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September 15, 2015, 14:07 
Calculate Density

#1 
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mehmet mumtaz ersan
Join Date: Nov 2014
Posts: 24
Rep Power: 11 
Hello Everyone;
About 4 months ago, i asked similar ques. in this forum and chris helps me... And i can answer que. about pressuredrop in my mind but about density still i need a help.... I want to analyze some kind of nozzle, Air is passed through inside this nozzle at 1,1 bar , 25 celcius , 120 lt/min So , i define 120lt/min for inlet volume flow and define Environment pressure for outlet volume flow, and start to calculate for density. Program shows me density is 1,20 kg/m3 as a result, BUT in my hand calculation; i find the density 1,28 kg/m3; P=d*R*T ; d=P/(R*T) P=1,1 bar=110 kPA , R =0,287 , T=25celcius=298 K so density is 1,28.... The difference between two calculation(one from cfd which is 1,2;one from my hand calculation which is 1,28) lead me to make misscalculations... Because if i use 1,20 kg/m3 as a density my mass flow is 0,0024kg/sec(1,2*0,002m3/sec=0,0024kg/sec), if i use 1,28 kg/m3 as a density my mass flow is 0,0026kg/sec.... to stop to confusion between my handcalculation and flowsimulation, i try to define inletpressure with 120lt/min,(i add a picturea for showing how i define inlet pressure,Redcircled area) but still i got the same result 1,20kg/m3 as a density... I am very confused, i think , i miss sth but what??? I am really glad that someone can help me..... 

September 17, 2015, 04:26 

#2 
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Join Date: Jul 2009
Posts: 616
Rep Power: 23 
Hi Mehmet,
The pressure in the inlet volume flow rate boundary condition is just an approximate pressure. This helps for faster convergence as the calculation starts with this pressure as initial condition on the inlet but it is not fixed and will change as the calculation progresses. If you go with your mouse over the ~P in the boundary condition you will see that it is "approximate Pressure". Your problem is that you specify a volume flow and temperature. Since Air is a compressible fluid a change in pressure can result in a different density. If you want to consider a specific mass flow then specify one. The reason for your problem is that you specify an environment pressure at the outlet but you are not sure if it is exactly that pressure or you cannot tell the total pressure and don't know the pressure loss between inlet and outlet. So if you specify the static or environment pressure in case of an outlet (then this is the same) you will fix the static pressure but the velocity can vary as the total pressure is not defined. So what the calculation does is that it only has a volume flow rate and temperature at the inlet but no specific pressure except that the total pressure has to be higher than at the outlet. The solver therefore cannot change the volume flow rate and temperature but the pressure on the inlet and on the outlet it can change the temperature. Not that correct described but I put it simple. Your hand calculation is correct but lets to it the other way around and you'll see where I come from. If you calculate with the density you expect (1,28) and solve for the pressure, you will see that the pressure is 1,0265 bar and not 1,1 bar! So if you want to have a fixed density then use the mass flow rate and temperature on the inlet. With a density of 1,28 and a volume flow rate of 120 lt/min you will get 0,1536 kg/min at 25°C. For the outlet you can specify the environment pressure and you will see that the total pressure will be different compared to your old simulation. Also try to have the surface goals on inlet and outlet for the values so you can see that directly rather than a surface plot. Use total pressure and static pressure for anything you are interested in pressure loss or also in your case mass flow rate and velocity and temperature. You always have to keep in mind: density, temperature volume flow rate and pressure are the values that can influence each other. If you specify the mass flow rate it is fixed and temperature or pressure change cannot change the mass flow rate as it is the mass balance that is kept. If you specify volume flow rate on the inlet you might get a different volume flow rate on the outlet if the pressure, temperature or density changes. Especially in nozzles density and temperature are changing due to the compression and possible local supersonic regions where it is not incompressible anymore. As soon as you experience a velocity of around 0,3 Mach number, you cannot expect an incompressible flow anymore. I hope this helps, Boris 

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