This study conducts both numerical and empirical assessments of thermal transfer and fluid flow characteristics in a Solar Air Collector (SAC) using a Delta Wing Vortex Generator (DWVG), and the effects of different height ratios (R_{h} = 0.6, 0.8, 1, 1.2 and 1.4) in delta wing vortex generators, which were not considered in the earlier studies, are investigated. Energy and exergy analyses are performed to gain maximum efficiency. The Reynolds number based on the outlet velocity and hydraulic diameter falls between 4400 and 22000, corresponding to the volume flow rate of 5.21–26.07 m^{3}/h. It is observed that the delta wing vortex generators with a higher height ratio yield maximum heat transfer enhancement and overall enhancement ratio. The empirical and numerical findings demonstrate that the exergy and thermal efficiencies decline in a specific range. The Nusselt number, pressure drop, energy, and exergy efficiencies enhance with rising Reynolds number, although the friction coefficient diminishes. The maximum heat transfer enhancement is 57%. According to the evaluation of exergy efficiency, the greatest efficiency of 31.2% is obtained at R_{h} = 1.4 and Reynolds number 22000.

In regards to renewable energy, solar energy would seem the most viable due to its plentiful nature and continuous environmental impact. The solar air heater is economically feasible due to its low cost, basic design and minimal upkeep. As an energy-producing device, it absorbs solar thermal energy by gripping the surface and converting it into hot air.

Various advanced approaches to preserving solar energy for heating purposes are employed extensively to decrease the reliance on fossil fuels in the winter [

Photothermal conversion mechanisms are divided into passive and active. The flow can be heated passively without requiring any external energy source. Heating a building, drying crops, and heating water inside a tank are examples of passive methods. In active methods of converting photothermal energy, heating the flow is done by using the energy inputs of the main system. Among the most common active methods are solar collectors, pools, and power plants [

A SAC takes advantage of the sun’s energy by absorbing it through its surface and converting it into warm air. Due to the high thermal resistance between the absorber and air, it has poor thermal efficiency [

Solar air collector is used in many heating applications, such as heating the buildings’ space, drying agricultural products, timber drying, industrial applications, etc., Solar air collectors are affordable and have a wide range of applications. Solar air collectors are environmentally friendly, pollution-free, portable, financially competitive, and safe. Mainly due to the heat transfer coefficient, the displacement of the bottom of the absorber plate and the circulating flow leads to higher temperature of the plate and more heat loss. In order to improve the efficiency of solar air heaters by using different designs of flow regimes such as porosity and artificial roughness, several efforts have been made. Solar collectors are divided into two types depending on the working fluid: liquid collectors and air collectors [

There has been increasing interest in improving the thermal performance of SACs by altering the design and application of these devices [

Yu et al. [

An experimental investigation on the impact of different factors of protrusion, such as configuration, location, and height on heat transfer augmentation was looked into on the solar air heater duct by Aman et al. [

When heat transfer properties are affected by flow and velocity patterns, it is vital to study ways to improve thermal exchange between the absorber and the air flow. The creation of a boundary layer on the area of the absorber has an instantaneous effect on heat exchange, with an augmentation of the boundary layer, resulting in a diminishment in the heat transferred [

The implementation of the protrusion resulted in an increase in both exergy and thermal efficiency due to the production of greater turbulence and the elimination of the vortices created in the corners of the SAC.

Karim et al. [^{3}/h, the performance of the collector rose to a significant degree, reaching 69%.

The evaluation of the thermal efficiency of a flat plate SAC has been reported by Agathokleous et al. [

In our previous work, four collectors with different baffle arrangements have been numerically and experimentally investigated. Among them, case A was chosen as the optimum collector for the installation of the wing vortex generators on the surface of the absorber. Furthermore, the flat wing vortex generator and perforated wing vortex generator in collector type A at different pitch ratios have been examined. The use of a large attack angle of 45° improved heat transfer, energy and exergy efficiencies [

The above review has demonstrated that a wide variety of numerical and experimental research has been done to organize the utilization of longitudinal wings and winglets for heat transfer. However, there have been very few investigations into the effects of the height ratio R_{h} of wing or winglet vortex generators on SACs.

In our previous studies, such as [

The current work is a continuation of our earlier investigations. In this study, the Flat Delta Wing Vortex Generators (FDWVG) with different height ratios R_{h} are fixed on the absorber of the SAC. To improve the exergy and energy performance and thermal achievement of the SAC, the optimal results such as the wing angle of attack and baffles arrangement of our previous studies have been utilized.

