 ## Fluids

Topics relate to Fluent, CFX, Turbogrid and more

• Ammar Ahmed
Subscriber

Hello everyone

I am trying to flow multiple fluids in cross directions thorugh C-D nozzle to observe the heat transfer between them. I have been using pressure based inlets till now. The temperature and pressure and velocity formed by the flow are almost simalar to the values calculated analytically but the issue i am facing is of flow rates. The flow rates I have found using CFD are not similar to those found analytically and the CFD values of flow rates about 10 to 11 times higher. Kindly If anyone has knowledge about calculating massflow rates correctly guide me.

Thanks

• Federico Alzamora Previtali
Ansys Employee

If your solution for numerical velocity is similar to analytical values, then what about density?

• Ammar Ahmed
Subscriber

Unfortunately it is not same for velocity too. the proper contours are not genertaed as compared to when pressure based inlets are used. As the fluids used are air and water, the density for air is ideal gas based and water's density is theone given in fluent Do you know anyway in which we can calculate velocity from amss flow raets with consideration of pressure as I am currently converting mass flow rate to velocity by formula: mdot =density*veloctiy*area

• Federico Alzamora Previtali
Ansys Employee

These differences that you are seeing, are those for air (ideal gas)? Or for Water? How are you modeling water? With constant density? If your nozzle has a large compression ratio, you might want to consider compressible liquid.

I'm not sure I understand your last question. The equation that you use is valid, but you can also get mdot or velocity directly using Report Definitions, no need to compute it yourself.

Finally, what do your residuals look like? Is your solution well converged?

• Ammar Ahmed
Subscriber

Hi Let me once again define my problem for better understanding. What I am doing is trying to simulate regnerative cooling in addition to film coooling  with air flow through C-D nozzle.
My geometry is My materials are:  My inlet conditions for the case are as following:
Film: Inlet pressure 9.5MPa with water as fluid: Air flow domain: Inlet pressure 8MPa with air as ideal gas Regenartive :domain: Inlet preesure with 9MPa with water as fluid The problem I am facing is when I try to match the mass flow rates obtained by report definition and from the analytical results. They are very much different as discribed below:

Film inlet mass flow rate from CFD : 9Kg/s, mass flow rate from analytical calculation: 0.12kg/s
regenerative  inlet mass flow rate from CFD :130 Kg/s, mass flow rate from analytical calculation: 2.57kg/s
air flow inlet mass flow rate from CFD : 102 Kg/s, mass flow rate from analytical calculation: 10 kg/s
The scaled residuals are almost parallel abnd below 1e-2 after 500 iterations

• Federico Alzamora Previtali
Ansys Employee

Is your inlet flow supersonic? If not, then you need not specify Supersonic/Initial Gauge pressure and should leave it as zero.

Also, remember that these are Gauge pressures, which are relative to your operating conditions.

Since you have analytical solutions, I would try to break down each component of the numerical mass flow rate and determine if the difference comes mostly from velocity or density.

• Ammar Ahmed
Subscriber

I ran the simulation with supersonic /inintial gauge pressure zero but I am not able to achieve proper results. When i ran the simulation with the condition that It only had one fluid domain (air flow thourgh C-D nozzle) it ran perfectly with inlet alternatively  as mass flow inlet and pressure based inlet, in case of mass flow inlet condition was the exact same value as achieved with the analytical results and in case of pressure based inlet the values were same as mentioned above for air flow but as soon as i added the solid domain above it to monitor the heat transfer, the results detoriated. Kindly guide what might be the probelm. 