Heat Transfer[MCQ’s]

Module 01

1. The literature of heat transfer generally recognizes distinct modes of heat transfer. How many modes are there?
a) One
b) Two
c) Three
d) Four
Explanation: There are three modes of heat transfer i.e. radiation, convection and conduction.

2. Consider system A at uniform temperature t and system B at another uniform temperature T (t > T). Let the two systems be brought into contact and be thermally insulated from their surroundings but not from each other. Energy will flow from system A to system B because of

a) Temperature difference
b) Energy difference
c) Mass difference
d) Volumetric difference
Explanation: Greater the temperature imbalance the higher would be the rate of energy transfer.

3. An oil cooler in a high performance engine has an outside surface area 0.12 m2 and a surface temperature of 65 degree Celsius. At any intermediate time air moves over the surface of the cooler at a temperature of 30 degree Celsius and gives rise to a surface coefficient equal to 45.4 W/ m 2 K. Find out the heat transfer rate?
a) 238.43 W
b) 190.68 W
c) 543.67 W
d) 675.98 W
Explanation: Q = (T– T1) A h = 0.12 (65-30) 45.4 = 190.68 W.

4. Unit of the rate of heat transfer is
a) Joule
b) Newton
c) Pascal
d) Watt
Explanation: Unit of heat transfer is Joule but the rate of heat transfer is joule per second i.e. watt.

5. Convective heat transfer coefficient doesn’t depend on
a) Surface area
b) Space
c) Time
d) Orientation of solid surface
Explanation: It is denoted by h and is dependent on space, time, geometry, orientation of solid surface.

6. The rate equation used to describe the mechanism of convection is called Newton’s law of cooling. So rate of heat flow by convection doesn’t depend on
a) Convective heat transfer coefficient
b) Surface area through which heat flows
c) Time
d) Temperature potential difference
Explanation: It is directly proportional to all of above except time.

7. How many types of convection process are there?
a) One
b) Three
c) Four
d) Two
Explanation: Forced, natural and mixed convection.

8. Thermal conductivity is maximum for which substance
a) Silver
b) Ice
c) Aluminum
d) Diamond
Explanation: Thermal conductivity of diamond is 2300 W/m K.

9. A radiator in a domestic heating system operates at a surface temperature of 60 degree Celsius. Calculate the heat flux at the surface of the radiator if it behaves as a black body
a) 697.2 W/m2
b) 786.9 W/m2
c) 324.7 W/m2
d) 592.1 W/m2
Explanation: As, q = Q/A = 5.67 * 10-8 (273+60)4 = 697.2.

10. Which of the following is an example of forced convection?
a) Chilling effect of cold wind on a warm body
b) Flow of water in condenser tubes
c) Cooling of billets in the atmosphere
d) Heat exchange on cold and warm pipes
Explanation: In forced convection, the flow of fluid is caused by a pump, fan or by atmospheric winds.

11. Heat transfer deals with the rate of
a. work transfer
b. temperature transfer
c. energy transfer
d. none of the above

12. Which among the following has lowest thermal conductivity among the others?
a. silver
b. water
c. mercury
d. copper

13. In gases, the transfer of heat takes place by
a. volumetric density
b. transporting energy with free electrons
c. unstable elastic collision
d. random molecular collision

14. How does the heat transfer take place in metals?
a. volumetric density
b. transporting energy with free electrons
c. unstable elastic collision
d. random molecular collision

15. Internal energy of a substance is associated with
a. microscopic modes of energy
b. macroscopic modes of energy
c. both a. and b.
d. none of the above

16. Upto the critical radius of insulation,
A) Added insulation will increase heat loss
B) Added insulation will decrease heat loss
C) Convective heat loss will be less than conductive heat loss
D) Heat flux will decrease

17. Unit of thermal diffusivity is
A) m²/hr
B) m²/hr °C
C) kcal/m² hr
D) kcal/m. hr °C

18. The rate of energy transferred by convection to that by conduction is called
A) Stanton number
B) Nusselt number
C) Biot number
D) Peclet number

19. Thermal conductivity of wood depends on
A) Moisture
B) Density
C) Temperature
D) All of the above

20. A designer chooses the values of fluid flow rates and specific heats in such a manner that the heat capacities of the two fluids are equal. A hot fluid enters the counter flow heat exchanger at 100°C and leaves at 60°C. A cold fluid enters the heat exchanger at 40°C. The mean temperature difference between the two fluids is
A) 20°C
B) 40°C
C) 60°C
D) 66.7°C

21. The unit of overall coefficient of heat transfer is
A) W/m²K
B) W/m²
C) W/mK
D) W/m

22. LMTD in case of counter flow heat exchanger as compared to parallel flow heat exchanger is
A) Higher
B) Lower
C) Same
D) Depends on the area of heat exchanger

23. The transfer of heat by molecular collision is smallest in
A) Solids
B) Liquids
C) Gases
D) None of these

24. In heat exchangers, degree of approach is defined as the difference between temperatures of
A) Cold water inlet and outlet
B) Hot medium inlet and outlet
C) Hot medium outlet and cold water inlet
D) Hot medium outlet and cold water outlet

25. In a shell and tube heat exchanger, baffles are provided on the shell side to
A) Improve heat transfer
B) Provide support for tubes
C) Prevent stagnation of shell side fluid
D) All of these

26. An ordinary passenger aircraft requires a cooling system of capacity.
A) 2 TR
B) 4 TR
C) 8 TR
D) 10 TR

27. Joule sec is the unit of
A) Universal gas constant
B) Kinematic viscosity
C) Thermal conductivity
D) Planck’s constant

28. Fourier’s law of heat conduction gives the heat flow for
A) Irregular surfaces
B) Nonuniform temperature surfaces
C) One dimensional cases only
D) Two dimensional cases only

29. The value of Prandtl number for air is about
A) 0.1
B) 0.3
C) 0.7
D) 1.7

30. The product of Reynolds number and Prandtl number is known as
A) Stanton number
B) Biot number
C) Peclet number
D) Grashoff number

31. Unit of thermal conductivity in M.K.S. units is
(a) kcal/kg m2 °C
(b) kcal-m/hr m2 °C
(c) kcal/hr m2 °C
(d) kcal-m/hr °C
(e) kcal-m/m2 °C.
Ans: b

32. Unit of thermal conductivity in S.I. units is
(a) J/m2 sec
(b) J/m °K sec
(c) W/m °K
(d) (a) and (c) above
(e) (b) and (c) above.
Ans: e

33. Thermal conductivity of solid metals with rise in temperature normally
(a) increases
(b) decreases
(c) remains constant
(d) may increase or decrease depending on temperature
(e) unpredictable.
Ans: b

34. Thermal conductivity of non-metallic amorphous solids with decrease in temperature
(a) increases
(b) decreases
(c) remains constant
(d) may increase or decrease depending on temperature
(e) unpredictable.
Ans: b

35. Heat transfer takes place as per –
(a) zeroth law of thermodynamics
(b) first law of thermodynamic
(c) second law of the thermodynamics
(d) Kirchoff’s law (e) Stefan’s law.
Ans: c

36. When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by
(a) conduction
(b) convection
(d) conduction and convection
Ans: a

37. When heat is transferred form hot body to cold body, in a straight line, without affecting the intervening medium, it is referred as heat transfer by
(a) conduction
(b) convection
(d) conduction and convection
Ans: c

38. Sensible heat is the heat required to
(a) change vapour into liquid
(b) change liquid into vapour
(c) increase the temperature of a liquid of vapour
(d) convert water into steam and superheat it
(e) convert saturated steam into dry steam.
Ans: c

39. The insulation ability of an insulator with the presence of moisture would
(a) increase
(b) decrease
(c) remain unaffected
(d) may increase/decrease depending on temperature and thickness of insulation
(e) none of the above.
Ans: b

40. When heat is Transferred by molecular collision, it is referred to as heat transfer by
(a) conduction
(b) convection
(d) scattering
Ans: b

41. Heat transfer in liquid and gases takes place by
(a) conduction
(b) convection
(d) conduction and convection
Ans: b

42. Which of the following is the case of heat transfer by radiation
(a) blast furnace
(b) heating of building
(c) cooling of parts in furnace
(d) heat received by a person from fireplace
(e) all of the above.
Ans: d

43. Heat is closely related with
(a) liquids
(b) energy
(c) temperature
(d) entropy
(e) enthalpy.
Ans: c

44. Pick up the wrong case. Heat flowing from one side to other depends directly on
(a) face area
(b) time
(c) thickness
(d) temperature difference
(e) thermal conductivity.
Ans: c

