Why does the simscape heat exchanger block not reduce in effectiveness as the mass flow rate increases?
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Niveditha Rajendrakumar am 17 Feb. 2022
Kommentiert: Yifeng Tang am 1 Jul. 2022
In my simscape model, I have an air (hot side) to water (cold side) heat exchanger (G-TL). The inlet to the cold side is connected to return pipe and the outlet to supply pipes (as in a heating system) which are connected to the same constant volume chamber. A centrifugal pump pushes the water to heat exchanger inlet, and it's added to ensure a controlled mass flow rate. My problem occurs when I increase the pump RPM/mass flow rate of water entering the heat exchanger (keeping the temperature difference across the gas side is a constant). I see an increased supply temperature indicating an increase in effectiveness when (if I'm not wrong), according to the E-NTU model the effectiveness should decrease.
Also I had added reservoirs to the gas and water side to give target temperatures and pressures to achieve (as I'd seen in another forum).This doesn't affect the mass flow rate on the water side but on the gas side atleast close to the heat exchanger block it does. Is this the reason my supply temperatures keep increasing as I increase the mass flow rates? If so, could you suggest an alternative way to go about modelling this part of my system (I tried caps instead of reservoirs to set initial (not target) values for the Hx, but it doesn't work).
Hope you can help, thanks!
Yifeng Tang am 1 Jul. 2022
It sounds like parametrization issue to me. Sounds like the pump is adding significant amount of heat to the liquid, resulting in higher inlet temperature, as flow rate is increased. This is typically a result of too much flow resistance downstream. I suggest you check the pressure drop over the heat exchanger and rest of the system and see if they are reasonable.
Also, including a more complete snapshot of your model, or the model itself, would help better identify the issue.
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