Turbulent Flow in Curved Square Duct: Prediction of Fluid flow and Heat transfer Characteristics

Transcription

1 Internatonal Research Journal of Engneerng and Technology (IRJET) e-issn: Volume: 4 Issue: 7 July p-issn: Turbulent Flow n Curved Square Duct: Predcton of Flud flow and Heat transfer Characterstcs R Grsh Kumar 1, Raesh V Kale 2 1M.E Student, RGIT, Andher, Mumba, Inda 2Professor, Dept of Mechancal Engneerng, RGIT, Andher, Mumba, Inda *** Abstract Fully developed Turbulent flows n curved square duct wth curvature of 3degree numercally and expermentally nvestgated. Turbulent flows n non crcular duct leads to generate the secondary motons n crosswse drecton. These secondary motons of second knd are generated due to the gradent of Reynolds Stresses n cross stream n straght duct. But these secondary motons are generated n curved square duct due to mbalance between centrfugal force and radal pressure gradents. Ths note ams to predct numercally these secondary motons and ther effects on flud flow and heat transfer characterstcs n the curved square duct. SST K ω model has been used to smulate the flow. And these results are valdated wth expermental measurements. It has been observed that there s good agreement between the numercal and expermental results. Key Words: Turbulent flow, Secondary motons, Reynolds Stresses, Square Duct, Heat transfer 1.INTRODUCTION Predcton of Turbulent flow characterstcs s of academc nterest snce few decades. In spte havng numerous applcatons, there are only few exclusve studes whch are focussed on secondary motons of turbulent flow. These secondary flows mean velocty wll be up to 5-1% of mean velocty of prmary flow. Expermental measurements of turbulent flow characterstcs n 9 curved square duct have been studed by Humphery, Whte law and Yee [1]. These studes are done wth the ad of non dsturbng Laser Doppler Velocmetry and they have reported that stronger secondary motons and hgher levels of turbulence s experenced near thcker boundary regon. Numerous studes undertaken over the past few years done by Leseur and Métas [2]; Mon [3]; Germano [4] demonstrate that LES s one of the most relable approaches for the predcton of complex turbulent flows of practcal relevance. Gavrlkas nvestgated the flow pattern nsde the straght rectangular duct through numerc smulaton and concluded that turbulent flow near the smooth corner s subected to remarkable structural change, mean stream wse vortces attan ther largest values n the regon where the vscous effects are sgnfcant [5]. In few maor applcatons the contnuous change of flud drecton nduces the centrfugal effects n flud flow. Tlak T et al and Humphrey smulated the flow wth varous boundary condtons and geometres and has gven the reason behnd formaton of secondary flows and relatonshps between duct geometry, aspect ratos and secondary flows [1, 6]. But there no exclusvely devoted study of these secondary flows n curved duct n relatonshp wth heat transfer. Present study focuses on presentng the relatonshp between heat transfer enhancement and Secondary motons or secondary flows. 2. GOVERNING EQUATIONS, METHODOLOGY AND TESTING DOMAIN The present case s of unsteady, three dmensonal and turbulent n nature. Therefore an averaged contnuty equaton, Reynolds averaged Naver stokes equaton and energy equatons are solved. 1. Contnuty equaton 217, IRJET Impact Factor value: ISO 91:28 Certfed Journal Page 3364 u 2. Naver Stokes equaton (1) 1 1 u ( uu ) 1 p u ( u u ( ) ) t (2) 3. Energy Equaton 1 1 T ( ut ) T uu (3) t x Where u tme averaged velocty and stress u s Reynolds 1 u 1 SST K-ω turbulence model s used n predctng the turbulent flow characterstcs n the heated Curved squared duct. And ths model s valdated wth DNS results of Gavrlakas(1992) for straght square duct at Reynolds number 441 and they show good agreement n the results. Computatonal doman s as shown n fgure.1 conssts of 3 curvature and all other dmensons are n terms of hydraulc dameter of the duct D h. It s square duct (Aspect Rato=1) wth 1mm sde. Ar has been used as workng flud wth the propertes of knematc vscosty (ʋ) = 1.6x1-6 m 2 /s and Prandtle number (Pr) =.7.

