Showing papers by "Raymond Viskanta published in 2006"

Mathematical Modeling of Direct Flame Impingement Heat Transfer

Abstract: Direct flame impingement (DFI) furnaces consist of large arrays of high velocity combusting jets with temperatures up to 1700 K and impinging on complex configuration surfaces of the work pieces. This results in serious convergence problems DFI modeling and computational efforts. A new method of modeling convective-diffusion transfer (CDT) and zone radiation transfer (RT) employing different calculation schemes with a multi-scale grid is presented. Relatively coarse grid calculation domain allows use of conservative and accurate zone radiation transfer method with only modest computational efforts. A fine grid calculation domain is used to predict convective -diffusion transfer for a representative furnace section, containing a small number of jets that allows to significantly decrease the computer time. The main difficulty of coupling between convective-diffusion transfer (CDT) and radiation heat transfer numerical computations is successfully overcome using a relatively simple algorithm. The method allows one to model the physicochemical process taking place in the DFI and reveals as well as explains many features that are difficult to evaluate from experiments. The results were obtained for high velocities (up to 400 m/s) and high firing rates. Maximum (available for natural gas-air firing) total heat fluxes up to 500 kW/m2 and convective heat fluxes of up to 300 kW/m2 were obtained with relatively 'cold' refractory wall temperatures not exceeding 1300 K. The combustion gas temperature range was 1400-1700 K. A simplified analysis for NOx emissions has been developed as post-processing and shows extremely low NOx emissions (under 15 ppm volume) in DFI systems. Good agreement between measurements and calculations has been obtained. The model developed may be regarded as an efficient tool to compute and optimize industrial furnaces designs in limited time.Copyright © 2006 by ASME

Melting of a Semitransparent Bed of Particles by Convection and Radiation

Abstract: A theoretical model has been developed to study heating and melting of porous media comprised of semitransparent particles. The heating and melting phases are treated by two different models. Heat transfer by conduction and radiation in the bed of particles is approximated using the effective thermal conductivity concept. The melt is assumed to accumulate at the top of the unmelted bed and not to run off after the melting has started. Radiative transfer in the one-dimensional melt layer is treated rigorously, and the spectral nature of radiation is accounted for. The convective and radiative heating of the bed and the melt layer are treated parametrically. The results of calculations for different bed thicknesses are reported, and sensitivity studies involving the heat transfer coefficient, surroundings temperature, porosity, and particle diameter are presented and discussed.

Physical Properties of Confined Silicon Structures Using EDIP

Abstract: Physically confined structures such as thin films and nanowires are becoming increasingly important in the energy and electronics sectors. This has resulted from the ability to tailor nanostructures to yield physical properties that are significantly different from bulk. The main focus of this work is to examine how physical confinement in one and two dimensions affects the phonon wave vector spectrum within the first Brillouin zone of silicon thin films and silicon nanowires. Dispersion curves as well as density of states (DOS) are obtained using the dynamical matrix approach and a harmonic approximation to the three-body environmentally-dependent interatomic potential (EDIP). It is also shown how these changes in the phonon spectrum for both films and wires affect the volumetric specific heat with respect to bulk. The simulations are carried out assuming ideal free-standing boundary conditions. It is shown that confinement effects on the phonon specific heat are only important below 5 mm for both silicon films and wires.Copyright © 2006 by ASME