Register to reply

Temperature change in an isentropic flow of an ideal gas

Share this thread:
citrouille
#1
Apr17-12, 04:52 PM
P: 3
I am a bit confused by the definition of an isentropic process in the flow of an ideal gas.

isentropic implies reversible & adiabatic.

for a process to be reversible, there are no losses to friction (viscosity in this case),
for a process to be adiabatic, there is no heat transfer with the surroundings.

That being said, when a compressible gas flows through a nozzle, there is a temperature change. When this happens, a temperature gradient occurs. Where there is a temperature gradient, heat transfer occurs within the gas. I was always taught that, according to the 2nd principle of thermodynamics, a heat transfer due to a temperature gradient (from hot -> cold areas) is irreversible...which is directly contradicting the definition of a reversible & adiabatic flow...

Hope this is a pertinent question...

Thanks a lot for any help.
Phys.Org News Partner Physics news on Phys.org
Step lightly: All-optical transistor triggered by single photon promises advances in quantum applications
The unifying framework of symmetry reveals properties of a broad range of physical systems
What time is it in the universe?
kmarinas86
#2
Apr17-12, 05:04 PM
P: 1,011
Quote Quote by citrouille View Post
I am a bit confused by the definition of an isentropic process in the flow of an ideal gas.

isentropic implies reversible & adiabatic.

for a process to be reversible, there are no losses to friction (viscosity in this case),
for a process to be adiabatic, there is no heat transfer with the surroundings.

That being said, when a compressible gas flows through a nozzle, there is a temperature change. When this happens, a temperature gradient occurs. Where there is a temperature gradient, heat transfer occurs within the gas. I was always taught that, according to the 2nd principle of thermodynamics, a heat transfer due to a temperature gradient (from hot -> cold areas) is irreversible...which is directly contradicting the definition of a reversible & adiabatic flow...

Hope this is a pertinent question...

Thanks a lot for any help.
Imagine a bunch of balloons containing gas. Imagine that they expand when they fly out of a window. The expansion of the gas cools the gas in the balloons. Yet, there is not any significant heat exchange between the balloons. That's because there's not enough time permitted for that exchange of heat to be significant. Thus, it is adiabatic.
citrouille
#3
Apr17-12, 05:07 PM
P: 3
Quote Quote by kmarinas86 View Post
Imagine a bunch of balloons containing gas. Imagine that they expand when they fly out of a window. The expansion of the gas cools the gas in the balloons. Yet, there is not any significant heat exchange between the balloons. That's because there's not enough time permitted for that exchange of heat to be significant. Thus, it is adiabatic.
I do not have a problem with the fact that the process is adiabatic (no heat transfer with the surroundings). My problem is with the fact that the transformation is reversible, and yet there is a spatial temperature gradient which would cause heat transfer within the gas (not with the surroundings). Heat transfer due to a temperature gradient within the gas is irreversible...maybe I'm thinking too much...sorry

kmarinas86
#4
Apr17-12, 05:22 PM
P: 1,011
Temperature change in an isentropic flow of an ideal gas

Quote Quote by citrouille View Post
Quote Quote by kmarinas86 View Post
Imagine a bunch of balloons containing gas. Imagine that they expand when they fly out of a window. The expansion of the gas cools the gas in the balloons. Yet, there is not any significant heat exchange between the balloons. That's because there's not enough time permitted for that exchange of heat to be significant. Thus, it is adiabatic.
I do not have a problem with the fact that the process is adiabatic (no heat transfer with the surroundings). My problem is with the fact that the transformation is reversible, and yet there is a spatial temperature gradient which would cause heat transfer within the gas (not with the surroundings). Heat transfer due to a temperature gradient within the gas is irreversible...maybe I'm thinking too much...sorry
If you count the separate balloons as members of the same "gas", you can easily see how there is not significant transfer of heat, not even between parts of the gas.
citrouille
#5
Apr17-12, 05:37 PM
P: 3
Quote Quote by kmarinas86 View Post
If you count the separate balloons as members of the same "gas", you can easily see how there is not significant transfer of heat, not even between parts of the gas.
That makes more sense ! Thanks a lot for clearing that up.


Register to reply

Related Discussions
Thermodynamics: isentropic efficiency and ideal gas model Engineering, Comp Sci, & Technology Homework 1
Thermodynamics, the ideal gas equation and temperature change Introductory Physics Homework 9
Relation between Gas flow rate, heat loss and change in temperature Classical Physics 0
Temperature change after flow through an oriface General Physics 2
Change of temperature of ideal gas Introductory Physics Homework 2