A Performance Evaluation of a Solar Air Heater Using Different Shaped Ribs Mounted on the Absorber Plate—A Review

In this paper, the effect of various shapes of ribs used in Solar Air Heaters (SAHs) was discussed. The review is concentrated on the geometry of the rib and its location on the SAH panel. Both numerical and experimental works were considered for discussion with dry air and Nano fluids as a working fluid. The influence of various shapes, such as an L shape, W shape, V shape, Multiple V shape, V shape with a gap, detachable & attachable ribs etc., was analyzed. The common fact observed from this analysis is that the implementation of artificial roughness in the absorber plate results in a considerable increase in the rate of heat transfer. Further, it is observed that ‘Multiple V-shaped with open between the ribs’ results in the maximum thermal enhancement when compared to the other shapes.


Introduction
Solar energy is abundantly available worldwide. Solar energy is an important alternative resource used in domestic and industrial applications like solar air heaters, solar lights, solar pumps, Photovoltaic applications, and many more, to meet the energy demand. Among these, the Solar Air Heater plays a major role in agriculture and rural areas for drying the crops, vegetables, heating the space etc. An SAH is simple to design and also economic. In the application of an SAH, it was found that using a smooth duct for the absorber plate led to a poor thermal efficiency by reason of less relative heat transfer. To increase the rate of thermal efficiency in a fluid, an artificial roughness is created closer to the surface, which results in recirculation of the fluid to create turbulence.
Implementing passive techniques in an absorber plate by using irregular forms like ribs, grooves, baffles, winglets, and twisted tapes, increases the heat transfer enhancement in solar duct [1][2][3]. Numerous authors [4][5][6][7] have conducted analytical and experimental investigations implementing passive techniques in absorber plates that resulted in heat transfer enhancement. The main goal of this review paper is to provide information about the importance of a rib molded on: (a) Different shaped 3 Rib-Groove shapes Reproduced with Permission from [3], Elsevier, 2015 Nanoparticles like Al2O3, CuO, SiO, and ZnO2 with volume fraction 1.4% taken.
Trapezoidal groove shape of rib shows the topmost heat transfer and best Nusselt number.

Experimental Investigation
Schematics of various geometries with the experimental investigation are shown in Table 2.

a. Broken Arc Rib (BAR)
Experimental and numerical simulations using the CFD technique were carried out by Gill R.S, et al. [5], and a broken arc-shaped rib collective with a staggered rib shape of a piece with the ratio of 12 was taken for analysis. Rib raggedness that was immovable with a staggered rib position of 0.4, roughness height of 0.043, gap size of 1.0, arc angle of 30°, and gap location of 0.65 with a roughness pitch of 10mm was considered. The staggered rib proportions were nominated from 1 to 6 with the Reynolds number ranging from 2000 to 16,000. The result shows that the comparison of the broken arc rib with a smooth channel revealed that the thermo-hydraulic performance of the duct attained the best performance and found that the highest value of the staggered rib size was 4.

b. Reverse L Shape Rib (RLSR)
Vipin B, et al. [6] numerically and experimentally investigated the reverse L shape rib with a relatively rough surface, with pitch varying from 7.14 to 17.86, the Re no ranging from 3800 to 18,000, and heat flux of 1000 W/m 2 taken for analysis, and the relative roughness height of e/D 0.042 was selected for the investigation. Here, the analytical results were related to observational results 3 Rib-Groove shapes Reproduced with Permission from [3], Elsevier, 2015 Nanoparticles like Al 2 O 3 , CuO, SiO, and ZnO 2 with volume fraction 1.4% taken.
Trapezoidal groove shape of rib shows the topmost heat transfer and best Nusselt number.  3 Rib-Groove shapes Reproduced with Permission from [3], Elsevier, 2015 Nanoparticles like Al2O3, CuO, SiO, and ZnO2 with volume fraction 1.4% taken.
Trapezoidal groove shape of rib shows the topmost heat transfer and best Nusselt number.

Experimental Investigation
Schematics of various geometries with the experimental investigation are shown in Table 2.

a. Broken Arc Rib (BAR)
Experimental and numerical simulations using the CFD technique were carried out by Gill R.S, et al. [5], and a broken arc-shaped rib collective with a staggered rib shape of a piece with the ratio of 12 was taken for analysis. Rib raggedness that was immovable with a staggered rib position of 0.4, roughness height of 0.043, gap size of 1.0, arc angle of 30°, and gap location of 0.65 with a roughness pitch of 10mm was considered. The staggered rib proportions were nominated from 1 to 6 with the Reynolds number ranging from 2000 to 16,000. The result shows that the comparison of the broken arc rib with a smooth channel revealed that the thermo-hydraulic performance of the duct attained the best performance and found that the highest value of the staggered rib size was 4.

b. Reverse L Shape Rib (RLSR)
Vipin B, et al. [6] numerically and experimentally investigated the reverse L shape rib with a relatively rough surface, with pitch varying from 7.14 to 17.86, the Re no ranging from 3800 to 18,000, and heat flux of 1000 W/m 2 taken for analysis, and the relative roughness height of e/D 0.042 was selected for the investigation. Here, the analytical results were related to observational results

Experimental Investigation
Schematics of various geometries with the experimental investigation are shown in Table 2.

a. Broken Arc Rib (BAR)
Experimental and numerical simulations using the CFD technique were carried out by Gill R.S, et al. [5], and a broken arc-shaped rib collective with a staggered rib shape of a piece with the ratio of 12 was taken for analysis. Rib raggedness that was immovable with a staggered rib position of 0.4, roughness height of 0.043, gap size of 1.0, arc angle of 30 • , and gap location of 0.65 with a roughness pitch of 10mm was considered. The staggered rib proportions were nominated from 1 to 6 with the Reynolds number ranging from 2000 to 16,000. The result shows that the comparison of the broken arc rib with a smooth channel revealed that the thermo-hydraulic performance of the duct attained the best performance and found that the highest value of the staggered rib size was 4.

