Evaporative Cooling

Last post we talked about the psychrometric chart and the process that moist air follows on the chart when it is cooled until condensation begins.  You may be familiar with cooling systems that are variously termed “evaporative coolers”, “swamp coolers”, “desert coolers” and other names.  These systems work on something sort of like the inverse of the condensation process that we talked about last time.  Instead of cooling the moist air until liquid water condenses out, these systems evaporate liquid water into the air in order to cool it. 

Condensation from the Atmosphere

Have you ever noticed the beads of water that form on a cold beverage container in warm humid weather?  Or been annoyed by the fog that obscures your bathroom mirror when you are ready to shave or apply makeup?  Or seen white clouds billowing out of the tailpipe of a car on a cold day? These are all examples of water vapor in the air condensing into a liquid.  In the first two cases, the liquid condensation had formed on a cool surface, but condensation also occurs any time that you can see clouds, fog, or steam, as in the third case.  Water vapor is colorless and transparent, so when you can see a cloud, it is visible because of light reflecting off of tiny droplets of liquid.  In other words, condensation has already occurred by the time that you can see anything.

Compression Ignition Engines

While electric cars are starting to gain a following, the vast majority of the cars on the road today are still powered by internal combustion engines.  The two main types of engines used for automobiles are gasoline engines (spark ignition) and diesel engines (compression ignition).  One of the main differences is that in a spark ignition engine a mixture of air and gasoline fumes is compressed, and lit off by a spark at the appropriate time.  In a compression ignition engine, on the other hand, only air is compressed and a fine mist of diesel fuel is injected into the hot air at the appropriate time.

Refrigerators and Cryotherapy

What does a common household refrigerator have in common with a home medical treatment? One answer might be that you get ice cubes out of your freezer to put on your black eye or sprained ankle. There might be other answers surrounding your treatment when the refrigerator tips over on top of you, but in this post we are going to talk about refrigerator operation and cryotherapy.

Duhamel's Theorem, part 2

As an additional example of Duhamel’s theorem that we talked about in the last post, we’ll use the solution to a sudden change in temperature at the surface of a semi-infinite solid to examine the temperature in a thin layer in the wall of the cylinder of an internal combustion engine.

Leveraging Solutions with Duhamel's Theorem

There are many transient heat transfer problems that can be solved analytically for a simple boundary condition, but would be difficult or impossible to solve with a more complicated, time-dependent boundary condition. In many cases, if the original solution can be formulated, possibly through parameterization, as the response to a unit step change on the boundary, Duhamel’s theorem can be used to extend the simple solution to a more complex case.

The Standard Atmosphere

Most people don’t need to worry too much about the temperature and pressure of the atmosphere, as long as there is some of it there to breathe.  However, pilots, airplane designers, meteorologists, and many other people need to know how the temperature and pressure typically change with altitude.  Of course, on any given day at a particular location, conditions might be different than the “typical” condition.  Still, the typical conditions are useful enough that there are several definitions of the Standard Atmosphere describing conditions clear out into what is usually considered outer space.