Julian Szekely

Bio: Julian Szekely is an academic researcher from Massachusetts Institute of Technology. The author has contributed to research in topics: Fluid dynamics & Turbulence. The author has an hindex of 35, co-authored 176 publications receiving 4999 citations.

Gas-solid reactions

Abstract: Preface In recent years considerable advances have been made in our understanding of gas-solid reaction systems. These advances are due in part to the development of more sophisticated mathematical models in which account is taken of such structural effects as pore size, grain size, and pore diffusion. Another important contributory factor has been the use of more sophisticated experimental techniques such as electron microscopy, X-ray diffraction, and porosimetry, which together with pore diffusion measurements provide information on the key structural parameters and make possible the critical assessment of this new generation of models. These new developments were motivated to a great extent by the societal and economic importance of gas-solid reaction systems due to their relevance for a broad range of processing operations, including iron oxide reduction, the combustion of solid fuels, the desulfurization of the fuel gases, and the incineration of solid wastes. The purpose of this monograph is to present in an integrated form a description of gas-solid reaction systems, where full account is taken of these new developments and where structural models of single particle systems, experimental techniques, the interpretation of measurements, the design of gas-solids contacting systems, and practical applications are treated in a unified manner. The actual approach to be developed here is based on methodology similar to that employed in chemical reaction engineering in the interpretation of rate data and the design of process systems in heterogeneous catalysis. More specifically, through the use of this methodology, the individual components of the overall reaction sequence are studied and examined in isolation and the description of the system is then synthesized from these components. This approach provides greatly improved insight and at the same time allows a much broader generalization of the results than is possible through the use of empirical models. While there is a close parallel between heterogeneous catalytic reaction systems and gas-solid reactions, the latter systems are rather more complicated because of the direct participation of the solid in the overall reaction. As the solid is consumed or undergoes chemical change, its structure changes continuously, making the system inherently transient. It follows that the analysis of gas-solid reactions involves an additional dimension, that of time, which is not necessarily needed in the study of heterogeneous gas-solid reactions. The inherently unsteady nature of gas-solid reaction systems introduces a number of complicating factors which render the tackling of these problems a definitely nonroutine task requiring originality. It is noted here that while the discussion in this text is devoted to gas-solid reactions, with little modification the treatment developed here should be applicable to liquid-solid reaction systems. The material presented here could form part of a one-semester graduate level course to be given to students either in metallurgy (materials engineering) or in chemical engineering. It is hoped, moreover, that the book will appeal to the growing number of practicing engineers engaged in process research, development, and design in the many fields where gas-solid reactions are of importance.

Mathematical modeling of the isothermal impingement of liquid droplets in spraying processes

Abstract: A mathematical representation has been developed and computed results are presented describing the spreading of droplets impacting onto a solid substrate. Problems of this type are of major practical interest in plasma spraying (PS) and in spray forming (SF) operations. While the present study was confined to the fluid flow aspects of the process, information has been generated on both the final splat dimensions and on the time required to complete the spreading process. Through this treatment, it is possible to relate these quantities (the splat size and the spreading time) to the operating conditions,i.e., droplet size and droplet velocity, and material properties. The theoretical predictions were found to be in good agreement with both Madejski’s asymptotic solution[17] and with available experimental results. For typical SF conditions (droplet sizes in the 100-µm range and droplet velocities in the 100 m/s range), the spreading times were of the order of microseconds,i.e., significantly shorter than the estimated solidification time.

Fluid flow phenomena in metals processing

Abstract: 458 pages 2012 Julian Szekely Fluid Flow Phenomena In Metals Processing 032314957X, 9780323149570 Elsevier, 2012 Fluid Flow Phenomena in Metals Processing outlines the fundamentals of fluid flow theory, emphasizing the potential applications of fluid flow concepts that are illustrated by actual problems drawn from the metallurgical literature.This book is divided into 10 chapters. Chapters 1 to 4 are devoted to the fundamentals of fluid flow, while Chapters 5 to 9 are concerned with the application of basic concepts to specific systems, such as electromagnetically driven flows, surface tension and natural convection driven flows, multiparticle systems, gas bubbles, and impinging jets. The discussion on flow measurements and introduction to physical modeling are provided in the last chapter.This publication is suitable for a one semester graduate level course for metallurgy and chemical engineering students. file download nugus.pdf

