Effect of wall thermal conductivity on the dimensionless Nusselt number of microchannel steam reforming reactors with oxidation catalyst segments
Description
The effect of wall thermal conductivity on the dimensionless Nusselt number is illustrated for microchannel steam reforming reactors with oxidation catalyst segments. In fluid dynamics, the Nusselt number is the ratio of convective to conductive heat transfer at a boundary in a fluid. Convection includes both advection and diffusion. The conductive component is measured under the same conditions as the convective but for a hypothetically motionless fluid. It is a dimensionless number. In each oxidation channel, the catalyst layer is reduced by half in amount. More specifically, the catalyst segments have a uniform distribution of length, and the spacing between catalyst segments is equal to their length. Accordingly, the catalyst segments are spaced apart from each other with a distance of one millimeter, and the catalyst segments are one millimeter in length. The extent of catalyst segmentation is balanced so as to reduce the likelihood of mechanical failure. Consequently, catalyst segmentation does not have a significant impact on the mechanical integrity. To facilitate computational modeling of transport phenomena and chemical kinetics in the flowing system of complex chemical reactions involving gas-phase and surface species, steady-state analyses are performed and computational fluid dynamics is used. ANSYS FLUENT is applied to the problem involving surface chemistry. ANSYS FLUENT handles thermodynamic properties, transport properties, gas-phase equation-of-state, and chemical kinetics. Two types of species are defined: gas-phase and surface. The sum of the site fractions of the species on the sites is unity. The Reynolds numbers are very small so that the gases flow through the channels in a laminar flow regime. The heat release and consumption due to a surface reaction must be included in the model. Endothermicity or exothermicity of surface reactions contribute to the energy balance at an interface. Heat fluxes in the solid phase are balanced by heat release at the surface. For the sake of simplicity, it is assumed that substantially all the pores have a similar diameter. The endothermic process is modeled in such a way as to take into account methanol steam reforming and decomposition and the water-gas shift reaction. To obtain the solution of the problem, numerical simulations are performed using computational fluid dynamics. The problem is solved using structured meshes. The mesh is refined in the computational domains of the catalytically active layers. A mesh independence study is carried out. The solution is independent of the mesh resolution. Boundary conditions are specified and physical properties are defined for the fluids, solids, and mixtures. Contributor: Junjie Chen, E-mail address: koncjj@gmail.com, ORCID: 0000-0002-5022-6863, Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P.R. China
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Steps to reproduce
The mathematical formalism developed to describe transport phenomena and chemical kinetics is implemented into ANSYS FLUENT. The governing equations are solved numerically for the conservation of mass and momentum and for energy and species.