Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in
CFD Based Thermal Efficiency Analysis of Solar Air Heater with Smooth Plate & Perforated Plate Anjali A. Vyas1 & Dr. Dinesh shringi2 1
Assistant Professor, Department of Mechanical Engineering, Rajiv Gandhi Institute of Technology, Mumbai-400053, India. 2 Associate Professor, Department of Mechanical Engineering, MBM Engineering college Jodhpur, Rajasthan, India. Abstract: This paper presents study of computational fluid dynamics (CFD) based thermal efficiency analysis to determine heat transfer characteristics of solar air heater. Artificial roughness is used in one side wall of solar air heater absorber plate to break laminar boundary sub layer. It enhances rate of heat transfer from the absorber plate to flow of air stream. A CFD-based investigation of 3-dimensional forced convective fluid flow over solar air heater rectangular duct with smooth plate &half perforated transverse baffles plate has been performed in ANSYS FLUENT software. The system & operating parameter studied are duct width-to-height ratio of 7.77; the baffle relative roughness pitch ratio is 7.06; the relative baffle height ratio is 0.495 for the Reynolds number ranges from 3000 to 9000. The effect of roughness geometry [i.e., relative roughness height of baffle (e/h), relative roughness pitch of baffle (p/e), open area ratio (β)] on the heat transfer coefficient and Nusselt number are predicted. Solar air heater with artificial roughness experimental model’s 3-dimensional geometrical modeling made with aid of ANSYS Workbench. The simulation results were predicted by ANSYS FLUENT 14.5 solver. Validation of results compared with performed experimental work and found to be in good agreement. Over the range of study the raise in heat transfer coefficient of half perforated absorber plate in range of 7-13 W/m2K over smooth absorber plate of solar air heater for variable mass flow rate. The 15% minimum raise in thermal efficiency of roughen plate compare with smooth plate of solar air heater. Keywords: Solar air heater, CFD, Artificial roughness, Thermal efficiency, ANSYS-FLUENT.
1. Introduction This paper present computational fluid dynamics approach to determine the thermal performance of flat plate solar air heater by considering the different system and operating parameters to obtain maximum thermal performance. The report
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ascertains about thermal performance for different mass flow rates, for absorber plate, at same solar flux intensity. Several methods are used to increase rate of heat transfer and thermal efficiency. Some of these are, use of fins, electro hydrodynamic method, packed bed, use of artificial roughness on absorber plate etc. Among these the easiest and most acceptable method to enhance the thermal and Thermo-hydraulic efficiency is the creation of artificial roughness on the absorber plate of solar air heater. It is observed that thermal resistance to the heat transfer is due to the formation of laminar sub-layer on absorber plate. Roughness elements have been used to improve the convective heat transfer by creating turbulence in the flow. However, it would result in an increase in friction losses and hence, greater power requirement by fan or blower. (Karmare,cfd.2010) There are different factors affecting the solar collector efficiency, e.g. collector length, collector depth, type of absorber plate, glass cover plate, wind speed, etc. Increasing the absorber area or fluid flow heat-transfer area will increase the heat transfer to the flowing air (Chaaneet al., 2013a-e). B. K. Maheshwari et al. [1] Roughness elements in the form of ribs (small height projections), baffles (thin elements of greater heights) or blocks (the thick elements) have been employed to enhance heat transfer in gas turbine blade cooling channels and solar air heaters. Chaube et al. [2] conducted two dimensional CFD-based analysis of an artificially roughened solar air heater having ten different ribs shapes, namely, rectangular, square, chamfered, triangular, and so forth, provided on the absorber plate. CFD code, FLUENT 6.1 and SST - turbulence model were used to simulate turbulent airflow. The best performance was found with rectangular rib of size 3 × 5 mm, and CFD simulation results were found to be in good agreement with existing experimental results. Experimental studies of evaluation performance of various design of solar air heating system with specific value adding parameter has been conducted more as compare to computational and numerical analysis of such system, due to complicated flow simulation methodology and
