III 8 marks Give brief answers for each of the following li

III (8 marks) Give brief answers for each of the following. li (101 How are conduction, convection and radiation distinguished? lii 201 Give one example each of free convection in nature and in technology, liii f3ll The total thermal capacity C pWc (/K) and thermal resistance R-LKA (KUW) in the case of unsteady heat transfer from a large plane wall of thickness L, considering a volume V with cross-sectional area A. Show that the product RC has units of time. l4 12ll The dimensionless parameters Biot number Bi and Nusselt number Nu are both evaluated using the expression hl k, where L is an appropriately chosen length scale. How does k differ in these two parameters? What use do we make of the parameters Bi and Nu in heat transfer?

Solution

1.Radiation is a phenomenon which does not require any medium where as conduction and convection requires medium to propagate. Conduction takes place in solid and liquid. Convection takes place in liquid only.

2. Free Convection(in nature): Boiling of water Free convection(in technology):Air Conditioner unit.

3.It can be well seen with the help of units of Nu and Bi.

4. Nusselt number is used in Convection phenomenon whereas Biot Number is used in Conduction phenomenon only. However it does not effect much if heat transfer by conduction is equal to heat transfer by convection.