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.

The Atmosphere’s Effect on Stainless Steel Slabs’ Oxide Formation in a CH4-Fuelled Reheating Furnace

“…The burner type affects the burn reaction itself and using oxyfuel practices, e.g., NO x emissions can be significantly reduced [11,12]. Further emission reductions can be achieved from flameless oxyfuel burn [12,13] or direct flame impingement (DFI) [14,15]. While traditional air/fuel burner nozzles can be utilised for oxygen-enriched practice [16], oxyfuel practice usually requires burner retrofitting due to flue gas recirculation [17].…”

Simple Expression for the Emittance of H2O–CO2 Mixtures in Zonal Methods of Radiation Transfer Modeling

“…The modeling of metal heating furnaces (or copper-smelting furnaces) requires a very large number of total emittances to eval uate direct exchange areas of volume and surface zones [10], Thus, for a zonal method, the advantage of the present approach is apparent.…”

Heat Transfer Due To Impinging Flames on Plane Surfaces

“…The experimental works pertain mostly to laminar flow and furnish the information that the average heat flux at plate surface increases with an increase in Reynolds number and a decrease in the ratio of separation distance to burner diameter. The theoretical studies in flame impingement heat transfer are relatively few in number [6][7]. Most of them deal with laminar and turbulent premixed flames impinging normally to a flat surface and predict the temperature field, velocity field and heat release rate near the plate surface.…”

“…Most of them deal with laminar and turbulent premixed flames impinging normally to a flat surface and predict the temperature field, velocity field and heat release rate near the plate surface. Malikovet al [6] predicted that the radiative heat flux at the plate surface is only 1/3 of the total flux , while the remaining 2/3 is due to the convection heat transfer from the hot jet impinging on the surface. All the works predict a non uniform temperature distribution near the plate surface with a maximum one shifted away from the stagnation point.…”