The thermal diffusivities for gases are generally
More than those for liquids
Less than those for liquids
More than those for solids
Dependent on the viscosity
Correct Answer :
A. More than those for liquids
Related Questions
In heat exchangers, degree of approach is defined as the difference between temperatures of
Cold water inlet and outlet
Hot medium inlet and outlet
Hot medium outlet and cold water inlet
Hot medium outlet and cold water outlet
According to Stefan Boltzmann law, the total radiation from a black body per second per unit area is directly proportional to the
Absolute temperature
Square of the absolute temperature
Cube of the absolute temperature
Fourth power of the absolute temperature
Film coefficient is defined as Inside diameter of tube
Equivalent thickness of film
Thermal conductivity Equivalent thickness of film Specific heat × Viscosity
Thermal conductivity Molecular diffusivity of momentum Thermal diffusivity
Film coefficient × Inside diameter Thermal conductivity
The heat transfer from a hot body to a cold body is directly proportional to the surface area and difference of temperatures between the two bodies. This statement is called
First law of thermodynamics
Newton's law of cooling
Newton's law of heating
Stefan's law
Which of the following is a case of steady state heat transfer?
I.C. engine
Air preheaters
Heating of building in winter
None of the above
Film coefficient is defined as the ratio of
Thermal conductivity to the equivalent thickness of the film of fluid
Temperature drop through the films of fluids to the thickness of film of fluids
Thickness of film of fluid to the thermal conductivity
Thickness of film of fluid to the temperature drop through the films of fluids
According to Wien's law, the wavelength corresponding to maximum energy is proportion to
Absolute temperature (T)
I²
F
T
The value of the wave length for maximum emissive power is given by
Kirchhoff's law
Stefan's law
Wines law
Planck's law
The critical thickness of insulation for a sphere is
k/h₀
2k/h₀
h₀/k
h₀/2k
In counter current flow heat exchanger, the logarithmic temperature difference between the fluids is ________ as compared to parallel flow heat exchanger.
Same
Less
Greater
None of these
Conduction is a process of heat transfer
From one particle of the body to another without the actual motion of the particles
From one particle of the body to another by the actual motion of the heated particles
From a hot body to a cold body, in a straight line, without affecting the intervening medium
None of the above
The value of Prandtl number for air is about
0.1
0.3
0.7
1.7
When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by
Conduction
Convection
Radiation
Conduction and convection
The ratio of Nusselt number and the product of Reynold's number and Prandtl number is equal to
Stanton number
Biot number
Peclet number
Grashoff number
Which of the following is expected to have highest thermal conductivity?
Steam
Solid ice
Melting ice
Water
The amount of radiation mainly depends upon the
Nature of the body
Temperature of the body
Type of surface of the body
All of these
Planck's law holds good for
Black bodies
Polished bodies
All coloured bodies
All of the above
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
1 : 1
2 : 1
1 : 2
4 : 1
The product of Reynolds number and Prandtl number is known as
Stanton number
Biot number
Peclet number
Grashoff number
When heat is transferred by molecular collision, it is referred to as heat transfer by
Conduction
Convection
Radiation
Scattering
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
Kirchoffs law
Stefan's law
Wien' law
Planck's law
Thermal conductivity of non-metallic amorphous solids with decrease in temperature
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
The highest thermal diffusivity is of
Iron
Lead
Concrete
Wood
The concept of overall coefficient of heat transfer is used in heat transfer problems of
Conduction
Convection
Radiation
Conduction and convection
Emissivity of a white polished body in comparison to a black body is
Higher
Lower
Same
Depends upon the shape of body
A steam pipe is to be insulated by two insulating materials put over each other. For best results
Better insulation should be put over pipe and better one over it
Inferior insulation should be put over pipe and better one over it
Both may be put in any order
Whether to put inferior OIL over pipe or the better one would depend on steam temperature
Joule sec is the unit of
Universal gas constant
Kinematic viscosity
Thermal conductivity
Planck's constant
Depending on the radiating properties, body will be transparent when
P = 0, x = 0 and a = 1
P=1, x = 0, and a = 0
P = 0, T= 1, and a = 0
X = 0, a + p = 1
The unit of overall coefficient of heat transfer is
kcal/m²
kcal/hr °C
kcal/m² hr °C
kcal/m hr °C
Thermal conductivity of wood depends on
Moisture
Density
Temperature
All of the above