Deck 9: Heat Transfer and Thermal Properties: Part A

Question 1

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
E)depends on temperature conditions.

Answer 1

higher

Question 2

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
E)none of the above.

Answer 2

hot medium outlet and cold water outlet

Question 3

In counter flow heat exchangers

A)both the fluids at inlet (of heat ex¬changer where hot fluid enters) are in their coldest state
B)both the fluids at inlet are in their hot¬test state
C)both the fluids .at exit are in their hottest state
D)one fluid is in hottest state and other in coldest state at inlet
E)any combination is possible depending on design of heat exchanger.

Answer 3

both the fluids at inlet are in their hot¬test state

Question 4

A steam pipe is to be insulated by two insulating materials put over each other. For best results

A)better insulation should be put over pipe and better one over it
B)inferior insulation should be put over pipe and better one over it
C)both may be put in any order
D)whether to put inferior OIL over pipe or the better one would depend on steam temperature
E)uN_Predictable.

Question 5

Fourier's law of heat conduction is valid for

A)one dimensional cases only
B)two dimensional cases only
C)three dimensional cases only
D)regular surfaces having non-uniform temperature gradients
E)irregular surfaces.

Question 6

According of Kirchhoff's law,

A)radiant heat is proportional to fourth power of absolute temperature
B)emissive power depends on temperature
C)emissive power and absorptivity are constant for all bodies
D)ratio of emissive power to absorptive power is maximum for perfectly black body
E)ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body.

Question 7

All radiations in a black body are

A)reflected
B)refracted
C)transmitted
D)absorbed
E)partly reflected and partly absorbed.

Question 8

According to Kirchoff's law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a

A)grey body
B)brilliant white polished body
C)red hot body
D)black body
E)none of the above.

Question 9

The concept of overall coefficient of heat transfer is used in case of heat transfer by

A)conduction
B)convection
C)radiation
D)conduction and convection
E)convection and radiation.

Question 10

Joule sec is the unit of

A)universal gas constant
B)kinematic viscosity
C)thermal conductivity
D)Planck's constant
E)none of the above.

Question 11

The value of Prandtl number for air is about

A)0.1
B)0.3
C)0.7
D)1.7
E)10.5.

Question 12

The value of the wavelength for maximum emissive power is given by -

A)Wien's law
B)Planck's law
C)Stefan's law
D)Fourier's law
E)Kirchhoff's law.

Question 13

Log mean temperature difference in case of counter flow compared to parallel flow will be

A)same
B)more
C)less
D)depends on other factors
E)none of the above.

Question 14

The energy distribution of an ideal reflector at higher temperatures is largely in the range of

A)shorter wavelength
B)longer wavelength
C)remains same at all wavelengths
D)wavelength has nothing to do with it
E)none of the above.

Question 15

Total emissivity of polished silver compared to black body is

A)same
B)higher
C)more or less same
D)very much lower
E)very much higher.

Question 16

According to Stefan-Boltzmann law, ideal radiators emit radiant energy at a rate proportional to

A)absolute temperature
B)square of temperature
C)fourth power of absolute temperature
D)fourth power of temperature
E)cube of absolute temperature.

Question 17

Which of the following property of air does not increase with rise in temperature

A)thermal conductivity
B)thermal diffusivity
C)density
D)dynamic viscosity
E)kuiematic viscosity.

Question 18

The unit of Stefan Boltzmann constant is

A)watt/cm² °K
B)watt/cmm⁴ °K
C)watt²/cm °K⁴
D)watt/cm² °K⁴
E)watt/cm² °K².

Question 19

In free con-vection heat transfer, Nusselt number is function of

A)Grashoff no. and Reynold no.
B)Grashoff no. and Prandtl no.
C)Prandtl no. and Reynold no.
D)Grashoff no., Prandtl no. and Reynold no.
E)none of the above.

Question 20

The thermal diffusivities for gases are generally

A)more than those for liquids
B)less than those for liquids
C)more than those for solids
D)dependent on the viscosity
E)same as for the liquids.

Question 21

The thermal diffusivities for solids are generally

A)less than those for gases
B)jess than those for liquids
C)more than those for liquids and gases
D)more or less same as for liquids and gases
E)zerci.

Question 22

Thermal diffusivity of a substance is

A)directly proportional to thermal con¬ductivity
B)inversely proportional to density of substance
C)inversely proportional to specific heat
D)all of the above
E)none of the above.

Question 23

The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body. This statement is known as

A)Krichoff's law
B)Stefan's law
C)Wien' law
D)Planck's law
E)Black body law.

Question 24

According to Stefan's law, the total radiation from a black body per second per unit area is proportional to

A)absolute temperature
B)T2
C)T5
D)t
E)l/T.