Numerical Analysis Of Heat Transfer And Fluid Flow Characteristics In Different V-Shaped Roughness Elements On The Absorber Plate Of Solar Air Heater Duct Jitesh Rana, Anshuman Silori, Rohan Ramola Abstract: Computational fluid dynamics (CFD) analysis of a solar air heater has been carried out using artificial roughness in the form of thin circular wire in V-shaped, V-shaped with gap, Multi v-shaped ribs and Multi v-shaped ribs with gap geometries. The effect of these geometries on heat transfer and friction factor was investigated, Comparison of CFD data with Dittuse-Boelter empirical correlation and Blasius equation for smooth duct has been done. Renormalization k-epsilon model (turbulence model) is used to predict heat transfer and friction factor in the duct as results based on this model have been found in good agreement with that of experimental. Keywords: Heat transfer, friction factor, solar air heater, CFD, artificial roughness 1. Introduction Solar air heater is one among solar thermal systems which has wide range of applications such as, space heating, winter home heating, seasoning of timber, drying of crops etc. but the problem with the solar air heater is that they are less efficient. From the study it was observed that a sublaminar layer is formed on the absorber plate which further resist the rate of heat transfer between the air and absorber plate and this is main reason for the poor efficiency of solar air heater. Many of the researcher has performed experimental study [1-3] as well as the CFD analysis on solar air heater with different roughness element to enhance the heat transfer rate. Gupta et al. [4] have carried out parametric study of artificial roughness geometry in the form of expanded metal mesh type on the absorber plate of solar air heater duct. Saini and Saini [6] by using arcshaped rib roughness elements attached to the underside of a heated plate reported the experimental prediction of heat transfer characteristics and turbulent fluid flow for a rectangular air channel. The experimental investigation of rectangular duct having its one broad wall heated and roughened with periodic discrete V-down rib' was carried out by Singh et al. [7] to study the heat transfer characteristics of solar air heater, and at pitch (P/e) of 8 maximum value of Nusselt number and friction factor was found. Jitesh Rana, Anshuman Silori, Rohan Ramola Department of Mechanical Engineering, DIT University, Dehradun (UK) E-mail: jiteshrana95@gmail.com A CFD based analysis of roughened duct with artificial roughness in form of arc shaped geometry was carried by Kumar and Saini [8] 3-D models was used carry out the heat transfer and flow analysis. FLUENT 6.3.26 commercial CFD code was used for simulation. Different Turbulent Models namely Standard k-ε, Shear stress transport k-u, Realizable k-ε and Renormalization group (RNG) k-ε were tested for smooth duct having same cross-section of roughened duct in order to find out the validity of the models. An Experimental investigations was carried out by Maithani and Saini [8] with V-ribs with symmetrical gaps ribs on absorber plate of solar air heater duct. 2. Nomenclature G d Gap distance, m Lv Length of single v shaped rib, m Gd/Lv Relative gap distance D Hydraulic diameter of duct, m e Rib height, m e/d Relative roughness height fs Friction factor of smooth duct f Friction factor of roughened duct g Gap width, m g/e Relative gap width H Depth of duct, m Nu Nusselt number of roughened duct Nu s Nusselt number of smooth duct Pr Prandtl number P Pitch of the rib, m P/e Relative roughness pitch W Width of duct, m w Width of single v-shaped rib, m W/w Relative roughness width ratio Greeks α Angle of attack, degree η Thermo-hydraulic performance parameter Γ Molecular thermal diffusivity Γ t Turbulent thermal diffusivity 253
