Heat Transfer Question Paper

Heat Transfer

Course:Bachelor Of Science In Engineering

Institution: Kenyatta University question papers

Exam Year:2009

KENYATTA UNIVERSITY
UNIVERSITY EXAMINATIONS 2009/2010
FIRST SEMESTER EXAMINATION FOR THE DEGREE OF BACHELOR OF
SCIENCE ENERGY ENGINEERING

SET 331:
HEAT TRANSFER

DATE: Monday, 21st December, 2009

TIME: 8.00 a.m. – 10.00 a.m.
------------------------------------------------------------------------------------------------------------
INSTRUCTIONS:
This paper consists of FIVE questions. Answer any THREE questions.
Thermo physical property tables are provided.
QUESTION 1
a)
Explain the following as used in radiation heat transfer
i)
Emissive power and Emissivity
ii)
Kirchhoff’s law of Emissivity
iii)
Stefan – Boltzmann law
iv)
Absorptivity
(5 marks)
b)
i)
A body at 1300oC in black surroundings at 650oC has an emissivity of 0.4
at 1300oC and emissivity of 0.7 at 650oC. Calculate the rate of heat loss
by radiation per M2.
a)
When body is grey with ?= 4
.
0
b)
When body is not grey
(8 marks)
Page 1 of 5

ii)
Calculate the heat transferred per M2 of surface area by radiation between
two brick wall s a short distance apart. When the temperature of the
surfaces are 30oC and 15oC. The emissivity of the brick may be taken as
0.93 and the surfaces may be assumed grey.

(7 marks)

QUESTION 2
a)
A 5 cm diameter cylinder is maintained at a constant temperature of 200oC and it
is completely enclosed by a wind tunnel test chamber, wall of which are at 10oC.
Air at 350oK is forced across the cylinder at a velocity of 50m/s. If the surface
emissivity of the cylinder is 0.7, determine the total heat transfer from the
cylinder (that is convective and radiative heat transfer). Take the convective heat
transfer coefficient to be 180W/mk and show a sketch of the system.
(7 marks)
b)
Current is passed through a cylindrical laboratory heater which is 30 mm diameter
and generation heat at a rate of 20 MW/m3. The heater is exposed to convection
environment at 25oC where the convection heat transfer coefficient is 0.5kW/m2.

(5 marks)
c)
A steel pipe 100 mm bore and 7 mm wall thickness carrying steam at 260oC is
insulated with 40 mm of moulded high temperature diatomaceous earth covering.
This covering is insulated with 60 mm asbestos. If the atmospheric temperature is
15oC. Calculate the rate at which heat is lost by the steam per m length of pipe.
The heat transfer coefficient for inside and outside surfaces are 550 and 15 w/m2k
respectively and the thermal conductivities of steel, diatomaceous earth and
asbestos are 50, 0.09 and 0.07 W/mk respectively. Calculate also the temperature
of the outside surface. Show sketch of the cross-section.
(8 marks)

QUESTION 3
a)
With aid of sketches, discuss the following:
i)
Parallel heat exchanger
ii)
Counter flow heat exchanger
(4 marks)
Page 2 of 5

b)
Exhaust gases flowing through a tubular heat exchanger at a rate of 0.3kg/s are
cooled from 600oC to 105oC by water initially at 10oC. The specific heat of the
exhaust gases and water may be taken as 1.13 and 4.9 KJ/KgK respectively. The
overall heat transfer coefficient from gases to water is 140W/m2k. Calculate the
surface area required when the cooling water flow is 0.5 Kg/s for:
i)
Parallel flow
ii)
Counter flow
(8 marks)
c)
A single – pass shell and tube counter – flow heat exchanger uses waste gas on
the shell side to heat a liquid in the tubes. The waste gas enters at a temperature
of 400oC at a mass flow rate of 40Kg/s. The liquid enters at 100oC at a mass
flow rate of 3Kg/s.
Assuming the velocity of the liquid does not exceed 1m/s, calculate using the data
below:
i)
The required number of tubes
ii)
The effectiveness of the heat exchanger
iii)
The exit temperature of the liquid
Neglect thermal resistances of the tube wall
Data:
Tube inside diameter 10 mm
Tube outside diameter 12.7 mm
Tube length = 4 m
Specific heat of waste gas = 1.04 kJ/Kg K
Specific heat of the liquid = 1.5 kJ/Kg K
Density of the liquid = 500 kg/m3
Heat transfer coefficient of shell side = 260 W/m2k
Heat transfer coefficient of the tube side = 580 W/m2k

(8 marks)

Page 3 of 5

QUESTION 4
A composite wall shown in the figure has a surface temperature of wall A at 500oC. The
surface temperature of wall D is 100oC. The heat transfer coefficient from outside wall
is 20W/m2k. Assuming one dimensional heat flour, calculate the heat transmission per
unit area and the interface temperature.

(10 marks)

Figure

B

A

D

C

Material Thermal
Conductivity
Thickness
K (W/mK)
mm
A
75
200
B
60
250
C
58
250
D
20
400

b)
A small hemispherical oven is built of an inner layer of insulating five brick 175
mm thick and an outer covering of 60 mm thick insulation. The inner surface of
the oven is at 900oC and the heat transfer coefficient for the outside surface is
15W/M2K. The room temperature is 22oC. Calculate the heat loss through the
Page 4 of 5

hemisphere if the inside radius is 0.8 m. Take thermal conductivities of the fire
brick and outer covering insulation as 0.33 and 0.07 W/mK respectively.

(7 marks)
c)
i)
Explain two mechanics through which heat transfer by conduction takes
place
in
solids
(2
marks)

ii)
State Fooriers law of conduction

(1 mark)

QUESTION 5
a)
i)
Differentiate between Forced convection and natural convection giving
examples.
(2
marks)
ii)
Define
heat
exchanger
effectiveness
(1
mark)
b)
Calculate the heat transfer coefficient for water flowing through a 25 mm
diameter tube at a rate of 2.5 kg/s when the bulk temperature is 60oC. For
turbulent flow of a liquid take

Nu = 0.03 Re0.8 X Pr0.6
Where the characteristic dimension of length is the tube diameter and all
properties are evaluated at mean bulk
temperature.
(7
marks)
c)
Calculate the heat loss by natural convection per m length from a horizontal pipe
of 160 mm diameter, the surface of which is at 277oC. The room temperature is
17oC.
Take for horizontal pipe
Nu = 0.527 (Pr)0.5(Pr + 0.952)-0.25(Gr)0.25
Where the properties are evaluated at surface temperature. Take Grashof number
Gr = 15.1 x106

(5 marks)
d)
i)
Briefly outline the purpose of using a finned tube instead of a plain tube in
a heat exchanger.

ii)
Define the term fin efficiency.

(5 marks)
Page 5 of 5

Heat Transfer Question Paper

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