ht sample prob

1. Multiple questions
1. The Fourier number (defined as a.t/L2
) is used in the analysis of problem involving heat
transfer by
A. forced convection
B. natural convection
C. transient conduction
D. steady state conduction
2. Dropwise condensation of steam on cooling surface is promoted
A. an oily surface.
B. when both the steam and the tube are clean.
C. only in presence of air.
D. none of these.
3. Conduction occurs in the buffer zone for a fluid flowing through a heated pipe, only when
Prandtl number is
A. 0.1
B. >1
C. < 1
D. l
4. Viscosity of gases __________ with increase in temperature.
A. increase very rapidly
B. increase slowly
C. decrease slowly
D. remain unaffected

2. 5. In Fourier's law, the proportionality constant is called the
A. heat transfer co-efficient
B. thermal diffusivity
C. thermal conductivity
D.
Stefan-Boltzman constant
6. Prandtl number is the reciprocal of
A. Thermal diffusivity/Momentum diffusivity
B. thermal diffusivity x Momentum
C. Thermal diffusivity x Mass diffusivity
D. Mass diffusivity x Momentum diffusivity
7. In Biot number, the characteristic length used is the ratio of the __________ of the solid.
A. volume to surface area
B. perimeter to surface area
C. surface area to volume
D. surface area to perimeter
8. In a heat exchanger with steam outside the tubes, a liquid gets heated to 45°C, when its flow
velocity in the tubes is 2 m/s. If the flow velocity is reduced to 1 m/s, other things remaining the
same, the temperature of the exit liquid will be
A. less than 45°C
B. more than 45°C
C. equal to 45°C
D. initially decreases and remains constant thereafter.
9. Unsteady state heat conduction occurs, when
A. temperature distribution is independent of time.
B. temperature distribution is dependent on time.
C. heat flows in one direction only.

3. D. three dimensional heat flow is concerned.
10. At steady state the temperature variation in a plane wall, made of two different solids I & II is
shown below :
The thermal conductivity of material I
A. is smaller than that of II.
B. is greater than that of II.
C. is equal to that of II.
D. can be greater than or smaller than that of II.
Objective type questions
1. jH factor for heat transfer is not a function of the __________ number.
2. With increase in temperature, the thermal conductivity of steel ______________.
3. The unit of heat transfer co-efficient is_____________
4. Units of prantel number________________.
5. At constant temperature, the thermal conductivities of gases __________ with rise in
pressure.
6. Prantel number is the ratio of the_________Diffusivity to ___________diffusivity.
7. The purpose of floating head in a heat exchanger is to_________________.
8. Double pipe heat exchangers are preferably useful, when compare with single Heat
exchanger___________.
9. For what value of Prandtl number, the Col-burn analogy is valid____________.
10. Nusselt number (for forced convection heat transfer) is a function of the __________
&___________numbers.
Answers
S.No Multiple questions Objective question
1 C Reynolds & Grashoff
2 A Decreases
3 A W/m2.K

4. 4 B Dimenstionless
5 C Decreases
6 A Momentum/thermal diffusivity
7 A Avoid buckling of tubes
8 B Lesser heat transfer area
9 B 0.6 to 120
10 A Prandtl & Reynolds numbers.