Numerical investigation to augmentation of heat transfer in solar air heater with arc shape ribs on

Abstract

Keywords: Artificial Roughness; CFD; Fluid Flow; Heat Transfer; Solar Air Heater; Solar Energy

Introduction

Due to its abundance and excellent quality, solar power has the ability to fulfill the modern world’s energy demands with little negative impacts on the environment. Optimal performance of technologically advanced usage of solar energy certain pieces of machinery demand high-quality energy.; the sun supplies more power to earth in more energy is consumed by the human population in a single

hour than the entire population consumes in a single year. In comparison to other sources of power, solar power has the distinct advantage of not adding to environmental pollution [1-2]. It was necessary to go to an alternative solution, which is renewable energy [3-6], which is considered one of the most important resources in the future if it is used, exploited, and worked on properly. Research is being conducted on solar energy and its applications to benefit from

*Corresponding author. *E-mail address: gaith7751@gmail.com, gaith.tw1997@gmail.com This paper was recommended for publication in revised form by Editor-in-Chief Ahmet Selim Dalkılıç Published by Yıldız Technical University Press, İstanbul, Turkey Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

it in various aspects of life, so solar air heaters are among the available applications that are easy to use, clean, and environmentally friendly, and last for a period because they are cheap and non-polluting. It is one of the uses of drying crops, heating homes and reheating production lines, and therefore it is fruitful [7, 8]. One drawback that accompanies these applications is the low heat transfer efficiency. Therefore, Utilization of synthetic It is rough. to Increment its efficiency is one of the applications that increment the heat transfer rate of the SAH themselves. The researchers directed the use of these applications to improve SAH. In order to enhance the functionality of SAH, there are a number of novel approaches. One of these methods is through the fins extended on the surface in a wavy shape, the second is through artificial roughness placed on the absorbent surface, and this technique is considered the most acceptable by many researchers. Garg [9] introduced the traditional SAH as a rough rectangular duct with three parts: an inlet, a test section, and an exit point. Jovini et al. [10] presented a test of hypotheses on the improvement of the Nusselt number of SP- solar air heaters, using different spring parameters coil diameter (2.5,5,7.5 ) mm, wire diameter (0.5,1,1.5) mm and step (2.5, 5,7.5) mm, where the researchers explained that the use of such roughness causes heat transmission to be more valuable, Compared to axial flow, the cross-flow pattern that is achieved is higher. Where the Nu increases by (4.67-3.11) and (18.81-5.01) for cross-flow areas, but for axial flow, the value of the Nusselt number is (3.12-5.59) and (7.51-28.39). One of the new methods mentioned is the manufacture of protrusions on the absorbent plate to improve heat transfer [11, 12]. Subsequently, SAH was developed and improved using the technique for surface roughness on absorbent materials by a number of researchers to gain access to a set of special information about artificial roughness. Review the references reached by the researchers [13, 15]. Chand et al. [16] a numerical study was presented using 2D CFD of a rectangular channel SAH with artificial roughness in the presence of vortex generators on three types: rectangular, triangle, and circular, with artificial roughness parameters of Re 3800–18000 and heat flow of 1100 W/m2. ANSYS 16 software was used to examine the variance in the Nusselt mean with the change of Re. It was found that the Nusselt rate increased with the presence of triangular vortices with little pressure loss. At Re = 3800, triangular vortices had the best thermal coefficient performance. Yadav and Sharma [17] experiment was carried out with the addition of a mixture of triangular ribs on an absorbent plate to a SAH with parameters Re = 3000-18000, e/D =0.045, and P/e =5-12.5. The study seeks in order to discover the influence of the step on both heat transmission and friction. Compared to the smooth layer, the most advanced level of thermal efficiency was attained at P/e = 10 by 2.02, the Nu was 3.05 and the coefficient of friction was 4.14. Jin et al. [18] a numerical analysis was conducted in order to investigate the properties of heat transmission and flux using various V-shaped roughness

