Microwave ovens operate by rapidly aligning and reversing water molecules within the food, resulting in volumetric energy generation and, in turn, cooking of the food. When the food is initially frozen, however, the water molecules do not readily oscillate in response to the microwaves, and the volumetric generation rates are between one and two orders of magnitude lower than if the water were in liquid form. (Microwave power that is not absorbed in the food is reflected back to the microwave generator, where it must be dissipated in the form of heat to prevent damage to the generator.) (a) Consider a frozen, 0.7-kg spherical piece of ground beef at an initial temperature of T₁ = -20°C placed in a microwave oven with T = 30°C and h = 15 W/m².K. Determine how long, in min, it will take the beef to reach a uniform temperature of T = 0°C, with all the water in the form of ice. Assume the properties of the beef are the same as ice, and assume 3% of the oven power (P = 1kW total) is absorbed in the food. (b) After all the ice is converted to liquid, determine how long, in min, it will take to heat the beef to T, = 80°C if 95% of the oven power is absorbed in the food. Assume the properties of the beef are the same as liquid water.
Microwave ovens operate by rapidly aligning and reversing water molecules within the food, resulting in volumetric energy generation and, in turn, cooking of the food. When the food is initially frozen, however, the water molecules do not readily oscillate in response to the microwaves, and the volumetric generation rates are between one and two orders of magnitude lower than if the water were in liquid form. (Microwave power that is not absorbed in the food is reflected back to the microwave generator, where it must be dissipated in the form of heat to prevent damage to the generator.) (a) Consider a frozen, 0.7-kg spherical piece of ground beef at an initial temperature of T₁ = -20°C placed in a microwave oven with T = 30°C and h = 15 W/m².K. Determine how long, in min, it will take the beef to reach a uniform temperature of T = 0°C, with all the water in the form of ice. Assume the properties of the beef are the same as ice, and assume 3% of the oven power (P = 1kW total) is absorbed in the food. (b) After all the ice is converted to liquid, determine how long, in min, it will take to heat the beef to T, = 80°C if 95% of the oven power is absorbed in the food. Assume the properties of the beef are the same as liquid water.
Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)
8th Edition
ISBN:9781305387102
Author:Kreith, Frank; Manglik, Raj M.
Publisher:Kreith, Frank; Manglik, Raj M.
Chapter1: Basic Modes Of Heat Transfer
Section: Chapter Questions
Problem 1.76P: 1.76 Explain a fundamental characteristic that differentiates conduction from convection and...
Related questions
Question
Transcribed Image Text:Microwave ovens operate by rapidly aligning and reversing water molecules within the food, resulting in volumetric energy generation
and, in turn, cooking of the food. When the food is initially frozen, however, the water molecules do not readily oscillate in response to
the microwaves, and the volumetric generation rates are between one and two orders of magnitude lower than if the water were in
liquid form. (Microwave power that is not absorbed in the food is reflected back to the microwave generator, where it must be
dissipated in the form of heat to prevent damage to the generator.)
(a) Consider a frozen, 0.7-kg spherical piece of ground beef at an initial temperature of T₁ = -20°C placed in a microwave oven with
To = 30°C and h = 15 W/m².K. Determine how long, in min, it will take the beef to reach a uniform temperature of T = 0°C, with all
the water in the form of ice. Assume the properties of the beef are the same as ice, and assume 3% of the oven power (P = 1 kW
total) is absorbed in the food.
(b) After all the ice is converted to liquid, determine how long, in min, it will take to heat the beef to Tf
power is absorbed in the food. Assume the properties of the beef are the same as liquid water.
Part A
To
Consider a frozen, 0.7-kg spherical piece of ground beef at an initial temperature of T; = -20°C placed in a microwave oven with
30°C and h = 15 W/m².K. Determine how long, in min, it will take the beef to reach a uniform temperature of T = 0°C,
with all the water in the form of ice. Assume the properties of the beef are the same as ice, and assume 3% of the oven power (
P = 1 kW total) is absorbed in the food.
=
ta = i
= 80°C if 95% of the oven
min
Expert Solution
This question has been solved!
Explore an expertly crafted, step-by-step solution for a thorough understanding of key concepts.
This is a popular solution!
Step 1: Define the given data and what is to find.
VIEWStep 2: Calculate the radius of the sphere of the beef and the surface area of the sphere.
VIEWStep 3: Determine the time required for the beef to reach a constant temperature.
VIEWStep 4: Determine the time required for the beef to reach the final temperature.
VIEWSolution
VIEW
Trending now
This is a popular solution!
Step by step
Solved in 5 steps
Knowledge Booster
Learn more about
Need a deep-dive on the concept behind this application? Look no further. Learn more about this topic, mechanical-engineering and related others by exploring similar questions and additional content below.Recommended textbooks for you
Principles of Heat Transfer (Activate Learning wi…
Mechanical Engineering
ISBN:
9781305387102
Author:
Kreith, Frank; Manglik, Raj M.
Publisher:
Cengage Learning
Principles of Heat Transfer (Activate Learning wi…
Mechanical Engineering
ISBN:
9781305387102
Author:
Kreith, Frank; Manglik, Raj M.
Publisher:
Cengage Learning