Periodic Heat Exchanger and Thermal Stress Simulations

Hello everybody,

I have 2 different Problems, that i want to address and hope to get some hints on how to solve it.

1: Periodic Heat Exchanger:
I want to model a Heat exchanger where the fluid flows there and back with a certain time period. To save Computation time I want to solve the fluid flow only for one period and calculate the heatflow afterwards with just "inserting" this transient periodic fluid behavior, instead of solving it every period. Is this even possible, or do you have some hints how to do this? In general: Is it possible to impose an arbitrary fluid flow for heat transfer purposes without solving the fluid domain?

2: Thermal Stress:
I want to model a material where the thermal expansion coefficient depends on the mechanical stress. If i simply enter this relation into Comsol i get a circulating reference error. But it is possible to enter a thermal expansion dependant on the displacement, so in principle these non-linearities should be doable by Comsol. I found that in this model Comsol solves for the displacement, is there a possibility to solve for the stress instead?

Thanks for your help!