Deck 2: Heat Transfer and Thermal Conductivity: Part A

Question 1

The concept of overall coefficient of heat transfer is used in heat transfer problems of

A)conduction
B)convection
C)radiation
D)all the three combined
E)conduction and comte_ction.

Answer 1

conduction and comte_ction.

Question 2

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).

Answer 2

cm (thickness)

Question 3

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.

Answer 3

all of the above.

Question 4

Which of the following has least value of conductivity

A)glass
B)water
C)plastic
D)rubber
E)air.

Question 5

Which of the following is expected to have highest thermal conductivity

A)steam
B)solid ice
C)melting ice
D)water
E)boiling water.

Question 6

Thermal conductivity of glass-wool varies from sample to sample because of variation in

A)composition
B)density
C)porosity
D)structure
E)all of the above.

Question 7

Thermal conductivity of a material may be defined as the

A)quantity of heat flowing in one second through one cm cube of material when opposite faces ^re maintained at a temperature difference of 1°C
B)quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C
C)heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces
D)all of the above
E)none of the above.

Question 8

Which of the following has maximum value of thermal conductivity

A)aluminium
B)steel
C)brass
D)copper
E)lead.

Question 9

Moisture would find its way into insulation by vapour pressure unless it is prevented by

A)high thickness of insulation
B)high vapour pressure
C)less thermal conductivity insulator
D)a vapour seal
E)all of the above.

Question 10

Heat is transferred by all three modes of transfer, viz, conduction, convection and radiation in

A)electric heater
B)steam condenser
C)melting of ice
D)refrigerator condenser coils
E)boiler.

Question 11

According to Prevost theory of heat exchange

A)it is impossible to transfer heat from low temperature source to t high temperature source
B)heat transfer by radiation requires no medium
C)all bodies above absolute zero emit radiation
D)heat transfer in most of the cases takes place by combination of conduction, convection and radiation
E)rate of heat transfer depends on thermal conductivity and temperature difference.

Question 12

The ratio of heat flow Q₁/Q₂ from two walls of same thickness having their thermal conductivities as K₁ - 2K₂ will be

A)I
B)0.5
C)2
D)0.25
E)4.0

Question 13

Heat transfer by radiation mainly depends upon

A)its temperature
B)nature of the body
C)kind and extent of its surface
D)all of the above
E)none of the above.

Question 14

Thermal diffusivity is

A)a dimensionless parameter
B)function of temperature
C)used as mathematical model
D)a physical property of the material
E)useful in case of heat transfer by radiation.

Question 15

Thermal diffusivity of a substance is .

A)proportional of thermal conductivity
B)inversely proportional to k
C)proportional to (k)
D)inversely proportional to K₂
E)none of the above.

Question 16

Unit of thermal diffusivity is

A)m²/hr
B)m²/hr°C
C)kcal/m² hr
D)kcal/m.hr°C
E)kcal/m² hr°C.

Question 17

Thermal conductivity of wood depends on

A)moisture
B)density
C)temperature
D)all of the above
E)none of the above.

Question 18

In convection heat transfer from hot flue gases to water tube, even though flow may be turbulent, a laminar flow region (boundary layer of film) exists close to the tube. The heat transfer through this film takes place by

A)convection
B)radiation
C)conduction
D)both convection and conduction
E)none of the above.

Question 19

Film coefficient is defined as Inside diameter of tube

A)Equivalent thickness of film
B)Thermal conductivity Equivalent thickness of film Specific heat x Viscocity
C)Thermal conductivity Molecular diffusivity of momentum Thermal diffusivity
D)Film coefficient x Inside diameter Thermalconductivity
E)none of the above.

Question 20

Heat conducted througfi unit area and unit thick face per unit time when temperature difference between opposite faces is unity,is called

A)thermal resistance
B)thermal coefficient
C)temperature gradient
D)thermal conductivity
E)heat-transfer.

Question 21

The rate of energy emission from unit surface area through unit solid angle, along a normal to the surface, is known as

A)emissivity
B)transmissivity
C)reflectivity
D)intensity of radiation
E)absorptivity.

Question 22

Emissivity of a white polished body in comparison to a black body is

A)higher
B)lower
C)same
D)depends upon the shape of body
E)none of the above.

Question 23

A grey body is one whose absorptivity

A)varies with temperature
B)varies with wavelength of the incident ray
C)is equal to its emissivity
D)does not vary with temperature and. wavelength of the incident ray
E)none of the above.

Question 24

Two balls of same material and finish have their diameters in the ratio of 2 : 1 and both are heated to same temperature and allowed to cool by radiation. Rate of cooling by big ball as compared to smaller one will be in the ratio of

A)1 :1
B)2: 1
C)1 : 2
D)4 : 1
E)1 : 4.

Question 25

A non-dimensional number generally associated with natural convection heat transfer is

A)Grashoff number
B)Nusselt number
C)Weber number
D)Prandtl number
E)Reynold number.