12 pts] List four reasons why fan noise is difficult to predict? 7. 12 pts] You
ID: 1766879 • Letter: 1
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12 pts] List four reasons why fan noise is difficult to predict? 7. 12 pts] You have a designed a device that uses a centrifugal blower for cooling. The blower currently operates at 2000 RPM and delivers 1.00 ACFM of air flow with a sound pressure of 35 dBA at the operator position. Unfortunately, the device is still overheating and the only solution you can come up with is to increase the fan speed until the device achieves 1.20 ACFM of cooling air flow. Estimate the new acoustic noise level for the device at the same operator position at the new flow rate 8. 9. Which of the following factors affect surface emissivity: a. Material type b. Surface finish c. Surface temperature d. All of the above 10. In which of these heat transfer problems is radiative heat transfer expected to play an important role: a. Heat dissipation from an incandescent light bulb b. Heat dissipation from the surface of a car driving at highway speeds c. Heat conduction through a vacuum insulated thermos wall d. Estimating the average surface temperature of a planet e. All of the above f. Only A, C, and D g. Only A and D 11. Which of the following statements are true for opaque materials: a. Reflectivity and Emissivity are equal for "grey" surfaces b. The sum of Absorbtivity and Emissivity is equal to 1 for "grey" surfaces c. The sum of Emissivity and Reflectivity is equal to 1 for "grey" surfaces d. All of the above e. Only A and CExplanation / Answer
9) All the Above.
Emissivity describes a material’s ability to emit or release the thermal energy which it has absorbed. A perfect radiator-known as a ‘black body’-will emit the entire amount of absorbed energy.
Factors which influence emissivity:
The emissivity of an object depends primarily on the type of material and its surface properties.
Furthermore, the emissivity can change depending on the wavelength.
The emissivity of metals and glass also changes as a function of temperature.
10) All the given points are an examples of Radiation only.
11) ALl the Above isRight
The body whihc has its absorptivity is equal to emissitivity is called as Greay Body provided temperature of surface of incident radiation and bode are same.
The body which has same emissitivity all over wave lenghts is called as Grey body.
For Opaque body Transmittivity is zero.
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