Reverse flow at boundary

andyross33 · August 22, 2013, 04:20

Hi,

I am seeing (heavy) reverse flow at my outlet boundary when there shouldn't be. I am modelling one side of a plate heat exchanger, with subcooled water at the inlet which flows vertically past a hot isothermal surface (which will eventually be simulated explicitly as the other side of the heat exchanger). I am including homogeneous phase change with IAPWS water/vapor definitions as I want to see how much vapor is formed.

The inlet condition is defined as 10C at a prescribed mass flow rate. The outlet is defined as an Opening at an opening pressure of -2 psig and flow direction set to Normal to Boundary. I have hand calculated the expected velocity through the domain and set that as the initial condition.

I monitor the mass flow rate at the outlet during the solve and it steadily climbs but in the positive direction (flow into the domain). I have tried various combinations of boundary conditions including Outlet and reversing the mass flow definition at the outlet vs the inlet but no luck. When I used the Outlet BC it placed a wall over 100% of the outlet.

This is not reverse flow caused by turbulence, as far as I can tell, because my flow is low speed and laminar with no geometry that would impart turbulence (flow past a vertical plate).

The weird thing to me is that I have modelled the other side of the heat exchanger separately, where I have superheated steam passing its heat to the wall and the solver has no issues. I do have to babysit it to ensure convergence but at least the flow seems correct.

I know phase change simulation can be tricky but this seems so elementary to me I'm kicking myself for not finding the culprit yet and I have been at it for several weeks now. Any thoughts anyone?

Thanks

ghorrocks · August 22, 2013, 20:37

Can you post an image of the flow you are getting?

andyross33 · August 27, 2013, 14:24

ghorrocks,

I decided to take your advice that I've seen you post on many other threads and re-read the CFX convergence documentation. I realized that my physical timescale was way too low. The residence time within my domain is about 3600 s and I'd been using a timescale of about 1e-3. As soon as I had bumped this up to within the range of the average residence time, things worked a lot better. The flow was no longer reversed and the model was far more stable.

Thanks for consistently hammering this point home, eventually people like me will get it

Cheers!

August 22, 2013, 04:20	Reverse flow at boundary	#1
andyross33 New Member Andrew Join Date: Nov 2010 Posts: 12 Rep Power: 16	Hi, I am seeing (heavy) reverse flow at my outlet boundary when there shouldn't be. I am modelling one side of a plate heat exchanger, with subcooled water at the inlet which flows vertically past a hot isothermal surface (which will eventually be simulated explicitly as the other side of the heat exchanger). I am including homogeneous phase change with IAPWS water/vapor definitions as I want to see how much vapor is formed. The inlet condition is defined as 10C at a prescribed mass flow rate. The outlet is defined as an Opening at an opening pressure of -2 psig and flow direction set to Normal to Boundary. I have hand calculated the expected velocity through the domain and set that as the initial condition. I monitor the mass flow rate at the outlet during the solve and it steadily climbs but in the positive direction (flow into the domain). I have tried various combinations of boundary conditions including Outlet and reversing the mass flow definition at the outlet vs the inlet but no luck. When I used the Outlet BC it placed a wall over 100% of the outlet. This is not reverse flow caused by turbulence, as far as I can tell, because my flow is low speed and laminar with no geometry that would impart turbulence (flow past a vertical plate). The weird thing to me is that I have modelled the other side of the heat exchanger separately, where I have superheated steam passing its heat to the wall and the solver has no issues. I do have to babysit it to ensure convergence but at least the flow seems correct. I know phase change simulation can be tricky but this seems so elementary to me I'm kicking myself for not finding the culprit yet and I have been at it for several weeks now. Any thoughts anyone? Thanks

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August 22, 2013, 20:37		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,854 Rep Power: 144	Can you post an image of the flow you are getting?

August 27, 2013, 14:24		#3
andyross33 New Member Andrew Join Date: Nov 2010 Posts: 12 Rep Power: 16	ghorrocks, I decided to take your advice that I've seen you post on many other threads and re-read the CFX convergence documentation. I realized that my physical timescale was way too low. The residence time within my domain is about 3600 s and I'd been using a timescale of about 1e-3. As soon as I had bumped this up to within the range of the average residence time, things worked a lot better. The flow was no longer reversed and the model was far more stable. Thanks for consistently hammering this point home, eventually people like me will get it Cheers!