b. Reverse L Shape Rib (RLSR)
Vipin B, et al. [6] numerically and experimentally investigated the reverse L shape rib with a relatively rough surface, with pitch varying from 7.14 to 17.86, the Re no ranging from 3800 to 18,000, and heat flux of 1000 W/m 2 taken for analysis, and the relative roughness height of e/D 0.042 was selected for the investigation. Here, the analytical results were related to observational results and found that the best achievement occurred for the range of parameters investigated. The authors revealed that thermal improvement in the Nusselt number was 2.9 times better than the normal duct.

c. Arc Shape Ribs Set in 'S' Shape (SR)
Experimental research was conducted by Khushmeet Kumar, et al. [7] with circular wire arc shape ribs, which were placed in an 'S' shape formation in the duct within an arc angle varying from (α) of 30 • to 75 • with a relative roughness pitch (P/e) 4 to 16mm and relative roughness width (W/w) of 1 to 4mm in the range of Re no 2400 to 20,000. Implementing this "S"-shaped rib results in a higher augmentation in the friction factor (f) and better Nusselt number (Nu), and it was found that the best thermal enhancement was achieved at an arc angle (α) of 60 • .

d. W-Shaped Rib (WSR)
AtulLanjewar, et al. [8] experimentally investigated thermal performances and friction factor (f) with novel 'W'-shaped ribs placed in a rectangular duct on its underneath on one side of the wall prepared at a 60 • inclination admiration to the fluid flow direction. The study considered a duct hydraulic diameter ratio equal to 8.0, a relative roughness height (e/Dh) fixed in the range from 0.018 to 0.03375, a relative roughness pitch (p/e) of the rib equal to 10, and an attack angle from 30 • to 75 • . The Reynolds number used ranged from 2300 to 14,000. They declared that the 'W' shape of the rib at a 60 • attack angle gave the best thermal performance and friction factor when related with the smooth duct.

e. Different Shape-Rib (DV-R)
Giovanni Tanda [9] conducted an experimental investigation of various shapes of rib-like (a) transverse continuous ribs, (b) angled continuous ribs, and used other novel shapes of (c) broken ribs and (d) discrete V-shaped ribs. The author considered that repeated ribs in a duct promote an effective performance to enhance the friction factor and thermal coefficients. The study experimentally examined a rectangular channel with uniform heat flux at one wall roughened by repeated ribs and the remaining three walls were fixed as smooth and insulated. The result revealed that all the implementing ribs roughened in channels achieved better agreement than the smooth duct.

f. Triangular-Rib with Triangular-Groove (TR)
Experimental research conducted by Smith Eiamsa-ard, et al. [10] using three types of rib-groove arrangement with the parameters of W/H equal to 20, height of the duct H equal to 9 mm, rib height set as e equal to 3 mm, and three different pitch ratios P/e of 6.6, 10, and 13.3. The results support the idea that using a Triangular-Rib with a Triangular-Groove-shaped rib shows that thermal enhancement obtained the highest values for all pitch ratios at a constant pumping power.

g. Protruded Roughness Geometry (PR)
BrijBhushan and Ranjit Singh [11] conducted an experimental investigation for better enhancement of the Nusselt number and friction factor using a protruded smoothness novel geometry in the duct. They compared the result with a smooth duct. The investigation revealed that the maximum augmentation in the heat transfer coefficient was found for an (S/e) way length of 31.25 and detailed values are shown in ( h. Transverse Ribs on One, Two, Three, and Four walls (TR) P.R. Chandra, et al. [12] experimentally investigated a heat transfer rib in all four walls, one by one, with the Re no ranging from 10,000 to 80,000. The pitch in the direction of the rib height ratio P/e was set as 8 and height of the rib in the direction of the channel hydraulic diameter ratio e/Dh was 0.0625. The result shows that the heat transfer roughness, G(e), augmented with roughness.  Reverse l shape rib increases thermal performance.

3
Arc shape wire ribs arranged in 'S' shape adapted from [7] Heat transfer augmentation and friction factor relative to arc shape significance 0.6667.
A survey was taken in an arc shape so they used a circular wire arc S shape rib for investigation.   Reverse l shape rib increases thermal performance.

3
Arc shape wire ribs arranged in 'S' shape adapted from [7] Heat transfer augmentation and friction factor relative to arc shape significance 0.6667.
A survey was taken in an arc shape so they used a circular wire arc S shape rib for investigation.  Best relative performance to the non-artificial channel is attained by the novel shape of transverse broken ribs. 6 Triangular-Rib with Triangular-Groove The aspect ratio of W/H Triangular-rib with 3 Arc shape wire ribs arranged in 'S' shape adapted from [7] Heat transfer augmentation and friction factor relative to arc shape significance 0.6667.
A survey was taken in an arc shape so they used a circular wire arc S shape rib for investigation.  Reverse l shape rib increases thermal performance.

3
Arc shape wire ribs arranged in 'S' shape adapted from [7] Heat transfer augmentation and friction factor relative to arc shape significance 0.6667.
A survey was taken in an arc shape so they used a circular wire arc S shape rib for investigation.   Reverse l shape rib increases thermal performance.

3
Arc shape wire ribs arranged in 'S' shape adapted from [7] Heat transfer augmentation and friction factor relative to arc shape significance 0.6667.
A survey was taken in an arc shape so they used a circular wire arc S shape rib for investigation.   Reverse l shape rib increases thermal performance.

3
Arc shape wire ribs arranged in 'S' shape adapted from [7] Heat transfer augmentation and friction factor relative to arc shape significance 0.6667.
A survey was taken in an arc shape so they used a circular wire arc S shape rib for investigation.

