Design of a Porous Bluff-Body Disc on Improving the Gas-Mixing Efficiency

Transcription

1 , July 5-7, 2017, London, U.K. Desgn of a Porous Bluff-Body Dsc on Improvng the Gas-Mxng Effcency Shun-Chang Yen, You-Lun Peng, and Kuo-Chng San Abstract A bluff body wth multple holes was desgned to transform the axal momentum nto radal and tangental momentum for ncreasng the swrl number (S). A numercal study of bluff-body structure wth multple holes was performed usng ANSYS Fluent computatonal flud dynamcs (CFD) analyss. The effects of hole number and jet nclnaton angles were consdered usng a fxed gas flow rate and a non-reactve gas. The concentraton dstrbuton behnd the mxng of central carbon-doxde (CO 2 ) jet and annular ar jet was utlzed to analyze the mxng effcency. Three bluff bodes wth dfferng hole numbers (H = 3, 6, and 12), three dfferent jet nclnaton angles (θ = 30, 45, and 90 ) were desgned for the analyss. The Reynolds normal stress ncreases wth the nclnaton angle. The Reynolds shear stress, averaged turbulence ntensty and a/veraged swrl number decrease wth the nclnaton angle. For the unsymmetrcal hole confguraton (.e., H = 3), the streamlne patterns shows a unsymmetrcal flow feld. The hghest mxng effcency (.e., the lowest ntegral gas fracton of CO 2 ) occurs at H = 3. Furthermore, the hghest swrl number concdent the strongest effect on the mass fracton of CO 2. The unsymmetrcal hole arrangement nduces a hgh swrled flow behnd the porous dsk. Index Terms Bluff body wth multple holes; Computatonal flud dynamcs (CFD); Swrl-jet flow; Mxng effcency T I. INTRODUCTION HE spray nozzles have been wdely utlzed n the daly lfe, such as the sprnkler, gas burner and fuel atomzer. The nozzle area, nozzle-matrx spacng and central-to-annular jet rato were adjusted to ntensfy the combuston ntensty. The mprovement of combuston effcency can dmnsh the energy dsspaton and exhaust emsson. The swrl flow was utlzed to ncrease the mxng rate between the fuel and ar jets. The swrl mechansm changes the axal-flow momentum to tangental flow. The ncreased tangental velocty ntensfed the swrl number n the flow feld. In addton, the ncreased swrl flow generated a nverse pressure gradent n the axal drecton and the vortex break-down occurred at a specfc crtcal value of swrl number. The ncreased swrl number mproved the flow mxng. Specfcally, the fuel-to-ar mxng was mproved effcently and the combuston stablty. Manuscrpt receved March 2, 2017; revsed Aprl 3, Shun-Chang Yen s wth the Department of Mechancal and Mechatronc Engneerng, Natonal Tawan Ocean Unversty, Keelung, 202 Tawan (correspondng author to provde phone: ext 3215; fax: ; e-mal: scyen@mal.ntou.edu.tw). You-Lun Peng was wth the Department of Mechancal and Mechatronc Engneerng, Natonal Tawan Ocean Unversty, Keelung, 202 Tawan. Kuo-Chng San s wth the Department of Aeronautcs and Astronautcs, R.O.C. Ar Force Academy, Kaohsung, 820 Tawan. A hgh adverse pressure gradent occurs n front of the bluff bodes when a unform and steady flow moved pass the bluff bodes, and then a low-pressure zone occurs behnd the bluff bodes. Furthermore, the separaton occurs near the leadng edge of bluff bodes and generates the separated shear layer. In the downstream, the nteracton between the shear layer generates a large-scale recrculaton. In the prevous studes, Strouhal [1] expermentally nvestgated the vortex sheddng behnd a crcular cylnder. He found that the Strouhal number (.e., a dmensonless parameter defned as fd/u) approaches a constant at a specfc range of Reynolds numbers. Von Karman [2] studed the vortex strp generated behnd a crcular cylnder and presented a quantfcaton descrpton usng ths sheddng regularty. In the expermental apparatus, Huang and Tsa [3, 4] utlzed the laser Doppler Velocmetry (LDV) and smoke-wre vsualzaton to nvestgate the flow structures usng dfferent blockage ratos and swrl numbers. They found that the bluff-body effect s sgnfcant when the blockage rato s greater than 0.1. Al-Abdel [5] used the LDV to nvestgate the unconfned flow behavors behnd a crcular dsc. He found that the recrculaton zone correlates wth the swrl number and the radal dffuson depends on the axal velocty and swrl number. In the practcal applcatons, Huang and Yen [6] utlzed the swrl flow perturbed usng an arfol van pack. They defned three flame modes: weak swrlng flame, lfted flame and turbulent reattached flame. San and Hsu [7] utlzed a rfled nozzle to ncrease the mxng effcency between the central fuel and annular ar jets. They classfed the flame patterns as jet, flckerng and turbulent flames. San et al. utlzed a rfled cone to ncrease the mxng rate between the central CO 2 -jet and the annular ar-jet. Three cold-flow structures were defned: jet flow, sngle bubble and dual bubble. Yen and Shh [8] utlzed a spral-grooved cone to ntensfy the bluff-body effect and ncrease the turbulence ntensty. They found these grooves can ncrease the mxng effcency and combuston ntensty; and however decrease the flame length. Many studes utlzed the numercal scheme/software to nvestgate the bluff-body flow behavors. Dally et al. [9] smulated the flow felds and mxng felds utlzng the standard models and found that two vortex occurred n the recrculaton zone. They also found that the modfed k-ε scheme and Reynolds stress turbulent flow model can predct more accurate poston of recrculaton zone. Wouters et al. [10] also utlzed the k-ε Reynolds stress model to analyze the effects of geometrcal shape on the gas mxng rato. Wegner et al. [11] utlzed the U-RANS (unsteady Reynolds averaged Naver-Stokes) to smulate a non-premxed swrl burner. The numercal results were compared wth the expermental and

2 , July 5-7, 2017, London, U.K. LES (large eddy smulaton) results. The comparson results show that the U-RANS algorthm can quanttatvely calculated the processng vortex core (PVC). Zhang, et al. [12] utlzed the LES algorthm to study the coherent structures behnd a bluff body. They found three vortex structures usng the Q-functon vsualzaton. These vortex structures are cylndrcal shell structure n the outer shear layer, rng structure and harpn-lke structures n the nner shear layer. Jenny et al. [13] utlzed three dfferent probablty densty functons (PDF) to study a constant-densty bluff-body stablzed flow. The three algorthms ncluded a standalone partcle-mesh method and two hybrd algorthms. Ther results show that the three numercal algorthms are satsfed wth the expermental data. The mxng mechansm s a crtcal ssue n combuston feld. Ths ssue ncludes the tme-shortenng and space-narrowng of fuel and ar. Ths study combned a bluff-body dsc and hole structures to adjust the ar jet near the nozzle ext. The spral hole structures drove the ar flowed n the tangental drecton and generated the swrl flow. The bluff bodes were frequently utlzed n the combustors to ncrease the flame detenton tme and ntensfy the mxng between fuel and ar. Namely, the bluff body s a passve devce utlzed to mprove the combuston ntensty. Ths nvestgaton utlzed the numercal smulaton to study the turbulent flow feld behnd the three-dmensonal mult-hole bluff body. The nvestgaton parameters ncluded the hole geometry, hole numbers and ar-jet ncdence angle. II. MATHEMATICAL MODEL Many ntrcate flud dynamc topcs have been solved usng the commercal software programs based on the fnte volume algorthm. Ths nvestgaton utlzed the software package ANSYS Fluent 15.0 to analyze the flow behavors behnd the porous bluff-body dsk. A. Hypotheses In the nozzle, the flow feld s very complcated. Therefore, the smplfed model must be presented to lower the computaton loadng. These hypotheses suggested n ths nvestgaton ncludes (1) three dmensonal unsteady flow feld, (2) turbulent flow feld, (3) Newtonan flow, (4) no slp boundary condton, (5) Cartesan coordnate system, (6) neglected gravty effect, and (7) constant room temperature. B. Governng Equatons The vector forms of three-dmensonal contnuty equaton (Eq. 1) and Naver-Stokes equatons (Eq. 2) are lsted as follows. ( U ) 0 (1) t U t U U pi g. Turbulent Models. Kolmogorov (1942) suggested the frst and complete turbulent model. In ths study, a standard k-ε model was utlzed and assumed that the turbulent flow feld was fully developed. The standard turbulent k-ε model s lsted as follows (Eq. 3). k ku t x1 x (3) t k G k G b. k x (2) Fg. 1. Desgn of pure-jet nozzle. (a) Pure-jet nozzle; (b) Sectonal vew, (c) Numercal model, and (d) Numercal model of hole structure. Fg. 2. Meshgrd graph.

