hi fluent user: i want to know how to define the backflow total temperature in modeling a furnace,the help file is " sets the total temperature of the inflow stream should the flow reverse direction",i don't know the meaning. thanks

You should input an estimate of the outlet temperature. Should reversed flow occur at the outlet boundary, the program is able to give that flow a more appropriate temperature value based on your input. If no reversed flow occurs this input has no effect.

thanks you for you anwser.if only there is whirlpool in the flow,the input haven't effect?

No, the input only has effect when there is flow recirculating back into the domain again. The best way to avoid this is to move your exit boundary away from areas with vortices, for example the flow recirculation after a bend. If you can't do that, then you have to specify the backflow parameters as accurately as possible.