2 Stream velocty m/s Velocty n m/s Internatonal Research Journal of Engneerng and Technology (IRJET) e-issn: Volume: 4 Issue: 7 July p-issn: temperature s 99kelvn when other walls temperature s mantaned at 33kelvn. And smulatons are carred out at Reynolds numbers 6 and 1. Inlet temperature of flud s gven as 33kelvn and outlet pressure s atmospherc pressure. 4.RESULTS AND DISCUSSION In ths secton of ntal part presents the numercal results, where we can vsualze the secondary motons and ther effects on the flud flow and heat transfer characterstcs. In second part we present the comparatve study between expermental and numercal results. Fg-1: Curved square Duct To have a fully developed turbulent flow at the begnnng of curvature, suffcent length of straght square duct smultaneously smulated n numercal smulaton case and expermental apparatus suffcent length straght square duct has been added n front of the curved secton of the duct. Ths length can be determned by usng the formula Entrance length (L e) = 4.4(R e)^(1/6) D h (4) Fg. no 3 depcts the three dmensonal components of velocty. These results are expermentally measured wth the help of fve hole pressure probe. From the fgure t clearly observable that magntude of normal components of velocty s 3-5% of the axal velocty at ntal part of the curvature Dmensonal Flow Fgure 2 shows the grd ndependent study for dfferent mesh szes and stream wse velocty s plotted aganst hydraulc dameter. And t s observed that 78x52x52 s optmum sze. And further smulatons are carred out wth ths grd Radal transverse Axal Vertcal dstance along the duct (Cm) ( along the Curvature) x52x52 75x45x45 84x45x Hydraulc Dameter n m Fg-2: Fully developed Stream wse velocty No slp boundary condtons are gven for velocty near the walls and ntally unform temperature s mposed on the all walls. And n second case concave wall temperature s 3 tmes hgher than the other walls temperature.e concave wall Fg-3: Velocty Components Fg.4 (a), 4(b) and 4(c) shows the ntensty of secondary flows ust before the curvature, along the curvature and after the curvature. Intensty (I) s calculated as I v 2 u bulk w 2 (5) From the fgures t s clearly observable that at the begnnng of the curvature ntensty s hgher near the concave wall as the whole flud s pushed towards concave wall due to the centrfugal force effect. As the flud flows downstream of the curvature secondary flows ntensty ncreases near the lateral walls as the flow reversal s gong to take place. And near the out let of duct ntensty becomes maxmum near the bottom wall due to the presence of the curvature of the duct. 217, IRJET Impact Factor value: ISO 91:28 Certfed Journal Page 3365

3 Internatonal Research Journal of Engneerng and Technology (IRJET) e-issn: Volume: 4 Issue: 7 July p-issn: Fg-4(a): Intensty of Secondary flow at begnnng of the Curvature Fg -4(b): Intensty of Secondary flow along the curvature Fg -5:velocty contour n a streamwse drecton n curved duct As already mentoned the man am of ths study s to nvestgate the effects of secodary flow due to curvature on heat trasfer characterstcs, dfferent temperature boundary condtons are mposed on the duct walls numercally and expermentally. Fgures 6(a), 6(b) and 6(c) depcts the temperature varaton contours before, along and after the curvature. And t s found that temperature varaton s unform untll the curvature. As the flud flow starts flowng along wth the curvature t clearly observable that temperature dffuson becomes non unform n crosswse drecton. Temperature near the convex wall s hgher compared to the concave wall, because more heat has been carred away by hgher velocty flow near the concave wall. And presence of secondary flows, enhanced the mxng process, becasue of whch hot flud near the walls s brought nto the core cold flud flow, whch nturn results n enhanced heattransfer. Fg -4(c): Intensty of Secondary flow after the Curvature As the flud near the concave wall has to be travel longer dstace comapared flud near the convex wall, fud near concave wall attans hgher veloctes due to law of conseraton of mss and t can be numercally vsualsed as shown n fgure (5). Where we can observe that down stream of the curvature the flud attans maxmum veloctes slghtly near to the bottom wall rather than beng the mddle of the duct. Fg -6(a):Temperature Contour before the Curvature 217, IRJET Impact Factor value: ISO 91:28 Certfed Journal Page 3366