3 , July 5-7, 2017, London, U.K. where G k presents the turbulent knetc energy caused from the average velocty, G b s the turbulent knetc energy caused from the buoyant force, and ε represents the effect of compressble turbulent pulse expanson on total dsspaton rate. C. Model Desgn Fgure 1(a) shows the physcal pure-jet nozzle [6]. The nner fuel jet and annular arflow compose the non-premxed flame mechansm. Fgure 1(b) shows the coordnate system utlzed n ths study, where x ndcated the axal drecton and r represents the radal axs. The nner dameter of central jet (D ) s 5 mm and the outer dameter of annular jet (D o ) s 30 mm. Fgure 1(c) shows the model and annular hole desgn utlzed n ths numercal study where the dameter of annular ar hole (D h ) s 2.5 mm. Fgure 1(d) shows the computaton mesh characterzed the spral hole structures. In ths numercal calculaton, the hole numbers (H) were chosen as 3, 6, 12 and the nclnaton angle (θ) of ths spral ar hole was set at 45, 60 and 90 degree. The open boundary condton was set at the boundary of calculaton doman whch s 5 tmes of the outer dameter of annular arflow (.e., D ). In the axal drecton, the calculaton doman s 12 tmes of the outer-flow dameter for transformng the flow to fully developed. At the nlet, the boundary condton was set at constant velocty of 0.2 m/s. The boundary condton for the open boundary was set at outlet pressure of 101,325 Pa (.e., 1 atm). Ths study utlzed the tetrahedral mesh to analyze the flow behavors behnd the porous bluff body. The boundary mesh was utlzed near the calculaton boundary. Ths nvestgaton utlzed the ANSYS Meshng to generate the calculaton mesh. The desgned model was nput nto ANSYS to mesh the flow feld. Fgure 2(a) shows the spral hole structure and reveals the defnton of nclnaton angle. Fgure 2(b)-2(d) show the perspectve vew for H = 6 at θ = 30, 45 and 90, respectvely. Fgure 2(e) show the top vew of the mesh structure for the flow feld. Fgure 2(f) shows the mesh structure near the jet ext. In ths calculaton doman, there s a sgnfcant change of velocty and pressure. Therefore, the mesh densty was ntensfed to ncrease the computaton accuracy. Fgure 2(g) shows the sde vew of the whole computaton doman. The arflow was drven from the bottom, passng through the hole structures, and then flew nto the free space. D. Element and CPU tme Ths nvestgaton focused on the flow velocty feld and the dstrbuton of mxng concentraton. In the computaton doman, fve ponts of A E (shown n the nset of Fg. 3(a)) were chosen to measure the flow propertes. The effect of grd numbers on the flow velocty s shown n Fg. 3(a). Fgure 3(a) shows that the flow velocty approaches stably when the grd number s larger than 1.2 x Fg. 3. (a) Average velocty vs. Grd number, and (b) CPU tme vs. Grd number. Fg. 4. (a) (d) Velocty vectors and (e) (h) Stream lnes.

4 , July 5-7, 2017, London, U.K. Fg. 5. Mass fracton of CO 2, N 2 and O 2. Fgure 3(b) shows the relaton of CPU tme and grd numbers. For consderng the factors of CPU tme and calculaton stablzaton, ths study utlzed the grd number of 1.5 x 10 6 to numercally analyze the whole flow feld. III. RESULTS AND DISCUSSION A. Velocty vectors and Stream lnes Fgure 4 shows the velocty vectors (Fgs. 4(a)-(d)) and stream lnes (Fg. 4(e)-(h)) behnd a pure jet and dfferent hole-numbered dsc. The test condtons were: (1) central-jet velocty (u c ) = 0.2 m/s; (2) annular-jet velocty u a = 0.2 m/s; (3) hole number (H) = 0 (.e., pure jet), 3, 6, 12; and (4) nclnaton angle (θ) = 45. Fgures 4(a) and 4(e) show that two recrculatons appear near the nozzle ext due to the entranment effect. Wth the effects of holes and nclnaton angel, Fgs. 4(b) and 4(f) show that two recrculatons occur behnd the left sde of 3-hole dsc. The un-symmetrc hole arrangement caused the un-symmetrc flow structures. In addton, Fg. 4(s) shows a four-way saddle pont occurrng at x/d = 7.7. Fgures 4(c) and 4(g) show that sx recrculatons (three pars) occurs behnd the sx-hole dsc. Fgure 4(g) shows a four-way saddle pont appear at x/d = 4.5. Fgures 4(d) and 4(h) show four symmetrc recrculatons (two pars) occurs behnd a 12-hole dsc. The four-way saddle pont occurs at x/d = 2.1. The dstance between the four-way saddle pont