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Liquids have simulated convection (as in heat rising), but it works completely different to in gases.


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This game has endless surprises. I have known for a very long time that heat rises (and spreads horizontally very fast) in gases, but honestly wasn't sure how exactly it worked in liquids, except knowing that the 1 degree rule applies strongly to liquids except when heat is rising. The surprise is that the devs implemented "heat rising" in liquid in a completely different way to gases.


Left hand side: 100 kg Oxygen Gas tiles at 0 or 100 C. Right hand side: 1100 kg Ethanol tiles at 0 or 70 C.
Top: Hot above cold (heat rising). Bottom: Cold above hot (heat sinking).


Convection in gases works in the following way. Gas tiles swap places randomly and rapidly horizontally (there used to be horizontally temperature smearing, but that was removed to fix a bug), and they swap places vertically occasionally but only if a hotter tile is beneath a colder tile. This is a form of true convection as the heat is being carried by the movement of the fluid. (the game doesn't form convection currents though and it's not based on density, hence it can't really be described as natural/free convection, but it is a form of heat convection, just with the motion being driven by something like macroscopic "Brownian motion" with a bias for heat rising)

Heat rising in liquids works in the following way: Normal conduction always applies horizontally, there is no random tile swapping. When a hotter tile is under a cooler tile, every few ticks they instantaneously and fully equalize in temperature (in a heat-conserving way), which bypasses thermal conductivity and the 1 degree rule, as far as I can tell there is no "convection" mechanism, as in the tile is not actually moving, only the heat is moving. Though this is clearly meant to simulate convection in liquids.

Because liquids have quite high thermal conductivity in all directions, the most practically significant effect of this comes from the 1 degree rule. In both gases and liquids the 1 degree rule is ignored when the "hot plate" is at the bottom of the room/pool, this causes the entire vertical column above the hot plate to reach the same temperature. In gas, the horizontal convection will also cause the tiles to reach the same temperature horizontally, but in liquid the 1 degree rule means that the temperature will not equalize horizontally.

Another thing to note is that vertical heat conduction in liquids is potentially astonishingly fast, in the video did you see how quickly the heat rose up the column? Though the potential heat transfer rate is so high I'm not sure what could actually make use of it.

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