Numerical Investigation
Schematics of various pitch distances with the numerical investigation are shown in Table 3.

a. Hyperbolic Rib (HR)
Deep Singh Thakur, et al. [13] carried out a simulation analysis using FLUENT 15.0 for the hyperbolic rib with various parameters, including a pitch distance of 10-20 mm, a roughness height of the rib ranging from e 0.5 mm to 2 mm, and a Reynolds number of 10,000. By comparing the above parameters of HR with various shaped ribs, such as triangular, rectangular, and another novel shape like semicircular rib geometries, it was shown that the optimum performance of the hyperbolic rib is attained when e is equal to 1 mm and P is equal to 10 mm at the Re no of 6000. In this case, the Hyperbolic shape of a rib avoids the frame of small eddies in the corners at upstream and downstream sides where the friction factor is not high at rib height from 0.5.mm to 1.5 mm and the heat transfer is enhanced at a Reynolds number of 6000.

b. Transverse Square Rib (TSR)
Anil Singh Yadav and Bhagoria J.L. [14] simulated a CFD code for a repeated transverse square sectioned rib. The authors examined 12 dissimilar position of a square sectioned rib with the following parameters: relative roughness height ranging from (e/D) 0.021 to 0.042, relative roughness pitch (P/e) ranging from 7.14 to 35.71, and Re no ranging from 3800 to 18,000. They concluded that Re 12, 000 produced a good thermo-hydraulic performance (THPP) equal to 1.88 and the best rib roughness shows an (e/D) value of 0.042 and (P/e) equivalent to 10.71.

c. Discrete Multi V-Rib Through Staggered Rib
Anil Kumar and Man-Hoe Kim. [15] examined the influence of the roughness width ratio by the CFD technique by using the RNG-k-e model and they analyzed the typical friction factor, Nusselt number, and overall thermal performance. The result of this case shows that the discrete multi V-rib using a staggered rib shape is 6% advanced when associated with further rib shapes. Anil Kumar and Man-Hoe Kim [15] found that the highest significance of the relative width ratio was 6.0.

d. Square Ducts with Internal Surfaces Ribbed
Simulation work carried out by Kamali R. and Binesh A.R. [16] for various profiles of the rib included triangular and square, a newly arranged method of trapezoidal with diminishing height in the flow direction, and another arrangement of trapezoidal with augmentative height in the flow direction. The results reveal that the presence of rib scattering of the heat transfer coefficient is sturdily pretentious by the incidences of the rib profile and it shows that the best heat transfer augmentation occurred in the trapezoidal rib with diminishing height in the downstream flow direction.

e. Broken V-Ribs (BV-R)
A numerical investigation was carried out by Promthaisong Pitak, et al. [17] in the 3D horizontal square channel with a broken V-rib, which was compared with and without a V-rib in the duct using open corner d/H values of 0, 0.01, 0.02, 0.03, 0.04, and 0.05. Promthaisong Pitak reported that using a broken V-Rib results in higher heat transfer and friction loss.

f. Textured Asymmetric Arc Rib (TAR)
H.T. Wang, et al. [18] carried out a simulation in a single phase channel with a 2D interior surface with the Re no ranging from 20,000 to 60,000. The optimized symmetric triangular rib was taken for comparison with smooth flow and found that certain developments are closely correlated to the rise in fluid flow and improve the performance of thermal efficiency. Transverse square rib adapted from [14] Thermal-hydraulic performance factor varies between 1.22 and 1.88.
Presence of an Inner square section of the rib will produce better thermal performance.
Discrete Multi V-Rib with Staggered Rib Adapted from [15] The overall thermal performance will be Staggered induct high velocity with turbulence in Transverse square rib adapted from [14] Thermal-hydraulic performance factor varies between 1.22 and 1.88.
Presence of an Inner square section of the rib will produce better thermal performance. of small eddies in the corners, heat transfer enhancement will increase, and a decrease of rib height causes reduction of friction factor.
Friction factor 2 Transverse square rib adapted from [14] Thermal-hydraulic performance factor varies between 1.22 and 1.88.
Presence of an Inner square section of the rib will produce better thermal performance. 3 Discrete Multi V-Rib with Staggered Rib Adapted from [15] The overall thermal performance will be 3.67 with a width ratio of 3.0.
Staggered induct high velocity with turbulence in Discrete multiple v rib increases heat  The overall thermal performance will be 3.67 with a width ratio of 3.0.
Staggered induct high velocity with turbulence in Discrete multiple v rib increases heat transfer coefficient.
decrease of rib height causes reduction of friction factor.
Friction factor 2 Transverse square rib adapted from [14] Thermal-hydraulic performance factor varies between 1.22 and 1.88.
Presence of an Inner square section of the rib will produce better thermal performance. 3 Discrete Multi V-Rib with Staggered Rib Adapted from [15] The overall thermal performance will be 3.67 with a width ratio of 3.0.
Staggered induct high velocity with turbulence in Discrete multiple v rib increases heat transfer coefficient.

4
Square Ducts with Internal Surfaces Ribbed Reproduced with Permission from [16], Elsevier, 2008 The P/e ratio of 12 provides the highest augmentation.
Inter rib distribution heat transfer strongly affects the shape of rib and recirculation just behind rib will be sensitive.

6
Textured Asymmetric Arc Rib Adapted from [18] p/e = 5 for the compound rib with d/e = 0.06 and p/e = 6 for triangular rib and asymmetric arc rib.
Progressive compound rib could progress the recital of heat transfer.

4
Square Ducts with Internal Surfaces Ribbed Reproduced with Permission from [16], The P/e ratio of 12 provides the highest augmentation.
Inter rib distribution heat transfer strongly affects the shape of rib and recirculation just behind rib will be sensitive.
decrease of rib height causes reduction of friction factor.
Friction factor 2 Transverse square rib adapted from [14] Thermal-hydraulic performance factor varies between 1.22 and 1.88.
Presence of an Inner square section of the rib will produce better thermal performance. 3 Discrete Multi V-Rib with Staggered Rib Adapted from [15] The overall thermal performance will be 3.67 with a width ratio of 3.0.
Staggered induct high velocity with turbulence in Discrete multiple v rib increases heat transfer coefficient.

4
Square Ducts with Internal Surfaces Ribbed Reproduced with Permission from [16], Elsevier, 2008 The P/e ratio of 12 provides the highest augmentation.
Inter rib distribution heat transfer strongly affects the shape of rib and recirculation just behind rib will be sensitive.

