Question 11. A water line is buried underground in dry soil that has an assumed initial temperature of 4.5 degree Celsius. The pipe may have no flow through it for long period of time, yet it will not be drained in order that no freezing occurs, the pipe must be kept at a temperature not lower than 0 degree Celsius. The pipe is to be designed for a 30 hour period at the beginning of which the soil surface temperature suddenly drops to – 17 degree Celsius. Workout the minimum earth covering needed above the water pipe so as to prevent the possibility of freezing during 36 hour cold spell. The soil in which the pipe is buried has the following properties
Density = 640 kg/m³
Specific heat = 1843J/kg degree
Thermal conductivity = 0.345 W/m degree

Question 12. The temperature and rate of heat conduction are undoubtedly dependent on

Question 13. Glass spheres of 2 mm radius and at 500 degree Celsius are to be cooled by exposing them to an air stream at 25 degree Celsius. Find the minimum time required for cooling to a temperature of 60 degree Celsius. Assume the following property values
Density = 2250 kg/m³
Specific heat = 850 J/kg K
Conductivity = 1.5 W/m K

Question 14. Which is true regarding lumped system analysis?
(i) Conductive resistance = 0
(ii) Convective resistance = 0
(iii) Thermal conductivity = 0
(iv) Thermal conductivity = infinity
Identify the correct statements

Question 15. What is the criterion for the applicability of lump system analysis?

Question 16. Which of the following is an example of lump system analysis?

Question 17. The boundary conditions in case of transient heat conduction in infinite thick solids are
(i) t (x = 0) = t i
(ii) t (0, T) = t a for T greater than zero
(iii) t (infinity, T) = t i for T greater than zero
Identify the correct statements

Question 18. A large steel ingot, which has been uniformly heated to 750 degree Celsius, is hardened by quenching it in an oil bath that is maintained at 25 degree Celsius. What length of time is required for the temperature to reach 600 degree Celsius at a depth of 1 cm? Thermal diffusivity for the steel ingot is 1.21 * 10¯⁵ m²/s. The ingot may be approximated as a flat plate

Question 19. At the penetration depth d, there will be 1% perturbation at a time given by

Question 20. The perturbation time varies as