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

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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:
(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

Risposte (2)

Juan Sagarduy
Juan Sagarduy il 14 Dic 2020
Hi Joris
In the cylinder a new through variable comes into play: the heat exchanged with the environment. The variations of enthalpy (*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
  1 Commento
Joris Naudin
Joris Naudin il 14 Dic 2020
Modificato: Joris Naudin il 15 Dic 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)

Accedi per commentare.

Sarah McElman
Sarah McElman il 23 Dic 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,


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