 Total marks: --
Total time: --
INSTRUCTIONS
(1) Assume appropriate data and state your reasons
(2) Marks are given to the right of every question
(3) Draw neat diagrams wherever necessary

Solve any four:
1 (a) What is meant by film condensation and dropwise condensation?
5 M
1 (b) What is Fin? What are the various types of fins?
5 M
1 (c) Explain the number of transfer units (NTU).
5 M
1 (d) Define Thermal Diffusivity and state its significance.
5 M
5 M

2 (a) Derive the relation for heat transfer through fin with insulated tip. State the assumptions clearly.
10 M
2 (b) Explain the term 'Time Constant' of thermocouple.
3 M
2 (c) A copper wire of radius 0.5mm is insulated uniformly with plastic (k=0.5 W/m K) sheathing 1mm thick. The wire is exposed to atmosphere at 30°C and the outside surface coefficient is 8 W/m2 K. Find the maximum safe current carried by the wire so that no part of the insulated plastic is above 75°C. Also calculate critical thickness of insulation. For copper, thermal conductivity = 400 W/m K, specific electrical resistance=2 X10-8 ohm-m.
7 M

3 (a) Using dimensional analysis, derive an expression for forced convection:- Nu=Constant X(Re)mX (Pr)n.
8 M
3 (b) Air at atmospheric pressure and 207deg;C flows with 6 m/s velocity through main trunk duct of air condisioning system. The duct is rectangular in cross-section and measures 40cm × 80cm. Determine heat loss per meter length of duct corresponding to unit temperature difference.
The relevant thermo-physical properties of air are: v=15×10-6, α=7.7×10-2 m2/hr, k=0.026 W/m-deg-k.
Use Nu=0.023 (Re)0.8 × (Pr)0.4.
8 M
3 (c) What is meant by Fouling in Heat Exchangers.
4 M

4 (a) Distinguish between specular and diffuse radiation.
4 M
4 (b) Prove that the total emissive power of black surface is π time the intensity of radiation.
6 M
4 (c) 16.5 kg/s of the product at 650°C (cp=3.55 kJ/kg K), in a chemical plant, are to be used to heat 20.5 kg/s of the incoming fluid from 100°C (cp=4.2 kJ/kg K). If the overall heat transfer coefficient is 0.95 kW/m2 K and the installed heat transfer surface is 44 m2, calculate the fluid outlet temperature for the counter flow and parallel flow arrangements.
10 M

5 (a) Derive the relationship between the effectiveness and the number of transfer units for a parallel flow heat exchanger.
10 M
5 (b) A thermocouple indicates a temperature of 800°C when placed in a pipeline where a hot gas is flowing at 870°C. If the convective heat transfer cofficient between the thermocouple and gas is 60 W/m2 K, find the duct wall temperature, ε (thermocouple)=0.5.
5 M
5 (c) A thin copper sphere with its internal surface highly oxydises, has a diameter of 20 cm. How small a hole must be made in the sphere to make an operating that will have an absorptivity of 0.9?
5 M

Write a short note (Any Two):
6 (a) (i) Heisler chart.
4 M
6 (a) (ii) Importance of numerical methods.
4 M
6 (a) (iii Heat Pipe.
4 M
6 (b) Draw the boiling curve and identify the different boiling regimes.
5 M
6 (c) A 15 mm diameter mild steel sphere (k=42 W/m °C) is exposed to coding airflow at 20°C resulting in the convective coefficient h=120 W/m2 °C.
Determine the following:
i) Time required to cool the sphere from 550°C to 90°C
ii) Instantaneous heat transfer rate 2 minutes after the start of cooling.
For mild steel take: ρ=7850 kg/m3, c=475 J/kg °C, α=0.045 m2/h.
7 M

More question papers from Heat Transfer