Deck 21: Heat Transfer and Fluid Mechanics

Question 1

Cpµ/K is termed as the __________ number.

A)Grashoff
B)Nusselt
C)Prandtl
D)Stanton

Answer 1

Prandtl

Question 2

The overall heat transfer co-efficient for a shell and tube heat exchanger for clean surfaces is U0 = 400 W/m².K. The fouling factor after one year of operation is found to be hd0 = 2000W/m².K. The overall heat transfer co-efficient at this time is

A)1200 W/m².K
B)894 W/m².K
C)333 W/m².K
D)287 W/m².K

Answer 2

333 W/m².K

Question 3

The Graetz number is concerned with the

A)Mass transfer between a gas and a liquid
B)Absorption with chemical reaction
C)Heat transfer in turbulent flow
D)Heat transfer in laminar flow

Answer 3

Heat transfer in laminar flow

Question 4

At what value of Prandtl number, conduction is negligible in the turbulent core of a fluid flowing through a heated pipe?

A)0.5
B)< 0.5
C)> 0.6
D)< 0.1

Question 5

The unit of conductance in SI unit is

A)W/m
B)W/m²
C)W/°K
D)W/m°K

Question 6

Pick out the wrong statement.

A)Bubble size increases with the dynamic viscosity of the liquid in case of nucleate pool boiling
B)Thermal conductivity of a dry material is more than that of the damp material
C)Ratio of its capacity to economy equals the steam consumption in kg/hr in an evaporator
D)Vaporisation of organic substances in evaporator mostly causes foam formation

Question 7

Pick out the correct statement.

A)Higher is the temperature of the radiating body, higher is the wavelength of radiation
B)Logarithmic mean area is used for calculating the heat flow rate through a thick walled cylinder
C)The wavelength corresponding to maximum mono-chromatic emissive power increases with rise in temperature
D)Solid angle subtended by the finite surface at the radiating element is called the angle of incidence

Question 8

Nucleate boiling is promoted

A)On polished surfaces
B)On rough surfaces
C)In the absence of agitation
D)None of these

Question 9

Which of the following has the lowest Prandtl number?

A)Molten sodium (a coolant used in fast breeder reactor)
B)Water
C)Transformer oil
D)Dilute H2SO4

Question 10

The thermal boundary layer at N_Pr > 1

A)Is thicker than hydrodynamic boundary layer
B)Is thinner than hydrodynamic boundary layer
C)And the hydrodynamic boundary layers are identical
D)Disappears

Question 11

In case of a multipass shell and tube heat exchanger, the temperature drop in the fluid

A)Is inversely proportional to the resistance across which the drop occurs
B)And the wall are proportional to individual resistances
C)And the wall is not related
D)None of these

Question 12

Which of the following is directly concerned with the heat transfer?

A)Strouhal number
B)Sherwood number
C)Euler number
D)Grashoff number

Question 13

The capacity of double-effect evaporator is less than half of the capacity of two single effects, each of which is operating over same terminal temperature difference, when the

A)Solution has an elevation of boiling point
B)Evaporators operate under vacuum
C)Evaporators operate at atmospheric pressure
D)None of these

Question 14

The equation, Nst = f/2, is the __________ analogy.

A)Colburn
B)Reynolds
C)Prandtl
D)None of these

Question 15

Nusselt number is a function of Prandtl number and __________ number of fluid in natural convection heat transfer.

A)Grashoff
B)Biot
C)Stanton
D)Reynolds

Question 16

Forced circulation evaporators are normally used for concentrating liquids having

A)Scaling characteristics
B)High viscosity
C)Both (A) & (B)
D)Neither (A) nor (B)

Question 17

Radiation energy is emitted by all the substances, which are above

A)0°K
B)0°C
C)100°C
D)Room temperature

Question 18

View factor is important in heat transfer by

A)Steady state conduction
B)Natural convection
C)Forced convection
D)Radiation

Question 19

Pick out the wrong statement.

A)Economy of a multiple effect evaporator is not influenced by the boiling point elevation
B)Two identical cubes of iron and copper will have the same heat content under the same conditions of temperature
C)Double pipe heat exchangers are mostly used in the field of refrigeration
D)Finned tube heat exchangers are suitable for heating air by steam

Question 20

'Duhring's plot' is of use in

A)Extractive distillation
B)Evaporation
C)Leaching
D)Absorption

Question 21

The dimensionless group in mass transfer that is equivalent to Prandtl number in heat transfer is

A)Nusselt number
B)Sherwood number
C)Schmidt number
D)Stanton number

Question 22

Pick out the wrong statement.

A)In drying a solid containing moisture above the critical moisture content the number of degrees of freedom is 2
B)Sherwood number in mass transfer corresponds to Nusselt number in heat transfer and Schmidt number to Prandtl number
C)Forced convection is relatively more effective in increasing the rate of mass transfer, if Schmidt number is larger
D)Hot gases at moderate pressure are usually in the shell side of shell and tube heat exchangers. At higher pressure, however, it is customary to put gas in the tube side

Question 23

Loss of heat from untagged steam pipe to the ambient air is by

A)Conduction
B)Convection
C)Radiation
D)All (A), (B) & (C)

Question 24

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

Question 25

Heat flux increases with temperature drop beyond the Leiden frost point in the plot of heat flux vs. temperature drop for a boiling liquid, because

A)Convection becomes important
B)Conduction becomes important
C)Radiation becomes important
D)Sub-cooled boiling occurs