what is the criticl Reynolds number when water flows through pipe? what is the critical Reynolds number when air flows through pipe? thanks

The critical Re for pipe flow is usually taken as ~ 2300, assuming you are referring to transition to turbulence. This Re is based on pipe diameter and does not depend on the type of fluid.

thanks for ur reply but Reynolds number = ud/kienmatic viscosity.. so Re depends on kinematic viscosity also..... this is my doubt..

Yes, it does - so use the correct kinematic viscosity for the appropriate fluid. The critical Re is still ~2300.

thank u......