4th International Conference on Mechanical, Electronics and Mechatronics Engineering (ICMEME'2) Dec. -16, 2 Pattaya (Thailand) Numerical Investigation of Heat Transfer and Fluid Flow Characteristics of Roughened Solar Air Heater Duct J.L. Bhagoria, and Ajeet Kumar Giri Abstract Numerical simulation is one of finest alternative technique to predict and investigate heat transfer and fluid flow characteristics of roughened solar air heater which aims to cost effective and time saving comparative to experimentation. This numerical simulation approaches to above by using computational fluid dynamics coded software (Fluent 6.3.26 Solver. Parameters are Reynolds number ranges (3 to ) and p/e ( to 2) and k-ɛ turbulence model fitted best by comparing the predictions of various turbulence models. Enhancement in heat transfer is found in between Re-1 to 12 at and at reattachment zone between ribs. Chamfered ribs giving maximum heat transfer enhancement and giving 3 to 4 times Nusselt Number enhancement and only 1. to 1.8 times friction increasing as compared to smooth duct flow. Furthermore effects on Nusselt Number, friction factor and thermo hydraulic performance also discussed in this analysis. Keywords Solar air heater, CFD, Artificial roughness, Heat transfer, Fluid Flow. S I. INTRODUCTION OLAR air heaters, because of their simplicity are cheap and widely used as energy collection devices. Here an effort has been made to increase the heat transfer through absorber plate by using roughness. A various experimental analysis in this area have been carried but only few computational analysis and investigation have been done. The presence of rib increases heat transfer because of interruption of the viscous sub layer, which enhance flow turbulence and reattachment results to a higher heat transfer. In this work, an attempt is done to predict the velocity and temperature which is responsible for heat transfer enhancement by reattachment of flow between ribs is considerably maximum. II. DATA FORMULATION OF SOLUTION DOMAIN AND CFD ANALYSIS Solution domain of solar air duct considered was having inner cross-sectional dimensions of mm x 2mm as shown in geometry Fig. 1. The flow system consists of mm (> WH) long entry section, mm long test section and 7 mm (> 2. WH) long exit section and selected test length or plate length mm. The entry and exit length of the flow have been kept as per consideration for fully developed flow and as per recommendation provided in ASHRAE Standard 93-77. On and average constant heat flux of 8 W/m 2 is considered Duct Height(H)=2mm Duct Width (W) =mm Hydraulic mean diameter, Dh = 28. mm Duct aspect ratio, W/H =2. Length of Test Section=mm Minimum Inlet Length for fully developed flow =mm Outlet length= 7mm Rib height height, e = 2 mm Reynolds number, Re = 3- p/e range= to 2 Uniform Heat at bottom Surface=8 W/m 2 Inlet Length for fully developed flow =mm Fig. 1 shows two dimensional view of problem in CFD, since 3D model increases time and computational complexities so 2D model is selected (Fig. 2) for analysis. In the present study, FLUENT Version 6.3.2 was used for analysis. The assumptions for mathematical model while CFD analyses are i. The flow is fully developed, steady, turbulent and three dimensional. ii. The thermal conductivity is not changing with temperature. iii. The working fluid is assumed incompressible iv. This was assumed in respect of experimentation of solar air heaters by various investigators. III. GEOMETRY AND GRID INDEPENDENT TEST A mesh model is created in GAMBIT with FLUENT /6 on the rectangular face with.1 and.2 and.3 interval size according to roughness height 2mm and interval size of.2 in the vertical direction and.1 interval sizes in the horizontal direction. In creating this mesh, it is desirable to have more cells near the roughness because we want to resolve the turbulent boundary layer, which is very thin compared to the height of the flow field. J.L. Bhagoria, Prof. and head,department of Mechanical Engineering, Maulana Azad National Institute of Technology (MANIT), Bhopal-4621, India. Email id: palak_bh@rediffmail.com Ajeet Kumar Giri (R/S), Department of Mechanical Engineering, Maulana Azad National Institute of Technology (MANIT), Bhopal-4621, India. Email id: ajeetgiri8@gmail.com. 6
4th International Conference on Mechanical, Electronics and Mechatronics Engineering (ICMEME'2) Dec. -16, 2 Pattaya (Thailand) 4.1 Velocity profile for rib IV. RESULTS AND DISCUSSION Fig 4.1 shows the velocity vectors for the square shape of ribs inserted in a solar air heater duct other four cases have also the similar velocity profiles, the flow over the triangular ribs appears to be the most complex this is because the rib face is perpendicular to the flow direction so triangular shape rib should not be preferable. Fig. 1 Geometry of Solar Air Heater with square ribs in Gambit Window Boundary Types selected in GAMBIT (for Fluent /6) is given below: TABLE I Edge Position Name Type Left Duct Inlet VELOCITY_INLET Right Duct Outlet PRESSURE_OUTLET Top Top Surface WALL Bottom Inlet Length WALL S.N o. TABLE II VARIOUS GEOMETRIES CREATED FOR ANALYSIS IN GAMBIT Type of rib Geome try Numb er Pitc h (p) mm p/ e Roughnes s height (e) mm 1. 