## 2 Materials and Methods  
### Experimental Setup  
The present study examines the effect of double-sided DWT VGs having varying values of blockage ratio (\(R_{b}\)) on convective heat-transfer behaviors and compares the results of using DWTs with those using plain tubes and a tube with a longitudinal strip (LS) without a wing. Experiments were performed with values of Reynolds number (_Re_) ranging from 5500 to 14,500. A schematic diagram of the enhanced heat-transfer test rig is shown in Figure 1. Convective heat-transfer behaviors were evaluated by employing a horizontal concentric tube heat exchanger as the test section.  
The horizontal concentric tube heat exchanger was fabricated from aluminum and comprised an inner tube with a length (\(L_{i}\)) of 2500 mm and an inner diameter (\(d_{i}\)) of 14.3 mm and an outer tube with a length (\(L_{o}\)) of 1950 mm and an inner diameter (\(D_{i}\)) of 23.4 mm. Two working fluids were used in the system: cold water for the outer tube and hot water for the inner tube with both liquids flowing in the counter-current direction. The cold water stream moved through an open loop system such that cold water was circulated once through the system and then discharged out into the environment. In contrast, the hot water stream moved through a closed loop system that recycled and recirculated the hot water. The mass flow rate was varied by regulating the rotameter within the accuracy of \(\pm\)0.5% of Reading (RD). During the experiments, hot water was held at a constant temperature of 60 \({}^{\circ}\)C with a varied flow rate ranging from 0.033 to 0.099 kg/s. To ensure a supply of hot water, an electric heater was utilized to control the temperature of water within a range of \(\pm\)0.1 K [17]. Cold water constantly flowed at 0.103 kg/s at an ambient temperature of 28 \({}^{\circ}\)C. A glass wool covering was used to insulate the outside of the heat exchanger.  
To investigate heat transfer and friction factor behaviors, temperature and pressure were measured. Figure 2 is a schematic of the heat-transfer test section with measurement points along the test section for registering temperature and pressure. Fourteen K-type thermocouples with an accuracy of \(\pm\)0.1 K (three-point calibration) were embedded in the test section: two thermocouples measured the inlet and outlet temperature of the inner tube; two thermocouples measured the inlet and outlet temperature of the outer tube; and ten thermocouples were positioned on the outer wall of the inner tube. Pressure differences were determined using a U-tube manometer filled with water. To ensure measurement accuracy, four pressure taps were mounted on the inlet and outlet of the inner tube.  
Figure 1: Schematic diagram of the enhanced heat-transfer setup.  
DWTs and LSs were fabricated using aluminum with the following dimensions: 0.7 mm in thickness, 12.6 mm in width (\(W\)), and 2000 mm in length (\(L\)). The DWTs are a modification of the LS with winglets punched out at the vertical center of the strip and made to protrude either upward or downward in an alternating fashion at a certain attack angle with respect to the strip base. When the winglet is punched out, it will create an attack angle between the wing/winglet with the remaining strip/base fin. Based on our previous study [16], the values of the attack angle (\(\alpha\) = 70\({}^{\circ}\)), winglet-pitch (\(p\) = 15 mm), and winglet width (\(c\) = 8 mm) were kept constant. The winglet heights (\(b\)) of the DWTs were 4, 5, and 6 mm, giving a blockage ratio (\(R_{b}\) = \(b/d_{i}\)) of 0.28, 0.35, and 0.42, respectively. DWTs were made across the entire strip. Details of the DWTs are shown in Figure 3.