The highest thermal diffusivity is of
Iron
Lead
Concrete
Wood
Correct Answer :
B. Lead
Related Questions
The unit of overall coefficient of heat transfer is
kcal/m²
kcal/hr °C
kcal/m² hr °C
kcal/m hr °C
The amount of radiation mainly depends on
Nature of body
Temperature of body
Type of surface of body
All of the above
The process of heat transfer from one particle of the fluid to another by the actual movement of the fluid particles due to difference of density caused by temperature of the particle is known as
Conduction
Free convection
Forced convection
Radiation
The product of Reynolds number and Prandtl number is known as
Stanton number
Biot number
Peclet number
Grashoff number
In regenerator type heat exchanger, heat transfer takes place by
Direct mixing of hot and cold fluids
A complete separation between hot and cold fluids
Flow of hot and cold fluids alternately over a surface
Generation of heat again and again
If the energy radiated per second per sq. cm. of the surface for wave lengths lying between λ, and λ + dλ is represented by (eλ.dλ), then eλ is called
Absorptive power
Emissive power
Emissivity
None of these
When α is absorptivity, ρ is reflectivity and τ is transmissivity, then for a diathermanous body,
α = 1, ρ = 0 and τ = 0
α = 0, ρ = 1 and τ = 0
α = 0, ρ = 0 and τ = 1
α + ρ = 1 and τ = 0
The ratio of surface convection resistance to the internal conduction resistance is known as
Grashoff number
Biot number
Stanton number
Prandtl number
A cube at high temperature is immersed in a constant temperature bath. It loses heat from its top, bottom and side surfaces with heat transfer coefficients of h₁, h₂ and h₃ respectively. The average heat transfer coefficient for the cube is
h₁ + h₂ + h₃
(h₁.h₂.h₃)1/3
1/h₁ + 1/h₂ + 1/h₃
None of these
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 emissive power of a body depends upon its
Temperature
Wave length
Physical nature
All of the above
Thermal conductivity of water in general with rise in temperature
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
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
Which of the following is expected to have highest thermal conductivity?
Steam
Solid ice
Melting ice
Water
Fourier's law of heat conduction gives the heat flow for
Irregular surfaces
Nonuniform temperature surfaces
One dimensional cases only
Two dimensional cases only
Unit of thermal conductivity in M.K.S. units is
K cal/kg m² °C
K cal m/hr m² °C
K cal/hr m² °C
K calm/hr °C
The transfer of heat by molecular collision is smallest in
Solids
Liquids
Gases
None of these
When absorptivity (α) = 1, reflectivity (ρ) = 0 and transmissivity (τ) = 0, then the body is said to be a
Black body
Grey body
Opaque body
White body
The ratio of the energy absorbed by the body to total energy falling on it is called
Absorptive power
Emissive power
Absorptivity
Emissivity
According to Kirchoff's law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a
Grey body
Brilliant white polished body
Red hot body
Black body
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
Upto the critical radius of insulation,
Added insulation will increase heat loss
Added insulation will decrease heat loss
Convective heat loss will be less than conductive heat loss
Heat flux will decrease
According to Newton's law of cooling, the heat transfer from a hot body to a cold body is
Directly proportional to the surface area
Directly proportional to the difference of temperatures between the two bodies
Either (A) or (B)
Both (A) and (B)
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
Convection
Radiation
Conduction
Both convection and conduction
The concept of overall coefficient of heat transfer is used in heat transfer problems of
Conduction
Convection
Radiation
Conduction and convection
In heat transfer, conductance equals conductivity (kcal/hr/sq.m/ °C/cm) divided by
Hr (time)
Sq. m (area)
°C (temperature)
K.cal (heat)
Heat is closely related with
Liquids
Energy
Temperature
Entropy
The logarithmic mean temperature difference (tm) is given by (where Δt1 and Δt2 are temperature differences between the hot and cold fluids at entrance and exit)
tm = (Δt1 - Δt2)/ loge (Δt1/Δt2)
tm = loge (Δt1/Δt2)/ (Δt1 - Δt2)
tm = tm = (Δt1 - Δt2) loge (Δt1/Δt2)
tm = loge (Δt1 - Δt2)/ Δt1/Δt2
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 unit of overall coefficient of heat transfer is
W/m²K
W/m²
W/mK
W/m