I’m a medical student in an interventional radiology rotation. Today I saw a procedure called “cryoablation” in which compressed argon is circulated through a needle that is inserted into a tumor. Argon expands rapidly when reaching the end of the needle and causes a decrease in temperature freezing the tumor from the inside out ( I understand how this works, I think). Then compressed helium is pumped through the same needle, but in this case the expansion of helium causes an INCREASE in temperature. How is this possible? I tried reading about the Joule-Thomson effect, but it is still really confusing to me. Maybe someone can explain it more simply. Thanks in advance!
In: Physics
If no one here is able to answer, it might be a good question for r/askscience or r/askengineers . You can request the answer in simple terms.
Ouf, Thermodynamics is complicated no matter how you look at it lol. It has to do with inversion temperature, Hydrogen and Helium have a much lower inversion temperature than other gases. So when they expand at constant enthalpy at room temperature they hit their inversion temperature where the gases experience temperature rise.
The inversion temperature in thermodynamics and cryogenics is the critical temperature below which a non-ideal gas (all gases in reality) that is expanding at constant enthalpy will experience a temperature decrease, and above which will experience a temperature increase.
**Helium and hydrogen are two gases whose Joule–Thomson inversion temperatures at a pressure of one atmosphere are very low (e.g., about 40 K, −233 °C for helium). Thus, helium and hydrogen warm when expanded at constant enthalpy at typical room temperatures**.
https://en.m.wikipedia.org/wiki/Inversion_temperature.
Tl;Dr Eli5: Helium at room temperature is far above that special temperature where it would have cooled down at expansion, thus it heats up at expansion because of the Joule Thompson effect
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It all comes down to a phenomenon called the Joule-Thomson effect.
Heat is one of the ways that energy is stored, and when pressure drops this energy is dispersed over a larger area and temperature drops.
However, the drop in pressure also means that atom knock into each other more seldomly and as this potential energy drops (the work that could be done by letting the gas decompress), heat increases (since energy has to be preserved).
If the first effect is dominant, then temperature drops. If the second is more dominant temperature increases. The temperature at which these forces are equivalent is the inversion temperature.
At room temperature Helium, Hydrogen and Neon heat up when expanded. All other gasses cool down.
It basically has to do with whether or not the gas requires energy beyond its pressure release to occupy more volume (gas cools) or whether it sheds its stored energy as pressure to occupy more volume (gas heats up). In the first case, the gas needs an extra push to spread out and draws that energy from its surroundings. In the latter case, it readily spreads and gives off heat as part of its stored potential energy (pressure).
This phenomenon is known as the Joule Thomson Effect. The temperature at which the direction of the energy transfer changes, is called the inversion temperature.
In the case of hydrogen and helium, the inversion temperature is close enough to room temperature to be a factor when designing pressure systems with them.
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