# Heat Transfer Test Questions - Set - 6

1)   What is the reason behind the fact that the thermal resistance of hollow cylinder for convection decreases with increase in the thickness of the insulation?

a. volume of insulating material increases and it transfer less heat energy
b. increase in surface area of the cylinder leads to less convective heat transfer
c. increase in surface area of the cylinder leads to more convective heat transfer
d. none of the above
 Answer  Explanation  Related Ques ANSWER: increase in surface area of the cylinder leads to more convective heat transfer Explanation: No explanation is available for this question!

2)   What is the critical thickness of insulation for hollow cylinder?

a. a thickness of insulation at which heat transfer rate from cylinder is minimum
b. a thickness of insulation at which heat transfer rate from cylinder is maximum
c. a thickness of insulation at which heat transfer from cylinder does not change with change in outside temperature
d. none of the above
 Answer  Explanation  Related Ques ANSWER: a thickness of insulation at which heat transfer rate from cylinder is maximum Explanation: No explanation is available for this question!

3)   When the thickness of an insulation is greater than the critical thickness of insulation, then the heat transfer rate of insulated pipe

a. will be more than the heat transfer rate from the same pipe without insulation
b. will be less than the heat transfer rate from the same pipe without insulation
c. will become equal to the heat transfer rate from the same pipe without insulation
d. none of the above
 Answer  Explanation  Related Ques ANSWER: will be less than the heat transfer rate from the same pipe without insulation Explanation: No explanation is available for this question!

4)   When the thickness of insulation is less than the critical thickness of the insulation, then the heat transfer rate of insulated pipe

a. will be more than the heat transfer rate from the same pipe without insulation
b. will be less than the heat transfer rate from the same pipe without insulation
c. will become equal to the heat transfer rate from the same pipe without insulation
d. none of the above
 Answer  Explanation  Related Ques ANSWER: will be more than the heat transfer rate from the same pipe without insulation Explanation: No explanation is available for this question!

5)   If ri and ro are the inner and outer radii of the the cylinder with insulating layer of some thickness and k is the thermal conductivity of the insulating material and hi and ho are the convective heat transfer coefficients of inside and outside fluids respectively. What is the formula for critical radius of insulation (rc)?

a. rc = (ro – ri) / k
b. rc = k / (ro – ri)
c. rc = k / hi
d. rc = k / ho
 Answer  Explanation  Related Ques ANSWER: rc = k / ho Explanation: No explanation is available for this question!

6)   What is the advantage of using the spherical vessel to store fluids at low temperature?

a. heat transfer in sphere is restricted in all direction by insulation
b. sphere has the smallest volume per unit surface area
c. sphere has the largest volume per unit surface area
d. none of the above
 Answer  Explanation  Related Ques ANSWER: sphere has the largest volume per unit surface area Explanation: No explanation is available for this question!

7)   What is the correct formula for thermal resistance (Rk) of a spherical shell of inner and outer radii as ri and ro respectively and k being the thermal conductivity?

a. Rk= (ri – ro ) / 4 π ri ro k

b. Rk= 4 π ri ro k / (ri – ro )

c. Rk= (ri – ro ) / 4 π k

d. Rk= ri ro / 4 π k (ri – ro )
 Answer  Explanation  Related Ques ANSWER: Rk= (ri – ro ) / 4 π ri ro k Explanation: No explanation is available for this question!

8)   What is the formula of the thermal resistance for convection at the outer surface of spherical shell of inner and outer radii as ri and ro respectively? Taking ho as convection heat transfer coefficient of outer fluid.

a. Rc = 4 π ho ro2
b. Rc = 1 / 4 π ho ro2
c. Rc = 4 π ho ro
d. Rc = 1 / 4 π ho ro
 Answer  Explanation  Related Ques ANSWER: Rc = 1 / 4 π ho ro2 Explanation: No explanation is available for this question!

