If you could produce a very thin tube, say less than the thickness of a human hair, designed to carry a hot liquid inside, the diameter is small so that the liquid would have a high surface area to volume ratio, and then suspended this tube in a vacuum tube, completely insulated so as it wouldn't gain heat from the environment, this tube will radiate thermal radiation and only be able to cool down the liquid through thermal radiation into the vacuum. If you could make this tube as long as you need and had no spacial constraint what so ever, could you use it as a radiator that doesn't need airflow to cool down the liquid inside? If that is possible, would it be possible to use reflectors and lenses to aggregate the thermal radiation and focus it somewhere else?

  • $\begingroup$ If radiation can get out, radiation can also get in. In this system, the liquid will cool down to whatever is the temperature of the surrounding environment. But yes, thermal radiation call allow something to cool off. $\endgroup$ – DanielSank Oct 4 '19 at 17:25

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