Velocity Outlet with Unknown Mixture?

yow · August 11, 2017, 00:57

Is this possible? I have an axisymmetric can with slots. Air is going into the slots and coming out, varying sinusoidally. A small amount of fuel is injected at constant velocity for combustion inside, and I'm assuming that the reaction negligibly affects the airflow in and out.

Right now, I'm using a UDF to vary the velocity at a velocity BC and a basic species model; I specified only air in/out from the velocity BC (O2 molar concentration = .21). When the velocity turns negative, will this BC force only air to leave the can?

CeesH · August 11, 2017, 05:06

A bit more basic question; why are you specifying both an inflow and outflow? conservation of mass should account for the outflow if the inflow is specified, and fixing both may easily lead to errors

yow · August 11, 2017, 15:25

Thank you for your reply. Since the simulation is transient, mass may build up inside the can, but, overall, the amount of air entering/exiting the can is much higher than the amount of fuel, so the pressure inside should stay nearly constant.

After reviewing my transient simulation, it seems that products are allowed to exit the air slots (correct me if I'm wrong...).

CeesH · August 14, 2017, 02:37

If you use a pressure outlet as a boundary condition, all phases can leave the domain. If you use degassing, only the secondary phase can leave the domain through the slots. For all other outlet b.c., I have not worked with them but I suppose both phases can leave.

Best,
Cees

August 11, 2017, 00:57	Velocity Outlet with Unknown Mixture?	#1
yow New Member Yang Join Date: Jun 2015 Posts: 25 Rep Power: 10	Is this possible? I have an axisymmetric can with slots. Air is going into the slots and coming out, varying sinusoidally. A small amount of fuel is injected at constant velocity for combustion inside, and I'm assuming that the reaction negligibly affects the airflow in and out. Right now, I'm using a UDF to vary the velocity at a velocity BC and a basic species model; I specified only air in/out from the velocity BC (O2 molar concentration = .21). When the velocity turns negative, will this BC force only air to leave the can? Last edited by yow; August 11, 2017 at 01:03. Reason: more information

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August 11, 2017, 05:06		#2
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	A bit more basic question; why are you specifying both an inflow and outflow? conservation of mass should account for the outflow if the inflow is specified, and fixing both may easily lead to errors

August 11, 2017, 15:25		#3
yow New Member Yang Join Date: Jun 2015 Posts: 25 Rep Power: 10	Thank you for your reply. Since the simulation is transient, mass may build up inside the can, but, overall, the amount of air entering/exiting the can is much higher than the amount of fuel, so the pressure inside should stay nearly constant. After reviewing my transient simulation, it seems that products are allowed to exit the air slots (correct me if I'm wrong...).

August 14, 2017, 02:37		#4
CeesH Senior Member Cees Haringa Join Date: May 2013 Location: Delft Posts: 607 Rep Power: 0	If you use a pressure outlet as a boundary condition, all phases can leave the domain. If you use degassing, only the secondary phase can leave the domain through the slots. For all other outlet b.c., I have not worked with them but I suppose both phases can leave. Best, Cees