Fall 2012 ME 395 - GSI Josh Lacey

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 important information about lab 5

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GSI Overlord
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PostSubject: important information about lab 5   Wed Oct 24, 2012 4:29 pm

A couple points that I am going to reiterate during the lab sections but that I want to have publicly available for people to refer to:

People have been asking me about COP calculations and what is reasonable and what is not. There are a few sanity checks that you can actually do with your data to ensure the values you are getting are legitimate.

1) Inspect the mass flow rates of your cycles. You should see that mass flow rate decreases along with compressor speed. If you see a point where this is not the case (I think the 10 Hz point many people took in part I of the lab will display this) then this should throw up a red flag. I think everyone did a good job of looking at sight glasses and the volumetric flowmeter during the lab, and remember, that flowmeter reading is worthless unless the substance going through it is a liquid (think about point 3 of your cycle on the T-S) diagram.

2) Calculate the coefficient of performance for the Carnot cycle. COP_Carnot = Tlow/(Thigh-Tlow) and remember the temperature here must be on an absolute scale (i.e. degrees Kelvin). If the COP you calculated is higher than the Carnot COP...you screwed up or the cycle itself wasn't at steady state (i.e. there were bubbles in the flowmeter sight glass). DO NOT report COP values that are better than the Carnot COP...this is a super easy check for the professors to make by looking at your T-S diagrams and data and they will immediately know if you fouled up your COP calculations.

The main point I'm getting at is that you will likely not want to use your 10 Hz data from part I now that everyone has (hopefully) seen it gives non-physical COP values due to the excessive bubbling in the flowmeter sight glass (you are approximating point 3 as a saturated liquid when in fact it's two-phase, and that will throw off your whole cycle).

Be very mindful of this as you are analyzing data from part I and part II and calculating your COPs. And, one more point:

3) We only have one measurement of volumetric flow (right before the capillary tube). Now, while volumetric flow through the system won't be conserved through the cycle, mass flow rate absolutely will be, so the density you are going to want to use is the density which corresponds to that point in the cycle (where the flowmeter is located). Assuming a saturated liquid (which you should confirm using the flowmeter sight glass) at this point, you can look up the specific volume in your handy thermo tables and take the inverse of that to get density and ultimately convert your volumetric flow rate to a mass flow rate. This also speaks to the fact about why cycles you measured where there is excessive bubbling are useless data...you'll never be able to get a reasonable density because you aren't a saturated liquid and you don't have any way to find a quality at that point. If you try to approximate your point 3 as a saturated liquid in this instance your cycle will be quite wrong, as will the rest of your calculations.
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alfarmer



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PostSubject: Re: important information about lab 5   Sun Oct 28, 2012 5:07 pm

We have a little bit of confusion regarding the volumetric flow rate. We have the analog Sho-Rite meter readings, with an absurdly high uncertainty that leads to significant error in number of compressor units and annual operating costs (on the scale of 24 ±6 units and $4,300 ±1,100 annual electricity cost based on one of our trials) before even considering other sources of uncertainty. We also have a volumetric flow rate given in the LabVIEW output, which is significantly different from our manual readings (because it's taken at a different point) and labeled as "Flow Evaporator #1 (ml/minute)". We don't know if/how this is supposed to be useful, since the fluid entering the evaporator is already a liquid/vapor mixture and the volume is changing; what point would that be measuring at and why would it even be measured in the first place?
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GSI Overlord
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PostSubject: Re: important information about lab 5   Mon Oct 29, 2012 12:10 am

With the configuration of the Hampden carts that you have been using, I believe it is supposed to be taking an electronic flow measurement just upstream of the analog flowmeter (right before the capillary tube, so both readings are at the same location in the cycle). I'm a bit surprised that the two measurements would be so different...if you have some time to stop by my office hours tomorrow or Tuesday maybe I can have a look at the data.

Another thing to consider is if there was any bubbling in the flowmeter during the data collection, then it's quite likely these two readings would be vastly dissimilar and the data for the cycle is probably bad.
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alfarmer



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PostSubject: Re: important information about lab 5   Mon Oct 29, 2012 1:25 am

Yeah, I might try to come by office hours then... we found that, even when there was no bubbling, the electronic flow measurement was 30-70% greater than the analog readout in all cases and never agreed within the uncertainty of either one for any trials.
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