During my numerical simulation for fluid flow & heat transfer in helical coiled tube, I observed that as the top diameter (base diameter and pitch is kept constant) is reduced, the pressure drop is increased and heat transfer is reduced. What could be the possible reasons? Could it be because as the top diameter is reduced, the Geometry (helical coiled tube) height increased?
Have a look at friction losses in the pipe. The pressure drop is largely due to the shear stress at the fluid-wall interface with a common representation of this being the Darcy friction factor (see link attached). The friction losses in such a system would be dependent on the pipe Reynolds number (or local Reynolds number as you have a variable diameter).
https://neutrium.net/fluid_flow/pressure-loss-in-pipe/
Thank @Rory Tyrrell, I don't see the link. The coils have variable curvature diameter not variable tube diameter
Have you checked the flow velocity along the curvature as you reduce the diameter (effectively increasing the curvature). High curvatures can cause the flow to generate higher levels of shear, thus a higher pressure drop. Although I'm not entirely sure on that theory. I can reconcile the reduction of heat transfer, that much is obvious as the smaller diameter has less heat transfer area.
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