Experimental investigation of mixing-enhanced swirl flows

Transcription

1 Journal of Mechanical Science and Technology 22 (8) 9~2 Journal of Mechanical Science and Technology DOI.7/ y Experimental invetigation of mixing-enhanced wirl flow Sam-Goo, Lee New & Renewable Energy Material Development Center, Chonbuk National Univerity, Jeonbuk, Korea (Manucript Received April 1, 8; Revied Augut 19, 8; Accepted September, 8) Abtract The experimental objective wa to compare diintegration characteritic from the internal mixing pneumatic nozzle under the different operating condition in term of wirl angle. For thi invetigation, upplied air preure and nozzle configuration ratio were fixed. Thi experimental comparion i of fundamental importance to the undertanding and modeling of turbulent atomization becaue the axiymmetric wirling flow involve relatively complex interaction. For the meaurement, four internal wirl mixing nozzle with axiymmetric hole at wirl angle of o, 3 o, 4 o, and 6 o to the central axi were employed, which i reponible for the enhancement of mixing in pneumatic jet. To illutrate the wirl phenomena quantitatively, the ditribution of mean velocitie, turbulence intenitie, and SMD (Sauter mean diameter, or D 32 ) variation with different configuration ratio were comparatively analyzed. It indicated that the atomization characteritic are performed well in the cae of 3 o of wirl angle, and that turbulence intenitie are gradually degenerated with the increae of radial ditance, howing a light increment of SMD at downtream region. In particular, meaurement howed that nozzle configuration i one of the ignificant geometrical parameter affecting the pray trajectorie. Keyword: Pneumatic jet; SMD (Sauter Mean Diameter); Turbulence intenity; Penetration; Conervation of momentum; Turbulence uppreion; Growth rate Introduction Undertanding the phyic of diintegrationmechanim in pneumatic atomizer will have a fundamental impact on the ability to characterize the optimum nozzle deign. From a practical tandpoint, it i eential to have a clear undertanding of the prerequiite aociated with good atomization. In previou experimental report concerned with wirl mixing atomizer, turbulent behavior ha been oberved. Feyedelem and Sarpkaya [1] tudied the turbulent flowfield created by a round wirling jet. They reported that wirl led to fater preading and quicker mixing of the jet, and the imilarity of the velocity Thi paper wa recommended for publication in revied form by Aociate Editor Jun Sang Park * Correponding author. Tel.: , Fax.: addre: glee239@chonbuk.ac.kr KSME & Springer 8 profile i achieved at ditance a cloe a 4d from the nozzle. But for trong wirl jet, imilarity i not reached within diameter downtream. Breaking up the liquid iuing from the pneumatic atomizer into multitudinou mall droplet i to increae the liquid urface area and to improve the diintegration. In other word, the atomization in two-phae flow i mot effectively achieved by generating a high relative velocity between the liquid jet and the urrounding air reulting from higher momentum by the mutual interaction between working fluid. Much of what i known about important parameter affecting the mixing proce ha been obtained from experiment with air to liquid ma flow ratio (ALR) and geometric configuration of the nozzle. Recent progre toward an undertanding of the diintegration proce ha been achieved in experimental approache of the wirling turbulent velocity, and improved reult have been obtained [2]. With the

2 2 S.-G. Lee / Journal of Mechanical Science and Technology 22 (8) 9~2 development of laer diagnotic, many experimental invetigation on turbulent mixing enhancement have been carried out. Han, et al. [3] invetigated the velocity field and turbulence tructure in the vortex uing an LDV ytem. In an attempt to undertand the diintegration mechanim affecting wirl flow, reearcher [4-] meaured mean and fluctuating velocitie. They howed that the droplet diameter i progreively reduced a the ALR i increaed. Kennedy [6] tudied the diintegration mechanim and pointed out that the SMD changed linearly with the urface tenion while the influence of the vicoity wa minimized. To tudy the flow pattern at the pray boundary region, Lee et al. [7] made phae Doppler particle analyzer. (PDPA) meaurement of the fluctuating quantitie and correlation between volume flux and number denity. In the experiment, the author howed that the maller droplet are inwardly entrained from the pray boundary. An attempt ha alo been made to invetigate the advantage of the internal mixing atomizer. Mullinger [8] howed it merit for the internal atomizer that the atomizing fluid can