push rod tube, to paint or not Options

[jacksun]

Hi,

got my first push rod tube cleaned up and would like to paint it with

hi-temp paint? any problems doing so?

tks

randal

[Highland]

I just watched both u-tube videos and it seems like no one talks about natural convection versus forced convection. I could see how the black paint leads to a higher surface temperature resulting in greater natural convection and significantly lower air output temperature. Of course with a fan or a car driving forward natural convection would be insignificant. The guy in the engineering video says 3 degrees is significant, but my guess is 3 degrees is within their measurement accuracy of not only their thermocouples, but the hair dryer’s ability to provide constant flow and temperature.

So my understanding is that you want air flow with no separation for good heat transfer. The fins have to be spaced far enough from each other so the adjacent boundary layers don't interfere with each other while maximizing the number of fins. I assume the VW engineers spaced the fins based on “design for manufacturing” so the range of surface roughness, flash, etc. would not affect the flow patterns and boundary layer thickness around the fins.

So back to the push rod tubes. The tubes measure a bit less than an inch. I could use some input here, but from what I can find a Reynolds number greater than 10 results in the start of air flow separation for a cylinder shape. If 60 mph air (either from forward travel or the fan) is passing around the tubes, that’ll give us a Reynolds number of about 700. I assume if you have the warm air guides installed most of the airflow whether standing still or moving is from the fan. Does anyone the air flow velocity of the fan? In any case there appears to be very little convection heat transfer occurring especially since the downstream tubes are getting turbulent air. If there is any heat transfer in radiation I’d rather have hot tubes than the tubes radiating toward the cylinders. I’m assuming the tubes are hot because they have so little mass compared to cylinder, shielding, and other parts.

So maybe black is not so good, but bare metal or a light color is okay? Although I agree with everyone else that it probably makes no real difference.