This factor is used to increase the exchange surface area inside the tube, if it is corrugated.
The factor that is displayed when you introduce a new geometry, and you select a corrugated tube, is related to the coefficient tube side, that is a coefficient that affects both the exchange coefficient and the pressure drops (as well as the Fin Corrugation Factor that plays the same function, only the side of the fins).
Currently, if the user inserts a geometry with a corrugated tube, he will find that the factor “Rapp. Internal Surfaces Tubes” by default at 1.6 (since this factor, in the above conditions has values that range from 1.6 to 1.8). In any case, the user may change it to suit his needs.