A building window pane that is 1.44 m high and 0.96 m wide is separated from the ambient air by a storm window of the same height and width. The air space between the two windows is 0.06 m thick. If the building and storm windows are at 20 and −10°C, respectively, what is the rate of heat loss by free convection across the air space?

the rate of heat loss by convection across the air space = 82.53 W

Explanation:

The film temperature

to kelvin = (5 + 273)K = 278 K

From  the " thermophysical properties of gases at atmospheric pressure" table; At = 278 K ; by interpolation; we have the following

 → v 13.93 (10⁻⁶) m²/s

→ k = 0.0245 W/m.K

→ ∝ = 19.6(10⁻⁶)m²/s

→ Pr = 0.713

The Rayleigh number for vertical cavity

=

=

For the rectangular cavity enclosure , the Nusselt number empirical correlation:

h = 1.99 W/m².K

Finally; the rate of heat loss by convection across the air space;

q = hA(T₁ - T₂)

q = 1.99(1.4*0.96)(20-(-10))

q = 82.53 W

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