6
Textured Asymmetric Arc Rib Adapted from [18] p/e = 5 for the compound rib with d/e = 0.06 and p/e = 6 for triangular rib and asymmetric arc rib.
Progressive compound rib could progress the recital of heat transfer.
height causes reduction of friction factor.
Friction factor 2 Transverse square rib adapted from [14] Thermal-hydraulic performance factor varies between 1.22 and 1.88.
Presence of an Inner square section of the rib will produce better thermal performance. 3 Discrete Multi V-Rib with Staggered Rib Adapted from [15] The overall thermal performance will be 3.67 with a width ratio of 3.0.
Staggered induct high velocity with turbulence in Discrete multiple v rib increases heat transfer coefficient.

4
Square Ducts with Internal Surfaces Ribbed Reproduced with Permission from [16], Elsevier, 2008 The P/e ratio of 12 provides the highest augmentation.
Inter rib distribution heat transfer strongly affects the shape of rib and recirculation just behind rib will be sensitive.

6
Textured Asymmetric Arc Rib Adapted from [18] p/e = 5 for the compound rib with d/e = 0.06 and p/e = 6 for triangular rib and asymmetric arc rib.
Progressive compound rib could progress the recital of heat transfer.

6
Textured Asymmetric Arc Rib Adapted from [18] p/e = 5 for the compound rib with d/e = 0.06 and p/e = 6 for triangular rib and asymmetric arc rib.
Progressive compound rib could progress the recital of heat transfer.
height causes reduction of friction factor.
Friction factor 2 Transverse square rib adapted from [14] Thermal-hydraulic performance factor varies between 1.22 and 1.88.
Presence of an Inner square section of the rib will produce better thermal performance. 3 Discrete Multi V-Rib with Staggered Rib Adapted from [15] The overall thermal performance will be 3.67 with a width ratio of 3.0.
Staggered induct high velocity with turbulence in Discrete multiple v rib increases heat transfer coefficient.

4
Square Ducts with Internal Surfaces Ribbed Reproduced with Permission from [16], Elsevier, 2008 The P/e ratio of 12 provides the highest augmentation.
Inter rib distribution heat transfer strongly affects the shape of rib and recirculation just behind rib will be sensitive.

6
Textured Asymmetric Arc Rib Adapted from [18] p/e = 5 for the compound rib with d/e = 0.06 and p/e = 6 for triangular rib and asymmetric arc rib.
Progressive compound rib could progress the recital of heat transfer.

Experimental Investigation
Schematics of various pitch distances with the experimental investigation are shown in Table 4.

a. Integral Chamfered Rib (ICR)
Rajendra Karwa, et al. [19] experimentally investigated ICR with relative roughness pitch from 4.58 to 7.09 and the rib chamfer angle fixed at 15 • . Airflow in a duct with depth values of 21.8 mm, 21.5 mm, and 16 mm was tested with relative roughness heights for the three roughness plates values of 0.0197, 0.0256, and 0.0441, respectively. The Reynolds number varied from 3750 to 16,350. The investigational study shows that the use of an integral chamfered rib significantly enriched the thermal efficiency from 10% to 40% in SAH.

b. Effect of a Gap in the Inclined Rib
An experimental investigation work by Aharwal K.R., et al. [20] on the rectangular duct with a ratio of 5.83 investigated the effect of an opening in the inclined shape of a rib. The authors declared that the inclined rib allowing for a gap (inclined discrete rib) shows that enhanced heat transfer is related to a continuous inclined rib arrangement. The result shows that a 1.0 relative gap width in an inclined discrete rib produced good heat transfer when related to the further relative opening width.

c. Combined Wavy-Rib and Groove Turbulators
SompolSkullong, et al. [21] carried out an experimental work on Combined Wavy-Rib and Groove turbulators with the Reynolds number ranging from 4000 to 21,000. To bring forth recirculation flow in the duct, triangular wavy ribs were used to repeatedly place it in a duct with heat-flux realistic on the upper wall Three tests were carried out with dissimilar rib pitch to duct height ratios and the PR set as 0.5; in addition, P/H was set as 2. A single rib was placed in the channel height ratio of b/H equal to 0.25 with three different types of rib preparations, namely (a) rib-groove combination placed on upper wall only, (b) inline methods rib-groove arrangement, and (c) combination of staggered and inline rib groove on opposite sides of the walls. The wavy ribs were situated at a 45 • attack angle inflow direction of the fluid. The authors declared that the highest thermal efficiency occurred at the upper wall of the ribbed-grooved specimen at PR equal to 0.5 and the combined rib-groove turbulator exhibited the over achievement of heat energy compared to the groove alone.

d. Staggered-Winglet Perforated Tapes (S-WTR)
Sompol Skullong, et al. [22] experimentally investigated the staggered-winglet perforated tapes with the Reynolds Number ranging from 4180 to 26,000. Winglet perforated tapes had a winglet inclination angle of 30 • with winglet blockage ratios B from 0.1 to 0.3.The highest thermal enhancement factor TEF equal to 1.71 was attained by applying the WPT with B RR equal to 0.15, P equal to 1.0, Re equal to 4180.The result reveals that the staggered-winglet perforated tapes with winglet perforated tapes yield a thermal enhancement factor that is about 1.2 times higher than the winglet non-perforated tape.

e. Pairs of Trapezoidal-Winglets Groove and TW (T-WGR)
Sompol Skullong, et al. [23] discerned an experimental investigation with a pair of trapezoidal winglets and groove shape of the rib, with the Re no ranging from 4500 to 22,000 and the single angle of attack of 45 • . Experimental outcomes show that the Trapezoidal-Winglets together with the Groove produce the maximum heat transfer and pressure drop increase in heat transfer over the smooth channel.

f. Continuous Rib Turbulator (CR)
Mohammed O.A. et al [24] carried out an experimental investigation using solid inclined, curved and vertical baffles for swirls flow underside of the absorber plate. Investigators declared that the ribs in the cavity area that gradual increase of the Nusselt number and also a solid curved baffles placed in absorber plate shows best performance.

g. Thin Ribs (TR)
Experimental and numerical research work was done by Sanjay K. Sharma, et al. [25] on the width to height W/H of 10 for the duct consuming obstacle ratio e/H of 0.1, with additional parameters like roughness height e/D of 0.055, roughness pitch P/e of 10, attack angle of 90 • ,and Reynolds number from 4000 to 16,000. The investigators claimed three important outcomes: (A) Rib arrangement 1 elucidated that midribs positioned at 3.3% and 6.67% truncation from the side walls provide the maximum thermal enhancement. (B) Arrangement 3 elucidated that middle ribs permanent at 5% truncation from side walls provide the better performances in terms of THPP results. (C) Arrangement 4 elucidated that with two transverse continuous ribs in between the truncated ribs, there was an indication of the maximum pressure drop.

