I am trying to design a pipe system with a pressure drop of less than 10 bar. The pump output pressure is 110bar, the output pressure is expected to be at least 100bar. However, when I try internal flow simulation and external flow simulation with the same boundary conditions applied, the results are totally different as shown below.

For internal flow simulation, it is shown that the pressure drop is less than 1bar.

(From pump output 110bar to outlet 109.39bar)

I am not sure if it is practical to have less than 1 bar pressure drop over these long pipes. Therefore I try external flow simulation.

For external flow simulation, the pressure drop went crazily down to 0 bar. The pressure drop is 110bar!

I am not sure why are the two simulations with same boundary conditions is different, I dunno which one is correct. Please give advice on how to get the correct pressure drop. Thanks!