# Numerical Investigation of Thermal-Flow Characteristics in Heat Exchanger with Various Tube Shapes
## 1 Introduction
They found that the thermal exchange coefficient was higher by 50% for elliptical finned-tubes than that for circular finned-tubes. [PERSON] and [PERSON] [28] performed experiments in one- and two-row plate-fin and oval-tube HEs to determine the heat transfer characteristics in the range of Re of (150-1300). They illustrated that replacing the conventional circular tube with an oval one substantially reduces the pressure losses and increases the fin efficiency; however, there was no major difference in the average thermal exchange coefficients between these two tube shapes. [PERSON] et al. [29] and [PERSON] et al. [30] investigated a lower fin having circular and oval tube patterns. They found a decrease in thermal exchange and pressure loss for the case of oval tubes. [PERSON] et al. [31] reported in their study that the thermal exchange increases and the pressure loss decreases with increasing tube ellipticity. VGs (vortex generators) may be employed to provide strong heat transfer processes that include interrupting the growth of boundary layer, formation of horseshoe vortices, and flow mixing. For a plate FTHE, [PERSON] et al. [32], [PERSON] et al. [33], and [PERSON] et al. [34]analyzed the effects of adding VGs in conjunction with the oval tube. They observed a significant hydrothermal enhancement by using this technique. [PERSON] et al. [35] studied five samples of oval THE, and they optimized the axis ratio (1:4, 1:3, 1:2, and 1:1) and flow angle of attack (\(0^{\circ}\leq\alpha\leq 150^{\circ}\)) for a range of \(5.3\times 10^{3}\leq\mathrm{Re}\leq 4\times 10^{4}\). In addition, a combined investigation consisting of CFD simulations and genetic algorithms was performed by [PERSON] and [PERSON] [36] to inspect the impact of tube rows, transversal and longitudinal tube pitch, air velocity, and fin spacing on the efficiency of elliptical plate FTHEs for a range of \(500\leq\mathrm{Re}\leq 1500\). In contrast to previous researching works, they considered the thermal and fluid flow characteristics in a staggered two-row bundle. Their outcomes reveal that at higher air velocity or lower water flow rate, increasing the axis ratio increases the hydrothermal efficiency, whereas at lower airspeed or higher water flow, the reverse effect occurs. Similarly, [PERSON] et al. [37] compared refrigerant-to-air finned-tube condensers with circular and oval tubes by using numerical simulations. According to the researchers, using elliptical tubes increased the coefficient of performance (COP) by 3.6-6.7%. [PERSON] et al. [38] investigated the impact of row numbers (2-10), tube geometry (oval and circular), and other parameters on the efficiency of the annular finned-tube bundle. They claimed that elliptical tubes with the lowest ellipticity (0.33) perform better than other tube shapes. [PERSON] et al. [39] evaluated the performance of plate FTHE using circular, oval, and flat tube shapes for seven rows with staggered or in-line tube banks. The overall performance of the cross-flow heat exchanger can be improved by reducing the flatness of a flat tube and the axis ratio of an oval tube. Moreover, based on the considered three performance evaluation criteria, they concluded that the oval geometry with the highest axis ratio is the optimum tube shape. Recently, the impact of tube geometry on thermal and mass transfer was experimentally and numerically investigated by [PERSON] et al. [40]. The convective thermal exchange coefficient of an oval tube is approximately 66% higher than that of a circular tube, and the dehumidifying coefficient is approximately 21% higher. [PERSON] et al. [41] evaluated the gain in energy efficiency owing to the use of oval tubes rather than circular tubes in the evaporator of a conditioning system. They reported that the optimized oval-tube outperforms the conventional circular-tube shape by approximately 13% and 15% in heat transfer rates, 9% and 13.5% in coefficients of performance (COP), and 13% and 16.6% in heat transfer coefficients for Re = 1030 and 2060, respectively.