Wikipedia
A working fluid is a pressurized gas or liquid that actuates a machine. Examples include steam in a steam engine, air in a hot air engine and hydraulic fluid in a hydraulic motor or hydraulic cylinder. More generally, in a thermodynamic system, the working fluid is a liquid or gas that absorbs or transmits energy.
Usage examples of "working fluid".
Such a compact heat source would power ramjets without fuel tanks, or it could vaporize a working fluid such as water.
The generators' working fluid was heated to a vapour by the ocean's warm surface water, passed through turbines, then chilled and condensed by the water from the bottom.
The drives were electric, generated by fusion-heated steam in a light-metal working fluid.
If they don't keep their working fluid circulating down to the collector and get the heat out of it when it comes back up, the hot end of the unit will melt.
The working fluid was liquid hydrogen at twenty-five degrees above absolute zero.
With a suitable working fluid, this was an economical method of power generation, and one that was extremely suitable for automatic operation and required no nearby water source.
Oh, yes, there would have to be a constant supply of nutriment and oxygen available, but as this is a quasi vegetable process in the patient with water instead of blood as the working fluid, there should be no problem in supplying a deeply buried brain.
Actually it's not air, it's helium, what we call the working fluid, but you can think of it as air if you want to.
You can be almost sure that any change of working fluid, even a fresh one right out of a sealed flask, will have picked up enough.
But the plasma rockets operate at such a high energy level they break their working fluid down into subatomic particles.