45. Metals are good conductors of heat because
(a) their atoms collide frequently
(b) their atoms-are relatively far apart
(c) they contain free electrons
(d) they have high density
(e) all of the above.
Ans: a

46. Which of the following is a case of steady state heat transfer
(a) I.C. engine
(b) air preheaters
(c) heating of building in winter
(d) all of the above
(e) none of the above.
Ans: e

47. Total heat is the heat required to
(a) change vapour into liquid
(b) change liquid into vapour
(c) increase the temperature of a liquid or vapour
(d) convert water into steam and superheat it
(e) convert saturated steam into dry steam.
Ans: d

48. Cork is a good insulator because it has
(a) free electrons
(b) atoms colliding frequency
(c) low density
(d) porous body
(e) all of the above.
Ans: d

49. Thermal conductivity of water in general with rise in temperature
(a) increases
(b) decreases
(c) remains constant
(d) may increase or decrease depending on temperature
(e) none of the above.
Ans: d

50. Thermal conductivity of water at 20°C is of the order of
(a) 0.1
(b) 0.23
(c) 0.42
(d) 0.51
(e) 0.64.
Ans: d

51. Temperature of steam at around 540°C can be measured by
(a) thermometer
(c) thermistor
(d) thermocouple
(e) thermopile.
Ans: d

52. Thermal conductivity of air at room temperature in kcal/m hr °C is of the order of
(a) 0.002
(b) 0.02
(c) 0.01
(d) 0.1
(e) 0.5.
Ans: b

53. The time constant of a thermocouple is
(a) the time taken to attain the final temperature to be measured
(b) the time taken to attain 50% of the value of initial temperature difference
(c) the time taken to attain 63.2% of the value of initial temperature difference
(d) determined by the time taken to reach 100°C from 0°C
(e) none of the above.
Ans: c

54. Thermal conductivity of air with rise in temperature
(a) increases
(b) decreases
(c) remains constant
(d) may increase or decrease depending on temperature
(e) none of the above.
Ans: a

55. Heat flows from one body to other when they have
(a) different heat contents
(b) different specific heat
(c) different atomic structure
(d) different temperatures
(e) none of the above.
Ans: d

56. The concept of overall coefficient of heat transfer is used in heat transfer problems of
(a) conduction
(b) convection
(d) all the three combined
(e) conduction and comte_ction.
Ans: e

57. In heat transfer, conductance equals conductivity (kcal/hr/sqm/°C/cm) divided by
(a) hr (time)
(b) sqm (area)
(c) °C (temperature)
(d) cm (thickness)
(e) kcal (heat).
Ans: d

58. The amount of heat flow through a body by conduction is
(a) directly proportional to the surface area of the body
(b) directly proportional to the temperature difference on the two faces of the body
(c) dependent upon the material of the body
(d) inversely proportional to the thickness of the body
(e) all of the above.
Ans: e

59. Which of the following has least value of conductivity
(a) glass
(b) water
(c) plastic
(d) rubber
(e) air.
Ans: e

60. Which of the following is expected to have highest thermal conductivity
(a) steam
(b) solid ice
(c) melting ice
(d) water
(e) boiling water.
Ans: b

Module 02

1. According to the Fourier’s law of heat conduction, the rate of heat transfer by conduction depends upon
a. area of cross section normal to the heat flow
c. both a. and b.
d. none of the above

2. What is the temperature gradient in the conduction heat transfer?
a. change in temperature per unit change in time
b. change in temperature per unit change in distance in the direction of heat flow
c. change in temperature per unit change in cross-sectional area normal to the direction of heat flow
d. change in temperature per unit change in cross-sectional area parallel to the direction of heat flow

3. Why is the negative sign introduced in the equation of Fourier’s law of heat conduction?
q = – kA (dT / dx)
a. because heat transfer rate is inversely proportional to temperature gradient
b. because value of thermal conductivity k is negative
c. because heat is transferred from higher temperature to lower temperature
d. none of the above

4. The value of thermal conductivity k depends upon
a. the material through which the heat is transferred
b. the intensity of heat energy which is being transferred
c. the area which is parallel to the heat transfer
d. all of the above

5. The materials which have low thermal conductivity are called as
a. thermal conductors
b. thermal resistors
c. thermal insulators
d. none of the above

6) How does the thermal conductivity of a material change with respect to change in temperature of the same material?
a. thermal conductivity of a material increases with increase in its temperature
b. thermal conductivity of a material decreases with increase in its temperature
c. thermal conductivity of a material remains same with change in its temperature
d. unpredictable

7) What is the correct formula for the temperature distribution in infinitely long fin?
Where,
m = √(hP/kA)
h = convective heat transfer coefficient
P = perimeter of the fin
k = thermal conductivity of fin
A = cross-sectional area of the fin
T = Temperature of the fin at the distance x from the base of fine
and the constants C1 and C2 depend on the boundary conditions
Ts = Base temperature of the fin
T∞ = Temperature of surrounding fluid
a. (Ts – T∞) / (T – T∞) = C2emx
b. (Ts – T∞) / (T – T∞) = C2e– mx
c. (T – T∞) / (Ts – T∞) = C2emx
d. (T – T∞) / (Ts – T∞) = C2e– mx

8) Heat transfer deals with the rate of
a. work transfer
b. temperature transfer
c. energy transfer
d. none of the above

9) If the body or element does not produce heat, then the general heat conduction equation which gives the temperature distribution and conduction heat flow in an isotropic solid reduces to(∂T/∂x2) + (∂T/∂y2) + (∂T/∂z2) = (1/α)(∂T/∂t)this equation is known as
a. Laplace equation
b. Fourier equation
c. Poisson equation
d. none of the above

10) For effective working of fins, the thickness of the fines should be
a. large
b. small
c. thickness of fin does not affect the fin effectiveness
d. unpredictable

11) Why is the negative sign introduced in the equation of Fourier’s law of heat conduction?
q = – kA (dT / dx)
a. because heat transfer rate is inversely proportional to temperature gradient
b. because value of thermal conductivity k is negative
c. because heat is transferred from higher temperature to lower temperature
d. none of the above

12) The cooling of a metal ball of volume V, in large quantity of fluid is analogous to
a. discharging of battery in an electric system
b. discharging of capacitor in an electric system
c. generation of heat in resistor in an electrical system
d. none of the above

13) What should be the Biot number to assume the body at uniform temperature?
a. the Biot number should be less than 0.1
b. the Biot number should be more than 0.1
c. the Biot number should be equal to 0.1
d. none of the above

14) What is the correct formula for The Biot number?
a. hl/k
b. k/hl
c. l/hk
d. hk/l

15) The Biot number or Biot modulus is given by
a. the ratio of external convection resistance to the internal conduction resistance
b. the ratio of internal conduction resistance to the external convection resistance
c. multiplying internal conduction resistance and external convection resistance
d. none of the above

16.‘a’ refers to ______ and ‘b’ refers to _____

c) None of the mentioned
d) All of the mentioned
Explanation: ‘a’ refers to Steady state heat transfer and ‘b’ refers to unsteady state heat transfer.

17. Statement 1: The heat transfer co-efficient in ice is four times that of water.
Statement 2: k in conductance is for thermal conductivity.
a) True, False
b) True, True
c) False, False
d) False, True
Explanation: Both the statements are true.

18. Frozen foods have a higher transfer co-efficient hence they lose heat faster than other food items when exposed to the outside environment.
a) True
b) False
Explanation: Frozen foods have a higher transfer co-efficient hence they lose heat faster than other food items when exposed to the outside environment.

19. Statement 1: When the average radius gives an error, log mean radius is used.
Statement 2: Log mean radius is given by rL= ( r0– ri )/(ln(r0/ri))
a) True, False
b) True, True
c) False, False
d) False, True
Explanation: Both the statements are true.

20. Statement 1: α=k/(ρCp)
Statement 2: α stands for thermal conductivity.
a) True, False
b) True, True
c) False, False
d) False, True
Explanation: α=k/(ρCp). α stands for thermal diffusivity.

21. Statement 1: NFo stands for Fourier number.
Statement 2: NFo= (αtT)/s2
a) True, False
b) True, True
c) False, False
d) False, True
Explanation: Both the statements pertaining to Fourier number are true.

