**1. Introduction**

A compact heat exchanger is a heat exchanger with a large area to volume ratio so that it has a high surface area of heat transfer to volume [1]. Compact heat exchangers are widely used in the air conditioning, refrigeration, chemical, petroleum, and automotive industries. Fin and tube heat exchanger is one type of compact heat exchanger that is often encountered. One example is the condenser in air conditioning, where air is used as a refrigerant cooling medium. However, the high thermal resistance on the airside results in a low heat transfer rate [2]. Therefore, to increase the heat transfer rate, the thermal resistance needs to be lowered by increasing the convection heat transfer coefficient [3].

The method of increasing the convection heat transfer coefficient has become an interesting thing to investigate [1]. In general, the method of increasing the convection heat transfer coefficient is divided into two, namely the active method and the passive method [4]. The active method is a method that uses external energy to increase the rate of convection heat transfer, for example, by electrostatic fields, fluid vibration, and flow pulsation [1, 5]. In contrast, the passive method is a method that does not use external energy to increase the convection heat transfer rate. Passive methods are more often used than active methods because they are simpler and more effective [6]. The increase in the convection heat transfer rate in the passive method is performed by adding an insert structure and surface modification, which results in the formation of swirl flow [4, 6].

transfer than that of the DWP VGs. Zhimin Han et al. (2018) conducted a threedimensional simulation study of the heat transfer characteristics through the perforated rectangular type of VGs [16]. In this study, the flow velocity was varied in the Reynolds number range of 214–10,703. The simulation results showed that giving holes to VGs can reduce pressure drop. The optimal thermo-hydraulic performance

*Numerical Investigation of Heat Transfer and Fluid Flow Characteristics in a Rectangular…*

In addition, M. Samadifar et al. (2018) studied the effect of a new type of VG with variations in the angle of attack on the increase in heat transfer in the plate-fin heat exchanger in the triangular channel [17]. Six types of VGs were used in this numerical simulation, namely rectangular VG, rectangular trapezius VG, angular rectangular VG, wishbone VG, intended VG, and wavy VGs. M. Samadifar et al. performed a numerical simulation approach with turbulent k-ω SST modeling. The simulation results showed that rectangular VGs provide a better heat transfer increase than other VGs, with an increase of 7%. The simulation results also showed that the best VGs installation is VGs with an angle of attack of 45°. Jiyang Li et al. (2019) investigated the increase in heat transfer in finless flat-tube heat exchangers due to the installation of double triangle, triangular, and rectangular VG [18]. In modeling, VGs were installed in front of the finless heat exchanger with a distance of 1 mm so that the condensation water does not hit VGs. The results showed that VGs could disturb the thermal boundary layer so that the mixing of cold and hot air increases, which results in an increase in heat transfer performance. In addition, the results also showed that the double triangle VG increases the heat transfer coefficient by 92.3% at an air velocity of 2 m/s. The double triangle VGs increase the heat transfer coefficient by 20% greater than that of the triangular and rectangular VGs

G. Lu and X. Zhai (2019) conducted a numerical investigation of the flow characteristics through the curved VG on the fin and tube heat exchanger [19]. G. Lu and X. Zhai varied the curvature and angle of attack of VG in their research. Flow characteristics were reviewed based on several non-dimensional parameters, namely Nu/

Their results showed that the best thermal–hydraulic performance was obtained for VG at a curvature of 0.25 with a value of R = 1.06 at a 15 ̊angle of attack. R.K.B. Gallegos and R.N. Sharma (2019) also conducted heat transfer experiments due to the installation of VG flapping flags on the rectangular channel [20]. Their experimental results showed that VG increases the flow instability and the turbulence rate so that the Nusselt number increases by 1.34 to 1.62 times. However, VG also causes an increase in pressure drop because of the resistance to VG. This can be identified by an

The use of VG causes an increase in thermal performance, but its use has an impact on an increase in pressure drop, which results in low hydraulic flow performance. This study discusses the effect of installing RWP VGs and CRWP VGs on thermal and hydraulic performance. Thermal performance is investigated through analysis of the field synergy angle, spanwise average Nusselt number, and convection heat transfer coefficient values. Meanwhile, the hydraulic performance is analyzed through an increased pressure drop. This study aims to determine the effect of the type of VG and the effect of giving a hole on VG on thermal–hydraulic performance.

Experiments on the effect of VG on heat transfer and pressure drop flow were carried out in a rectangular channel made of glass with a thickness of 1 cm and a

increase in the friction factor, which increased by 1.39–3.56 times.

1/3 with a Reynolds number range of 405–4050.

was observed for VGs with a hole diameter of 5 mm.

