Branching of a two-phase fluid

I am currently trying to expand the Electric Vehicle Thermal Management model by adding a heat pump for heating operation instead of the PTC element. I would like to have the two-phase fluid flow through some kind of valve into another circuit. Since there seems to be no closing valves for 2P, I have tried to insert a valve orifice.

Below you can see my modified circuit.

Since it doesn't work like that yet, I created a much simplified model. Now I simply want to test the two shutoff valves.

SV1 should be open and SV2 closed. I realize this via a MATLAB function in the control block. cmd_SV1 or cmd_SV2 is either 1 or 0 and thus opens or closes the orifice.

function [cmd_SV1,cmd_SV2] = modefunction(delta_T_cabin)

if delta_T_cabin > 0    % cooling
    cmd_SV1 = 1;
    cmd_SV2 = 0;
else                    % heating
    cmd_SV1 = 0;
    cmd_SV2 = 1;
% elseif                  % mixed
% elseif                  % off
end

The scenario:

• Cabin temperature: 30 °C
• Target temperature: 21 °C
• delta_T_cabin = 9 °C (For the original model)

Now, strangely enough, the following case occurs: With two valves, a temperature difference of -21 °C is passed to the function because the cabin temperature is suddenly 0 °C. This means that my valves are not controlled correctly and an error occurs.

Now my questions:

Is it possible to switch a two-phase fluid like this at all? If so, how do I solve the problem? Why is my cabin temperature suddenly changed? Without SV2 but with SV1 the simulation runs without any problems.

Thanks for your help!

Kind regards

Kai

Answer 1

[Yifeng Tang]

Hi Kai,

You are using the "Orifice (2P)" block to model the valve, right? Good choice, I'd say, but there are a few things you need to pay attention to.

1. the S signal to the orifice block is supposed to be the spool position, with a unit of length (m/cm/mm/inch, etc.). The parametrization of the block will translate this position to opening area. This is similar to how most of the hydraulic valve blocks are parametrized. Giving it command of 0 and 1 without specifying a unit consistent with the block parameters, may give you unexpected results.

2. Trying to open or close a valve instantaneously can cause numerical problems as well. And it's not really physical. Consider add a 1st order transfer function or add derivatives in the Simulink-PS converter to slow it down a bit.

3. A bang-bang type of control algorithm with no deadzone in the midde is usually not optimal for refrigeration loop, either. You may want add a threshold around your target (delta_T=0), beyond which the valves will act and switch direction.

[Kai Franke]

Great! Thanks for the quick reply. I'll see if I can get the problem solved with these tips. Thank you!