The hydrothermal behavior of air inside a solar channel heat exchanger equipped with various shaped ribs is analyzed numerically. The bottom wall of the exchanger is kept adiabatic, while a constant value of the temperature is set at the upper wall. The duct is equipped with a flat rectangular fin on the upper wall and an upstream V-shaped baffle on the lower wall. Furthermore, five hot wall-attached rib shapes are considered: trapezoidal, square, triangular pointing upstream (type I), triangular pointing downstream (type II), and equilateral-triangular (type III) cross sections. Effects of the flow rates are also inspected for various Reynolds numbers in the turbulent regime (

The insertion of roughness elements in channel heat exchangers is well-known as an excellent technique to enhance the overall performances (Chen et al. [

Various research studies have been achieved on this subject, among other works, Patankar et al. [

Other similar studies can be found in literature as Ghanbari et al. [

Other investigations examined the installation of porous baffles in ducts. Abbasi et al. [

Furthermore, many studied have been conducted suggesting various newly designed baffles, such as the Flat and trapezoidal [

The engineering analysis of previous studies can classify the heat transfer enhancement methodology into two different techniques. Several studies have used heat exchangers with smooth channels containing transverse/longitudinal VGs (baffles and/or fins) in various shapes. On the other hand, other used rough walls (ribbed surface) rather than extended surfaces (VGs). Both cases demonstrated significant improvement in heat transfer. This study is a combination of these heat transfer reinforcement techniques, in order to achieve a wider hydrothermal performance inside the heat exchanger. This is the motivation of the current work, in which a detailed analysis of the flow structure and its effect on the efficiency of a solar ribbed, finned and baffled channel air-heat exchanger is highlighted. This work aims to enhance the hydrothermal behavior of air within channel heat exchangers. An attempt is made to highlight the influence of flow rates, as well as the design of ribs. Several shapes of ribs are considered, including the trapezoidal, square, triangular pointing upstream (type I), triangular pointing downstream (type II), and the equilateral-triangular (type III) shape. The study is concerned with the following:

The current filed lines are shown in various models of ribs subject to a turbulent Newtonian flow.

The effect of duct blockage on the mean velocity and its two components (axial and transverse) is shown at various stations.

Hydrodynamic analysis of direct and reverse flows and their relationship to dynamic pressure.

The simultaneous effect of VGs and ribs on the exchanger performance.

In order to highlight the effectiveness of the proposed model, a comparison with referenced baffled and finned heat exchangers without ribs [

The hydrothermal characteristics of air flowing through a solar finned and baffled channel heat exchanger [_{w}_{h}_{in}_{out}

Our simulation is based on real experimental studies, this is why the physical and geometrical parameters are the same as those of Demartini et al. [

For turbulent flow conditions, and the hydrothermal behavior is two-dimensional. The fluid nature is Newtonian as well as incompressible with a constant value of the velocity profile at the inlet [_{in}_{w}

The governing equations of the problem under investigation are written as [

The continuity:

The momentum:

The energy:

The model of standard type

For the top wall of the exchanger (

For the bottom wall of the exchanger (

Reynolds number (Re)

where _{h}

where, Nu is the average Nusselt number for the ribbed and baffled channel computed as:

with the local Nusselt number (

Nu_{0} is the Nusselt number for the smooth channel (

And, _{0} is the friction factor for the smooth channel (

The computational approach of finite volume [

The grid dependency tests were conducted by considering various mesh cases, where the nodes’ number of grids varied from 35 points to 145 points along with the depth of the exchanger and 95 nodes to 370 nodes along with the length, as determined in the referenced papers [

The originality of the present work is the implementation of incompressible Newtonian fluid with the ribbing and baffling techniques to enhance the overall performance of the heat exchanger. Therefore, we compared our work against an experimental data of a simple baffled channel without ribs [

The streamlines inside the channel heat exchanger at

Furthermore, the second V-baffle yields a smooth airflow along with the main flow direction, which increases the axial velocity and significantly reduces the reattachment length. In general, and for the five types of baffles, a recirculation zone is formed in each region where the roughness is located. The size of these recirculation zones is very significant for the case of triangular-shaped baffles.

The effects of the shape of VGs on the turbulent airflow behavior are highlighted in

Finally, these obstacles augment the length of the flow patterns and intensify the vortex magnitude due to the changes in the streamline’s orientation.

The fields of the axial velocity (

After the second baffle, the flow is accelerated again to reach the values of about 419.66%–426.91% over the inlet velocity. We note that these maximum values depend on the geometrical shape of the baffle. The maximum axial velocity is reached with the triangular case in type II, while the square baffle yielded the lowest axial velocity. At the same Re and compared with the II-model triangular baffle, the decrease in the axial velocity was about 1.61%, 0.41%, 0.63%, and 1.69% for the square, triangular in I-model, trapezoidal, and the triangular-shaped ribs in III-model, respectively.