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Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in computational development in hardware in Indian industry. The major advantage of computational numerical simulation work areas follows; it can specifically determine extremely large volume of results at virtually without additional expense along with parametric analysis. It can quantify the parameter which is difficult by expensive experimental testing work along with valuable time. S. Kumar et al. [3] performed three-dimensional CFD-based analysis of an artificially roughened solar air heater having arc shaped artificial roughness on the absorber plate. FLUENT 6.3.26 commercial CFD code and Renormalization group (RNG) -đ?œ€turbulence model were employed to simulate the fluid flow and heat transfer. Overall enhancement ratio with a maximum value of 1.7 was obtained, and results of the simulation were successfully validated with experimental results. S. Karmare et al. [4] carried out CFD investigation of an artificially roughened solar air heater having metal grit ribs as roughness elements on the absorber plate. Commercial CFD code FLUENT 6.2.16 and Standard -đ?œ€turbulence were employed in the simulation. Authors reported that the absorber plate of square cross-section rib with 580angle of attack was thermos-hydraulically more efficient. B. Gandhi et al. [5] employed wedgeshaped ribs roughness in their simulation works. Simulation of artificially roughened solar air heater by using FLUENT showed reasonably good agreement with the experimental observations except for the friction factor. A. Yadav et al. [6] employed triangular shaped rib roughness on the absorber plate to predict heat transfer behavior of an artificially roughened solar air heater by adopting CFD approach. ANSYS FLUENT 12.1 and RNG -đ?œ€turbulence model were employed in their simulation.From1.4 to 2.7 times enhancement in the Nusselt number was observed as compared to smooth solar air heater. A. Yadav et al. [7] carried out CFD investigation of an artificially roughened solar air heater having circular transverse wire rib roughness on the absorber plate. A two-dimensional CFD simulation was performed using ANSYS FLUENT 12.1 code as a solver with RNG -đ?œ€turbulence model. The maximum value of thermal enhancement factor was reported to be 1.65 for the range of parameters investigated .
2. EXPERIMENTAL SECTION 2.1. Collector analysis The performance of solar air heater depends on the optical and thermal properties of the collector system. This studied contains a computational fluid dynamics simulation background used to characterize the collector thermal efficiency. The
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insulated box reduces heat losses from the back and sides of the collector (Duffie and Beckman, 1991). An experimental test facility has been designed and fabricated in accordance of ASHRAE Standard (1977) for testing of solar collectors [], to study the effect of perforated baffle on the heat transfer and fluid flow characteristics of air flow in the rectangular duct. A schematic diagram of the experiment setup is shown in figure 1&2.The solar sir heater system consist of an entrance section, test section, exit section, mixing section, a flow measuring orifice meter and a centrifugal blower with control valve. The duct of 300mm wide made of smooth faced plywood and wooden boards. The rectangular duct has internal dimension of
Figure 1 Solar air heater with perforated baffle 2870 X 300 X 38.4mm which include the entrance section, test section and exit section of length 650mm, 1620mm and 600mm, respectively as shown in figure 1. The test section of one of the parallel ducts carries 3.25mm thick aluminum plate in 12 equal width pieces as absorber plate with twelve aluminum sections 0.9mm thick installed as baffles. Each baffle consists of one row of holes as shown in Figure 2. The sun facing sides of both the absorber plates are smooth and blackened. Glass plates of 4mm thickness have been used as cover over the absorber plates at a height of 60 mm.
