Oil
at 120⁰C
is used to heat water at 30⁰C
in a 1-1 co-current shell and tube heat exchanger. The available heat exchange
area is S1. The exit temperatures of the oil and the water streams
are 90⁰C
and 60⁰C
respectively. The co-current heat exchanger is replaced by a 1-1
counter-current heat exchanger having heat exchange area S2. If the
exit temperatures and the overall heat transfer coefficients are same, the
ratio of S1 and S2 is
(A)
∞
(B)
1.1
(C)
0.91
(D)
0
GATE
2011
Answer: (B)
In
film type condensation over a vertical tube, local heat transfer coefficient is
(A)
Inversely proportional to local film
thickness
(B)
Directly proportional to local film
thickness
(C)
Equal to local film thickness
(D)
Independent of local film thickness
GATE
2011
Answer: (A)
Consider
two black bodies with surfaces S1 (area = 1 m2) and S2
(area = 4 m2). They exchange heat only by radiation. 40% of the
energy emitted by S1 is received by S2. The fraction of
energy emitted by S2 that is received by S1 is
(A)
0.05
(B)
0.1
(C)
0.4
(D)
0.6
GATE
2011
Answer: (B)
In
a counter-flow double pipe heat exchanger, oil (ṁ = 2 kg/s, Cp = 2.1 kJ/kg°C)
is cooled from 90⁰C
to 40⁰C
by water (ṁ = 1 kg/s, Cp = 4.2 kJ/kg°C) which enters the inner tube
at 10⁰C.
The radius of the inner tube is 3 cm and its length is 5 m. Neglecting the wall
resistance, the overall heat transfer coefficient based on the inner radius in
(kW/m2.K) is
(A)
0.743
(B)
7.43
(C)
74.3
(D)
2475
GATE
2012
Answer: (A)
Heat generated
at a steady rate of 100 W due to resistance heating in a long wire (length = 5
m, diameter = 2 mm). This wire is wrapped with an insulation of thickness 1 mm
that has a thermal conductivity of 0.1 W/m K. The insulated wire is exposed to
air at 30⁰C.
The convective heat transfer between the wire and surrounding air is
characterized by a heat transfer coefficient of 10 W/m2.K. The
temperature in °C at the interface the wire and the insulation is
(A)
211.2
(B)
242.1
(C)
311.2
(D)
484.2
GATE
2012
Answer: (C)
For the
enclosure formed between two concentric spheres as shown below (R2 =
2R1), the fraction of radiation leaving the surface area A2
that strikes itself is
GATE 2012
Answer: (D)
The
one-dimensional unsteady state heat conduction equation in a hollow cylinder
with a constant heat source q is
If A and B are arbitrary constants,
then the steady state solution to the above equation is
GATE
2012
Answer: (B)
If the Nusselt
number (Nu) for heat transfer in a pipe varies with Reynolds number (Re) as
Nu∞Re0.8, then for constant average velocity in the pipe, the heat
transfer coefficient varies with the pipe diameter D as
(A)
D-1.8
(B)
D-0.2
(C)
D0.2
(D)
D1.8
GATE
2012
Answer: (B)
A solid sphere
with an initial temperature Ti is immersed in a large thermal
reservoir of temperature T0. The sphere reaches a steady temperature
after a certain time t1. If the radius of the sphere is doubled, the
time required to reach steady state will be
(A)
t1/4
(B)
t1/2
(C)
2t1
(D)
4t1
GATE
2012
Answer: (D)
For heat
transfer across a solid fluid interface, which one of the following statements
is not true when the Biot number is very small compared to 1?
(A)
Condition resistance in the solid is
very small compared to convection resistance in the fluid
(B)
Temperature profile within the solid is
nearly uniform
(C)
Temperature drop in the fluid is
significant
(D)
Temperature drop in the solid is
significant
GATE
2012
Answer: (D)
In
a double pipe counter-current heat exchanger, the temperature profiles shown in
the figure were observed. During operation, due to fouling inside the pipe, the
heat transfer rate reduces to half of the original value. Assuming that the
flow rates and the physical properties of the fluids do not change, the LMTD
(in ˚C) in the new situation is
(A)
0
(B)
20
(C)
40
(D)
indeterminate
GATE
2013
Answer: (C)
A
hole of area 1 cm2 is opened on the surface of a large spherical
cavity whose inside temperature is maintained at 727°C. The value of
Stefan-Boltzmann constant is 5.67×10-8 W/m2 K4.
Assuming black body radiation, the rate at which the energy is emitted (in W)
by the cavity through the hole, up to 3 digits after the decimal point, is
_____________.
GATE
2013
Answer: 5.67 watt
Calculate
the heat required (in kJ, up to 1 digit after the decimal point) to raise the
temperature of 1 mole of a solid material from 100˚C to 1000˚C. The specific
heat (Cp) of the material (in J/mol-K) is expressed as Cp
= 20 + 0.005T, where T is in k. Assume no phase change. _________.
GATE
2013
Answer: 21.7 KJ
In
a pool boiling experiment, the following phenomena were observed
P. Natural convection
Q. Film boiling
R. Transition boiling
S. Nucleate boiling
What was the correct
sequence of their occurance?
(A)
P, Q, R, S
(B)
S, R, Q, P
(C)
Q, R, P, S
(D)
P, S, R, Q
GATE
2013
Answer: (D)
The
effectiveness of a heat exchanger in the ε-NTU method is defined as
GATE
2013
Answer: (D)
The
bottom face of a horizontal slab of thickness 6 mm is maintained at 300°C. The
top face is exposed to a flowing gas at 30°C. The thermal conductivity of the
slab is 1.5 W m-1 K-1 and the convective heat transfer
coefficient is 30 W m-2 K-1. At steady state, the
temperature (in °C) of the top face is _____________.
GATE
2014
Answer: 271.07 °C
An oil with a
flow rate of 1000 kg/h is to be cooled using water in a double-pipe
counter-flow heat exchanger from a temperature of 70°C to 40°C. Water enters
the exchanger at 25°C and leaves at 40°C. The specific heats of oil and water
are 2 kJ kg-1 K-1 and 4.2 kJ kg-1 K-1,
respectively. The overall heat transfer coefficient is 0.2 kW m-2 K-1.
The minimum heat exchanger area (in m2) required for this operation
is ___________.
GATE
2014
Answer: 3.85 m2
A brick wall of
20 cm thickness has thermal conductivity of 0.7 W m-1 K-1.
An insulation of thermal conductivity 0.2 W m-1 K-1 is to
be applied on one side of the wall, so that the heat transfer through the wall
is reduced by 75%. The same temperature difference is maintained across the
wall before and after applying the insulation. The required thickness (in cm)
of the insulation is _____________.
GATE
2014
Answer: 17.14 cm
Steam economy of
a multiple effect evaporator system is defined as
(A)
Kilogram of steam used per hour
(B)
Kilogram of steam consumed in all the
effects for each kilogram of steam fed
(C)
Kilogram of steam used in all the
effects for each kilogram of water vaporized per hour
(D)
Kilogram of water vaporized from all the
effects for each kilogram of steam fed to the first effect
GATE
2014
Answer: (D)
In a complete
opaque medium, if 50% of the incident monochromatic radiation is absorbed, then
which of the following statements are correct?
(P) 50%
of the incident radiation is reflected
(Q) 25%
of the incident radiation is reflected
(R) 25
% of the incident radiation is transmitted
(S) No
incident radiation is transmitted
(A) P and
S only
(B) Q and
R only
(C) P and
Q only
(D) R and
S only
GATE
2014
Answer: (A)
No comments:
Post a Comment