Heat Flow and Temperature
The heat flow through the chamber wall is very high indeed, 1-20 MW/m2 is not uncommon.
The amount of heat that can flow into the coolant is controlled by many factors including the temperature difference between the chamber and the coolant, the heat transfer coefficient, the thermal conductivity of the chamber wall, the velocity in the coolant channels and the velocity of the gas flow in the chamber or the nozzle.
Two boundary layers form; one in the hot gas in the chamber and the other in the coolant within the channels.
Very typically most of the temperature drop occurs in the gas boundary layer since gases are relatively poor conductors. This boundary layer can be destroyed however by combustion instabilities, and wall failure can follow very soon afterwards.
The boundary layer within the coolant channels can also be disrupted if the coolant is at subcritical pressure and film boils; the gas then forms an insulating layer and the wall temperature climbs very rapidly and soon fails. However, if the coolant engages in nucleate boiling but does not form a film, this helps disrupt the coolant boundary layer and the gas bubbles formed rapidly collapse; this can triple the maximum heat flow. However, many modern engines with turbopumps use supercritical coolants, and these techniques can be seldom used.
Regenerative cooling is seldom used in isolation, film cooling, curtain cooling, transpiration cooling, radiation cooling are very frequently employed as well.
Read more about this topic: Regenerative Cooling (rocket)
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