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Evaporation of Ammonia

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Old   December 26, 2022, 10:27
Smile Evaporation of Ammonia
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Rahmani Zakaria
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Hi everyone, I am trying to simulate the evaporation of Ammonia (Nh3). A tube-in-tube evaporator was designed for this simulation with 2 inlets and an outlet, where ammonia liquid enters the expansion chamber and mixing with hydrogen. I choose the mixture model with 3 phases as multiphase model upon the details providing by Ansys theory guide.

Primary phase: Hydrogen
Secondary phase: Ammonia-Liquid
Secondary phase: Ammonia-Vapor
Tsat of ammonia @1bar = -33C
P_total = 25bar

Boundary conditions:
Nh3 inlet: Velocity inelt 1m/s, T= -33C
H2 inlet: Velocity inlet 5m/s, T=-5C
mixture outlet: Outflow
Wall: Convection: 20 W/m^2.K, T=10C

Time step size: t=0.0001s
Number of time steps: 250
Max iterations/time step: 10

My questions:
- where should I add the total pressure of the mixture: the operating pressure or the gauge pressure?
- the total pressure is P_tot = 25bar, when ammonia become in contact with hydrogen it is supposed to reduce its partial pressure to P_Nh3 = 1bar and T= -33 C and allow ammonia to evaporate according to the Dalton's law.
the question Ansys will do that?
- I have also a problem with the convergence, there is a fluctuation in residuals then the solution diverges. Then I change the time from transient to steady, the fluctuations do not appear but the residuals still constant and the solution diverge after 60 iterations.
More details in attachments, thanks in advance.
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convergance, evaporation model, mixture model, partial pressure

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