Thermal Conductivity

In keeping with the theme of material properties from the last post, today we’ll talk about thermal conductivity which is a material property that relates to the conduction of heat.

Applied Contact Temperature

We’ve talked before about the contact temperature and the effect that different materials have on the temperature that you feel when you first touch them.  Today we’ll look at that effect in a more applied way.

Snow Sublimation

If you live in an area with snow, you’ve probably experienced a snowfall followed by several cold clear days. If it lasted long enough, you may have noticed that the snow gradually starts to disappear (sublimate) even when the air temperature never gets above the melting point. In this post, we’ll look at the speed of that effect.

More Lumped Capacitance

Last post we looked at another extension of lumped capacitance beyond our original exploration here and here.  In the last post, we looked at a case where the heat transfer coefficient increased linearly with time, which might be a good approximation for the initial moments of the heat transfer for an object in a duct with the fan just starting up.  Today we’ll look at a case where the heat transfer coefficient is decreasing.

Lumped Capacitance Revisited

Lumped capacitance is a very useful heat transfer approximation that we mentioned originally here,  and explored in more depth with extensions here and here.  Today, we’ll look at another extension of the basic lumped capacitance approach.

Heat from Breath

In an earlier post, we looked at how much water is lost through breathing.  Today, we’ll consider the amount of body heat lost through breathing.  Of course, depending on surroundings, air movement, and  how one is dressed, a lot of (or a little) heat might be lost through the skin, but we’re just considering the part that goes out with your breath.

A Cold Snap, Part 2

In the last post, we talked about the ground temperature during a cold snap.  Today, we’ll look at the same situation from two different viewpoints.

A Cold Snap

Following a series of relatively warm days, we had a sudden (over a period of a few hours) drop in temperature and things then stayed very cold for several days.  This led me to wonder how fast the ground would cool under such circumstances.

For the purposes of getting a simple, quick feel for the effect of the air temperature change, we’ll look at a somewhat simplified version of the actual events.  Of course this result won’t exactly match what happened in real life, but it will provide a reasonable estimate.

Fundamentals of Thermal Resistance

The Thermal Resistance Analogy
Thermal resistance is a convenient way of analyzing some heat transfer problems using an electrical analogy in order to make complicated systems easier to visualize and analyze.  It is based on an analogy with Ohm’s law which is:

In Ohm’s law for electricity, “V” is the voltage which drives a current of magnitude “I”.  The amount of current that flows for a given voltage is proportional to the resistance (R_elec).  For an electrical conductor, the resistance depends on the material properties (copper tends to have a lower resistance than wood, for example) and the physical configuration (thick short wires have less resistance than long thin wires).

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.

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.

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.

Quenching in a Finite Bath

In an earlier post we looked at the transient 1 dimensional temperature profiles and

temperature gradients across a piece of metal being quenched in a tank large enough that the temperature of the quench fluid never changed.  In this post, we’ll look at a different case: a piece of steel small enough that the temperature is uniform across the piece which is being quenched in a bath small enough that the temperature of the quench fluid goes up as the steel cools down.  We’ll assume that the quench fluid is well-stirred so that it can also be characterized by single temperature.

An Alternate View of Condensation

Usually when we think of condensation, we think of the droplets of water that form on a cold glass on a humid day, or perhaps we think of dew making the grass wet on a summer morning, or maybe the fog that forms on the bathroom mirror after you have a hot shower.  In those familiar cases of condensation, the water vapor is mixed in with dry air in a very dilute mixture.  For example, at 75 deg F, 60% relative humidity, the water vapor comprises only about 1.1% of the total moist air, by mass.  Even at 75 deg F, 100% relative humidity, (at which point the vapor is about to start condensing to liquid) the water vapor is only about 1.8% of the mixture.  So, we’re accustomed to condensation of water from a very dilute mixture of water vapor in air.  In this blog, we’ll consider the condensation of pure water vapor, and see some surprising forces.

Thermal Gradients from Quenching

Many heat treating operations involve a quench—that is, an immersion in a fluid at a lower temperature in order to achieve a rapid cooling rate. It is commonly used for hardening in ferrous metals. Quench fluids include air, water, oils, and many others. From a heat transfer standpoint, quenching of a hot metal has a lot of interesting aspects: determining the heat transfer coefficient (possibly with phase change) at the surface, calculating transient temperature profiles inside the material (with implications for thermal stresses and metallurgical properties), effects of thermal transport properties (possibly time-dependent, or spatially non-uniform) on the heat transfer, and others. In this post, we’ll discuss transient temperature profiles and temperature gradients induced by quenching a one-dimensional (wide enough and long enough that the main effects are controlled by the thickness) piece of tool steel.