# Enhanced Heat Transfer Performance of the Tube Heat Exchangers Using Carbon-Based Nanofluids
## 3 Experiment
### Heat Transfer Performance Experiments  
Figure 3 displays the experimental diagram of THE heat transfer performance. A threaded electric heater was placed in the stainless-steel circular tube (OD: 31.5 mm), and the internal gap between the heater and the stainless-steel circular tube was filled with iron powder. Then, a T-type thermocouple (L = 45 cm, \(\upomega\) = 2 mm; Jetec, Taiwan; accuracy: \(\pm\)0.75%) with a stainless-steel protective tube was installed on the inner wall of the stainless-steel circular tube to measure the temperature of the heating tube (\(T_{h}\)), and the two ends were sealed with heat-resistant epoxy resin to form an inner tube with heating function. Finally, the inner tube was placed in a PVC circular tube (ID: 35 mm) with an inlet and outlet at both ends and sealed with heat-resistant epoxy resin to form a THE. The inner tube used a programmable power supply (DC 5-300V/0.01-3A/300 W; MP-3 AP, Major Science, Taiwan; accuracy: \(\pm\)1.0 W) to provide 80 W and 120 W heating sources in a constant power mode. T-type thermocouples (0.3 mm \(\times\) 2 C; Jetec, Taiwan; accuracy: \(\pm\)0.75%) and pressure transducers (0-1.0 bar; JPT 131S-LJ, Jetec, Taiwan; accuracy: \(\pm\)0.25%) were mounted at the inlet and outlet of the working fluid side of the to measure temperature and pressure. The inlet temperature of the working fluid was controlled at 30 \({}^{\circ}\)C using a temperature controller (TTM-i4N, TOHO, Kanagawa, Japan; accuracy: \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Item** & **Temp.** (\({}^{\circ}\)**C**) & **Water** & \
\begin{tabular}{c} **CBNF** \\ **0.05 wt\%** \\ \end{tabular} &
\begin{tabular}{c} **CBNF** \\ **0.2 wt\%** \\ \end{tabular} \\ \hline \multirow{3}{*}{\(\rho\)} & 30 & 996.63 & 997.08 & 997.94 \\  & 40 & 993.24 & 993.45 & 994.25 \\ (kg/m\({}^{3}\)) & 50 & 989.02 & 988.81 & 989.99 \\  & 60 & 984.28 & 983.52 & 984.62 \\  & 30 & 0.80 & 0.88 & 0.90 \\ \(\mu\) & 40 & 0.70 & 0.78 & 0.82 \\ (mPa-s) & 50 & 0.65 & 0.68 & 0.70 \\  & 60 & 0.55 & 0.58 & 0.60 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: List of \(\mu\) and \(\rho\) for working fluids. Figure 3: System diagram of heat transfer experiments of THE. \(\pm\)0.3%). The flow rate of the working fluid was controlled at the target value (1.5, 2.5, 3.5, 4.5, and 5.5 LPM) using a flow control valve (0-10 LPM; FCV-10, Regal Joint, Kanagawa, Japan; accuracy: \(\pm\)3.0%). A plate heat exchanger (K025-10F-P4, Kaori, Taiwan) with an isothermal unit (20 L, \(-\)20-100 \({}^{\circ}\)C; RC20, Yotec, Taiwan; accuracy: \(\pm\)0.5 \({}^{\circ}\)C) was used to cool the temperature of the working fluid entering the heating water tank to below 30 \({}^{\circ}\)C in order that the electric heater in heating water tank can stably control the inlet temperature of the at 30 \({}^{\circ}\)C. The temperature and humidity of the experimental environment were controlled at 25 \(\pm\) 1.0 \({}^{\circ}\)C and 65 \(\pm\) 5% RH, respectively, by the temperature/humidity control system. The external side of the plate heat exchanger, heating water tank, THE, and connecting pipes was covered with PU foam insulation material (pipeline insulation thickness, 1.25 cm; equipment insulation thickness, 2.5 cm) to maintain experimental stability. All temperature, pressure, and flow data were measured and recorded using a data logger (TRM-20, TOHO, Kanagawa, Japan; accuracy: \(\pm\)0.1%) with an interval time of 5 s. The power consumption of the water pump (1/8 HP, maximum: 68 LPM; 5 MD-HC, Little Giant, Wayne, Indiana, USA) was measured and recorded using a power meter (KEW 6305, Kyoritsu, Tokyo, Japan; power accuracy: \(\pm\)1.0%) with an interval time of 5 s. The time for each experimental parameter was 20 min, and stable data acquired over the last 10 min were recorded, averaged, and used as the experimental data. The same experimental parameters were repeated five times, and the average of five readings was the final experimental result.