How to define it. You can select the maxium velocity , and hydrodynamic diameteers

You can use the conventional form of Reynolds formula, Re = 4 m/ (pi) Dh mu

but use the more general form of Dh, which equals four times the net flow volume divided by the wetted area (take care: volume and area not area and perimeter)

The net flow volume is volume of shell - volume of tubes - volume of helical baffles

@Mohamed Salem Sir, that I understood, but I am not able to define the flow area for the helical path in which shell side fluid is moving.

flow area can be all contact area, outside surface of tubes, inside surface of shell and helical baffles

Regards