The case study in this work is a rectangular SAC with the dimension of 1000 L × 800 W × 100 H (mm^{3}) which is made of a galvanized metal plate with a thickness of 1 mm as illustrated in

The radial blower (item 1) has a capacity of 0.5 KW and discharges air from the blower through the inlet pipe (item 2) with a diameter of 63.5 mm. In order to record the inlet and exit temperature, two digital Thermometers (item 3) are attached. Also, the pressure loss of the inlet and exit of the collector is determined by two VSI VM281 digital pressure meters (item 4). To detect the temperature of the surface, eighteen PT100 K-type thermocouples (item 15) are mounted at the bottom of the SAC. The heat flux radiation from the sun is measured with the TES 1333 Solar Power Meter (item 9) which is mounted at the outside of the SAC. Furthermore, the air velocity at the SAC outlet is measured using a Prova AVM-301 anemometer (item 11). The baffles with dimensions of L × H = 100 mm × 700 mm and crafted of polycarbonate with a thickness of 3 mm are installed inside the SAC. The test section is isolated by wood and glass wool, hence the amount of heat loss from channel walls, collector bottom, and surfaces of the inlet and outlet has not been significant. In addition, due to the isolation, the heat loss to the environment has been negligible.

To make the absorber surface rough, FDWVGs are glued to the absorber of the collector. A schematic of the absorber with VGs is depicted in

Row | d (cm) | b (cm) | a (cm) | R_{h} (a/d) |
---|---|---|---|---|

1 | 6.25 | 5 | 3.75 | 0.6 |

2 | 5 | 0.8 | ||

3 | 6.25 | 1 | ||

4 | 7.5 | 1.2 | ||

5 | 8.75 | 1.4 |

In the current investigation, thermal characteristics, energy and exergy efficiencies, and the Overall Enhancement Ratio (OER) in the SAC are determined. The thermal energy balance is established by contrasting air enthalpy with the heat provided from outside. The thermal energy gained by the absorber plate may be written as

In order to calculate the heat transfer coefficient, Newton’s law of cooling, expressed by

The experimental Nusselt number is determined by

The Reynolds number (Re) is defined as

The value of the friction factor is derived as

∆P expresses the pressure drop along the length of the collector and can be written as ∆P = P_{in} – P_{out}, where P_{in} and P_{out} are the inlet and outlet pressure respectively. Additionally, ρ, u, L, and D denote the density of the fluid, the velocity of the fluid, the length, and the hydraulic diameter of the collector, respectively. _{bulk} = (T_{in} + T_{out})/2 in both numerical and empirical procedures.

To determine the practical use of the augmented SAC, the performance of the SAC is estimated

The collector thermal efficiency (η_{e}) is one of the most essential factors in this investigation. It can be determined as [_{in}, while the temperature of the air at the exit of the collector is known as T_{out}. The aperture area of the collector is denoted by A_{c}, while I stands for the effective solar irradiation over the collector area. According to the second law [_{s} and T_{e}, respectively.

Exergy destruction generally occurs because of irreversibility (due to the temperature difference, or sudden expansion), within a component system, and is an internal phenomenon that can be derived as follows:

The uncertainty analysis is performed to estimate the errors in calculated parameters. Non-dimensional parameter uncertainties are ±5% for the Nusselt number, ±5 for the Reynolds number, and ±7 for friction.

Measurement device | Unit | Range | Uncertainty |
---|---|---|---|

SOLAR POWER METER TES 1333 | 2000 | ±10 | |

Anemometer PROVA AVM-301 | 0–45 | ±0.3 | |

Digital pressure meter VSI VM281 | −1–5 | % 0.5 | |

Digital thermometer PT3001 | −50–300 | 0.1 | |

PT100 thermocouple K-type | −40–250 | 0.1 | |

Thermostat and heat indicator Ecotec |
0.1 |

A numerical study is conducted in this research to compare the experimental data with the numerical results. Numerical simulations were done using ANSYS-FLUENT 13, which uses FVM to solve fluid flow equations. It is based on turbulent flow inside SAC and the

In which,

Heat transfer from radiation was simulated using the S-2-S model; which considers only surface to surface radiation. The outgoing heat flux from a specific surface s consists of both directly emitted and reflected heat. The amount of reflected heat is influenced by the incoming heat from the surrounding environment, and this can be described in relation to the heat flux departing from all other surfaces. The heat that bounces off the surface can be written as

For the investigation, the following assumptions were made:

Heat loss from channel walls, collector bottom, and surfaces of inlet and outlet not considered because of the isolation.

The absorber does not emit any long-wave radiation outside of SAC.