Numerical Investigation
Schematics of the various angles of ribs with the numerical investigation are shown in Table 5.

Numerical Investigation
Schematics of the various angles of ribs with the numerical investigation are shown in Table 5.

a. 45 • Inclined V Shape Rib (IVSR)
Ahmed M., et al. [26] conducted numerical simulations for a duct using six different rib configurations. Inline and staggered arrangement of the ribs is fixed in the top and bottom walls of the channel with an angle of 45 • inclined and 45 • V shape. All these shapes of ribs are associated with the 90 • transverse ribs. Ahmed M. et al. [26] declared that 45 • V-shaped ribs enhance the performance, inducing a strong rotation and increasing the heat transfer in the cross-sectional passage compared with 90 • transverse ribs.

b. Discrete V-Down Rib (DV-DR)
Numerical investigations were carried out by Sukhmeet Singh, et al. [27] with a discrete V-down artificial raggedness rib compared with flat plate SAH to increase the pumping power flow rate. The limitations used were relative roughness pitch, position, width, height, and the angle of attack, which have a combined effect on heat transfer and fluid friction. The report shows that roughness parameters of the discrete V-down shape of rib for an assumed Reynolds number show the highest energetic efficiency.

c. Slit Rib (SR)
PankajParihar, et al. [28] numerically investigated heat transfer enhancement and pressure drop characteristics in a rectangular duct using slit ribs. The hydraulic diameter of the duct to rib height ratio was fixed as 0.094, whereas the rib pitch varied from 6 to just before 12 and the Reynolds number ranged from 10, 000 to 50,000. Results attained the highest Nusselt number at an angle of α = 65 • when the Reynolds number was equal to 50,000 at a pitch ratio p/e of 12, and the lowest friction factor was obtained at the irregularity angle of 85 • with Re equal to 50,000.

d. Nine Different Rib Shapes Compared with Two Turbulent Models
Simulation work was done by Alok Chaube, et al. [29] with nine different shapes of rib with Shear Stress Transport (SST), and the k-ω turbulence model used in the analysis also compared heat transfer enrichment and the friction factor in the range of Re 3000 to 20,000. They reveal that a rectangular rib size of 5 ×3 mm resulted in the best heat transfer coefficient befalls at reattachment of the separated flow.

e. Forward Chamfered Ribbed (FCR)
Rajnesh Kumar, et al. [30] numerically studied a forward chamfered ribbed parameter of (e/w) 0.24 to 1.5 with relative roughness height varying from (e/D) 0.018 to 0.043 and e /e ranging from 0 to 1.0. They declared that their result showed that the maximum Nusselt number increase occurred in the case of (e/w) 1.5, (e/D) 0.043, and (e /e) value 0.75.  The e/w, relative 10 roughness height (e/D), and e′/e ranges from 0.24 to 1.5, 0.018 to 0.043, and 0 to 1.0, respectively. Different values of e/D, the Nu enhances, and friction factor is maximum in case of e/D value of 0.043.

Experimental Investigation
Schematics of the various angles of ribs with the numerical investigation are shown in Table 6.

a. Arc-Shaped Wire Rib (ASR)
Saini S.K. and Saini R.P. [31] conducted an experimental investigation for enhancing turbulences by using an arc-shaped wire roughness element with relative roughness height (e/D) and arc angle (a/90), and the (Re) varied from 2000 to 17,000. The author reveals moral agreement achieved between experimental measures and predicted values of Nu no and friction factor declared in SAH.  The e/w, relative 10 roughness height (e/D), and e′/e ranges from 0.24 to 1.5, 0.018 to 0.043, and 0 to 1.0, respectively. Different values of e/D, the Nu enhances, and friction factor is maximum in case of e/D value of 0.043.

Experimental Investigation
Schematics of the various angles of ribs with the numerical investigation are shown in Table 6.
SST turbulence model used to compare the different shapes for turbulence models.
Turbulence intensity is initiate maximum at peak of the local heat transfer. Heat transfer is initiate at the reattachment points.  The e/w, relative 10 roughness height (e/D), and e′/e ranges from 0.24 to 1.5, 0.018 to 0.043, and 0 to 1.0, respectively. Different values of e/D, the Nu enhances, and friction factor is maximum in case of e/D value of 0.043.

Experimental Investigation
Schematics of the various angles of ribs with the numerical investigation are shown in Table 6.

a. Arc-Shaped Wire Rib (ASR)
Saini S.K. and Saini R.P. [31] conducted an experimental investigation for enhancing turbulences by using an arc-shaped wire roughness element with relative roughness height (e/D) and arc angle (a/90), and the (Re) varied from 2000 to 17,000. The author reveals moral agreement achieved between experimental measures and predicted values of Nu no and friction factor declared in SAH.

