Laminar flow is governed by the unsteady Navier-Stokes equations. The laminar option does not apply a turbulence model to the simulation and is only appropriate if the flow is laminar. This typically applies at low Reynolds number flows (for example, for pipe flow the laminar flow regime is ~ Re < 1000). Energy transfer in the fluid is accomplished by molecular interaction (diffusion). In the case of high speed flows, the work of the viscous stresses can also contribute to the energy transfer. You should always check in the CFX-Solver Output file that the Reynolds number is in the laminar flow regime. If you set up a simulation using laminar flow, but the real flow is turbulent, convergence is difficult and the simulation will not reach the correct solution.