Technically, this isn't a question concerning supercritical fluids. It's more about converting thermal energy into velocity by playing "keep-away" with molecules that want to go supercritical. The idea is simple. In an fully contained system, we can more or less say volume is static. Temperature becomes what determines both pressure and state of matter once we have that system of static volume. We can now simulate the introduction and removal of thermal energy. Let's say our system is a figure-8 loop and contains CO2 at the right pressure to invoke a super-critical state relatively near to ambient external temperatures. We can now introduce a low-energy heat source to generate a state change with relative ease. As the CO2 begins to go supercritical, we can introduce a low-energy source of cooling to from outside of this closed system, as well. What I'd like to know is how to describe the transfer of this energy. It seems to me, that an unnaturally large portion of the thermal energy we introduce will be converted directly into kinetic energy until it leaves the system as thermal energy again on the "cooling side" of our figure-8. I'm trying to determine the potential velocity of molecules within this system, based on the differential between my source of heat and my heat absorption. I'm just not sure exactly how to go about such a thing. Supercritical fluids are purportedly frictionless, but a system that is very nearly supercritical should be very nearly frictionless correct? If that's the case, shouldn't it be hypothetically possible to generate extreme velocities within this system by increasing both the heat source and heat absorption rates to high levels? And if so, how do we trend these rates to accurately achieve a desired velocity?