Fourier's law of heat conduction is valid for
One dimensional cases only
Two dimensional cases only
Three dimensional cases only
Regular surfaces having non-uniform temperature gradients
Correct Answer :
A. One dimensional cases only
Related Questions
Heat transfer by radiation mainly depends upon
Its temperature
Nature of the body
Kind and extent of its surface
All of the above
The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body
Move actually
Do not move actually
Affect the intervening medium
Does not affect the intervening medium
Pick up the wrong case. Heat flowing from one side to other depends directly on
Face area
Time
Thickness
Temperature difference
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
Depending on the radiating properties, a body will be opaque when
P = 0, x = 0 and a = 1
P=1, x = 0 and a = 0
P = 0, x = 1 and a = 0
X = 0, a + p = 1 Where a = absorptivity, p = reflectivity, X = transmissivity.
The radiation emitted by a black body is known as
Black radiation
Full radiation
Total radiation
All of these
Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be
One
Two
Three
Four
Fouling factor is used
In heat exchanger design as a safety factor
In case of Newtonian fluids
When a liquid exchanges heat with a gas
None of the above
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
Depending on the radiating properties, a body will be black when
P = 0, x = 0 and a = 1
P= 1, T = 0 and a = 0
P = 0, x = 1 and a = 0
X = 0, a + p = 0 Where a = absorptivity, p = reflectivity, X = transmissivity.
The energy distribution of an ideal reflector at higher temperatures is largely in the range of
Shorter wavelength
Longer wavelength
Remain same at all wavelengths
Wavelength has nothing to do with it
All radiations in a black body are
Reflected
Refracted
Transmitted
Absorbed
The most commonly used method for the design of duct size is the
Velocity reduction method
Equal friction method
Static regains method
Dual or double method
The heat transfer takes place according to
Zeroth law of thermodynamics
First law of thermodynamics
Second law of thermodynamics
Kirchhoff's law
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
The concept of overall coefficient of heat transfer is used in heat transfer problems of
Conduction
Convection
Radiation
Conduction and convection
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)
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
The unit of overall coefficient of heat transfer is
kcal/m²
kcal/hr °C
kcal/m² hr °C
kcal/m hr °C
Heat transfer takes place as per
Zeroth law of thermodynamics
First law of thermodynamic
Second law of the thermodynamics
Kirchoff's law
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.
Heat flows from one body to other when they have
Different heat contents
Different specific heat
Different atomic structure
Different temperatures
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
Thermal conductivity of solid metals with rise in temperature normally
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
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
Temperature of steam at around 540°C can be measured by
Thermometer
Thermistor
Thermocouple
None of these
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 value of Prandtl number for air is about
0.1
0.3
0.7
1.7
The critical radius is the insulation radius at which the resistance to heat flow is
Maximum
Minimum
Zero
None of these