Last time we tackled breakfast beverages and how they can illustrate thermal topics. Breakfast time is one opportunity to find examples of thermal magic in our everyday lives. Once you start down the path of Thinking Thermally, a whole new world of awareness opens up to you and you’ll find examples of heat transfer in all sorts of places.
Sorry, I digress; back to our thermal breakfast. I love English muffins. If I’m having bread with breakfast and it’s not homemade biscuits, English muffins are my next choice. The nooks and crannies are what make them so good. You get crunch and softness in the same bite. There’s some thermal magic going on with the nooks and crannies, too. With English muffins we’re seeing Newton’s Law of Cooling at work. We learned about Newton’s Law of Cooling in Level I, but let’s review.
Q= h•ΔT•A
To refresh your memory, “h” represents The Coefficient of Convective Cooling, which is made up of a host of factors that impact the amount of heat transferred by convection. The one we consider most often is velocity. Even the neophyte thermographer understands the more rapidly fluid moves across a surface the more transfer we have. That’s why we feel cooler if we fan ourselves. Another variable that impacts “h” is surface condition.
If we have a microscopically smooth surface for fluid to flow across, we experience maximum heat transfer via convection. The reason for this is the fluid can more easily make contact with the surface because the surface is smooth with very little variance. When we have a less smooth surface, the fluid makes contact with the surface intermittently and a lower rate of heat transfer takes place. This is the case with the nooks and crannies in our tasty English muffin.
No matter which method we use to heat our treat, convection is one of the modes of heat transfer that takes place. In a toaster, a toaster oven or even an oven broiler (my late grandmother’s preferred method to toast bread), the heat source heats the air and the air (which is a fluid) flows across the surface of our English muffin. Due to the irregularity of its surface, we find the English muffin toastier on the peaks than in the nooks and crannies. There is more consistent contact between the hot air and the peaks of the muffin than the air and the nooks and crannies. The “h” factor is higher on the peaks, lower in the nooks and crannies.
Who knew we could learn so much about heat transfer from our breakfast experiences? Keep Thinking Thermally!
In case you missed part 1: http://www.thesnellgroup.com/community/ir-talk/b/think-thermally/archive/2012/12/11/think-thermally-174-at-breakfast.aspx