# MERM 29.1 practice problem



## uzairsyedahmed (Jul 31, 2013)

This may be a little basic but I am burshing up on it and would be a little rusty.

but for 29.1 problem, for a) why am I getting a different results when I use equation 25.92 as opposed to reading air table and pressure ratios ( as given in solution)? my understanding was that it should give the same results if you use property relation or if you use tables as done in chapter 28.

Thanks!


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## CRNewsom (Aug 1, 2013)

If you could paraphrase the question, perhaps I could help more. I no longer have the practice problem books.


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## uzairsyedahmed (Aug 1, 2013)

Thanks

the question sis air expands isentropically at 10 ft^3/sec from 200 psi and 1500 F to 50 psi. What is the final air temperature.

The answers are different if you chose to do it with the air tables or with property relations even though in many of the examples both methods were used to obtain answers. so question really is, when can we NOT use property relations for isentropic process(like above) and have to use the air gas tables to get the answer.

THanks again!


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## ikesdsu (Aug 12, 2013)

I believe you have to use gas tables when it is superheated. But it is just a guess. I don't have my books anymore.


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## FatherTime (Aug 14, 2013)

The answers from the two methods can differ significantly, particularly at higher temperatures. If they are close, then it's just a lucky break.

The equations can (should) be used if air is considered to be an ideal gas. This implies the specific heats are constant, enthalpy change is a function of temperature change only, and so on. If the problem statement contains the phrase, "ideal gas", then that's your clue that the equations can (and, should) be used.

If "ideal gas" is not stated, then the methodology is ambiguous. Either method might be acceptable, and hopefully, the available answer choices won't be too close. Without the "ideal gas" qualification, I think an argument for using the tables (where specific heats are not constant, because properties are based on actual measurements) is stronger than the argument for using the equations (which require making an unstated/unsubstantiated assumption).


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## uzairsyedahmed (Aug 17, 2013)

@Fathertime,

Great answer! I think you are right about using it when ideal gas is mentioned and the air tables when it is not. it doesnt take me that much longer to do it through the tables (even with the interpolations) so I would go coservatively with air tables unless ideal gas is specified and temperatures arent too high. The book has always gone with the answer from the Air table, so I guess you wont be going wrong if you used air tables.

Thanks agian!


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## FullxCollapse (Aug 22, 2013)

I'm rusty and studying myself; however,I think the ideal gas hint is the fact that the process is isentropic, is it not? Then you may utilize the heat capacity ratio (1.32 for air @1500ºF) and the EQ: 

T2 = T1(P2/P1)(1 − 1/γ)


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## uzairsyedahmed (Aug 22, 2013)

fullixCollapse,

Look up that problem. it was isentropic and the answers differed for both. I guess its prudent to use the air gas tables.


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## FullxCollapse (Aug 23, 2013)

I'll check it out. Haven't reached the Thermodynamics portion of my studying yet but I'll keep it in mind once I get there. I'm sort of shooting from the hip on this one.

-John


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