CFD from stationary to time dipendent

I built a model of pipe in axialsymmetric configuration, with internal laminar flow of water with pre-determinate velocity.
The water flow, during its pass, are heating by resistor, simulated as heat source with the total power put into the system.
I obtained a stationary solution, but now i want to study the evolution of system's temperature for three second when the velocity flow change suddenly to zero.