The short answer is that a pipe flow model with the OD and velocity will significantly underpredict the pressure drop of an annular flow. However, there are well established ways for treating annular flows using the wetted-perimeter modifications to pipe flow that should be accurate. Any reasonable fluid dynamics text will explain these.

Dear Baxter,

Thanks for your answer. But will the friction factor be different?

There is a alternative definition of the Reynolds number that will allow you to use the same plot of friction factor vs. Re as you would use. So, technically, the friction factor chart does not change, but the Reynolds number does, so the friction factor itself does change (because of the different Re number).