counter current heat exchanger - pipe TL temperature difference (model.T-W.T) not accurate

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I am modelling a counter current heat exchanger in simscape. In the pipe element, the inner Temperature is always identical with the Temperature at the downstream conserving port (or downstream temperature is equal to the calculated inner temperature probably) Can someone explain why the equation of the heat flux of the thermal port W uses this inner Temperature (model.T) instead of the mean Temperature (A.T+B.T)/2 ? Since phi_w is calculated by htc*surface*(W.T-T), when using counter current pipes, the delta the delta of temperatures is smaller than it should be compared to other models and manual calculations. Therefore I have to icrease the heat transfer coefficient which kind of works fine, but i absolutely don't like it because temperature delta and htc are not fitting to the "real" model

Answers (1)

Yifeng Tang
Yifeng Tang on 29 Jun 2022
Interesting point and application you described. I agree, for counter flow heat exchangers, the deltaT you get between the thermal port (i.e. the pipe internal temperature), is smaller than say pipe1_A.T - pipe2_B.T (or something like that).
Simscape block are mostly "lump" components, meaning you have a variable for all the mass inside the component and all the mass are at the same state (like perfectly mixed, right away after anything enters the component). Then the upwind scheme will dictate the outlet port to have the properties of the component. This is what you observed in the model.
One way to get a more "real" model is to use more than one pipe on each side of your heat exchanger, so you have more "lump" and the deltaT would be more reasonable.
Another way is to use the heat exchanger blocks in newer versions of the Simscape Fluids.
And a quick comment on (A.T+B.T)/2: it's not necessarily a good idea. I believe it'll be too "static". Imagine you have a pipe filled with hot water, with both ends shut by a valve. The other ends of the valves are cold water tanks. Once you open the valve, the temperature for the pipe will remain hot for a while, and (A.T+B.T)/2 won't be valid until much later in time.

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