mec551 soalan
TRANSCRIPT
-
8/10/2019 MEC551 soalan
1/7
CONFIDENTIAL
EM/JAN 2012/MEC551
UNIVERSITI TEKNOLOG I MARA
FINAL EXAMINATION
COURSE
COURSE CODE
EXAMINATION
TIME
THERMAL ENGINEERING
MEC551
JANUARY 2012
3 HOURS
INSTRUCTIONS TO CAN DIDATES
1
2.
This question paper consists of two (2) parts : PART A (5 Questions)
PART B (3 Questions)
Answer ALL questions from PART A and two (2) questions from PART B in the Answer
Booklet. Start each answer on a new page.
3. Do not bring any material into the exam ination room unless permission is given by the
invigilator.
4.
Please check to make sure that this examination pack consists of :
i) the Que stion Paper
) an Ans wer Booklet - provided by the Faculty
i) a Property Tables Booklet - p rovided by the Faculty
DO NOT TURNT ISP GEUNTILYOU RE TOLD TO DO SO
This examination paper consists of
7
printed pages
Hak Cipta Universiti Teknologi MARA CONFIDENTIAL
-
8/10/2019 MEC551 soalan
2/7
CONFIDENTIAL
2
EM/JAN 2012/MEC551
PART A
QUESTION 1
w
A flat plate of thickness 0.8 cm , area o f
A
- ISO cm
2
a n c
j thermal conductivity m is
subjected to constant heating at a rate of Q=
8 0 0
W
a
t its bottom surface. When steady
state operation is reached the upper surface temperatures of the plate is measured to be
80 C.
x
A = 1 5 0 cm
2
T = 80 C
T
1 A ' A
T
0.8 cm
For steady one dime nsional conduction heat transfe rthroug h the plate,
a) calculate the heat flux q
Q
(W/m
2
) at the lower surface,
b) express the differential equation and the boundary conditions for steady one
dimensional heat conduction through the plate, and
c) obtain the variation of temperature on the plate and calculate the temperature at the
bottom of the surface.
(10 marks)
QUESTION 2
a) W hat is the physical significance of the Nusselt Num ber? How it is defined?
(3 marks)
b) An engine oil at 80 C flows over a 10-m long flat plate who se temp erature is 30 C
with a velocity of 2.5m/s. Determine the rate of heat transfer over the entire plate per
unit width. (kW /m)
Given : Prop erties of the eng ine oil at T
f
= 55C :
density., p = 867kg/m
3
kinem atic viscosity, = 7.045X1Q
-5
m
2
s
conductivity,k = 0.1414W/mC
Prandlt number, P r= 1SS1
Hak Cipta Universiti Teknologi MARA
(7 marks)
CONFIDENTIAL
-
8/10/2019 MEC551 soalan
3/7
CONFIDENTIAL
3
EM/JAN 2012/MEC551
QUESTION
3
a) Sketch and define the tem perature profiles for parallel flow and counter flow in a
double-pipe heat exchanger.
(4 m arks)
b) 0.8 kg/s of wa ter is heated from 35 to 90 C with oil flow of 0.9 kg/s in a counter-flow
heat exchanger. The specific heat of oil is 2.0 kJ/kg.K. The oil enters the heat
exchang er at a temp erature of 180 C and leaves at 110C. The O verall heat transfer
coefficient is 450 W/m
2
.K. Calculate the area of heat exchanger using LMTD method.
Given: Specific heat of water,
Cw
= 4180 J/kg.K.
(6 marks)
QUESTION 4
An ideal vapor compression cycle uses R134a as a refrigerant and has a flow rate of
120 kg/min. The pressure in the evaporator of the refrigerator is 0.14 Mpa and the pressure
in the condenser is 0.8 Mpa. Draw a T-s diagram and if the isentropic efficiency of the
compressor is 100 percent, find:
a) the refrigerating effect (kW ),
b) the work input (kW), and
c) the COPret of the system .
(10 marks)
Hak Cipta Universiti Teknologi MARA
CONFIDENTIAL
-
8/10/2019 MEC551 soalan
4/7
CONFIDENTIAL 4
EM/JAN 2012/MEC551
QUESTION 5
Liquid propane (C
3
H
8
) at 25 C is burned in a space he ater at a rate of 0.05 kg /min with the
stoichiometric amount of air. The air enters this space heater steadily at 1 atm from the
comp ressor at 22 7 C. The c omb ustion products leave at 1 atm and 72 7 C but only 98
percent of the carbon burns to C0
2
and the remaining 2 % burns to form CO but all the
hydrogen in the fuels bum to H
2
0 .
a) W rite a balanced com bustion equation.
b) Determine:
i) the air fue l ratio (kg air/kg fue l), and
ii) the ma ss flow rate of air (kg/min).
What are the main factors contribute to the incomplete combustion process? Which
substance is the m ain product of incomplete com bustion, CO or OH ? , and explain.
