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Two-Phase Fluid Simscape : different values between Simscape Results Explorer and Sensors

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Joris Naudin
Joris Naudin on 14 Dec 2020
Answered: Sarah McElman on 23 Dec 2020
Hello hello
I have a nice and running (without any errors or warnings !) model of a piston expanding some steam.
However, when I want to read the temperature and enthalpy in the piston chamber, I see 2 different results which for me should be the same:
1 - One is taken through sensors on the line ;
2 - One is taken directly from the Simscape Results Explorer of the piston itself (mechanical translational converter).
Since there is no valve or anything between 1 and 2, they should be exactly the same readings no ? Well they are not: see the temperature and specific enthalpy (which by the way the temperature acts super weirdly with those spikes but I already put a different post for that one: https://www.mathworks.com/matlabcentral/answers/540911-two-phase-fluid-weird-temperature-peak?s_tid=prof_contriblnk)
(Blue is the mechanical converter and red is the sensor)
If you want to check the sensors also if anything's wrong :
It's not a super issue blocking me but still my curiosity sense is triggered on why this is happening so if you have an idea why please share it !
Cheers friends

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Answers (2)

Juan Sagarduy
Juan Sagarduy on 14 Dec 2020
Hi Joris
In the cylinder a new through variable comes into play: the heat exchanged with the environment. The variations of enthalpy (int.energy+p*vol) in the cylinder are due to expansion/compression in the cylinder and this exchange.
But the heat balance (heat_in = heat_fluid_cyl+heat_to_ambient) is always respected.
Hope that helped. Br Juan

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Joris Naudin
Joris Naudin on 14 Dec 2020
But why is the value of specific enthalpy and temperature different between 1 and 2 ?
Since the state of the steam inside the cylinder should be the same as the line before ? Plus a single line does not have any volume so it really should carry the same information of the environment it is connected to, no ?
It's like in this poor drawing: this model being a 0-D simulation, the volume in (1) shares the same state as the volume in (2)

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Sarah McElman
Sarah McElman on 23 Dec 2020
Hi Joris,
Yes, it can be confusing! It's possible to observe differences in measurements of the same variable between quasi-steady components (or ports) and dynamic components, or due to [numerical] conduction. You can see an example of different measurements of the same variable in a model of flow reversal here:
Additionally, you can find more information about the Simscape Fluids numerical scheme here:
Hope that helps,
Sarah

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