Combined convection and other effects in heat transfer in horizontal flows

For many internal flow situations the effect of buoyancy is to cause significant modifications of the internal flow field and heat transfer rate. The present study focuses on combined forced and free convection under laminar flow conditions in horizontal cylindrical ducts. There, the superposition of secondary, buoyancy induced circulations to the basic forced flow, and the presence of peripheral and axial conduction in the duct wall, give rise to a three-dimensional conjugate heat transfer problem. Such a combined convection feature had not been investigated previously. A novel parallel predictive and experimental study of combined convection for laminar flow in cylindrical ducts is carried out here. The finite volume code FLOW3D from Harwell is used to treat the fully elliptic three-dimensional thermal-flow problem. The choice derives from a complete and detailed survey of the numerical techniques used in the context of combined convection. The predictive work relates specifically to a new experimental study, which has the object of obtaining fresh data for combined convection in horizontal duct flow. A 3 m long, 16 mm I.D. copper pipe is used, with uniform peripheral electrical heating. Wall temperature measurements are taken at twelve axial positions. The experiment covers the range of stable mixed convection and strictly laminar flow conditions, with the Reynolds number ranging from 500 to 1000, and the modified Rayleigh number, Raq, from 1x10⁵ to 5x10⁶. The scope of the experiment is to provide data for comparison with numerical predictions. These, in turn, are designed to model the experimental conditions very accurately, including, in particular, the effects of peripheral and axial wall conduction. The study is complemented with various analyses intended to understanding properly both pure forced and natural convection modes. These are investigated separately in the first part of the work. Overall, this work provides fresh experimental and predictive evidence on various features relevant to the onset and the development of buoyancy induced secondary flows in round ducts under heating conditions. More specifically, the effects, of conduction in the duct wall are highlighted, and demonstrated to have a definite influence on wall temperature, and Nusselt number distributions, even in the case of long, thin-walled ducts

Air entrainment in the primary impact of single drops on a free liquid surface

Air-bubble entrainment produced by the impact of water drops on a liquid pool is investigated with the use of high-speed imaging. A wide range of drop volumes and impact velocities is considered to determine how the entrainment mechanisms change with varying the impact conditions. Five different entrainment regimes are distinguished on the basis of the observed flow phenomena. Their characteristic features are described in terms of bubble formation, crater evolution, jetting and secondary drop ejection. A regime map is reconstructed in the Froude-Weber space. Results obtained in the present study show good agreement with the phase diagrams reported in the literature and contribute to complete the scenario of the entrainment regimes. Quantitative data about the size and the residence times of the entrained nuclei are also presented

Laminar Flow Heat Transfer with Axial Conduction in a Circular Tube: a Finite Difference Solution

A finite difference technique is developed for evaluating the rate of heat transfer in the thermal entrance region of ducts with axial conduction. The pipe is assumed to extend from minus to plus infinity, while the velocity profile is fully developed. The boundary condition selected is a uniform heat flux for z greater than or equal to 0, while the wall temperature is kept uniform and equal to its minus infinity value for z less than 0. The results show that axial conduction and heat losses from the unheated section of the pipe significantly affect the temperature profile at the inlet of the heated part lower than the local mixed mean temperatures immediately downstream from the start of the heating. It is found that the performance of this type of heat exchanger can be described by a new parameter as an alternative to the usual Nusselt number

Convective Heat Transfer Coefficients in the Circulation

Convective heat transfer in the vessels of the circulatory system is investigated numerically. In the modeling, account is taken of the non-Newtonian rheological properties of blood and the presence of a cell-depleted plasma layer at the vessel wall. The latter is found to produce a remarkable enhancement of the heat transfer rate in the small vessels, while the effects due to the rheological behavior of blood are comparatively low. A comparison with experimental data available in the open literature is finally attempted

A fast algorithm for Direct Numerical Simulation of natural convection flows in arbitrarily-shaped periodic domains

