boundary conditions for refrigerator condenser simulation

[Anonymous]

Hello everyone,

Please see the attachment to see my problem. 

• Simulation is starting from position 1 (inlet) where the temp. is 70, quality is 1, velocity=5 m/s.
• Now till the point 2 the temperature should decrease sensibly. and no change in quality.
• As soon as it reaches point 2, the temperature should become constant.
• and 2-3 only phase change should happen, means quality change of refrigerant.
• point 3 is our outlet. 
• Pressure drop throughout the condenser tube is assumed to be negligible. 
• we can increase the length of tube as much as we want.

Now, my question is

• what should be my exact boundary condition for this type of simulation?
• Pressure at initial stage should be given as Psat. according to the temp. at point 2. so where should I give this pressure value?

please help me in simulating this type of problem.

thanks in advance.

regards,

Akshay Kumar

[Jon.Wilde]

I think what you have in the first line might be enough to define the inlet - refer to the CFZ I gave you before also 😉

What is going to cause the temperature to drop? You need an additional condition to drive it down to then cause the phase change.

[Anonymous]

Sir,

yes you are right that I need an additional conditions to drop it down to point 2.

explanation to this phenomena : 

• As hot fluid if flowing through the copper tube, and air is in environment. So, temperature drops down initially to point 2 ( due to natural convection).
• but if we are working below the critical temp. of refrigerant. then each fluid at different pressures has its saturation temperature on the saturation line.
• so, first it will cool down sensibly, but as soon as it intersects the saturation line, where it matches to its saturation pressure, the phase change will start.
• and as soon as phase change stops. again it starts cooling down sensibly. from point 2 to lower temp. 

And my doubt is the same what should be that condition and how should I apply it? 🙂 

the .CFZ file that you had given to me, in that also you can see the phase change was happening at constant temperature. In that problem or sample the fluid had already intersected to the saturation line. that's why in that problem temperature of fluid was constant and only phase change was happening from the start of simulation. 

hope I have cleared my doubt.

regards,

Akshay Kumar

[Jon.Wilde]

I understand, no worries 🙂

You need to apply something outside the solid to lower the fluid temperature and you have two options.

A film coefficient, 5 W/m2/K at ambient is a sensible figure for natural convection.

A temperature, which is like an infinite heat sink which will remove as much heat as necessary to drive down the temperature. This might be too much but the decision is really yours in the end.

Maybe test both?

Thanks,

Jon

[Anonymous]

thank you sir for these suggestion I will try these first.

as each condensation process depends on the pressure, with which the fluid is coming out of the compressor to the inlet of the condenser. because the temp. value at which the fluid will change its phase purely depends on the pressure of fluid. 

for example: suppose for R134a, pressure 1.061 Mpa, then its saturation temperature is defined 40 C. So in this case whatever is the initial temperature of  fluid coming out of compressor to condenser it will start phase change at 40 only not before or after. If temp. is ( 140 C and 160 C) If their pressure is 1.6 MPa then this is sure that both fluid will start phase change at 40 C.

Hope I have cleared the phenomena. Now my doubt is

• So how to associate that pressure with the fluid at which it is coming out of the compressor to the inlet of condenser? as So far I have defined ambient pressure at outlet, which do not defines the condition of fluid at inlet perfectly.

regards,

Akshay Kumar

[Anonymous]

Hello Sir,

Please see the attachment. It has been clearly mentioned there that reference pressure,   temp. and quality affects the fluid properties. they have also written that it sets the saturation temperature, which is dependent on reference pressure,  which I was talking about earlier in this thread.

Now it seems my problem can be solved soon. 

Please make me learn how to do this.

regards,

Akshay Kumar

[Jon.Wilde]

To answer your boundary condition question, try setting the inlet pressure and have the flow rate pulling from the outlet - not sure this will work, give it a quick test 🙂

[Anonymous · Accepted solution]

  

thank you sir for providing that .CFZ file which cleared my biggest problem which was the unchanging phase of refrigerant at a particular temperature.

I got the solution in the .CFZ file which you gave me eariler. So, the solution is: while assignig the material for coolant there is one section below the fixed or variable option called use scenerio environment. unpick that option and enter the saturation pressure there corrosponding to the saturation temperature at which you want the phase change of refrigerant. leaving the temperature option as it is the default value.

Is it the correct solution? because as of now I am getting good results according to the theory (convection then phase change then convection ).

Please confirm me if I am wrong here.

I am attaching the screenshots of the results which I am getting as of now, iterations are still in progress. please find the attachment.

• first one is temperature change results you an see it stopped at 40 degree Celsius.
• it started changing it's phase right at moment.

Regards,

Akshay Kumar

[Jon.Wilde]

I am not sure it is necessary, the fluid reference properties might have already been correct, right?

Only change them if necessary...

It looks like you already have a working solution, yes? 🙂

[Anonymous]

yes it is working perfectly. 

I investigated that file again, in that file the phase change was happening at 70 degree Celsius because the reference pressure in the scenario environment was given as 2.168 MPa that matched with the saturation table of r134a exactly. Now I am confirmed about it. 

thank you @Jon.Wilde sir for your help..

regards,

Akshay Kumar

[Jon.Wilde]

Very cool, thanks for the update.

(Thank you for helping out other people on here too 😉 )