Modeling of gas mass fraction across local restriction for moist air - Simscape

Hi there,

I have a model using a Local Restriction (MA) as part of the Simscape model. It is actually the standard example ssc_vehicle_hvac in Matlab 2021a. When I plot gas mass fractions of input and output of the restriction, .A.x_g.series.values and .B.x_g.series.values, the results are not the same. I ran the model and plot as below. Can you please tell me how Simscape models mass fraction, resulting into the difference? The model can be called by typing ssc_vehicle_hvac in Matlab command window.

Thanks,

figure;
hold on;
plot(simlog_ssc_vehicle_hvac.Recirculation_Flap.Restriction_AB.B.x_g.series.values);
plot(simlog_ssc_vehicle_hvac.Recirculation_Flap.Restriction_AB.A.x_g.series.values);
legend('Reci flap, orifice inlet','Reci flap, orifice outlet');

Answer 1

[Pratyush]

Hi John,

The difference in mass fractions at the input and output of a Local Restriction (MA) block in Simscape, as observed in the "ssc_vehicle_hvac" example, can be caused by:

• Non-equilibrium flow conditions leading to varying mass fractions.
• Mass transfer effects that alter the gas mixture's composition.
• Changes in pressure and temperature across the restriction impacting solubility and reaction rates.
• Numerical discretization and solver settings introducing small discrepancies.
• Model assumptions and simplifications that may not fully capture complex behaviors.

These factors can result in different gas mass fractions at the inlet and outlet of the restriction within the HVAC system model. To pinpoint the exact cause, one would need to analyze the specific dynamics and interactions within the model.