[nirmal_singh]

Hello,

I am solving a double pipe heat exchanger, where I have input temperature for both pipes.

My solution gives me total heat exchanged and outlet temperatures.

All residual values are below 10 and monitor points have also converged. But If I perform an energy balance using inlet and computed outlet temperatures (Average values), and pluging　them into m*Cp*dT equation, the balance has a huge mismatch.

Please help me as I am in middle of an important task.

Thanks in advance

[nirmal_singh]

I recently realized that the outlet temperature of outer temperatures of pipes are getting influenced by the ambient temperature setting of the system!

I tried to solve the problem by creating an ambient with 0 heat transfer coeff., but this also seems not to work.

[Boris_M]

Have you considered the temperature dependency of Cp?
It is not a constant value but changes over the temperature range you have as dT.
So ideally you would need to use the integral of Cp over dT as the integral form of the equation suggests.

[nirmal_singh]

I have considered the average value of Cp over workign temperature range. This will introduce some error in energy balance,　But when I am doing energy balance on paper, the mismatch is of order to about 800W.

[Boris_M]

Can you post both temperatures at inlet and outlet as well as the total heat exchange and the fluid you've used. If it is water, then I have the Cp values if it is some other special user defined fluid then it'll be hard to calculate correct values.

[nirmal_singh]

The properties and temperatures are:

Liq side (Outer shell) : Tin = 41 oC, Tout 35 oC, Cp 1270 J/KgK
Gas (Inner shell) : Tin = 29 oC, Tout 34.2 oC, Cp 950 J/KgK

The mass flow rate is same for both fluids.

[nirmal_singh]

The main problem is that default ambient is set to 35 oC and if I change the default ambient to 30 oC, the Tout of Liq side will change to 30 oC. I have not used default ambient properties anywhere inside the system, But I don`t know why this is happening and How this is happening!

[Boris_M]

There is something wrong then in your model or your setup. The default ambient temperature set in the general settings/wizard is not influencing the outlet temperature of the fluid. You can set the inlet temperature of the fluid to any other value in the boundary condition and it is either heated or cooled but not forced to the ambient temperature at the outlet.

In such a case always go back to a very simple task and test it there.
Try the heat exchanger tutorial model that comes with the software.
You can specify the ambient temperature to any value and specify the inlet temperature of the gas and water fluid subdomains to another temperature, the temperature at the outlet is the result of the heat transfer and diffusion of the heat in the fluid.
There are only two reasons why this might happen.

1. The pipe is very long and with wrong goal definitions the convergence is reached before the hotter fluid has reached the end of the pipe and changes the temperature there. The fluid is moving through the pipe and heating up in a steady state simulation just like in a transient simulation as it is an iterative process and the fluid moves towards the outlet from iteration to iteration but it takes some time to reach it.
By default the temperatures on inlet and outlet are set to default temperatures. So if you don't set the inlet temperature differently then the fluid will not enter the pipe with a different temperature than the default one. And the outlet temperature is the initial temperature at the outlet but will change over time once the hot fluid reaches the outlet.

2. If you have an error warning of "Vortex crosses the pressure opening" the amount of fluid entering the outlet from the outside will have the default (or in the outlet BC defined temperature) and mixes with the hot outflowing fluid as part of the vortex is going in and part is going out. This will reduce the average temperature as well as some other parameters such as concentration if a mixture is used etc. Always try to avoid this error by extending the pipe to have the vortex inside the computational domain. Usually this happens after a bent if the outlet is right after that bent or if there was something that caused some turbulation or dead zone behind an object right in the outlet like with the ball valve tutorial where there is a step behind the ball of the valve due to the angle it is at.

[Boris_M]

Oh, sorry. Forget my last comment. I posted in so many questions and mostly for FloEFD/SWFS that I forgot this was about FloTHERM :-)

These settings are from FloEFD.

[nirmal_singh]

Thanks Boris.

I'll check for the errors that you have mentioned.