I am simulating a microchannel and using the microfluidic module. I use the transport of diluted species to see the concentration profile. I use both 2d and 3d simulations. When i do the 2d vs 3d i see the concentration profiles are virtually similar. In 2d you don't put the height of the channel, does comsol assumes any 2d model and does the simulation assuming its in 3d space and makes a 3d profile?
This may not be 100% accurate but what I can remember from comsol/FEA simulations. A 3D simulation is how you imagine it to work correctly. In your simulation, if the height is uniform/has no effect on the results, then you can simplify the simulation to a 2D one. In this situation, the math is only done in the x,y direction and the z direction is ignored. Since the result of anything happening in z=0, this is okay to do and hence why you are getting the same result.
This also applies for 1D simulations. If the sum of everything in y=0, then you can just simulate x