Tire Heating on Landing

When an airplane first touches the runway, there is an interesting moment of drama because the ground (relative to the airplane’s wheel) is moving past quite rapidly, while the wheel just at the instant of touchdown, is not moving at all.  Of course, there is a great deal of friction between the non-rotating wheel and the moving ground, so the wheel is accelerated rapidly in rotation until it “catches up” to the ground speed and rolls without sliding.  However, the wheel has some rotational inertia and so requires a little bit of time to get up to speed.  During that time, the tire is sliding across the ground and generating a lot of heat. Today we’ll talk about a rough approximation for the heat generated and transferred to the tire during an airplane landing.

Heat Transfer in the Kitchen, Part 3

This will be the third (and final, at least for now) post about heat transfer in the kitchen.

Heat Transfer in the Kitchen, Part 2

Last post we were talking about heat transfer in the kitchen and we’ll continue exploring that topic today.

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.

Water in the Exhaust, Part 2

     In the last post we examined the water vapor that is present after the combustion of hydrocarbon fuels.  This time let’s look a little closer at the effects of excess air and water vapor in the combustion air on the amount of water vapor in the exhaust.

Water in the Exhaust

     Have you ever noticed white clouds of exhaust billowing out of a car’s tailpipe on a cold day? When hydrocarbon fuels (like gasoline, diesel fuel, oil, propane, natural gas, etc.) are burned in air, normally the vast majority of the exhaust consists of carbon dioxide, water vapor, and nitrogen.  All three of these gases are colorless and transparent, so when you see white exhaust coming out of a car or see an airplane’s contrail, it is light reflecting off the surfaces of tiny droplets of liquid water that have condensed from the water vapor.  In this post we’ll examine the amount of water produced by combustion, and the conditions under which it condenses.

Adiabatic Flame Temperature

Normally when a fuel burns, the energy of combustion goes partly into raising the temperature of the exhaust gases, and partly into heat that is removed from the reactants via conduction, convection, and radiation.  It is often helpful to consider the two extremes of this process:
(1) How much heat would be obtainable if you could remove so much that the exhaust gases came out at the same temperature as the reactants started at? 
(2) How hot would the exhaust gases get if you removed NO heat whatsoever?
The answer to that second question is called the adiabatic flame temperature, and it is the subject of our post today.

Is Incompressible Good Enough?

       In fluids, aerodynamics, and thermodynamics, sometimes it is important to distinguish whether a gas stream needs to be treated as compressible flow or incompressible flow. In this context, these terms have a little different implication than they might have in common use. Gases are pretty much always compressible in the sense that their density changes significantly with changes in pressure. However, in the specialized context of “compressible flow” we are talking about situations where the gas velocity is high enough that the kinetic energy of the flow plays a significant part in determining the properties and changes in properties of the gas. A common rule of thumb is that a flow should be treated as compressible if the velocities involved exceed about 1/3 the speed of sound in the fluid. Of course, that is a general guideline, not a sharp limit. In this post we’ll explore that guideline a little bit.

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.