Pressure drop in the transitional flow regime inside smooth tubes with twisted tape inserts

Abstract

The reduction in energy consumption and improvement of productivity in industries, particularly in industrial heat exchangers, has become a concern worldwide. One of the readily available methods of achieving this is by operating heat exchangers in the transitional flow regime. This regime encourages the use of small pump size as compared to the turbulent flow regime; thus encouraging energy efficiency by the reduction of associated energy consumption. This paper presents the results of an empirical investigation of fully developed diabatic friction factors of flow in a smooth circular heat exchanger equipped with twisted tape insert with a twist ratio of 5. This experiment was conducted using water as the working fluid over a range of Reynolds numbers of 500 to 10 000 spanning the laminar, transitional and turbulent flow regimes and Prandtl numbers of 3.9 to 6.7 with a square-edge inlet. The results were also validated with correlations of previous studies on smooth tubes. The diabatic friction factor results with heat flux of 2 kW/m2 are presented in this paper. The commencement of transition was observed at a Reynolds number of 2 700 and ended at a Reynolds number of 3 187 for the smooth tube. The transition range during the enhanced tube experimentation started at a Reynolds number of 750 and ended at 2 082. The commencement of transitional flow regime was earlier with twisted tape insert compared with ordinary smooth tube. The early transition of this enhanced tube indicates that heat exchangers can now be designed and operated at relatively low pressure thus encouraging energy management.