Heat Transfer in the Kitchen

When I tell people (and by 'people', I mean 'non-engineers') that my field of specialization is heat transfer, I usually get some incredulous response on the theme of, "There is a whole field where someone would study nothing but heat transfer?" with the implied sub-text of "sane someone" accompanied by lots of extra question marks.  I have to admit that in everyday experience, most people don't have to think quantitatively about heat transfer.  However, almost everybody connects (non-quantitatively) with heat transfer in the kitchen, so in this post we'll explore heat transfer in cooking.
There are three modes of heat transfer:  (1) conduction, which is the diffusion of heat through a solid, or through a liquid at rest 

conduction

convection

  (2) convection, which is the transfer of heat between a solid to a moving fluid and 












(3) radiation, which is the movement of heat through electromagnetic waves.

radiation

A couple of things to note: (1) In real life, there are often multiple modes of heat transfer present in any real process.  For example, if you wanted to boil a pan of water over a gas flame, the hot gases that comprise the flame are transferring heat to the bottom of the pan by both convection and radiation.  The heat is conducted through the solid pan, then convection moves the heat from the inside surface of the pan into the water. 

(2) Heat always moves from a hotter place to a cooler place.  This is actually a fundamental law of nature, and is one way of stating the Second Law of Thermodynamics, but that is a discussion for another day.

For now, let's think about some familiar kitchen processes in terms of heat transfer and thermodynamics principles.

Boiling, simmering, and poaching are all common cooking processes that involve boiling water. 

Two aspects of these processes are endearing for kitchen use (1) the process occurs at a constant temperature without any complicated effort to control the heat, and (2) the convective heat transfer of a moving liquid is very high relative to the heat transfer from air.  Regarding the first one, notice that the burners on most stoves don't have a thermostat, or any complicated mechanism for controlling the temperature like most ovens have.  In fact, they usually just have numbers to indicate various relative heating levels.  You set a heating level, and the heat just pours out at that level, regardless of what might be going on in the pan above.  If the pot or pan above has water in it, the liquid water will soon start boiling into steam.  The phase change of the water absorbs the heat, so that lots of heat just makes it boil faster, but doesn't raise the temperature above the local boiling temperature of water.  So the process of boiling provides a cheap, easy, and almost foolproof temperature control for your cooking process.

There are a couple of little wrinkles in cooking with boiling.  First, the boiling temperature of water depends somewhat on the local pressure.  So, at higher elevations water boils at a lower temperature.  For example, at 10,000 feet above sea level, the boiling temperature of water is about 193 deg F rather than the 212 deg F boiling temperature at sea level.  You are familiar with another wrinkle if you've ever gotten distracted and let a pot of boiling food boil all the way dry.  Once the water is no longer there to absorb the heat by changing phase, the temperature in the pot will rise very rapidly, scorching the food, and sometimes scorching the pan or even starting a fire.