Internal R&D – the burnt stripe effect

Extended material is available  –  AMPERE 2009 presentation on the burnt stripe effect  –  IMPI 2007 presentation on “puddles and droplets”.

The so-called cold rim effect is strong for thin semidry loads with radii less than about 35 mm (at 2450 MHz), which thus get a centre heating that cannot be modified much by the design of the microwave heating system. But a particular amplification of the heating in a typical narrow zone across the diameter in load items such as potato chips may also occur – see the picture below to the left.


If there would be only drying-out by the microwave energy, this would be faster in the whole central region. Since the permittivity then decreases and by that the absorption capability of these dryer parts, a negative feedback would then occur and even out the drying. But as is seen in the photo to the left,  the burnt stripe effect is indeed a runaway phenomenon, where an already dried-out region absorbs additional power so burn marks are created. It is concluded that liquid water transport is necessary for the effect to occur.
What happens is illustrated in the numerical modelling scenario in the image to the right. This shows a circular 24 mm diameter thin potato chip at 100 °C (permittivity ε = 50–j16 ) having an elliptical inclusion with axes 18 and 4 mm, representing a partially depleted region (permittivity ε = 8–j1,6). This region is marked with a black dotted curve. The potato chip is irradiated from the left with a free space TM-polarised 2450 MHz wave with incidence angle θ = 82°. The magnetic source field is thus y-directed and the main current x-directed; the dominating electric field is z-directed.

The following phenomena can be distiguished: