Thermal conductivity of water at 20°C is of the order of
0.1
0.23
0.42
0.51
Correct Answer :
D. 0.51
Related Questions
According to Stefan Boltzmann law, ideal radiators emit radiant energy at a rate proportional to
Absolute temperature
Square of temperature
Fourth power of absolute temperature
Fourth power of temperature
Thermal diffusivity of a substance is given by (where h = Thermal diffusivity, ρ = Density of substance, S = Specific heat, and k = Thermal conductivity)
h = k/ ρS
h = ρS/k
h = S/ρk
h = kρ/S
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
Stefan Boltzmann law is applicable for heat transfer by
Conduction
Convection
Radiation
Conduction and radiation combined
Thermal diffusivity of a substance is
Directly proportional to thermal conductivity
Inversely proportional to density of substance
Inversely proportional to specific heat
All of the above
Emissivity of a white polished body in comparison to a black body is
Higher
Lower
Same
Depends upon the shape of body
An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m²). The critical thickness of insulation will be
25 mm
40 mm
160 mm
800 mm
The heat transfer by conduction through a thick cylinder (Q) is given by (where T₁ = Higher temperature, T₂ = Lower temperature, r₁ = Inside radius, r₂ = Outside radius, l = Length of cylinder, and k = Thermal conductivity)
Q = [2πlk (T₁ - T₂)]/2.3 log (r₂/r₁)
Q = 2.3 log (r₂/r₁)/[2πlk (T₁ - T₂)]
Q = [2π (T₁ - T₂)]/2.3 lk log (r₂/r₁)
Q = = 2πlk/2.3 (T₁ - T₂) log (r₂/r₁)
A steam pipe is to be lined with two layers of insulating materials of different thermal conductivities. For the minimum heat transfer,
The better insulation must be put inside
The better insulation must be put outside
One could place either insulation on either side
One should take into account the steam temperature before deciding as to which insulation is put where
The automobile radiator is a heat exchanger of
Parallel flow type
Counter flow type
Cross flow type
Regenerator type
In free convection heat transfer transition from laminar to turbulent flow is governed by the critical value of the
Reynold's number
Grashoff's number
Reynold's number, Grashoff's number
Prandtl number, Grashoff's number
The rate of heat flow through a body is Q = [kA (T₁ - T₂)]/x. The term x/kA is known as
Thermal coefficient
Thermal resistance
Thermal conductivity
None of these
The total emissivity power is .defined as the total amount of radiation emitted by a black body per unit
Temperature
Thickness
Area
Time
Which of the following is expected to have highest thermal conductivity?
Steam
Solid ice
Melting ice
Water
The concept of overall coefficient of heat transfer is used in heat transfer problems of
Conduction
Convection
Radiation
Conduction and convection
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
The process of heat transfer from one particle of the body to another by the actual motion of the heated particles, is called
Conduction
Convection
Radiation
None of these
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
The value of the wavelength for maximum emissive power is given by
Wien's law
Planck's law
Stefan's law
Fourier's law
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
A perfect black body is one which
Is black in colour
Reflects all heat
Transmits all heat radiations
Absorbs heat radiations of all wave lengths falling on it
The use of heat exchangers is made in
Radiators in automobile
Condensers and boilers in steam plants
Condensers and evaporators in refrigeration and air conditioning units
All of the above
The ratio of the energy absorbed by the body to total energy falling on it is called
Absorptive power
Emissive power
Absorptivity
Emissivity
The insulation ability of an insulator with the presence of moisture would
Increase
Decrease
Remain unaffected
May increase/decrease depending on temperature and thickness of insulation
Moisture would find its way into insulation by vapour pressure unless it is prevented by
High thickness of insulation
High vapour pressure
Less thermal conductivity insulator
A vapour seal
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
The critical radius is the insulation radius at which the resistance to heat flow is
Maximum
Minimum
Zero
None of these
Thermal conductivity of water ________ with rise in temperature.
Remain same
Decreases
Increases
May increase or decrease depending upon temperature
In free convection heat transfer, Nusselt number is function of
Grashoff number and Reynold number
Grashoff number and Prandtl number
Prandtl number and Reynold number
Grashoff number, Prandtl number and Reynold number
Thermal diffusivity is a
Function of temperature
Physical property of a substance
Dimensionless parameter
All of these