Tube-in-tube helical heat exchangers performance optimization by entropy generation minimization approach

Highlights: • Helical heat exchanger performance is optimized in terms of entropy generation rate. • Laminar and turbulent flows in the inner and outer tubes of the device were studied. • Optimum diameter ratio for the inner tube and the annulus were determined. • Optimal flow characteristics for this type of heat exchanger were found. - Abstract: There are several different types of heat exchangers that are used for various applications. Helically coiled heat exchanger is one of widely used types of heat exchangers in industrial applications as it offers certain advantages, such as smaller size, higher heat transfer rate, efficient performance in high pressure and temperature differentials and lower cost. This study aims at finding the optimal geometry and operational conditions of helically coiled heat exchangers for both laminar and turbulent flows based on the second law of thermodynamics. In order to fulfill the main goals of this work, first, a dimensionless function for entropy generation number comprising four dimensionless variables, i.e. Prandtl number (Pr), Dean number (De), the ratio of helical pipe diameter to the tube diameter (δ) and the duty parameter of heat exchanger, is derived. Next, the entropy generation number is minimized to develop analytical expressions for the optimal values of δ, De number (for laminar flow) and Reynolds number (Re, for turbulent flow) of the heat exchangers.