Page 163 - Process Modelling and Simulation With Finite Element Methods

150 Process Modelling and Simulation with Finite Element Methods

into the FEMLAB GUI to carry on. The definitions of u0 and v0 will now have
the desired effects. Annoyingly, the AddEdit Couplings dialogue window no
longer launches with this alteration. But the computational model now solves.
Advice from COMSOL is that there is a simpler workaround for this
apparent bug in the coupling variables, which allows you to solve this problem
without playing around with m-files. You basically define a weak boundary
mode (as opposed to a weak subdomain mode). The make sure to use non-
conservative formulation for the convection-diffusion application mode. The
reason for doing this is that the coupling variable or expression you define only
needs to be accessed on a subdomain level, not on the boundary level.
Pull down the Mesh menu and select Parameters.

Mesh Mode
>>More
Number of Elements in Subdomain: 1 1000 2 10
Apply
OK
0
As is typical for FEMLAB, the pairs 1 1000 2 10 put 1000 elements in
subdomain 1 and 10 in subdomain 2.
This results in a 1013 element meshing. Now for the Solver. Pull down the
Solver Menu and select Parameters. Check the Stationary Nonlinear solver
box, apply, and click on the Solve button. With no recycle and steady state,
FEMLAB finds the same solution for the reactor as before. You can ramp up the
recycle ratio gradually using the Re-Start button on the Toolbar. By f=10%, a
modest variation in the position of the crossover point is noted. The effect of the
recycle is to load even more U into the reactor, as well as to speed up the
throughput somewhat.
The transient solution is a hard problem, but potentially more interesting as
the “capacitor” takes a long time to charge in the buffer tank. Before undertaking
the transient solution, however, we need to make some modest alterations to the
PDE coefficients:
Pull down the Multiphysics menu, select surface mode, then pull down the
Subdomain menu and select View as Coefficients. Finally, select Parameters.

Subdomain Mode (Surface Mode, View as Coefficients)
Select the dweak tab, domain 2
Replace US-time, VS-time, WS-time with zeros in the
US,VS,WS coefficient boxes
APPlY
OK