Click Materials from the Setup tab:
In this step, we create a new air material with an appropriate value of emissivity.
Questions about applying settings?
To open the Material quick edit dialog, click Edit from the Materials context menu:
On the Material quick edit dialog, Type = Fluid
On the Material line, Click Edit...
Enter the Name = Air_Emissivity
Select My Materials from the Save to database menu.
Click the Emissivity button.
Value = 0.3
Click Apply.
Click Save.
Click OK.