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.
There are two heat transfer analysis approaches that we use to analyze time-dependent heating and cooling. In the simplest case, we can treat the entire body as being at a single temperature that changes with time. This analysis can be used if the convective heat transfer coefficient is fairly low, the body is fairly small, and the thermal conductivity of the body is high. So, you might use this approach to estimate the time for a cold olive to warm up to room temperature while sitting out on the counter.

A more complicated approach is required if the body and convective heat transfer coefficient are large. In this case, the body won’t be characterized by a single temperature, but instead the temperature will vary across the inside of the body, and will also change with time.

Nowadays, microwave ovens are often as important (more?)  in the kitchen as conventional ovens.  Microwave ovens generate radio waves (electromagnetic radiation with a frequency of 2450 MHz and wavelength of about 12.2 cm) and heat the food with it. Compared to the electromagnetic radiation that we talked about earlier in the context of radiation heat transfer, microwaves are at a much lower frequency and much longer wavelength. (0.0001 to 0.1 mm is the usual range of wavelengths considered in radiation heat transfer).  Microwaves typically pass right through plastics, ceramics, and glass, but are absorbed strongly by water, and to a lesser extent, fats and sugars.

In typical food products microwaves will penetrate a centimeter or two before being mostly absorbed.  As they are absorbed by the water or fat in a food, their energy is transformed into heat.  From the standpoint of heat transfer engineering, we usually leave the generation and distribution of microwaves to the radio engineers and pick up our analysis after the microwaves have been absorbed.  We can then treat microwave heating as a heat generation term in the heat transfer or thermodynamics problem.

Usually, the distribution of microwaves inside the oven cavity is not perfectly uniform, the moisture in the food is not distributed uniformly, and/or the food is thicker than two centimeters.  The rotating turntable in many microwave ovens helps compensate for the uneven distribution of microwaves, but some places typically still get hotter than others. Of course, as soon as this begins to occur, heat begins to move by conduction within the food to even out the temperature.  Sometimes cooking instructions tell you to stop the oven and stir the food occasionally, or to let the food sit for a short time before eating.  These both serve to let the temperature even out.