While on a camping trip in the High Sierras with your new girl/boyfriend, with l

Question

While on a camping trip in the High Sierras with your new girl/boyfriend, with little to do in the evening, you want to impress her him by conducting a scientific experiment, by determining the thermal properties of a hotdog (hey. we are engineers, after all). The hot dog (pc = 3.822 Times 106 J/m3 K) is 12.5 cm long and 2.2 cm in diameter. The initial temperature of the hot dog is 20 degree c when you drop it in the 94 degree C boiling water after embedding 2 thermocouples in it. After 2 min. the temperature at the center and surface of the hot dog are 59 degree C and 88 degree C, respectively. Determine the thermal diffusivity of the hot dog. Determine the thermal conductivity of the hot dog. Find the convection heat transfer coefficient.

Explanation / Answer

There are two main methods of heat transfer: conduction and convection. (A third method, radiation, is also recognized, but it's outside the scope of this discussion.) Conduction Conduction is probably the most basic and intuitive way of achieving heat transfer: Something hot touches something cool and the cool thing heats up. For instance, the water in a pot boils when the flame from the stovetop heats the pan, and the heat from the pan is transfered to the water via conduction. If you drop an egg into that boiling water, the heat from the water is then transfered to the egg. As the outer parts of the egg heat up, that heat is transfered inward, so that it is the hotter parts of the egg that end up cooking the cooler, interior parts of itself. So the transfer of heat from one part of an object to another part of the same object is also considered conduction. How efficiently heat is transfered in this way depends on the conductivity of the items involved. Copper is an extremely good conductor of heat, which means heat moves through copper cookware and is transfered to the food very quickly. By comparison, water or even stainless steel are relatively poor conductors of heat. For that matter, food itself is a poor conductor of heat, which is why we see that a roast will continue cooking for several minutes even after we take it out of the oven. Convection Whereas conduction is a static process, convection is a more efficient method of heat transfer because it adds the element of motion. A convection oven heats food faster than an ordinary one because it has a fan that blows the hot air around. Convection ovens can reduce cooking times by 25% or more compared with ordinary ovens. They also tend to increase the browning of food by concentrating more heat on the food's outer surface. The movement of steam or the motion of boiling water in a pot are also examples of convection. Stirring a pot of soup would be considered a form of convection, as it redistributes the heat from the bottom of a pot throughout the soup. Convection is also the reason that frozen items thaw more quickly under cold running water than if they are simply submerged in water.

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