CEE 5244 Lecture Notes - Lecture 23: Cavity Magnetron, Defrosting, Thermal Conductivity
5 Apr 2016
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This is actually hard for the microwave to do. Some few molecules will be loose and can be wiggled, These will quickly heat up their surroundings, making more liquid. Runaway process in little pockets: ice still unaffected: defrost cycling: allowing time for diffusion. Rather than let a few pockets run away with all the heat, turn off magnetron and allow time for thermal diffusion: thermal diffusion is natural time it takes heat to propagate through a medium. Thermal conductivity, (how fast does heat move) (w/m/k) Heat capacity, cp (how much heat does it hold) (j/kg/k) Mass, m (how much stuff is there) (kg) Size, r like a radius (how far does heat have to travel) (m: just working off units, derive a timescale: Where is density, in kg/m3: (cid:1) m/((4/3)(cid:1)r3) (cid:1) m/4r3. Faster if: cp is small, is large, r is small (these make sense) For typical food values, (cid:1) 6 minutes (cid:1) (r/1 cm)2.