Experimental Investigation
Schematics of the various angles of ribs with the numerical investigation are shown in Table 6.

a. Arc-Shaped Wire Rib (ASR)
Saini S.K. and Saini R.P. [31] conducted an experimental investigation for enhancing turbulences by using an arc-shaped wire roughness element with relative roughness height (e/D) and arc angle (a/90), and the (Re) varied from 2000 to 17,000. The author reveals moral agreement achieved between experimental measures and predicted values of Nu no and friction factor declared in SAH.

b. Combination of Inclined and Transverse Ribs (CI&TR)
Varun, et al. [32,33] experimentally looked into a combination of inclined and transfer ribs in a duct with the Re no (Re) ranging from 2000 to 14,000, relative roughness pitch varying from (p/e) 3 to 8, and relative roughness height set as (e=D) 0.030. The result declared that the relative roughness pitch of 8 has the highest thermal efficiency. Varun et al. [33] investigated the same parameter using the Taguchi method to optimize the thermal efficiency and revealed that this method can be effectively used for envisaging the solar air heater performance.

b. Combination of Inclined and Transverse Ribs (CI&TR)
Varun, et al. [32,33] experimentally looked into a combination of inclined and transfer ribs in a duct with the Re no (Re) ranging from 2000 to 14,000, relative roughness pitch varying from (p/e) 3 to 8, and relative roughness height set as (e=D) 0.030. The result declared that the relative roughness pitch of 8 has the highest thermal efficiency. Varun et al. [33] investigated the same parameter using the Taguchi method to optimize the thermal efficiency and revealed that this method can be effectively used for envisaging the solar air heater performance.

Numerical Investigation
Schematics of multiple ribs with the numerical investigation are shown in Table 7.

a. Multi V Shape Rib (MVSR)
Anil Kumar [34] and Dongxu Jin et al. [35] both conducted CFD analysis of the Nusselt number and friction factor. Anil Kumar [34] used a different shape of artificial smoothness in the impressive of the thin circular wire in various shapes, namely (a) V-shaped, then arranged in (b) Multi V-shaped, and introduced another novel (c) Multi V-shaped rib with gap geometries for diverse turbulent models to investigate the heat transfer and pressure drop. The author choose Renormalization k-epsilon models for their analysis. Maximum heat transfer and pressure drop occurred in the novel Multi V-shaped ribs with a gap, which were compared with further shapes like V-shaped ribs and achieved good agreement. Dongxu Jin et al. [35] examined that effect of the span wise V-rib number in connection with a variable parameter such as attack angle, relative pitch, and relative height. They concluded that the maximum thermal performance was achieved at an attack angle of 60 o , which provides the maximum value of the friction factor, and the authors revealed that using multi V-shaped ribs in a duct causes more stream wise helical vortex flow.

b. Non-Uniform Cross-Section in the Form of Saw-Tooth
A three-dimensional CFD investigation was carried out by Sukhmeet Singh, et al. [36]. They conducted a numerical experiment with the Re number ranging from 3000 to 15,000 in the transverse 2 Combination of Inclined and Transverse Ribs Reproduced with Permission from [33], Elsevier, 2009 Relative roughness pitch (p/e) 3-8 and relative roughness height (e = D) 0.030.

1.
Relative roughness pitch of 8 has the highest thermal efficiency.

2.
Taguchi method used for optimal thermal efficiency.

b. Combination of Inclined and Transverse Ribs (CI&TR)
Varun, et al. [32,33] experimentally looked into a combination of inclined and transfer ribs in a duct with the Re no (Re) ranging from 2000 to 14,000, relative roughness pitch varying from (p/e) 3 to 8, and relative roughness height set as (e=D) 0.030. The result declared that the relative roughness pitch of 8 has the highest thermal efficiency. Varun et al. [33] investigated the same parameter using the Taguchi method to optimize the thermal efficiency and revealed that this method can be effectively used for envisaging the solar air heater performance.

Numerical Investigation
Schematics of multiple ribs with the numerical investigation are shown in Table 7.

a. Multi V Shape Rib (MVSR)
Anil Kumar [34] and Dongxu Jin et al. [35] both conducted CFD analysis of the Nusselt number and friction factor. Anil Kumar [34] used a different shape of artificial smoothness in the impressive of the thin circular wire in various shapes, namely (a) V-shaped, then arranged in (b) Multi V-shaped, and introduced another novel (c) Multi V-shaped rib with gap geometries for diverse turbulent models to investigate the heat transfer and pressure drop. The author choose Renormalization k-epsilon models for their analysis. Maximum heat transfer and pressure drop occurred in the novel Multi V-shaped ribs with a gap, which were compared with further shapes like V-shaped ribs and achieved good agreement. Dongxu Jin et al. [35] examined that effect of the span wise V-rib number in connection with a variable parameter such as attack angle, relative pitch, and relative height. They concluded that the maximum thermal performance was achieved at an attack angle of 60 o , which provides the maximum value of the friction factor, and the authors revealed that using multi V-shaped ribs in a duct causes more stream wise helical vortex flow.

b. Non-Uniform Cross-Section in the Form of Saw-Tooth
A three-dimensional CFD investigation was carried out by Sukhmeet Singh, et al. [36]. They conducted a numerical experiment with the Re number ranging from 3000 to 15,000 in the transverse

Numerical Investigation
Schematics of multiple ribs with the numerical investigation are shown in Table 7.

a. Multi V Shape Rib (MVSR)
Anil Kumar [34] and Dongxu Jin et al. [35] both conducted CFD analysis of the Nusselt number and friction factor. Anil Kumar [34] used a different shape of artificial smoothness in the impressive of the thin circular wire in various shapes, namely (a) V-shaped, then arranged in (b) Multi V-shaped, and introduced another novel (c) Multi V-shaped rib with gap geometries for diverse turbulent models to investigate the heat transfer and pressure drop. The author choose Renormalization k-epsilon models for their analysis. Maximum heat transfer and pressure drop occurred in the novel Multi V-shaped ribs with a gap, which were compared with further shapes like V-shaped ribs and achieved good agreement. Dongxu Jin et al. [35] examined that effect of the span wise V-rib number in connection with a variable parameter such as attack angle, relative pitch, and relative height. They concluded that the maximum thermal performance was achieved at an attack angle of 60 o , which provides the maximum value of the friction factor, and the authors revealed that using multi V-shaped ribs in a duct causes more stream wise helical vortex flow.