*DOI: http://dx.doi.org/10.5772/intechopen.96117*

but also an increase in pressure drop.

Nu0, f/f0 and R = (Nu/Nu0)/(f/f0)

**2. Physical model**

**55**

**2.1 Experimental set-up**

Vortex generator (VG) is an insert that produces vortices due to the formation of swirl flow, which increases the heat transfer rate [7–9]. The vortex can be divided into two, namely the transverse vortex and the longitudinal vortex [9]. The transverse vortex has a vortex axis that is perpendicular to the main flow. Meanwhile, the longitudinal vortex has a vortex axis parallel to the main flow. Longitudinal vortices are more efficient in increasing convection heat transfer because they can improve thermal performance better than transverse vortices with the same pressure drop. Longitudinal vortex causes increased fluid mixing, boundary layer modification, and flow instability resulting in increased convection heat transfer coefficient [10].

Various studies regarding the use of VGs to improve convection heat transfer have been carried out. A. Datta et al. (2016) conducted a numerical investigation of heat transfer on a rectangular microchannel installed with VGs with angle position variations in two VGs with a Reynolds number range of 200–1100 [11]. The simulation results proved that the increase in heat transfer is directly proportional to the increase in the Reynolds number and the angle of attack of VG. Installation of angle of attack of 30̊° with Reynolds number 600 is the best combination. In addition, H. Y Li (2017) conducted an experimental and numerical study on the case of fluid flow in a pin-fin heat sink mounted with a delta winglet vortex generator (DW VGs) [12]. The study was conducted to determine the effect of Reynolds number, angle of attack of VGs, and height of VGs on convection heat transfer. The results show that the increase in the Reynolds number causes a decrease in thermal resistance resulting in an increase in the convection heat transfer coefficient. The results of these studies also indicate that the angle of attack of 30̊° is the best. Meanwhile, the optimum VGs height is 3/2 H.

In 2017, H.E. Ahmed et al. conducted a heat transfer study on a triangular duct with a DWP VGs in three-dimensional modeling with nanofluid flow [13]. The simulation results showed an increase in heat transfer and pressure drop of 45.7% and < 10% respectively due to the installation of VGs and 3% Al2O3 nanoparticles. Overall, the use of VGs and nanofluids can improve heat transfer with lower pressure drops. In addition, Syaiful et al. (2017) conducted a numerical study of the installation of CDW VGs on rectangular channels [14]. The results showed that the increase in the heat transfer coefficient due to the installation of CDWP VGs is much better than DWP VGs. However, the use of CDWP VGs results in a higher increase in pressure drop. In general, the results showed that the increase in convection heat transfer coefficient and pressure drop due to the installation of three rows of CDW VGs are 42.2–110.7% and 180–266.9%, respectively.

Then, M. Oneissi et al. (2018) conducted a numerical study on the increase in heat transfer due to the installation of DWP VGs and inclined projected winglet pair VGs with the k-ω turbulent model [15]. In this three-dimensional simulation, the increase in heat transfer was viewed from the distribution of the Nusselt number, the coefficient of friction, and the vortices. The simulation results showed that the inclined projected winglet pair produces 7.1% better performance in increased heat

#### *Numerical Investigation of Heat Transfer and Fluid Flow Characteristics in a Rectangular… DOI: http://dx.doi.org/10.5772/intechopen.96117*

transfer than that of the DWP VGs. Zhimin Han et al. (2018) conducted a threedimensional simulation study of the heat transfer characteristics through the perforated rectangular type of VGs [16]. In this study, the flow velocity was varied in the Reynolds number range of 214–10,703. The simulation results showed that giving holes to VGs can reduce pressure drop. The optimal thermo-hydraulic performance was observed for VGs with a hole diameter of 5 mm.

In addition, M. Samadifar et al. (2018) studied the effect of a new type of VG with variations in the angle of attack on the increase in heat transfer in the plate-fin heat exchanger in the triangular channel [17]. Six types of VGs were used in this numerical simulation, namely rectangular VG, rectangular trapezius VG, angular rectangular VG, wishbone VG, intended VG, and wavy VGs. M. Samadifar et al. performed a numerical simulation approach with turbulent k-ω SST modeling. The simulation results showed that rectangular VGs provide a better heat transfer increase than other VGs, with an increase of 7%. The simulation results also showed that the best VGs installation is VGs with an angle of attack of 45°. Jiyang Li et al. (2019) investigated the increase in heat transfer in finless flat-tube heat exchangers due to the installation of double triangle, triangular, and rectangular VG [18]. In modeling, VGs were installed in front of the finless heat exchanger with a distance of 1 mm so that the condensation water does not hit VGs. The results showed that VGs could disturb the thermal boundary layer so that the mixing of cold and hot air increases, which results in an increase in heat transfer performance. In addition, the results also showed that the double triangle VG increases the heat transfer coefficient by 92.3% at an air velocity of 2 m/s. The double triangle VGs increase the heat transfer coefficient by 20% greater than that of the triangular and rectangular VGs but also an increase in pressure drop.