22. NFo= (αtT)/s2. Here ‘s’ stands for half slab thickness. Based on this data, ‘s’ for a chapatti/ dosa will be _____
a) Half slab thickness
b) Full slab thickness
c) One and a half slab thickness
d) Double slab thickness
Explanation: ‘s’ for a chapatti/ dosa will be Full slab thickness. This is because ‘s’ equals half slab thickness only when heating takes place from both sides. But, for a chapatti/ dosa, heating takes place only from one end hence‘s’ becomes ‘2s’.

23. _____ is the equation for unsteady state heat conduction through solid materials.
a) dT/dx= α (d2 T)/ (dt)2
b) dT/dx= α (d2 T)/ (dx)2
c) dT/dt= α (d2 T)/ (dx)2
d) dT/dt= α (d2 T)/ (dt)2
Explanation: dT/dt= α (d2 T)/ (dx)2 is the equation for unsteady state heat conduction through solid materials.

24. For a copper pipe, k = 0.3534, temperature difference = 60, B = 0.0125 and A = 850. What is the value of q?
a) 1442 W
b) 1442 kW
c) 1565 W
d) 1565 kW
Explanation: q=kA ∆T/B.

25. The heat is absorbed by
A) Condenser
B) Evaporator
C) Compressor
D) Thermostat
Ans: B

26. The Stefan Boltzman law states that
A) E α T
B) E α T2
C) E α T3
D) E α T4
Ans: D

27. The body which absorbs all radiations incident upon it, is called as
A) Black body
B) White body
C) Opaque body
D) Transparent body
Ans: A

28. If the body is at thermal equilibrium, then the
A) Emissivity = absorptivity
B) Emissivity ˃ absorptivity
C) Emissivity ˂ absorptivity
D) None of the above
Ans: A

29. In radiative heat transfer, a gray surface is one
A) which appears gray to the eye
B) whose emissivity is independent of wavelength
C) which has reflectivity equal to zero
D) which appears equally bright from all directions
Ans: B

30. Heat transfer takes place according to
A) First Law of Thermodynamics
B) Second Law of Thermodynamics
C) Third Law of Thermodynamics
D) Zeroth Law of Thermodynamics
Ans: B

31. Heat is mainly transferred by conduction, convection and radiation in
A) insulated pipes carrying hot water
B) refrigerator freezer coil
C) boiler furnaces
D) condensation of steam in a condenser
Ans: C

32. The value of Biot number is very small (less than 0.01) when
A) The convective resistance of the fluid is negligible
B) The conductive resistance of the solid is negligible
C) The conductive resistance of the fluid is negligible
D) None of these
Ans: B

33. The ratio of energy transferred by convection to that by conduction is called
A) Stanton number
B) Nusselt number
C) Biot number
D) Preclet number
Ans: C

34. What happens when the thickness of insulation on a pipe exceeds the critical value?
A) Heat transfer rate increases
B) Heat transfer rate decreases
C) Heat transfer rate remain constant
D) none of these
Ans: B

35. In heat exchangers, the value of logarithmic mean temperature difference should be
A) maximum possible,
B) minimum possible,
C) zero;
D) constant
Ans: A

36. Air at 20° C blows over a plate of 50 cm x 75 cm maintained at 250° C. If the convection heat transfer coefficient is 25 W/m2 °C, the heat transfer rate is
A) 215.6 kW
B) 2156 kW
C) 2.156 kW
D) 21.56 Kw
Ans: C

37. Units for thermal conductivity
A) J/kg.K
B) J/mol.K
C) J.ohm/sec.K2
D) W/m.K
Ans: D

38. The process of heat transfer from one particle of the body to another without actual motion of the particle is called
B) Conduction
C) Convection
D) None of these
Ans: A

39. Fourier law of heat conduction is based on the assumption that
A) Heat flow through a solid is one dimensional
B) Heat flow is in steady state
C) Both (A) and (B)
D) None of the options
Ans: C

Module 03

1. Regarding one dimensional heat transfer, choose the correct statement.
Explanation: In case of one dimensional heat flow steady state is a function of x coordinate only while unsteady state is a function of x coordinate and time only.

2. Which statement is true regarding steady state condition?
a) There is a variation in temperature in the course of time
b) Heat exchange is constant
c) It is a function of space and time coordinates
d) Internal energy of the system changes
Explanation: Heat influx is always equal to heat efflux. It is a function of space coordinates only.

3. Which of the following is an example of steady state heat transfer?
a) Boilers and turbines
b) Cooling of I.C engine
c) Chilling effect of cold wind on a warm body
d) Electric bulb cools down by the surrounding atmosphere
Explanation: System is a perfect black body.

4. Heat transfer in a long, hollow cylinder which is maintained at uniform but different temperatures on its inner and outer surfaces may be assumed to be taking place in which direction?
a) Axial only
b) Unpredictable
d) No heat transfer takes place
Explanation: Ambient temperature is uniform on the periphery of cylinder and temperature is uniform. So it takes place in the radial direction only.

5. Heat transfer takes place according to which law?
a) Newton’s law of cooling
b) Second law of thermodynamics
c) Newton’s second law of motion
d) First law of thermodynamics
Explanation: Second law states about heat transfer between source and sink.

6. Heat transfer takes place in liquids and gases is essentially due to
b) Conduction
c) Convection
d) Conduction as well as convection
Explanation: Convection is a process by which thermal energy is transferred between solid and fluid flowing through it.

7. The appropriate rate equation for convective heat transfer between a surface and adjacent fluid is prescribed by
a) Newton’s first law
b) Wein’s displacement law
c) Kirchhoff’s law
d) Newton’s law of cooling
Explanation: The rate equation used to describe the mechanism of convection is called Newton’s law of cooling when the solid surface is cooled by the fluid.

8. Identify the wrong statement
a) The process of heat transfer is an irreversible process
b) For heat exchange, a temperature gradient must exist
c) A material medium is not necessary for heat transmission
d) Heat flow doesn’t depend on temperature
Explanation: Heat flows from higher to lower temperature.

9. During a cold winter season, a person prefers to sit near a fire. Which of the following modes of heat transfer provides him the maximum heat?
a) Conduction from the fire
b) If it is near the fire, convection sounds good
d) Radiation will provide quick warmth
Explanation: Heat transfer by radiation can occur between two bodies even when they are separated by a medium colder than both of them.

10. Most unsteady heat flow occurs
a) Through the walls of the refrigerator
b) During annealing of castings
c) Through the walls of the furnace
d) Through lagged pipe carrying steam
Explanation: Under steady state condition, with time there is a change in temperature i.e. temperature field is a function of space and time

11. Generally the external thermal resistance between the surface of the body and the environment is
a. less than the internal conduction resistance in the body
b. more than the internal conduction resistance in the body
c. same as the internal conduction resistance in the body
d. none of the above
ANSWER: b. more than the internal conduction resistance in the body

12. Suppose that a hot metal ball is immersed in cold water, then temperature distribution in the body depends upon
a. thermal conductivity of the body
b. convective heat transfer from the body surface to water
c. both a. and b.
d. none of the above
ANSWER: c. both a. and b.

13. Suppose that a hot metal ball is suddenly immersed in cold water. What is the condition for the ball to maintain it at a uniform temperature?
a. the conduction resistance in a ball should be very large compared to the convection resistance for heat transfer from surface of ball to water
b. the conduction resistance in a ball should be very small compared to the convection resistance for heat transfer from surface of ball to water
c. the conduction resistance in a ball should be equal to the convection resistance for heat transfer from surface of ball to water
d. cannot say
ANSWER: b. the conduction resistance in a ball should be very small compared to the convection resistance for heat transfer from surface of ball to water

14. What is lumped heat capacity analysis?
a. the analysis of a system in which it is assumed to be at nonuniform temperature
b. the analysis of a system in which it is assumed to be at uniform temperature
c. the analysis of a system in which it is assumed to be at either uniform or nonuniform temperature
d. none of the above
ANSWER: b. the analysis of a system in which it is assumed to be at uniform temperature

15. The Biot number or Biot modulus is given by
a. the ratio of external convection resistance to the internal conduction resistance
b. the ratio of internal conduction resistance to the external convection resistance
c. multiplying internal conduction resistance and external convection resistance
d. none of the above
ANSWER: b. the ratio of internal conduction resistance to the external convection resistance

