Heat and Mass Transfer

What is the correct formula for the rate of heat transfer (q) through a plane wall of thickness L and surface are A, when temperature difference between two surfaces is ΔT ? The thermal conductivity is k.

• q = (k A ΔT) / L

• q = k A L ΔT

• q = (ΔT) / (k A l)

• none of the above

Thermal conductivity of the same material varies with

• a. the thickness of the material

• b. the temperature of the material

• C. both a. and b.

• d. thermal conductivity of the same martial does not vary at all

What does a composite wall mean?

• two walls of different materials are connected in series without any gap between them

• three walls of different materials are connected in series without any gap between them

• more than three walls of different materials are connected in series without any gap between them

• all of the above

When a composite wall of three layers in series having thermal resistances R1, R2 and R3 respectively. The heat transfer takes place normal to the surface of the layers. How is the total thermal resistance of the composite system calculated?

• 1 / (R1 + R2 + R3)

• ((1 / R1) + (1 / R2) + (1 / R3) )

• (R1 + R2 + R3)

• none of the above

What is the formula for the thermal resistance (R) of a hollow cylinder of internal radius Ri, outer radius Ro and the length l ? Thermal conductivity of the cylinder material is k. The heat flows from inner side to outer side.

• R = ln (Ro / Ri) / 2πkl

• R = ln (Ri / Ro) / 2πkl

• R = (Ro + Ri) / 2πkl

• R = (RoRi) / 2πkl

At the critical radius of insulation of a hollow sphere, the heat transfer will be

• minimum

• maximum

• does not change

• unpredictable

The critical radius of a hollow sphere having thermal conductivity k and h_o as convective heat transfer coefficient of outer fluid is given by

• h_o / k

• k / h_o

• 2k / h_o

• h_o / 2k

A hollow sphere of internal diameter ID = 20 cm and outer diameter OD = 30 cm contains hot fluid. What should be the critical radius of insulation for maximum rate of heat transfer? Thermal conductivity k = 0.86 W/mK and convection heat transfer coefficient of outer fluid h_o = 20 W/m²K.

• 0.086 cm

• 0.86 m

• 8.6 mm

• 8.6 cm

Heat transfer from higher temperature to low temperature takes place according to

• Fourier law

• First law of thermodynamics

• Zeroth law of thermodynamics

• Second law of thermodynamics

For a given volume and specified heat input which material will have the smallest temperature rise _____.

• water

• mild steel

• aluminum

• copper