Thermal conductivity of non-metallic amorphous solids with decrease in temperature
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
Correct Answer :
B. Decreases
Related Questions
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
Thermal conductivity of solid metals with rise in temperature normally
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
The value of the wave length for maximum emissive power is given by
Kirchhoff's law
Stefan's law
Wines law
Planck's law
When heat is transferred by molecular collision, it is referred to as heat transfer by
Conduction
Convection
Radiation
Scattering
A grey body is one whose absorptivity
Varies with temperature
Varies with wavelength of the incident ray
Is equal to its emissivity
Does not vary with temperature and. wavelength of the incident ray
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
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
In case of liquids and gases, the heat transfer takes place according to
Conduction
Convection
Radiation
None of these
Temperature of steam at around 540°C can be measured by
Thermometer
Thermistor
Thermocouple
None of these
Thermal conductivity of wood depends on
Moisture
Density
Temperature
All of the above
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
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
A heat exchanger with heat transfer surface area A and overall heat transfer coefficient U handles two fluids of heat capacities Cmax and Cmin. The number of transfer units (NTU) used in the analysis of heat exchanger is specified as
A.Cmin/U
U/A.Cmin
A.U.Cmin
A.U/Cmin
Thermal diffusivity of a substance is given by (where h = Thermal diffusivity, ρ = Density of substance, S = Specific heat, and k = Thermal conductivity)
h = k/ ρS
h = ρS/k
h = S/ρk
h = kρ/S
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
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
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
Joule sec is the unit of
Universal gas constant
Kinematic viscosity
Thermal conductivity
Planck's constant
Pick up the wrong case. Heat flowing from one side to other depends directly on
Face area
Time
Thickness
Temperature difference
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 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 time constant of a thermocouple is
The time taken to attain the final temperature to be measured
The time taken to attain 50% of the value of initial temperature difference
The time taken to attain 63.2% of the value of initial temperature difference
Determined by the time taken to reach 100°C from 0°C
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)
The rate of energy transferred by convection to that by conduction is called
Stanton number
Nusselt number
Biot number
Peclet number
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
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 glass wool varies from sample to sample because of variation in
Composition
Density
Porosity
All of the above
Thermal conductivity of water in general with rise in temperature
Increases
Decreases
Remain constant
May increase or decrease depending on 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)