Plot the time versus the initial temperature and

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Unformatted text preview: here heat transfer takes place by convection with a heat transfer coefficient of h 140 W/m2 · °C. Determine the temperature at the center of the cylinder 5 cm from the end surface 8 min after the start of cooling. 4–76E A hot dog can be considered to be a cylinder 5 in. long and 0.8 in. in diameter whose properties are 61.2 lbm/ft3, Cp 0.93 Btu/lbm · °F, k 0.44 Btu/h · ft · °F, and 0.0077 ft2/h. A hot dog initially at 40°F is dropped into boiling water at 212°F. If the heat transfer coefficient at the surface of the hot dog is estimated to be 120 Btu/h · ft2 · °F, determine the center temperature of the hot dog after 5, 10, and 15 min by treating the hot dog as (a) a finite cylinder and (b) an infinitely long cylinder. 4–79 Reconsider Problem 4–78. Using EES (or other) software, investigate the effect of the initial temperature of the ice block on the time period before the ice block starts melting. Let the initial temperature vary from 26°C to 4°C. Plot the time versus the initial temperature, and discuss the results. 4–80 A 2-cm-high cylindrical ice block (k 2.22 W/m · °C and 0.124 10 7 m2/s) is placed on a table on its base of diameter 2 cm in a room at 20°C. The heat transfer coefficient on the exposed surfaces of the ice block is 13 W/m2 · °C, and heat transfer from the base of the ice block to the table is negligible. If the ice block is not to start melting at any point for at least 2 h, determine what the initial temperature of the ice block should be. 4–81 Consider a cubic block whose sides are 5 cm long and a cylindrical block whose height and diameter are also 5 cm. Both blocks are initially at 20°C and are made of granite (k 2.5 W/m · °C and 1.15 10 6 m2/s). Now both blocks are exposed to hot gases at 500°C in a furnace on all of their surfaces with a heat transfer coefficient of 40 W/m2 · °C. Determine the center temperature of each geometry after 10, 20, and 60 min. 4–82 Repeat Problem 4–81 with the heat transfer coefficient at the top and the bottom surfaces of each block being doubled to 80 W/m2 · °C. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 260 260 HEAT TRANSFER 5 cm To reduce the cooling time, it is proposed to cool the carcass with refrigerated air at –10°C. How would you evaluate this proposal? 5 cm 5 cm Ti = 20°C 5 cm 5 cm Ti = 20°C Hot gases, 500°C FIGURE P4–81 4–83 A 20-cm-long cylindrical aluminum block ( 2702 0.896 kJ/kg · °C, k 236 W/m · °C, and kg/m3, Cp 9.75 10 5 m2/s), 15 cm in diameter, is initially at a uniform temperature of 20°C. The block is to be heated in a furnace at 1200°C until its center temperature rises to 300°C. If the heat transfer coefficient on all surfaces of the block is 80 W/m2 · °C, determine how long the block should be kept in the furnace. Also, determine the amount of heat transfer from the aluminum block if it is allowed to cool in the room until its temperature drops to 20°C throughout. 4–84 Repeat Problem 4–83 for the case where the aluminum block is inserted into the furnace on a low-conductivity material so that the heat transfer to or from the bottom surface of the block is negligible. 4–85 Reconsider Problem 4–83. Using EES (or other) software, investigate the effect of the final center temperature of the block on the heating time and the amount of heat transfer. Let the final center temperature vary from 50°C to 1000°C. Plot the time and the heat transfer as a function of the final center temperature, and discuss the results. Special Topic: Refrigeration and Freezing of Foods 4–86C What are the common kinds of microorganisms? What undesirable changes do microorganisms cause in foods? 4–87C How does refrigeration prevent or delay the spoilage of foods? Why does freezing extend the storage life of foods for months? 4–88C What are the environmental factors that affect the growth rate of microorganisms in foods? 4–89C What is the effect of cooking on the microorganisms in foods? Why is it important that the internal temperature of a roast in an oven be raised above 70°C? 4–90C How can the contamination of foods with microorganisms be prevented or minimized? How can the growth of microorganisms in foods be retarded? How can the microorganisms in foods be destroyed? 4–93C Consider the freezing of packaged meat in boxes with refrigerated air. How do (a) the temperature of air, (b) the velocity of air, (c) the capacity of the refrigeration system, and (d) the size of the meat boxes affect the freezing time? 4–94C How does the rate of freezing affect the tenderness, color, and the drip of meat during thawing? 4–95C It is claimed that beef can be stored for up to two years at –23°C but no more than one year at –12°C. Is this claim reasonable? Explain. 4–96C What is a refrigerated shipping dock? How does it reduce the refrigeration load of the cold storage rooms? 4–97C How does immersion chilling of poultry compare to forced-air chilling with respect to (a) cooling time, (b) moisture loss of poultry, and (c) microbial grow...
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