Basics of Conduction and Convection

Practice flashcards covering the fundamental modes of heat transfer, thermal conductivity, insulation concepts, and heat exchangers.

Heat Transfer

The transmission of thermal energy from one region to another as a result of temperature gradient.

Conduction

The mode of heat transfer in which heat energy travels from particle to particle in the direction of decreasing temperature without any bodily movement of the materials from their normal position.

Convection

The mode of heat transfer in which the material particles conveying the heat are themselves carried from one place to another until the whole mass of the substance becomes uniformly heated.

Radiation

The mode of transmission of heat in which the heat energy travels from a hot place to a relatively cold place without any necessity of a physical medium.

Thermal Conductivity ($K$)

The amount of heat that flows in one second through a cm-cube ($cm^3$) of the material from the hot face to the cold face when there is a steady temperature difference of $1\,°C$ between the faces.

Fourier’s equation

The equation represented as $\frac{Q}{t} = \frac{KA(\theta_1 - \theta_2)}{x}$, where $K$ is a constant depending on the nature of the material.

Unit of Thermal Conductivity ($K$)

The standard units expressed as $W\,m^{-1}\,K^{-1}$ or $J\,s^{-1}\,m^{-1}\,K^{-1}$.

Factors affecting thermal conductivity

Variables including temperature, density, direction of heat flow, moisture, air pressure, and convection in insulators.

Bad conductors of heat

Materials such as wood, brick, glass, granite, wool, cotton, felt, rubber, and ebonite that retard the flow of heat.

Thermal Resistance ($R_{th}$)

The quantity $\frac{dx}{kA}$, which is analogous to electrical resistance in Ohm's law.

Thermal Conductance

The reciprocal of the thermal resistance.

Thermal Insulation

A material which retards the flow of heat with reasonable effectiveness or the use of a material to reduce heat transfer between objects in thermal contact.

Critical Thickness

The thickness up to which heat flow increases and after which heat flow decreases.

Critical Radius

The term for critical thickness when applied specifically to cylinders and spheres.

Free or natural convection

A type of convection that occurs where the fluid circulates by virtue of the natural differences in densities of hot and cold fluids.

Forced convection

Convection that occurs when work is done to blow or pump the fluid.

Heat Exchangers

Equipment designed to transfer energy from a hot fluid (body) to a cold fluid (body) with a maximum rate and minimum investment and running costs.