The model and what it showed
The grill on its table, inside a volume of room air, with the element, the stainless steel body, the thermostat and the plastic parts. Conduction, convection and radiation were solved together from a cold start for 40 minutes, until thermal equilibrium.
- Of the 1200 W, 47 % leaves with the hot plume, 35 % is radiated to the room and 17 % goes into the table. The element gives 80 % of its power as radiation.
- The air inside is strongly stratified: about 305 °C at thermostat height, 205 °C some 25 mm lower. A thermocouple reading depends on where it sits.
- The thermostat tip settled 96 K below the mean air temperature: 82 % of the heat it picked up escaped by conduction to its outer part, in the room air.
- With the real geometry, the open end of the thermostat socket sits 2.5 mm from the hot body and reached 172 °C, above the continuous-use limit of PA66-GF30 (about 120–140 °C).
On the exposed part of the thermostat, outside the grill, the model gave 117 °C and the laboratory measured 118 °C. That temperature depends on the whole heat path through the thermostat, so the agreement checks the model, not just one number.
Where model and measurement disagreed, the reason was found: the air thermocouple was touching the grill rack, which absorbs the element's radiation directly and is much hotter than the air.
Not the colours, but the heat paths: once it is known how the heat reaches a plastic part, it is clear which change is worth making.
