[mumtaz ersan]

Hello everyone,

i draw Y-pipe which works like a mixing-chamber,
This y-pipe has got 2 entrance and one exit, from one of them oxygen is entering, from the other one Air is entering through to the y-pipe,
i set 24lt/min(0,0005 kg/sn.) oxygen is entering from one side, and 96 lt./min(0,0019 kg/sn.) Air is entering from the other side, these two gasses are mixing in my y-pipe and then pass through the outside of the Y-pipe from-the exit...

My question is like these ;

if i examine mass fraction of oxygen in the exit surface of my y-pipe , i get the pic. of A but if i add 2 meter pipe to my y-pipe and then i examine the exit surface of my assembly i get the pic. B. these are quite different At the pic. of B, there is no Oxygen and it seems %100 air, i know then if the distance is raise, homogeneous of flow is raise but is this result "OK", where is my Oxygen???

Can anyone explain, why mass fraction of oxygen is dissapear????

[Boris_M]

Hi Mehmet,

my guess is that the geometry is too simple for the flow to pass through the full model in the short simulation time you have. It will take some time for the flow to pass the full model and I guess your calculation stopped too early. If you create goals on the models inlets and outlets for velocity, pressure and mass fraction and deactivate the stopping criteria in the calculation control options for the goals then just let it calculate till the values don't change anymore. Also have an eye on the outlet massfraction as it should be 100% of air for some time and as soon as the oxygen reached the outlet the value should change. As soon as that change becomes converged and also the other parameters haven't changed anymore you can stip the calculation.

In your case the calculation might have taken 40 iterations but for the oxygen-air mixture to reach the outlet it probably would have taken 80 iteration as an example. In the shorter pipe of course the mixture reached the outlet earlier. If you add 2 meters then this is a long pipe for the flow to reach the end.

Boris

[mumtaz ersan]

Hi Boris,

Actually what is mass fraction???

i know that; it is examine the homogenous of the flow which is mixture of two different fluid,
But i mean; how should i comment the results,
For example ;

From one inlet surface air is entering at 96lt./dak(0.0019 kg./sn) and substence concentration is %100 air,

From other inlet surface oxygen is entering at 24lt./dak(0.00005 kg/sn) and substence concentration is %100 oxygen,

After mixing these two gasses , i set the substence concentration at the exit-surface 0.5oxygen and 0.5air,

Now, my question ,in which result i should satisfy???

The result which i get from simulation like this:At the exit surface , simulation shows me only one colour and it symbolize that 0.2 oxygen , 0.8 air .(it is simply mathematic 96+24=120 and 24/120=0.2 like i get from simulation.).If i see this result , should i satisfy????

OR

The result which is like below explanation;
It is almost one colour but the result show me 0.5 oxygen and 0.5 air


Which result is better???????

[Boris_M]

Hi Mehmet,

Mass fraction is the ratio of the mass of a substance in your plot reference compared to the rest of substances.
For example in a cut plot like in your images it shows the ratio between all teh substances compared to the selected parameter. If you have oxygen, xenon and nitrogen as an example the parameter "mass fraction of oxygen" would show you the (mass of oxygen)/(overall mass of substances) in this plane of the plot. So 0.45 Oxygen is 45% oxygen and 55% rest substances. You can switch the parameter also to mass fraction of nitrogen to maybe find there is 23% nitrogen and then obviosly the rest is 32% xenon in that section. Mass fraction is used because it considers the mass but you can also use volume fraction but then you would neglect the density and with that the compressibility etc.

What do you mean you set the substance concentration after mixing them? Isn't the mixing done in the y-junction? You can only "suck" out the mass flow with a certain mass fraction but that wouldn't make sense except you want to find out how much you have to put in at the inlet in reality. So in such a simulation case you would define a pressure with 100% of the initial substances at the inlets and the outlet is a mass flow outlet with a 50/50 mix sucked out. It should then calculate how much mass flow you have of each substance at the inlet in order to roughly achieve this.

If you have 0.2 oxygen and 0.8 air and it doesn't change over time then the calculation is converged. You can also put a preview plot in the plane of the y-junction and see if the concentration is still progressing towards the outlet.

If you define a mass flow at the inlets, then the massflow at the outlets of each substance should match the inlet mass flow. That's called mass balance which is a law of nature. If you define the volume flow you do not consider the compressibility and temperature effects in the mixture. You bascially force the calculation to have a certain volume flow with the given inlet temperature but the pressure at the inlet can change during the calculation so the temperature as well as the pressure influence the value of the mass flow according to p*V=m*R*T for the ideal gas. R is the gas constant of the substance, m is the mass, T is the temperature in Kelvin, p is the pressure and V is the volume. You can substitute the V and m to volume flow rate V_dot and mass flow rate m_dot, the equation is the same. As you can see the pressure and temperature are influencing the mass flow rate if the volume flow rate is given. The temperare is defined in the boundary condition but the pressure will result out of the calculatin which can be a back pressure as there is more of the other substance flowing into the y-junction if every diameter of inlet and outlet is the same.

I hope this helps,
Boris

[mumtaz ersan]

Thanks,

Now , it is better to understand...