Drolets disersion and deosition measurements in an axisymmetric sudden exansion low Sylvain Aguinaga 1,2,3,4, Jacques Borée 1, Olivier Simonin 2, Jean-Paul Bouchet 3, Vincent Herbert 4 1: Laboratoire d Etudes Aerodynamiques (L.E.A.) ENSMA/CNRS, Chassenueil-Futuroscoe, France 2: Institut de Mécanique des Fluides de Toulouse (I.M.F.T.)/CNRS/INP/UPS, Toulouse, France 3: Centre Scientiique et Technique du Bâtiment (C.S.T.B.), Nantes, France 4: PSA Peugeot Citroën, Velizy Villacoublay, France, sylvain.aguinaga@gmail.com Abstract This study is about drolets disersion in an axisymmetric sudden exansion low. It resents in the same exeriment acility both drolets disersion and deosition at the wall. The main hase low is characterized by PIV measurements. Drolets are injected inside the shear layer, they rogressively interact with the main hase turbulence. Two exansion rates and two inlet velocities are considered, allowing thus dierent resonses o drolets to the turbulent structures o the shear layer. Their disersion in the sudden exansion is analyzed with PIV and PTV measurements. A new method or measuring articles deosition at the wall based on the UV luorescence o a dye is resented. The drolets deosition rates are analyzed and comared to the articles disersion in the sudden exansion. The drolets wall normal velocity PDF are calculated close to the wall and analyzed in order to link the deosition rate to the articles disersion in the near wall region. 1. Introduction Two-hase low is becoming an imortant issue in automotive alications, not only or engine alications (combustion) but also or external aerodynamic or under-hood lows. Rain, og, or rojections rom wheels have a signiicant imact on security (visibility rom the rear and ront windshield, rear-view mirrors) and the aestheticism o the vehicle. Reliability issues are also concerned regarding the interaction o drolets with electrical organs located under the hood. The automotive industry needs to have reliable tools in order to understand and redict such two-hase lows. The low around a vehicle as under the hood is turbulent and comlex, made o searations and reattachment oints, shear layers and recirculation zones. The ath o drolets in such lows is quite diicult to redict. Indeed regarding the ratio o the drolets inertia to the strength o the local turbulence, drolets artially resonse to the turbulence. This henomenon increases the random motion o drolets due to the turbulence. As a consequence it is diicult to redict where water drolets will diserse and then deosit. The urose o this study is to evaluate exerimentally the main henomena o water drolets disersion in a tyical low and then obtain an exerimental database or hysical analysis and comarisons to numerical simulations. The resent study deals with an axisymmetric sudden exansion low. It has been identiied as a tyical academic low case which can be encountered in automotive alications. It is comosed o two tubes o dierent diameters connected together. When the main low rom the irst tube reaches the ste it decelerates and orm a otential core bounded by a shear layer. This shear layer exands to the wall into a reattachment oint. Between the ste and this reattachment oints a recirculating low is created [Szczeura(1985)]. So this low includes three mean tyical low conigurations that can be encountered in automotive alications. Drolets disersion in such a coniguration has already been studied [Ahmadi and Chen(1998)] [Hardaluas et al(1992] [Sbrizzai et al(2004)]. Precedent studies have shown that the interaction o drolets with this low is quite comlex due to the rogressive interaction o the drolets with the main hase turbulence. [Ahmadi and Chen(1998)] show that the random motion o drolets may lead them whether or not inside the recirculation zone where they would either deosit at the wall or be - 1 -
retrained by turbulence inside the low. O course the magnitude o this eect is strongly linked to the ratio o articles' inertia to the local main hase turbulence. The Stokes number is an interesting arameter to evaluate this henomenon: τ St = τ 2 ρ d τ = is the article resonse time, where ρ is the density o articles, 18µ d is their diameter and µ is the dynamic viscosity o the main hase. τ is a luid turbulence time scale. 