That is not an easy problem. Your steel tube starts off above the boiling point of water, so when your immerse it in the water, the water will start to boil. I don't know how well Flow handles a situation like that. I don't think it is very easy to handle in Simulation either. Once the steel cools below the boiling point of the water, you could estimate film coefficients, but they will probably be pretty poor estimates unless you can find estimates from someone who has either experimental data or ran a good CFD study on a similar problem.