16. What is the correct formula for The Biot number?
a. hl/k
b. k/hl
c. l/hk
d. hk/l

17. What should be the Biot number to assume the body at uniform temperature?
a. the Biot number should be less than 0.1
b. the Biot number should be more than 0.1
c. the Biot number should be equal to 0.1
d. none of the above
ANSWER: a. the Biot number should be less than 0.1

```18. The cooling of a metal ball of volume V, in large quantity of fluid is analogous to
a. discharging of battery in an electric system
b. discharging of capacitor in an electric system
c. generation of heat in resistor in an electrical system
d. none of the above
ANSWER: b. discharging of capacitor in an electric system

19). 200 kg of solids (on dry basis) is subjected to a drying process for a period of 5000 seconds. The drying occurs in the constant rate period with the drying rate as, Nc = 0.5 x 10⁻³ kg/m².s. The initial moisture content of the solid is 0.2 kg moisture/kg dry solid. The interfacial area available for drying is 4 m²/1000 kg of dry solid. The moisture content at the end of the drying period is (in kg moisture/kg dry solid)
A.0.5
B.0.05
C.0.1
D.0.15

20). It is desired to concentrate a 20% salt solution (20 kg of salt in 100 kg of solution) to a 30% salt solution in an evaporator. Consider a feed of 300 kg/min at 30°C. The boiling point of the solution is 110°C, the latent heat of vaporisation is 2100 kJ/kg and the specific heat of the solution is 4 kJ/kg.K. The rate at which the heat has to be supplied in (kJ/min) to the evaporator is
A. 3.06 x l0⁵
B. 6.12 x 10⁵
C. 7.24 x 10⁵
D. 9.08 x 10⁵

21). Fqra cold viscous feed, backward feed gives __________ than forward feed.
A.  A higher capacity
B.  A lower capacity
C.  Lower economy
D.  None of these

22). In a multipass shell and tube heat exchanger, tube side return pressure loss is equal to __________ the velocity head.
A. Twice
B. Four times
C. Square root of
D. Square of

23). In the process of heat transfer through extended surfaces or fins, the entire surface area is at
a. the same constant temperature
b. different temperatures
c. maximum base temperature
d. minimum temperature

24.  An engine is fitted with pin fins having thermal conductivity k = 200 W/mK. The diameter and length of the fin is 2 cm and 50 cm respectively. Calculate the temperature at 10 cm from the fin base if fine base temperature is 500 0C and fin is in contact with air at 50 0C. Take h = 12 W/m2K
Consider that the fin is infinitely long
a. 80.67 0C
b. 100 0C
c. 85.67 0C
d. 185.67 0C

25). What is the purpose of using fins in a particular heat transfer system?
a. to decrease rate of heat transfer
b. to increase rate of heat transfer
c. to maintain rate of heat transfer at a constant rate
d. cannot say
ANSWER: b. to increase rate of heat transfer

26). Temperature at the end tip of the fin having uniform cross-sectional area is
a. maximum
b. minimum
c. similar to the heat generation temperature
d. unpredictable

27. A very long copper rod 20 mm in diameter extends horizontally from a plane heated wall maintained at 100 degree Celsius. The surface of the rod is exposed to an air environment at 20 degree Celsius with convective heat transfer coefficient of 8.5 W/m2 degree. Workout the heat loss if the thermal conductivity of copper is 400 W/m degree
a) 10.71 W
b) 20.71 W
c) 30.71 W
d) 40.71 W
Explanation: P/A = 4/d and m = (h P/k A) ½ = (4 h/k d) ½ = 2.061.per meter.

28. Common applications of finned surfaces are with
(i) Electrical motors
(ii) Economizers for steam power plant
(iii) Convectors for steam and cold water heating systems
(iv) Cooling coils
Identify the correct option
a) i, ii and iv
b) i, ii and iii
c) i, ii, iii and iv
d) i and ii
Explanation: It should be for hot water heating systems.

29. The extended surface used for the enhancement of heat dissipation is
a) Convective coefficient
b) Fourier number
c) Fin
d) No finned surface
Explanation: The surface area exposed to the surroundings is frequently increased by the attachment of protrusions to the surfaces, and the arrangement provides a means by which heat transfer rate can be improved.

30. It is said that fins can take a variety of forms
(i) Longitudinal fins of rectangular cross section attached to a wall
(ii) Cylindrical tubes with radial fins
(iii) Conical rod protruding from a wall
Identify the correct statement
a) i only
b) i and ii
c) ii and iii
d) i, ii and iii
Explanation: Option b is also known as annular fins.

31. A steel rod (k = 30 W/m degree) 1 cm in diameter and 5 cm long protrudes from a wall which is maintained at 10 degree Celsius. The rod is insulated at its tip and is exposed to an environment with h = 50 W/m2 degree and t a = 30 degree Celsius. Calculate the fin efficiency
a) 56.57%
b) 66.57%
c) 76.57%
d) 86.57%
Explanation: Fin efficiency = tan h ml/ml, where m = (h P/k A) ½ = 25.82 per meter.

32. If the fin is sufficiently thin, so heat flows pertain to
a) One dimensional heat conduction
b) Two dimensional heat conduction
c) Three dimensional heat conduction
d) No heat flow is there
Explanation: As, δ is less than b, so one dimensional heat conduction is there.

33. If heat dissipation for one fin is given by 377.45 k J/hour, then what is the heat dissipation for 12 fins?
a) 7529.4 k J/hour
b) 6529.4 k J/hour
c) 5529.4 k J/hour
d) 4529.4 k J/hour
Explanation: For 12 fins, the heat dissipation will be equal to 12 (377.45) = 4529.4 k J/hour.

34. In order to achieve maximum heat dissipation, the fin should be designed in such a way that has a
a) Maximum lateral surface towards the tip side of fin
b) Minimum lateral surface near the center line
c) Maximum lateral surface at the root side of fin
d) Maximum lateral surface near the center of fin
Explanation: Fins are so designed that lateral surface at the root side of the fin is maximum. This aspect results into higher heat dissipation.

35. A steel rod (k = 30 W/m degree) 1 cm in diameter and 5 cm long protrudes from a wall which is maintained at 10 degree Celsius. The rod is insulated at its tip and is exposed to an environment with h = 50 W/m2 degree and t a = 30 degree Celsius. Calculate the rate of heat dissipation
a) 2.658 W
b) 3.658 W
c) 4.658 W
d) 5.658 W
Explanation: Q = k A m tan h ml (t 0 – t a) = 3.658 W.

36. On a heat transfer surface, fins are provided to
a) Increase turbulence in flow for enhancing heat transfer
b) Increase temperature gradient so as to enhance heat transfer
c) Pressure drop of the fluid should be minimized
d) Surface area is maximum to promote the rate of heat transfer
Explanation: Fins are provided to a heat exchanger surface to augment the heat transfer by increasing the surface area exposed to the surroundings

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37. Fin increases the rate of heat transfer by increasing the
a) Temperature difference
b) Surface area
c) Film coefficient
d)None
Ans: (b)

38. The fin efficiency of infinite length is
a)Infinity
b) 100 %
c) zero
d) None
Ans: (c)

39. In case of a fin, q.max is
a) hAc(tb–ta)
b) hAs (tb–ta)
c) -kAc Ət/Əx
d) None
ANS: (b)

40. Efficiency of the fin is given by
a) q.with fin / q.without fin
b) q.without fin/ q.with fin
c) q.without fin/ q.max
d) None,
ANS: (c)

41. Boundary conditions for a fin of infinite length are
a) θ=θ0=(tb–ta) at x=0 and θ=θL=(tb–ta) at x=L
b) θ=θ0=(tb–ta) at x=0 and θ=0 =(tb–ta) at x=∝
c) θ=θ0=(tb–ta) at x=0 and θ=θ∝=ta at x=∝
d) None
ANS: (b)

42. Two boundary conditions for a fin of finite length with tip insulated are
a) θ=θ0=(tb–ta) at x=0 and θ=θL=(tb–ta) at x=L
b) θ=θ0=(tb–ta) at x=0 and θ=θL=tb at x=L
c) θ=θ0=(tb–ta) at x=0 and Əθ/Əx =0 at x=L
d) None
ANS: (c)

43. To increase the effectiveness of a fin, the film coefficient ‘h’ should be
a) high
b) less
c) equal to 100
d)None
ANS: (b)

44. Minimum effectiveness of a fin should be
a) > (10)0.5
b) > (20)0.5
c) > (5)0.5
d) None
Ans: (c)

45.The relation between effectiveness and efficiency of a fin is
a) η= ε As/Ac
b) η= ε Ac/As
c) ε = η As/Ac
d) None
ANS: (b)

