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Old   February 21, 2012, 08:59
Default Pressure drop w constant fluid properties
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Hello everybody

I am finishing my first CFD proyect and I have two questions which I struggle to give answer.

The problem is a very simple heat exchanger. One pipe with cold water at the inlet and at some points heat is applied uniformly. Assumed no buoyancy. Wall is smooth and with no slip.
I simulate different a matrix of volume flow rates and inlet water temps. For 5 l/min, water inlet temp varies from 50C to 150C; same for 10 l/min and 20 l/min.

My questions are:

1.- If the water properties (density, viscosity, heat capacity, thermal conductivity) are constant, should the pressure drop be constant for a given volume flow rate?
2.- If I simulate water accordingly with variable density and viscosity, should the pressure drop increase with the increase of density or decrease?

I dont know if I explained correctly. Dont hesitate to ask if needed.

Thank you very very much in advance!
Regards

EDIT: I am using CFX
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Old   February 23, 2012, 09:01
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anyone can help me please?
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Old   February 23, 2012, 09:29
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you can answer those questions using Bernoulli's equation. I don't know how CFX is going to treat 150C water, but if it is in the liquid phase you need some serious pressure else it will put it in the vapor phase.
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Old   February 23, 2012, 10:21
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I fully agree with the pressure thing, but im doing a validation to compare other model wich was simulated as fluid even though temp reaches +150, so i guess it will withsand the pressure. Also CFX has no problem with it.

I think I cant reallly use Bernouilli because there is a heat exchange process and there are viscosity effects wich are not included in the equation. So i still dont know if there will be pressure drop if density/viscosity remain constant.

I am calculating the pressure drop as difference of pressures at the inlet and outlet. Isnt that right?

Thanks a lot for your help!!
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Old   February 23, 2012, 10:42
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but, in order for CFX to pull the correct values for the simulation you need to define the inlet pressure. Just because CFX is solving it certainly doesn't mean it is correct - thus, the validation.. The easiest way to get an idea of the Q would be mdot(dh) depending on your Temp difference between inlet and outlet

If everything is constant you can use Bernoulli. If there is heat exchange you will have density changes and Cp, K...etc..then, you can't use bernoulli.

Yes, pressure drop is the difference between inlet and outlet. Is your outlet into atmosphere where the pressure is 0 gauge?
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Old   February 23, 2012, 11:13
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Thanks for your really fast answering!
Quote:
Originally Posted by mettler View Post
but, in order for CFX to pull the correct values for the simulation you need to define the inlet pressure. Just because CFX is solving it certainly doesn't mean it is correct - thus, the validation..
I couldnt agree more but they asked me to do it this way, so I cant do much about it...

Quote:
Originally Posted by mettler View Post
The easiest way to get an idea of the Q would be mdot(dh) depending on your Temp difference between inlet and outlet?
wont mass flow rate remain constant?

Quote:
Originally Posted by mettler View Post
Yes, pressure drop is the difference between inlet and outlet. Is your outlet into atmosphere where the pressure is 0 gauge?
The reference pressure is 1 atm and the relative pressure at the outlet is 0 atm, so yes

The biggest problem is that they are telling me that in the simulation done before, water was defined with constant properties. But for a given volume flow rate, pressure drop changes with the inlet temperature... and that its driving me crazy because i dont get correlated results for the same flow rates and temperatures.

As said, I must use fluid water. I tried to parametrize Density vs Temp and asume it would remain linear after 120C (which is wrong...i know).
But is it right the fact that the lower the density, the lower the pressure loss? Or the conclussions will be so erratic that nothing could be concluded from them?

Thank you very much once again!!
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Old   February 23, 2012, 11:30
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your reference pressure for the inlet is 1 atm? Water at 150C and 1 atm won't be liquid.

mdot is constant.. enthalpy (h) is not.. mdot(h2-h1)

constant properties can be assumed if the inlet and outlet temperature are close.

Have you defined an inlet pressure?
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Old   February 23, 2012, 12:21
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I think I already know what happened! I think that when they defined the water properties, they have selected the IAPWS EOS option (International Association for the Properties of Water and Steam Equations of State), so they think the properties were constant. Dont know exactly how it works, if it mixes vapour and fluid or just gives temperature dependent properties, but I have seen that pressure drop now is changing with temperature changes! It is already something

For the inlet i have defined: temperature, mass flow. My idea is that i define an outlet pressure and let the program calculate the inlet one, is it right?
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Old   February 23, 2012, 12:32
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do you know the pressure at the exit? I guess if you are defining the outlet pressure you can back-track to get the inlet given the temperature there. But, I believe for an 'equation of state' you need both P and T, since the 'state' will vary with different T or P.

What are you getting for an inlet pressure? It should be quite high if it is liquid coming in.
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Old   February 23, 2012, 17:27
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Quote:
Originally Posted by mettler View Post
do you know the pressure at the exit? I guess if you are defining the outlet pressure you can back-track to get the inlet given the temperature there. But, I believe for an 'equation of state' you need both P and T, since the 'state' will vary with different T or P.
I know the pressure drop and that the relative pressure at the outlet is zero, so I know: Pin, Tin, Pout, Tout. I tried to replicate the same results and I couldnt. I tried with this IAPWS EOS and i got correlated pressure drops and very similar temperature gain.

Quote:
Originally Posted by mettler View Post
What are you getting for an inlet pressure? It should be quite high if it is liquid coming in.
Actually the inlet relative pressure is quite low! At the highest temperature (150c) it ranges from 50Pa to 2000Pa. Do you think the IAPWS is assuming steam even though i declared it as a fluid liquid?
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Old   February 24, 2012, 09:51
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I still think you need to define an inlet pressure. I'd think it was steam if I was just told a temperature. Also, if it is steam the temperature change will be quite low until enough energy is released to convert it back to a liquid. That would depend on the quality of the steam coming in too.

Can you define a pressure?
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Old   February 29, 2012, 10:55
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Sorry for not answering before. I was quite busy with other projects.

No inlet pressure is provided by the customer because the heater is still at an early development phase. I agree that for the higher temperatures it must be steam (if not there should be a huge pressure which im not sure that the pipe could handle).

My question is, does the CFX IAPWS EOS property "tell" the solver wether the fluid is water or steam? I might post this question in the CFX forum.

Thank you very much for your help once again
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Old   February 29, 2012, 11:01
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I am not sure if CFX would "assume" it is steam given the temperature. But, if you have no temperature change from one end of the pipe to the other, then it is steam - as most of the energy is being used for phase change. I will tell you that from my experience with high temperature steam it is tough to compensate for the large fluctuations that you get in the pressures -you end up with slow flow thru the pipe, followed by a burst of high flow.
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