🚀 go-pugleaf

HELP ME TO UNDERSTAND WHAT HAPPENS IN A MICROWAVE OVEN.

I wanted to develop another "take home" activity to be assigned to students.
But my expectations were not confirmed by data. Can you verify observation?
Use any solid food material (to avoid convection which would complicate matters
in liquids). Can you help me to understand the data? I took a uniform piece of
meat (bologna sausage whose diameter and height were 12 and 8.5 centimeters,
resectively) and cooked it in the center of the oven, directly below its single
magnetron, for 6 minutes. The expectation was that the upper layers would
become warmer than the lower layers according to the exponential absorption
formula. The actual changes in temperature along the axis, as measured with
four thermometers after the exposure, were as shown below. The distance d
refers to the depth, it is the axial distance from the top surface, at which
the temperatures were measured. Standard glass thermometers were used but
errors in determinations of temperature differences could not possibly be
larger than one degree Celsius.

        d  (cm) 1.0     3.3     5.3     7.8
        Dt (C)   15       3       1       5

The distribution of temperature, recorded between 1 and 2 minutes after the
exposure, remained essentially unchanged for about 6 minutes or so. Then the
inner layer temperatures were seen to increase slowly while the outer layer
temperatures decreased according to an expected heat transfer pattern. After
about one hour the temperature difference between the first and third
thermometers was less than 3 degrees C. The thermal equilibrium, at the room
temperature (24 C), was reached after more than three hours.

Why was the lower layer always warmer than the inner layers? Moving the
sample closer to the magnetron, and placing a large plastic container with
6.5 cm of water below the sample, did not help to eliminate the minimum in
the distribution of temperatures. (The rectangular plastic container was 18 cm
by 14 cm. The change in the temperature of water was from 22 C to 45 C, in 6
minutes, when there were no meat, and from 22 C to 39 C when meat was placed
above water.)

A taller meat cylinder (same diameter but height of 14 cm) was used in one
experiment and the distribution of absorbed heat (also expressed in terms of
local changes of temperature) was as below. There were no water below that
taller sample and the cooking time was 5 minutes.

        d  (cm) 1.5     5.3     9.1    12.6
        Dt  (C) 11.5    2.0     2.5    11.5

The microwave oven, Specemaker III, manufactured in 1985 by General Electric,
and rated at 1.05 kW, was used in all experiments.

                     Ludwik Kowalski    Kowalski@apollo.montclair.edu