Thermodynamics (Mass flow rate & Volume flow rate)

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Discussion Overview

The discussion revolves around the calculation of mass flow rate and volume flow rate in a thermodynamics context, specifically involving steam entering a nozzle. Participants explore the relationship between specific volume and density, and how to derive density from specific volume for the calculations.

Discussion Character

  • Homework-related
  • Conceptual clarification

Main Points Raised

  • One participant attempts to calculate the volume flow rate using the formula voldot = AV, arriving at a value of 0.2 m^3/s but expresses uncertainty about how to find the mass flow rate due to needing density.
  • Another participant acknowledges the relationship between specific volume and density, suggesting that specific volume is the inverse of density, but expresses confusion about how to derive density from specific volume.
  • Further clarification is sought regarding the relationship, with one participant questioning why density cannot simply be obtained by taking the inverse of specific volume, despite it being a straightforward equation.
  • Participants express their confusion and seek reassurance about their understanding of the concepts involved.

Areas of Agreement / Disagreement

Participants generally agree on the relationship between specific volume and density, but there is disagreement and confusion regarding the correct application of this relationship in calculations.

Contextual Notes

Participants express uncertainty about the definitions and relationships between specific volume and density, indicating a potential lack of clarity in the underlying concepts necessary for solving the problem.

jaredogden
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Homework Statement



Steam at 400oC enters a nozzle with an average velocity of 20 m/s. If the specific volume and the flow area at the inlet are measured as 0.1 m^3/kg and 0.01 m^2 respectively, determine (a) the volume flow rate in m^3/s, and (b) the mass flow rate in kg/s


Homework Equations



voldot = AV
mdot = (rho)(voldot)


The Attempt at a Solution



voldot = (0.01m^2)*(20m/s)
= 0.2 m^3/s

mdot = ?*(0.2m^3/s)

I am unsure what to do here. I am given specific volume 0.1 m^3/kg but I need density (kg/m^3) so if specific volume is V/m and density is m/v.. gosh I can't even think straight this late I feel like I'm going to pull some illegal math move and just do the reciprocal of specific volume to get density..
 
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Hi jaredogden,
jaredogden said:
I am unsure what to do here. I am given specific volume 0.1 m^3/kg but I need density (kg/m^3) so if specific volume is V/m and density is m/v.. gosh I can't even think straight this late I feel like I'm going to pull some illegal math move and just do the reciprocal of specific volume to get density..
Sounds like you're feeling a bit uncomfortable about how specific volume and density are related. Try this:
http://en.wikipedia.org/wiki/Volume_(thermodynamics)#Specific_volume
 
Well I know that specific volume is the inverse of density but I remember seen on a website that specific volume is the inverse of density but you can't get density by doing the inverse of specific volume so I was confused.
 
jaredogden said:
Well I know that specific volume is the inverse of density but I remember seen on a website that specific volume is the inverse of density but you can't get density by doing the inverse of specific volume so I was confused.
If specific volume is the inverse of density then why can't you get density by taking the inverse of specific volume? It's just a straightforward equation that relates one to the other, isn't it?
 
Well that was my thinking and when I read that I got really confused and didn't understand what the guy running that website was saying. Thanks for the help though.
 

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