Iranian (Iranica) Journal of Energy & Environment Heat Exchange Optimization by Adding Baffles to Streaming Duct of Solar Air Collector

This numerical and experimental work aims to improve the heat transfer inside a solar thermal collector. By incorporating rectangular baffles in the middle of the distributed air passing channel at different angles of inclination (  = 90  ,  = 180  ,  = 180  and  = 90  ). That is called the model H. These experiments were carried out in the Biskra region of Algeria in good natural conditions with an average solar radiation approximately constant I= 869 W/m 2 varying from 11:30 to 14:00. After the completion of the experimental investigation, a computational fluid dynamics (CFD) model was created that matches this experimental model with the same experimental boundary conditions. In the numerical study, ANSYS Fluent 18.1 was used to conduct simulations and compare the results of the thermal and hydraulic performance of the collector. It was concluded that the effectiveness of the CFD model, meaning that the theoretical and numerical data were very close to each other for all mass flow rates. As the mass flow increased the heat transfer process increased, while the absorber plate temperature inside the collector for experimental and numerical studies decreased. Addition of baffles increased heat transfer, due to the creation of turbulent flow that leads to crack the dead thermal layers near the absorber plate, which leads to an increase in heat transfer from the absorber plate to the air.

INTRODUCTION 1   Solar energy is one of the renewable, environmentally friendly and easy-to-use energies that ensures future development.One of the most important means used to benefit from this energy, as it transfers energy from solar radiation to the air passing through its channel through the absorber plate, which is easy to install and simple to use.In the literature, there are many researchers who tried to improve these transformers, some of them conducted experimental investigations and others used numerically by adding a new gemetrical shape of baffles [1][2][3][4] and changed the form of the duct [5][6][7][8].Among these works, we mention the study of Hu et al. [9] where prepared numerical and experimental work of the heat tranfer inside a channel divided into serval compartments, the pricipal characteristics where changed in this study is the geometrical dimension of the compartments.In the same contex Akpinar and Koçyiğit [10] conducted an experimental investigation of the heat exchange inside solar air collectoe with baffles, then compared the results with smooth plate collector.This study showed the the thermal and hydrolic perfermence are related with the geometrical shape of baffles.Also, Wang et al. [11] made an experimental study where added the S shepe of baffles inside their collector, from the results the the effect of the baffles on the thermal perfermances is confirmed.On the other side double-pass solar air collector was studied by Wijeysundera and Tjioe [12].
In this study, many of the operating conditions were modified and they were compared with a single-channel solar thermal collector.Through its results, it confirmed the superiority of the double-pass solar collector compared to the single-channel solar collector.Based on the mentioned works, in this study, rectangular barriers were added perpendicular to the air inside the collector channel, and the heat transfer was studied by numerical and experimental methods.

Experimental investigation
The experimental work was carried out in the Biskra region, specifically at the University of Biskra, near the Technological Hall with inclination angle β=38.
The studied collector receiver consists of a single channel, to which were added rectangular baffles made of galvanic located in the middle of the channel at angles of inclination of =90, =180, =180and =90 as depicted in Figures 1 and 2. Also, this collector is well thermally insulated with layers of polystyrene to ensure the correctness of the results reported in the literature [13].The purpose of adding baffles is to create turbulent flow and increase heat transfer within the transformer; this was the target focused by Menasria et al. [14].Geometric dimensions of the building is tabulated in Table 1.
The experiment was done on 01/12/2021, from 11:30 to 14:00 where the mass flow rate changed from 0.011 to 0.068 kg/s.Every quarter of an hour we measured absorber plate temperature, bottom plate temperature, solar radiation, inlet and outlet temperatures.All measuring devices were available in the laboratory, where five measuring points were placed on the absorbent plate and like them in parallel on the bottom plate of Figure 1.

CFD set-up
The same experimental studied model was created for numerical studies using ANSYS Fluent 18.1, the generated model is a two-dimensional model with the same dimensions as the experimental model and the baffles situated in it.The CFD model consists of three basic steps, which are:  • Design the model using Design Modeler Figure 3, where S= 10 cm, d= 4 cm, and e= 2 cm • Meshing the model using the same software, the division is carried out into several nodes, which are controlled by the stability of the extracted results.The most accurate division was adopted at the walls to observe the effect of the walls on the thermal properties, where did the number of nodes reached to 22775 nodes as illustrated in Figure 4.
• After the division we pass to the introduction of the boundary conditions extracted from the experiment, the determination of the turbulence model (RNG k-), and the introduction of the physical properties of the materials.Through the studie conducted by Bensaci et al. [15] and Lee et al. [16] the boundary conditions were constant temperature on the absorber plate and bottom plate, velocity, turbulence intensity, temperature at the inlet, and hydraulic diameter.

