Sangeeta Kohli

Numerical Simulation of the Jet Impingement Cooling of a Circular Cylinder

A numerical investigation was carried out on circular jet impingement heat transfer from a constant temperature circular cylinder to understand the major parameters which influence the fluid flow and heat transfer characteristics. In this study, air was considered as the working fluid. The flow was considered to be three-dimensional, incompressible, and turbulent. To select a suitable turbulence model for the parametric study, numerical simulations were carried out with standard k-ϵ, standard k-ω, RNG k-ϵ, Realizable k-ϵ, and SST k-ω turbulence models for modeling Reynolds stress terms. Simulations were also carried out using four low Reynolds number models. The results obtained using these models were compared with the available experimental results of jet impingement heat transfer from circular cylinder. It was identified that the RNG k-ϵ model predicts heat transfer characteristics better compared to all other turbulence models considered in this study. Using this turbulence model, a parametric study was carried out for the Reynolds number (Re d ), defined based on the diameter of the nozzle ranging from 10,000 to 50,000. The ratio of distance between the nozzle exit and the cylinder surface to the diameter of the jet (h/d) was varied from 4 to 16 and the ratio of nozzle diameter to cylinder diameter (d/D) varied from 0.11 to 0.25. For a fixed Re d and d/D, the stagnation point Nusselt number increases as h/d decreases. The stagnation point Nusselt number decreases as d/D increases for a fixed value of Re d and h/d. The effects of change in h/d and d/D are significant only near the stagnation region.

Thermodynamics and Kinetics of Gasification

This chapter deals with the basic thermodynamics and chemical kinetics pertaining to the various physicochemical phenomena that are collectively termed as the phenomenon of gasification. Although the phenomena associated with the gasification of various feedstocks differ from each other in detail, the underlying thermodynamics is more or less common and is attempted to be captured here. The technology of gasification also has a wide variety, and this results in different phenomena having varying grades of importance in each. Thermodynamics of a phenomenon is described in terms of conservation equation for mass and the first law, often discussed under the headings of stoichiometry and energetics of a phenomenon, and in terms of the second law, which determines the equilibrium state at the end of the phenomenon, and thus defines the product compositions in gasification when the reactor is maintained at a given pressure and temperature. The variety of phenomena involved in gasification, namely drying of feedstock, its pyrolysis, homogeneous and heterogeneous reactions which form part of the gasification in the form of oxidation and reduction reactions, proceed at different rates in a given system, and also vary widely between different types of gasification systems. Hence, it is important to study the kinetics of these phenomena, in addition to the study of thermodynamic equilibrium states pertaining to these phenomena. Owing to the fact that each of these phenomena is extremely complex, in mathematical modelling of these phenomena, often apparent mechanism and their thermodynamics and kinetics are studied. This leads to a variety of models and thermodynamic and kinetic data in the literature, often in apparent conflict with each other. This chapter also attempts to identify some of these conflicts through the experience of the authors in modelling gasification phenomena.