Fall 2012 ME 395 - GSI Josh Lacey

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 Having trouble interpreting the LabView Data file

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nmhunsa



Posts : 2
Join date : 2012-09-12

PostSubject: Having trouble interpreting the LabView Data file   Thu Oct 18, 2012 1:28 am

Josh,

I am trying to pull temperatures and pressures out of the data file but I am confused about which labels in the data actually correspond to what.

What is the difference between the 'Heat exchanger' and the 'Condenser' in the file? These two devices hold similar (but slightly different) values for temperature. It is my understanding that these two device names are interchangeable for our purposes. What am I missing here?

Also, are certain points in the cycle not given sensors on purpose? E.g., it seems we are not given data for the in and out of the 'expansion coil' (throttle device). Is it intended that we use the laws of thermodynamics to get these points?

Thanks,

-Nick
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GSI Overlord
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Join date : 2012-08-30

PostSubject: Re: Having trouble interpreting the LabView Data file   Thu Oct 18, 2012 1:53 am

Part of the 'fun' of this lab is figuring out exactly what temperatures and pressures to use.

Remember that the condenser and heat exchanger are both part of the 'condenser system'. I think it is probably more sensible to use the condenser in and out temperatures and pressures for the determination of thermodynamic state because you don't actually have a pressure reading at the heat exchanger (and due to realistic pressure losses in the line, the pressure at the condenser and heat exchanger won't quite be the same).

There are definitely assumptions you will have to make to obtain what I'll call 'point 4' of the thermodynamic cycle (the beginning of the evaporation process right after the process fluid has been flash frozen in the expansion valve). These assumptions are grounded in the laws of thermodynamics, but the main issue you'll have to figure out is the quality of the mixture at point 4 (remember, it's part vapor and part liquid there!). You can probably approximate the temperature going in as close to what was coming out of the condenser/heat exchanger and what was going out as approximately what was going into the evaporator. Just remember to mention all your assumptions and exactly how you found each point in the thermodynamic cycle when you are writing up the report.
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tuesday ninja



Posts : 5
Join date : 2012-09-15

PostSubject: Re: Having trouble interpreting the LabView Data file   Thu Oct 18, 2012 2:48 pm

Which data should we be using to determine entropy values, the gage readings we took in lab, or the LabVIEW data? Our problem is that the gage readings differ quite drastically from the LabVIEW data, specifically the temperature on the gage differs from that of the LabVIEW program.
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GSI Overlord
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PostSubject: Re: Having trouble interpreting the LabView Data file   Thu Oct 18, 2012 3:12 pm

You will have to exercise some judgement on this as you consider how to draw the thermodynamic cycle. If one of the readings is far 'off' or has more uncertainty in it, you might decide not to use it and just go with the other reading. Or, you might average together the electronic and gage readings and figure that into the overall uncertainty. Ultimately, you'll be able to decide a legitimate way to proceed based on how the cycle looks and if the points are in appropriate places, thinking about the relative position of each step with relation to the vapor dome. Remember too that under the vapor dome, processes occur at constant temperature (most likely your measured temperatures will not adhere to this, however you'll need to make simplifying assumptions when drawing the cycle). Also remember that some of your 'idealized' processes will occur along isobars (constant pressure). ALWAYS be certain to state your simplifying and procedural assumptions in the final report on how you developed the T-S diagrams and ALWAYS be sure to state the uncertainty associated with the measurements and method you chose.
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