## 3 Experimental Facility
The outside tube wall was insulated with phenolic foam rubber to mitigate heat loss to the environment. Figure 2: Experimental heat transfer enhancement setup. (**a**) Experimental facility. (**b**) Testing section layout. The pressure drop, which was measured by a differential pressure sensor (Dwyer DM-2004, DwyerOmega, Michigan City, IN, USA; accuracy: \(\pm\)2% for 250 Pa, \(\pm\)1% for 250-1250 Pa) under isothermal conditions or without a heating conduit, was used to calculate the friction factor. In the experiment, pressure probes were placed approximately 50 mm upstream and 150 mm downstream of the test section. The distance between the two twisted tapes was about 1800 mm. Before conducting the tests, each pressure gauge was calibrated using a micromanometer. The volumetric flow rate was measured with an orifice flow meter, which was itself calibrated using a thermo-anemometer. (TSI/Alnor 9565-A, TSI Incorporated, Shoreview, MN, USA; accuracy: \(\pm\)3% for reading, \(\pm\)0.015 m/s, range: 0-50 m/s). For each test run, data on the volumetric flow rate and pressure drop of air under steady-state conditions were recorded, with the inlet air temperature maintained at 25 \({}^{\circ}\)C. The Reynolds numbers and various flow friction characteristics were determined based on the inlet temperature. Fluid properties were evaluated using the overall mean temperature collected during the experiments. The average Nusselt numbers were then calculated and analyzed accordingly. In addition, the temperature measurements and pressure differentials of the entire system were processed using a data logger linked to a host computer. A data logger was employed to visualize temperature at each measurement site. The experimental setup was run for more than ten minutes after adjusting the flow to specified values to ensure data reliability. This facilitates temperature and micromanometer reading stabilization before data collection. The experimental configuration and operating parameters are listed in Table 2.  
In the experiments, a uniform wall heat flux condition was set at the test tube's inner surface. The entry fluid temperature (\(T_{i}\)) was kept at 25 \({}^{\circ}\)C. During the experiments, heat (\(Q_{air}\)) was passed through the air within the tube by convective heat transfer from the electrical heating wire. Consequently, \(Q_{air}\) is considered equivalent to the convective heat transfer (\(Q_{conv}\)) of the tube wall. \[Q_{air}=Q_{conv} \tag{1}\] \begin{table}
\begin{tabular}{l l} \hline
**Parameter** & **Values** \\ \hline Testing fluid & air \\ \hline Inlet temperature, \(T_{i}\) & 25 \({}^{\circ}\)C \\ \hline Heat flux, \(q^{\prime\prime}\) & 1000 W/m\({}^{2}\) \\ \hline V-rib attack angle, \(\beta\) & 15\({}^{\circ}\), 30\({}^{\circ}\), and 45\({}^{\circ}\) \\ \hline Twist ratio, \(y/w\) & 4.0 \\ \hline Tape thickness, \(\delta\) & 0.9 mm \\ \hline Tape width, \(W\) & 60 mm \\ \hline Twisted tape length & 1500 mm \\ \hline Copper tube thickness, \(t\) & 2 mm \\ \hline Inner copper tube diameter, \(D\) & 62 mm \\ \hline Outer copper tube diameter & 66 mm \\ \hline Test tube length, \(L\) & 1500 mm \\ \hline Diameter of nichrome heating wire & 0.7 mm \\ \hline Reynolds number & 6000, 8000, 10,000, 12,000, 14,000, 16,000, \\  & 18,000, and 20,000 \\ \hline \end{tabular}
\end{table} 
Table 2: Experimental configuration and operating parameters.