G. Lu and X. Zhai (2019) conducted a numerical investigation of the flow characteristics through the curved VG on the fin and tube heat exchanger [19]. G. Lu and X. Zhai varied the curvature and angle of attack of VG in their research. Flow characteristics were reviewed based on several non-dimensional parameters, namely Nu/ Nu0, f/f0 and R = (Nu/Nu0)/(f/f0) 1/3 with a Reynolds number range of 405–4050. Their results showed that the best thermal–hydraulic performance was obtained for VG at a curvature of 0.25 with a value of R = 1.06 at a 15 ̊angle of attack. R.K.B. Gallegos and R.N. Sharma (2019) also conducted heat transfer experiments due to the installation of VG flapping flags on the rectangular channel [20]. Their experimental results showed that VG increases the flow instability and the turbulence rate so that the Nusselt number increases by 1.34 to 1.62 times. However, VG also causes an increase in pressure drop because of the resistance to VG. This can be identified by an increase in the friction factor, which increased by 1.39–3.56 times.

The use of VG causes an increase in thermal performance, but its use has an impact on an increase in pressure drop, which results in low hydraulic flow performance. This study discusses the effect of installing RWP VGs and CRWP VGs on thermal and hydraulic performance. Thermal performance is investigated through analysis of the field synergy angle, spanwise average Nusselt number, and convection heat transfer coefficient values. Meanwhile, the hydraulic performance is analyzed through an increased pressure drop. This study aims to determine the effect of the type of VG and the effect of giving a hole on VG on thermal–hydraulic performance.

## **2. Physical model**

#### **2.1 Experimental set-up**

Experiments on the effect of VG on heat transfer and pressure drop flow were carried out in a rectangular channel made of glass with a thickness of 1 cm and a

The method of increasing the convection heat transfer coefficient has become an interesting thing to investigate [1]. In general, the method of increasing the convection heat transfer coefficient is divided into two, namely the active method and the passive method [4]. The active method is a method that uses external energy to increase the rate of convection heat transfer, for example, by electrostatic fields, fluid vibration, and flow pulsation [1, 5]. In contrast, the passive method is a method that does not use external energy to increase the convection heat transfer rate. Passive methods are more often used than active methods because they are simpler and more effective [6]. The increase in the convection heat transfer rate in the passive method is performed by adding an insert structure and surface modifi-

Vortex generator (VG) is an insert that produces vortices due to the formation of swirl flow, which increases the heat transfer rate [7–9]. The vortex can be divided into two, namely the transverse vortex and the longitudinal vortex [9]. The transverse vortex has a vortex axis that is perpendicular to the main flow. Meanwhile, the longitudinal vortex has a vortex axis parallel to the main flow. Longitudinal vortices are more efficient in increasing convection heat transfer because they can improve thermal performance better than transverse vortices with the same pressure drop. Longitudinal vortex causes increased fluid mixing, boundary layer modification, and flow instability resulting in increased convection heat transfer

Various studies regarding the use of VGs to improve convection heat transfer have been carried out. A. Datta et al. (2016) conducted a numerical investigation of heat transfer on a rectangular microchannel installed with VGs with angle position variations in two VGs with a Reynolds number range of 200–1100 [11]. The simulation results proved that the increase in heat transfer is directly proportional to the increase in the Reynolds number and the angle of attack of VG. Installation of angle of attack of 30̊° with Reynolds number 600 is the best combination. In addition, H. Y Li (2017) conducted an experimental and numerical study on the case of fluid flow in a pin-fin heat sink mounted with a delta winglet vortex generator (DW VGs) [12]. The study was conducted to determine the effect of Reynolds number, angle of attack of VGs, and height of VGs on convection heat transfer. The results show that the increase in the Reynolds number causes a decrease in thermal resistance resulting in an increase in the convection heat transfer coefficient. The results of these studies also indicate that the angle of attack of 30̊° is the best. Meanwhile,