Figure 2 Perforated Absorber plate
3. Governing equation and data reduction 3.1 Geometrical and Mesh domain: The solar air heater duct is divided into three sections, namely, adiabatic entrance section of 650mm long, test
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Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in section of 1620 mm long and exit sections of 600mm. The internal cross section of the duct is 298.5Ă—38.4 mm2. The longitudinal section of the duct is shown in figure 1. The test section carries 3.25-mm-thick aluminum plate in 12 pieces each 135 mm long to reduce the effect of axial conduction. One aluminum L sections 0.9 mm thick have been inserted between the first and second, and so on for twelve pieces of the aluminum plate, as shown in the figure 1, to serve as baffles in the test section of the duct. The height (e) of the each baffle is 19 mm while the duct height (H) is fixed at 38.4 mm. The width (b) of the baffles is about 2 mm less than width (W) of the duct. The surfaces of the aluminum plate and baffles were highly smooth. Open area ratio of the perforated baffles, β is 26%.The open area ratio (β) of the perforated baffles is the ratio of the area of the perforations to the baffle frontal area given by = n / d^ /be where ‘n’ is the number of the holes drilled through a baffle and d is the diameter of the hole. Reynolds number range of 3000 – 9000, Duct aspect ratio (W/H) 7.77, Baffle height-to-duct height ratio (e/H) 0.495, Baffle thickness-to-height ratio (tb/e) 0.047, relative roughness pitch of baffle (p/e) 7.06. Unstructured computational quad grid with tetrahedron element structure generated in ANSYS 14.5 is used for numerical simulation. The grid made in fine sizing mode with medium smoothing in fluid domain. Dense fine size of mesh generated near the abrupt change in area. A grid for SAH with perforated plate of 196000 cells and for SAH with smooth plate of 323556 cellsis adopted for analysis after a checking of Nusselt number and Temperature values.
Figure 3bMesh Model of SAH with perforated plate 3.2 Governing Equation and Boundary condition: Computational fluid dynamics governing equations are used to investigate the interactive motion of large number of individual particles inside the fluid domain. Which defines a various parameters e.g. velocity, pressure, Temperature at individual point inside fluid domain. Governing equations are non –linear partial differential equations, which governs the flow of fluid and derived from basic physical laws. The principle governing equations used are as follows: Continuity equation:
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Meshing model of illustrates in figure 3a and 3b are for smooth plate and perforated plate SAH model respectively. Figure 3a and 3b elaborate the difference in dense meshing in test section region develops due to roughness on absorber plate.
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Figure 3a Mesh model of SAH with smooth plate
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Three-Dimensional computational domain of solar air heater with perforated baffle duct is divided into inlet, outlet, symmetry, plate and wall boundaries. The working fluid air is used and assumed to be
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Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in incompressible for operating range of duct with minimal variation. The material of the absorber plate is taken as aluminum. Thermo-physical properties are given in Table 1 for working fluid air and absorber plate material (aluminum). Mass flow inlet boundary condition consider at inlet section. The upper wall in the test section is considered as an absorber plate and assigned a heat flux of 735 W/m2.The walls are assigned no-slip boundary condition and are adiabatic. The working fluid (Air) enters at a temperature of 310K in the beginning. At the outlet, pressure outlet boundary condition is applied with a static pressure value of 1.013 x 105 N/m2. Table 1.Thermo-physical properties of working fluid & absorber plate Parameter Fluid – Air Absorber plateAluminum Specific heat Cp 1006.43 871.00 in J kg/K Thermal 0.0242 202.40 Conductivity k in W/mK Density Ď in 1.225 2719.00 kg/m3 Viscosity Îź in 1.7894 x105 2 N/m Fluid flow Turbulence model and Solution Method: In FLUENT solver, 3D steady RNG − k − Îľ turbulence model is taken for analysis since it gives results very close to the experimental results as reported by researchers [10, 11]. The modeled turbulent kinetic energy k , and its rate of dissipation Îľ , is obtained from the following transport equations for Renormalization-group (RNG) − đ?œ€, model: ∂
ku =
