Heat transfer

Types of heat transfer
There are 2 primary mechanisms by which heat is transferred from a hotter material to a cooler material:

Conduction
This occurs when two solid objects are touching, within fluids, or between touching fluids and solids. Some materials conduct heat much better than others. For example, metals generally conduct heat better than wood. Good thermal conductors, at room temperature, often feel cool to the touch, as they conduct body heat away from the hand of the warmer person.

Example
A brick house absorbs heat from solar radiation during the day, on the surface of the bricks. This heat slowly conducts through the bricks and heats the house only after it reaches the inside edge. This may take several hours. Thus, houses can actually be hottest after the Sun has set.

Convection
This type of heat transfer requires a fluid, which may be either a gas or liquid, and a force acting on the fluid, such as gravity. Convection occurs due to density changes in fluids when the temperature of those fluids change. In general, fluids become less dense when heated, but there are exceptions. In the case of a fluid which does become less dense when heated, the hotter portion of the fluid rises and the cooler portion sinks. Just the reverse would happen if the fluid became more dense when heated. If the fluid does not change density when heated, then no convection will occur, although much slower conduction within the fluid will still occur.

Example
Within the mantle of the Earth, hot magma rises to the crust, releases it's heat by conduction or by volcanism, then sinks back down to the hotter core and repeats the process. This sets up convection cells, where this process repeats in a stable flow pattern for millions or even billion of years. One result is that the location where hot magma rises produces a "hot spot" which creates a series of volcanoes as it punctures the crust as the continental and oceanic plates drift across. The Hawaiian Islands were created by just such a hot spot.

Chemical phase changes
Heat may cause solids to melt to become liquids, or liquids to boil to become gases. Some heat is absorbed by the phase change itself, and some may then be carried away by the fluid via convection. The combination of phase changes and convection cools an object quicker than convection alone.

Example
Blacksmiths often drop red-hot or white-hot metals into water in order to "quench" them. This causes the water to boil and carry the heat away as steam.

Radiation
Radiation occurs from all objects above absolute zero to every other object in its line of sight. This occurs when molecular vibrations cause particles to be emitted from the surface of an object. Many different particles may be emitted, depending on the temperature of the object. Different frequencies are thus emitted; including infrared, visible light, ultraviolet, radio waves, microwaves, X-rays, gamma rays, etc. Radiation is unique in that it is the only type of heat transfer which can occur across a vacuum. Radiation is an extremely quick mechanism for transferring heat (it occurs at the speed of light), provided the hotter object is very hot (thousands of degrees). Cooler objects don't radiate much energy. The radiation received by an object will be inversely proportional to the square of the distance from the radiating object.

Example
Radiation from stars, such as our Sun, is the method by which light and heat is given off to the planets.

Reaching equilibrium
Heat transfer within a material or between two materials occurs in both directions, but more heat is transferred from the hotter material to the cooler than from the cooler to the hotter. This process continues until the temperatures approach the same value within the material or between the two materials. At this point an equal amount of heat transfer will occur in both directions, and the system will remain at thermal equilibrium until some change is made to the system.

Lessons

 * Wikibooks: Heat Transfer
 * Cooling Towers: Design and Operation Considerations
 * Wikibooks: Automotive Air Conditioner: Material and Energy Balances (nothing yet available)
 * LearnChemE screencasts on Heat Transfer
 * Process Engineering Fundamentals - Heat transfer (Youtube)