How to avoid heat transfer to reservoir in simscape two phase system?

How can I avoid transferring heat from a reservoir when no mass is transferred to it?

I'm building a model of an LNG (liquefied natural gas) fueling station with Simscape two-phase fluid components. The idea is to have vehicles come refuel at intervals. The LNG tank of the station is represented by a 2-port constant volume chamber, and the vehicles refueling are represented by a reservoir. When the vehicles connect, a controlled mass flow rate source pumps fluid into the reservoir, to represent filling the vehicles' fuel tanks. My problem is that since the reservoir is always connected, heat is transferred from the infinite reservoir to my very finite LNG tank, even when no mass moves in between.

I need to be able to disconnect the reservoir from the system, to represent the situation when no vehicle is there to refuel from the tank. Or if its possible to stop the heat flow when there is no mass flow, that would be easiest. I've found no components for either of those options though.

Another idea I have is to run two simulations in tandem, one model including only the LNG tank and the ambient air heating it slowly, and the other model including the much greater influx of heat that occurs only occasionally, during refueling. If I could pause the first simulation, to run the other for the event of refueling, and copy the relevant parameters that change in between the two, I think that could work.

Answer 1

[Ray Joseph]

Olle,

Yes, that is a great question. There is a thermal input on a 2P Receiver Accumulator. Can a switch or controlled restriction be put on this line?

I am new to Simscape and am trying to determine rules and tools for domains. I have not yet found details on the thermal connectors.

I also have not found a 2P reservoir like you mentioned.

[Olle Levin]

Are you working in Simscape Fluids? I'm using the two-phase components of the Simscape foundation library, which are different. There is no accumulator component in this library, for example. Also, when I search for a receiver accumulator in the Matlab help, I find no such component in any library.

Anyhow, in my case the problem is not with a thermal port. The heat is supplied as a 'Through'-variable from one component to another, seemingly independent of the other 'Through'-variable - the mass flow.

Answer 2

[Ray]

Olle,

Yes, navigating the documentation is a challenge for me.

I am guessing you are managing all your thermal ports to assure they have known behavior. Here is a link to controlling thermal flow: https://www.mathworks.com/help/physmod/simscape/ref/variablethermalresistance.html

For flow resistence: https://www.mathworks.com/help/physmod/simscape/elements-1.html

There is a fixed flow resistance, an infinite resistance and a variable resistance shown on the above page.

The 2-P constant volume chamber has two mass ports and 1 thermal port. https://www.mathworks.com/help/physmod/simscape/ref/2portconstantvolumechamber2p.html These are the two means of transferring heat. The heat port and the mass port. If no mass is moving, heat will not be transferred through the mass ports; there is not a mechanism for heat transfer in the mass port other than heat carried within the mass. This raises the question of what do you have connected to the constant volume chamber?

I will see if I can re-find the link to the accumulator.

Ray

[Olle Levin]

"If no mass is moving, heat will not be transferred through the mass ports;"

- see, that is where you are wrong. That is exactly what is happening to me - despite there being no mass flow, heat is flowing from the reservoir component to the constant volume chamber. I imagine this is because there is a temperature gradient between them.

Maybe I should make clear that the conserving ports carry two (2) 'Through'-variables in the two-phase case: mass flow rate and energy flow rate. Evidently they are independent to some extent.

Here is the mass flow, only ever positive or zero.

Here is the energy flow, clearly negative while the mass flow is zero. That is what I want to prevent.

Answer 3

[Ray]

Olle,

I found the receiver at: SimscapeFluids_lib/Two-Phase Fluid/Tanks & Accumulators

What is the component you are using to connect the reservoir and vessel?

The 3-zone pipe (2P) help page states: "Opening through which the thermal liquid flows into or out of the pipe. Ports A and B can each function as either inlet or outlet. Thermal conduction is allowed between the thermal liquid ports and the fluid internal to the pipe (though its impact is typically relevant only at near zero flow rates)."

This seems to be exactly what you identified as thermal leakage in the pipe.

Now, how to eliminate thermal leakage. Here is a kludge: The the reciever tank. Minimize the volume. Input to the AV, output through BV. No heat condution at AV.

Ray

[Olle Levin]

Ah, I found the component, in 2018b - I have 2018a installed, and the component was only introduced in the latest version of Matlab. Anyhow, I have found other workarounds that will help me. As for minimizing the volume of the tank, that would not be possible since the tank is what I want to observe. The volume is one of the relevant parameters for my study.

Answer 4

[matlab newcommer]

Can you please link these workarounds? Maybe they will help me to because I have a similar problem.

Thank you!

Answer 5

[Ray Joseph]

Please clarify "link these workarounds". I don't know what you are looking for. If you describe your situation, 'linkages' may be provided in better light.

Ray

Answer 6

[matlab newcommer]

Englisch is not my native language so I hope I got the point... if not I am sorry.

It seems like the heat flow reacts like a unwanted drop and I have the same problem with the thermal block "Controlled Heat Flow Rate". When the power changes at the input the temperature decreases for a few seconds and afterwards it reacts as expected. Because of that I thougth I get the point with the help of that workaround.

Answer 7

[Garrett Hennig]

I am experiencing the same phenomonon in the gas domain. I boiled down my system to the simplest elements to identify the issue: a Resivoir(G) , a Mass Flow Rate Source (G), and a Constant Volume Chamber (G). The resivoir temperature is set to something high and the initial chamber temperature is set low. The mass flow rate is set to 0 and the chamber thermal port is capped with an Perfect Insulator. When the simulation is run to equilibrium, I would have expected no change yet the chamber temperautre approached the resivoir temperature. I confrimed the mass flow is 0 yet the energy flow rate is non-zero. I thought it may be thermal conduction, but when I set the gas conductivity very low, the results dont change. How can there be energy transfer without transport, nor conduction?