parameters. The study determined that at the temperature, the thermal performance coefficient was found to be excellent. a P/e ratio of 10, with a value of 2.35. Luo et al. [19] a study on solar air heaters using dimple and delta winglet VGs showed a 36.23% improvement in heat transfer and a 36.29% friction factor. Tamna et al. [20] the thermal properties of a staggered V baffled absorbent plate (SAH) with artificial roughness were investigated in the study, where the lowest step, PR = 0.5, achieves optimal thermal efficienc. Fadala et al. [21] a review study showing ways to improve heat transfer using synthetic fillers for a solar air heater. Where the research dealt with aspects, including the use of artificial roughness technique after its types, as well as impact of bilateral and unilateral efficiency of the process of SAH. The inclusion of RWVG led to a thermal enhancement (TEF) in their research. Zhang et al. [22] researchers found that a new VG, placed inside a helical channel, enhances heat transmission with minimal pressure loss, highlighting the importance of VG installation in enhancing heat transfer. Gupta et al. [23] according to the research, punched RWVG greatly enhanced heat transfer in fin-andtube heat exchangers, which frequently led to a reduction in pressure. Li et al. [24] the study investigated the effects of increasing pressure drop on the pumping power and heat transfer in a fin-and-tube heat exchanger, hence improving its thermal-hydraulic efficiency. Promvonge et al. [25] researchers looked at the thermo-hydraulic efficiency of two different configurations of vortex generators—one with rectangular blades and one with delta winglets—and found that the V-shaped delta winglet VG was marginally more effective. According to studies, solar air heating (SAH) can have better thermal performance if its surface is artificially roughened with ribs. A number of earlier investigations investigated the impact of height in relation to the roughness by altering the side form to a circle, rectangle, triangle, semicircle, or NACA shape. But this research intends to close that knowledge gap by looking at how these variables affect the flow and heat transfer properties of the collector. The study explores the possibility of using arc-shaped ribs that can reach a height of 1.2 cm to improve the heat efficiency of a SAH, even when the coefficient of friction increases, resulting in more pumping power.

Numerical Modeling

Geometrical Modeling and Mesh A SP-SAH with artificial roughness was modeled in three dimensions using SolidWorks 2021 software. The resulting data was converted to ANSYS DESIGN MODELER 17 for investigation of the heat transfer rate. Figure 1 shows a demonstration of the SAH duct. The channel is separated into three portions based on its length. L1=600 mm entry, L2=1200 mm test, L3=300 mm exit, W=300mm, and H=30mm high. Step p = 5–20 cm has four

values, and the angles of attack are 30o, 45o, and 60o. In addition, Re=3000-15000 with 4 values The aluminum absorbent plate featured in Table 1 has a thickness of 0.5 mm, an arc of artificial roughness, and a heat flux of 1000 W/m2. Test Independent of Infrastructure During a quantitative investigation, obtaining an accurate the outcome of Nu and for the proposed geometric configuration is vital, and ANSYS ICEM CFD V.17 is used for meshing - grid element distribution. Performing a mesh grid-independent test to validate the accuracy of forecasting Nu and values yields an appropriate computation analysis outcome. Variable element sizes [four distinct values situated between 0.4 mm to 0.1 mm] were utilized to ascertain the ideal element cell size for the current operation. Table 2 outlines the validation outcomes. Figure 2 depicts the mesh

Table 1. Details of SAH using synthetic roughness units in a controlled environment Geometrical parameters

Figure1. (a) The geometry in ANSYS design modeler (b) Schematic view of VGs arc staggered (c) Computational domain.

Figure 2. (a) Elements size of rough surface with arc shaped (b) The process of creating a computational mesh for different geometric shapes at p = 10 cm and α=60o.

Table 2. Grid-independent review S. No Size of the elements (mm)

formed for the VG arc shape with a discrete pitching of p = 10 cm and an attack angle of 60 degrees. According to the proposed element size, a nonuniform triangular grid is generated using a as well as convergent and fine mesh configurations engagement between 4864379 and 9859884 element cells. The largest Nu divergence was found in the 9859884-element cell, whereas the average Nu variation showed the least deviation at +2.1%. Consequently, the 9859884 element cell is utilized for analytical research. Equation of Governance Method of CFD involve utilizing numerical methods to resolve equations that ensure the conservation of mass, momentum, and energy. These solutions are obtained using the realizable (RNG) and k-e models that are most effective when addressing eddy flows. To accurately describe the heat transfer process close to a wall with a steady heat flow, an enhanced wall manipulation is employed. Solving these equations with algorithms and numerical approaches. Below is a concise overview of the governing equations in three dimensions:

2. Equation of momentum:

(2) Where the Reynolds stress tensor through the Boussinesq approximation as:

4. For the RNG k-ε model, the transport equation [27]:

In the equation, the term of turbulence kinetic energy generation is due to mean velocity gradient it defines as

Where µeff The term “effective turbulent viscosity” refers to the calculated value of the turbulent viscosity. (9)

Data Reduction A computational fluid dynamics (CFD) analysis of the Nu, f, and TPF rates is carried out in the publication. The following equation can be used to figure out the ave. Nu for the roughness of the artificial surface in solar air heaters:

Where µt turbulent viscosity is combining k and ε as following: (10) In addition, buoyancy and Gb respectively. G1ε = 0.09, G3ε = 1.92, In addition, G2ε = 1.44 that constant respectively and ak = 1.39 aε = 1.39 [28].

