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Homework2_Solution

Homework2_Solution - 4 What is irradiation and what are its...

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BAEN-366 SPRING/09 Homework #2 Read Chapter 1 Page 2 – 13 Answer the following questions 1) What is the difference between natural and forced convection? Give an example of each. Natural convection is the flow caused by buoyancy forces due to the differences in density of temperature variations. Example: A pie cooling inside of a room, where the air heats up and rises above the cooler air. Forced convection is a flow caused by external means like fans, pumps, etc. Example: A fan on the ceiling. 2) What is the Newton’s law of cooling? Write the equation and define each element of this equation with the appropriate units. q’’= h (Ts-T∞) q’’ = heat flux [w/m 2 ] h = convection heat transfer coefficient [w/m 2 K] Ts = surface temperature [K] T∞ = fluid temperature [K] 3) What is the Stefan-Boltzmann Law? Write the equation and define each element of this equation with the appropriate units. E b = σ Ts 4 E b = Emissive power σ = Stefan-Boltzmann constant [w/m 2 K 4 ] Ts = Surface temperature [K]
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Unformatted text preview: 4) What is irradiation and what are its units? Irradiation is the rate at which all radiation is incident upon a specific object, and it’s measured on a unit area of the surface. [w/m 2 ] For 1.13 change the following Hand temperature = 20C a) Vehicle speed = 40km/h Air at -10C h = 50 W/m 2 K b) water velocity = 0.5 m/s Water temperature = 15C h = 1000 W/m 2 K Solution a) q’’ = h (Ts-T∞) q’’ = (50 W/m 2 K) * (20 – (-10))K = 1500 W/m 2 b) q’’ = h (Ts-T∞) q’’ = (1000 W/m 2 K) * (20 – 15)K = 5000 W/m 2 Note: Since (Ts-T∞) represents only the difference between temperatures, it doesn’t matter if it is expressed in °C or °K, for this case. For 1.25 change the following Diameter = 0.8m Energy dissipated = 200W Emissivity = 0.85 E b = εσ Ts 4 q = A εσ Ts 4 A = π (0.8) 2 = 2.01m 2 q = A εσ Ts 4 200W = (2.01m 2 )(0.85)(5.67x10-8 W/m 2 K 4 )Ts 4 200W = Ts 4 9.68x10-8 W/K 4 Ts = (2.0645x10 9 ) ¼ = 213.16 K...
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