Flow resistance and convective heat transfer performances of airflow through helical-tube bundles

Based on CCA technique and design concept of compact helical tube heat exchanger, this article carried out a series of experiments to study the flow and convective heat transfer characteristics for air flowing through helical-tube bundles under uniform heat fluxes. The single helical tube outer diameter d_o is fixed at 2.2 mm and the spatial structure of various helical-tube bundles remain the constant. The experimental results show that the flow resistance is generally influenced by helical diameter ratio D/d_o and slightly affected by variation of helical pitch. Furthermore, the Nusselt number will increase by 30% with the decrease of helical diameter ratio D/d_o at the range from 13.64 to 9.10. For constant helical diameter ratio D/d_o, the heat transfer coefficient increases with the increase of helical pitch ratio P/d_o at the range of 4.55–6.82, but reduces at the range of 6.82–9.10. In addition, temperature difference correction factor is exploited to modify the heat transfer caused by non-uniform temperature field. At last, two empirical correlations are proposed to summarize the friction factor and convective heat transfer coefficient for air flowing through helical-tube bundles.