A metal wire of 0.01 m dia and thermal conductivity 200 W/m.K is expos...
To calculate the Biot number, we need to compare the resistance to heat transfer inside the wire (due to conduction) with the resistance to heat transfer at the surface of the wire (due to convection). The Biot number is defined as the ratio of the internal resistance to the external resistance.
Given data:
- Diameter of the wire (D): 0.01 m
- Thermal conductivity of the wire (k): 200 W/m.K
- Convective heat transfer coefficient (h): 100 W/m2.K
Let's calculate the Biot number step by step:
1. Calculate the characteristic length:
The characteristic length for a wire is its diameter. In this case, the characteristic length (L) is equal to the diameter of the wire, which is 0.01 m.
2. Calculate the internal resistance (Ri):
The internal resistance is due to conduction inside the wire. For a cylindrical shape, the resistance is given by the formula:
Ri = ln(r2/r1)/(2πkL), where r1 and r2 are the inner and outer radii of the wire.
In this case, the wire is solid, so r1 = 0 and r2 = D/2 = 0.01/2 = 0.005 m.
Ri = ln(0.005/0)/(2π(200)(0.01)) = ln(0.005)/(2π(200)(0.01))
3. Calculate the external resistance (Re):
The external resistance is due to convection at the surface of the wire. For a cylindrical shape, the resistance is given by the formula:
Re = 1/(h2πDL), where D is the diameter of the wire and L is the characteristic length.
Re = 1/(100(2π)(0.01)(0.01))
4. Calculate the Biot number (Bi):
Bi = Ri/Re = (ln(0.005)/(2π(200)(0.01))) / (1/(100(2π)(0.01)(0.01)))
Simplifying the expression:
Bi = (100(2π)(0.01)(0.01)ln(0.005)) / (2π(200)(0.01))
Bi = 100ln(0.005)/200
Calculating the value:
Bi = 0.025
Therefore, the Biot number is 0.025.