"CFD SIMULATION OF HEAT TRANSFER AUGMENTATION IN CIRCULAR TUBE"
CFD SIMULATION OF HEAT TRANSFER AUGMENTATION IN A CIRCULAR TUBE FITTED WITH REGULARLY SPACED HELICAL TWIST INSERTS IN LAMINAR FLOW UNDER CONSTANT HEAT FLUX P.Sivashanmugam *, S.Suresh and P.K.Nagarajan Depart ment of Chemical Engineering, National Institute of Technology, Tiruchirappalli-620015, India * Corresponding author Email: email@example.com Abstract CFD simulat ion for the heat transfer augmentation in a circular tube fitted with regularly spaced helical inserts in laminar flo w conditions with the Reynolds fro m has been explained in this paper using Fluent version 6.2.16. The data obtained by simulation are matching with the literature value for plain tube with the discrepancy of less than 9%.for Nusselt number and 15 % for frict ion factor. The simu lated results for the tube fitted with regularly spaced helical inserts are agreeing with the literature values 10% for Nusselt number and 18% fo r frict ion factor. Key words: Heat transfer augmentation; Regularly spaced helical twist; Twist ratio; CFD simu lation; Fluent; 1. Introduction Heat transfer enhancement is the process of improving the performance of a heat transfer system by increasing the heat transfer coefficient. In the past decades, heat transfer enhancement technology has been developed and widely applied to heat exchanger applicat ions; for example, refrigeration, auto motives, process industry, chemical industry etc. To date large numbers of attempts have been made to reduce the size and costs of the heat exchangers. An increase in heat transfer coefficient generally leads to another advantage of reducing the temperature driving force, which increases the second law efficiency, and decreases entropy generation. Also heat augmentation techniques play a vital role for laminar flo w, s ince the heat transfer coefficient is generally low in plain tubes. Among many techniques (both passive and active) investigated for augmentation of heat transfer rates inside circu lar tubes, tube fitted with fu ll length twisted tape inserts (also called a s swirl flow device) has been shown to be very effective, particularly in laminar flow due to impart ing of helical path to the flow. A great deal of experimental works on heat transfer augmentation studies using twisted tape have been reported in the literature. The aim of this study is to model the heat transfer augmentation and friction factor characteristics for the circular tube fitted with regularly spaced helical inserts using CFD, which enable us to find out Nusselt number, friction factor for g iven flo w rate and twist ratio in laminar flow and this will be co mpared with the experimental data. 2. Computational Model The flo w through tube fitted with a helical screw inserts under constant heat flu x is governed by the follo wing model equations. The following model equations were used for simulation 3. Geometry and grid arrangement The geometry and the gird were generated using GAMBIT® the preprocessing module of the FLUENT code. The geometry and the gird for both the plain tube without helical twist and with helical t wist are created in GAM BIT and imported into FLUENT. The geometry consist of a cylindrical tube of diameter 25.45 mm diameter and length of 1800 mm. The plain tube with helical twist configuration consists of a pla in tube with above diameter and length, and a helical twist with twist ratio of 1.95 and 4.89 . Fig.1. shows the grid for the plain tube configuration. A boundary layer mesh is created near the wall to capture the wall effects in FLUENT. The mesh is a quadrilateral face mesh and it is swept through the volume of the cylinder. The cooper tool in the volu me mesh option is used for generating the gird. Figure shows the boundary layer in wh ite and the other mesh in yellow co lor Figure 1. Grid for the plain tube configuration The geometry for the twisted insert (Fig.2.) is also created in GAMBIT using the sweep with twist option, the cylindrical volu me is subtracted by the twist and the required fluid do main is obtained. The volu me has to be split or divided suitably so that one can get a hexahedral mesh. Here also necessary boundary layer is used to predict the flo w in the near wall reg ions. Necessary fluid exit, entry and wall boundary conditions were given before numerical simu lation Figure 2. Grid for the plain tube with regularly spaced helical twist insert 4. Modeling parameters Nu merical values of the mass flow rate and heat flu x used in a nu mber of the simu lations are given in Tab le.1. Table.1 Numerical values of the parameters used for simulation Mass Flow 0.003 0.005 0.006 0.008 0.01 0.0116 0.0133 0.015 0.0166 0.02 0.03 rate, kg/sec Heat 240.03 340.77 459.15 563.38 749.85 1001.57 1363.26 1512.68 1893.56 2130.05 2445.25 flu x w/ m2 5. Numerical procedure and