10-2 N.S/M2 AND THERMAL CONDUCTIVITY = 8.540 W/MK. THE PRANDTL NUMBER...
10
-2
N.s/m
2
and thermal conductivity = 8.540 W/mK. The Prandtl number
of the mercury at 300 K is:
[GATE-2002]
(a) 0.0248
(b) 2.48
(c) 24.8
(d) 248
GATE-3. The average heat transfer coefficient on a thin hot vertical plate
suspended in still air can be determined from observations of the
change in plate temperature with time as it cools. Assume the plate
temperature to be uniform at any instant of time and radiation heat
exchange with the surroundings negligible. The ambient temperature
is 25°C, the plate has a total surface area of 0.1 m
2
and a mass of 4 kg.
The specific heat of the plate material is 2.5 kJ/kgK. The convective
heat transfer coefficient in W/m
2
K, at the instant when the plate
temperature is 225°C and the change in plate temperature with time
dT/dt = – 0.02 K/s, is:
[GATE-2007]
(a) 200
(b) 20
(c) 15
(d) 10
Data for Q4–Q5 are given below. Solve the problems and choose
correct answers.
Heat is being transferred by convection
from water at 48°C to a glass plate whose
surface that is exposed to the water is at
40°C. The thermal conductivity of water is
0.6 W/mK and the thermal conductivity of
glass is 1.2 W/mK. The spatial Water
gradient of temperature in the water at the
water-glass interface is dT/dy =1 × 10
4
K/m.
[GATE-2003]
Page 41 of 97
GATE-4. The value of the temperature gradient in the glass at the water-glass
interface in k/m is:
(a) – 2 × 10
4
(b) 0.0
(c) 0.5 × 10
4
(d) 2 × 10
4
GATE-5. The heat transfer coefficient h in W/m
2
K is:
(a) 0.0
(b) 4.8
(c) 6
(d) 750
GATE-6. If velocity of water inside a smooth tube is doubled, then turbulent
flow heat transfer coefficient between the water and the tube will:
(a)
Remain
unchanged
[GATE-1999]
(b) Increase to double its value
(c) Increase but will not reach double its value
(d) Increase to more than double its value
Previous 20-Years IES QuestionsIES-1.
A sphere, a cube and a thin circular plate, all made of same material
and having same mass are initially heated to a temperature of 250
o
C
and then left in air at room temperature for cooling. Then, which one of
the following is correct?
[IES-2008]
(a) All will be cooled at the same rate
(b) Circular plate will be cooled at lowest rate
(c) Sphere will be cooled faster
(d) Cube will be cooled faster than sphere but slower than circular plate
IES-2.
A thin flat plate 2 m by 2 m is hanging freely in air. The temperature of
the surroundings is 25°C. Solar radiation is falling on one side of the
rate at the rate of 500 W/m
2
. The temperature of the plate will remain
constant at 30°C, if the convective heat transfer coefficient (in W/m
2
°C)
is:
[IES-1993]
(a) 25
(b) 50
(c) 100
(d) 200
IES-3.
Air at 20°C blows over a hot plate of 50 × 60 cm made of carbon steel
maintained at 220°C. The convective heat transfer coefficient is 25
W/m
2
K. What will be the heat loss from the plate?
[IES-2009]
(a) 1500W
(b) 2500 W
(c) 3000 W
(d) 4000 W
IES-4. For calculation of heat transfer by natural convection from a
horizontal cylinder, what is the characteristic length in Grashof
Number? [IES-2007]
(a) Diameter of the cylinder
(b) Length of the cylinder
(c) Circumference of the base of the cylinder
(d) Half the circumference of the base of the cylinder
IES-5.
Assertion (A): For the similar conditions the values of convection heat
transfer coefficients are more in forced convection than in free
convection. [IES-2009]
Page 42 of 97
Reason (R): In case of forced convection system the movement of fluid
is by means of external agency.
(a) Both A and R are individually true and R is the correct explanation of A
(b) Both A and R individually true but R in not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true
IES-6.
Assertion (A): A slab of finite thickness heated on one side and held
horizontal will lose more heat per unit time to the cooler air if the hot
surface faces upwards when compared with the case where the hot
surface faces downwards.
[IES-1996]
Reason (R): When the hot surface faces upwards, convection takes
place easily whereas when the hot surface faces downwards, heat
transfer is mainly by conduction through air.
(b) Both A and R are individually true but R is not the correct explanation of A
IES-7.
For the fully developed laminar flow and heat transfer in a uniformly
heated long circular tube, if the flow velocity is doubled and the tube
diameter is halved, the heat transfer coefficient will be:
[IES-2000]
(a) Double of the original value
(b) Half of the original value
(c) Same as before
(d) Four times of the original value
IES-8.
Assertion (A): According to Reynolds analogy for Prandtl number equal
to unity, Stanton number is equal to one half of the friction factor.
Reason (R): If thermal diffusivity is equal to kinematic viscosity, the
velocity and the temperature distribution in the flow will be the same.
(c) A is true but R is false
[IES-2001]
IES-9.
The Nusselt number is related to Reynolds number in laminar and
turbulent flows respectively as
[IES-2000]
(a)
Re
-1/2
and Re
0.8
(b) Re
1/2
and Re
0.8
(c) Re
-1/2
and Re
-0.8
(d) Re
1/2
and Re
-0.8
IES-10. In respect of free convection over a vertical flat plate the Nusselt
number varies with Grashof number 'Gr' as
[IES-2000]
(a)
Gr
and
Gr
1/4
for laminar and turbulent flows respectively
(b)
Gr
1/2
and Gr
1/3
for laminar and turbulent flows respectively
(c)
Gr
1/4
and Gr
1/3
for laminar and turbulent flows respectively
(d)
Gr
1/3
and Gr
1/4
for laminar and turbulent flows respectively
IES-11. Heat is lost from a 100 mm diameter steam pipe placed horizontally in
ambient at 30°C. If the Nusselt number is 25 and thermal conductivity
of air is 0.03 W/mK, then the heat transfer co-efficient will be: [IES-1999]
(a) 7.5 W/m
2
K
(b) 16.2 W/m
2
K
(c) 25.2 W/m
2
K
(d) 30 W/m
2
K
IES-12. Match
List-I
(Non-dimensional
number) with List-II (Application) and
select the correct answer using the code given below the lists:
List-I
List-II
[IES
2007]
Page 43 of 97
A. Grashof number
1. Mass transfer
B. Stanton number
2. Unsteady state heat conduction
C. Sherwood number
3. Free convection
D. Fourier number
4. Forced convection
Codes:
A B C D A B C D
(a)
4 3 1 2 (b)
3 4 1 2
(c) 4 3 2 1 (d)
3 4 2 1
IES-13. Match List-I (Type of heat transfer) with List-II (Governing
dimensionless parameter) and select the correct answer:
[IES-2002]
List-I
List-II
A. Forced convection
1. Reynolds, Grashof and Prandtl
number
B. Natural convection
2. Reynolds and Prandtl number
C. Combined free and forced convection
3. Fourier modulus and Biot number
D. Unsteady conduction with
4. Prandtl number and Grashof
convection at surface
number
Codes:
A
B
C
D
A
B
C
D
(a)
2
1
4
3
(b)
3
4
1
2
(c)
2
4
1
3
(d)
3
1
4
2
IES-14. Match List-I (Phenomenon) with List-II (Associated dimensionless
parameter) and select the correct answer using the code given below
the lists:
[IES-2006]
A. Transient conduction
1. Reynolds number
B. Forced convection
2. Grashoff number
C. Mass transfer
3. Biot number
D. Natural convection
4. Mach number