40µm water drolets are injected close to the ste, right into the shear layer where they will diserse. This diameter leads to an interesting range o Stokes number along the ath o the drolets rom their injection in the shear layer where St 100 to the reattachment oint where St 0.1. Between these two extremes the Stokes number is close to 1, which means that articles have a artial resonse to turbulence and will be able or not to go inside the recirculation zone. Close to the wall the luid-articles interaction is more comlex due to a high gradient o turbulence inside the boundary layer. This interaction aects the deosition rocess. I the *2 τ u dimensionless resonse time τ = is high (i.e. τ > 100 ) articles will cross the boundary υ layer without interacting with the turbulence thanks to their inertia. I it is small (i.e. 1 τ < 40 ) articles will have a comlex behavior relative to the wall turbulence and the robability o their deosition decreases with the square o τ. A lot o authors have already studied that roblem esecially in the case o channel lows [Kallio and Reeks(1989)] [Marchioli et al(2003)] and it is still an oen issue. Several DNS studies are still conducted in order to understand the hysical henomena included in the deosition rocess. Close to the reattachment oint, the deosition rocess is exected to be even more comlex due to the low coniguration at this location. So, it can be interesting to have both inormation on articles disersion in this area and the deosition rate at the wall associated. This aer will irst resent the exerimental setu with the low generation and the measurement devices used, including a new method or deosition rate measurements based on a UV luorescent dye and images treatment. Finally, the results will resent the sudden exansion two hase low with a articulate emhasis on the link between the drolets disersion by large eddies and the deosition rate at the wall. The PDF tool will be then used in order to highlight the deosition mechanisms. 2. Exerimental Setu & Measurements technique 2.1. Generation o the two-hase low The exerimental acility (Figure 1) is comosed o a irst Plexiglas tube, Φ 1 =74mm and 1m long, connected to a bigger tube either Φ 2 = 284mm or Φ 2 = 190mm and 1.5m long which constitute the sudden exansion. Two exansion rates and ste size h are then available and then will be reerred as coniguration 1 ( Φ 2 = 284mm h=105mm) and coniguration 2 ( Φ 2 = 190mm h=58mm). The low is generated thanks to a centriugal um which creates an asiration through a lenum connected to the second tube. The lenum disconnects the sudden exansion low rom the um asiration. The inlet includes a convergent with honeycomb and a seciic device which < - 2 -
ensures the transition to a turbulent boundary layer. The mean low velocity in the irst tube is either 10m/s or 20m/s (later they will be reerred as 10m/s or 20m/s coniguration). Each diameter o the second tube is available either in Plexiglas or glass or laser and UV measurements. The ste, the irst tube and the inlet can be disconnected rom the second tube in order to access the inside wall or cleaning or remove deosited water drolets. z is the sudden exansion axis oriented with the low, y g = where g is the gravity. The ste g is located at z = 0 and the sudden exansion axis is the ( x = 0, y = 0 ) line. Most o the measurements are done in the x = 0 lane. The subscrit stands or luid quantities, or drolets. The velocity is decomosed in mean and luctuating velocity as u = U u'. The water drolets are injected at the wall o the irst tube, 2 cm ustream the ste i.e. x = 0, y = 0. 037m, z = 0. 02m. It has been ound to be the best way to seed the shear layer with drolets. Indeed, i the low is seeded at the inlet, many drolets would deosit on the irst tube and orm a ilm which will roduce huge dros as reaching the edge o the ste. Unortunately this coniguration breaks the axisymmetry or the disersed hase low. An ultrasonic atomizer rototye o the PolySray comany is used. Inormation about this device can be ound at [Dumouchel et al(2003)]. It is comosed o two arts linked by two iezos which oscillate at a high requency ( 40kHz). Water is driven at the to o the vibration art and orm a ilm. Instabilities at the surace o the ilm lead to drolets emission. This method ensures a narrow 3 diameter distribution (with mean at 40µm, τ 5 10 ). The drolets size distribution has been measured with a Malvern Sraytec and via a shadowgrahy method. The initial velocity o drolets without wind has been measured thanks to PIV. It has been ound that their velocity is lower than 1m/s. The atomizer is ed thanks to a syringe um roviding accurate and low eeding rates, in the resent case 500ml/h. 2.2. PIV & PTV Measurements 2.2.1. PIV System Figure1: Exerimental Facility Figure 2 Drolets disersion visualization in a laser sheet Reliable inormation on the main hase are required in order to understand articles interaction with turbulence. A PIV system has been used, it is comosed o a LaVison FlowMaster camera (1280 er 1024 ixels resolution 12bits) and 120mJ YAG double Pulse Laser. The laser sheet is shot rom a straight window located at the rear side o the lenum (Figure 2). Seeding is made owing to a smoke generator which roduces a og o 1µm articles. A seeding tent has been build around the inlet in order to seed a huge volume o air. This device rovides a uniorm seeding in the - 3 -
