seemingly wrong power given by Simscape block Compressor(G) and Turbine(G)

Hello. I am recently playing with the Brayton Cycle Gas Turbine example. After I entered the component maps and other inputs, the calculated dynamics seemed much off. In specific, I entered steady-state data with the fluid power of both the compressor and the turbine of about 300kW, but the gas generator speed dropped continuously. I then looked at the logged data and found that the power was much different: the fluid power of the turbine was lower than expected and even lower than that of the compressor. I checked other variables like pressures, temperatures, and isentropic efficiencies, but they all seemed correct, which should give correct powers in theory. I then calculated the fluid power using the mass flow rate times the specific enthalpy change across the component and found them correct and congruent with the temperature and specific heat capacity data, with the turbine fluid power higher than that of the compressor. However, the energy inflow and outflow (Phi_A and Phi_B in the block) were inconsistent with the enthalpy data, which are shown below. The above analysis led me to suspect that the problem is from the Simscape block itself. I performed the same procedure to the original example and observed that the fluid power was still lower than that calculated from enthalpy change, shown below. Could anybody provide some hints on this issue?

[Yifeng Tang]

There is a "scale factor" in the example. Have you tried playing with that value first to get the system to work roughly at the power you want?

It's usually tricky to set one component at a time in a large model. One often need to set up test harness models to make sure the parameters of a certain component are set properly before integrating into the bigger model.

[William]

Yes, I am aware of the "scale factor", but since it affects too many parameters like flow area and shaft inertia simultaneously, I keep it as one and modify the parameters of interest directly. I am not particularly concerned with the exact number produced by the compressor and the turbine block but the contradictory results (at least in my view). In my case, the power (fluid_power) is consistent with the energy flow (Phi_A + Phi_B), but not with the enthalpy difference (mdot * \delta H). From what I understand, Phi_A = mdot * H_A. If this equation is indeed one of the equations set by the block, then it is very unlikely for simulink to produce such erroneous result. That's why I am curious about how Phi_A or Phi_B is calculated.

[Yifeng Tang]

I see what you are asking now. Through a component like compressor or turbine, the change in the energy flow (Phi) is usually the same as shaft work, i.e. Phi_A + Phi_B + W = 0, with positive sign meaning energy flow or power into the component. The energy flow (Phi) is closely related to the mass flow (mdot), but NOT the same as mdot*H_A. There is an additional mdot*V^2/2 term, V being the bulk flow velocity at the port, to account for the kinetic energy term. To view this part of the code, you may go the the source code of foundation library components like flow resistance, local restriction, pipe, etc, and get the almost the very end. You may find some code like this:

The "founcation.gas.port_convection" subroutine defines the total energy flow Phi. You can select the whole string and right click to open it. Towards the end, you will see code that includes the V^2/2 term and the upwinding numerical scheme:

Hope this explains the difference you saw.