Reynolds number is the ratio of
Energy transferred by convection to that by conduction
Kinematic viscosity to thermal diffusivity
Inertia force to viscous force
None of the above
Correct Answer :
C. Inertia force to viscous force
Related Questions
The amount of radiation mainly depends on
Nature of body
Temperature of body
Type of surface of body
All of the above
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
An ordinary passenger aircraft requires a cooling system of capacity.
2 TR
4 TR
8 TR
10 TR
The highest thermal diffusivity is of
Iron
Lead
Concrete
Wood
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
Thermal conductivity of water at 20°C is of the order of
0.1
0.23
0.42
0.51
Sensible heat is the heat required to
Change vapour into liquid
Change liquid into vapour
Increase the temperature of a liquid of vapour
Convert water into steam and superheat it
Temperature of steam at around 540°C can be measured by
Thermometer
Thermistor
Thermocouple
None of these
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 radiation emitted by a black body is known as
Black radiation
Full radiation
Total radiation
All of these
The unit of overall coefficient of heat transfer is
kcal/m²
kcal/hr °C
kcal/m² hr °C
kcal/m hr °C
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
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
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 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
The concept of overall coefficient of heat transfer is used in case of heat transfer by
Conduction
Convection
Radiation
Conduction and convection
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
Unit of thermal diffusivity is
m²/hr
m²/hr °C
kcal/m² hr
kcal/m. hr °C
Thermal conductivity of air with rise in temperature
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
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
Thermal diffusivity is a
Function of temperature
Physical property of a substance
Dimensionless parameter
All of these
The natural convection air cooled condensers are used in
Domestic refrigerators
Water coolers
Room air conditioners
All 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
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
The critical thickness of insulation for a sphere is
k/h₀
2k/h₀
h₀/k
h₀/2k
The heat transfer from a hot body to a cold body is directly proportional to the surface area and difference of temperatures between the two bodies. This statement is called
First law of thermodynamics
Newton's law of cooling
Newton's law of heating
Stefan's law
The transfer of heat by molecular collision is smallest in
Solids
Liquids
Gases
None of these
Two plates spaced 150 mm apart are maintained at 1000°C and 70°C. The heat transfer will take place mainly by
Convection
Radiation
Forced convection
Free convection
According to Stefan's law, the total radiation from a black body per second per unit area is proportional to
Absolute temperature
T²
T⁵
T
In heat transfer, conductance equals conductivity (kcal/hr/sq.m/ °C/cm) divided by
Hr (time)
Sq. m (area)
°C (temperature)
K.cal (heat)