Value for convection coefficient (h) 1

[ddace]

I am desperatly trying to establish a good ball-park figure for the value of convection coefficient h in a forced air flow (fan) condition. This will be used to determine heat regection of an exhaust system inside a trailer(enclosure).

I understand that h~10 is a rule of thumb for natural convection. But what is the rule of thumb when it comes to forced convection at the surface of the isolation blancket that covers my exhaust ?

1. 26 000 cfm
2. ~ 500 ft/min
3. surface ~ 150 sq-ft (isolation blanket)
4. surface temp 160F
5. enclosure temp 113F

n.b

I do not want to go through the Reynolds No. equations

 

[25362]

to ddace,

I believe the ROT "10" figure is for metric units, namely, kcal/(h*m[sup]2[/sup]*K) or SI units W/(m[sup]2[/sup]*K). This would mean about 2 Btu/(h*ft[sup]2[/sup]*[sup]o[/sup]F) or 1.7 Btu/(h*ft[sup]2[/sup]*[sup]o[/sup]F), respectively.

For natural convection the figure would, of course, depend on the reported temperatures, and would be about
0.6 Btu/(h*ft[sup]2[/sup]*[sup]o[/sup]F)=3.4 W/(m[sup]2[/sup]*K).

For air (wind) velocities of 5.7 mph (500 fpm), the htc for convection (no radiation effects) depends on the dimensions of the warm exhaust surface, however we may assume a ballpark figure of 1+0.225v=1+0.225*5.7=2.28 Btu/(h*ft[sup]2[/sup]*[sup]o[/sup]F)
=11.1 kcal/(h*m[sup]2[/sup]*[sup]o[/sup]C)=12.9 W/(m[sup]2[/sup]*K).

Kindly comment.

 

[prex]

10 is of course 10 W/m²°K.
Take 20 in moving air conditions: this includes radiation (as the 10 value) and is the ballpark figure used for calculating thermal losses from buildings (so not too far from your temperature range) in windy conditions.

prex

http://www.xcalcs.com

Online tools for structural design

 

[apfelsaft]

hi all,
i've just calculated it (no radiation effect)(it's a cylinder?) :
Cylinder diameter 0.6 m - 13.0 W/m2K
0.5 m - 13.6 W/m2K
0.4 m - 14.5 W/m2K
0.3 m - 15.8 W/m2K
Ciao Apfelsft