HW5QuestionsAnswersME525SP2011

HW5QuestionsAnswersME525SP2011 - 1 ME 525 SP2011 Homework#5...

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1 ME 525 SP2011 Homework #5 Questions and Answers Due Thursday, March 31, 2011 Prof. Lucht (E-mail address: [email protected]) Question: I have a doubt relating to Session 19; In both the problems you solved you assumed M2=1, but u2 turns out to be greater than 1000m/s. Basically, will the speed of sound be that much at the temperature we are dealing with? Or is M2>1, as in a supersonic wave? I dodn't quite get it. Please explain. Answer: Yes the speed of sound will be that much at flame temperatures. Question For homework five, problem one, we need to plot the selected radical's concentrations. The data files provided list the selected radical's mole fractions but I cannot find a corresponding pressure. Can I just assume the reaction was occurring at ambient pressure? Answer: I am not quite sure whether the flame is at 1 atm or 1 bar. You can assume that it is at 1 atm. Question: I'm not seeing how the density is calculated in correlation to the thermal diffusivity for Example 8.2 From Table C.1, I can get k and c_p, but the density of air at 1280K doesn't correlate to what's in the example. Also, in the example, when calculating the average density, do you know where the MW of 29 comes from? It seems like it should be 44, the MW of the fuel?? Thanks again. Answer: The MW of 29 is air. It is .21(32)+.79(28)=28.8. The air density is the appropriate term to use if you go back and derive the equation they use to simplify omega_f. As for your other question: In the expression for thermal diffusivity, you are looking for rho_unburned. This means you need to use T_unburned instead of the averaged temperature. From the first page of the example, T_u=300 K , instead of the averaged 1770 K.
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