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Pressure velocity coupling equation is done with semi-implicit method for pressure linked equations (SIMPLE) algorithm and Second-order upwind scheme is used as discretization scheme for all the transport equations. The first order numerical scheme should be adequate for good results. But we considered second order accuracy for better prediction of pressure drop, momentum equation. 3.3 Data reduction: The heat transfer coefficient between air and absorber plate h = {Q/ A T m − T∞ } (11) Where, Heat transfer rate to air Q = ṁ C T − T , Heat transfer area A = W Ă— L. The bulk mean air temperature is T∞ . The mean temperature of absorber plate T m has been calculated as area weighted average mean value of plate. The Nusselt number calculated based on the heat transfer coefficient as: Nu = h D /k (12) The Reynolds number calculated based on the mass flow rate m as: Re = G D /Îź (13) Thermal efficiency (Éł) of solar air heater is the ratio of useful heat gain đ?‘„ and incident solar radiation (I) on the collector plate area; that is, đ?‘„
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4. Results and discussion
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The turbulent viscosity đ?œ‡ is computed by combing − đ?œ€ as follows:
= đ?œ‡+đ?œ‡,
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Collector efficiency performance CFD simulations were conducted for rectangular solar air heater with perforated baffles and smooth absorber plate in ANSYS FLUENT 14.5. The results of perforated plate simulation are presented and compared with smooth plate. The enhancement of thermal performance for variable flow rate is discussed follows. 4.1 Heat Transfer: Thermal behavior of SAH is predicted by heat transfer coefficient and Nusselt
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Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in number plate against the variable Reynolds number. Heat transfer coefficient and Nusselt number increase with increase in Reynolds number of perforated plate compare to smooth plate shown in figure 4 and 5. This augments the overall heat transfer profile of solar air heater with perforated plate system due to increase in heat transfer area.
Figure 6 Reynolds Number V/s Nusselt number plot
Figure 4 Reynolds Number V/s Heat transfer coefficient plot
4.3 Friction factor: The effect of roughness on flow friction factor in the range of Reynolds number investigated. figure 7 shows that the friction factor decreases with increas in reynolds number due to suppression of laminar sublayer for fully developed turbulent flow in duct. It is observed that the maximum average friction factor 0.58 occurred of rough plate at 2725 reynolds number.
Figure 5 Reynolds Number V/s Nusselt number plot
Figure 7 Reynolds Number V/s Friction Factor plot
4.2 Thermal efficiency: It can be seen in figure 6 that thermal efficiency raises with increase in Reynolds number, which is due to increase in heat transfer coefficient at the increased air velocity of perforated plate compare to smooth plate in solar air heater duct.
4.4 Thermal performance: The ratio of nussselt number of rough plate to the smooth plate against Reynolds number is shown in figure 8. It is seen that nusselt number ratio increases with increase in Reynolds number.
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Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in Figure 10b Temperature profilecontour plot of SAH with perforated plate 4.6 Velocity profile: Air velocity passing across the duct is increased in roughness region of perforated plate compare to smooth plate shown in figure 11a and 11b respectively. Due to turbulence created in rough region air velocity increases and which enhances the heat transfer coefficient.
Figure 8 Reynolds Number V/s Nusselt Number Ratio plot Figure 11a Velocity profile contour plot of SAH with smooth plate
Figure 9 Reynolds Number V/s Friction Factor Ratio plot 4.5 Temperature profile: the figure 10a and 10b illustrate the thermal layer inside the duct of perforated and smooth plate SAH model in same scale.
Figure 11b Velocity profile contour plot of SAH with perforated plate 4.7 Turbulence profile: Heat transfer phenomena can be comprehended by the contour plot of kinetic energy based on Reynolds number. As Reynolds number increases turbulent kinetic energy and turbulent dissipation rate also increases, which will increases turbulent intensity of the fluid flow across the roughened duct. The maximum value of turbulent kinetic energy is observed near the absorber plate on the downstream side of the perforation hole in attached baffle of absorber plate. It can be illustrated in figure 12a and 12b.
