So anyway a throttling process, also called a Joule-Thompson process, is a thermodynamic process in which a fluid is forced through a restriction without heat transfer and experiences a drop in pressure. Since there is no loss of energy from the fluid it’s specific enthalpy is conserved, which is interesting because it can result in counterintuitive effects in real fluids. In an ideal gas the temperature on either side of the restriction is constant, but in real fluids the temperature would go up or down depending on the properties of the fluid. Pure helium is a strange gas in this process, experiencing an increase in temperature after a throttling process, which feels counterintuitive because you’d expect most gases to get colder when they expand. If we examine the mathematical definition of specific enthalpy we see that it is

h = u + pv

Where h is the specific enthalpy (in Joules per kg), u is the specific internal energy (in Joules per kg), p is the pressure (in Pascals), and v is the specific volume (cubic meters per kg, equal to 1 over density). If the enthalpy is held constant while the pressure decreases, either the specific volume must increase or the specific internal energy must increase or some combination of the two. The specific internal energy is closely related to the temperature, so in some sense the temperature of the fluid after the throttling process is related to how much it expands when the pressure is released and how specific internal energy is related to temperature. For pure helium it does expand as the pressure is reduced, but it has so little energy storing capacity it gets warmer at the same time! Of course this depends on the actual pressures and temperatures, so it doesn’t always work out this way. Other fluids that get hotter like this are generally incompressible liquids, so it’s pretty weird that helium does it at all. Isn’t that neat! Physics is so cool.

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