Heat Transfer Concept Page - 1

Definition
Conductors and insulators
Substances through which heat can pass easily are called good conductors of heat. Examples: metals, mercury, etc.
Substances through which heat cannot pass easily are called insulators of heat. Examples: glass, wool, etc.
Definition
Conduction
Conduction is heat transfer between two bodies in contact and at different temperatures. It usually occurs between solids. Heat transfer by conduction is a slow process.
Definition
Thermal conductivity
Thermal conductivity is defined as the rate at which heat passes through a specified material, expressed as the amount of heat that flows per unit time through a unit area with a temperature gradient of one degree per unit distance.(W/(mK)) and BTU/(hrftF) are some units of thermal conductivity.
Formula
Coefficient of thermal conductivity
Formula :
Coefficient of thermal conductivity:=QdA(θ2θ1)t
Dimension : [M1L1T3K1]
Unit : Wm1K1
Definition
Coefficient of thermal conductivity
Coefficient of thermal conductivity:=QdA(θ2θ1)t
Cefficient of thermal conductivity of the material of solid is equal to the rate of flow heat per unit area per unit temperature gradient across the solid.
Definition
Proportionality of rate of heat transfer and temperature gradient at steady state
In steady state, the rate of flow of heat H from one point to another is proportional to the temperature difference (T1T2) and the area of cross section A and is inversely proportional to length L:
H=KAT1T2L
The constant of proportionality K is the thermal conductivity of the material.
Definition
Heat flow in series conductors

The attached figures show the heat flow in parallel conductors. Series configuration in conductors is defined by same heat flow in the two conductors across all cross sections.
In the given figure,
q=k1A(T1T2)L1=k2A(T2T3)L2
Formula
Series thermal resistance
Rseries=Ri
where Ri=LkiA is the resistance of the ith layer
Definition
Heat flow in parallel conductors

The attached figures show the heat flow in parallel conductors. Parallel configuration in conductors is defined by same temperature at the two junctions of the conductors. Total heat flowing through a cross section is the sum of heat flow in each conductor.
In the given figure, qtotal=q1+q2
qtotal=k1A1(T1T2)L+k2A2(T1T2)L
Formula
Parallel thermal resistance
1Rparallel=1Ri
where 

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