AnsweredAssumed Answered

Question about going from steady-state to transient study

Question asked by Dan Hofstetter on Aug 29, 2013
Latest reply on Sep 3, 2013 by Jared Conway

I'm confused about running transient airflow studies with contaminant gases.  I have been attempting to set up a study where a fan is used to evacuate contaminant gas from a tank, but I noticed while solving that the velocity field is not necessarily well-resolved during the beginning of the transient study.  Wouldn't this cause inaccurate results when looking at contaminant gas concentrations in the tank versus time?


To overcome this, I attempted to set up a steady-state study without contaminant gas and 1) run it until the automatic convergence criteria were all met, then 2) set up a time-dependent study with transferred boundary conditions and then add an initial condition where the tank has a concentration of 100 ppm of contaminant gas.  I can't seem to get it set up correctly though.  Some problems I have noticed:


1.   I tried to run the transient study with transferred initial conditions from the steady-state one (From the General Settings>Initial Conditions>Parameter Definition dialog).  However, I don't think the initial conditions are getting transferred correctly, because the transient calculation always starts re-solving the velocity field from scratch, i.e. the flow vectors are not the same as from the end of the steady-state solution.


2.  I attempted to continue the calculation for the steady-state solution, but with the new initial condition in the tank unsuppressed and as a time-dependent study, but the solver always starts the calculation from the beginning.


3.  For a transient-only study, I tried setting up an initial condition for a solid body inside the tank  with 100 ppm of contaminant gas being turned on after a number of time steps.  So at t=0 the concentration in the tank was 0 ppm, at t=20 the concentration was still 0 ppm, then at t = 21 the concentration should have jumped to 100 ppm.  I don't see this happening in the transient study though, the contaminant gas concentration inside the tank never rises above 0 ppm.


Can anyone offer any suggestions?  I am going to take a look at the Tracer Gas analysis that the HVAC module offers, but I would rather do this without additional expense for add-ins if possible.  It seems to me that the contaminant gas will act as a passive scalar, which means once the flow field is resolved, I shouldn't have to continue to calculate the flow parameters for the simulation, but only continue the calculation with the final values for velocity in all mesh cells with the new values of contaminant gases until the final time step is reached.