Convective heat transfer to temperature dependent property fluids in the entry region of corrugated tubes

The present experimental investigation is aimed to the analysis of the thermal performances of corrugated wall tubes employed in a broad variety of industrial applications in order to intensify the convective heat transfer. Both axial symmetrical and helical corrugations with different pitch values, have been considered in the present analysis. In particular, the phenomena arising due both to the wall roughness and to fluid property variation are investigated in the thermal entrance region in the Reynolds number range 90–800. The comparative analysis of the behaviour of the two types of corrugation enables to draw interesting conclusions about the effects of the corrugation pattern on the heat transfer enhancement mechanism. The data show that the helical corrugation induces significant swirl components to which, however, an as much significant heat transfer enhancement is not associated. The variation of the fluid physical properties with temperature, promotes the transition to an unstable flow. Regarding to this phenomenon a critical local Reynolds number, proportional to the dimensionless corrugation pitch has been identified. The dependence of the local Nusselt number on the corrugation pitch has been investigated, too.