3. CFD flow simulation and modeling of rectangular duct The present work has been carried out using a CFD code ANSYS 15.0.The duct has been made in design modeler. The flow system is a rectangular duct, which is divided into test, entry and exit sections. Computational fluid dynamics (CFD) codes make it possible to numerically solve heat, flow, mass and energy balances in complicated flow geometries such as a Multi v-shaped with gap, Multi v- shaped, V-shaped as a solar air heater duct.the 3-d view of the duct and the air flow direction is shown in Fig. 1. Table.1 shows in the values of flow and roughness parameters for this study. Fig1. Duct in ANSYS fluent 15.0 Table.1.Geometrical parameters of duct used in ANSYS FLUENT 15.0 v-shaped v-shaped with gap Multiple rib Multiple rib with gap P/e=10,e/D=0.043, α=60 ) P/e=10,e/D=0.043, Ng=4 α=60 ) (W/w=6, P/e=10,e/D=0.043, α=60 ) (W/w=6, P/e=10, e/d=0.043, α=60, g/e=1.0, G d/lv=0.69) (a) (b) (c) (d) Fig.2. roughness geometry used for analysis (a) V- shaped, (b) V- shaped with gap, (c) V shaped multiple ribs, (d) V shaped multiple ribs with gap. 254
The arrangement of roughness elements in the form of V- shaped ribs, Multi v-shaped ribs and Multi v-shaped ribs with gap fixed on the underside of the absorber plate has been considered. The duct used for CFD analysis having the height (H) of 25 mm and width (W) of 300 mm. Other detailed of V-shaped ribs (W/w=1, P/e=10, e/d=0.043, α=60 ), Multi v-shaped ribs(w/w=6,p/e=10, e/d=0.043, α=60 ) and Multi v-shaped ribs with gap(w/w=6,gd/lv=0.69,g/e=1.0,p/e=10, e/d=0.043, α=60 ) has been considered. A uniform heat flux of 1000 W/m2 was considered for analysis. Roughness was considered at the underside of the top of the duct to have roughened surface while other three sides were considered as smooth surface. Various roughness geometry used for analysis is shown in Fig.2. 3.1 Analysis in FLUENT 3.1.1 Grid generation The complete solution of the fluid flow is the compilation of the results in each cell. In the current analysis non-uniform grid is generated in the computational domain, in the meshing module of ANSYS 15.0 as shown in Fig. 2. Very fine mesh is used near the boundary of the domain such that the laminar sub-layer can be reduced. (a) (b) Fig.2. Non-uniform grid on computational domain,(b) Non-uniform mesh at rib region 3. 2 Governing equations Steady state two dimensional continuity, energy equation and time independent incompressible Navier Stokes equation are used to solve the fluid phenomenon in artificially roughened solar air heater rectangular duct. In the Cartesian tensor system these governing equations can be written as Continuity equation: Momentum Equation (Newton s Second Law of Motion): Energy (2) (1) Where Γand Γ t are molecular thermal diffusivity and turbulent Thermal diffusivity, respectively. (3) 255
3.3.Mathematical solution in Ansys The flow equations are solved using RNG k e turbulence model as it shows very good agreement with the empirical relations. The convergence criteria of 10-3 for the residuals of continuity equations, 10-6 for the residuals of velocity components and 10-6 for the residuals of energy. A CFD code ANSYS 15.0 is used which by using a control volume technique it convert the partial differential governing equations into algebraic equations. 3.4. Validation of CFD model The CFD results are validated in the form of average Nusselt number and friction factor for smooth duct with the correlations proposed by Dittus-Boelter and Blasius. Comparison of experimental and predicted values of Nusselt number and friction factor is shown in Fig. 3. The CFD results that are obtained are in good agreement to ensure the accuracy of the data being collected from the experimental investigation. The Nusselt number for a smooth rectangular duct is given by the Dittus-Boelter equation as; Nu s =0.023Re 0.8 Pr 0.4 (4) The friction factor for a smooth rectangular duct is given by the modified Blasius equation as; f s =0.085Re -0.25 (5) 4. Results and discussion The major objectives of this investigation is to see the effects of V-shaped ribs, V-shaped ribs with gap, Multi v- shaped ribs and Multi v-shaped ribs with gap roughened solar air heater duct with the help of Computational fluid dynamics (CFD) software on heat transfer and friction factor. 