generally be upplied to the mixing region at a higher preure than the external mixing type. The aim here i 1) to hed ome light on the turbulent mixing flow and diintegration characteritic, and 2) to determine the geometrical effect iuing from the internal mixing wirl jet. l l o 1 mm d p = 1.mm Air Inlet l ap α D do α = 118 l wp Water Inlet Fig. 1. Specification of nozzle ued for the experiment Experiment arrangement, procedure, and diagnotic The nozzle configuration ued to etablih counterwirling mixing of an axiymmetric jet i chematically hown in Fig. 1. The body of the prototype nozzle for generating a counter-flowing pray wa fabricated of bra. The dicharge orifice diameter ( d o ) i 2 mm, wirl chamber diameter ( D ) i 9 mm, and the length to diameter ratio of the dicharge orifice i.6 (l o 1.3 mm). The working fluid were flowed through the tangential port that reult in an angular velocity between two fluid, interacted together in the mixing chamber and injected into the quiecent ambient air at room temperature. The experimental apparatu i hown chematically in Fig. 2. Continuou and teady flowing water and the pulation-free air are upplied to the mixing chamber from the preurized torage tank. Working fluid were properly filtered and regulated. A number 1) Air compreor 2) Air torage & preure gauge 3) Surge tank 4) Air filter 9) Travering ytem ) Laer (Ar+ ion) 11) Tranmitting optic 1 12) Tranmitting optic 2 6 ) Preure regulator 6) Water torage tank 7) Flowmeter for the air & liquid 8) Nozzle 13) Receiving optic 1 14) Receiving optic 2 ) Signal proceor 16) Computer Fig. 2. Experimental etup and diagnotic. of valve, preure gauge, and flow meter were et up to control the flow rate. Experiment were conducted for a liquid flow rate kept contant at 7.9 g/ and the air preure were gradually increaed from kpa to kpa, and ALR could be varied from.4 to.132. A phae Doppler particle anemometer wa intalled to meaure the droplet pray behavior. Information i provided here on the individual particle ize between 1 µm and µm paing through the meaurement volume. The focal length of the tranmitting and receiving optic were 4 mm and

3 S.-G. Lee / Journal of Mechanical Science and Technology 22 (8) 9~2 211 mm, repectively. The photo-multiplier detector voltage of 14 V wa optimized to provide the greatet enitivity, and 4 cattering wa made in the forward direction. Alo, a Bragg cell wa ued to hift the frequency of one beam by 4 MHz to provide directional enitivity. The data acceptance rate in thi experiment wa too low for ditance le than mm from the nozzle exit. The reaon for the low S/N ratio were uually attributed to the preence of nonpherical particle in the PDPA probe volume. Becaue the PDPA work on the principle of light cattering by pherical particle, ignal of non-pherical particle will be rejected by the intrument. The data acceptance rate varied from 6% to 98% depending on the experimental condition and the location of the probe volume in relation to the pray geometry. Radial profile of a geometric equence pace at each meaurement location were obtained at ix axial poition downtream from the nozzle exit, repectively. The z coordinate correpond to the downtream direction at the nozzle exit and y ignifie radial outward direction. The meaurement volume can be poitioned eaily at variou tation without moving the diagnotic ytem in three orthogonal direction by uing a computer-controlled travering ytem that permit poitioning to within.2 mm. The droplet quantitie were calculated by collecting, ample data at each point. The ampling time depend on the local number denity of drop, and ec. wa et a the upper limit to record data. Precaution for accurate meaurement were taken to avoid poible ource of error during the experiment, uch a mitracking the particle, nozzle vibration, and the reading of the flow meter, etc. Alo, mit of mall droplet were dicharged to an exhaut ytem to prevent plahing. To etablih the repeatability of the data received, each profile wa meaured at leat twice at different time. 3. Reult and dicuion Component of mean axial and tangential velocity tatitic provide more detailed information of flow field characteritic in wirling jet. The radial profile of axial and tangential mean velocity ditribution both at uptream and downtream region with four different wirl angle are illutrated in Fig. 3 through 6, repectively. Fig. 3 how the typical decay rate of mean axial velocity, revealing their axial momentum. Regardle of thoe different wirl angle, Axial velocity (m/) 2 = o =4 o =6 o Radial ditance (mm) Fig. 3. Variation of axial mean velocity ditribution for different wirl angle meaured at uptream location of Z/do =. the droplet from both region are qualitatively conidered to be an explicit flow-imilarity. Reult along the axi at uptream region are particularly helpful in defining the nature of axial momentum depite it wirl component, a hown in Fig. 3. Initially, there i an almot maximum velocity zone that i aociated with the potential core region. It indicate the highet momentum ranging from 17-2 m/ at the centerline, but abruptly decreae a a function of radial ditance up to 1-7 m/. However, the magnitude of tangential velocity component how quite intereting phyical phenomena a indicated in Fig. 4. A the droplet move downtream, the propertie in the central region reveal farther downtream propagation due to eay acce of atomizing air, wherea the acceleration at the pray boundary (approximately r= mm) are dicernibly le by the lo of axial momentum and the urrounding drag. A hown in Fig. 4, the minimum value of tangential velocity i along the central axi, which i attributed to the downtream momentum. However, at approximately r=6 mm the tangential velocity increment reache it highet, revealing a wirl inclination for the turbulent jet to be propagated into the outward. Thee finding can be attributed to the fact that the pray behavior near the axi eem to have higher momentum and i ubequently ubject to higher acceleration even though it geometric condition are diimilar. Thi pray behavior explain relatively well why the principle of conervation of axial momentum remain contant even though the liquid droplet pread out at an angle due to the centrifugal force. But the effect of nozzle configuration for are quite

4 212 S.-G. Lee / Journal of Mechanical Science and Technology 22 (8) 9~2 2 2 Tangential Velocity (m/) = o =4 o =6 o Tangential velocity (m/) = o = 3 o = 4 o = 6 o Radial ditance (mm) Fig. 4. Variation of tangential mean velocity ditribution for different wirl angle meaured at uptream location of Z/do =. Axial velocity (m/) 2 At downtream (Z/d o =6) = o =4 o =6 o 2 Radial ditance (mm) Fig.. Variation of mean velocity ditribution for different wirl angle meaured at downtream location of Z/do=6. viible, indicating that the ditribution for the cae of are much maller even in the central part. For the cae hown here, the difference both in axial and tangential mean velocity variation can be a poible prognoi to predict and deign the optimal wirl atomizer. The effect of wirling flow can be mot clearly een by comparing two profile, i.e., in axial and tangential component. Even though the pray pattern are imilar a hown in Fig. 3 through 6, big difference in magnitude of velocity a a function of radial ditance are apparent between two component. However, the growth rate, or the pray diperion angle in Fig. 4 and 6 baed on the lope of the radial poition doe not indicate bigger difference among thoe wirl angle. However, radial ditribution for both axial location a hown in Fig. 3 and are een to be geometrically ymmetric about the pray axi, howing nearly qualitatively conitent value. 2 Radial ditance (mm) Fig. 6. Variation of mean tangential velocity ditribution for different wirl angle meaured at downtream location of Z/do=6. In particular, the tangential ditribution at the downtream region are found to be relatively uniform, having a maller magnitude a hown in Fig. 6. Thee variation are indeed mall when compared to the change oberved in the axial mean velocity ditribution. Thi behavior indicate that the momentum to be pread out tangentially i conitent with the principle of conervation of angular momentum, not the ame phenomenon a illutrated in Fig. 3 and. Tangential velocitie in the center, epecially, how minimum value, which comprie the maximum in axial velocity a hown in Fig. 3 and. Thi i mainly becaue the effect of downward axial penetration and momentum tend to ubide the pray diperion inclination. On the other hand, the pray trajectory in tangential component at uptream region exhibit a progreive diperion a it goe to the outer region up to r=6 mm, hifting the location of maximum velocity at all condition. After reaching a maximum value, the velocitie are luggihly decreaed. But, at the downtream region a hown in Fig. 6, the radial poition of maximum tangential value in magnitude i lightly hifted to r=11 mm. Similar but le obviou peak can be oberved. At thi location, the axial momentum decreae and, thereby, the wirl inclination increae, upporting the fundamental and typical concept of wirl atomizer. The axial turbulence intenitie and droplet diameter of SMD variation in Fig. 7 through meaured under different wirl angle condition diplay the ame trend oberved in Fig. 3 to 6, indicating quite comparable pray tranport.

