Consideration of first cell height of prism layer for laminar flow in heat transfer
 

I am doing simulations on heat transfer enhancement for laminar flow in pipe using vortex generator.
I am using ANSYS workbench for meshing

Pipe diameter is 0.025 meter
Fluid is water
I am calculating surface temperature(wall of pipe) for Nu calculation.
My problem is in consideration of first cell height for prism layer on wall of pipe.
I had tried first cell thickness as 1e-4,5e-5,2e-5.I had given 5 prism layers.
For all meshes calculated Surface temperature is different. It is increases as thickness decreases.
Is there any way for calculating first layer height in laminar flow OR it is trial error method in which we will decrease first layer height up to which temperature is independent of mesh



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