| Transport properties can be visualized for every point of the fixed bed. The estimation of main momentum and heat transport properties (i.e., density, viscosity, heat capacity and thermal conductivity) is done by means of the implementation of an additional set of equations via user defined functions that are solved together with the momentum, mass and energy balances. Figure 27 shows the density profile for the studied case.
Figure 27: Density profile in an axial cut of the studied case. Density scale expressed in kg/m3.
To conclude, CFD proves to be a useful tool to simulate fluid flow through complex geometries as a fixed bed. The implementation of transport properties estimation parameters via user defined functions into the used equations in commercial available solvers makes possible the idea of using this technique to simulate the behavior of supercritical fluids in fixed bed equipments such as supercritical extractors or fixed bed reactors. At low values of Re, gravity forces become clearly important in the behavior of the flow pattern inside the fixed bed for a supercritical fluid. This makes that the buoyancy terms have to be included into the calculations for momentum, heat and mass transfer parameters when needed.
Free convection in laminar flow regime is the main heat transfer mechanism in the fixed bed under the operating conditions mentioned; therefore, numerical calculations confirm the ideas extracted from the dimensionless analysis of the problem. The numerically obtained local values of Nup inside the channels of the fixed bed increase with the dimensionless group [GrH · Pr].
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