At the moment I am using the standard k-e model. But I don't know what turbulent intensity to set at and the scale length too. What about the near wall function, which one to use if I am usinf the k-e model???

Please read the User's Guide. It provides an overall guideline on how to set the BCs for turbulent quantities. Yet, it's just a general guideline, and can't tell you much about your specific situation. For instance, you may have a big disturbance just updtream of an inlet boundary on which you want to specify the BCs forturbulent quantities. The, the rule of thumb given in the manual won't help much.

Anyway, it's best to discuss this with your support engineer, if the User's Guide cease to be useful.

If your flow is quite turbulent, say a combustor then a turbulence intensity of 10% is ok, or if it is a wind tunnel type flow then an intensity 1-5% is more likely. If you want to set the length scale then a sort of rule of thumb is to use a length scale which corresponds to about 10% of a charcteristic dimension. Generally I use 10% of the length of the smallest length of the inlet.

If my flow type is in a cleanroom, what turbulent intensity will it likely to be???

What you could do if you model does not take long to run is set the inlensity to 0.5% run it then double the value and see if there is any great difference in the flow field, then double it again and so on and keep checking.

It may be easier to measure it though.

I take it that the air inlet will be through a mesh of some sort, use 10% of the mesh size as you length scale.