G. Ziskind

A uniform temperature heat sink for cooling of electronic devices

Experimental investigation of a heat sink for cooling of electronic devices is performed. The objective is to keep the operating temperature at a relatively low level of about 323–333 K, using a dielectric liquid that boils at a lower temperature, while reducing the undesired temperature variation in the both streamwise and transverse directions. The experimental study is based on systematic measurements of temperature, flow and pressure, infrared radiometry and high-speed digital video imaging. The heat sink has parallel triangular microchannels with a base of 250 lm. Experiments on flow boiling of Vertrel XF in the microchannel heat sink are performed to study the effect of mass velocity and vapor quality on the heat transfer, as well as to compare the two-phase results to a heat sink cooled by single-phase water flow. 2002 Elsevier Science Ltd. All rights reserved.

Resuspension of particulates from surfaces to turbulent flows—Review and analysis

Abstract The paper reviews the state of the art of aerosol resuspension research. Five different theoretical models of particle reentrainment are described. Accordingly, the expressions for resuspension from a surface exposed to fluid flow are explained. The advantages and shortcomings of the models are compared. Experimental results from the literature are summarized and presented in the form of tables. Dimensional analysis is applied to the experimental results, introducing the wall shear velocity as a universal parameter which determines the flow character. The advantages and limitations of the existing models of aerosol resuspension are assessed by means of a comparison between theory and experiments, recast in terms of dimensionless groups. Critical analysis shows that, in general, presently available experimental data do not support the existing theoretical models. Models of adhesion of small particles to solid surfaces are also reviewed. The role of van der Waals and electrical interactions in formation of contact with a surface is analyzed, together with the influence of elastic and plastic deformations. The effects of surface roughness, particle type and system history are discussed. The work analyzes the application of boundary-layer turbulence, especially the presence of quasi-periodic repeating patterns of coherent motion, to resuspension. Various mechanisms for generating the hydrodynamic force in turbulent and shear flows at different Reynolds numbers are discussed. Dimensionless expressions for hydrodynamic and surface forces and moments are developed allowing comparison and evaluation of their relative importance. Possible mechanisms of resuspension are proposed.

Adhesion moment model for estimating particle detachment from a surface

Abstract In this paper particle detachment from a surface by a hydrodynamic moment is analyzed. The detachment occurs when this moment exceeds the moment exerted on the particle by surface forces. An expression for the moment of surface forces is derived from the existing adhesion models. This moment is a product of the force acting on the particle and the variable contact radius, which decreases when the applied force increases. Accordingly, a condition for particle detachment from a smooth surface is obtained. In addition, particle detachment from a rough surface is considered. We show that a single asperity contact is similar to the contact of a particle with a smooth surface, but the detaching moment is reduced, because of the lower adhesion force and smaller contact radius. We also consider a particle in contact with two and three asperities, and obtain a condition for particle detachment from a rough surface. It is also shown that the hydrodynamic moment can cause particle detachment, while the hydrodynamic lift force is smaller than the adhesion force by several orders of magnitude. On the other hand, the lift force exceeds the weight of the particle. Hence, the detached particle is eventually removed from the surface by this force.

Numerical and Experimental Study of Solidification in a Spherical Shell

The present study explores numerically and experimentally the process of a phase-change material (PCM) solidification in a spherical shell. At the initial state, the PCM liquid occupies 98.5% of the shell. The upper segment of 1.5% contains air, which flows in as the solidification progresses. In the experiments, a commercially available paraffin wax is used. Its properties are engaged in the numerical simulations. The investigation is performed for solidification in spherical shells of 20 mm, 40 mm, 60 mm, and 80 mm in diameter at the wall uniform temperature, which varied from 10°C to 40°C below the mean solidification temperature of the phase-change material. Transient numerical simulations are performed using the FLUENT 6.2 software and incorporate such phenomena as flow in the liquid phase, volumetric shrinkage due to solidification, and irregular boundary between the PCM and air. The numerical model is validated versus the experimental results. Shrinkage patterns and void formation are demonstrated. Dimensional analysis of the results is performed and presented as the PCM melt fractions versus the product of the Fourier and Stefan numbers. This analysis leads to a generalization that encompasses the cases considered herein.