In 2017, H.E. Ahmed et al. conducted a heat transfer study on a triangular duct with a DWP VGs in three-dimensional modeling with nanofluid flow [13]. The simulation results showed an increase in heat transfer and pressure drop of 45.7% and < 10% respectively due to the installation of VGs and 3% Al2O3 nanoparticles. Overall, the use of VGs and nanofluids can improve heat transfer with lower pressure drops. In addition, Syaiful et al. (2017) conducted a numerical study of the installation of CDW VGs on rectangular channels [14]. The results showed that the increase in the heat transfer coefficient due to the installation of CDWP VGs is much better than DWP VGs. However, the use of CDWP VGs results in a higher increase in pressure drop. In general, the results showed that the increase in convection heat transfer coefficient and pressure drop due to the installation of three

Then, M. Oneissi et al. (2018) conducted a numerical study on the increase in heat transfer due to the installation of DWP VGs and inclined projected winglet pair VGs with the k-ω turbulent model [15]. In this three-dimensional simulation, the increase in heat transfer was viewed from the distribution of the Nusselt number, the coefficient of friction, and the vortices. The simulation results showed that the inclined projected winglet pair produces 7.1% better performance in increased heat

rows of CDW VGs are 42.2–110.7% and 180–266.9%, respectively.

cation, which results in the formation of swirl flow [4, 6].

*Heat Transfer - Design, Experimentation and Applications*

coefficient [10].

**54**

the optimum VGs height is 3/2 H.

length of 370 cm, a width of 8 cm, and a height of 18 cm, as shown in **Figure 1**. The blower sucks air into the channel from the inlet side through a straightener composed of pipes with a diameter of 5 mm and wire mesh to equalize the flow velocity. The flow velocity in the channel was varied in the range of 0.4 m/s to 2.0 m/s with an interval of 0.2 m/s using a motor regulator controlled by an inverter (Mitsubishi Electric-type FR-D700 with an accuracy of 0.01 Hz) and measured with a hotwire anemometer (Lutron type AM-4204 with an accuracy of 0.05). In this study, the airflow flowed through VGs with variations in the number of rows (one, two, and three rows) as well as variations with/without holes to investigate the effect on heat transfer rate and pressure drop. The VGs were mounted on a flat plate that was heated at a constant rate of 35 W using a heater that was regulated by a heater regulator and monitored by a wattmeter (Lutron DW-6060 with an accuracy of 0.01). Thermocouples K type was used to measure surface temperature, inlet and outlet temperatures, which were connected to data acquisition (Advantech type USB-4718 with accuracy 0.01) and were monitored and stored in the CPU. In the pressure drop test, two pitot tubes were installed at the inlet and outlet of the test section and connected to a micro manometer (Fluke 922 with accuracy 0.01) to monitor the pressure drop due to the installation of VGs. Flow visualization tests were also carried out to observe the longitudinal vortex formed as a result of VGs insertion. The longitudinal vortex was formed when the smoke resulting from the evaporation of oil in the heater was flowed through VGs and captured by the transverse plane formed by the luminescence of the laser beam. The camera was used to record the longitudinal vortex structure that was formed.

with a diameter of 5 mm. **Table 1** shows the geometric parameters of the CRWP and RWP VGs. **Figure 3** is a top view of the RWP and CRWP VGs. VG with the angle of attack (α) 45° arranged in-line in common flow-down orientation with a longitudinal pitch of 125 mm. The distance between the first row and the inlet channel is 125 mm. Meanwhile, the leading-edge transverse distance between winglet pairs VG is 20 mm. The rectangular channel modeled in this simulation has dimensions of length (P), width (L), height (H) of channels of 500 mm, 75.5 mm,

*Numerical Investigation of Heat Transfer and Fluid Flow Characteristics in a Rectangular…*

**Figure 4** shows the computational domain used in this modeling. This domain

In this 3-D flow modeling, air was assumed to be steady state, incompressible and has constant physical properties. Flow can be laminar or turbulent based on its

consists of an inlet extended region and an outlet extended region. An inlet extended region was provided to ensure that the airflow entering the channel is a fully developed flow. Meanwhile, an extended region outlet was added so that the

air does not experience reverse flow in the channel.

*DOI: http://dx.doi.org/10.5772/intechopen.96117*

*Geometry of RWP and CRWP VG with and without holes.*

**a (mm)**

**cv (mm)**

**dv (mm)**

**ev (mm)**

RWP with holes 45 60 15 — 15 20 — 10 40 —

**ch (mm)**

45 59 —— ———— 40 58

45 59 15 14.56 — — 20 9.85 40 58

45 60 —————— 40 —

**dh (mm)**

**eh (mm)**

**t (mm)**

**R (mm)**

**(°)**

*Geometry parameters of vortex generator (VG).*

65 mm, respectively.

**2.3 Governing equations**

**Figure 2.**

**VGs α**

CRWP without holes

CRWP with holes

RWP without holes

**Table 1.**

**57**
