Abstract
Mass and heat transfer coefficients (MTC and HTC) in automotive exhaust catalytic monolith channels are estimated and correlated for a wide range of gas velocities and prevailing conditions of small up to real size converters. The coefficient estimation is based on a two dimensional computational fluid dynamic (2-D CFD) model developed in Comsol Multiphysics, taking into account catalytic rates of a real catalytic converter. The effect of channel size and reaction rates on mass and heat transfer coefficients and the applicability of the proposed correlations at different conditions are discussed. The correlations proposed predict very satisfactorily the mass and heat transfer coefficients calculated from the 2-D CFD model along the channel length. The use of a one dimensional (1-D) simplified model that couples a plug flow reactor (PFR) with mass transport and heat transport effects using the mass and heat transfer correlations of this study is proved to be appropriate for the simulation of the monolith channel operation.
Highlights
The growing concern about the environmental impact of the exhaust emission pollutants has led to the need for the development of increasingly efficient exhaust gas after-treatment systems and catalytic converters [1].In gasoline engine applications, the toxicity of exhaust emissions is reduced by employing catalytic converters as an afterburning reactor [2]
The mass transfer coefficients of each species in the gas mixture and the heat transfer coefficient were calculated along the monolith channel length according to the thin film theory by developing and running detailed 2-D computational fluid dynamic (CFD) models in Comsol Multiphysics and combining their results with a 1-D model in which mass and heat transfer coefficients account for transport phenomena
The parameter a is related to the Kt value at which the heat transfer coefficient reaches a plateau or alters less significantly with the cell length, away from the cell entrance; the parameter b is related to the initial value that the Kt takes at the cell entrance, and the parameters c and d are related with the curvature of the profile and the distance from the entrance that is demanded for the Kt profile to level off
Summary
The growing concern about the environmental impact of the exhaust emission pollutants has led to the need for the development of increasingly efficient exhaust gas after-treatment systems and catalytic converters [1]. The mass and heat transfer coefficients introduced in the 1-D models are often expressed in terms of dimensionless numbers Sherwood (Sh) and Nusselt (Nu) and they depend on various parameters, such as shape and dimensions of the channel, fluid properties, flow rates, washcoat characteristics and reaction rates [7,15]. It is very difficult, if not impossible, to experimentally measure local concentrations and temperatures, and especially wall concentrations and temperatures in narrow channel monoliths [16], but the mass and heat transfer coefficients may be computed from the radial temperature and concentration gradients computed by using two or three dimensional models [3,7]. The proposed correlations have been developed for a wide range of velocities and conditions and they can be used in simulation studies from lab up to real size automotive catalytic converters and for a wide range of monolithic reactors operating with gas feeds
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