PARTICLE RESUSPENSION FROM SURFACES: REVISITED AND RE-EVALUATED

This review paper discusses the field of particle resuspension from surfaces, with special attention paid to the developments of the last decade. We present both general models of particle resuspension which appear in the literature, and studies on specific aspects of the problem. Among the topics discussed in the paper, one can find such diverse subjects as effect of particle and substrate material properties, modeling of particle adhesion, hydrodynamic mechanisms of particle removal, particle detachment from smooth and rough surfaces, and particle motion after its detachment. Also presented are particle resuspension from multilayer deposits and resuspension phenomena in liquid environments. Modern methods of particle removal and other related topics of interest are discussed.

Chimney-enhanced natural convection from a vertical plate: experiments and numerical simulations

Abstract This study deals with natural-convection heat transfer from a vertical electrically heated plate, which is symmetrically placed in a chimney of variable height. The heated plate serves as a thermal pump for ventilation of a symmetrical enclosure beneath the chimney. In order to provide a comprehensive picture of the phenomena, three main approaches are used in parallel: temperature and velocity measurements, flow visualization, and numerical simulation. Temperature measurements are done by thermocouples distributed inside the plate and through the chimney. Velocity measurements are performed by means of a precise anemometer. Visualization is performed using smoke of incense sticks, with video recording and consequent image processing. Computer simulations of unsteady flow and temperature fields are performed in 3D and compared with measurements and visualization, with special attention paid to velocity fluctuations. Analysis is presented on the dependence of the temperature distribution on the flow field. The air flow rate on the heating plate in the chimney increases with the chimney height and is adequately predicted by the numerical simulation of the system.

Ventilation by natural convection of a one-story building

Abstract The objective of the present paper is to study passive ventilation of a one-story detached building. The flow of air is induced by a hot element of the building heated by solar energy. The hot element could be a part of a roof or a wall of the building, or a chimney through which the air is sucked from the building. The method does not require electrical power or mechanical installations, thus it can be applied in remote areas and buildings that are not connected to electric power, like desert-located buildings. The method may be used also for removal of toxic gases, like radon, from the ground floor of the building, without additional expenses. Experiments and simulations have been performed, in steady and transient states, in a scaled-down laboratory model. The results obtained from the simulations and fully supported by measurements and visualization, indicate that it is possible to obtain effective ventilation by the proposed method. Numerical simulations for steady and transient ventilation in real-size buildings are presented and discussed.

Particle behavior on surfaces subjected to external excitations

Abstract Oscillatory motion of a particle on a surface may be caused by mechanical vibrations of the surface, acoustic oscillations and shock waves in the surrounding fluid, and also by turbulent flow near the surface. Whatever the source of the excitation, it causes particle oscillations. The character of these oscillations depends on the direction and frequency of the external force, as well as on the stiffness of the bond. Several linear and nonlinear oscillation models are introduced and analyzed in order to show whether particle removal is possible for soft and hard particles on smooth and rough surfaces under various conditions. Application to existing methods of surface cleaning is discussed.

Natural convection inside ventilated enclosure heated by downward-facing plate: experiments and numerical simulations

The present paper deals with heat transfer inside a ventilated enclosure. The enclosure is heated by a horizontal downward-facing constant-temperature hot plate. In order to provide a comprehensive picture of the problem, three main approaches are used in parallel: temperature measurement, flow visualization, and numerical simulation. The measurements are done by thermocouples distributed uniformly at the vertical mid-plane of the enclosure. The visualization is performed using the smoke of incense sticks, with video recording and consequent image processing. The computer simulations of the flow and temperature fields are performed both for steady and transient ventilation and compared with the results of the measurements and visualization. Applicability to ventilation is discussed.

Passive ventilation and heating by natural convection in a multi-storey building

Abstract Passive ventilation and heating in a multi-storey structure, by natural convection in a heated vertical duct were studied. Experimental study and computer simulations were first performed in a scaled-down laboratory model, divided into three levels, and connected by a duct, in which an electrically heated plate was used. The experiments included temperature and velocity measurements at each inner space, and inside the duct. The results obtained from the simulations and supported by the measurements, indicated that effective ventilation and heating, by the proposed method, were achievable in the laboratory structure. For a real-size structure of a five-storey building, which has a duct heated by solar irradiation, computer simulations were performed. Temperature fields and average temperatures were obtained at all levels of the building. The results have shown that even at low solar irradiation fluxes ventilation was achieved in summer, and heating in winter. The study has demonstrated that the proposed method is operable and feasible.