Conduction Flashcards
Types of conduction
1) steady state conduction
2) unsteady state conduction
Steady state conduction
No accumulation of heat
Q constant throughout
Unsteady conduction
Heat gets accumulated
Heat generation and heat consumption
Parabolic type of profiles
Thermal conductivity
Independent of temperature gradient except porous solids where radiation can affect
Dependent on temperature
Usually an average k value from both the temperatures are used
Thermal conductivity of metals
Decreases with increase in temperature since the mean free path for the free electrons to move reduces because of particle vibration
Decreases with impurities example alloys have less conductivity than pure metals
Thermal conductivity of liquids
Decreases with temperature except water which reaches a maximum
Thermal conductivity of gases
Incresases with increase in temperature
Proportional to square of T for mono atomic gases
Decreases with increase in molecular weight
Resistance
Length/ conductivity * area
K/W
Conductance
Log mean area of cylinder
Log mean radius of cylinder
Geometric mean area of sphere
Geometric mean radius of sphere
Unsteady state Heat conduction
Heat generation
Heat consumption
Dimensional number associated with unsteady state heat transfer
Biot number
Fourier number
Biot number
Conductive resistance of solid/ convective resistance of fluid
hL/k
Bi < 0.1
Lumped system
Bi > 0.1
Distributed system
Fourier number
Conduction rate/ storage rate