Numerical simulations were done using ANSYS-FLUENT 13, which uses FVM to solve fluid flow equations. It is based on turbulent flow inside SAC and the k-ϵ model is accurate for both planer and round jets. Heat transfer from radiation was simulated using the S-2-S model. A cluster of 40 face cells was established on a glass surface in order to reduce the amount of time needed for calculations and guarantee dependable precision for the simulation. The mesh density for various elements is shown in

To perform grid independence analysis, the simulations were initiated with a mesh size of 290000 cells, and the mesh resolution was refined approximately 1.3 times each time. Finally, the cases with 290000, 370000, 480000, 590000, 84000 were examined for the independency analysis. The averaged Nusselt number _{h} = 1, exhibited consistency, and the results were overlapping as mesh got finer. Therefore in order to minimize the computational costs, this particular mesh was selected for the numerical model.

An example of Boussinesque approximation, a review and some information about wall functions, their applications and optimizations can be found in [

Boundary | Type | Properties |
---|---|---|

Absorber | Wall | |

FDWVG | T = Temperature at the periphery | |

Opaque wall | ||

Glass cover | T = Temperature at the periphery | |

Baffles | Wall | Adiabatic |

Channel | Wall | Adiabatic |

Inlet | Inlet velocity | |

Exit | Pressure outlet |

Several tests were performed in order to verify the numerical results with empirical data that have been collected. Data from experiments, such as velocity and temperature were converted to non-dimensional values and compared to numerical simulation.

The performance of the SAC is evaluated numerically in this section, along with simulation results. The influence of R_{h} on the flow patterns, temperature distribution, pressure drop, and thermal and exergy efficiencies of the collector is also investigated. The R_{h} of the VGs ranges from 0.6 to 1.4 with height ratio of 0.6, 0.8, 1, 1.2, and 1.4. There were three VGs in each way. The angle of attack was set to 45°. The volume flow rate fluctuated between 5.21 to 26.07

The variation of Nu _{h} = 0.6, 0.8, 1, 1.2, and 1.4 is presented in _{h}) produces strong flow circulation and separation, which leads to higher turbulence intensity, resulting in heat transfer enhancement [

_{h} = 1.4 yields a higher pressure drop. The collector roughened with different height ratios of the vortex generators; R_{h} = 0.6, 0.8, 1, 1.2, and 1.4, supplies the average pressure drop greater than the smooth one by around 4%, 9%, 29%, 45%, and 50%, respectively.

The influence of FDWVG with five height ratios; R_{h} = 0.6, 0.8, 1.0, 1.2, and 1.4, on the friction factor

The friction factor of the collector with FDWVG is higher than the one with the smooth case. It is found that the friction factors achieved from the five height ratios have a similar tendency and lead to a decrease with a rise in mass flow rate and height ratio. In all the cases, the greatest values of the friction factor occurred at a lower mass flow rate.

The average growth of the friction factors in the roughened collector is about 8%, 18%, 30%, 43%, and 52% times higher than the smooth one with R_{h} = 0.6, 0.8, 1.0, 1.2, and 1.4, respectively. The collector with height ratio R_{h} = 1.4 yields the highest value of friction factor. The vortex generator with a bigger surface area produced a higher recirculation zone. Consequently, this can be attributed to the increased flow blockage, which leads to higher turbulence intensity, resulting in higher frictional coefficients [

_{h} = 1.4 yields a higher energy of around 31.2%. It is implied that there is more energy absorption and less heat loss.

The exergy efficiency of the collector _{h} = 1.4. The case of R_{h} = 1.2 is similar to the previous case (R_{h} = 1.4). While for the rest of cases (R_{h} = 0.6 − 1) including the smooth channel the maximum point enhancement occurs at Re = 15400. Similar to energy efficiency, the exergy efficiency increases with Re increasing for all the cases. It is also observed that the FDWVG with R_{h} = 1.4 yields a higher energy of around 37.3%.

A summary of some of the very recent investigations on the efficiency optimization of solar collectors by using passive methods is presented in

Refs. | Configuration | Thermal enhancement (%) | Year | |
---|---|---|---|---|

Xiao et al. [ |
Inclined trapezoidal vortex generators | 55 | 24.0 | 2020 |

Yassien et al. [ |
Net of tubes below the absorber | 80.2 | 6.8 | 2020 |

Akhbari et al. [ |
Triangular channel absorber | 23 | 5.0 | 2020 |

Wang et al. [ |
“S”-shaped ribs with gap | 65 | 32.4 | 2020 |

Zhao et al. [ |
Aluminum honeycomb 45% PV cover | 64 | 12.3 | 2020 |

Sari et al. [ |
Delta winglet vortex generators and baffles | 19.84 | 37 | 2020 |

Sari et al. [ |
Perforated delta wing vortex generator | 22.1 | 20.5 | 2022 |

Current study | Different delta wing height ratios | 31.2 | 54.83 | – |

To evaluate the heat transfer augmentation, the Overall Enhancement Ratio (OER) of the system at different R_{h} is analyzed in _{h} for all the cases. The overall enhancement ratio of the collector with different height ratios of R_{h} = 0.6, 0.8, 1.0, 1.2, and 1.4 are in the range of 0.90–1.01, 0.82–1.01, 0.77–1.10, 0.72–1.18 and 0.66–1.19, respectively. It was also discovered that; the cases of R_{h} larger than one, resulted in an overall enhancement ratio more than unity in the range of Reynalds numbers between 13200 and 22000.