46. Efficiency for a fin of finite length with tip insulated is
a) Cosh(mL)/mL,
b) Sinh(mL)/mL
c) Cosh(mL)/Sinh(mL)
d) None
ANS: (d)

47. The extended surface used for the enhancement of heat dissipation is
A)Convective coefficient
B)Fourier number
C)Fin
D)No finned surface

48. A steel rod (k = 30 W/m degree) 1 cm in diameter and 5 cm long protrudes from a wall which is maintained at 10 degree Celsius. The rod is insulated at its tip and is exposed to an environment with h = 50 W/m2 degree and t a = 30 degree Celsius. Calculate the fin efficiency
A)56.57%
B)66.57%
C)76.57%
D)86.57%

49. In order to achieve maximum heat dissipation, the fin should be designed in such a way that has a
A)The maximum lateral surface towards the tip side of the fin
B)The minimum lateral surface near the centerline
C)The maximum lateral surface at the root side of the fin
D)The maximum lateral surface near the center of the fin

50. A steel rod (k = 30 W/m degree) 1 cm in diameter and 5 cm long protrudes from a wall which is maintained at 10 degree Celsius. The rod is insulated at its tip and is exposed to an environment with h = 50 W/m2 degree and T a = 30 degree Celsius. Calculate the rate of heat dissipation
A)2.658 W
B)3.658 W
C)4.658 W
D)5.658 W

51. On a heat transfer surface, fins are provided to
A)Increase turbulence in flow for enhancing heat transfer
B)Increase temperature gradient so as to enhance heat transfer
C)Pressure drop of the fluid should be minimized
D)Surface area is maximum to promote the rate of heat transfer

52. Which one of the following configurations has the highest fin effectiveness?
A) Thin, closely spaced fins
B) Thin, widely spaced fins
C) Thick, widely spaced fins
D) Thick, closely spaced fins

53. A fin has 5mm diameter and 100mm length. The thermal conductivity of fin material is 400Wm-1k-1 . One end of the fin is maintained at 130ºC and its remaining surface is exposed to ambient air at 30ºC. if the convective heat transfer coefficient is 40Wm-2K-1, the heat loss (in W) from the fin is
A) 0.08
B) 5.0
C) 7.0
D) 7.8

54. A fin should be surrounded by a
a)Any fluid
b) Liquid
c) Gas
d) None
ANS: (c)

55. Parameter ‘m’ of a fin is given by the relation
a) (kAc/hP)0.5
b) (PAc/hk)0.5
c) (hP/kAc)0.5
d) None
ANS: (c)

56. Temperature distribution in case of a fin of infinite length is
a) linear
b) Parabolic
c) Exponential
d) None
ANS: (c)

58. The sequence of the modes of heat transfer in case of a heat exchanger are
c) Conv+Cond+Conv
d) None,
ANS: (c)

59. The effectiveness of a parallel and counter flow heat exchanger is of same value
b) In a Condenser
c) in a Pre-heater
d) None
ANS: (b)

60. Correction factor ‘F’ is used to calculate the rate of heat transfer in case of a
a) Parallel flow heat exchanger
b) Counter flow heat exchanger
c) Cross flow heat exchanger
d) None
ANS: (c)

61. NTU in case of a heat exchanger is given by
a) UA/C
b) Cmin/UA
c) UA/Cmax
d) None
ANS: (d)

62. Temperature variation in a heat exchanger is
a) Linear
b) Parabolic
c) exponential
d) None
ANS: (b)

63. The heat capacity ratio ‘C’ in a heat exchanger is zero in case of a
b) Pre-heater
c) Condenser
d) None
ANS: (c)

64. In case of a 1:1 heat exchanger, which method of analysis is used
a) LMTD
b) NTU
c) Any one of LMTD or NTU
d) None
ANS: (c)

Module 04

1. What is the correct formula for the rate of heat transfer by convection form a surface of area A at temperature T to the surrounding fluid at temperature T0?
Where h is convection heat transfer coefficient
a. qc = h A (T – T0)4
b. qc = h A (T4 – T04)
c. qc = h A (T – T0)
d. none of the above
ANSWER: c. qc = h A (T – T0)

2. What is the mode of heat transfer from the hot surface to the adjacent layer of fluid which surrounds the surface?
a. conduction mode of heat transfer
b. radiation mode of heat transfer
c. convection mode of heat transfer
d. none of the above
ANSWER: a. conduction mode of heat transfer

3. Convective heat transfer coefficient is also known as
a. film convection factor
b. film coefficient
c. film conductance
d. none of the above

4. Suppose a hot water is placed inside a closed metal vessel of thickness 5 mm. There is atmospheric air outside the vessel at temperature lower than the temperature of hot water. The heat is transferred from the hot water to atmospheric air. What is the mode of heat transfer in this condition?
a. conduction then convection
b. convection then conduction
c. convection then conduction then convection
d. conduction then convection then conduction
ANSWER: c. convection then conduction then convection

5. The primary driving force for natural convection is
(A) Shear stress forces
(B) Buoyancy forces
(C) Surface tension forces
(D) None of them

6.For the fluid flowing over a flat plate with Prandtl number greater than unity, the thermal boundary layer for laminar forced convection
(A) is thinner than the hydrodynamic boundary layer
(B) has thickness equal to zero
(C) is of same thickness as hydrodynamics boundary layer
(D) is thicker than the hydrodynamic boundary layer

7.For flow of fluid over a heated plate, the following fluid properties are known: Viscosity=0.001 Pa/s, Sp. Heat at constant pressure=1 kJ/kgK, Thermal Conductivity=1W/mK; The hydrodynamic boundary layer thickness at a specified location on the plate is 1mm, thermal boundary layer thickness at the same location is
(A) 0.001mm
(B) 0.01mm
(C) 1mm
(D) 1000mm

8.For laminar forced convection over a flat plate, if the free stream velocity increases by a factor of 2, the average heat transfer coefficient
(A) remains same
(B) decreased by a factor of 20.5
(C) rises by a factor of 20.5
(D) rises by a factor of 4

9.Nusselt number is given by
(A) hl/k
(B) 2hl/k
(C) 3hl/k
(D) 4hl/k

10. The convective heat transfer coefficient in laminar flow over a flat plate
a) Increases with distance
b) Increases if a higher viscosity fluid is used
c) Increases if a denser fluid is used
d) Decreases with increase in free stream velocity
Explanation: It mostly increases if a denser fluid is used.

11. For laminar flow over a flat plate, the average value of a Nusselt number is prescribed by the relation
Nu = 0.664 (Re) 0.5 (Pr) 0.33
Which of the following is then a false statement?
a) Density has to be increased four times
b) Plate length has to be decreased four times
c) Specific heat has to be increased four times
d) Dynamic viscosity has to be decreased sixteen times
Explanation: The dynamic viscosity has an inverse relation to 1/6 power. To double the convective heat transfer coefficient, the dynamic viscosity has to be decreased 64 times.

12. For turbulent flow over a flat plate, the average value of Nusselt number is prescribed by the relation
Nu = 0.664 (Re) 0.5 (Pr) 0.33
Which of the following is then a false statement?
The average heat transfer coefficient increases as
a) 1/5 power of plate length
b) 2/3 power of thermal conductivity
c) 1/3 power of specific heat
d) 4/5 power of a free stream velocity
Explanation: The average heat transfer coefficient reduces with length as 1/5th power of the length.

13. A nuclear reactor with its core constructed of parallel vertical plates 2.25 m high and 1.5 m wide has been designed on free convection heating of liquid bismuth. Metallurgical considerations limit the maximum surface temperature of the plate to 975 degree Celsius and the lowest allowable temperature of bismuth is 325 degree Celsius. Estimate the maximum possible heat dissipation from both sides of each plate. The appropriate correlation for the convection coefficient is
Nu = 0.13 (Gr Pr) 1/3
a) 143 MW
b) 153 MW
c) 163 MW
d) 173 MV
Explanation: Q = 2 h A d t = 153 MW.

14. Consider the above problem, find the value of Grashoff number
a) 101.3 * 10 12
b) 102.3 * 10 12
c) 103.3 * 10 12
d) 104.3 * 10 12
Explanation: Grashof number = l β g d t/µ 2.

15. A thin walled duct of 0.5 m diameter has been laid in an atmosphere of quiescent air at 15 degree Celsius and conveys a particular gas at 205 degree Celsius. Base your calculations on one meter length of the duct, estimate the convective coefficient of heat transfer
