What is the name of the effect where a real gas cools upon expansion through a porous plug?

The effect where a real gas cools upon expansion through a porous plug is the Joule–Thomson effect. It occurs during throttling, when gas flows from higher pressure to lower pressure through a valve, porous plug, or similar restriction, usually without exchanging heat with its surroundings.

The effect is named after James Prescott Joule and William Thomson, later Lord Kelvin, whose experiments in 1852 established the phenomenon. Unlike a turbine expansion, throttling extracts no useful external work; the gas's enthalpy remains constant. Changes in molecular attraction and repulsion can then shift energy between molecular motion and intermolecular potential energy, changing the temperature.

Cooling is not universal. At ordinary room temperature, nitrogen and oxygen generally cool, while hydrogen, helium, and neon warm because they are above their relevant inversion temperatures. An ideal gas has no Joule–Thomson temperature change at all. The effect is therefore specifically associated with real-fluid behavior and should not be confused with Joule expansion, in which gas expands freely into a vacuum.