Why flow in Jule Thomson expansion doesn't choke

JT doesn't involve 'work,' while flow choking does. In JT, gas density increases as gas cools. In flow choking, gas density decreases as the gas expands at the restriction. These are two opposite phenomena that don't occur together.

In simple one line, in Joule Thomson expansion the process is isenthalpic and does not involve external work. Sound waves at the sonic velocity of the medium can't compress the medium and do flow choking by doing work externally.

It may be noted that JT cooling at constant enthalpy occurs by switching between kinetic and potential energy without involving ' work'.In JT expansion a small amount of gas forces through a tiny opening or 100-150 micrometer pores of porous plug into a large volume downstream almost like a free expansion without involving any ‘ work’.

What is choked flow?

Choked flow occurs when a fluid flows through a restriction and reaches sonic velocity. As the flow of fluid increases, at restriction the specific volume of the fluid increases, when the fluid velocity reaches sonic velocity the sound waves compress the fluid and stop further expansion of fluid or increase of flow. This involves external work done by sound waves on the fluid. The final result is that the flow gets restricted even if the downstream pressure continues to decrease or the upstream pressure continues to increase. At choked flow, the flow velocity at the restriction reaches Mach number (the ratio of the flow velocity to the speed of sound) is equal to 1.

In conclusion, choked flow is a critical phenomenon at Mach = 1. At this point, the sound waves stop further expansion of fluid or further increase of fluid flow at restriction.

Explanation

The Joule-Thomson effect occurs when a pressurized fluid expands through a throttling valve, causing a decrease in temperature. This phenomenon is described by the Joule-Thomson coefficient, which determines whether the fluid will experience cooling or heating as it expands. In the case of flow through a throttling valve, the fluid does not choke because the flow is isenthalpic, meaning that the enthalpy remains constant during the process. In other words, the sum of internal energy and work energy always remains constant.

As said. in the case of the Joule-Thomson effect, the flow is isenthalpic and does not involve any external work. This implies that no work is done on the fluid, so the sound waves present in the fluid are unable to compress or obstruct the flow as they would in a compressible flow situation. In other words, the sound waves are unable to choke the flow at the constriction by compression when the flow is isenthalpic.

This is why flow choking does not occur in the Joule-Thomson effect, as the process is isenthalpic and the sound waves are unable to impede the flow of the fluid through the throttling valve.

In summary, the Joule-Thomson expansion doesn't choke because the enthalpy remains constant during the expansion. This means that no external work is done, and the potential energy of the gas increases. This increase in potential energy causes the kinetic energy to decrease, which in turn causes the temperature of the gas to fall.