A comparative study of gray and non-gray methods of computing gas absorption coefficients and its effect on the numerical predictions of oxy-fuel combustion

Authored by: Z. C. Wheaton, D. E. Stroh, G. Krishnamoorthy, M. Sami, S. Orsino and P. Nakod

Corresponding Author: G. Krishnamoorthy

Current affiliation:
Department of Chemical Engineering,
PO Box 7101, Harrington Hall Room 323,
241 Centennial Drive,
University of North Dakota,
Grand Forks, ND 58202-7101, USA

A mail can be sent to the Corresponding Author at gautham.krishnamoorthy@engr.und.edu

This work is a collaboration between a major computational software supplier (ANSYS) and academic researchers from the University of North Dakota in the U.S.A. CFD models developed for air-breathing combustion do not necessarily transfer to oxygen-firing with flue gas recycling. This paper treats necessary modifications to the weighted sum of grey gases (WSGG) method for radiative heat transfer in a gas fired furnace. The reader will immediately comprehend the importance of thermal radiation transfer due to the increase in carbon dioxide and water (if flue gas is not dried).

IFRF members will be pleased to note that the OXYFLAM and HTAC experimental data are used for comparison to the computational results.

Key Words:
Oxy combustion; Radiative heat transfer; weighted-sum-of-gray-gas (WSGG); CFD simulation

Summary:
Computational fluid dynamics (CFD) simulations are performed to model the radiative heat transfer in natural gas fired furnaces operating under oxygen-enriched conditions with flue gas recirculation. Gray and non-gray formulations of recently proposed weighted-sum-of-gray-gas (WSGG) models are employed to compute the gas absorption coefficients within two representative furnaces (OXYFLAM and HTAC) and their accuracies and impact on the predicted velocity, temperature and radiative heat fluxes evaluated by comparisons against experimental data. As a result of the nearly homogeneous and isothermal conditions encountered within the furnaces, the differences in the predicted incident radiative flux profiles between the gray and non-gray models is not significant. However, in the OXYFLAM furnace where higher temperatures and larger temperature gradients are encountered, the differences between the gray and non-gray radiation calculations manifest themselves in the temperature distributions downstream of the flame region.

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