how to setup radiate emissivity in CFX
 

I have one electric process heater which some heating elements will be insert one vessel, the gas will flow through this heater,I don't know how to set up radiate emissivity of heating elements and vessel in CFX?

If radiation heat transfer is important you have many options:
1) If the body radiates and it just leaves the domain you can impose a simple source term /heat transfer equation on the hot surface to account for radiation.
2) If the body radiates and it is absorbed in the domain you will need to use a radiation model. There are a few options depending on how opaque the fluid is and exactly what radiation physics you need to include.