Reynolds number (RN) is given by (where h = Film coefficient, l = Linear dimension, V = Velocity of fluid, k = Thermal conductivity, t = Temperature, ρ = Density of fluid, cp = Specific heat at constant pressure, and μ = Coefficient of absolute viscosity)
RN = hl/k
RN = μ cp/k
RN = ρ V l /μ
RN = V²/t.cp
Correct Answer :
C. RN = ρ V l /μ
Related Questions
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
Reynolds number (RN) is given by (where h = Film coefficient, l = Linear dimension, V = Velocity of fluid, k = Thermal conductivity, t = Temperature, ρ = Density of fluid, cp = Specific heat at constant pressure, and μ = Coefficient of absolute viscosity)
RN = hl/k
RN = μ cp/k
RN = ρ V l /μ
RN = V²/t.cp
An ordinary passenger aircraft requires a cooling system of capacity.
2 TR
4 TR
8 TR
10 TR
In case of liquids and gases, the heat transfer takes place according to
Conduction
Convection
Radiation
None of these
Which of the following would lead to a reduction in thermal resistance?
In conduction, reduction in the thickness of the material and an increase in thermal conductivity.
In convection, stirring of the fluid and cleaning the heating surface.
In radiation, increasing the temperature and reducing the emissivity.
All of the above
Which of the following statement is wrong?
The heat transfer in liquid and gases takes place according to convection.
The amount of heat flow through a body is dependent upon the material of the body.
The thermal conductivity of solid metals increases with rise in temperature
Logarithmic mean temperature difference is not equal to the arithmetic mean temperature difference.
Cork is a good insulator because it has
Free electrons
Atoms colliding frequency
Low density
Porous body
The amount of heat flow through a body by conduction is
Directly proportional to the surface area of the body
Directly proportional to the temperature difference on the two faces of the body
Dependent upon the material of the body
All of the above
The heat transfer takes place according to
Zeroth law of thermodynamics
First law of thermodynamics
Second law of thermodynamics
Kirchhoff's law
The thermal diffusivities for solids are generally
Less than those for gases
Less than those for liquids
More than those for liquids and gases
More or less same as for liquids and gases
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
All radiations in a black body are
Reflected
Refracted
Transmitted
Absorbed
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)
Planck's law holds good for
Black bodies
Polished bodies
All coloured bodies
All of the above
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
According to Prevost theory of heat exchange
It is impossible to transfer heat from low temperature source to t high temperature source
Heat transfer by radiation requires no medium
All bodies above absolute zero emit radiation
Heat transfer in most of the cases takes place by combination of conduction, convection and radiation
The concept of overall coefficient of heat transfer is used in case of heat transfer by
Conduction
Convection
Radiation
Conduction and convection
Which of the following statement is correct?
A grey body is one which absorbs all radiations incident on it.
At thermal equilibrium, the emissivity and absorptivity are same.
The energy absorbed by a body to the total energy falling on it, is called emissivity.
A perfect body is one which is black in colour.
The thickness of thermal and hydrodynamic boundary layer is equal if Prandtl number is
Equal to one
Greater than one
Less than one
Equal to Nusselt number
According to Dalton's law of partial pressures, (where pb = Barometric pressure, pa = Partial pressure of dry air, and pv = Partial pressure of water vapour)
Pb = pa - pv
Pb = pa + pv
Pb = pa × pv
Pb = pa/pv
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
Wien's law
Stefan's law
Kirchhoff's law
Planck's law
Heat transfer in liquid and gases takes place by
Conduction
Convection
Radiation
Conduction and convection
The expression Q = ρ AT4 is called
Fourier equation
Stefan-Boltzmann equation
Newton Reichmann equation
Joseph-Stefan equation
The total emissivity power is .defined as the total amount of radiation emitted by a black body per unit
Temperature
Thickness
Area
Time
The heat is transferred by conduction, convection and radiation in
Melting of ice
Boiler furnaces
Condensation of steam in condenser
None of these
A composite slab has two layers of different materials with thermal conductivities k₁ and k₂. If each layer has the same thickness, then the equivalent thermal conductivity of the slab will be
k₁ k₂
(k₁ + k₂)
(k₁ + k₂)/ k₁ k₂
2 k₁ k₂/ (k₁ + k₂)
The emissive power of a body depends upon its
Temperature
Wave length
Physical nature
All of the above
The critical thickness of insulation for a sphere is
k/h₀
2k/h₀
h₀/k
h₀/2k
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
Which of the following is the case of heat transfer by radiation?
Blast furnace
Heating of building
Cooling of parts in furnace
Heat received by a person from fireplace