help on a convective heat transfer transient model

cmbv

Hello CFD experts I desperately need your help I am trying to solve a 2D transient forced convection heat transfer model. It has a very simple shape, imagin a cross secional area of a thin plate heated up by forced convection, or if you like two parallel plates. A flow warms up the plates at a given rate. This warming up is not constant, it varies with time. I have written some subroutines that take care of the transient boundary conditions. My concern is that the whole period is about 10 hours long, and the boundaries are constantly changing, that is mass flow, temperature and pressures. Now as far as I am concern the best practice to choose a time step, is to make sure that a given particle in the flow touches at least two times a cell during its flow path. I have deliberately build a coarse mesh in order to get the most out of it, however if I calculate the proper time step it would give me something around 1e-4 s average. If you take into account that each time step takes about 10-15 s to converge it would take my computer about two years to finish! Please could you offer some advice? Regards C