4 Stream wse velocty Temperature Internatonal Research Journal of Engneerng and Technology (IRJET) e-issn: Volume: 4 Issue: 7 July p-issn: Expermental CFD.2 Along the curvature@re hydraylc dameter Fg -8: Temperature Profle n transverse drecton Fg -6(b):Temperature contour alog the Curvature Fg -6(c): Temperature contour after the Curvature Fgures 7 and 8 presents the comparve study of CFD(numrcal) and Expermental results on Streamwse velocty and Temperature. It s wtnessed that temperatures near the convex wall s hgher compared to the regon near to the concave wall due to lower magntudes of veloctes. 5. CONCLUSIONS Turbulent flud flow and heat transfer characterstcs n the curved square duct s nvestgated numercally and expermentally. All 3 dmensonal components of veloctes are compared wth each other and t s found that normal components of veloctes are n the range of 3-5% of axal component. Velocty, temperature and Pressures are measured at dfferent cross secton postons and valdated wth expermental results at dfferent Reynolds numbers. And t s observed that veloctes near concave wall regons are hgher compared to convex wall as the flud has to travel longer dstance. Because of whch more heat s beng carred away near the concave wall. Secondary flows whch are generated due to mbalance between the centrfugal force and radal pressure gradents enhances the heat transfer by carryng the heat from the near wall regon to core cold flud. These Secondary motons ntensty s hgher near the lateral walls and enhances mxng process of flud whch ncreases the heat transfer rate ACKNOWLEDGEMENT All credt goes to the Supreme Personalty of Godhead Krshna and hs devotees lke Sr Vashnava (Dr.Rahul Trved) by ther causeless mercy only I could complete ths work. Thank you my Lord..8 REFERENCES.4 CFD Expermental 5 1 Hydraulc Damter Fg -7: Stream wse velocty profle n transverse drecton [1] Humphrey, J., Whtelaw, J., and Yee, G., 1981, Turbulent flow n a square duct wth strong curvature. J. Flud Mech. 13, pp [2] Mon., P., 1998, Numercal and physcal ssues n large eddy smulaton of turbulent flows, JSME Int. J., Ser. B 41(2), pp [3] Germano., M., 1998, Fundamentals of large eddy smulatons. In: Advanced Turbulent Flow Computatons. Sprnger-Verlag, pp [4] Temmerman, L., Leschzner, M., A., Mellen, C., P., and Frohlch., J., 23, Investgaton of wall-functon approxmatons and subgrd-scale models n large eddy smulaton of separated flow n a channel wth 217, IRJET Impact Factor value: ISO 91:28 Certfed Journal Page 3367

5 Internatonal Research Journal of Engneerng and Technology (IRJET) e-issn: Volume: 4 Issue: 7 July p-issn: streamwse perodc constrctons, Int. J. of Heat and Flud Flow, Vol. 24, pp [5] Ghosal, S., Lund T. S., Mon P., and Akselvoll, K., 1995, A dynamc localzaton model for large-eddy smulaton of turbulent flows. J. Flud Mech., 285, pp [6] Camarr, S., Salvett, M. V., Koobus, B., and Derveux, A., Large-eddy smulaton of a bluff body flow on unstructured grds. Int. J. Numer. Meth. Fluds, 22, 4, pp , IRJET Impact Factor value: ISO 91:28 Certfed Journal Page 3368