Figure 2. Schematic illustration of the highly exothermic reactor with a catalytic single-channel reaction chamber. The thick arrows indicate the direction of flow, whereas the thin arrows indicate the direction of heat transfer. All external surfaces exposed to the surroundings are subjected to convective and radiative heat losses. Symmetry allows the simulation of only half of each system.
For the single-channel combustor, an orthogonal staggered grid is used to perform these simulations, containing 200 and 80 points in the axial and transverse directions, respectively. The grid is finer where the gradients are steeper, at the entrance and in the vicinity of the active catalytic surface. The solid wall is discretized such that the elements at the fluid-solid interface have the same axial size as the corresponding elements on the fluid sides, resulting in a 200 and 20 grid resolution in the axial and transverse directions, respectively. For the heat-recirculating combustor, a non-uniform grid is used, with elements near the catalytic wall being smaller than those at the symmetry plane, in order to capture more accurately the sharp gradients at the catalytic wall while minimizing the computational intensity. As a result, the grid used contains a 260 and 160 grid resolution in the axial and transverse directions, respectively. Prior to performing these simulations, computations are performed using grids with varying nodal densities to determine the optimum node spacing and density that would give the desired accuracy and minimize computation time. The final grid density is determined when the centerline profiles of temperature and species concentration do not show obvious difference. Solutions obtained with the above grids are reasonably accurate, and larger grid densities yield no obvious advantage. The second-order upwind scheme is used to discretize the mathematical model, and the semi-implicit method for pressure-linked equations algorithm is employed to solve for the pressure and velocity fields. The simulation convergence is judged upon the residuals of all governing equations. Numerical convergence is in general difficult. In order to assist convergence and compute extinction points, natural parameter continuation is implemented.
The heat integrated reactor with multiple parallel channels is illustrated schematically in Figure 3 for thermochemically producing hydrogen from methanol by steam reforming. The heat integrated reactor can be used to produce hydrogen for fuel cells for the production of electricity. The reactor consists of two sets of flow channels: one where the steam reforming reaction takes place and one where combustion of the same fuel provides the heat necessary to carry out the reaction. Heat is transferred across the channel walls, and the system results in a compact configuration. The two sets of uniform parallel reaction channels are separated by the walls that separate the combustion region from the reforming region, and are in close thermal contact as to facilitate the efficient transfer of heat from the combustion region to the reforming region. In this manner, greater heat integration and utilization is accomplished inside the reactor. The reactor operates in the so-called co-current mode, namely combustion and reforming mixtures flow in the same direction, and the combustion and reforming channels are arranged alternately. Combustion takes place over a structured catalyst comprising copper-oxide and zinc-oxide. Steam reforming is a catalytic reaction and takes place over a structured catalyst comprising copper and zinc-oxide. The washcoat for applying the combustion and reforming catalysts is high surface area aluminum oxide, and it must be in close contact with the channel walls to facilitate efficient heat transfer. On account of the symmetry of the system, only two half combustion and reforming channels associated with the surrounding walls are modeled.