Conductive Heat Transfer

Heat transfer takes place as conduction if there is a temperature gradient in a solid or fluid

Conduction will take place if there exist a temperature gradient in a solid (or stationary fluid) medium.

Energy is transferred from more energetic to less energetic molecules when neighboring molecules collide. Conductive heat flow occur in direction of the decreasing temperature since higher temperature are associated with higher molecular energy.

Fourier's Law express conductive heat transfer as

q = k A dT / s         (1)

where

q = heat transferred per unit time (W, Btu/hr)

A = heat transfer area (m², ft²)

k = thermal conductivity of the material (W/m.K or W/m ^oC, Btu/(hr ^oF ft²/ft))

dT = temperature difference across the material (K or ^oC, ^oF)

s = material thickness (m, ft)

Example - Heat Transfer by Conduction

A plane wall constructed of solid iron with thermal conductivity 70 W/m^oC, thickness 50 mm and with surface area 1 m by 1 m, temperature 150 ^oC on one side and 80^oC on the other.

Conductive heat transfer can be calculated as:

q = (70 W/m^oC) (1 m) (1 m) ((150 ^oC) - (80 ^oC)) / (0.05 m)

    = 98,000 W

    = 98 kW

Thermal Conductivity and Common Units

• Btu/(h ft^{2 o}F/ft)
• Btu/(h ft^{2 o}F/in)
• Btu/(s ft^{2 o}F/ft)
• MW/(m² K/m)
• kW/(m² K/m)
• W/(m² K/m)
• W/(m² K/cm)
• W/(cm^{2 o}C/cm)
• W/(in^{2 o}F/in)
• kJ/(h m² K/m)
• J/(s m^{2 o}C/m)
• kcal/(h m^{2 o}C/m)
• cal/(s cm^{2 o}C/cm)

• Convert between thermal conductivity units

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