Deck 18: Heat Transfer Mechanisms in a Spaced Plate System

Question 1

According to Wien's law, the wavelength corresponding to maximum energy is proportion to

A)absolute temperature (T)
B)I²
C)f
D)t
E)1/r.

Answer 1

absolute temperature (T)

Question 2

Depending on the radiating properties, a body will be white when (where a = absorptivity, p = reflectivity, x = transmissivity)

A)p = 0, x = 0 and a = 1
B)p=l,T = 0anda = 0
C)p = 0, x = 1 and a = 0
D)x = 0, a + p = 1
E)a = 0, x + p = 1.

Answer 2

p=l,T = 0anda = 0

Question 3

Depending on the radiating properties, a body will be black when (where a = absorptivity, p == reflectivity, X = transmissivity.)

A)p = 0, x = 0 and a = 1
B)p= l,T = 0anda = 0
C)p = 0, x = 1 and a = 0
D)x = 0, a + p = 0
E)a = 0,x + p= 1.

Answer 3

p = 0, x = 0 and a = 1

Question 4

Depending on the radiating properties, a body will be opaque when (where a = absorptivity, p = reflectivity, X = transmissivity.)

A)p = 0, x = 0 and a = 1
B)p=l,x = 0anda = 0
C)p = 0, x = 1 and a = 0
D)x - 0, a + p = 1
E)a=0,x + p= 1.

Question 5

The total emissivity power is .defined as the total amount of radiation emitted by a black body per unit

A)temperature
B)thickness
C)area
D)time
E)area and time.

Question 6

The ratio of the energy absorbed by the body to total energy falling on it is called

A)absorptive power
B)emissive power
C)absorptivity
D)emissivity
E)none of the above.

Question 7

40% of incident radiant energy on the surface of a thermally transparent body is reflected back. If the transmissivity of the body be 0.15, then the emissivity of surface is

A)0.45
B)0.55
C)0.40
D)0.75
E)0.60.

Question 8

The emissive power of a body depends upon its

A)temperature
B)wave length
C)physical nature
D)all of the above
E)none of the above.

Question 9

Two plates spaced 150 mm apart are maintained at 1000°C and 70°C. The heat transfer will take place mainly by

A)convection
B)free convection
C)forced convection
D)radiation
E)radiation and convection.

Question 10

Absorptivity of a body will be equal to its emissivity

A)at all temperatures
B)at one particular temperature
C)when system is under thermal equi-librium
D)at critical temperature
E)for a polished body.

Question 11

In regenerator type heat exchanger, heat transfer takes place by

A)direct mixing of hot and cold fluids
B)a complete separation between hot and cold fluids
C)flow of hot and cold fluids alternately over a surface
D)generation of heat again and again
E)indirect transfer.

Question 12

A perfect black body is one which

A)is black in colour
B)reflects all heat
C)transmits all heat radiations
D)abslprbs heat radiations of all wave lengths falling on it
E)fully opaque.

Question 13

Planck's law holds good for

A)black bodies
B)polished bodies
C)all coloured bodies
D)all of the above
E)none of the above.

Question 14

If the temperature of a solid surface changes form 27°C to 627°C, then its emissive power changes in the ratio of

A)3
B)6
C)9
D)27
E)81.

Question 15

Depending on the radiating properties, body will be transparent when

A)p = 0, x = 0 and a = 1
B)p=l,x = 0,anda = 0
C)p = 0, T= l,anda = 0
D)X = 0, a + p = 1
E)a = 0,x + p= 1.

Question 16

A grey body is one whose absorptivity

A)varies with temperature
B)varies with the wave length of incident ray
C)varies with both
D)does not vary with temperature and wave length of the incident ray
E)there is no such criterion.

Question 17

Maximum water velocity in tubes of a 1-2 shell and tube heat exchanger may be around __________ metres/second.

A)1
B)10
C)20
D)30

Question 18

For the same heat load and mass flow rate in the tube side of a shell and tube heat exchanger, one may use multipass on the tube side, because it

A)Decreases the pressure drop
B)Decreases the outlet temperature of cooling medium
C)Increases the overall heat transfer coefficient
D)None of these

Question 19

A tank painted with which of the following coloured paints, would heat up maximum by radiation from sun?

A)Yellow paint
B)White paint
C)Black paint
D)Grey paint

Question 20

Walls of a cubical oven are of thickness l, and they are made of material of thermal conductivity k. The temperature inside the oven is 100°C and the inside heat transfer coefficient is „3k/l'. If the wall temperature on the outside is held at 25°C, what is the inside wall temperature in degree centigrade?

A)35.5
B)43.75
C)81.25
D)48.25

Question 21

Three materials A, B and C of equal thickness and of thermal conductivity of 20, 40 & 60 kcal/hr. m. °C respectively are joined together. The temperature outside of A and C are 30°C and 100°C respectively. The interface between B and C will be at a temperature of __________ °C.

A)40
B)95
C)70
D)50

Question 22

A graph between __________ is called Wilson plot.

A)(1/U) Vs. (1/v 0.8)
B)(1/v 0.8) Vs. U
C)v 0.8 Vs . U
D)(1/U) Vs. (1/V)

Question 23

The thermal radiation emitted by a body is proportional to T n, where T is its absolute temperature. The value of 'n' is exactly 4 for

A)Black painted bodies only
B)All bodies
C)Polished bodies only
D)A black body

Question 24

For a laminar flow of fluid in a circular tube, 'h1' is the convective heat transfer co-efficient at velocity 'V1'. If the velocity is reduced by half and assuming the fluid properties are constant, the new convective heat transfer co-efficient is

A)1.26 h1
B)0.794 h1
C)0.574 h1
D)1.741 h1