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

Abstract

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 (\( D_{h} = \frac{{4A_{c} }}{{P_{e} }} \)) 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 \(\Delta P\) and HTC. The results demonstrate that flattening the circular tube enhances the amounts of \(\Delta P\) 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