Main Question or Discussion Point
I might be going crazy or searching at the wrong places. Is there an empirical formula for solving the hydrodynamic entry length of laminar flow through a circular annulus? Thanks!
The entry length for a circular tube is going to be longer than for an annulus if the Re is defined in terms of the outer diameter for both. So, to be conservative, why not just use the value K=20.Thanks for your help, Chet. (Also, thanks for answering my question on displacement, momentum, enthalpy, etc. thicknesses in circular pipes.)
The gap between the cylinders is not small compared to the inner cylinder though. I have the gap to be 0.015 m, the the inner cylinder radius is 0.012 m, but the outer cylinder radius is 0.027 m. Can I still use the estimate of entry length for flow between parallel plates? The estimation of entrance length is given as L_e = Re*D/K, where K = 20 for circular tube and K = 100 for parallel plates. From simulations done on COMSOL, I know that fully developed velocity profile is achieved within the tube length. The entry length for a circular tube overestimates the entry length/point of fully developed velocity profile.