Basically, after some heat exchanger, a fluid is pumped to a radiator and then releases its heat to the void of space. But I'm having a hard time seeing how that is done fast enough. The fluid should radiate its heat immediately, so some heat is put back into the spaceship before it can reach the outer tubes of the radiator.
So now I'm going to provide my own "research" about this, because for one thing, I want to know if my simple understanding of thermodynamics is correct.
I understand heat is a scalar, and power is a vector. Heat is measured in joules and power is measured in joules/second (the watt). It's very annoying tho because there are many types of energy, so I will refer to them as heat energy/power or electic energy/power.
There are two types of thermal management. One is internal heat moving/rearranging, and the other is radiation.
Ultimately all heat must be lost as radiation, because space is a vacuum. (I know there are some evaporative cooling things even in space, but that expends a liquid and I'd rather stick with pure electrically powered thermal management).
Very simple example. Spherical spacecraft has a wall and inner air. Wall mass is 200 kg. Air mass is 2 kg. This craft could have a uniform temperature of 400 K. Or the walls could have a temperature of 401 K and the air is 300 K.
This would have the same total heat energy because 400x202 = 80,800, and 401x200 + 300x2 = 80,800. (I actually don't understand why heat energy, the joule, is not measured in K x kg for this purpose.)
Correct me if I'm wrong. If this is wrong so far then I'll always be totally lost unless it's corrected. I know total heat energy is measured in joules but I'd prefer to know how to calculate that based on mass and temperature.
Anyway, so those two examples were to illustrate that you can "rearrange" internal heat but you haven't actually reduced the total heat of the spacecraft. I made that example because I wanted something where the outer walls act like a hot radiator, but the internal air is cool and comfortable for people. (Ignoring that the walls will radiate inwards as well as outwards, for now.)
So now imagine a nuclear power plant inside the space ship lol. That's a lot of heat generation. If you are generating 1 GW of electrical power then you are also generating more than 1 GW of heat power (because all things have inefficiency).
Btw, it's been done before. The RORSATs. It's not 1 GW, but it is a nuclear power plant on an unmanned spacecraft.
I understand you can use heat exchangers, which then pump the fluid thru some insulated pipe, and this pipe leads to a radiator. The problem is, the fluid should be radiating away heat all the time.
So after the heat exchanger, the fluid is say 600 K. It starts radiating away heat immediately, into the surrounding pipe, which warms up and then transfers that heat to the surrounding air of the cabin.
Then the fluid reaches the radiator, where it is maybe 400 K. It's still radiating away heat, but slower now because of lower temperature.
So then what? Does it just sit there until it reaches some cold temperature like 250 K, then it comes back to help cool the overall temperature of the spacecraft? That sounds like it would take a long time. And the fluid needs to be pumped at some constant rate anyway.
I also understand the Stefan–Boltzmann law, where radiated joules per second is equal to $\sigma T^4$. To me this seems like you want the fluid to be as hot as possible, so it radiates heat away as quickly as possible.
But I don't understand how you can pump heat to concentrate heat somewhere. The heat exchanger simply "equalizes temperatures" so it's not going to be hotter than the initial ambient temp of whatever it's cooling. So if the steam is 600 K, the fluid in your heat exchanger will be 600 K or a little less after it passes thru the steam.
What I'd like to do is somehow pump heat out of something at any temperature. So the air might be 100 kg at 310 K, but I'd like to take 10 K out of it and into some small amount of fluid at some rly rly hot temperature like 1200 K. But a heat exchanger will never do that. The fluid in its pipe will reach 310 K at the most, and never get hotter.
Anyway, I'm lost now. I have looked at similar questions and answers (link, link), but they never seem to describe exactly how the radiator works or how heat transfer actually works. I feel like if I don't understand these at the component level, I'll never really understand it at all.