a) 5.086 W/m2 K
b) 6.086 W/m2 K
c) 7.086 W/m2 K
d) 8.086 W/m2 K
Explanation: h = 1.37 (d t/l) 0.25 = 5.086 W/m2 K.

16. Free correction modulus is given by
a) p 2 β g c P
b) p 2 β g c P/k
c) p 2 β g c P/µ k
d) p 2 β g c P
Explanation: It contains only fluid properties and is called the free convection modulus.

17. The free convection coefficient is given by
h = C 1 d t m/l 1 – 3m
The value of exponent for laminar flow is
a) 0.5
b) 0.6
c) 0.7
d) 0.8
Explanation: For laminar flow h = C 1 (d t/l) 0.25.

18. For inclined plates we multiply Grashoff number with
a) Cos 2 α
b) Sin 2 α
c) Sin α
d) Cos α
Explanation: It should be multiplied with cos α, as α is angle with the horizontal.

19. The free convection coefficient is given by
h = C 1 d t m/l 1 – 3m
The value of exponent for turbulent flow is
a) 0.43
b) 0.33
c) 0.23
d) 0.13
Explanation: For turbulent flow h = C (d t).

20. Assume fluid flowing in a tube forcefully. The velocity boundary layer develops along the tube. Thickness of this boundary layer increases in the flow direction until the boundary layer reaches the tube centre. This region from the tube inlet to the point at which the boundary layers merge at the centerline is called as
a. laminar entry region
b. hydraulic entry region
c. hydrostatic entry region
d. hydrodynamic entry region

21. What is hydrodynamically developed region in fluid flowing inside a pipe?
a. a region where velocity profile of the fluid is partially developed
b. a region where velocity profile of the fluid is fully developed
c. a region where velocity profile of the fluid changes according to the distance
d. none of the above
ANSWER: b. a region where velocity profile of the fluid is fully developed

22. What is the ratio of the buoyancy force to the viscous force acting on a fluid called?
a. Prandtl number (Pr)
b. Reynolds number (Re)
c. Nusselt number (Nu)
d. Grashof number (Gr)

23. In natural convection, the Nusselt number (Nu) depends on
a. Pr and Re
b. Gr and Re
c. Gr and Pr
d. none of the above

24. Which of the following condition is correct for natural convection?
a. (Gr / Re2) = 1
b. (Gr / Re2) << 1
c. (Gr / Re2) >> 1
d. none of the above
ANSWER: c. (Gr / Re2) >> 1

25. If there are no externally induced flow velocities, then the Nusselt number (Nu) does not depend upon
a. Prandtl number (Pr)
b. Reynolds number (Re)
c. Grashof number (Gr)
d. none of the above

26. The Grashof number in natural convection plays same role as
a. Prandtl number (Pr) in forced convection
b. Reynolds number (Re) in forced convection
c. Nusselt number (Nu) in forced convection
d. none of the above
ANSWER: b. Reynolds number (Re) in forced convection

27. Which of the following is true for turbulent flow?
a) G r > 108
b) G r > 109
c) G r > 103
d) G r > 1015
Explanation: The product G r is often referred to as Rayleigh number, and its value sets the criterion of turbulent character of flow.

28. Mc Adam relation is given by
a) Nu = 0.023 (Re) 0.8 (Pr) 0.4
b) Nu = 0.023 (Re) 0.8 (Pr) 0.3
c) Nu = 0.023 (Re) 0.8 (Pr) 0.2
d) Nu = 0.023 (Re) 0.8 (Pr) 0.1
Explanation: Here, n = 0.4 if the fluid is being heated.

29. Investigate the effect of following condition on the average value of heat transfer coefficient in flow through a tube
Two fold increases in flow velocity by varying mass flow rate
It may be presumed that there is no change in the temperature of the liquid and the tube wall, and that the flow through the tube is turbulent in character
a) 64.1% decrease
b) 64.1% increase
c) 74.1%decrease
d) 74.1% increases
Explanation: h 2/h = (V 2/V 10.8 = .0741 = 74.1%

30. Consider the above problem for two fold increase in the diameter of the tube, the flow velocity is maintained constant by change in the rate of liquid flow
a) 15%
b) 14%
c) 13%
d) 12%
Explanation: h 2/h = (d 1/d 20.2.

31. Calculate the rate of heat loss from a human body which may be considered as a vertical cylinder 30 cm in diameter, and 175 cm high while standing in a 30 km/hr wind at 15 degree Celsius. Human has a surface temperature of 35 degree Celsius
a) 588.86 W
b) 688.86 W
c) 788.86 W
d) 888.86 W
Explanation: Q = h A d t, h = Nu k/d.

32. A heat treat steel plate measures 3 m by 1 m and is initially at 30 degree Celsius. It is cooled by blowing air parallel to 1 m edge at 9 km/hr. If the air is at 10 degree Celsius. Estimate the convection heat transfer from both sides of the plate
a) 477.7 W
b) 547.7 W
c) 647.7 W
d) 747.6 W
Explanation: Q = h A d t, h = Nu k/d.

33. Consider the above problem, find the value of Reynolds number
a) 166003
b) 177003
c) 188003
d) 199003
Explanation: Re = p V l/µ = 16603

34. The oil pan of 1.6 engine approximates a flat plate 0.3 m wide by 0.45 m long and protrudes below the framework of the automobile. The engine oil is at 95 degree Celsius and the ambient air temperature is 35 degree Celsius. If the automobile runs at 36 km/hr, make calculations for the rate of heat transfer from the oil-pan surface. Assume negligible resistance to conduction through the oil pan
a) 190.37 W
b) 180.37 W
c) 170.37 W
d) 160.37 W
Explanation: Q = h A d t, where h = (Nu) (k)/l and Nu = 0.664 (Re) 0.5 (Pr).33.

35. Air flows through a 10 cm internal diameter tube at the rate of 75 kg/hr. Measurements indicate that at a particular point in the tube, the pressure and temperature of air are 1.5 bar and 325 K whilst the tube wall temperature is 375 K. Find heat transfer rate from one meter length in the region of this point
The general non-dimensional correlation for turbulent flow in the tube is
Nu = 0.023 (Re) 0.8 (Pr) 0.4
Where the fluid properties are evaluated at the bulk temperature
a) 147.35 W
b) 157.35 W
c) 167.35 W
d) 177.35 W
Explanation: Q = h A d t, where h = (Nu) (k)/l = 177.35 W.

36. Consider the above problem, find the value of Nusselt number
a) 40.46
b) 50.56
c) 60.66
d) 70.76
Explanation: Nu = 0.023 (Re) 0.8 (Pr) 0.4 = 40.46

Module 05

1. What is the correct formula for the rate of heat transfer by convection form a surface of area A at temperature T to the surrounding fluid at temperature T0?
Where h is convection heat transfer coefficient
a. qc = h A (T – T0)4
b. qc = h A (T4 – T04)
c. qc = h A (T – T0)
d. none of the above

2. What is the mode of heat transfer from the hot surface to the adjacent layer of fluid which surrounds the surface?
a. conduction mode of heat transfer
b. radiation mode of heat transfer
c. convection mode of heat transfer
d. none of the above

3. Convective heat transfer coefficient is also known as
a. film convection factor
b. film coefficient
c. film conductance
d. none of the above

4. Suppose a hot water is placed inside a closed metal vessel of thickness 5 mm. There is atmospheric air outside the vessel at temperature lower than the temperature of hot water. The heat is transferred from the hot water to atmospheric air. What is the mode of heat transfer in this condition?
a. conduction then convection
b. convection then conduction
c. convection then conduction then convection
d. conduction then convection then conduction

5. Compared to hot air, cool air is
a.light
b.good conductor
c.more strong
d.more dense

6. Warm air rises because it is
a.more dense than cool air
b.equally dense as cool air
c.less dense than cool air
d.completely dense

7. Sunrays warm up the
a.land before the sea
b.sea before the land
c.land and sea at the same time
d.land but not sea

8. If air is heated, it
a.compresses
b.expands
c.falls down
d.gets dense

9. Weather is an example of
a.conduction current
b.convection current
d.air current

10. The primary driving force for natural convection is
A)Shear stress forces
B)Buoyancy forces
C)Surface tension forces
D)None of them

11. The convective heat transfer coefficient in laminar flow over a flat plate
a) Increases with distance
b) Increases if a higher viscosity fluid is used
c) Increases if a denser fluid is used
d) Decreases with increase in free stream velocity
Explanation: It mostly increases if a denser fluid is used.

12. For laminar flow over a flat plate, the average value of a Nusselt number is prescribed by the relation
