what is the critical reynolds in a pipe flow for both air and water?

It depends on a type of a flow. Say aprox. from 1600 up to 6000 (for Hogen-Poiseuille flow) But, it's just by heart.

i thk 2000 is the critical Reynolds no.. for all pipe flows..below that laminar and above 3000 its turbulent..pls correct me if iam wrong..

thank you...

we're zooming in.... some number around 2300 is often quoted in textbooks as the critical Reynolds number (based on pipe diameter) for fully developed incompressible pipe flow

(don't remember which velocity is used to define that Reynolds number -- cross-sectional average or center velocity?)

2300 is the most widely quoted critical Re number that I have seen too. But researchers have reported postposing the critical Re to 10,000 by making the inlet to the pipe smooth.

for commercial pipes ,,,, re 2300-4000 transition occurs for all fluids(air and water)