THERMAL CONDUCTIVITY OF MATERIALS
Qx = -k A dt / dx
The constant of proportionally kx is called the co efficient of thermal conductivity, which is physical property of the substance and is defined as the ability of a substance to conduct heat.
The quantity of heat transferred per unit time per unit area of isothermal surface is defined as the heat flux.
The heat flow vector q can be written as:
Q = I qx + jqy + kqz
Since conduction is a molecular phenomenon, viscosity in fluid motion is analogous to the thermal conductivity in heat transfer.
The thermal conductivity is a physical property of substance and like viscosity, it is primarily a function of temperature and / or position, and nature of the substance. If varies significantly with pressure only in the case of gases subjected to high pressure. However, for many engineering problems, material are often considered to possess a constant thermal conductivity (isotropic).
Thermal conductivities of common substance at 200C
Substance k(W/mK)
Silver, pure 407.0
Copper pure 386.0
Aluminum, pure 75.6
Mild steel 37.2
Stainless steel 19.3
Wood 0.15
Tables show pure metal have the highest valve of thermal conductivities while gases and vapours the lowest; insulating materials and in organic liquids have thermal conductivities that lie in between those of metal and gases.