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Explain briefly: i) Thermal conductivity ii) Thermal diffrrsivity iii) Thermal contact

resistance. (06 Mar\<s)

The walls of a house in cold region consist of three layers, an outer brick'work 15 cm thiik,an inner wooden panel 1.2 cmthick, the intermediate layer is made of an insulating material

7 cm thick. The thermal conductivity of brick and wood are A.7 Wink and 0.18 Wlmk

respectively. The inside and outside temperatures of the composite wall are 21"C and -15"C

respectively. If the layer of insulation offers twice the thermal resistance of the brick wall,calculate,i) Heat loss per unit area of the wall.ii) Thermal conductivity of insulating material.

Sixth Semester B.E. Degree Examination, December 2010

Heat and Mass TransferMax- Marks:100

Note: l. Answer any FIVE full queslions, selecting at leasttwo questions from each Part.

2. Use of heat transfer data hand book is permitted.

PART _ A

(06 Marks)

An insulated stearn pipe having outside dihmeter of 30 mm is to be covered-.with two layers

of insulationo each having a thickness of 20 mm. The thermal conductivity of onematefial is

3 times that of the other. Assuming that the inner and outer surface temperatures ofcomposite insulation are fixed, how much heat transfer will be increasbd when the better

insulation material is next to the pipe than when it is at the outer layer? (08 Marks)

Define fin efficiency and fin effectiveness with respect to a fin with insulated tip. (04 Marks)

What is the physical significance of critical thickness of insulation? Derive an expression forcritical thickness of insulation for a sphere. (06 Marks)

The handle of a ladle used for pouring molten metal at 327"C is 30 cm long and is made of2.5 cm x 1.5 cm mild steel bar stock (K: 43 W/mK). In order to reduce the grip temperature

it is proposed to make a hollow handle of mild steel plate of 0.15 cm thick to the same

rectangular shape. If the surface heat transfer coeffrcient is 14.5 Wlm'zrc and the ambient

temperaflre is at 27'C, estimate the reduction in the temperature of grip. Neglect the heat

transfer from the inner surface of the hollow shape. (10 Marks)

3 a. Obtain an expression for instantaneous heat kansfer and total heat transfer for lumped heat

b.

c.

analysis treatment of heat conduction problems.Explain the physical significance of Biot number and Fourier number.

(08 Marks)(04 Marks)

An aluminium sphere weighing 5.5 kg and initially at a temperature of 290"C is suddenly

immersed in a fluid at 15"C. The convective heat transfer coefficient is 58 Wlm2K. Estimate

the time required to cool the aluminium to 95"C using the lumped capacity method ofanalysis (For aluminium, p:2700 kg/*', C : 900 JlkgK, K: 205 W/mK) (08 Marks)

4 a. What do you mean by hydrodynamic and thermal boundary layer? How does the ratio $or

vary with prandtl number? (06 Marks)b. Using Buckingham's ,r-theorem, obtain the relationship between various non-dimensional

numbers for free convection heat transfer. (08 Marks)c. Air at 20"C flows over a thin plate with a velocity of 3 m/sec. The plate is 2 m long and 1 m

wide. Estimate the boundary layer thickness at the trailing edge of the plate and the totaldrag force experienced by the plate.

I of2

(06 Marks)

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6a.

b.

c.

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PART - B

Water at 25oC flows through a tube of 50 mm diameter. Determine the flow rate that will

result in a Reynolds number of 1600. The tube is provided with a nichrome heating element

on its surface and receives a constant heat flux of 800 Wm length of the tube. Determine the

average heat transfer coefficient between the water and the tube wall, assuming fully

develiped conditions. Also determine the length of the tube for the bulk temperature of

water to rise from 25oC to 50oC' (12 Marks)

Air stream atZT,C moving at 0.3 m/sec across 100 w incandescent bulb glowing at 127"C.

If the bulb is approximated by a 60 mm diameter sphere, estimate the heat transfer rate and

the percentage of power lost due to convection. Use correlation Nu : 0'37 Rli ' (08 Marks)

Define effectiveness and NTU of a heat exchanger. Explain why minimum heat capacity

value is used in the definition of effectiveness for the maximum possible rate of heat

transfer. (04 Marks)

Derive an expression for LMTD in case of parallel flow heat exchanger stating the

assumptions made. (08 Marks)

A counter flow heat exchanger is employed to cool 0.55 kg/sec (Cp : 2.45kJ/kgK) of oil

from il5oC to 40"C by the use of water. The inlet and outlettemperature of cooling water

are 15oC and 75oC respectively. The overall heat transfer coefficient is expected to be

1450 Wm2oC. Using NTU method, calculate the following:i) The mass flow rate of water.ii) The effectiveness ofheat exchanger.

iii) The surface area required'

Explain :

i) Filmwise condensation and dropwise condensation.

ii) Subcooled boiling and saturated boiling. (06 Marks)

A square array of 400 tubes 15 mm outer diameter is used to condense steam at atmospheric

pr.rr*". The tube walls are maintained at 88oC by a coolent flowing through the tubes.

Calculate the amount of steam condensed per hour per unit length of the tubes' (08 Marks)

ti

b.

la.

b.

c.

8a.

b.

State and explain Fick's law of diffusion.

For a black body enclosed in a hemispherical

body is rc time the intensity of radiation.State and explain:i) Kirchoffls law.ii) Planck's law.iii) Wein's displacement law.iv) Lambert's cosine law.Explain briefly the concept of a black body.

(08 Marks)

(06 Marks)

space show thit emissive power of the black(08 Marks)

(08 Marks)(04 Marks)c.

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