1 square 1 2 2. 2 square 2 1 2 3. 3 square 24 12 2 4. 4 square 3 2. square 36 18 2 6. 6 square 4 2 2 7. 7 semicircular 1 2 8. 8 semicircular 2 1 2 9. 9 semicircular 24 12 2 1. 1 semicircular 3 2 11. 11 semicircular 36 18 2 12. 12 semicircular 4 2 2 13 13 chamfered 1 2 14 14 chamfered 2 1 2 chamfered 24 12 2 16 16 chamfered 3 2 17 17 chamfered 36 18 2 18 18 chamfered 4 2 2 19 19 triangular 1 2 2 2 triangular 2 1 2 21 21 triangular 24 12 2 22 22 triangular 3 2 23 23 triangular 36 18 2 24 24 triangular 4 2 2 For grid independence test, the number of cells taken from,67 to 6294 in various steps and it was viewed that after 42,1 cells, further increase in cells has negligible effect on the results. Fig.2 Velocity profile for square ribs 4.2 Heat Transfer in Roughened Duct. The heat transfer and flow gets affected because of ribs in the solar air heater. Nusselt number just at the vicinity of rib has been found to be low. This may be because that heat transfer takes place at that rib area isdue to conduction only, While at point where flow reattaches Nusselt Number is quite high. The increase in Nusselt is due to the variation in flow pattern downstream of the rib. Temperature profile is shown in the figure given below. Fig. 4 Temperature profile for chamfered ribs 7
4th International Conference on Mechanical, Electronics and Mechatronics Engineering (ICMEME'2) Dec. -16, 2 Pattaya (Thailand) For semicircular rib Nusselt number Nu 4 4 3 2 1 2 4 6 8 1 12 14 16 p/e 1 p/e 2 Fig. 3 Variation in heat transfer coefficient along the Solar plate, With Re=1, I=8 W/m 2 4.3 Comparison between various ribs at different p/e CFD analysis is predicting performance of various ribs at different p/e which is shown in figure given below and it is seen that with particular range of p/e heat transfer is maximum and while shifting both sides of p/e performance decreases. To justify the heat transfer features for the investigated rib shapes, the Nusselt number ratios along the bottom wall between two successive ribs are plotted. Nusselt Number Nu 4 4 3 2 1 for square rib 1 2 Renyolds number Re for for p/e 1 for for for p/e 2 for Fig. Variation of Nusselt number with Reynolds number for Squre Ribs Fig.6 Variation of Nusselt number with Reynolds number for Semicircular Ribs Nusselt Number Nu For triangular rib 4 4 3 2 1 2 4 6 8 1 12 14 16 p/e 1 p/e 2 Fig. 9 Variation of Nusselt number with Reynolds number for Triangular Ribs Nusselt number Nu 4 3 2 1 For chamfered rib p/e 1 p/e 2 2 4 6 8 1 12 14 16 Fig.1 Variation of Nusselt number with Reynolds number for Chamfered Ribs 8
4th International Conference on Mechanical, Electronics and Mechatronics Engineering (ICMEME'2) Dec. -16, 2 Pattaya (Thailand) It is also found that Chamfered ribs showing highest heat transfer enhancement with minimum friction. Analysis also shows that the lowest heat transfer rate is seen in case of duct provided with semicircular ribs. 4.4 Comparison of average heat transfer and friction characteristics With (P/e =12) Reynolds number range, the trapezoidalshaped ribs have the highest friction loss; whereas, the trapezoidal-shaped ribs have the lowest pressure drop. Furthermore, the triangular-shaped ribs have higher friction factor than that of square-shaped ribs, based on the law of the wall similarity Tanda[12]. developed semi empirical formulas to predict the heat transfer coefficient and friction factor in a square duct roughened by the square-shaped ribs on one wall. For chamfered rib.18.16.14.12.1.8.6.4.2 2 4 6 8 1 12 14 16 p/e 1 p/e 2 For square rib Fig.13 Variation of Friction factor with Reynolds number for Chamfered ribs.18.16.14.12.1.8.6.4.2 2 4 6 8 1 12 14 16 Fig.11 Variation of Friction factor with Reynolds number for Square Ribs For semicircular rib p/e 1 p/e 2 Fig. 14.18.16.14.12.1.8.6.4.2 For triangular rib 2 4 6 8 1 12 14 16 p/e p/e 1 p/e 2 Variation of Friction factor with Reynolds number for triangular rib.18.16.14.12.1.8.6.4.2 2 4 6 8 1 12 14 16 smooth duct p/e 1 p/e 2 Fig.12 Variation of Friction factor with Reynolds number for Semicircular ribs V. CONCLUSION This investigation shows that Chamfered shape ribs giving maximum heat transfer enhancement and minimum friction as compared to other ribs geometries with only 1. to 1.8 times friction increasing as compared to smooth duct flow, which is very small and unaccountable. Maximum heat transfer is found near reattachment zone. Experiments also confirm this. K-ɛ turbulence model found good for close results on comparing the predictions of various turbulence models during analysis. Since 3D model requires much higher memory and computational time compared to 2D ones and 2D model results are found closer to the experimental ones so it is sufficient to employ 2D model. REFERENCES [1] J.L. Bhagoria, J.S. Saini, S.C. Solanki. Heat transfer coefficient and friction factor correlations for rectangular solar air heater duct having transverse wedge shaped rib roughness on the absorber plate. Renewable Energy (22) 341 369 9
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