As noted in .3, microwave ovens operate byrapidly aligning

Chapter 5, Problem 5.33

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As noted in 5.3, microwave ovens operate byrapidly aligning and reversing water molecules withinthe food, resulting in volumetric energy generation and,in turn, cooking of the food. When the food is initiallyfrozen, however, the water molecules do not readilyoscillate in response to the microwaves, and the volu-metric generation rates are between one and two ordersof magnitude lower than if the water were in liquidform. (Microwave power that is not absorbed in thefood is reflected back to the microwave generator,where it must be dissipated in the form of heat to pre-vent damage to the generator.)(a) Consider a frozen, 1-kg spherical piece of groundbeef at an initial temperature of Ti??20C placedin a microwave oven with T??30C and h?15 W/m2?K. Determine how long it will take thebeef to reach a uniform temperature of T?0C,with all the water in the form of ice. Assume theproperties of the beef are the same as ice, andassume 3% of the oven power (P?1 kW total) isabsorbed in the food.(b) After all the ice is converted to liquid, determinehow long it will take to heat the beef to Tf?80Cif 95% of the oven power is absorbed in the food.Assume the properties of the beef are the same asliquid water.(c) When thawing food in microwave ovens, one mayobserve that some of the food may still be frozenwhile other parts of the food are overcooked. Explain why this occurs. Explain why mostmicrowave ovens have thaw cycles that are associ-ated with very low oven powers.