2 replies to this topic

[ahmadijaz65]

Hi

 

I am performing an experiment using both Nitrogen and Helium. In the first case when i use Nitrogen in a tank at 25 °C and 250 bar. Nitrogen undergoes expansion from 250 bar to 40 bar and attain the final temperature of -75.77 °C while in the second run, i have used the Helium instead of Nitrogen. Under the similar conditions, helium attains the temperature of -87.70 °C.

 

Why there is that much temperature difference in the final value of nitrogen and helium when the experiment is performed under the similar operating conditions.

 

Is it the heat capacity values which is responsible for this temperature difference or there is a some other valid reason?

[serra]

real fluids have (in general) different characteristics,

a mollier chart (or a table of values) can be useful,

Hin=Hout for constant enthalpy, but you may consider different processes.

[PingPong]

Helium is a mono-atomic molecule with a specific heat ratio ɣ of about 1.67

 

Nitrogen is a di-atomic molecule with a specific heat ratio ɣ of about 1.4

 

Consult a thermodynamics or physical chemistry book to find out why that is.

 

 

If both gases would behave as ideal gases then one can simply calculate the final temperature due to adiabatic (isentropic) expansion using Poisson's Equations

 

T₂/T₁ = (p₂/p₁)^(ɣ-1)/ɣ

 

and due to its higher ɣ helium's final temperature T₂ will be lower than that of nitrogen.

 

At 250 bar no gas behaves as an ideal gas, but the difference in behavior between helium and nitrogen will nevertheless be similar. Use  p-h diagrams of helium and nitrogen to verify.