Boundary Conditions To achieve the current geometry, The boundary conditions were implemented. At the beginning of the entrance, the temperature was 300 K and the velocity was constant. The Reynolds number is computed using Equation (12). The heat flow on the surface of the duct is equal to (1000 W/m2). As shown in Table 3. In addition, the air pressure at the exit remains constant at 1.013*105 Pa, the standard atmospheric pressure as show in Table 4 properties of physical thermos hydraulic. The following is how the border condition is applied: i. At entry point in the X-direction

(12) The average f of a synthetic roughness SAH is found by [29]: (13) Where ΔP is the difference in pressure between the test portion and the rest of the heater. An instrument for gauging the enhancement of heat transmission in a SAH with artificial roughness relative to a smooth channel is the thermal performance factor [29]:

Table 4. The experimental setup and numerical simulation make use of the thermo-physical features of the material Properties

Findings And Analysis

Numerical Validation This was done by comparing the corresponding rates found by correlation equations in rectangular channel smooth plates with the numerical results of the RNG, perturbation model for smooth stream quantities. Figure 3 shows the Nu and f as a function of Re, based on a comparison with the corresponding rates and numerical data for the uniform duct. The study’s convergence was satisfactory

Heat Transfer The impact of roughness pitch, angle of attack, Nusselt number, friction factor, and thermal enhancement factor on absorbent plates are investigated in this study using a staggered arc arrangement. The enhancement of heat transmission with the Nu and Nu ratio in Figure 4 and 5 Illustrates the heat transfer through forced convection in a SAH, as indicated by the Nu. The figure shows a discrepancy

between the Nu and the Re. It is observed that when fixing e/ H=0.271. As Re rises, so does the value of Nu. at the roughness parameters p/H=1.667-6.667 with an angle of attack of 30o, 45o, and 60o. In addition, this is due to the increase in the Re That is, the turbulence increases and leads to the

formation of the ribs, breaking the thickness of the adjacent shear layer and in the direction of flow, a decrease in thermal resistance results in an increase in the amount of heat transfer, shown by the Nu. It should be noted that as p/H grows, Nu decreases because the regions in the higher p/H

Figure 4. The Nu fluctuation in response to changes in Re, p/H, and angle of attack values.

values are few because the number of edges is small. It is noticed that when the values of the angle of attack increase, The flow becomes separated when a roughness element is present, leading to less heat transmission and the Nu worth increasing. Nu the highest possible value was 215 and Nu ratio maximum value was 7.21 at a pH of 3.33, an angle of 60°, and a Re of 15000. An increase in Re leads to an elevation in the kinetic energy of the turbulent flow., which in turn causes a greater rate of turbulent dissipation. This, in turn, causes the turbulence to become more intense and causes Nu to grow. Through Figure 6, it is possible to observe the kinetic energy contour diagrams in order to acquire a deeper comprehension of the activity of heat transfer. TKE is plotted for p/H=3.33 and angles of attack of 30o, 45o, and 60o with different rates of Re. Maximum TKE values are obtained

near the top of the staggered arc for a 60° angle of attack, where TKE decreases with increasing distance from the wall. An increase in TKE was observed up to a maximum value of (1.071, 5.112, and 7.481) m2/s2 for Re = 3500, 9000, and 15000, respectively. A high TDR region is observed at the beginning and end of the staggered arc. The obstruction of fluid flow leads to the release of TDR caused by eddy generators. However, the higher TDR value in the current is higher because of the second flow, which is caused by the generation of eddies. When TKE increases with Re, Nu increases. Figure 7 presents the contour of the velocity vector in understanding the flow behavior along the YZ axis. To show the number of eddies generated at the angle of attack of 60o, and the step p/H = 3.33 and fixed e/H = 0.271 at x/L=0.60 begin of ribs and at x/L=0.64 end of ribs shows the best

Figure 6. TKE contour for SAH at (a) angle of attack 30o (b) angle of attack 45o (c) angle of attack 60o.