(10 marks)
Hak Cipta Universiti Teknologi MARA
CONFIDENTIAL
-
8/10/2019 MEC551 soalan
5/7
CONFIDENTIAL
5
EM/JAN 2012/MEC551
PA R T B
QUESTION 1
Figure Q1 il lustrates a combustion system using propane as fuel and supplied with 40 % of
excess air to generate energy for a feed water heating. Both reactants are assumed to be at
the standard reference state. Due to non-uniform mixing, 7 % of the carbon content of the
fuel reacts poorly with oxygen and forms carbon m onoxide.
The Feed water heating is achieved by using a single tube, 2-shell and 4-pass heat
exchanger. T he feed w ater supply is at 50 C and 2 8 kg/min feed rate, and exits in a heated
state of 250C. The hot gas from the combustion chamber reaches the heat exchanger at
700 C, slightly less than the combustion product temperature due to internal cooling
mechanisms, and exits at 300 C. The tube is thin-walled, 12 cm in diameter, with a heat
transfer coefficient of 300 W/m
2
.K. At the shell side, the heat transfer coefficient is 60
W /m
2
.K.
For this particular system:
a) Calculate the fue l feed rate in (L/min) if the fuel density is 0.5 kg/liter,
b) Determine the thermal power of the combustion process (MW ), and
c) If the specific heat at constant pressure of the feed wa ter is 49 00 J/kg.K, determine the
minimum length of the heat exchanger based on the length of the tube (m).
Air
25C & atm
5 kg/min
O
Combustion system
Flue Gas
300C
Burner
Combustion
products
827C
Heat
Exchanger
700C
6
Propane
25C &
atm
FW exit 250C FW inlet 50C
Figure Q1
(25 marks)
Hak C ipta Universiti Teknologi MARA
CONFIDENTIAL
-
8/10/2019 MEC551 soalan
6/7
CONFIDENTIAL 6 EM/JAN 2012/MEC551
QUESTION 2
In a manufacturing process, a transparent tinted is being bonded to a composite substrate
as shown in Figure Q2. To cure the bond at temperature T
u
the respective substrate has
been heated, which give the uniform temperature of
T
=
79 C w hile the free surface of the
tinted (T
s
= 4 0 C) is exp osed to air at
7,
and a co nvection heat transfer coefficient /?.For the
calculated
T
fl
|
m
,
the properties of the air at atm are as follows;
DENSITY
p[kg/m
3
]
1.092
THERMAL
CONDUCTIVITY
K[W/(m.C)]
0.02735
DYNAMIC
VICOSITY
u[kg/(m.s)]
1.963X10
5
KINEMATIC
VISCOSITY u
[m
2
/s]
1.798X10
5
PRANDTL
NUMBER
Pr
0.7228
Under condition of steady state and one dimensional heat transfer, calculate:
a) the rate of heat transfer, Q (W),
b) the interface tem pera ture,
T
(C),
c) the air tem perature , T . (C),
d) if the flow is assum ed to be comp letely turbulent over the entire plate, calculate the
approximate total heat transfer (W), and
e) give your prediction for the rate of heat transfer if the velocity of the air is increased
about 45 %.
V.
=
m/s
^ :
0.5m
1.2m
y
p r-
t
jr.
___
jc%TOtfreVrr
B
/>
2 1m 7 1 1m
To
Figure Q2
> -
Given:
Nu = 0.664Re
5
Pr
1/3
(for laminar flow)
Nu = 0.037Re
08
Pr
1/3
(for turbulent flow)
Nu = [0.037Re
08
-871] Pr
1/3
(for mixed flow)
K
A
=27 1W/m.c
K
B
=
11 5
W/m.c
K
ti
nted=0 4W/m.c
(25 m arks)
Hak Cipta Universiti Teknologi MARA
CONFIDENTIAL
-
8/10/2019 MEC551 soalan
7/7
CONFIDENTIAL 7
EM/JAN 2012/MEC551
QUESTION 3
An autom otive air-cond itioner is used to treat air cabin at 1 atm , 30 C, and 80 percent
relative humidity at a rate of 10 m
3
/min, by passing the air through the cooling system which
consist of cooling co ils. The cooling process produces water in the condensate at 14 C. The
air leaves the cooling coils and re-enters the cabin as saturated condition. The cooling
system uses R-134a as the working fluid and applies an ideal vapour-compression
refrigeration cycle, operates between saturated pressure of 1.4 bar and 8 bar. The
refrigerant enters the compressor in saturated phase at a rate of 16.9 kg/s and it is
isentropically comp ressed.
For the automotive air-conditioner system described above, sketch the schematic of the
complete system and calculate:
a) the cooling load of the cooling coil (kW ),
b) the exit temp erature of the air cabin (C),
c) the rate of condensate removal (kg /s), and
d) the required work input to the system (kW).
(25 marks)
END OF QUESTION PAPER
Hak C ipta Universiti Teknologi MARA
CONFIDENTIAL