low. The treatment sotware used was FlowManager rom Dantec. Background removal (subtraction o the mean image o the set) was used when walls were visible. Velocity ields were obtained using the adative correlation algorithm. It is comosed o three inter-correlation stes, the irst one with 64*64ixels interrogations windows, the second and third ones with 32*32 ixels interrogations, each time with an overlaing o 50%. Each ste constitutes an initial guess or interrogation windows deormations and dislacements or the next ste. This algorithm allows obtaining accurate velocity ields where both high and low velocities are resent like in the shear layer area. The number o image airs required to obtain accurate statistics was obtain thanks to a test o convergence o a velocity ield located on the shear layer. It has been ound that 500 image airs are enough to obtain statistics with an error lower than 2% comared to the statistics calculated with 1000 image airs. PIV ields size was 175mm er 140mm with a satial resolution o 2mm. Additional zoom ields with 109mm er 87mm windows, 1.3mm resolution, were also taken close to the wall at the reattachment oint, in the corner o the ste and in the mid o the recirculation zone PIV ields size was 175mm er 140mm with a satial resolution o 2mm. Additional zoom ields with 109mm er 87mm windows, 1.3mm resolution, were also taken close to the wall at the reattachment oint, in the corner o the ste and in the middle o the recirculation zone. 2.2.2. PTV System Considering the small size o articles and the low drolets injection rate, it is assumed that there are no two-way couling eects, i.e. the disersed hase low doesn't change the main hase low. So drolets velocity statistics can be comared with the ormer PIV measurements. Figure 2 shows the drolets disersion in a laser sheet ( x = 0 lane). The concentration o drolets is much more imortant inside the shear layer than in the recirculation zone. As a consequence PIV can be used or drolets measurements inside the shear layer, but there are not enough articles to have accurate measurements inside the recirculation zone. That's why Particle Tracking Velocity (PTV) measurements have been used inside the recirculation zone. Treatments are made thanks to an algorithm develoed in the rame o Laure Vignal PhD thesis [Vignal(2006)] This algorithm can be searated in three dierent oerations. The irst is a hase searation which is achieved with a local threshold unction alied to the intensity o the image. Ater this stage, articles are detected and centers o all articles are calculated with a mass center method. The second oeration is the article airing between two amilies o articles detected in each rame. For a more accurate airing, a reliminary PIV measurement is made in a large mesh (128*128 ixels) to give the aroximated velocity. From its direction and intensity, a dislaced interrogation window is considered (20*20 ixels), and the best airing o all articles in this searched. The airing is based on a minimization o velocity variances in this interrogation window. Ater this article airing, recise velocity is calculated by cross-correlation o small regions around article images (commonly 8*8 ixels). Sub-ixel dislacements are used in order to avoid "eak-locking" eect. 2.3. Wall deosition measurements method A lot o authors have already studied the roblem o wall deosition measurements, esecially in the case o channel lows. The most amous exerimental database is the Liu & Agarwal's one [Liu and Agarwal(1974)] where deosition was evaluated measuring the concentration o an uramine dye inside water which cleaned the channel. [McCoy and Hanratty(1977)] roosed a comilation o many exeriments in channels lows and established ittings or the well known deosition curve which lots the deosition rate against τ the dimensionless resonse time o articles. Many - 4 -
