Heat transfer between warm air and cold water

[wbogen]

For your consideration:
A very large (100s of meters) vertical metal (stainless steel) wall maintained at, say, 35F.
Warm (72F) moist air flowing over the wall (natural convection, not forced).
Condensation flowing down the wall (at 35F?).

How to find the heat transfer rate between the warm air and the wet surface of the wall and the rate between the water layer and the wall?

Thanks for any help you can provide.

 

[LostHippie]

condensation will begin as droplet formation, with a much higher heat trasnfer rate, once the droplets become large enough that the centrifugal forces overcome the adhesive and cohesive forces that bind the water to the endcap and itself, then it will begin to flow, ideally into a "sheet" of condensate. This will form an extra convective layer b/w the wall and the inside air, I'll look up some of the equations when i get back to my heat transfer book later today. It should be noted that at the near 0g area around the axis you may start to form a "string" or extended bubble of condensate initiated from the center of each endcap...and may need some sort of disturbance to move it out of the "gravitational doldrums" and into an area of higher centripetal forces.

You very well could get low level drizzle if you have a tropical environment around the inside surface...though more than likely you will get a cylinder of clouds around the axis...simply becuase the bergeron process (the main method of precip formation) relies on nucleation points as well as near-freezing temps...although you may get some drizzle from coalescence.

 

[wbogen]

To Xera: Oops, miscalculated: Hyperion 'Lake' would actually cover the US about 1800 meters deep.

 

[wbogen]

To LostHippie: Yes, I thought that fog and water might accumulate near the axis at the endcaps. I suppose it would be best _not_ to have heatpipes too close to that area or that water may freeze into tons of ice that, if perturbed somehow, may 'calve' and come crashing down, slowly but dangerously (as it hits the floor of the cylinder which is zipping by at 140 m/sec or more than 300 mph).

 

[wbogen]

So if a raindrop forms 1000m 'up' (halfway between axis and floor), it will have a tangential velocity of 70 m/sec (r=1000m, w=0.07 radians/sec rotation, velocity=wr). Since it is now much denser than the air around it, it will plow in a somewhat straight line toward the floor. As it gets lower the air it meets has a higher tangential velocity so the drop will increase its own. But if it only gets up to 120 m/sec while the floor is spinning around at 140 m/sec that means the rain will hit at 20 m/sec: gale force, not a drizzle.

This might place an upper limit on the size one might make such a space city.

 

[btrueblood]

"I understand that a radiator can still work on a surface exposed to the Sun"

Ok, good.

"but I'm using the sides of the cylinder for energy input, not as radiators"

Argh. How are you going to PREVENT the surfaces from radiating? They will radiate, you can't change that. You can adjust the surface emissivity somewhat, but you can't make it zero. My point would be to "adjust" (in theory) the surface emissivity to get a thermal balance that just allows your endcaps to be whatever function you want them to be.

 

[wbogen]

The sides will be 3/4 covered with PV solar panels (and 1/4 with fixed mirros bringing in sunlight to illuminate the interior). The PV panels (if using current PV technology) will convert sunlight at about 12% efficiency to electricity (3.4 GW) for use in the city. The rest of the energy (24.64 GW of heat) will, yes, radiate away from the PV panels. The cylinder will have 2 or 3 meters of soil between the interior and 3K space acting as insulation (and radiation shielding). I guess I could somehow pipe heat/refrigerant from the interior to radiators beyond the PV panels, but why bother if I can use the endcaps as low-maintenance radiators and weather-generators?

Actually, maybe I _would_ want to do that, if the weather effects from using the endcaps is too severe.