Figure 10aTemperature profile contour plot of SAH with smooth plate
Figure 12a Turbulence profile contour plot of SAH with smooth plate
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Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in Figure 14a Wall Heat transfer coefficient profile contour plot of smooth plate
Figure 12b Turbulence profile contour plot of SAH with rough plate 4.8 Temperature profile of Absorber plate: Figure 13a and 13b represent the heat contains collected by absorber plate through solar isolations. The character of role player of perforated baffle signifies in figure 13b as the absorbed heat dissipated maximum level to flowing fluid inside the duct.
Figure 13a Temperature profile contour plot of smooth absorber plate
Figure 13b Temperature profile contour plot of perforated absorber plate 4.9 Wall Heat transfer coefficient profile: The figure 14a and 14b illustrate wall heat transfer coefficient at contacting surface area of fluid with wall. It exhibits the enhancement of wall heat transfer coefficient in perforated plate area compare with smooth plate of solar air heater.
Figure 14b Wall Heat transfer coefficient profile contour plot of perforated plate
5. Conclusion This paper presents the Three-dimensional CFD investigation of Thermal efficiency, heat transfer and fluid flow in a rectangular duct with perforated plate and smooth plate of solar air heater has been tested for the same Reynolds number range of 3000-11000. The major findings of the study are: 1. Computational Fluid Dynamics (CFD) method is used in the present work for thermal efficiency analyzing with artificial roughness under consideration. The CFD results give the good agreements with experimental results. Hence, CFD techniques can be used for analyzing complex type of solar air heater system with roughness element. 2. Thermal efficiency enhancement of solar air heater due to use of perforated plate in place of smooth plate. The % raise of thermal efficiency of SAH with perforated plate is about 24-35% compare to SAH with smooth plate for Reynolds number range of simulation. 3. Heat transfer coefficient improvement of perforated plate is 2.234 times of smooth plate for the operating range of Reynolds number. It has been found heat transfer coefficient of rough plate in range of 13-35 W/m2K and smooth plate in range of 7-16 W/m2K. 4. Nusselt number enhancement of roughen absorber plate with increase in Reynolds number due to increase in heat transfer surface area. It gives Nusselt number of rough plate in range of 3998 W/m2K and smooth plate in range of 20-44 W/m2K for range of simulation.
References [1] B. K. Maheshwari, R. Karwa, and N. Karwa, “Experimental study of heat transfer enhancement in an asymmetrically heated rectangular duct with perforated baffles,” International communication of Heat and Mass transfer, vol.32, pp. 275-284, 2005. [2] A. Chaube, P. K. Sahoo, and S. C. Solanki, “Analysis of heat transfer augmentation and
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Imperial Journal of Interdisciplinary Research (IJIR) Vol-3, Issue-2, 2017 ISSN: 2454-1362, http://www.onlinejournal.in flow characteristics due to rib roughness over absorber plate of a solar air heater,” Renewable Energy, vol. 31, no. 3, pp. 317– 331, 2006. [3] S. Kumar and R. P. Saini, “CFD based performance analysis of a solar air heater duct provided with artificial roughness,” Renewable Energy, vol. 34, no. 5, pp. 1285– 1291, 2009. [4] S. V. KarmareandA. N. Tikekar, “Analysis of fluid flow and heat transfer in a rib grit roughened surface solar air heater using CFD,” Solar Energy, vol. 84, no. 3, pp. 409– 417, 2010. [5] B. K. Gandhi and K. M. Singh, “Experimental and numerical investigations on flow through wedge shape rib roughened duct,” Journal of the Institution of Engineers, vol. 90, pp. 13–18, 2010. [6] A. S. Yadav and J. L. Bhagoria, “A CFD analysis of a solar air heater having triangular rib roughness on the absorber plate,” International Journal of ChemTech Research, vol. 5, no. 2, pp. 964–971, 2013. [7] A. S. Yadav and J. L. Bhagoria, “A CFD based heat transfer and fluid flow analysis of a solar air heater provided with circular transverse wire rib roughness on the absorber plate,” Energy, vol. 55, pp. 1127– 1142, 2013.
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