I'm wondering where the boiled liquid is going, you said the tank is not a pressure vessel, so is it just lost?
I think OP just meant the liquid *looked* like it was boiling. I'm betting it looked like it was boiling because the pumps are continually circulating the liquid around the blades.
The fluid IS boiling but it boils at a much lower boiling point that water. Since they mention 37 degrees for the room temperature, which I assume is Celsius, I imagine they are using a fluid that boils somewhere between 40-60 Celsius, of which there are a number of options from 3M. Immersion cooling is really neat, just requires a vapor-seal for the container and a condenser to condense the evaporated fluid. So long as you don't exceed your heat exchange capacity, you will minimally pressurize the container.