Heat- and fluid-flow phenomena in weld pools

Abstract: A mathematical formulation is presented for the transient development of the fluid-flow field and the temperature field in a liquid pool, generated by a spatially variable heat flux falling on an initially solid metal block. This physical situation is an idealized representation of a TIG (tungsten-inert-gas) welding process. In the formulation allowance is made for electromagnetic, buoyancy and surface forces and the resultant equations are solved numerically.It is found that both the convective flow field and the temperature field are markedly affected by the nature of the heat flux and the flux of electric current falling on the free surface.In the absence of surface-tension effects a broadly distributed heat flux and corresponding current distribution cause a situation where both electromagnetic and buoyancy forces are important in determining the fluid-flow field; however, in these systems the fluid-flow field does not play a significant role in defining the heat-transfer process. In contrast, a sharply focused heat flux and current density on the free surface give rise to strong electromagnetically driven flows, which play an important role in determining the shape of the weld pool.Calculations are also done exploring the effect of surface-tension-driven flows. It is found that surface-tension gradients may produce quite high surface velocities and can have a profound effect on determining the weld-pool shape.

Fluid flow, heat transfer, and solidification of molten metal droplets impinging on substrates: Comparison of numerical and experimental results

Abstract: A mathematical representation has been developed, and computed results are presented describing the spreading and solidification of droplets impacting onto a solid substrate. This impingement is of major practical interest in plasma spraying and spray forming operations. Experiments in which molten metal drops were made to impinge onto a substrate were used to test the model. High-speed videography was used to record the spreading process, which typically took a few milliseconds for the experimental conditions employed. A comparison was made of the theoretical predictions with the experimental measurements; these were found to be in very good agreement, suggesting that the theoretical treatment of the model is sound. These calculations permit the prediction of the time and extent of the spreading process, the solidification rate, and the effect of process parameters, such as droplet size, droplet velocity, superheat, and material properties, provided that a value of the thermal contact coefficient is known. The most important finding of the modeling work is that for large droplets (∼5-mm diameter) with low impinging velocities (∼2 m/s), spreading and solidification appear to take place at comparable rates; in contrast, for small (∼100−µm diameter) particles impacting at a high velocity (∼100 m/s), the time scale for spreading appears to be shorter than the time scale for solidification (within the range of parameters of this study.)

The surface evolver

Abstract: The Surface Evolver is a computer program that minimizes the energy of a surface subject to constraints. The surface is represented as a simplicial complex. The energy can include surface tension, gravity and other forms. Constraints can be geometrical constraints on vertex positions or constraints on integrated quantities such as body volumes. The minimization is done by evolving the surface down the energy gradient. This paper describes the mathematical model used and the operations available to interactively modify the surface.

Behavior of metal alloys in the semisolid state

Abstract: During dendritic solidification of castings and ingots, a number of processes take place simultaneously within the semisolid region. These include crystallization, solute redistribution, ripening, interdendritic fluid flow, and solid movement. The dendritic structure which forms is greatly affected by convection during the early stages of solidification. In the limit of vigorous convection and slow cooling, grains become spheroidal. Alloys with this microstructure possess rheological properties in the semisolid state which are quite different from those of dendritic alloys. They behave thixotropically, and viscosity can be varied over a wide range, depending on processing conditions. The metal structure and its rheological properties are retained after solidification and partial remelting. The semisolid alloys can be formed in new ways, broadly termed «semisolid metal (SSM) forming processes». Some of these are now employed commercially to produce metal components and are also used to produce metal-matrix composites

Capillary effects during droplet impact on a solid surface

Abstract: Impact of water droplets on a flat, solid surface was studied using both experiments and numerical simulation. Liquid–solid contact angle was varied in experiments by adding traces of a surfactant to water. Impacting droplets were photographed and liquid–solid contact diameters and contact angles were measured from photographs. A numerical solution of the Navier–Stokes equation using a modified SOLA‐VOF method was used to modeldroplet deformation. Measured values of dynamic contact angles were used as a boundary condition for the numerical model. Impacting droplets spread on the surface until liquid surface tension and viscosity overcame inertial forces, after which they recoiled off the surface. Adding a surfactant did not affect droplet shape during the initial stages of impact, but did increase maximum spread diameter and reduce recoil height. Comparison of computer generated images of impacting droplets with photographs showed that the numerical model modeled droplet shape evolution correctly. Accurate predictions were obtained for droplet contact diameter during spreading and at equilibrium. The model overpredicted droplet contact diameters during recoil. Assuming that dynamic surface tension of surfactant solutions is constant, equaling that of pure water, gave predicted droplet shapes that best agreed with experimental observations. When the contact angle was assumed constant in the model, equal to the measured equilibrium value, predictions were less accurate. A simple analytical model was developed to predict maximum droplet diameter after impact. Model predictions agreed well with experimental measurements reported in the literature. Capillary effects were shown to be negligible during droplet impact when We≫Re1/2.