a. A horizontat
29.
plate of 800 mm long, 70 mm wide is maintained at atemperature of 140"C in a
large tank of full of water at 60oC determine total hat loss from plate.
Reg. No.
(oR)
B.Tech. DEGREE EXAMINATION, NOVEMBER 2017
Third/Fourth/Fifttr Semester
b. Air
at 30oC flows over a flat plate at a velocity of 4 m/s. Ptate is maintained at 90oC. The
plate dimension is 90x30 cm2. Calculate the heat transfer for the fotlowing conditions
First half of the plate
For tulIplate
Next half of plate
(i)
(ii)
(iiD
15ME3O2 _ HEAT AND MASS TRANSFER
(For the candidates admitted during the academic year 2015 2016 onwards)
(Use of Heat and Mass Transfer data book permitted)
Note:
a. Two large parallel
plates of size 1 m by I m spaced 0.5 m apart are located in a very large
room. The walls of which are maintained at a temperature of 27oC. One plate is maintained
at a temperature of 900'C and other at 400oC. The emissivities of plates we 0.2 and 0.5
respectively. If these plates exchange heat between themselves and surroundings, find the net
heat transfer to each plate and to the room. Consider only the plate surfaces facing each
other.
30.
(1)
Part - A should be answered in OMR sheet within first 45 minutes and OMR
(ii)
over to hall invigilator at the end of 45n minute.
Part - B and Part - C should be answered in answer booklet.
Time: Three Hours
Max. Marks: 100
PART-A(20x1=20Marks)
b- Emissivities of two
1. Heat is mainly transferred by conduction, convection and radiation in
large parallel plates maintained at temperature of 800oC and 300'C are
0.3 and 0.5 respectively. Find
Net radiant heat exchange per square meter of these plates
Find the percentage of reduction in heat transfer when a polished aluminium shield
of emissitvity of 0.05 is placed between
Temperature of the shield.
(A) Insulated pipes carrying hot water (B)
(D)
Refrigerator freezer coil
Condensation of steam in a condenser
(B)
Added insulation will decrease heat loss
(C)
(D)
Heat flux will decrease
(A) WmK
(B) wK
(D) Wim
(C)
them
(iii)
ALL Questions
Answer
(oR)
(i)
(ii)
sheet should be handed
Boiler furnace
2. Upto critical radius of insulation
(A)
Added insulation will increase heat
loss
3
a. Discuss in detail
1.
the various regimes of pool boiling heat transfer.
b. 4 counter flow heat exchanger
3.
(oR)
employed to cool 0.55 kgls
(, o :2.45 kI t tcg X)
of oil from
(c) Kiw
115"C to 40'C by circulating water. The inlet and outlet temperature of water and 15"C and
75oC. The overall heat transfer coefficient is 1450 Wm2K, using NTU method, determine
Mass flow rate of water
Effectiveness of heat exchanger
Surface area
4. An increase in convective coefficient
(i)
(ii)
(iii)
(A) Result in higher effectiveness
5.
32.a.i. Explain the various mechanisms of mass transfer.
6.
(oR)
Water in the tark2D mm deep is filled with water to 10 mm level and exposed to dry air at
40o C. calculate the time required for all the water to evaporate. What is the time required for
evaporation if the air temperature is 80'C? Take diameter of tank as 2 meter. Take D (water
7.
to air'10.0925m2 I hr.
over a fin
will
(B) Result in lower effectiveness
(D) Influence only the fin efficiency
(C)
Not affect effectiveness
(A)
number is generally associated with natural convection heat transfer.
Prandtl
(B) Weber
Nusselt
(D) Grashoff
(C)
ii. State Fick's law of difflrsion. What are its limitations?
b.
Convection heat loss will be less
than conduction heat loss
The SI unit of thermal conductivity is
The prandtl number if liquid metals are usually
(A) Less than 0.5
(B) Greater than 0.5
(C) Near about 1
(D) Decreasing with temperature
The velocity profile for fully developed laminar flow in a tube is
Linear
(B) Exponential
Hyperbolic
(D) Parabolic
(A)
(C)
{.*:F**
8.
