issues on Heat exchanger in simscape TL-MA

the model created seems not to behave phisically correct: heat exchanger reduces both temperature of TL and moisture air.

it should heat up air, cooling down fluid instead.

[Yifeng Tang]

took a quick look and noticed something: the moist air flow rate is 50kg/s (that's a LOT for air flow), while the port area is ~2500 mm^2. This would amount to about 20000m/s air speed (!!!!), which doesn't make sense to me. The local speed of moist air is part of the enthalpy flow calculation, so maybe this is why? Could you please check the parameters and inputs, make them reasonable and see if the problem persists?

[carlo ferro]

Hi Yifeng,

we have checked the initial conditions and now seems to be correct, in the air we have a square section as inlet of 2500mm (square section of 500 per side) with a velocity of 15 m/s entering. assuming the density of 1.15 kg/m3 we obtain a mass flow of 0.043 kg/s. regarding the fluid it has been imposed a diameter of the pipe of 5mm (section 19.63mm2) and velocity of 5.09m/s with a volumetric flow of 0.0001 m3/s and a density of water.

still the block doesn't run, the output message is: Failed to determine size of heat exchanger from the specified nominal operating condition. Try reducing Nominal rate of heat transfer or increasing Nominal mass flow rate..

even reducting rate of heat transfer or increasing mass flow rate nothig happens.

[Yifeng Tang]

This message is complaining about the nominal conditions (temperature, flowrate, heat transfer rate, etc.) you set for the heat exchanger are inconsistent. Most likely, under the provided nominal conditions, the liquid will be cooled down to a temperature lower than the air inlet, OR air hotter than liquid inlet, i.e. violating the 2nd law of thermodynamics :p Please take a look at that.

[carlo ferro]

thank for your reply Yifeng. Im fully aware of thermodynamics but less confident with the Simscape environment, despite the Onramp course taken. the thermal fluid reveives a net income of 500 W if I am not wrong. assuming an equal heat exchanged in the thermal heat exchanger it should provide me a heating effect on the air flowing through. maybe I have made a msitake chosing the reservoir as inlet and outlet, i intend the outlet temperature as a non fixed value to be updated during the iteration.

appreciate your understanding.

carlo

[Yifeng Tang]

I took a look at the TL_MA_s model you shared. The nominal flowrate for the TL side of the Heat Exchanger block, is Liquid_Mass_Flow (0.0001) kg/s, which is VERY small. 500W of nominal heat transfer will raise the temperature by 1000 degrees ??

[carlo ferro]

That was a mistake I have corrected the day before yesterday with positive results. not it converges rapidly without issues. I am now moving to a TL_2P_MA system

[Yifeng Tang]

just summarized the discussion into "Answer". happy modeling!

Answer 1

[Yifeng Tang]

Setting reasonable parameters to discribe the nominal conditions for the heat exchanger seem to resolve the issue. Pay particular attention to units and type of information needed (mass flow vs. volumetric flow). The port size matters as well because the local velocity calculated based on port size and mass flow is also part of the energy flow calculation.