Energy Balance

[hs.simanto]

Hi!

I was trying to simulate the efficiency of a tube to tube (air to water) heat exchanger. high temp. of air was introduced through inner tube and low temp. of water was assigned counter to the air flow. Simulation was done in steady state condition.

my main design setup was like this.

But this pipe is too long compared to its diameter. So, i tired to solve this as axisymmetric in Y with fine mesh.



I was looking for a relatively tight energy balance.

As there is no heat loss, so energy loss by hot air will be equal to energy gain by cold water for steady state condition. My results are showing the system efficiency is much more higher than 100%.

Here is a small summary of my calculations:

From energy balance: Heat loss by hot fluid= Heat gain by cold fluid

Heat loss by hot fluid                                          Qair= maCpa(Tin-Tout)

Heat gain by cold fluid                                        Qwater= mwCpw(Tout-Tin)

Efficiency                                                                        E= (Qwater / Qair) *100

ma =Mass flow rate of air= va*area*density=26*0.000305*1.2=0.009516 Kg/s

Cpa =1.0006 KJ/kg.K

mw = (Volume flow rate per min/60)*density = (0.002182/60)*998=0.03629 Kg/s

Cpw = 4.2 Kj/kg.K

Calculated by Wall Calculator at output air and water edge :

Twater output =39.36 C

Tair output = 36.82  C

Efficiency=311.128%

Using X-Y plot along the output diameter and taking average value of temperature (Ignoring the corner nodes in solid-liquid interface)

Twater output =34.182 C

Tair output = 37.95 C

Efficiency=142.349%

In both cases, energy balance is not satisfied.

I have attached my CFZ file for counter flow approach. Can't understand what when wrong in my setup/approach,

Please help!

Thanks!

[srhusain]

I would suggest the following steps to modify the geometry to start with:

1. Remove the outermost solid (you can add it back later to see if it causing a balance degradation)
2. Extend the inner and annular fluid domain about 3 diameters on both ends beyond the intervening solid annular wall.

This may help with the energy balance issue by preventing the intervening solid material from interfering with the fluid thermal calculations in terms of axial conduction and estimation of the bulk temperatures.

If things improve, you can restore the outer solid wall and re-run- it might turn out that extending the fluid domains is all you need.

Hope this helps.

[Royce_adsk]

Beyond what Syed added I would also suggest the following article to review:


http://knowledge.autodesk.com/support/simulation-cfd/troubleshooting/caas/sfdcarticles/sfdcarticles/...

[hs.simanto]

Thanks for you suggestions!

I'm trying to apply those to my model. I'll let you know about my observations as soon as i can.

Thanks! 🙂