Boundry layer cooling

[krazyglue]

I am a CNC programmer by trade. I program big machine tools to make the things that people think up. I work to millionths of an inch sometimes.

I recently started working for a company involved in space flight. Making rocket engines.

Part of the engine is that they spray some sort of liquid usually one of the propellants around the inside of the engine casing to keep it cool and burning through. The Saturn V f-1 engine used this and i am sure the space shuttle and many others do also.

How does this work? Is it just a transfer of heat by passing a huge amount of fluid over a surface or some other means. Its a really thin layer from what i hear.

 

[PottedMeat]

Interesting stuff - wikipedia has:

Regenerative Cooling -> Film / Curtain Cooling

http://en.wikipedia.org/wiki/Regenerative_cooling

"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 and curtain cooling are very frequently employed as well."

 

[CycloWizard]

Boundary layers in heat transfer are simply the portion of the fluid near the heat exchange interface that supports a temperature gradient (and, therefore, heat transfer). The thickness of this layer depends on a lot of factors (rate of heat transfer, rate of heat generation, thermal conductivity of both fluids and the interfacial material, viscosity of the fluids, flow patterns of both fluids, and the geometry of the interface for starters).

In a rocket engine, you have a very high heat flux. Therefore, to cool it, you need a lot of coolant. It should pass by the interface very quickly to increase the heat transfer rate. The faster it's flowing across the surface, the thinner the boundary layer will be.