Investigation of potential of improvement of helical coils based on avoidable and unavoidable exergy destruction concepts

An inevitable problem challenges heat exchanger designers is that the heat transfer augmentation in a thermal system is always achieved at the expense of an increase in pressure loss. Thus, the trade-off by choosing the most proper configuration and best flow condition has become the critical problem for design work. The brief survey on literature shows that optimal Reynolds number of laminar forced convection in a helical tube, was specified based on minimum entropy generation. Therefore, the present study analyzes the thermodynamic potential of improvement for steady, laminar, fully developed, forced convection in a helical coiled tube subjected to uniform wall temperature based on the concept of avoidable and unavoidable exergy destruction. The influence of various parameters such as coil curvature ratio, dimensionless inlet temperature difference, dimensionless passage length of the coil, and fluid properties on avoidable exergy destruction have been investigated for water as working fluid. Results show considerable potential of thermodynamic optimization of helical coil tubes. In addition, a relation for determining the amount of optimum Dean Number is proposed for the range considered in the present study.

► Based on this investigation, a new correlation for prediction of optimal Dean Number has been proposed for helical coil heat exchangers subjected to uniform wall temperature.
► Avoidable exergy destruction concept has been used to investigate the influence of all effective geometrical and thermal parameters on the potential of improvement of helical coil heat exchangers.
► The results show that up to 16% of total exergy destruction can be avoided.