Thermodynamic investigation and optimization of laminar forced convection in a rotating helical tube heat exchanger

Based on the second law of thermodynamics, an entropy generation investigation is carried out under given dimensionless parameters, i.e. heat exchanger duty, heat flux, with respect to heat transfer and frictional pressure drop in a rotating helical tube heat exchanger with laminar convective flow. The entropy generation from heat transfer across a finite temperature difference – Ψh decreases with increasing Dean number which represents the impact of centrifugal force induced secondary flow in enhancing heat transfer. Another aspect of increasing Dean number is that intensified momentum transfer in the radial direction also raises the entropy generation from frictional pressure drop – Ψf, the superposed effect of which yields a decreasing–increasing trend of the total entropy generation-Ψ, a local minimum located in between. The rotation of the helical tube in streamwise (co-rotation) or counter streamwise (counter-rotation) direction leads to a decrease in Ψh and a increase in Ψf which complicates the situation that whether or where the minimum of total entropy generation exists is dependent on whether Ψ is dominated by Ψh or Ψf or somewhere in between. No difference is discerned between pairs of cases with constant wall temperature and uniform wall heat flux but the same set of variables and parameters. A multi-objective optimization targeting Ψh and Ψf simultaneously is implemented using the non-dominated sorting genetic algorithm II (NSGA II). Five solution sets are selected and compared with the conventional optimization in regard of Ψ distinguishing the Ψh-dominated region from the Ψf-dominated region, the dimensionless variable η1 is found to be the most suitable representative in describing the trade-off between Ψh and Ψf. The Pareto solution sets is dominated by Ψh within the variable and parameter space under discussion. On the Pareto frontier, the counter rotational cases are distributed where the impact of Ψf is relatively higher while co-rotational cases dominate almost all the rest part. The proposed investigation procedure is a synthetic analysis concerning optimization of both Ψ and its components Ψh and Ψf, via which the dominating compartment and the key impact factors for irreversibility minimization can be obtained as a guidance for practical design of rotating helical tube heat exchangers.