Nu = 0.664 (Re) 0.5 (Pr) 0.33
Which of the following is then a false statement?
a) Density has to be increased four times
b) Plate length has to be decreased four times
c) Specific heat has to be increased four times
d) Dynamic viscosity has to be decreased sixteen times
Explanation: The dynamic viscosity has an inverse relation to 1/6 power. To double the convective heat transfer coefficient, the dynamic viscosity has to be decreased 64 times.

13. For turbulent flow over a flat plate, the average value of Nusselt number is prescribed by the relation
Nu = 0.664 (Re) 0.5 (Pr) 0.33
Which of the following is then a false statement?
The average heat transfer coefficient increases as
a) 1/5 power of plate length
b) 2/3 power of thermal conductivity
c) 1/3 power of specific heat
d) 4/5 power of a free stream velocity
Explanation: The average heat transfer coefficient reduces with length as 1/5th power of the length.

14. A nuclear reactor with its core constructed of parallel vertical plates 2.25 m high and 1.5 m wide has been designed on free convection heating of liquid bismuth. Metallurgical considerations limit the maximum surface temperature of the plate to 975 degree Celsius and the lowest allowable temperature of bismuth is 325 degree Celsius. Estimate the maximum possible heat dissipation from both sides of each plate. The appropriate correlation for the convection coefficient is
Nu = 0.13 (Gr Pr) 1/3
a) 143 MW
b) 153 MW
c) 163 MW
d) 173 MV
Explanation: Q = 2 h A d t = 153 MW.

15. Consider the above problem, find the value of Grashoff number
a) 101.3 * 10 12
b) 102.3 * 10 12
c) 103.3 * 10 12
d) 104.3 * 10 12
Explanation: Grashof number = l β g d t/µ 2.

16. A thin walled duct of 0.5 m diameter has been laid in an atmosphere of quiescent air at 15 degree Celsius and conveys a particular gas at 205 degree Celsius. Base your calculations on one meter length of the duct, estimate the convective coefficient of heat transfer
a) 5.086 W/m2 K
b) 6.086 W/m2 K
c) 7.086 W/m2 K
d) 8.086 W/m2 K
Explanation: h = 1.37 (d t/l) 0.25 = 5.086 W/m2 K.

17. Free correction modulus is given by
a) p 2 β g c P
b) p 2 β g c P/k
c) p 2 β g c P/µ k
d) p 2 β g c P
Explanation: It contains only fluid properties and is called the free convection modulus.

18. The free convection coefficient is given by
h = C 1 d t m/l 1 – 3m
The value of exponent for laminar flow is
a) 0.5
b) 0.6
c) 0.7
d) 0.8
Explanation: For laminar flow h = C 1 (d t/l) 0.25.

19. For inclined plates we multiply Grashoff number with
a) Cos 2 α
b) Sin 2 α
c) Sin α
d) Cos α
Explanation: It should be multiplied with cos α, as α is angle with the horizontal.

20. The free convection coefficient is given by
h = C 1 d t m/l 1 – 3m
The value of exponent for turbulent flow is
a) 0.43
b) 0.33
c) 0.23
d) 0.13
Explanation: For turbulent flow h = C (d t).

21. For the fluid flowing over a flat plate with Prandtl number greater than unity, the thermal boundary layer for laminar forced convection
A)is thinner than the hydrodynamic boundary layer
B)has thickness equal to zero
C)is of same thickness as hydrodynamics boundary layer
D)is thicker than the hydrodynamic boundary layer

22. For flow of fluid over a heated plate, the following fluid properties are known: Viscosity=0.001 Pa/s, Sp. Heat at constant pressure=1 kJ/kgK, Thermal Conductivity=1W/mK; The hydrodynamic boundary layer thickness at a specified location on the plate is 1mm, thermal boundary layer thickness at the same location is
A) 0.001mm
B) 0.01mm
C) 1mm
D) 1000mm

23. For laminar forced convection over a flat plate, if the free stream velocity increases by a factor of 2, the average heat transfer coefficient
A)remains same
B)decreased by a factor of 20.5
C)mnrises by a factor of 20.5
D)rises by a factor of 4

24. Nusselt number is given by
A)hl/k
B)2hl/k
C)3hl/k
D)4hl/k

25. Assume fluid flowing in a tube forcefully. The velocity boundary layer develops along the tube. Thickness of this boundary layer increases in the flow direction until the boundary layer reaches the tube centre. This region from the tube inlet to the point at which the boundary layers merge at the centerline is called as
a. laminar entry region
b. hydraulic entry region
c. hydrostatic entry region
d. hydrodynamic entry region

26. What is hydrodynamically developed region in fluid flowing inside a pipe?
a. a region where velocity profile of the fluid is partially developed
b. a region where velocity profile of the fluid is fully developed
c. a region where velocity profile of the fluid changes according to the distance
d. none of the above

27. What is the ratio of the buoyancy force to the viscous force acting on a fluid called?
a. Prandtl number (Pr)
b. Reynolds number (Re)
c. Nusselt number (Nu)
d. Grashof number (Gr)

28. In natural convection, the Nusselt number (Nu) depends on
a. Pr and Re
b. Gr and Re
c. Gr and Pr
d. none of the above

29. Which of the following condition is correct for natural convection?
a. (Gr / Re2) = 1
b. (Gr / Re2) << 1
c. (Gr / Re2) >> 1
d. none of the above

30. If there are no externally induced flow velocities, then the Nusselt number (Nu) does not depend upon
a. Prandtl number (Pr)
b. Reynolds number (Re)
c. Grashof number (Gr)
d. none of the above

31. The Grashof number in natural convection plays same role as
a. Prandtl number (Pr) in forced convection
b. Reynolds number (Re) in forced convection
c. Nusselt number (Nu) in forced convection
d. none of the above

32. Which of the following is true for turbulent flow?
a) G r > 108
b) G r > 109
c) G r > 103
d) G r > 1015
Explanation: The product G r is often referred to as Rayleigh number, and its value sets the criterion of turbulent character of flow.

33. Mc Adam relation is given by
a) Nu = 0.023 (Re) 0.8 (Pr) 0.4
b) Nu = 0.023 (Re) 0.8 (Pr) 0.3
c) Nu = 0.023 (Re) 0.8 (Pr) 0.2
d) Nu = 0.023 (Re) 0.8 (Pr) 0.1
Explanation: Here, n = 0.4 if the fluid is being heated.

34. Investigate the effect of following condition on the average value of heat transfer coefficient in flow through a tube
Two fold increases in flow velocity by varying mass flow rate
It may be presumed that there is no change in the temperature of the liquid and the tube wall, and that the flow through the tube is turbulent in character
a) 64.1% decrease
b) 64.1% increase
c) 74.1%decrease
d) 74.1% increases
Explanation: h 2/h = (V 2/V 10.8 = .0741 = 74.1%

35. Consider the above problem for two fold increase in the diameter of the tube, the flow velocity is maintained constant by change in the rate of liquid flow
a) 15%
b) 14%
c) 13%
d) 12%
Explanation: h 2/h = (d 1/d 20.2.

36. Calculate the rate of heat loss from a human body which may be considered as a vertical cylinder 30 cm in diameter, and 175 cm high while standing in a 30 km/hr wind at 15 degree Celsius. Human has a surface temperature of 35 degree Celsius
a) 588.86 W
b) 688.86 W
c) 788.86 W
d) 888.86 W
Explanation: Q = h A d t, h = Nu k/d.

37. A heat treat steel plate measures 3 m by 1 m and is initially at 30 degree Celsius. It is cooled by blowing air parallel to 1 m edge at 9 km/hr. If the air is at 10 degree Celsius. Estimate the convection heat transfer from both sides of the plate
a) 477.7 W
b) 547.7 W
c) 647.7 W
d) 747.6 W
Explanation: Q = h A d t, h = Nu k/d.

38. Consider the above problem, find the value of Reynolds number
a) 166003
b) 177003
c) 188003
d) 199003
Explanation: Re = p V l/µ = 16603

39. The oil pan of 1.6 engine approximates a flat plate 0.3 m wide by 0.45 m long and protrudes below the framework of the automobile. The engine oil is at 95 degree Celsius and the ambient air temperature is 35 degree Celsius. If the automobile runs at 36 km/hr, make calculations for the rate of heat transfer from the oil-pan surface. Assume negligible resistance to conduction through the oil pan
a) 190.37 W
b) 180.37 W
c) 170.37 W
d) 160.37 W
Explanation: Q = h A d t, where h = (Nu) (k)/l and Nu = 0.664 (Re) 0.5 (Pr).33.

