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December 4, 2001, 23:38 
Question on Boundary Conditions

#1 
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Hi,
I am studying the air flow through the cowl plenum of a car. In simple language the problem is as follows: 1. Air flows into the AC blower though a part called the cowl plenum. The air path offers certain resistance, depending on the geometry of the system. 2. From the blower the air is goes out into the cabin through the duct system. The purpose of the study is to to a comparative evaluation of different cowl component geometries. So if proposal A gives me 300 cfm of airflow, what would proposal B give me, given the same everythingelse. I am not interested simulating the flow through the blower, hence want to use a reasonable approximation through a boundary condition at the blower inlet. What will be a good boundary condition? The inlet to the cowl is ambient air (consider the car to be stationary). 

December 5, 2001, 15:16 
Re: Question on Boundary Conditions

#2 
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In order to estimate how much the flowrate will change if you redesign the cowl you will also need to estimate the resistance/losses in the other parts of the system  if, for example, the "blower" produces 90% of all the resistance/losses then you will have difficult to increase the flow much by reducing losses in other components.
You can compare one cowl to another cowl and say which is better by analysing only the cowl, but to estimate the effect on the whole system you need to model the other parts somehow also. 

December 8, 2001, 07:06 
Re: Question on Boundary Conditions

#3 
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Jonas,
Thanks for your answer. You are absolutely right about it. But what I was, or actually am, trying to do is to come up with a 'magical' boundary condition treatment to include the effect of the components that I am ignoring in the present simulation. I have done analysis on some of these components earlier and have a good idea of their resistance to flow. Flow through the system is determined by the intersection of the blower curve (pressure as a function of flowrate) and the system resistance curve. System resistance comprises of many components, such as the cowl grille, the cowl plenum, the heat exchangers, the ducts and the outlets. Since the system is a series system, the total resistance is sum of all the individual resistances. Now the way I am currently doing it is by running a simulation for a case for which we know something about the solution from the measurements. Then I iteratively match the experiment in my simulation. At this point I extract the total pressure drop in the system. I then make changes to the system and rerun iteratively to get the same total pressure drop (to simulate that I have the same blower, hence the same capacity to flow air)across this part system. Fortunately, I have two experimental data points, and I will get an idea on how close an answer this approach can give me. It does not seem to be bad at this point. By the way, I have had very good correlation with experimental testing in predicting the correct flow through these kind of system. Hence matching the experimental data might work. 

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