very fast reaction of dissociation A ↔2 B takes place at atmospheric pressure. The effective thermophysical properties of the gas in the interval of dissociation are evaluated and the governing boundary-layer equations are solved numerically by a finite-difference method with control volume formulation for a wide range of values of the independent variables which have a significant influence on the phenomenon. In the case of undisturbed fluid temperature T ∞ smaller than T 0.5 , corresponding to a rate of dissociation α =0.5, three different heat transfer regimes, marked by two critical heat fluxes, may be distinguished as the surface heat flux rate increases. The theoretical results obtained for the critical heat fluxes as well as the coefficient of convection are expressed in terms of correlations among dimensionless parameters defined through the mixture effective properties.

Preliminary experimental validation of a landmine detection system based on localized heating and sensing

In this paper we present results of experimental validation of a new methodology for anti-personnel mine (APM) detection for humanitarian demining, proposed by the authors and previously validated only by simulation. The technique is based on local heating and sensing by contactless thermometers (pyrometers). A large sand box (2.6m3) has been realized and fitted with a cart moving on rails and holding instrumentation. Accurate mine surrogates have been hidden in the sand together with confounders. Preliminary measurements are consistent with simulations and prove validity of the approach.

Thermal detection of buried landmines by local heating

A new concept for buried antipersonnel mine (APM) detection is proposed. The detection system is based on a focused heat source and a contactless thermometer mounted on a suspended carriage scanning the ground surface. Presence of heat diffusivity anomalies (mine candidates) is assessed from anomalies in the surface temperature measured after the local heating. After describing the structure of the proposed system, and the physical functioning principles, we discuss a case study, based on numerical simulations, which proves the feasibility of the device. Main advantages of the system proposed are the simplicity of the equipment (that is cheap to realize, and easy to use, maintain and repair), and power efficiency.