# Simscape Thermal Liquid Domain - Across variable T of parallel flows

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Joël Habersatter on 30 Nov 2023
Answered: Yifeng Tang on 3 Jan 2024
Hello there,
I am working with the Thermal-Liquid Domain of Simscape to model a liquid cooling network. During my work, I noticed the following problem:
Let's say my very simple network consists of two custom elements (Element 1 and Element 2) that instoduced heat power to the fluid (see picture). Element1 adds 1000W and Element 2 adds 3500W. Clearly, the temperature of the fluid at port B of Element1 should be larger than the one at port B at Element2. However, the simulation always outputs the same temperature difference for both elements i.e. B.T of Element1 is the same as B.T of Element2. These temperatures are also the same after the junction (where the 2 parallel branches come together again). Therefore, it is clear to me that both B.T temperatures already refer to the mixed fluid. What I would obviously like to have is that the temperatures of the upper and lower branch stay dissimilar until after the junction. Is there a possibility to somehow achieve that?
Any hint is appreciated. Thank you in advance!

Ganesh on 5 Dec 2023
The issue arises as you are simply connecting the fluid flow together. Due to this, the Simscape model tries to retain fluid properties throughout the pipe and chooses automatically to set the fluid property of the mixed system.
A workaround for this would be to use a “T-Junction” from Simscape, which would help in differentiating the fluid flows individually to that of the mixed system.
Please find attached the documentation on “T-Junctions”:

Yifeng Tang on 3 Jan 2024
Adding a pipe block after each of the custom components will work just fine as well.
Indeed, ALL ports connected to the same junction will have the same pressure and temperature. This is behind the modeling principle of Simscape (see this Doc page). What a bit special for thermal-fluids systems is that the temperature/energy value at the port also uses an upwind scheme, meaning the state you see at the port is what's coming from the upstream, as explained on this Doc page. If the flow is almost zero, the state will be a blend between the internal states of the blocks connected to it. When there are more than one upstream flowing into the port, like what you have, it's a blend of the incoming streams.

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