Ideal Gas Law - NCEES Thermo # 502 and 504

tmacier said:

Good morning,
As I am preparing for the exam I find myself relearning many areas, using the MERM as the main method of teaching.

I am working the NCEES 2008 Thermo problems 502 and 504

The solutions appear to being using a version of PV=nRT that brings area and mass flow rate into it.

Looking thru the MERM I cannot find these associations at all.

Can someone point me to the specific section so I can get a better understanding of these solutions?

Or explain to me what I am missing-

m(dot)=pAV Were did this come from?

m(dot)RT=PAV Were did this come from?

Thanks in advance for your time.

Tim

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tmacier said:

Good morning,
As I am preparing for the exam I find myself relearning many areas, using the MERM as the main method of teaching.

I am working the NCEES 2008 Thermo problems 502 and 504

The solutions appear to being using a version of PV=nRT that brings area and mass flow rate into it.

Looking thru the MERM I cannot find these associations at all.

Can someone point me to the specific section so I can get a better understanding of these solutions?

Or explain to me what I am missing-

m(dot)=pAV Were did this come from?

m(dot)RT=PAV Were did this come from?

Thanks in advance for your time.

Tim

Click to expand...

Baltimore Joe said:

tmacier said:

Good morning,
As I am preparing for the exam I find myself relearning many areas, using the MERM as the main method of teaching.

I am working the NCEES 2008 Thermo problems 502 and 504

The solutions appear to being using a version of PV=nRT that brings area and mass flow rate into it.

Looking thru the MERM I cannot find these associations at all.

Can someone point me to the specific section so I can get a better understanding of these solutions?

Or explain to me what I am missing-

m(dot)=pAV Were did this come from?

m(dot)RT=PAV Were did this come from?

Thanks in advance for your time.

Tim,

You are exactly correct. They are using PV=mRT and merging the continuity equation as follows:

1) PV=mRT

2) dot both sides: PV(dot) = m(dot)RT

3) Continuity equation: m(dot) = pAV = pV(dot) : NOTE - the V in the middle equation is velocity of the fluid, and the V(dot) in the third equation is the volumetric flow rate of the fluid

4)Therefore:

pAV = pV(dot)

V(dot) = AV

5) Substitute this back into: PV(dot) = m(dot)RT

PAV= m(dot)RT

6) Now you can solve for the Area

I hope this helps.

NerdHerd

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