I have some urgent questions

How to specify empty boundary conditions.

what is the backflow turbulence dissipation rate and backflow kinectic energy in pressure outlet boundary conditions.

bob

These are the conditions used if the fluid flow back into the domain. They will not be used unless you get reversed flow on your pressure outlet, and you will be told during iteration if this occurs.

So I should set them as zero right bob

bob:

see, it is perfectly ok to have reversed flow in a problem... you will have to specify the correct (realistic bc's) at the section -- For example for a pressure outlet if by reversed flow air is coming in to your domain, then you should set the coming fluid at the correct ambient temperature -- if your ambient temperature is changing then use a profile file with the correct temperature data or you can even use a udf .. -- but the bottom line is you should provide all the information that you have about the possible reversed flow and the conditions at which it might happen .. i hope this helps !