function calculator gives wrong result in post
I'm trying to track down a couple issues and I'm wondering if anybody might have seen something similar.
I'm modelling several ducts with slightly different internal geometry (to improve heat transfer). The CFX-Pre settings are identical, with a moderate heat flux on one duct wall, and a developed inlet temperature and velocity profile.
First, when I use the function calculator in CFX post to measure the average inlet temperature, it is off by 1.441K. The mesh is fine enough to numerically approximate the function much better than that. No ideas what is going on here!
The second issue I have is some of the ducts gain a couple K as expected along their length from the heat flux, but with a different geometry the temperature is decreased at the outlet! I am using an opening BC with the opening temperature as the average of the temperature at the opening. There is insignificant backflow in all geometries so I could probably change this to an outlet.
I am using thermal energy without viscous dissipation (10m/s speeds means viscous heating effects are small). Laminar flow, air ideal gas. The duct is a rectangular prism. The inlet is adjacent to the side with the wall flux, and opposite the outlet. The other sides are symmetrical or adiabatic without slip.
I got the first part, I was using average when I should have been using area average. Also, to do control volume calculations I need to use massflow average of temperature.
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