Old Exam questions - Chapter 5-11

Old Exam questions - Chapter 5-11 - Old Exams Short Answer...

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Old Exams Short Answer – please answer the questions concisely and completely 1. What is an isothermal surface? What can be said about the heat flux at any location on this surface? 2. What is the physical interpretation of the Fourier number (i.e., the Fourier number describes what important quantity?) 3. What is the physical interpretation of the Sherwood number (i.e., the Sherwood number describes what important quantity)? 4. What is the physical interpretation of the Nusselt number (i.e., the Nusselt number describes what important quantity)? How does the Nusselt number differ from the Biot number? 5. Briefly describe what a “semi-infinite solid” is with respect to one dimensional heat transfer. 6. What is the “film temperature” that is used in convection problems? 7. Under what conditions is the Nusselt number for internal flow equal to a constant value? Problems 1. Stainless steel ball bearings ( ρ = 8085 kg/m 3 , k = 15.1 W/m•K, C p = 0.480 kJ/kg•K) have a diameter of 1.2 cm. They are to be cooled in air, and then quenched in water. They leave the oven at a uniform temperature of 900 o C, and are then exposed to air at 27 o C for a time, and then immersed in the water. If the surface temperature of the bearings can not fall below 850 o C before quenching in the water, determine how long they can spend in the air-cooling stage. The air provides a heat transfer coefficient of 120 W/m 2 •K. 2. Chicken carcasses with an average mass of 1.7 kg ( ρ = 0.95 g/cm 3 , k = 0.45 W/m•K, α = 0.13 x 10 -6 m 2 /s) are initially at a uniform temperature of 15 o C, and are to be chilled in an agitated brine solution at 0 o C. The average heat transfer coefficient between the chicken and the brine is experimentally determined to be 450 W/m 2 •K. USDA regulations require that the carcasses be chilled to 4 o C or below within 4 hours to prevent microbial growth. Treating the chicken carcasses as spherical shapes, determine how long it will take the center of the carcasses to reach 4 o C. 3. A thin-walled pipe (D = 50 mm) transports water at a flow rate of 0.3 kg/s. The water enters the pipe at 64 o C, and is cooled to 30 o C by a coolant flowing (crossflow) over the pipe. The coolant is at a constant temperature of 15 o C. What is the length of tubing required if the coolant is air and its velocity is 20 m/s? 4.
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Old Exam questions - Chapter 5-11 - Old Exams Short Answer...

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