Comparative study of fluid flow and heat transfer in corrugated curved tube-in-tube heat exchangers at various configurations using Al2O3/water nanofluids

In the existing study, an experimental investigation of corrugated curved tube-in-tube ( CCTT ) heat exchangers with different geometrical configurations was performed. This study aimed to evaluate the coupled effects of Al 2 O 3 water-based nanofluids, curvature, and surface corrugation on thermal–hydraulic performance. Although previous studies focused on individual enhancement techniques, this work combines multiple unconventional geometries with internal corrugation to identify the dominant mechanisms governing heat transfer enhancement and flow resistance. Ten curved tube-in-tube configurations, including five corrugated and five smooth designs (conical, spiral, helical, converging–diverging, and diverging–converging), were fabricated and tested under counterflow conditions. The experiments were conducted over Reynolds number ( Re ) ranges of 10,700–36,300 for the inner tube and 2400–15,400 for the annulus. The results reveal that the interaction between corrugation-induced turbulence and geometrical curvature, as a passive enhancement technique, plays a critical role in disrupting thermal boundary layer development, leading to a significant increase in heat transfer performance. The diverging–converging configuration exhibited the highest performance, achieving Nusselt number ( Nu ) enhancements of up to 52.8% and 44.2% for the inner tube and annulus, respectively, with corresponding friction factor ( f ) penalties of 35.7% and 79.9%. The addition of Al 2 O 3 nanoparticles to water significantly increased the heat transfer characteristics by 37.2% and 29.4% for the inner and annulus sides, respectively, at a 3% volume concentration. Furthermore, the thermal performance index ( TPI ) results indicate that the investigated geometry–corrugation combinations provide favorable thermohydraulic performance, offering useful design insights for compact heat exchanger applications. The maximum TPI values reached 1.65 and 1.47 for the diverging–converging configurations for the inner tube and annulus, respectively.