5 S.-G. Lee / Journal of Mechanical Science and Technology 22 (8) 9~ = o.4 = 3 o = 4 o.4 u' rm / U m.3.2 = 6 o u' rm / U m.3.2 = o =4 o.1 Radial ditance (mm) Fig. 7. Ditribution of turbulence intenitie for different wirl angle meaured at uptream location of Z/do=. 1 =6 o.1 2 Radial ditance (mm) Fig. 9. Ditribution of turbulence intenitie for different wirl angle meaured at downtream location of Z/do=6. = o 1 SMD (µm) =4 o =6 o 4 Radial ditance (mm) Fig. 8. Ditribution of SMD for different wirl angle meaured at uptream location of Z/do=. The maximum axial turbulence intenitie for both cae in Fig. 7 and 9 are located in the center, while having comparatively maller value toward the pray boundary. Interetingly, for both cae a indicated in Fig. 7 and 9, the magnitude of turbulence intenitie for ha the highet value among thoe four wirl angle. Thi phenomenon how a different trend in axial mean velocity variation a dicued in Fig. 3 and. Actually, it ha nearly the minimum value in mean velocity, cauing the droplet with weaker axial momentum to have larger active turbulent diintegration for. Comparion of Fig. 7- reveal that equivalent peak turbulence level occur in approximately the center region. Thi explain why the pray acquire larger velocity fluctuation for all the cae in the center a an acceleration tage baed on the principle of conervation of axial momentum. But, an intereting reult can be drawn from thi. Even with a higher axial SMD (µm) = o =4 o =6 o 4 2 Radial ditance (mm) Fig.. Ditribution of SMD for different wirl angle meaured at downtream location of Z/do=6. momentum cloe to the nozzle exit, the downtream turbulence intenitie are much higher than the uptream. Thi i preumably caued by the nonpherical particle or le diintegrated droplet at uptream. Alo, the overall turbulence uppreion effect i evident at the region depite higher repone to the axial momentum. The turbulence data together with the correponding SMD variation in Fig. 7- illutrate that the jet repone to wirl angle i noticeably robut and comparatively enitive to the cae of 3 o of wirl angle. At higher turbulence intenity level a hown in Fig. 7 and 9, the lower the SMD diameter on the whole a compared in Fig. 8 and. Conequently, a the pray are in the proce of diintegration, it i conidered that better atomization of the droplet could be oberved at the downtream region, which i thought to be one of the characteritic in a wirling mixing nozzle. In particular, the

6 214 S.-G. Lee / Journal of Mechanical Science and Technology 22 (8) 9~2 uptream droplet even in the center part are virtually larger than that of the downtream region. A hown for all the cae, the initial increae in SMD i due to the poibility of coalecence from the le atomized droplet in pite of trong penetration, howing that SMD decreae from approximately 9- at uptream to 7-8 at downtream. In contrat to the uptream tranport, however, the SMD in the pray periphery gradually increae, which might be explained by the entrainment of mall droplet from the outer part to the central region. Thi reult wa reported in the previou reearch that the maller droplet in the peripheral region tend to be entrained inwardly and the larger one are inclined to remain in the region. The maller droplet at uptream are le abundant in the center region, and the preence of relatively maller one near the boundary i acribed to a ditinctive feature in thi wirl nozzle. Alo, it i intereting to oberve that the variation in SMD for i the lowet among thee nozzle. It i alo confirmed that the particle with lower axial momentum caue brik turbulence fluctuation to bring about better atomization in the pray field. 