For the vertical velocity (^{4}.

The variation of the dynamic pressure is illustrated in

The turbulent kinetic energy (

The distribution of the temperature fields is also provided at

The axial velocity profiles (

The obtained results reveal that the rectangular baffle yields the higher recirculation lengths than those with the V-obstacle, regardless of Re and the baffle design. In addition, the hot obstacle creates an abrupt variation in the velocity, while the insulated V-baffle yields a progressive change in the velocity, which participates in the considerable reduction of the reattachment lengths. The comparison of the recirculation length for the different geometrical configurations of ribs reveals that the triangular obstacle (type II) yields the longest recirculation cell for all Re studied here. Also, the results suggest that the flow rates impact significantly on the vortex size behind the baffles, where the augmentation of Re produced an increased length in the recirculation areas.

The performance (

The hydrothermal characteristics of air inside a solar duct provided with various designed baffles, i.e., flat and V, under a staggered arrangement have been inspected numerically. Five various roughness situations were considered: trapezoidal, square, triangular pointing upstream (type I), triangular pointing downstream (type II), and equilateral-triangular (type III).

From the several computations, low pressure values were observed behind the baffles due to the existence of recirculation cells. However, the pressure increased in the space between the baffle tip and the duct wall.

The velocity magnitude behind the rectangular-shaped VG is considerably higher than that behind the V-shaped VG, resulting thus in a difference in the reattachment length and vortex size for the two geometrical models. The obtained results reveal that the rectangular baffle yields the higher recirculation lengths than those with the V-obstacle, regardless of Re and the rib design. In addition, the hot obstacle creates an abrupt variation in the velocity, while the insulated V-baffle yields a progressive change in the velocity, which participates in the considerable reduction of the reattachment lengths.

The comparison of the recirculation length for the different geometrical configurations of ribs reveals that the triangular obstacle (type II) yields the longest recirculation cell for all Re studied here. Also, the results suggest that the flow rates impact significantly on the vortex size behind the baffles, where the augmentation of Re produced an increased length in the recirculation areas.

The comparison between the various roughness situations revealed that the triangular baffles (type II) are able to provide the most significant amounts of axial and vertical velocities. At

The most significant value of kinetic-energy of turbulence is reached with the triangular-type ribs, while the lowest kinetic-energy is given with the two first geometrical models. More precisely, the square-shaped ribs allowed the lowest amount of k compared with the other cases. The comparison between the different cases in terms of rates of turbulence dissipation suggested that the highest dissipations rates are yielded by the triangular type roughness geometries.

The higher temperature gradients were located in the baffled regions, while the lower ones were found behind the VGs. This means that the main recirculation loops have great impact on the temperature distribution, since they generate good agitation of fluid particles between the hot wall and the core areas. The low thermal exchange areas are located behind the vortex generators.

The performance evaluation gave an enhancement in the

The rough surface presence with triangular-type ribs proved their effectiveness compared to those reinforced with square or trapezoidal ribs.

The highest performance value was given for the II-triangular rib case in all Re values, while the square-shaped ribs showed a significant decrease in the

The

Also, a comparison was made with heat exchangers that have non-rough walls and contain cross-shaped VGs presented previously, in order to highlight the effectiveness of the rough surface presence in the baffled and finned channels. The comparison was made at the largest value of the flow rate and as expected, the proposed exchanger with its five different models showed a significant improvement in

This highlights the necessity of roughness heat transfer surfaces for finned and baffled channels to improve significantly the performance of the air-heat exchangers they contain.

Constant in k-

Constant in k-

Specific heat, J kg^{−1} K^{−1}

Hydraulic diameter of the exchanger, m

Average coefficient of friction

Factor of friction for smooth exchanger

Height of exchanger, m

Height of attached VG, m

Height of attached rib, m

Kinetic energy of turbulence, m^{2} s^{−2}

Length of exchanger, m

Inlet–-1st VG space, m

2nd VG–-exit space, m

Average Nusselt number of the ribbed and baffled exchanger

Average Nusselt number for the smooth exchanger

Pressure, Pa

Dynamic pressure, Pa

Number of Prandtl

Number of Reynolds

Space between VGs, m

Temperature, K

Inlet fluid temperature, K

Wall temperature, K

X-velocity, m s^{−1}

Intake velocity, m s^{−1}

Average velocity, m s^{−1}

Y-velocity, m s^{−1}

Width of exchanger, m

Thickness of the fixation base of the rib, m

Thickness of the upper face of the rib, m

Attack angle, degree

Thermal enhancement factor

Thermal-conductivity, W m^{−1} K^{−1}

Dynamic viscosity, kg m^{−1} s^{−1}

Turbulent viscosity, kg m^{−1} s^{−1}

Density, kg m^{−3}

Constant in k-equation

Constant in

Wall-shear-stress, Pa

Atmosphere

Fluid

Intake

Outlet

Turbulent

Wall

Local