<p>This study examines a passive heat-transfer enhancement method employing flow-driven propeller inserts that rotate freely under fluid motion to generate adaptive swirling flow inside the tube. Experiments were conducted in a counterflow double-pipe heat exchanger with a 26&#xa0;mm inner tube fitted with three-blade propeller inserts (propeller diameter of 24&#xa0;mm), using water over a Reynolds-number range of 2,800–17,500, with systematic variations in the number and axial placement of the rotating elements. Unlike previous studies that primarily examined individual rotating inserts, the present study focuses on the thermo-hydraulic response associated with multiple inline self-rotating propellers. The objective was to quantify their influence on the Nusselt number, friction factor, and overall thermal-hydraulic performance. The results showed that the swirl generated by the rotating propellers resulted in measurable heat-transfer enhancement. This improvement is reflected in the measured Nusselt-number ratios (Nuₚ/Nu_₀), which ranged from 1.16 to 1.88 at low Reynolds numbers and from 1.02 to 1.24 at higher values. The friction-factor ratio (fₚ/f_₀) increased from 1.06 to 1.20 across the configurations, and this moderate rise occurred despite a high blockage ratio (Br = 0.852). Despite this increase, arrangements with three and four propellers maintained a thermal-performance ratio above unity throughout the entire range, reaching a maximum of 1.81 at low Reynolds numbers. Empirical correlations for the Nusselt number and friction factor, expressed as functions of the Reynolds number and propeller count, predicted the measurements within ± 10.2% and ± 3%, respectively, supporting the design of multi-stage swirl enhancement in double-pipe heat exchangers.</p>