Thanks, here is a simplified version of the model: the heat exchanger itself is in grey and has internal features, a cold fluid flows inside and exchanges heat with the hot fluid. So far the simulations I've been running were only steady state in Fluent, then I imported the temperature profile (not directly, I used external data for some adjustments) onto the solid body only in a static structural simulation and used the temperature map to calculate the thermal stress. 

But after completing the transient simulation, I am not sure how to specify two temperature distributions at two time steps.