Solar Air Collectors (SACs) are widely used nowadays in residential, commercial, and industrial applications. Many attempts have been made to optimize them and enhance their efficiency. The studies have mainly targeted a constant vortex generator height ratio, and a gap existed for the effect of the vortex generator height ratio on the efficiency of SACs. Hence, in this paper, numerical and empirical investigations were conducted to examine the influence of the vortex generator height ratio on SAC implementation. The impacts of R_{h} on thermal performance, pressure drop, friction factor, energy, and exergy efficiencies, and overall enhancement ratio have been looked into.

The VGs with a height ratio of 1.4 exhibited superior performance compared to the other VGs with a lower height ratio in terms of heat-transfer rate, friction factor, and overal enhancement ratio. The results show that the overall enhancement ratio improved as the R_{h} of the VGs increased. It is also observed that the efficiency solar air collectors increased as the mass flow rates rose, and this led to a more effective heat transfer to the airflow.

It has been seen that the energy and exergy efficiencies increased with ascending Reynolds number and height ratio.

The numerical findings were validated using experimental data. The main results can be listed as follows:

The collector roughened with different height ratios of the vortex generators; R_{h} = 0.6, 0.8, 1, 1.2 and 1.4, supplies the average pressure drop greater than the smooth one by around 4%, 9%, 29%, 45%, and 50%, respectively.

The average growth of the friction factors in the roughened collector is about 8%, 18%, 30%, 43%, and 52% times higher than the smooth one with R_{h} = 0.6, 0.8, 1.0, 1.2, and 1.4, respectively.

R_{h} = 1.4 offers a significant enhancement in Nu number. The Friction factor and pressure drop increase with R_{h}.

At higher Re, the VGs with R_{h} = 1.4 yield higher energy and exergy efficiencies which indicates higher energy absorption and lower heat loss.

FDWVG with R_{h} = 1.4 yields higher energy and exergy performance to be around 31.2% and 37.3%, respectively.

The overall enhancement ratio for all the cases is above unity for Re number ranging from 13200 to 22000, and R_{h} = 1–1.4.

Collector aperture area

Top glass cover’s specific heat capacity

Specific heat capacity of air

Hydraulic Diameter (

The rate Energy

The rate of exergy

Irreversibility or exergy destruction rate

Vortex Generator pitch ratio

Friction factor coefficient

Effective sun irradiation on the collector’s surface

Collector Length feature (

Air mass flow rate (

Nusselt number (

Nusselt number of reference collector

Nusselt number ratio (

Pressure

The universal gas constant

Reynolds number (

Height ratio

Entropy

The temperature at the periphery

The component of velocity in the corresponding direction

Feature of length

Boltzmann’s constant

Heat flux receive on the surface from the surroundings

Transmittance of the transparent cover

Thermal expansion coefficient

Rate of plate absorption

Dissipation rate of turbulent kinetic energy

Heat absorber plate emissivity

The emissivity of the top glass cover

Thermal efficiency

Exergy performance

Glass cover thermal conductivity coefficient (

Dynamic viscosity

Density (

Emissivity

Turbulent Viscosity

Turbulent Prandtl number for kinetic energy

Turbulent Prandtl number for dissipation

Environment

Flat Delta-Wing Vortex Generator

Friction Factor

Finite Volume Method

Fluid

Inlet

Overall Enhancement Ratio

Outlet

Pumping Power

Sun

Solar

SAC

Vortex Generators

The authors thank Islamic Azad University–Shahrood Branch for their facilities and cooperation to perform this study.

The authors received no specific funding for this study.

The authors confirm contribution to the paper as follows: study conception and design: Ghobad Shafiei Sabet, Ahmad Fakhari; data collection: Ali Sari, Seyed Mehran Hoseini; analysis and interpretation of results: Ghobad Shafiei Sabet, Ahmad Fakhari, Nasrin Afsarimanesh; draft manuscript preparation, English correction and grammatical revision: Dominic Organ. All authors reviewed the results and approved the final version of the manuscript.

The data and materials utilized in our research manuscript are accessible upon request from corresponding author, Ghobad Shafiei Sabet. Unreleased data is not available owing to confidentiality agreements with participants or constraints imposed by the data provider. We are dedicated to ensuring transparency and reproducibility in our research endeavors and will exert utmost diligence in granting access to the data, while adhering to ethical and regulatory limitations.

The authors declare that they have no conflicts of interest to report regarding the present study.