4. Concluion Velocity, turbulence intenity and SMD meaurement have been obtained in a pneumatic wirl flow. Reult were obtained for different wirl angle to tudy the effect of geometric configuration on the turbulence flow-field characteritic. Pending a more detailed examination of the flow-field development under the increae of ALR a well a the length-todiameter ratio of nozzle tip, the current finding repreent trong characteritic in wirl flow. Although the magnitude of axial velocity ditribution for the cae of i much maller even in the central part, it explain the poitive effect for diintegration. Tangential velocitie in the center, epecially, how minimum value, which comprie the maximum in axial velocity. Thi i mainly becaue the effect of downward axial penetration and momentum tend to ubide the pray diperion inclination. Thee finding can be attributed to the fact that the pray behavior near the axi eem to have higher momentum and i ubequently ubject to higher acceleration even though it geometric condition are diimilar. Epecially, the turbulence intenitie for the cae of are higher than the other cae, indicating that trong fluctuation of finer droplet would be developed in thi cae. The SMD variation illutrate that the jet repone to wirl angle i noticeably robut and comparatively enitive to the cae of 3 o of wirl angle. In particular, the uptream droplet even in the center part are virtually larger than that of the downtream region. There i the poibility that the initial increae in SMD i due to the coalecence from the le atomized droplet in pite of trong penetration. Thu, the nozzle configuration with a wirl angle of 3 o to the central axi i recommended for better diintegration compared to the other one. Nomenclature d o : Final dicharge orifice diameter d p : Diameter of paage for the fluid D : Swirl chamber diameter l o : Length of final dicharge orifice (1.3 mm) l : Length of wirl chamber ( mm) l ap : Length of air paage ( mm) l wp : Length of liquid paage ( mm) : Swirl angle of the inlet paage for the fluid ALR : Air to liquid ma ratio D 32 : Sauter mean diameter R : Radial ditance U m : Maximum axial velocity at the centerline u rm : Root mean quare of the axial fluctuating component U : Axial mean velocity W : Tangential mean velocity Z : Axial ditance from the nozzle tip Reference [1] M. S. Feyedelem and T. Sarpkaya, Free and Near Free Surface Swirling Turbulent Jet, AIAA Journal, 36 (3) (1998) [2] A. Manour and Norman Chigier, Diintegration of liquid heet, Phy. Fluid A., (199) [3] Y. O. Han, J. G. Leihman and A. J. Coyne, Meaurement of the Velocity and Turbulence Structure of a Rotor Tip Vortex, AIAA Journal, 3 (3) (1997), [4] S. G. Lee, B. J. Rho, J. Y. Jung and S. J. Kang, Analyi of Turbulent Flow and Diintegration Characteritic Featuring the Counter-Swirl Pneumatic Nozzle, 4 th JSME-KSME Thermal Engineering Conference, Oct. 1-6, Kobe, Japan. [] S. G. Lee and B. J. Rho, Atomization characteritic

7 S.-G. Lee / Journal of Mechanical Science and Technology 22 (8) 9~2 2 in pneumatic counterflowing internal mixing nozzle, KSME International Journal, 14 () (1), [6] J. B. Kennedy, High number SMD Correlation for preure atomizer, Journal of Engrg., for Ga Turbine and Power. (1986) [7] S. G. Lee, B. J. Rho and K. K. Song, Turbulent diintegration characteritic in twin fluid counter flowing atomizer. 39 th ALAA Aeropace and Science Meeting and Exhibit, AIAA 1-47, (1), Reno Nevada, USA. [8] P. J. Mullinger, The deign and performance of internal mixing multijet twin fluid atomizer, Journal of the Intitute of Fuel. (1974)