4.1 Temperature Profile Variation of the static temperature for various rib geometries of solar air heater duct in three dimensional are shown in Fig.4 at Reynolds number 16000. A uniform heat flux of 1000w/m2 is applied on the top surface of the absorber plate. Air is allowed to enter at the inlet section of rectangular duct and flows under the absorber plate. In practical applications the nature of most of the flow is generally turbulent. In the turbulent region, very close to surface the velocity of the particles becomes almost zero and as a result low kinetic energy is found at this region. This region is known as laminar sub-layer. To minimize the effect of this region artificial roughness is used inside the duct which creates turbulence in the region breaking the laminar sub layer and resulting in the increase of heat transfer rate. Throughout the duct air temperature increases. The maximum temperature is indicated in red and minimum in blue. Fig.5. temperature profile of smooth duct at Reynolds number of 16000. The Nusselt number (Nu) and friction factor (f) in experimental investigation and in CFD analysis were calculated using the following relations, Nu r =hd/k (6) f r =(ΔP/l)D/2ρvU 2 (7) where D is called hydraulic diameter. Fig. 3 Comparison of CFD data with Dittuse Boelter empirical correlation and Blasius equation for smooth duct 256
( a) (b) (c) (d) Fig.4. Temperature profile of (a) V-shaped ribs, (b) V-shaped ribs, (c) Multi v-shaped ribs, (d) Multi v-shaped ribs with gap at Reynolds number 16000 Fig.5. temperature profile of smooth duct at Reynolds number of 16000 257
4.2 Velocity Profile Fig.6.shows the contour plot of velocity and turbulent intensity for Re = 16,000 and α = 60 at relative roughness pitch (P/e) of 10.(a) V-shaped ribs, (b) V-shaped ribs, (c) Multi v-shaped ribs, (d) Multi v-shaped ribs derived from the CFD analysis of a solar air heater. In the present Computational fluid dynamics (CFD) based analysis above mentioned geometries are considered underside of the absorber plate. At the inlet section of the rectangular duct the atmospheric air enters and flows under absorber plate which is uniform distribution of heat flux.it was observed in multiple rib that on increasing the relative gap width ratio (W/w) there was increase in number of leading ends and number of secondary flow cells, While on Creating gap in the Multi v-shaped ribs it allow the release of the secondary flow along with the rib joins the main flow to accelerate it, which energizes the retarded boundary layer flow along surface which result in heat transfer resulting in enhancement. It was also observed that due to the presence of sufficient number of ribs occurrence of reattachment and circulation of flow increases the laminar sub layer thickness decreases to large extent due to generation of vortices this causes increase in turbulent intensity. The peak values of turbulent intensity are located near the heated absorber plate and between first and second rib. The phenomenon of reattachment and generation of vortices leads to enhancement of heat transfer in solar air heater. (a) (b) (c) 258
(d) Fig. 6.The contour plot of velocity for Re = 16,000 and α = 60 at relative roughness pitch (P/e) of 10,(a) V-shaped ribs, (b) V- shaped ribs, (c) Multi v-shaped ribs, (d) Multi v-shaped ribs 4.3 Heat flux 4.4. Flow Pattern Fig.7. heat fulx on top surface of the absorber plate Fig.8. flow over the ribs Fig 8 shows the flow over the ribs, the attachment and detachment of adjacent ribs helps in enhancement of heat transfer and flow characteristics. 259
