Glowing aluminized steel manifolds
Glowing aluminized steel manifolds
(OP)
We supply a lot of exhaust tube manifolds for the small engine industry. In the last few years, many of these manifold tubes have been converted from steel to aluminized steel. Engine exhaust gas temperatures in these tube are hot (about 1100 F or even higher). Some of our customers have noticed that the aluminized steel tubes tend to glow bright red where the old standard steel tubes did not. Then after the aluminized steel ages (surface turns dark grey), the glowing problem goes away (or at least isn't as apparant). Is there some characteric of aluminized steel that could explain this? The affect of the darken coating might explain some of this but I'm confused by the reaction of a virgin steel tube vs. a virgin aluminized steel tube.





RE: Glowing aluminized steel manifolds
Is the Aluminum applied pre or post fabrication?
RE: Glowing aluminized steel manifolds
RE: Glowing aluminized steel manifolds
The only other part is that the appearance may be a factor, you can simply see it more clearly in the brighter material.
= = = = = = = = = = = = = = = = = = = =
Plymouth Tube
RE: Glowing aluminized steel manifolds
I understand that radiation is a factor, but what surprises me is why a new 'plain steel' manifold (unpainted and 'bright steel' in color) seems to react quite a bit differently than an new aluminimum coated steel.
Wall thickness of both pipes are approximately the same but not exact.
RE: Glowing aluminized steel manifolds
RE: Glowing aluminized steel manifolds
When in doubt turn out the lights - an aged part will probably have a similar glow to new. Plus its a valid excuse to get in a good nap!
RE: Glowing aluminized steel manifolds
I have had the experience I quoted, which differs somewhat from yours, but I was using actual thermocouples to measure the temperature of the tube wall. If the emissivity of the surface varies over time, how can you trust an optical measurement?
RE: Glowing aluminized steel manifolds
I'm not certain that I believe there is all that much difference in emissivity of type II alumininized steel and new, clean CRS steel. I haven't found any published value for aluminized steel emissivity and steel's published data seems to vary quite a bit depending upon surface finish.
I did run a quick and dirty calibration curve for our Raytek non-contact thermometer, changing it's emissivity settings, then plotting temperature deviation from actual temperature. While aged aluminized steel differs substancially in it's characteristic curve, the two curves for aluminized steel and straight, (washed)CRS steel lay almost on top of one another (or at least until we get to very low emissivity settings). Yes, I know this isn't a very good way to verify emissivity and I can't seperate instrument error from the effects of the real emissivity. However, if I believe my data I would have set the value for our thermometer's emissivity setting at 0.6 for both materials, then set E at .95 for the aged aluminized steel. [In reality, I always paint a spot on the part surface with a flat black, high temperature paint when attempting to take IR temperature readings of exhaust systems.]
RE: Glowing aluminized steel manifolds
Many times we have muffler chambers with extreme differences in heat tint on the same muffler depending on internal tube routing. The emissivity of the IR device needs to be adjusted to read accurately between the two chambers as well as adjusted for aging.
RE: Glowing aluminized steel manifolds
When you care black a spot and set for 0.95. I have used carbon black in MEK for painting these spots.
Or if you really want the right number use a fine gauge contact thermocouple.
= = = = = = = = = = = = = = = = = = = =
Plymouth Tube
RE: Glowing aluminized steel manifolds
1) Probably the two most important heat transfer mechanisms involved is conduction and convection. Heat radiation is probably less important for a couple of reasons -- a) The tube diameters are relatively small (1 to 1-1/18 inch diameter and usually less than a foot long) so surface area isn't that great; and b) the manifold tubes are surrounded by other hot surfaces (engine, heat shields, and muffler) so the amount of radiated heat transfer between relatively equal hot surfaces is minimized.
2) Since aluminized steel is essentially almost all CRS, then manifolds made of either aluminized steel or uncoated CRS, should have nearly identically heat transfer properties (excluding any minor radiation effect). That probably means that both aluminized steel manifolds and uncoated crs manifolds are likely to reach approximately the same maximum temperature for any given engine running condition.
3) A new aluminized steel manifold won't immediately darken upon heating. A new uncoated steel darkens immediately at temperatures above approx. 400F (first blue, then grey before starting to glow).
Can I assume, that the darkening surfaces of heated crs causes a shift in it's emissivty value (upward) and in turn affects the amount of visible light being absorbed by a glowing surface? So once glowing, a material with a higher emissivity value, appears darker than a material with a lower emissivity value even if both materials reach roughly the same temperature.
RE: Glowing aluminized steel manifolds
You are correct. Emissivity is the compliment of reflectivity, so a material with a higher emissivity will have a lower reflectivity, and will appear darker when there is an appreciable amount of reflected light. That is, in a dark room, at temperatures high enough to glow, the material with the higher emissivity will appear brighter (since most of the light coming from the surface is being emitted). In a bright room, the material with the higher emissivity can appear darker (if most of the light coming from the surface is reflected). This will depend on how much radiation is being emitted and how much is being reflected.
rp
RE: Glowing aluminized steel manifolds
I wonder?
Best Regards
Qalander(Chem)
RE: Glowing aluminized steel manifolds
There are still a multitude of other possibilities, including, surface micro-quality, which affects radiation and convection, as well as potentially different interior surfaces, which can affect heat transfer between the gas and the pipe.
Without careful measurements and analysis, this discussion can endless.
TTFN
FAQ731-376: Eng-Tips.com Forum Policies