Discussion Forum

I am trying to model a simple glass furnace using Incompressible navier-stokes and heat transfer modules of COMSOL 3.5a.

As I proceeded further on my model, to make it more physically realistic, I added a temperature dependent viscosity term and enabled convective heat transfer. My viscosity values are based on the VFT relation, and for the type of glass that I am using, I entered a value as follows:

10^(-2.49474+(4099.768/(T[1/K]-544.0237)))[Pa*s]

This causes "negative inside sqrt" error either during calculation of various expressions or the residual term, depending on the type of solver that I use, whenever I enable convective heating.

When I disable convective heat transfer, the solver seems to work fine but gets stuck at certain times and I observe convergence value getting bigger and bigger at those times. When I give a constant value for viscosity (5-10-20-30), the bigger viscosity I give, the less problematic the convergence becomes during solver operation.

Has anyone faced with a similar problem? Any help would be greatly appreciated.

PS: One of my colleagues suggested that I should change my time unit from seconds to minutes or even hours, as glass processes are slow in real life. I do not know how to do that in COMSOL as well.