Friction factor Nusselt number INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY RESEARCH VOLUME 5, ISSUE 09, SEPTEMBER 2016 ISSN 2277-8616 4.5. Effect of different shaped roughness geometries on Nusselt number (Nu) The various geometries were selected i.e. V-shape ribs, v shape rib with gap, Multi v-shaped shape and Multi v- shaped shape with gap.the size of the ribs geometries were selected on the basis of guidelines available in the literature. Computational fluid dynamics (CFD) based analysis of the V-shaped ribs, v shape rib with gap Multi v- shaped ribs and Multi v-shaped ribs with gap roughened is carried out and the various result related to Nusselt number and friction factor of solar air heater duct as a function of Reynolds numbers have been plotted and compared with those of the smooth duct under similar conditions. Fig.9.shows variation of Nusselt number with Reynolds number as a function of different shaped roughness geometries, the highest values of Nusselt number is found for the multi v-shaped rib arrangement with gap than other shaped roughness geometries. 300 250 200 150 100 50 0 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 Reynolds number V -shape rib V-shaped rib with gap Multiple V rib Multiple V rib with gap Smooth Fig.9. Variation of Nusselt number with Reynolds number for different geometries 4.6. Effect of different shaped roughness geometries on Friction factor (f) The secondary flow exerts a measurable effect in disturbing the axial flow profile, which increases the friction coefficient in noncircular duct. For selected Reynolds number Fig.10 shows CFD results of the friction factor (f) as a function of different shaped roughness geometries. It is noticed that the CFD values and experimental values of friction factor (pressure drop) very close to each other and for the multi v-shaped rib arrangement highest values of Friction factor is observed. 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0-2000 3000 8000 13000 18000 Reynolds number V -shape rib V-shaped rib with gap Multiple V rib Multiple V rib with gap Smooth Fig.10 Variation of Friction factor with Reynolds number for different geometries 5. Conclusions CFD analysis of the solar air heater duct with V-shaped ribs, V-shaped ribs with gap, Multi v-shaped ribs and Multi v-shaped ribs with gap as an artificial roughness geometry on the absorber plate has been carried out and heat transfer and friction factor Characteristics has been presented, from the Present work the following conclusion can be drawn: 1. Validation of CFD result for the smooth duct has been carried out with Dittus-Boelter empirical relationship. 260
2. Among different CFD models, Renormalization k- epsilon model have been used as its results have been found to have good agreement with that of experimental. 3. The maximum enhancement in heat transfer and friction factor is observed in the Multi v-shaped ribs with gap as compared to other V-shaped ribs roughness geometries of solar air heater duct. 4. The enhancement in heat transfer is found to be increased to 1.6,4.5,5.4 times that of the heat Transfer with that of the heat transfer of smooth surface for V- shaped, Multi v-shaped and Multi V-shaped with gap ribs roughened solar air heater. 6. References [1] Kumar A, Saini RP, Saini JS. Heat and fluid flow characteristics of roughened solar air heater ducts e a review. Renewable Energy 47 (2012) 77- [2] A.S. Yadav, 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 55 (2013) 1127 1142. [3] Varun, R.P. Saini, S.K. Singal, Investigation of thermal performance of solar air heater having roughness elements as a combination of inclined and transverse ribs on absorber plate, Renewable Energy 33 (2008) 1398 1405 [4] Gupta MK, Kaushik SC Performance evaluation of solar air heater having expanded metal mesh as artificial roughness on absorber plate. Int J Thermal Sci 48 (2009) 1007 1016 [5] Saini, R.P., Saini, J.S.,. Heat transfer and friction factor correlations for artificially roughened ducts with expanded metal mesh as roughened element. Int. J. Heat Mass Transfer 40 (1997) 973 986 [6] Singh S, Chander S, Saini JS. Heat transfer and friction factor correlations of solar air heater ducts artificially roughened with discrete V-down ribs. Energy; 36 (2011) 5053-64. [7] Kumar S, Saini RP. CFD based performance analysis of a solar air heater duct provided with artificial roughness. Renewable Energy 34 (2009) 1285-91. [8] Maithani R., Saini, J.S., "Heat transfer and fluid flow behaviour of a rectangular duct roughened with V-ribs with symmetrical gaps", Int. J. Ambient Energy, 2015, DOI: 10.1080/01430750.2015.1100681. 261