# Problem with changing mass flow rate

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 June 4, 2013, 05:28 Problem with changing mass flow rate #1 New Member   Join Date: Jun 2013 Posts: 2 Rep Power: 0 hi my problem relates to ground heat exchanger, where the water in the pipes exchange heat with the ground and vice versa. Make a steady calculate the input values ​​into the pipe defined mass flow rate 1.2 kg.s-1 and a temperature of 328.15 K. After 200 iterations when the solution is converged, switch model for unsteady state and set the input to the pipe by means of mass flow rate 0.1 kg . s-1 and a temperature of 293.15 K. time step is set after 86400 s immediately after starting the calculation begins diverge solutions and writes "divergence detected AMG solver: temperature" and several faces shows reversed flow. I think the problem is the rapid change in mass flow and large time steps. Any ideas how it could be solve this problem, while maintaining a low mass flow and large time steps? thx for your attention

 June 4, 2013, 10:18 #2 Senior Member   OJ Join Date: Apr 2012 Location: United Kindom Posts: 473 Rep Power: 19 Do you mean you set timestep as 86400 s? That is insane! You can not guess the timestep. You need to either to a timestep sensitivity study, or use adaptive timestepping algorithm to let FLUENT find the right timestep. There are no shortcuts in transient simulations, unless of course you don't want correct results OJ

 June 4, 2013, 12:52 #3 New Member   Join Date: Jun 2013 Posts: 2 Rep Power: 0 If i set lower time step (60, 100, 1000 or 3600 s) or adaptive timestepping the results is same. Immediately, solution diverges. I used to explain the time step 86400s since the problem is solved in day mode after a period of 151 days.

 June 4, 2013, 14:53 #4 Senior Member   Join Date: Mar 2009 Location: Indiana, US Posts: 185 Rep Power: 16 To give you some perspective, for a simple unsteady flow analysis ( no energy), the timesteps used are never more than 0.1-1 s. In your case it would have to be even smaller. Regards Luke