hw04-hints

# hw04-hints - ABE 484/584 CHOI HW4 1[Self-cleaning Oven An...

This preview shows pages 1–5. Sign up to view the full content.

ABE 484/584 CHOI HW4 1. [ Self-cleaning Oven ] An engineer at a leading manufacturer of household appliances has newly proposed a self-cleaning oven that involves the use of a composite window separating the oven cavity from the room air. The composite is to consist of two high A = 2L B and thermal conductivities are k A = 0.15 W/m K and k B = 0.08 W/m K. During the self-cleaning process, the oven wall and air temperatures, T w and T a , are 400 o C, while the room air temperature, T , is 25 o C. The inside convection and radiation heat transfer coefficients, h i and h r , as well as the outside convection coefficient, h , are approximately 25 W/m 2 K. What is the minimum window thickness, L = L A + L B , needed to ensure a temperature that is 50 o C or less at the outer surface of the window (for safety reasons)? Estimate L when ignoring the radiative effect. Discuss the results. Department of Agricultural and Biosystems Engineering, The University of Arizona Material A k A = 0.15 Material B k B = 0.08 W/m K L A T s,o < 50 o C AIR T = 25 o C h = 25 W/m 2 K L B AIR T a = 400 o C h i = 25 W/m 2 K T w = 400 o C h r = 25 W/m 2 K

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
ABE 484/584 CHOI Department of Agricultural and Biosystems Engineering, The University of Arizona
ABE 484/584 CHOI 2. [ Microwave Oven ] A microwave oven operates on the principle that the application of a high frequency field causes electrically-polarized molecules in food to oscillate. The net effect is a uniform generation of thermal energy within the food, which enables it to be heated from refrigeration temperatures (0 o C) to 90 o C in as short a time as 30 seconds. Consider the process of cooking a slab of beef of thickness 2L in a microwave oven and compare it with cooking in a conventional oven, where each side of the slab is heated by radiation for a period of approximately 30 min. In each case, the meat is to be heated from 0 o C to a minimum temperature of 90 o C. Sketch the temperature distribution at selected times for each of the cooking processes. In particular, consider the time t o at which heating is initiated, a time t 1 during the heating process, the time t 2 corresponding to the conclusion of heating, and a time t 3 well into the subsequent cooling process. Assume: (1) 1D in x, (2) uniform internal heat generation for the microwave, and (3) uniform surface heating for the radiant oven. Department of Agricultural and Biosystems Engineering, The University of Arizona +x +L -L T(t) 90 o C 45 o C 0 o C +x +L -L T(t) 90 o C 45 o C 0 o C +x +L -L Slab of beef of thickness 2L with microwave (uniform internal) heating or radiant (uniform surface) heating

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
ABE 484/584 CHOI 3. [ Conduction/Convection/Heat Generation/Energy Balance ] A plane wall is a composite of two materials, A and B. The wall of material A has uniform heat generation q''' = 1.5 x 10 6 W/m 3 , k A = 75 W/m K, and thickness L A = 50 mm. The wall material has no heat generation with k
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### Page1 / 10

hw04-hints - ABE 484/584 CHOI HW4 1[Self-cleaning Oven An...

This preview shows document pages 1 - 5. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online