Question

Stainless steel ball bearings (r 5 8085 kg/m3 and cp 5 0.480 kJ/kg·°C) having a diameter of 1.2 cm are to be quenched in water at a rate of 800 per minute. The balls leave the oven at a uniform temperature of 900°C and are exposed to air at 25°C for a while before they are dropped into the water. If the temperature of the balls drops to 850°C prior to quenching, determine the rate of heat transfer from the balls to the air.

Answer 1

Answer:

125 W

Explanation:

Data:

Let the Biot number be:

$L_{c} = \frac{V}{A} = \\\frac{\pi /6D^{3} }{\pi D^{2} }\\ = \frac{D}{6} \\= \frac{1.2cm}{6} \\= 0.002m$

Then:

$Bi = \frac{hL_{c} }{k} = 0.0166$

We know that:

L$L_{c} = 0.002$

K= 15.1 W/m²K

Then, substituting gives:

125 W