Heat transfer and fluid flow characteristics of a dual-tube heat exchanger with alternating flattened tubes

Dual-tube heat exchangers (DTHEs) have a broad range of applications owing to their simple structure. Hence, their design is pivotal to be efficient in enhancing heat transfer coefficient (HTC) and reducing pressure drop ( D h = 4 A c P e ) while their fabrication process is maintained simple. The current paper examines the thermal performance of circular, flattened, and alternating flattened tubes (AFTs) in a DTHE. The amounts of HTC and Δp in the inner tube of DTHE are estimated by changing the inlet flow rate and alternating angle of AFTs ( θ = 30°, 45°, 60°, and 90°). This work is conducted experimentally and numerically at various Reynolds numbers (Re) ranging from 500 to 1750 for hot oil flowing in the inner tube and Re = 2000 for water as a coolant flowing in the outer tube. The performance evaluation criterion (PEC) is defined for the simultaneous evaluation of Δ P and HTC. The results demonstrate that flattening the circular tube enhances the amounts of Δ P and HTC; however, the PEC can be improved significantly. It is revealed that increasing θ augments the PEC of AFT-based DTHE by 54% compared to the one with a circular tube. Besides, an enhancement in θ from 60⁰ to 90⁰ leads to rapid growth in PEC for different amounts of Re. For instance, at Re = 1750, PEC changes from about 1.44 to about 1.54 when θ is increased from 60⁰ to 90⁰, experiencing a 6.9% improvement. The present AFT-based DTHE can be employed for small-scale applications, such as crystallization, concentration, pasteurization, air conditioning, solar water heaters, etc. Graphical abstract