I'm working on a Ansys Fluent of an Aluminium coil annealing furnace with the goal of estimating the heat transfer coefficients at different temperatures and ventilator/fan speed conditions. The furnace transfers heat coils by blowing hot air through round nozzles at high speeds, i.e. similar to a impinging jet. I have simulated cases with different inlet air temperatures and velocities and then calculated the HTC from the average temperature and heat flux of the coil at each surface as well as the inlet air temperature. An ideal gas law was used for the material properties of air. Velocity inlets and pressure outlets were used for the inflow and outflow of air.
The results show higher HTC for higher fan speeds, which is expected. But it also shows a considerable lowering of HTCs with increasing temperature. On closer investigation, it seems to be that the HTCs have a dependence on the mass flow rate of the jets, which decreases with increasing temperature. Is this expected in theory?