A parallel algorithm is presented for the Direct Numerical Simulation of buoyancy-induced flows in open or partially confined periodic domains, containing immersed cylindrical bodies of arbitrary cross-section. The governing equations are discretized by means of the Finite Volume method on Cartesian grids. A semi-implicit scheme is employed for the diffusive terms, which are treated implicitly on the periodic plane and explicitly along the homogeneous direction, while all convective terms are explicit, via the second-order Adams-Bashfort scheme. The contemporary solution of velocity and pressure fields is achieved by means of a projection method. The numerical resolution of the set of linear equations resulting from discretization is carried out by means of efficient and highly parallel direct solvers. Verification and validation of the numerical procedure is reported in the paper, for the case of flow around an array of heated cylindrical rods arranged in a square lattice. Grid independence is assessed in laminar flow conditions, and DNS results in turbulent conditions are presented for two different grids and compared to available literature data, thus confirming the favorable qualities of the method

Natural convection in asymmetric triangular enclosures heated from below

Triangular enclosures are typical configurations of attic spaces found in residential as well as industrial pitched-roof buildings. Natural convection in triangular rooftops has received considerable attention over the years, mainly on right-angled and isosceles enclosures. In this paper, a finite volume CFD package is employed to study the laminar air flow and temperature distribution in asymmetric rooftop-shaped triangular enclosures when heated isothermally from the base wall, for aspect ratios (AR) 0.2 64 AR 64 1.0, and Rayleigh number (Ra) values 8 7 105 64 Ra 64 5 7 107. The effects of Rayleigh number and pitch angle on the flow structure and temperature distributions within the enclosure are analysed. Results indicate that, at low pitch angle, the heat transfer between the cold inclined and the hot base walls is very high, resulting in a multi-cellular flow structure. As the pitch angle increases, however, the number of cells reduces, and the total heat transfer rate progressively reduces, even if the Rayleigh number, being based on the enclosure height, rapidly increases. Physical reasons for the above effect are inspected

Temperature and Velocity Distributions in a Church with Floor Heating in Various Seasons

Experiences carried out within a large church in Bologna, Italy, equipped with a floor radiant panels heating plant are presented. High velocity air flows are detected, not compatible with thermal comfort. Experimental data form the basis for understanding and controlling thermal instabilities in tall halls

Bifurcations of Natural Convection Flows from an Enclosed Cylindrical Heat Source

A numerical analysis of transitional natural convection from a confined thermal source is presented. The system considered is an air-filled, square-sectioned 2D enclosure containing a horizontal heated cylinder. The resulting flow is investigated with respect to the variation of the Rayleigh number, for three values of the aspect ratio A. The first bifurcation of the low-Ra fixed-point solution is tracked for each A-value. Chaotic flow features are detailed for the case A = 2.5. The supercritical behaviour of the system is investigated using nonlinear analysis tools and phase-space representations, and the effect of the flow on heat transfer is discussed

Review on the Influence of Biological Deterioration on the Surface Properties of Building Materials: Organisms, Materials, and Methods

A strong attention is recently paid to surface properties of building materials as these allows controlling solar gains of the building envelope and overheating of buildings and urban areas. In this regard, deterioration phenomena due to biological aggression can quickly damage solar-reflecting roof surfaces and thus increase sharply solar gains, discomfort, air-conditioning costs and waterproofing degradation. The same deterioration problem has deleterious effect on cultural heritage, ruining its huge historic and artistic value. This work is aimed at providing an overview on the different organisms that affect the surface of most used building materials, to support the design of new building materials with long-lasting surface properties and to find a way to preserve cultural heritage. Artificial ageing is the long-term aim of this investigation, in which what in nature happens after months or years is compressed in a very short time by forcing the growth of microorganisms through a strict control on the different conditioning factors. Both natural and artificial ageing are eventually outlined in the last part of this work to provide a comprehensive idea of what is necessary to study in a complete way biological ageing protocols on building materials. Several characterization techniques are also introduced to analyse the influence of microorganisms on the surface of different building materials

Considerazioni in merito alla depolverazione per mezzo di campi acustici

Si esamina la bibliografia relativa alle tecniche di depolverazione di fumi industriali mediante imposizione di un campo acustico di adeguata frequenz