evaluation of the number of eddies generated that results in an augmentation of heat transfer. The rest of the figures (b & c) show the formation of vortices and their comparison with the angle of attack of 60o at Re=15000. Figure 8 shows study investigates the influence of the assault angle on an arc-shaped staggered configuration of the absorber plate in a (SAH). The study demonstrates that the rib forms at specific relative pitch intervals and heights result in increased circulation in the flow direction. A robust vortex is formed in close proximity to the first rib Due to the acute angle at the apex and the additional inclination. The flow direction causes intense turbulence in close proximity to the wall surface, outcomes in an increased rate of heat transfer. The arc form with a 60-degree angle exhibits more pronounced representations compared to other arc shapes, suggesting the fact that a Nu rises as the Re increases, owing to enhanced fluid recirculation in the flow direction. The impact that the angle of attack and the p/H ratio have on the SAH absorbent plate along the YZ axis is understood. And also, when Re = 15000, with an angle of attack of 60, a strong vortex was formed in the wall of the first rib because it was affected by the angle of attack at followed by a secondary vortex, which is located at the tip of the rib. As a result, this resulted in severe disturbances near the surface of the wall and thus a high rate of heat transfer. Figures 9 displaying the temperature distributions under the conditions mentioned p/H = 3.33, e/H=0.271, different case of angle of attack, and Re = 15000 with staggered arrangement. According to Figure 9, the temperature

distributions in the arc-shaped improved duct at an angle of 60° of contact are more evenly spread out compared to the other two examples. A greater disparity in temperature in the increased duct adjacent to the absorber plate promotes efficient thermal exchange between the absorber layer and the working fluid. Furthermore, the temperature distribution in the vicinity of the VGs alternates between warm and cold fluids, which promotes efficient heat transfer between the fluids. The current SAH improves heat transmission efficiency by rerouting cool fluid in the core flow, which cleans the heated absorber wall and lowers its temperature. Frication Factor The use of the VG staggered arc increases the amount of turbulence that is produced, disrupting the layer of viscous fluid near the surface. This disruption leads to a reduction in the frictional resistance, as the Re increases. The friction factor values of the rough layer are greater than those of the Unruffled layer, and the turbulence intensity becomes more with the Re, Figures 10 and 11 display the coefficient of friction values and the ratio of friction factor with the change of Re, with different parameters from p/H=1.6676.667 and the angle of attack 30o, 45o, and 60o. Where it reached its highest value at p/H=3.33. The values of f and f/ fs start decreasing due to the process of separating the flow that occurs under the rib and the process of reconnecting the shear layer at a lower p/H. When the values of the angle of attack increase, A rise in an angle of attack As a result of a growth in the coefficient of friction all the way up to its peak value at the angle of attack 60o and Commences retreat

Figure 8. Contour of velocity profile at Re=15000 for (1) angle of attack 30o (2) angle of attack 45o(3) angle of attack 60o (4) Smooth duct.

The rib experiences the smallest angle of attack because of its large arc angle value, and a portion of it remains unaffected by the impediment. Where the highest value of f =

Conclusion

The ANSYS Fluent program was used to predict the efficiency of a SAH with arc-shaped ribs using a three-dimensional numerical model. The results were evaluated using streamlines, the average Nusselt number, the average friction coefficient, and the thermal-hydraulic performance parameter. A CFD investigation was conducted using the RNG k-ε turbulence model with different Reynolds numbers, p/H, angle of attack, and α. The study found significant findings:

1. This study’s CFD analytical results were confirmed by

the correct experimental outcomes anticipated by the RNG-k, which was validated for a smooth duct.

2. Nusselt number increase with Re. At p/H = 3.33 and

an angle of 60° at Re = 15000, and e/H=0.271 the most important factors of Nu and Nu ratio were 215 and

3. The f reduces as the Re augment. Where the highest

value of f = 2.5 and f/fs = 3.39 was obtained at p/H =3.33 and e/H =0.271 at an angle of attack of 60o.

4. It found arc staggered VGs at p/H = 3.33 and an angle

of 60° at Re = 15000 provide maximum thermal performance factor 3.88 for augmentation heat transfer.

Data Availability Statement

The authors confirm that the data that supports the findings of this study are available within the article. Raw data that support the finding of this study are available from the corresponding author, upon reasonable request.

Conflict Of Interest

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethics

There are no ethical issues with the publication of this manuscript.

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