# fan coefficients

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 November 7, 2011, 05:17 fan coefficients #1 Member   supercommandodhruv Join Date: Sep 2011 Posts: 57 Rep Power: 5 Hello all, I am trying to simulate a fan in an open channel with a radiator in front of the fan, modeled as a porous zone. The flow in the channel is entirely driven by the fan. I am using the fan boundary condition. My mesh is made of almost 3 million cells. Some details are as follows: p (inlet and outlet) type totalPressure; U (inlet) type pressureInletOutletVelocity; value uniform (0 0 0); U (outlet) type inletOutlet; inletValue uniform (0 0 0); value uniform (0 0 0); solver: porousSimpleFoam The pressure curve of fan is taken as linear for simplicity, with 2 coefficients in the f List. the equation is of the form delP = a - bx, where x is the velocity. My observations are that as I change the coefficients of the fan in the same geometry, my calculations take longer and longer to converge. So, if I take the first coefficient (value of "a" in the fan equation above) as (say) 500, my solution converges in 6000 steps, but if I increase the value of the coefficient to 800, the solution takes almost 9000 steps to converge. So, it takes very long time to reach the duty point of the fan. I do not understand why this happens . Can anyone help me with this? Regards, Dhruv.

 November 8, 2011, 05:44 #2 Senior Member   Eugene de Villiers Join Date: Mar 2009 Posts: 725 Rep Power: 12 Dhruv, I might be wrong here, but the probable reason this thing takes so long to converge is because your fan has to accelerate the flow from a stationary starting point. The faster the fan turns, the more the flow has to be accelerated before it reaches equilibrium, leading to longer integration times. Try starting with a initial velocity field closer to the final one and see if this improves matters. Eugene

 December 8, 2011, 07:10 Thanks... but a different approach worked. #3 Member   supercommandodhruv Join Date: Sep 2011 Posts: 57 Rep Power: 5 Hi Eugene, Thanks for the reply. I did get a faster convergence to one of my problems, but not in some cases. But I would like to point out that the problem was the explicit and implicit solution method applied to porousZones. When I changed my fvSolution file to implicit from explicit, my problem reached convergence in 2500 steps (rather that 9000-10000 steps in explicit), and also was much more stable and accurate. Do you know, why is there so much of a difference between these two solution methods? Regards, Dhruv.

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