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.