### comsol

#### COMSOL won't solve the the diffusion equation in axisymmetric using coefficient form PDE

So this should be extremely simple. I want to solve the 3D diffusion equation, u_t = nabla^2 u, using the axisymmetric solver. This is the problem description, in COMSOL: Set up: Axisymmetric problem, Coefficient Form PDE, time dependant Geometry: Circle radius 3, Circle radius 1 Coefficient Form PDE IC: Initial value = 1/( (4/3)*pi ) in sphere radius 1, zero elsewhere. Coefficient Form PDE BC: Zero flux on boundary of outer sphere. Therefore initially the total concentration (or heat) inside is 1 - and should stay at 1 for all time. However as you can see from this file (attached *.mph file here https://www.dropbox.com/s/fv2feo3gr18ohwe/diffusion_eq.mph?dl=0) the concentration increases and settles at roughly 3 times the initial total concentration. My question is how can I make comsol actually solve the diffusion equation using the Coefficient Form PDE? I cannot do this in 1D as this is a reduction of my full problem that breaks symmetry in the spherical theta coordinate. My problem is axisymmetric, and takes too long to run in 3D, hence the need for an axisymmetric solution. I also very much need to use the Coefficient Form PDE solver as my full problem requires it.

### Related Links

COMSOL solving geometry

COMSOL won't solve the the diffusion equation in axisymmetric using coefficient form PDE

using stationary solution as the initial conditions in time dependent model

COMSOL SchrÃ¶dinger Equation