For In-class Problem #1, we calculated the minimum
temperature of air that could be blown over a sheet of
glass that could cool it without causing it to crack. In
this problem, it was assumed that the glass has a
thermal conductivity k = 1.4 W/m-Kand the heat
transfer coffecient for the air blown over the surface was
$h_c$=5.2 W/mĀ²-K. In this problem, the goal is to
determine whether this was a realistic estimate for h
and to use the predicted value of to determine the
cooling rate of the glass under the following conditions:
Support rolls
Forming ro
Molten
glass
$T_\infty$=27 C
$T_s$ = 107 C
0.5 m/s
conveyer
ģ“
The glass sheet is 1.5 m wide, 1.5 mm thick, and has been sliced
off just past the forming rolls to create a sheet that is 2 m long. In
this estimate, assume that the support rolls do not interfere with
the air movement over the glass as it moves along the conveyr
belt at 0.5 m/s. At a point where the glass has cooled down to
107Ā°C, the bulk air is stagnant and at 27Ā°C.
2.0 m
$T_\infty$=27 C
a) What is the average heat transfer coefficent for the glass sheet at this point on the conveyor belt? Evaluate the
properties at the film temperature.
b) What is the rate of heat loss in watts from the sheet at this point on the conveyer belt?
c) If the sheet was poly(methyl methacrylate) (Plexiglas) with a thermal conducticity of 0.12 W/m-K, but otherwise has
the same dimensions, temperature as the glass sheet, and the air is at the same conditions, what is the rate of heat loss
from the Plexiglas sheet?