A non-dimensional number generally associated with natural convection heat transfer is
Grashoff number
Nusselt number
Weber number
Prandtl number
Correct Answer :
A. Grashoff number
Related Questions
LMTD in case of counter flow heat exchanger as compared to parallel flow heat exchanger is
Higher
Lower
Same
Depends on the area of heat exchanger
The concept of overall coefficient of heat transfer is used in heat transfer problems of
Conduction
Convection
Radiation
Conduction and convection
The value of the wave length for maximum emissive power is given by
Kirchhoff's law
Stefan's law
Wines law
Planck's law
Depending on the radiating properties, body will be transparent when
P = 0, x = 0 and a = 1
P=1, x = 0, and a = 0
P = 0, T= 1, and a = 0
X = 0, a + p = 1
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
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
Heat is transferred by all three modes of transfer, viz. conduction, convection and radiation in
Electric heater
Steam condenser
Boiler
Refrigerator condenser coils
A cube at high temperature is immersed in a constant temperature bath. It loses heat from its top, bottom and side surfaces with heat transfer coefficients of h₁, h₂ and h₃ respectively. The average heat transfer coefficient for the cube is
h₁ + h₂ + h₃
(h₁.h₂.h₃)1/3
1/h₁ + 1/h₂ + 1/h₃
None of these
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 case of heat transfer by
Conduction
Convection
Radiation
Conduction and convection
The total radiation from a black body per second per unit area is ________ fourth power of the absolute temperature. This statement is known as Stefan Boltzmann law.
Equal to
Directly proportional to
Inversely proportional to
None of these
Which of the following has least value of conductivity?
Glass
Water
Plastic
Air
The heat transfer takes place according to
Zeroth law of thermodynamics
First law of thermodynamics
Second law of thermodynamics
Kirchhoff's law
Metals are good conductors of heat because
Their atoms collide frequently
Their atoms are relatively far apart
They contain free electrons
They have high density
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 emissivity for a black body is
0
0.5
0.75
1
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
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)
The critical temperature is the temperature
Below which a gas does not obey gas laws
Above which a gas may explode
Below which a gas is always liquefied
Above which a gas will never liquefied
The value of the wavelength for maximum emissive power is given by
Wien's law
Planck's law
Stefan's law
Fourier's law
The heat transfer by conduction through a thick sphere is given by
Q = 2πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 4πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 6πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 8πkr1 r2 (T1 - T2)/ (r2 - r1)
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
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
The critical thickness of insulation for a sphere is
k/h₀
2k/h₀
h₀/k
h₀/2k
Thermal diffusivity is a
Function of temperature
Physical property of a substance
Dimensionless parameter
All of these
The process of heat transfer from one particle of the fluid to another by the actual movement of the fluid particles caused by some mechanical means, is known as
Conduction
Free convection
Forced convection
Radiation
Temperature of steam at around 540°C can be measured by
Thermometer
Thermistor
Thermocouple
None of these
The heat of sun reaches to us according to
Conduction
Convection
Radiation
None of these
Log mean temperature difference in case of counter flow compared to parallel flow will be
Same
More
Less
Depends on other factors
The highest thermal diffusivity is of
Iron
Lead
Concrete
Wood