# HT 2009

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"HT 2009"

Code   No.: 4181
FACULTY OF ENGINEERING
B.E. III/IV Year (Mech.) II Semester      (Main) Examination,           April/May    2008
HEAT TRANSFER
Time:   3 Hours]                                                                  [Max. Marks:     75

Answer all questions of Part A.
Answer five questions from Part B.
Part A - (Marks:     25)

1. Derive a differential equation for heat flow through a pipe neglecting axial conduction.
2. A plane slab is maintained at temperatures Tl and T2 on either side. The thermal
conductivity of slab various according to K =Ko (1 + I3T). where Ko & 13 constants.
are
Derive an expression   for temperature   distribution     within the slab.
3.   Give three examples of heat generating       systems. A plane slab generates       heat
uniformly. Sketch the temperature distribution within the slab if surface temperature
on either side are same.

4. What is the physical significance of Biot number and what is the implication of Biot
No = O?

5. Explain why heat transfer rates are higher when the flow is turbulent              compared to
laminar, in case of flow over a flat plate.
6. State Buckingham 1ttheorem, and its importance in convection heat transfer.
7.   Differentiate between dropwise and filmwise condensation.          What are the conditions
under which dropwise condensation    would occur?
8.   Draw the boiling curve and mark various      regimes.     Why boilers are operated        in the
Nucleate boiling region?

9.   A heat exchanger    heats a fluid from 30°C to 50°C while hot fluid cools from 100°C
to 60°C.
Which of the following two arrangement      gives smaller     surface    area required?
(a) Parallel flow
(b) Counter flow.

10. State planks law of radiation & draw the Plank's distribution between emissive
power and wavelength of radiation at various temperature. How would you calculate

[P.T.O.
2                                                  4181

Part B - (Marks:             5 x 10   = 50)
11. A steam pipe (K = 45 W /m-K) has 70 mm ID and 85 mm OD. It is covered with
two layers of insulation, the inside being asbestos (K = 0.15 W/m-K) & of 35 mm
thick   and over insulation        is magnesia          25 mm thick    (K    = 0.075    W /m-K).    The heat
transfer    coefficients    for inside and outside being 220 W / m2 -oK and 6.5 W / m2 -oK
respectively. If the steam temperature of steam is 350°C and the ambient
temperature is 30° C; calculate the following:                                                                  10
(a) Heat loss from pipe for unit length
(b) Interface temperature.
12. Calculate the amount of energy required to solder together two very long pieces
of copper wires 1.625 mm diameter with solder that melts at 195°C. The
. surrounding   air temperature  is 24°C and heat transfer coefficient between the
wire and surrounding    being 17 W /m2-K. The thermal conductivity of copper is
335W/m-K.                                                                        10

13. An iron plate of thickness          5 cm is initially at 250° C. Suddenly                both surfaces     are
exposed     to a fluid at 50° C and         h   = 500    W /m2-K.    Determine                                   10
(a) Centre line temperature           of plate after 120 seconds
(b) Heat transferred to the fluid from one square                    meter surface        area of plate
during this time.
Properties     of Iron ore
K   = 60    W /m-K, Cp      = 460   J /kgO K, P = 7850 kg/m3, a               = 1.6   x 10- 5 m2/S

14. Air at 295° K flows over a flat plate at a velocity of 180 m/min. If the plate is
0.5 mx 0.25 m in size, fmd the average heat transfer coefficient and heat loss
per hour if air flow is parallel to 0.5 m side of plate.                                                        10

15. (a) Using electrical circuit analogy derive an expression                      for net radiation     transfer
between two infinitely long parallel plates.                                                                  5
(b) A 10 cm diameter          pipe carrying        steam at 400°C passes              through   a large room
at 27°C.                                                                                                       5
The emissing       of pipe E    = 0.8
Estimate     the heat loss from the pipe.

16. Using Nusselts theory of laminar                film condensation       derive an expression         for
Nusselt's number.                                                                                               10
3                                  4181

17. A surface condenser condenses 2S000 kg/hr steam at 3SoC. The cooling water
inlet and outlet temperatures    are lSoC and 2SoC respectively. The tubes used
for in condenser are 2 cm diameter and the velocity of water inside the tubes
is 2 m/S. Assume heat transfer coefficient on steam side = SOOO 1m2 -K. Heat
W
transfer coefficient on waterside = 4000 W1m2 K. Neglect resistance due to
tubewall. Determine                                                             10

(a) Total surface   area in m2 required
(b) Number of tubes required
(c) Length of each tube.

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