7.21 The top surface of a heated compartment consists of very smooth (A) and highly roughened (B) portions, and the surface is placed in an atmospheric    airstream.

 

 

 

In the interest of minimizing total convection heat transfer from the surface, which orientation, (1) or (2), is preferred? If Ts = 100°C,  Too  = 20°C,  and  uoo  = 20 m/s, what is the convection heat transfer from the entire surface for this orientation?

 

(c)    The proposed design operates in a strip constant- temperature mode for which the airstream velocity is related to the  measured  power.  Consider now an alternative mode wherein the strip is provided with a constant power, say, 30 mW/mm, and the airstream velocity is related to the measured strip

 

 

 

 

 

 

0.5 m

1 m

1.5  m

 

A              B

or              or

B              A

 

 

Ts

u¥, T¥

 

 

 

u¥, T¥

 

 

 

 

(1)

 

 

 

 

 

(2)

temperature Ts. For this mode of operation, show the graphical relationship between the strip temper- ature and airstream velocity. If the temperature can be measured with an uncertainty of ±0.2°C,   what is the uncertainty in the airstream velocity?

(d)    Compare the features associated with each of the anemometer operating modes.