Total No. of Questions 10]
[Total No. of Pages 4
[5353] 112
T.E. (Mechanical)
HEAT TRANSFER
(End Semester)
(2012 Pattern)
Time :2½ Hours] [Max. Marks :70
Instructions to the candidates:
Solve Q.1 or Q.2. Q.3 or Q.4, Q5. or Q.6, Q.7 or Q.8, Q.9 or Q.10.
Draw Neat diagrams wherever necessary.
Use of scientific calculator is allowed.
Assume suitable data where ever necessary.
Figures to the right indicate full marks.
P3249
SEAT No.
P.T.O.
[5353] 112 2
OR
Q4) A cylindrical fin is 3mm in diameter and 3cm long. Calculate the value of
the temperature at fin tip if the fin is made of.
Copper 350 and
ii) Teflon 0.35
Assume the heat loss from fin tip is negligible.
Take h 10 w/m2k, Tbase 120°C.
Surrounding fluid temperature is 20°C
Steel balls of 12mm diameter are annealed by heating to 877°C and then
slowly cooling to 127°C in an environment where temperature is 52°C.
The heat transfer coefficient is 20w/m2k. Calculate the time required by
the balls to reach the desired temperature. Use following properties, for
steel. Density 7800 kg/m3, Cp 600 J/kg K 40 w/mk
Q5) Explain the following with their applicability.
Nusselt number
ii) Grashoff's Number
iii) Rayleigh number
iv) Prandtl number
Liquid mercury flows at a rate of 1.6 kg/sec through a copper tube of
20mm diameter. The mercury enters the tube at 15°C and leaves at 35°C.
Calculate the tube length if the tube wall temperature is 50°C. The
properties of mercury at 25°C are.
13582 kg/m3. Cβ 140 J/kg k 8.69 1.5×10-7m2/s
Pr 0.0248
Use Nu 7 0.025 (Re Pr) 0.8
OR
Q6) A rectangular plate of length 7 cm and width 4cm is maintained at 115°C.
It is exposed to still air at 25°C on both sides. Calculate convective heat
transfer rate if smaller side of the plate is held vertical compare heat
transfer when larger side is held vertical.
Use correlation Nu 0.59 (Gr. Pr) 0.25
For air, at 70°C k 0.03 w/mk Pr 0.697;
Kinematic viscosity 2.076 × 10-6 m2/s
[5353] 112 3
Explain the concept of thermal boundary layer.
Show with neat sketch direction of natural convection Fluid flow
(Development of thermal boundary layer) when
Plate is kept vertical and surrounding fluid temperature is higher
than plate.
ii) Cylinder is kept vertical and surrounding fluid temperature is lower
than cylinder.
Q7) Two large parallel planes and are maintained at temperature of
1500k and 600k respectively εA 0.9 εD 0.4 Two radiation shields
with emissivity 0.5 and with emissivity 0.2 are inserted in
between them such that C and D are placed one after the other.
Calculate.
Heat transfer rate without radiation shields
ii) Heat transfer rate with radiation shields.
iii) Temperature attained by planes and
State and explain any 4 properties/ rules of radiation shape factor.
OR
Q8) If the shape factor between two adjacent sides of rectangular room is
0.22, find the shape factor between opposite faces.
Define radiosity and irradiation.
Differentiate between filmwise and dropwise condensation.
ii) Design criteria for Heat exchanger
Q9) Derive the expression for effectiveness of parallel flow heat exchanger
by using NTU method using standard notations.
A counter flow shell and tube type heat exchanger is used to heat water
at a rate of 0.8 kg/sec from 30°C to 80°C with hot oil entering at 120°C
and leaving at 85°C. Calculate the size of heat exchanger required. Overall
heat transfer coefficient is 125 w/m2°C. Take specific heat for water as
4180 J/kg °C.
[5353] 112 4
OR
Q10)a) Hot air at 66°C is cooled up to 38°C by means of cold air entering at
15.5°C. Mass flow rates of hot and cold air are 1.25 kg/s and 1.6kg/s
respectively sp. heat of hot and cold air 1.05kJ/kg k U 80 w/m2k.
Find the area of the heat exchanger for parallel flow configuration.
If the same exchanger is operated in counter flow mode, find the exit
temperatures of both the fluids.
Explain 'Film boiling 'Phenomenon in pool boiling process and show
this region on the pool boiling curve.