40. Air flows through a 10 cm internal diameter tube at the rate of 75 kg/hr. Measurements indicate that at a particular point in the tube, the pressure and temperature of air are 1.5 bar and 325 K whilst the tube wall temperature is 375 K. Find heat transfer rate from one meter length in the region of this point
The general non-dimensional correlation for turbulent flow in the tube is
Nu = 0.023 (Re) 0.8 (Pr) 0.4
Where the fluid properties are evaluated at the bulk temperature
a) 147.35 W
b) 157.35 W
c) 167.35 W
d) 177.35 W
Explanation: Q = h A d t, where h = (Nu) (k)/l = 177.35 W.

41. Consider the above problem, find the value of Nusselt number
a) 40.46
b) 50.56
c) 60.66
d) 70.76
Explanation: Nu = 0.023 (Re) 0.8 (Pr) 0.4 = 40.46.

Boiling and Condensation/Heat Exchangers/Heat Pipe

1. Some examples of heat exchanger are
(i) Condensers and evaporators in refrigeration units
(ii) Evaporator of an ice plant and milk chiller of a pasteurizing plant
(iii) Automobile radiators and oil coolers of heat engines
a) i only
b) ii and iii
c) i, ii and iii
d) i and ii
Explanation: All are the examples of heat exchanger.

2. Heat exchangers are classified into how many categories?
a) 1
b) 2
c) 3
d) 4
Explanation: Nature of heat exchange process, relative direction of motion of fluid, mechanical design of heat exchange surface and physical state of heat exchanging fluids.

3. Based upon the nature of heat exchange process, the heat exchangers are classified into how many categories
a) 1
b) 2
c) 3
d) 4
Explanation: Direct contact, regenerators and recuperators.

4. The energy transfer between the hot fluid and cold fluids is brought about by their complete physical mixing in
a) Direct contact heat exchanger
b) Regenerators
c) Recuperators
d) Boilers
Explanation: In this type of heat exchanger, there is a simultaneous transfer of heat and mass.

5. Which of the following is not an example of recuperators type heat exchanger?
b) Condensers
c) Chemical factories
d) Oil heaters for an aero plane
Explanation: Recuperators are not used in chemical factories.

6. In how many categories heat exchangers are classified on the basis of direction of flow of fluids?
a) 4 categories
b) 3 categories
c) 2 categories
d) 1 categories
Explanation: Parallel, counter and cross flow.

7. In how many categories heat exchangers are classified on the basis of mechanical design of heat exchanger surface?
a) 2
b) 4
c) 1
d) 3
Explanation: Concentric tubes, shell and tube and multiple shell.

8. In how many categories heat exchangers are classified on the basis of physical state of heat exchanging fluids?
a) 1
b) 2
c) 3
d) 4
Explanation: Condenser and evaporator.

9. Many types of heat exchangers have been developed to meet the widely varying applications. Based upon their
(i) Operating principle
(ii) Arrangement of flow path
(iii) Design
Identify the correct statements
a) i, ii and iii
b) i and ii
c) ii and iii
d) i and iii
Explanation: Heat exchanger is a process equipment designed for the effective transfer of heat energy between two fluids.

10. Capacity ratio is defined as the product of
a) Mass and temperature
b) Mass and specific heat
c) Specific heat and temperature
d) Time and temperature
Explanation: The product mass and specific heat of a fluid flowing in a heat exchanger is known as capacity ratio.

11. Which of the following is not associated with a heat exchanger?
a) Fouling
b) NTU
c) Capacity ratio
Explanation: The correction factor i.e. Mc Adam’s is associated with laminar film condensation on a vertical plate.

12. The engine oil at 150 degree Celsius is cooled to 80 degree Celsius in a parallel flow heat exchanger by water entering at 25 degree Celsius and leaving at 60 degree Celsius. Estimate the exchanger effectiveness
a) 0.56
b) 0.66
c) 0.76
d) 0.86
Explanation: Effectiveness = (t h 1 – t h 2) C h /C MIN (t h 1 – t c 2).

13. Consider the above problem, if the fluid flow rates and the inlet conditions remain unchanged, workout the lowest temperature to which the oil may be cooled by increasing length of the exchanger
a) 46.62 degree Celsius
b) 56.62 degree Celsius
c) 66.62 degree Celsius
d) 76.62 degree Celsius
Explanation: Effectiveness = 1 – [exponential [- NTU (1 – C)]/1 + C].

14. In a surface condenser, the water flowing through a series of tubes at the rate of 200 kg/hr is heated from 15 degree Celsius to 75 degree Celsius. The steam condenses on the outside surface of tubes at atmospheric pressure and the overall heat transfer coefficient is estimated at 860 k J/m2 hr degree. Find the effectiveness of the heat exchanger. At the condensing pressure, stream has a saturation temperature 0f 100 degree Celsius and the latent heat of vaporization is 2160 k J/kg. Further, the steam is initially just saturated and the condensate leaves the exchanger without sub-cooling i.e. only latent heat of condensing steam is transferred to the water. Take specific heat of water as 4 k J/kg K
a) 0.224
b) 0.706
c) 2.224
d) 3.224
Explanation: Effectiveness = 1 – exponential (- NTU) and Effectiveness = C (t h 1 – t h 2)/C MIN (t h 1 – t c 2).

15. Consider the above problem, find the tube length. Let the diameter of tube is 25 mm
a) 14.5 m
b) 15.5 m
c) 16.5 m
d) 17.5 m
Explanation: NTU = U (π d l)/C.

16. For evaporators and condensers, for the given conditions, the logarithmic mean temperature difference for parallel flow is
a) Does not depend on counter flow
b) Smaller than counter flow
c) Greater than counter flow
d) Equal to counter flow
Explanation: The temperature of one of the fluid remains constant during the flow passage.

17. Water (specific heat = 4 k J/kg K) enters a cross flow exchanger (both fluids unmixed) at 15 degree Celsius and flows at the rate of 7.5 kg/s. It cools air (C = 1 k J/kg K) flowing at the rate of 10 kg/s from an inlet temperature of 120 degree Celsius. For an overall heat transfer coefficient of 780 k J/m2 hr degree and the surface area is 240 m2, determine the NTU
a) 4.2
b) 5.2
c) 6.2
d) 7.2
Explanation: NTU = U A/C MIN = 5.2.

18. Consider the above problem, find the capacity ratio of the heat exchanger
a) 0.555
b) 0.444
c) 0.333
d) 0.222
Explanation: Capacity ratio = 10/30 = 0.333.

19. Which of the following is/are example/s of heat exchanger?
a. Feed water heater in which a stream of steam is directly mixed with cold water and the mixture leaves at uniform temperature
b. Feed water heater in which a stream of steam and cold water are not mixed and separated by partition through which heat flows
c. both a. and b.
d. none of the above

20. Which of the following phases of designing of heat exchangers does designer consider corrosive nature of the fluid in?
a. The thermal analysis
b. The mechanical design
c. The design for manufacture
d. none of the above

21. Which of the following is NOT a type of heat exchanger?
a. Recuperator
b. Regenerator
c. Mixer
d. none of the above

22. The two fluids are not mixed and kept separated as they both flow through heat exchanger in
a. Transfer type heat exchanger or recuperator
b. Storage type heat exchanger or regenerator
c. Direct contact type heat exchanger or mixer
d. none of the above

23. Which of the following is/are example/s of direct contact type heat exchanger?
a. jet condenser
b. desuperheater
c. cooling tower
d. all of the above

24. The steam condenser in a thermal power plant is a heat exchanger of the type
A)direct contact
B)regenerator
C)recuperator
D)none of these

25. The normal automobile radiator is a heat exchanger of the type
A)direct contact
B)parallel flow
C)counter flow
D)cross flow

26. Choose the correct statement with respect to a counter-flow heat exchanger
A)both the fluids at the inlet are in their coldest state
B)both the fluids at the exit are in their hottest state
C)both the fluids at the inlet are in their hottest state
D)one fluid is hottest and the other is coldest at inlet

27. The requirement of transfer of a large heat is usually met by
A)increase the length of the tube
B)decreasing the diameter of the tube
C)increase the number of tubes
D)having multiple tube or shell passes