computational methodology The governing differential transport equations were converted to algebraic equations before being solved numerically. After the specificat ion of the boundary condition, the solution control and the initialization of the solution have to be given before the iteration starts. The solution controls like the pressure velocity coupling and the discrimination of the different variables and the relaxation factors have to be specified. The solutions sequential algorith m (called the segregated solver) used in the numerical computation requires less memory that the coupled solver. Since we are using the segregated solver for our problem, the default under relaxation factors are used and the SIMPLE scheme for the pressure velocity coupling is used and the second disc rimination is used for the mo mentu m and the standard scheme is used for the pressure. 6. Results and discussion 6.1 Plain tube The results obtained in this simu lation study are presented and discussed in this section. Fig.3 shows variation of Nusselt number with Reynolds number for p lain tube whereas Fig.4 shows variation of friction factor with Reynolds number for plain tube. By referring to the Fig.3 one can observe that as Reynolds number increases Nusselt number increases whereas friction factor decreases with Reynolds number (Fig.4) The data obtained by simu lation are matching with the literature value fo r plain tube with the discrepancy of less than 9%.for Nusselt number (Sieder and Tate) and 15 % for frict ion factor. 100 Nusselt Number (Nu) 10 Simulated Nusselt Number Literature Nusselt Number 1 100 1000 10000 Reynolds number (Re) Figure 3. Plain tube simulated Nusselt Number vs literature value 1 Literature friction factor Simulated friction factor Friction factor (f) 0.1 0.01 0.001 100 1000 10000 Reynolds Number (Re) Figure 4 . Plain tube simulated friction factor vs lterature value 6.2 Effect of twist ratio and spacer length Variation of Nusselt number with Reynolds number, and Friction factor with Reynolds number for the tube fitted with regularly spaced helical inserts are shown in Figs.5 and 6 for twist ratio 1.95, Figs 7 and 8 for twist ratio 2.93. Figs 5 and 7 shows that Nusselt number for a given Reynolds number increases with decreasing twist ratio indicating enhanced heat transfer coefficient due to enhanced swirl flo w as the Reynolds number increases for given twist ratio. The Nusselt number also increases with Reynolds number indicat ing enhanced heat transfer coefficient due to increased convection as flow increases. Figs 6 and 8 show that friction factor for given Reynolds number increases with decreasing twist ratio and decreases conventionally with increasing Reynolds number. The simulated results are agreeing with the literature values (Sivashanmugam and Suresh) within 10% for Nusselt number and 18% for frict ion factor respectively. 100 Nusselt number (Nu) Experimental 100 Simulated 100 Experimental 300 10 Simulated 300 1 100 1000 10000 Reynolds number (Re) Figure 5.Simulated and Experimental Nusseltnumber for helical twist with twist ratio 1.95 with spacerlength 100 and 300mm 1 Experimental 100 Simulated 100 Experimental 300 Friction factor (f) Simulated 300 0.1 0.01 100 1000 10000 Reynolds number (Re) Figure 6.Simulated and Experimental friction factor for helical twist with twist ratio 1.95 with spacer length 100 and 300 mm 100 Nusselt number (Nu) Experimental 100 10 Simulated 100 Experimental 300 Simulated 300 1 100 1000 10000 Reynolds number (Re) Figure 7. Simulated and Experimental value of Nusselt number for helical twist with twist ratio 2.95 with spacer length 100 and 300 mm 1 Experimental 100 Simulated 100 Friction factor (f) Experimental 300 Simulated 300 0.1 0.01 100 1000 10000 Reynolds number (Re) Fig.ure 8.Simulated and Experimental friction factor for helical twist with twist ratio 2.95 with spacer length 100 and 300 mm 7. Conclusion CFD simu lation for the heat transfer augmentation in a circular tube fitted with regularly spaced helical inserts in laminar flow conditions with the Reynolds fro m has been exp lained in this paper using fluent version 6.2.16. The data obtained by simulation are matching with the literature value for plain tube with the discrepancy of less than 9%.for Nusselt number and 15 % for frict ion factor. The simu lated results for the tube fitted with regularly spaced helical inserts are agreeing with the literature values 10% for Nusselt number and 18% for friction factor. References Fluent Manual E.N.Sieder ,G.E. Tate, 1936 Ind. Eng. Chem. 28, 1429. Sivashanmugam, P and S Suresh; Experimental studies on heat transfer and friction factor characteristics in laminar flow through a circular tube fitted with regularly spaced helical screw-tape inserts has been accepted for publication in the Journal of Experimental Thermal and Fluid Science