Page 4 of 4
21NF3/4/515M8302
Page
I of4
Heat transfer rate will
(A) Be higher in turbulent flow
(C) Depend only on the fluid
(B)
(D)
Be lower in turbulent flow
Depend only on fluid viscosity
21NF3/4/515ME302
9. Emissivrff of a white polished
(A) Higher
(C) Same
body in comparison to a black body is
(B) Lower
(D) Depends upon the shape of the body
10. A grey body is one whose absorptivity
(A) Varies with temPerature
(C) Varies with both
20. Fict's law of diffirsionMa:
(A)
(C)
(B)
-Dob,.dco
dx
(D)
-Dab.dCo
Dob.dCo
dx
Dab.dx
(B) Varies with the wavelength of incident ray
(D) Does not vary with temPerature and
wavelength is the incident raY
PART-B(5x4=20Marks)
and 0.08 respectively.
11. For a glass plate, transmissivity and reflectivity are specified as 0.86
The absorptivity of the glass plate is
(B) 0.08
(A) 0.86
(D) 0.06
1.0
(c)
:
at a temperature of 400 K and 300 K (emissivity
t2. If two surfaces are maintained
radiative heat transfer per unit area will be
(A)
(c)
eez
464
Answer AI{Y
Questions
21. Derive an expression for critical radius of insulation.
22.
0.9
FM
A solid steel ball 50 mm diameter is initially at 450"C is quenched in a controlled
environment with convective coefficient of 115 Wm2K. Determine the time taken to reach a
temperature of 150"C atthe centre of the ball. Take Cp=420J I kgK, density:8000 kdm'
), the
Wm2, o =5.67x10-8 W I m2Ka .
(B) 8r2
(D) s67
and thermal
conductivitY:46 WmK.
23. Give the physical significance of Reynolds number and Grashoff number'
mean temperature difference should be
13. In heat exchanger the value of logarithmic
-
(A) Mr-t*r(C) Zero
possible
24. A steam pipe of 0.1 m diameter runs horizontally in a room at23oC. Take the outside surface
oC, determine the heat loss per meter lenglh of the pipe.
temperaturl of the pipe as 165
(B) Minimum Possible
(D) Constant
25. Explain intensity of radiation and Wein's displacement law'
14. In shell and tube heat exchanger, baffles are mainly used to
(B) Increase the heat transfer rate
(A) Increase the mixing of fluid
iCi Deflect the flow in the desired (D) Reduce fouling of the tube surface
26. 1000 kg/hr of
(D)
17. The unit of
mass
(A) *2
27.
(C)
,2
(D)
LTo.s
2g.
Pressure gradient
Velocity gradient
19. Sherwood number is a function of
(A)
(C)
Page2
of4
Re, Sc
Re, Gr
layer
composite slab is made of three layers 15 cm, 10 cm and 12 cm thickness. The first
with
is made of material with R =1.45 W I mK for 600/o of the area and rest of the material
R=2.5 W I mK. The second layer is made up of material with ,R =1.25 W I mK for 50% of
material
area and rest of the material with R = 18.5 W I mK. The third layer is made of single
of R = 0.76W I mK. The composite slab is exposed to warm air to 26"C and cold air at
The outside heat transfer
-20"C. The inside heat transfer coefficient is 15 Wm2K.
temperature.
coefficient is 20 Wm2K. Determine heat flow rate and interface
kss
2
(oR)
(B) Temperaturegradient
(D) Concentration gradient
(B)
(D)
ALL Questions
a. A
18. Mass transfer in fluid takes place when there exist a
(A)
(C)
of
Explain mass concentration and molar concentration'
Answer
s
m
m
the type of flow and area
PART-C(5x12=60Marks)
diffusion coefficient or diffusivity is
(B) kg*2
,s
I kg"C)is to be cooled from 80oC to 40"C flow of 1000
heat exchanger.
16. In nucleate pool boiling the heat flux is proportional to the excess temperature as
(B) arl.o
(A) ar3.o
LT2l
tcl
kg/h at 30"c. if the heat transfer coefficient is 24 w/m2oc, find
direction
15. The excess temperature in free convection boiling is
(B) 1Ooc
(A) 5"C
(D) 5o'c
(c) 3ooc
(C)
oil (rr=2.0g
of 50 mm diameter and one meter long provided with 12 longitudinal straight frns
protrude
(k =75.6W I mK) is placed in atmosphere at 40"C. the fins are 0.8 mm thick and
the hns
25 mm from the cylinder surface. The heat transfer coefficient from the cylinder and
rate
to the atmosphere air is 23.25 Wm2K. The surface temperature is 150oC, calculate the
of heat transfer and the temperature at the tip of the fin'
b. A cylinder
Re,Pr
Re, Ga
21NF3/4/515ME302
Page 3
of4
2lNF3/4/515ME302