Jupiter brain is known as a colossal computer the size of a planet, if heat dissipation is a concern when constructing it how come most models lack an ocean of distilled reverse osmosis UV treated pure water?
Several reasons. First of all, having an ocean (or other large bodies of water) would mean to have evaporation and rainfall. I don't know about your computer, but mine hates water.
Second, the higher the amount of water in your atmosphere, the more heat it keeps in. You want your computer to get rid of its heat and on a planetary scale, the only way to do this is to radiate it out into space.
So in order to radiate out as much heat as possible you want it to have no atmosphere at all. No atmosphere means no atmospheric pressure, which means any amount of water evaporates.
Moreover...even now, with our current level of technology which is far below that necessary for the construction of such megastructures (whether Dyson spheres or computational stations the size of planets) there's significant research going on into alternative chemistries and methods for computing in higher temperature ranges:
Yuji Zhao, head of the semiconductor materials and devices group at Arizona State University, is poised to open new worlds. Literally. “That’s our dream,” said Zhao, an expert in electrical and computer engineering at Arizona State University. “That’s what we’re planning.”
Zhao recently won a grant from NASA to build a computer brain made from an exotic material being hailed as the new silicon with the ability to withstand intense heat and radiation. Electronics made from the material would enable NASA next-generation missions to planets seen only from great distances or for short periods. The material, called gallium nitride, outstrips silicon in speed, temperature and power handling.
“This material has unique features,” said Zhao, an assistant professor in the School of Electrical, Computer and Energy Engineering in the Ira A. Fulton Schools of Engineering. “It’s radiation-hardened material, so it can resist a very high-radiation environment.”
No one has ever made a CPU from gallium nitride.,“We will be one of the first ever to do that,” Zhao said. “It’s very exciting.”
NASA’s Hot Operating Temperature Technology program, HOTTech for short, awarded Zhao a three-year, $750,000 grant to develop a high-temperature gallium nitride microprocessor for space applications.
“This program is aimed at NASA missions that are close to the sun, or to planets that have very high-surface temperatures, like Mercury and Venus,” he said. “When they have electronic devices or systems on the surface, they need high-temperature electronics. That is what we are doing.”
Silicon does not do well at high temperatures. “It sucks,” Zhao said.
“The material we are developing, we have results for the solar cells we have up to 500 degrees C, 300 degrees C, and don’t see a decrease at all,” he said. “In some cases if we design our device in a very unique way, their performance will be peak at 500 degrees C. This is very unique about this material; it’s designed for high temperatures.”
Gallium nitride actually performs better when it’s hot. “For space it makes sense to use this material to develop a high-temperature CPU,” Zhao said. “There’s a lot we don’t know about this material.”
NASA is sitting up and taking notice of gallium nitride. Currently, Zhao is prototyping high-temperature resistant solar panels made from gallium nitride for another NASA grant project.
“Our progress on solar cells has started to give NASA notice and to really pay attention to this material,” he said. “On the comments I had for my review on other NASA projects they said, ‘I never believed in this material, but your stuff is starting to make me believe it.’ We are very happy about this comment, but we still have a very long time to go.”
That being the case, there's no conceptual reason at this early stage in conception to even begin to consider thermal management strategies, as we've no real idea what the computation system's requirements might be: perhaps shortly before initiating such a project, we will have designed newer, yet higher temperature systems which operate best in a range from 800°C - 1200°C, and if so, perhaps the bigger issue will not be (as it currently is) heat dissipation but rather heat capture.
elPolloLoco is partly right; Standing bodies of water aren't especially useful, but there is a major flaw in that answer:
Heat is a problem. Water is really good at transferring heat. With no atmosphere you can't use convection for cooling, so all heat transfer is either radiative or conductive. Both of those are not great in the internal spaces. What you would want is a circulatory system to transfer heat from internal spaces to external radiators, preferably radiators that are out of sunlight. These could be water pipes, or you could use a heat pipe filled with some kind of refrigerant with a low boiling point.
Inch for inch, a heatpipe has the highest heat transfer possible, so long as there is a big enough radiator on the far end to handle the load. However, there might be problems with heatpipes over great distances, in which case water pipes with circulation pumps every few kilometers are going to be a better bet.
So you would probably want some kind of reservoir, but it wouldn't need to take the form of an ocean; it could just be a lot of big tanks.