Uniform mass flow rate at different outlets.

mehcaecfd · May 12, 2016, 03:31

Dear CFD users,

I am working on a problem, where my objective to calculate mass flow rates at 4 outlets and if mass flow rate at these outlets is not uniform then make it uniform (same for all outlets). I made a simplified region like I have shown in image attached herewith. This is a application used to provide sufficient oil to crank shaft, where lubrication channel is drilled in crank shaft and it is used to provided oil. Anybody has idea, how can I do it?? I am confused since I have actually gauge pressure at inlet as BC (2 bars) and I do not know mass flow rate at inlet. So, actually I am fixing pressure at inlet and getting mass flow rate as output from CFD. Region 2 in attached images is rotating at 6000 rpms and its 4 ends shown as surfaces to be monitored as mass flow rate are the surfaces where I want to make uniform mass flow rate. I am using MRF for rotation and using two fluids (VOF approach).

marmot · May 12, 2016, 11:39

not sure if I understand 100% but check if with VOF you can use flow split outlet, than assign .25 to each one, but just a quick guess right now,

fluid23 · May 12, 2016, 13:32

I don't think flow split outlet is appropriate. They aren't boundary conditions, so you shouldn't have to specify them. You know inlet and outlet pressure, plus your rotation rate... all you need to do is setup report monitors at each surface to get flow rate.

Your image is a little confusing, I am not sure how your rotating region is defined. But assuming you aren't off base on your domain definition, then you should already be setup and ready to go once you get mass flow report monitors.

mehcaecfd · May 13, 2016, 01:32

Dear MBdonCFD and marmot,

thank you very much for your response. Yes I know that, I can report the mass flow rate at those outlets.But my problem is, they are not coming same. I get a huge mass flow rart at outlets. I have attached a imae, where I am getting mass flow rates. At first two outlets they are very high and at last two outlets they are small. I also want to ask, if it is a problem of VOF, since I take oil (density 850 kg/m3) as primary fluid and air as secondary fluid. This is a rotating lubrication channel inside a rotating shaft.
So, in brief I want to know following things.

1. How can I make the four outlets same. I mean all 4 outlets have same mass flow rates? I may try to change pipe dimensions. But I want to know if anybody has tried by a shorter way in starccm?
2. I am giving pressure at inlet. Will it cause a problem in getting uniform mass flow rates?
3. Will it be feasible to try to use a single phase fluid instead VOF. I will try it and check it.

fluid23 · May 13, 2016, 09:40

1. If your task is to DESIGN a system where you get equal mass flow rates, then yes change pipe dimensions. However, if your task is to VERIFY that you get equal mass flow rates then you could have a mesh issue. VOF typically requires very fine resolution in order to resolve the fluid interfaces.

2. I don't see why it would. You may end up with some resonance/oscillation, but an inlet/outlet pressure will develop a specific mass flow for your system.

3. I ALWAYS recommend starting simple and building in complexity. If it make sense to you to use single phase, then do so.

marmot · May 13, 2016, 10:08

This is a good problem, OK your outlets are an interface to another region not a true outlet, good idea to start with single-phase.

If possible you should create the CAD in the CAD modeler with design parameters this way you can easily make changes to the dimensions or even us a java macro to run a set of cases to learn how the flow rates are changing. I have not tried or have an Optimate license but perhaps a good application for it.

mehcaecfd · May 18, 2016, 03:34

Thank you MBdonCFD and marmot for your suggestions. Yes, I did a single phase simulation with stationary case and I got sufficient amount of oil at all outlets. Then I gave the rotation to the walls with VOF simulation and I found that oil was not able to reach all outlets except first one, so this proevd that I can not use single phase flow for my actual problem.This was confirmed by reducing speed as well. So, conclusion is that, because of huge centrifugal forces, oil was pushed outwards, starving the remaining outlets. This suggests me to change my design either by changing the dimension or by changing the way the channel goes inside shaft. I did not create CAD model in starccm, but I imported it. I do not have optimate license to optimize channel, but I believe that, If I create CAD in starccm, then I can play with diameter of channel to provide uniform flow rate to all outlets.

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May 12, 2016, 11:39		#2
marmot Senior Member kevin alun Join Date: Sep 2011 Location: Germany Posts: 106 Rep Power: 15	not sure if I understand 100% but check if with VOF you can use flow split outlet, than assign .25 to each one, but just a quick guess right now,

May 12, 2016, 13:32		#3
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	I don't think flow split outlet is appropriate. They aren't boundary conditions, so you shouldn't have to specify them. You know inlet and outlet pressure, plus your rotation rate... all you need to do is setup report monitors at each surface to get flow rate. Your image is a little confusing, I am not sure how your rotating region is defined. But assuming you aren't off base on your domain definition, then you should already be setup and ready to go once you get mass flow report monitors.

May 13, 2016, 09:40		#5
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	1. If your task is to DESIGN a system where you get equal mass flow rates, then yes change pipe dimensions. However, if your task is to VERIFY that you get equal mass flow rates then you could have a mesh issue. VOF typically requires very fine resolution in order to resolve the fluid interfaces. 2. I don't see why it would. You may end up with some resonance/oscillation, but an inlet/outlet pressure will develop a specific mass flow for your system. 3. I ALWAYS recommend starting simple and building in complexity. If it make sense to you to use single phase, then do so.

May 13, 2016, 10:08		#6
marmot Senior Member kevin alun Join Date: Sep 2011 Location: Germany Posts: 106 Rep Power: 15	This is a good problem, OK your outlets are an interface to another region not a true outlet, good idea to start with single-phase. If possible you should create the CAD in the CAD modeler with design parameters this way you can easily make changes to the dimensions or even us a java macro to run a set of cases to learn how the flow rates are changing. I have not tried or have an Optimate license but perhaps a good application for it.

May 18, 2016, 03:34		#7
mehcaecfd New Member mehcaecfd Join Date: May 2016 Posts: 6 Rep Power: 10	Thank you MBdonCFD and marmot for your suggestions. Yes, I did a single phase simulation with stationary case and I got sufficient amount of oil at all outlets. Then I gave the rotation to the walls with VOF simulation and I found that oil was not able to reach all outlets except first one, so this proevd that I can not use single phase flow for my actual problem.This was confirmed by reducing speed as well. So, conclusion is that, because of huge centrifugal forces, oil was pushed outwards, starving the remaining outlets. This suggests me to change my design either by changing the dimension or by changing the way the channel goes inside shaft. I did not create CAD model in starccm, but I imported it. I do not have optimate license to optimize channel, but I believe that, If I create CAD in starccm, then I can play with diameter of channel to provide uniform flow rate to all outlets.