The removal of pollutants from high-temperature gases is often required in combustion processes. In this example, the selective reduction of NOx by ammonia occurs as flue gas passes through the channels of a monolithic reactor. The chemical reactions take place on a V2O5/TiO2 catalyst. The full reactor is a modular structure made up of blocks of reactive channels and supporting solid walls.

A separate model, NOx Reduction in a Monolithic Reactor, solves the coupled mass transport, heat transfer, and fluid flow equations representing the NOx reduction process in the reactor. Load this example from the Chemical Reaction Engineering Module Application Libraries and extend it by coupling the temperature field to a structural mechanics analysis to evaluate the thermal stresses in the reactor. For a more detailed description of the reaction kinetics, see Analysis of NOx Reaction Kinetics.

where σ is the stress, D is the elasticity matrix, and ε represents the strain. The equation for the thermal strain is given by:

where α is the coefficient of thermal expansion, T is the temperature (K), and Tref is the strain-free reference temperature (K).

Figure 2 shows the x-component of the stress tensor plotted in a number of cross sections. The highest negative values are found in the channel blocks closest to the center, signaling compressive stresses in this part of the reactor. This is a direct result of the temperature field in the reactor as shown in NOx Reduction in a Monolithic Reactor; the material in regions with high temperatures expand more than material at lower temperatures. The opposite effect is observed in channel blocks located near the outer surface of the reactor. In this relatively cold region the monolith structure experiences tensile stresses. The maximum compressive stress is evaluated to approximately 7.0 MPa and the maximum tensile stress is approximately 9.0 MPa. This indicates moderate stress levels in the monolith working under the stationary conditions presented in this example. More pronounced stress levels are expected as thermal gradients increase, for instance, during system startup.

Add a Solid Mechanics interface to extend the model with an analysis of the thermal stresses in the reactor materials.

Two Linear Elastic Material models are needed for the analysis, one for the supporting wall material and one for the channel blocks. Note that the wall material is isotropic while the block material is orthotropic.

Add a Thermal Expansion feature to each Linear Elastic node. The features receive the temperature field solved by the Heat Transfer in Fluids interface as Model Input.

Clear all physics interfaces except Solid Mechanics to solve only for the displacement field.

The Datasets node allows you great freedom to define tailored sets of plot data to be used by the plot nodes. Here, use the Mirror 3D feature to reflect the results in the symmetry planes of the model.

Mirror the data in the symmetry plane that is at a 45 degree angle to the xy-plane.