9)   The critical radius of a hollow sphere having thermal conductivity k and ho as convecting heat transfer coefficient of outer fluid is given by

a. ho / k
b. k / ho
c. 2k / ho
d. ho / 2k
 Answer  Explanation  Related Ques ANSWER: 2k / ho Explanation: No explanation is available for this question!

10)   At the critical radius of insulation of a hollow sphere, the heat transfer will be

a. minimum
b. maximum
c. does not change
d. unpredictable
 Answer  Explanation  Related Ques ANSWER: maximum Explanation: No explanation is available for this question!

11)   The buoyancy forces which give rise to the natural convection are called as

a. convection forces
b. fluid forces
c. body forces
d. none of the above
 Answer  Explanation  Related Ques ANSWER: body forces Explanation: No explanation is available for this question!

12)   The intensity of mixing of fluid in natural convection is

a. more than the intensity of mixing of fluid in forced convection
b. less than the intensity of mixing of fluid in forced convection
c. equal to the intensity of mixing of fluid in forced convection
d. unpredictable
 Answer  Explanation  Related Ques ANSWER: less than the intensity of mixing of fluid in forced convection Explanation: No explanation is available for this question!

13)   What is the relation between convection heat transfer coefficients of natural convection and forced convection?

a. convection heat transfer coefficient of natural convection is lower than the convection heat transfer coefficient of forced convection
b. convection heat transfer coefficient of natural convection is more than the convection heat transfer coefficient of forced convection
c. convection heat transfer coefficients in both natural and forced convection are the same for same system
d. unpredictable
 Answer  Explanation  Related Ques ANSWER: convection heat transfer coefficient of natural convection is lower than the convection heat transfer coefficient of forced convection Explanation: No explanation is available for this question!

14)   Below figure shows a natural convection heat transfer on a vertical flat plate surrounded by a fluid.
What is the relation between the upward velocity of the fluid and the distance from the bottom of the plate, when plate is hotter that fluid? a. as the distance from the bottom of the plate increases, the upward velocity of the fluid near the plate surface decreases
b. as the distance from the bottom of the plate increases, the upward velocity of the fluid near the plate surface increases
c. the upward velocity of the fluid near the plate surface is same all over the plate
d. unpredictable
 Answer  Explanation  Related Ques ANSWER: as the distance from the bottom of the plate increases, the upward velocity of the fluid near the plate surface increases Explanation: No explanation is available for this question!

15)   Assume a natural convection heat transfer on a vertical flat plate surrounded by a fluid. Where will be the fully developed turbulent layer of fluid established, if the plate is hotter than the fluid?

a. At the bottom of the plate
b. At the middle of the plate
c. At the top of the plate
d. Nowhere
 Answer  Explanation  Related Ques ANSWER: At the top of the plate Explanation: No explanation is available for this question!

16)   Below figure shows a natural convection heat transfer on a horizontal flat plate and fluid is above it. Which condition satisfies the figure below? a. Plate temperature is lower than the fluid temperature
b. Plate temperature is higher than the fluid temperature
c. Plate temperature is equal to the fluid temperature
d. unpredictable
 Answer  Explanation  Related Ques ANSWER: Plate temperature is higher than the fluid temperature Explanation: No explanation is available for this question!

17)   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
 Answer  Explanation  Related Ques ANSWER: hydrodynamic entry region Explanation: No explanation is available for this question!

18)   What is hydro-dynamically 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  Explanation  Related Ques ANSWER: a region where velocity profile of the fluid is fully developed Explanation: No explanation is available for this question!

19)   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)
 Answer  Explanation  Related Ques ANSWER: Grashof number (Gr) Explanation: No explanation is available for this question!

20)   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
 Answer  Explanation  Related Ques ANSWER: Gr and Pr Explanation: No explanation is available for this question!

