We have been numerically investigating the thermal-fluid characteristic of sCO2 laminar flowing in a 0.5 mm diameter circular microtube in upward direction. Depending on sharp density change along the tube, this case can be admitted as a compressible flow. When applied pressure-based solver option, the pressure firstly decreases until a certain point along the tube, then increases towards the outlet. So, this situation is impossible. Can this situation result from pressure-based solver? Can it be fixed by using density-based solver? I would like to be grateful if you could share your recommendations. (Boundary conditions: mass flow inlet, pressure outlet (75 bar), constant wall heat flux)
You need to have the correct properties of the sCO2 as a function of temperature and pressure. You can get them from another software such as EES and then integrate them within the CFD solver.
Dear Sir, thank you for your reply. We have already defined the properties of the sCO2 by using Fluent Console (NIST tables for CO2 is current in the Fluent database). Although the entire graph is very reasonable, the pressure change along the flow direction is interesting as seen in the attached file. When calculated theoretically (considered friction depending on both wall shear and momentum change) by using Ansys data (wall shear stress, density, axial velocity, etc.), the pressure drop is found as expected (full decreasing along flow direction).
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