2 replies to this topic

[SPC]

Hi dear all,

 

    I need to ask very basic question about expansion of gas & liquid, for exmaple across orifice.

 

I am sure about Mass flow will not change in case of Liquid expansion, Please correct me if I  am wrong.

 

What about gas mass flow rate?

 

Please find attached herewith a simple schematic for the same.

 

Hope for a help.  Expansion of Fluid Behaviour.xlsx   9.93KB   26 downloads

[ahmadikh]

 The "Conservation of Mass" Law has no exception (Here we are not talking about conversion of mass to energy or vice versa, of course). Remember that in case of liquid, we assume that the density of the liquid only depends on the temperature and not on the pressure. As such, since there is no temperature change across the orifice it shows that volumetric flow remains the same and as a result the mass flowrate is the same! On the other hand,  in case of gas expansion it appears that since the volumetric flowrate has been changed, so the mass flowrate has changed as well.

 

So, the confusion is coming from missing the point that "how does the density change?". In case of liquid it remains the same, so it may appear to you that mass flowrate is constant. However, in case of gas expansion as the volumetric flow may be significantly larger, you may think that the mass flow is not the same. But remember, the density has been decreased in such a way that if you multiply the density by volumetric flow for both the upstream and downstream you will get the same result!

 

So, no exception for mass conservation Law.

 

Hope this helps...

[SPC]

Hi 

ahmadikh,

    

         Thanks a lot for removing a big confusion.

Conclusion is : 1) Gas or Liquid both will follow " Law of Conservation of mass" . Total Mass In = Total mass out.

 

                         2) As Gas is compressible fluid, hence density of gas is changed to new density to downstream of Orifice

 

                                  rho2 = P2 * M / R * T2   (P2 - is changed)

 

                            Hence, Volumetric flow rate of gas(Downstream of Orifice)  = mass of gas / density of gas( downstream of Orifice)

 

                           As, Liquid is incompressible, Although P2 is changed, but No Change in density as not dependent not pressure & hence                           no change in Volumetric flow rate. 

 

 

Once again lot of thanks for simplifying my this basic concept.

 

Thanks a lot.