Pressure drop in plain micro channel
 

I am new to FLUENT. I am trying to simulate laminar fluid flow in a 2D plain channel of dimension 250 microns x 120 microns in ANSYS FLUENT (velocity = 0.1 m/s, Water). Theoretical pressure drop is around 21 Pa, while in simulation (unsteady) it is coming around 33 Pa. Kindly tell me where could it went wrong?

I assume that the theoretical pressure drop was derived for a fully developed flow, while your simulation has a uniform velocity profile at the inlet.
This causes higher shear stress near the inlet in your simulation and increases the pressure drop compared to the analytical value.

Use periodic boundary conditions for your simulation instead of velocity inlet/pressure outlet to match the analytical value for the pressure drop.
Additionally, I dont see any reason for an unsteady simulation from the setup you described.