and nozzle ext decreases wth the ncrease of hole number due to the ntensfcaton of turbulence ntensty. Namely, the spral hole structure drove the axal flow to tangental flow and mprove the nteracton between central and annular flows. B. Concentraton Dstrbutons Fgure 5 shows the dstrbutons of mass fracton for CO 2, N 2 and O 2. For the pure jet nozzle, the CO 2 s accumulated near the jet ext. The CO 2 was lfted from the jet ext as the CO 2 exhaled from the holes. Fgure 5(d) shows the un-symmetrc arrangement of hole confguraton. For the mass fracton of N 2 and O 2, very low fracton of N 2 and O 2 were entraned n the central zone near the jet ext. The mass fracton of N 2 and O 2 n the downstream agrees wth the standard atmospherc condton. IV. CONCLUSIONS Bluff-body dscs drlled wth multple holes were examned to ncrease the mxng effcency n the cold flow feld. Ths desgn was numercally performed usng the commercal ANSYS Fluent software. The test condton settng a fxed ar flow and a non-reactve CO 2 gas and changng the number of holes, and jet nclnaton angles. The swrl number (S) and turbulence ntensty (T.I.) were ntroduced to ndcted the effects of these factors on the CO 2 dstrbuton. The followng conclusons were summarzed from the calculated results. 1) For a pure jet (.e., H = 0), H = 6 and H = 12 porous dscs, the streamlne patterns show the symmetrcal confguratons. For a H = 3 holed dsc show a unsymmetrcal streamlne pattern. 2) For the un-symmetrc arrangement of hole confguraton (.e., H = 3), very low fracton of N 2 and O 2 were entraned n the central zone near the jet ext. 3) The hghest mxng effcency occur at H = 3 (.e., the

5 , July 5-7, 2017, London, U.K. lowest ntegral gas fracton of CO 2 occurrng at H = 3) due to the unsymmetrcal hole arrangement. 4) For a H = 3 dsk, the hghest swrl number concdent the strongest effect on the mass fracton of CO 2. The unsymmetrcal hole arrangement nduces a hgh swrled flow behnd the porous dsk. ACKNOWLEDGMENT Ths research was supported by the Mnstry of Scence and Technology (Republc of Chna, Tawan), under Grant No. MOST E REFERENCES [1] V. Strouhal, "Ueber ene besondere art der tonerregung (On an unusual sort of sound exctaton)," Annalen der Physk und Cheme, vol. 5, no. 10, 1878, pp [2] T. Von Kármán, Aerodynamcs. New York: McGraw-Hll, [3] R. F. Huang and F. C. Tsa, "Observatons of swrlng flows behnd crcular dscs," AIAA J., vol. 39, 2001, pp [4] F. C. Tsa, "Flow structure and mxng characterstcs of swrlng wakes," Ph.D. dssertaton, Dept. Mech. Eng., Natonal Tawan Unversty of Scence and Technology, Tape, Tawan, [5] Y. M. Al-Abdel and A. R. Masr, "Recrculaton and flowfeld regmes of unconfned non-reactng swrlng flows," Expermental Thermal and Flud, vol. 27, 2003, pp [6] R. F. Huang and S. C. Yen, "Aerodynamc characterstcs and thermal structure of nonpremxed reactng swrlng wakes at low Reynolds numbers," Combuston and Flame, vol. 155, 2008, pp [7] K. C. San and H. J. Hsu, "Characterstcs of flow and flame behavor behnd rfled/unrfled nozzles," ASME Journal of Engneerng for Gas Turbnes and Power, vol. 131, 2009, pp [8] S. C. Yen and C. L. Shh, "Improvng combuston ntensty and modulatng flame behavors usng helcal-grooved cones," Journal of Mechancs, vol. 29, 2013, pp [9] B. B. Dally, A. R. Masr, R. S. Barlow, and G. J. Fechtner, "Instantaneous and mean compostonal structure of bluff-body stablzed nonpremxed flames," Combuston and Flame. vol. 114, 1998, pp [10] H. A. Wouters, P. A. Nooren, T. W. J. Peeters, and D. Roekaerts, "Smulaton of a bluff-bodystablzed dffuson flame usng second-moment closure and Monte Carlo methods," Symposum on Combuston, vol. 26, no. 1, 1996, pp [11] B. Wegner, A. Maltsev, C. Schneder, A. Sadk, A. Drezler, and J. Jancka, "Assessment of unsteady RANS n predctng swrl flow nstablty based on LES and experments," Internatonal Journal of Heat and Flud Flow, vol. 25, 2004, pp [12] P. Zhang, C. Han, and Y. Chen, "Large eddy smulaton of flows after a bluff body: Coherent structures and mxng propertes," Journal of Fluds and Structures, vol. 42, 2013, pp [13] P. Jenny, M. Muradoglu, K. Lub, S. B. Pope, and D. A. Caughey, "PDF smulatons of a bluff-body stablzed flow," Journal of Computatonal Physcs, vol. 169, 2001, pp