Che-205: Heat Transfer Fundamentals(Spring 2017)Lecture 1-iii

Dr. Muhammad Wasim Tahir

Department of Chemical EngineeringUniversity of Engineering & Technology Lahore

Source: http://www.enexio.com/products/wet-cooling-towers/ December 30, 2016

3. Modes of heat transfer

Conduction:

HT due to particle inreaction alone. No bulk movement

Convection:

HT due to actual movement of bulk of fluid

Radiation:

HT by electromegnatic waves (photons). Require no medium

3. Modes of heat transfer

Conduction

Fourier‘s law of heat conduction:

The rate of heat conduction through a plane layer is proportional to the temperature difference across the layer and the heat transfer area, but is inversely proportional to the thickness of the layer.

That is,

Heat conduction rate α (A·ΔT)/(Δx)

Where, ΔT = (T2 ̶ T1)

3. Modes of heat transfer

Thermal conductivity (k):Rate of heat transfer through a unit thickness of material per unit area per unit degree rise in temperature.

o k is a measure of material‘s ability to conduct heat

e.g. for water, k = 0.608 (W/m·°C) at room T

and for iron, k = 80.2 (W/m·°C) at room T

o Higher value of k indicative of good condutor

o Lower value of k is indicative of poor conductor or good onsulator

3. Modes of heat transfer

Thermal conductivity (k):

Data derived from

3. Modes of heat transfer

Thermal diffusivity (α):Thermal diffusivity of a material can be viewed as the ratio of theheat conducted through the material to the heat stored per unitvolume.

α = (Heat conducted)/(Heat stored)

α = k/(ρ CP)

Larger value of thermal diffusivity indicates faster propogation of heat through the material

3. Modes of heat transfer

Example 3.1:

3. Modes of heat transfer

Example 3.2:

3. Modes of heat transfer

Convection

Rate of convective heat transfer between a solid surface and adjacent fluid is given by Newton’s law of cooling.

Heat convection rate = hA (Ts ̶ Tf)

Where, h = Heat transfer or film coefficient given in (W/m2·°C)

3. Modes of heat transfer

Example 3.3: