If the temperature of a solid surface changes from 27°C to 627°C, then its emissive power changes in the ratio of
6
9
27
81
Correct Answer :
D. 81
Related Questions
Sensible heat factor is given by (where S.H. = Sensible heat, and L.H. = Latent heat)
S.H/(S.H + L.H)
(S.H + L.H) /S.H
(L.H - S.H)/S.H
S.H/(L.H - S.H)
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
The transfer of heat by molecular collision is smallest in
Solids
Liquids
Gases
None of these
The highest thermal diffusivity is of
Iron
Lead
Concrete
Wood
Joule sec is the unit of
Universal gas constant
Kinematic viscosity
Thermal conductivity
Planck's constant
Heat is transferred by all three modes of transfer, viz. conduction, convection and radiation in
Electric heater
Steam condenser
Boiler
Refrigerator condenser coils
Pick up the wrong case. Heat flowing from one side to other depends directly on
Face area
Time
Thickness
Temperature difference
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
Heat transfer by radiation mainly depends upon
Its temperature
Nature of the body
Kind and extent of its surface
All of the above
Which of the following property of air does not increase with rise in temperature?
Thermal conductivity
Thermal diffusivity
Density
Dynamic viscosity
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
In a shell and tube heat exchanger, baffles are provided on the shell side to
Improve heat transfer
Provide support for tubes
Prevent stagnation of shell side fluid
All of these
A designer chooses the values of fluid flow rates and specific heats in such a manner that the heat capacities of the two fluids are equal. A hot fluid enters the counter flow heat exchanger at 100° C and leaves at 60° C. A cold fluid enters the heat exchanger at 40° C. The mean temperature difference between the two fluids is
20°C
40°C
60°C
66.7°C
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
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)
The transfer of heat by molecular collision is known as
Conduction
Convection
Radiation
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
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
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 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
Heat is closely related with
Liquids
Energy
Temperature
Entropy
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
In case of liquids and gases, the heat transfer takes place according to
Conduction
Convection
Radiation
None of these
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 expression Q = ρ AT4 is called
Fourier equation
Stefan-Boltzmann equation
Newton Reichmann equation
Joseph-Stefan equation
In counter flow heat exchangers
Both the fluids at inlet (of heat exchanger where hot fluid enters) are in their coldest state
Both the fluids at inlet are in their hottest state
Both the fluids at exit are in their hottest state
One fluid is in hottest state and other in coldest state at inlet
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 rate of energy emission from unit surface area through unit solid angle, along a normal to the surface, is known as
Emissivity
Transmissivity
Reflectivity
Intensity of radiation
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