b. Non-Uniform Cross-Section in the Form of Saw-Tooth
A three-dimensional CFD investigation was carried out by Sukhmeet Singh, et al. [36]. They conducted a numerical experiment with the Re number ranging from 3000 to 15,000 in the transverse rib with non-uniform ribs like a square, trapezoidal, and circular rib. The k-ε turbulence model solution method was taken for the simulation investigation. They found that the maximum Nusselt number values occurred for the trapezoidal rib and a low friction factor occurred for the saw teeth rib.

c. Circular Transverse Wire Rib (CTWR)
Anil Singh Yadav and Bhagoria J.L. [37] extended their work in the numerical investigation for another shape of circular transverse wire rib by varying the Re no with relative roughness pitch (P/e) and relative roughness height (e/D) by using Renormalization-group (RNG) in the k-ε model. They showed that average Nusselt number and average friction factor values escalated with rib height; however, this value decreased with pitch distance. The maximum thermal enhancement factor established was 1.65 greater than the smooth duct.

d. Conical Protrusion (CPR)
TabishAlam, et al. [38] conducted a numerical study using a conical protrusion rib and investigated the effect of relative ribs pitch p/e 6 to 12 and relative ribs height e/D 0.020 to 0.044 in the range of Re 4000 to 16,000. The result found that a maximum thermal efficiency of 69.8% was achieved and the efficiency enhancement factor (EEF) was 1.346.

e. Discrete Double-Inclined Rib (DDR)
Ying Shuang Wang, et al. [39] completed an analytical work on the rectangular micro channel with a Discrete Double-Inclined rib. This double-inclined rib shape enhances heat transfer with the increasing height of the rib and number of double-inclined ribs, but the fluid flow resistance also increases at a similar time.

f. Downstream Ribs Arrangement
An analytical study was conducted by GongnanXie, et al. [40] in downstream ribs with six big continuous ribs mounted on one side of the wall with a pitch distances ratio of P/e equal to 20. It was initially designed in (Case A). Four cases were further designed, introducing half-size and same-size ribs downstream of the big ribs (Case B-E, respectively), shown in (Table 7, S. No 6). They declared that the downstream ribs are the best for decreasing the pressure loss and progressing the flow structure. rib with non-uniform ribs like a square, trapezoidal, and circular rib. The k-ε turbulence model solution method was taken for the simulation investigation. They found that the maximum Nusselt number values occurred for the trapezoidal rib and a low friction factor occurred for the saw teeth rib.

c. Circular Transverse Wire Rib (CTWR)
Anil Singh Yadav and Bhagoria J.L. [37] extended their work in the numerical investigation for another shape of circular transverse wire rib by varying the Re no with relative roughness pitch (P/e) and relative roughness height (e/D) by using Renormalization-group (RNG) in the k-ε model. They showed that average Nusselt number and average friction factor values escalated with rib height; however, this value decreased with pitch distance. The maximum thermal enhancement factor established was 1.65 greater than the smooth duct.

d. Conical Protrusion (CPR)
TabishAlam, et al. [38] conducted a numerical study using a conical protrusion rib and investigated the effect of relative ribs pitch p/e 6 to 12 and relative ribs height e/D 0.020 to 0.044 in the range of Re 4000 to 16,000. The result found that a maximum thermal efficiency of 69.8% was achieved and the efficiency enhancement factor (EEF) was 1.346.

e. Discrete Double-Inclined Rib (DDR)
Ying Shuang Wang, et al. [39] completed an analytical work on the rectangular micro channel with a Discrete Double-Inclined rib. This double-inclined rib shape enhances heat transfer with the increasing height of the rib and number of double-inclined ribs, but the fluid flow resistance also increases at a similar time.

f. Downstream Ribs Arrangement
An analytical study was conducted by GongnanXie, et al. [40] in downstream ribs with six big continuous ribs mounted on one side of the wall with a pitch distances ratio of P/e equal to 20. It was initially designed in (Case A). Four cases were further designed, introducing half-size and same-size ribs downstream of the big ribs (Case B-E, respectively), shown in (Table 7, S. No 6). They declared that the downstream ribs are the best for decreasing the pressure loss and progressing the flow structure. rib with non-uniform ribs like a square, trapezoidal, and circular rib. The k-ε turbulence model solution method was taken for the simulation investigation. They found that the maximum Nusselt number values occurred for the trapezoidal rib and a low friction factor occurred for the saw teeth rib.

c. Circular Transverse Wire Rib (CTWR)
Anil Singh Yadav and Bhagoria J.L. [37] extended their work in the numerical investigation for another shape of circular transverse wire rib by varying the Re no with relative roughness pitch (P/e) and relative roughness height (e/D) by using Renormalization-group (RNG) in the k-ε model. They showed that average Nusselt number and average friction factor values escalated with rib height; however, this value decreased with pitch distance. The maximum thermal enhancement factor established was 1.65 greater than the smooth duct.

d. Conical Protrusion (CPR)
TabishAlam, et al. [38] conducted a numerical study using a conical protrusion rib and investigated the effect of relative ribs pitch p/e 6 to 12 and relative ribs height e/D 0.020 to 0.044 in the range of Re 4000 to 16,000. The result found that a maximum thermal efficiency of 69.8% was achieved and the efficiency enhancement factor (EEF) was 1.346.

e. Discrete Double-Inclined Rib (DDR)
Ying Shuang Wang, et al. [39] completed an analytical work on the rectangular micro channel with a Discrete Double-Inclined rib. This double-inclined rib shape enhances heat transfer with the increasing height of the rib and number of double-inclined ribs, but the fluid flow resistance also increases at a similar time.

f. Downstream Ribs Arrangement
An analytical study was conducted by GongnanXie, et al. [40] in downstream ribs with six big continuous ribs mounted on one side of the wall with a pitch distances ratio of P/e equal to 20. It was initially designed in (Case A). Four cases were further designed, introducing half-size and same-size ribs downstream of the big ribs (Case B-E, respectively), shown in (Table 7, S. No 6). They declared that the downstream ribs are the best for decreasing the pressure loss and progressing the flow structure. Average friction factor originates in 0.00317 for relative height of 0.042 and relative pitch of 7.14.

