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Tutorial 7. Modeling Flow Through Porous Media

Introduction

Many industrial applications involve the modeling of flow through porous media, such as filters, catalyst beds, and packing. This tutorial illustrates how to set up and solve a problem involving gas flow through porous media.

The industrial problem solved here involves gas flow through a catalytic converter. Catalytic converters are commonly used to purify emissions from gasoline and diesel engines by converting environmentally hazardous exhaust emissions to acceptable substances.

Examples of such emissions include carbon monoxide (CO), nitrogen oxides (NOx), and unburned hydrocarbon fuels. These exhaust gas emissions are forced through a substrate, which is a ceramic structure coated with a metal catalyst such as platinum or palladium.

The nature of the exhaust gas flow is a very important factor in determining the performance of the catalytic converter. Of particular importance is the pressure gradient and velocity distribution through the substrate. Hence CFD analysis is used to design efficient catalytic converters: by modeling the exhaust gas flow, the pressure drop and the uniformity of flow through the substrate can be determined. In this tutorial, FLUENT is used to model the flow of nitrogen gas through a catalytic converter geometry, so that the flow field structure may be analyzed. This tutorial demonstrates how to do the following:

_ Set up a porous zone for the substrate with appropriate resistances.

_ Calculate a solution for gas flow through the catalytic converter using the pressure based solver.

_ Plot pressure and velocity distribution on specified planes of the geometry. _ Determine the pressure drop through the substrate and the degree of non-uniformity of flow through cross sections of the geometry using X-Y plots and numerical reports.

Problem Description

The catalytic converter modeled here is shown in Figure . The nitrogen flows in through the inlet with a uniform velocity of m/s, passes through a ceramic monolith substrate with square shaped channels, and then exits through the outlet.

While the flow in the inlet and outlet sections is turbulent, the flow through the substrate is laminar and is characterized by inertial and viscous loss coefficients in the flow (X) direction. The substrate is impermeable in other directions, which is modeled using loss coefficients whose values are three orders of magnitude higher than in the X direction.

Setup and Solution Step 1: Grid

1. Read the mesh file (catalytic . File /Read /Case...

2. Check the grid. Grid /Check FLUENT will perform various checks on the mesh and report the progress in the console. Make sure that the minimum volume reported is a positive number. 3. Scale the grid.

Grid! Scale...

(a) Select mm from the Grid Was Created In drop-down list.

(b) Click the Change Length Units button. All dimensions will now be shown in millimeters.

(c) Click Scale and close the Scale Grid panel. 4. Display the mesh. Display /Grid...

(a) Make sure that inlet, outlet, substrate-wall, and wall are selected in the Surfaces selection list. (b) Click Display.

(c) Rotate the view and zoom in to get the display shown in Figure . (d) Close the Grid Display panel.

The hex mesh on the geometry contains a total of 34,580 cells.

Step 2: Models

1. Retain the default solver settings. Define /Models /Solver...

2. Select the standard k-¦Å turbulence model. Define/ Models /Viscous...

Step 3: Materials 1. Add nitrogen to the list of fluid materials by copying it from the Fluent Database for materials. Define /Materials...