21)   The total radiation leaving a surface per unit time per unit surface area is called as

d. none of the above

a. emitted, reflected and absorbed
b. emitted, reflected and transmitted
c. transmitted, reflected and absorbed
d. none of the above
 Answer  Explanation  Related Ques ANSWER: emitted, reflected and transmitted Explanation: No explanation is available for this question!

a. to increase radiant heat transfer
b. to decrease radiant heat transfer
c. to maintain radiant heat transfer constant
d. none of the above
 Answer  Explanation  Related Ques ANSWER: to decrease radiant heat transfer Explanation: No explanation is available for this question!

24)   Which surface can be best described as a radiation shield?

a. the surface which has very high absorptivity
b. the surface which has very high transmissivity
c. the surface which has very high reflectivity
d. none of the above
 Answer  Explanation  Related Ques ANSWER: the surface which has very high reflectivity Explanation: No explanation is available for this question!

25)   Which object can be used as a radiation shield?

a. polished transparent glass
b. mirror
c. rubber
d. all of the above
 Answer  Explanation  Related Ques ANSWER: mirror Explanation: No explanation is available for this question!

26)   Which of the following gases is/are not capable to emit and to absorb radiant heat energy?

a. Nitrogen (N2)
b. Water vapour (H2O )
c. Carbon monoxide (CO)
d. all of the above
 Answer  Explanation  Related Ques ANSWER: Nitrogen (N2) Explanation: No explanation is available for this question!

27)   Which of the following gases is/are capable to emit and to absorb radiant heat energy?

a. Carbon dioxide (CO2)
b. Sulphur dioxide (SO2)
c. Ammonia (NH3)
d. all of the above
 Answer  Explanation  Related Ques ANSWER: all of the above Explanation: No explanation is available for this question!

28)   Gases absorb and emit radiant energy

a. in all wavelengths over the entire spectrum (λ = 0 to ∞)
b. only between narrow ranges of wavelengths
c. only at single constant wavelength
d. unpredictable
 Answer  Explanation  Related Ques ANSWER: only between narrow ranges of wavelengths Explanation: No explanation is available for this question!

29)   Which among the following factors is/are taken into account in calculation of radiation emitted or absorbed by a gas layer?

a. thickness of the gas layer
b. shape of the gas enclosure
c. pressure in gas enclosure
d. all of the above
 Answer  Explanation  Related Ques ANSWER: all of the above Explanation: No explanation is available for this question!

30)   How is the incident radiation absorbed by a gas layer?

a. A gas layer absorbs incident radiation slowly
b. A gas layer absorbs incident radiation rapidly
c. doesn't matter
d. unpredictable
 Answer  Explanation  Related Ques ANSWER: A gas layer absorbs incident radiation slowly Explanation: No explanation is available for this question!

31)   What are the compact heat exchangers?

a. the heat exchangers having small surface area per unit volume
b. the heat exchangers having large surface area per unit volume
c. the heat exchangers having small surface area per unit weight
d. the heat exchangers having large surface area per unit weight
 Answer  Explanation  Related Ques ANSWER: the heat exchangers having large surface area per unit volume Explanation: No explanation is available for this question!

32)   The heat exchanger is said to be compact when its area density is

a. equal to 700 m2/ m3
b. less than 700 m2/ m3
c. more than 700 m2/ m3
d. unpredictable
 Answer  Explanation  Related Ques ANSWER: more than 700 m2/ m3 Explanation: No explanation is available for this question!

33)   Which of the following can be considered as more compact efficient heat exchanger?

b. Stirling engine regenerator
c. Ceramic regenerator in gas turbine
d. none of the above
 Answer  Explanation  Related Ques ANSWER: Stirling engine regenerator Explanation: No explanation is available for this question!

34)   The compact heat exchangers are commonly used in

a. gas to gas heat transfer
b. gas to liquid heat transfer
c. both a. and b.
d. none of the above
 Answer  Explanation  Related Ques ANSWER: both a. and b. Explanation: No explanation is available for this question!

35)   Usually, the flow arrangement of two fluids in compact heat exchangers is

a. parallel flow
b. counter flow
c. cross flow
d. none of the above
 Answer  Explanation  Related Ques ANSWER: cross flow Explanation: No explanation is available for this question!