Experimental Investigation
Schematics of multiple ribs with the experimental investigation are shown in Table 8.

Experimental Investigation
Schematics of multiple ribs with the experimental investigation are shown in Table 8.

a. Flow-Attack-Angle in V-Down Rib (AV-D R)
6 Downstream Ribs Arrangement Reproduced with Permission from [40], Elsevier, 2013 Different cases designed by introducing half-size and same-size ribs downstream of the big ribs.
It found downstream ribs decrease the pressure loss and increase the flow structure.
3 relative pitch of 7.14 and relative height of 0.042.
wire rib with parameter of P/e = 10.71 and e/D = 0.042 affords better thermal enhancement.
Average friction factor originates in 0.00317 for relative height of 0.042 and relative pitch of 7.14. 4 Conical Protrusion Reproduced with Permission from [38], Elsevier, 2017

Experimental Investigation
Schematics of multiple ribs with the experimental investigation are shown in Table 8.

Experimental Investigation
Schematics of multiple ribs with the experimental investigation are shown in Table 8.

a. Flow-Attack-Angle in V-Down Rib (AV-D R)
Sukhmeet Singh, et al. [41] conducted experimental work for five rib roughened plates with a flow-attack-angle (a) of 30 • to 75 • . The authors tested a duct with the aspect ratio AR of 12, with the relative roughness height set as 0.043 and relative roughness pitch as 8, with ranges of Re no from 3000 to 15,000. The thermo-hydraulic performance parameter power (g), friction factor (f), and Nusselt number (Nu) were considered for analysis. The report found that the highest values show an angle of 60 • and are related with the continuous V-down rib for the same rib-roughness parameters.

b. Discrete Multi V-shaped and Staggered Ribs
Ravi Kant Ravi and Saini R.P. [42] experimentally analyzed the discrete multi V-shaped and staggered ribs with the Re no ranging from 2000 to 20,000 and the relative staggered rib size ranging from 1 to 2.5. It was shown that implementing the rib on every side of the plate in double pass mode results in higher heat transfer. Rajaseenivasan T. et al. [43] compared circular turbulators with V-type turbulators in inline and staggered methods for both experimental and theoretical work, and the ambient temperature reached an upper value of 66 • C in type-f, shown in (Table 8), with the flow velocity of 57.7 kg/h. The Nu no accelerated with the Re no and reached the maximum of 210 for type-f turbulators at a Re no of 11,615. The report stated that using a Zigzag arrangement of a circular type of turbulator produces the best thermal enhancement factor.

d. Criss-Cross Pattern Formed by 45 • Angled Rib
Prashant Singh, et al. [44] experimentally and numerically conducted a study using an inline and staggered Criss-Cross inclined rib with the Re no ranging from 30,000 to 60,000. The result declared by the author showed that best Nusselt number in a duct was achieved in the ranges of 2.7 and 3.1 for inline and staggered methods of the rib and that the best thermal hydraulic performance achieved between 1.2 and 1.5.  3000 to 15,000. The thermo-hydraulic performance parameter power (g), friction factor (f), and Nusselt number (Nu) were considered for analysis. The report found that the highest values show an angle of 60° and are related with the continuous V-down rib for the same rib-roughness parameters.

b. Discrete Multi V-shaped and Staggered Ribs
Ravi Kant Ravi and Saini R.P. [42] experimentally analyzed the discrete multi V-shaped and staggered ribs with the Re no ranging from 2000 to 20,000 and the relative staggered rib size ranging from 1 to 2.5. It was shown that implementing the rib on every side of the plate in double pass mode results in higher heat transfer.

c. Circular and V-Type Turbulators (V-Type)
Rajaseenivasan T. et al. [43] compared circular turbulators with V-type turbulators in inline and staggered methods for both experimental and theoretical work, and the ambient temperature reached an upper value of 66 °C in type-f, shown in (Table 8), with the flow velocity of 57.7 kg/h. The Nu no accelerated with the Re no and reached the maximum of 210 for type-f turbulators at a Re no of 11,615. The report stated that using a Zigzag arrangement of a circular type of turbulator produces the best thermal enhancement factor.

d. Criss-Cross Pattern Formed by 45° Angled Rib
Prashant Singh, et al. [44] experimentally and numerically conducted a study using an inline and staggered Criss-Cross inclined rib with the Re no ranging from 30,000 to 60,000. The result declared by the author showed that best Nusselt number in a duct was achieved in the ranges of 2.7 and 3.1 for inline and staggered methods of the rib and that the best thermal hydraulic performance achieved between 1.2 and 1.5.

e. Multiple-Arc Shaped with Gaps (ARCR)
N.K. Pandey, et al. [45] experimentally examined the Multiple-arc shaped with a gap using an Re no ranging from 2100 to 21,000 with an (e/D) roughness height ranging from 0.016 to 0.044 (four values), roughness pitch (p/e) choices of 4 to 16 steps of four values, an Arc angle (∞) set from 30° to 75° steps of four values, a roughness width (W/w) from 1 to 7 steps of five values, a relative gap distance (d/x) of 0.25-0.85 steps of four values, and a relative gap width (g/e) from 0.5 to 2.0 steps of four values. The results show that the extreme augmentation in Nu no of 5.85 times and friction factor of 4.96 times was achieved in the evaluation compared to the smooth duct.

Conclusions
The aim of this study was to review various shapes of artificial roughness geometry implemented in solar air heaters. A comparison of numerical and experimental analyses of different shapes of ribs with various parameters shows that artificial roughness results in enhanced heat transfer.
The salient information is.

Conclusions
The aim of this study was to review various shapes of artificial roughness geometry implemented in solar air heaters. A comparison of numerical and experimental analyses of different shapes of ribs with various parameters shows that artificial roughness results in enhanced heat transfer.
The salient information is.

Conclusions
The aim of this study was to review various shapes of artificial roughness geometry implemented in solar air heaters. A comparison of numerical and experimental analyses of different shapes of ribs with various parameters shows that artificial roughness results in enhanced heat transfer.
The salient information is.