Modeling
In this study, many relations that have to do with hydraulic or thermal properties in both dimensional and non-dimensional forms were used.

Heat flux
( This relation becomes: ( The phenomenon that occurs inside the channel is a forced convection heat transfer, so that the heat transfer coefficient becomes one of the most important characteristics, which is given as follows: (3)

Hydraulic diameter
The hydraulic diameter depended on the cross-section and section perimeter (4)

RESULTS AND DISCUSSION
In this manuscript, the focus was on the thermal properties of the transformer (heat transfer from the absorber plate to the passing air) [13], where the results were a detailed study of the heat of the absorber plate in its various parts.In addition to the temperature of the bottom plate, and the coefficient of heat transfer by convection and to take a closer look the heat fields were extracted and the velocity fields, to understand the phenomena inside the studied collector.
Figure 5 shows the temperature of the absorber plate in terms of its length at different mass flow rates.The first observation we notice is that all curves have the same changes, which is the rise in the temperature of the plate from the inlet to the outlet, and all the maximum values were recorded at the last point of the measurement at the length x= 1.50 m.The second observation we noticed was that as the mass flow increased, the average temperature of the absorber plate decreased; which indicates an increase in heat transfer from the device to the air.The effect of the baffels on the heat transfer inside the collector was evident on the temperature of the absorber plate, as in the middle region there is a decrease in the temperature of the absorber plate in the region between x= 0.752 m to x= 1.128 m, which is the area of the positioning of the four baffels, and this is due to the creation of turbulent flow in that area and the cracking of layers dead thermal near to the walls.
As for the temperature of the bottom plate Figure 6, it has the same trend of change curve with the absorber plate due to its gaining thermal resistance from the air by convection.The highest temperature was recorded at the position x= 1.5, and in the flows, the highest temperature was recorded in the lowest flow rate m= 0.011 kg/s.represents the change of the heat transfer coefficient in terms of mass flow in both the experimental and numerical investigations.Through this picture, we notice that there is a great convergence between the experimental results and the results of CFD, which leads to the validity of the designed model, as the greater the mass flow rate increases the heat transfer coefficient, it means, an increase in the heat transfer, and this confirms the heat absorber plate results.The highest value reached by the heat transfer coefficient was h= 32.14 W/m 2 .K at mass flow rate of 0.068 kg/s, and that is in the experimental investigation.As for the CFD, at the same flow rate the heat transfer coefficient was h= 30.62 W/m 2 .K .
We now pass to another result that is no less important than the above, which is the temperature difference between the inlet and outlet in terms of the mass flow by the two methods CFD analysis and experimental investigation as illustrated in Figure 8.This curve confirms the validity of the numerical model based on the convergence between the experimental and numerical values.Due to the decrease in the temperature of the  Figure 9 represents the velocity change field inside the channel and in the baffles area, where we see the effect of these baffles on the air velocity (the effect of baffles is an evident in the speed field change) [15,16], especially the first baffle, as it is considered a stage of change in the air temperature due to the turbulence caused by these baffles.

CONCLUSION
The problem that was addressed in this study is the effect of baffles inside the channel of the solar thermal collector on the absorber plate and the heat transfer in general.Through the experimental and numerical results, it was found that the baffles have a great effect on the heat of the absorber plate and on the heat transfer, as their presence ensures an increase in the heat transfer and a decrease in the heat of the absorber plate due to the turbulence that it causes to the air, and the results also showed on the mass flow rate.It is inversely proportional to the heat of the absorber plate and directly to the heat transfer.The last conclusion of this study is to prove the effectiveness of the CFD model, through the convergence of its results with the experimental results, where we can rely on it to study other situations of baffles without resorting to the experimental investigation.

Figure 5 .Figure 6 .Figure 7
Figure 5. Temperature of the absorber plate in terms of its length at different mass flow rates

Figure 7 .Figure 8 .
Figure 7. Heat transfer coefficient in terms of mass flow rates