methods or deosition rate measurements can be ound in the literature, via wall cleaning, wall ilm low rate measurements, or catching solid articles on adhesive suraces. Those methods in the case o drolets are not useul or comlex geometries (car windshields or examle). 2.3.1 Fluorescence and image treatment Here is resented a method or the drolet deosition rate measurement at the wall based on a water ilm thickness measurement technique already resented by [Ribes et al(2005)]. It is roosed to use this method to measure low water volumes deosited at the wall, and through time tracking, to assess the deosition rate. A luorescent dye, the Uvitex 2BT is used. Under Ultra Violet (UV) illumination this dye emits a aint blue light by luorescence. The key is to link this luorescence to the local thickness o the deosit ilm or drolets. The Beer Lambert law links the thickness to the absorbance or a given wave length I O A = λ log I t I t is the transmitted intensity through a thickness e o a solution, I 0 is the incident intensity. The absorbance is also given by the ollowing law: = C e Aλ UV ε λ C UV is the concentration o the dye in the water (here 0.6/1000), andε λ is the molar absortion o the Uvitex at the wave length λ. It has been ound via sectrohotometry that the maximum absorbance is or λ =348nm. So thickness can be measured thanks to the ollowing law I t = I 0 ( 1 ex( CUV e ε λ )). I the emitted luorescence intensity I is modeled by a linear law I = Φ I then one inally obtains Figure 3 From the image to the volume, examle o image treatments or water volume measurement t I ( 1 ex( C )) = ΦI0 e ε λ A digital camera gets a light intensity I and give a digital image which is a matrix o grey levels g l = G I oset. G is the gain o the camera, oset is the so called black-level (grey level or zero light intensity) and γ is a non linearity coeicient o the CCD. Finally the local thickness can be linked to the grey levels o an image via the simliied law: the simliied transer law is ( ) γ UV [ A( 1 ( B e) ) C] γ g l = ex A deends on the illumination source and camera settings (aerture, gain, integration time). B deends on Uvitex intrinsic characteristics (molar absortivity, concentration in water), C is linked g l, - 5 -
to the illumination source and camera oset. The A, B and C coeicients are ound thanks to a calibration. Eight dierent calibrated water ilm thicknesses were made: 100µm, 150µm, 200µm, 250µm, 300µm, 400µm, 600µm and 1mm. Each samle is laced into the measured area under UV illumination and the mean grey level over the samle is measured. A routine based on least squares method is used to interolate the eight measured samles and to deduce A, B and C coeicients. The calibration curve then obtained is the one used to transorm grey levels rom any image taken in the very same conditions into ilm thicknesses. An examle o calibration is given igure 4. An image o a graduated ruler is also taken to calculate the scale actor o ixels. Knowing then the area o a ixel and the local thickness associated, one can deduce, by summation all over the ixels, the volume o water resent in any region o interest. The tyical area o interest or [Ribes et al(2005)] in automobile alications is 1m wide. Here the tyical window is 15cm wide. Volumes measured start rom 10µl to more than 10ml. These changes o scale induce extremely rigor in the calibration method and measurements. Indeed, additional image treatments have been required. An otical ilter has been used, the ilter cuts all the wavelengths below 420nm, it revents rom direct relections o the illumination source. Then the only source o light which is recorded comes rom the luorescence. Figure 4 -Beer-Lambert law interolation Figure -5 Validation tests A limitation o the Uvitex is the eect known as "hotobleaching": the luorescence o the dye doesn't remain constant through the time, it is ading. The more owerul the source o UV is, the aster the ading goes. It has been tried to model this henomenon, but it is a non linear unction o the time and o the ilm thickness. So, solution is to limit this eect. Initially, the UV illumination was rovided by two Phillis 36W luorescent tubes (blacklight). These tubes allow an imortant and uniorm illumination centered at λ =365nm. But the hotobleaching eect is too imortant with such an illumination in the coniguration used or deosition measurements. For examle the 400µm calibrated thickness loses 35% o its initial luorescence ater 30s, 55% ater 60s. As a consequence, it has been decided to use Light Emitting Diodes (LEDs) as a UV source. The main advantage is that the illumination can be ulsed, with a short time exosure, just during the time the image is taken, and with good reroducibility at each ulse. A weakness o the LEDs is the wave length o the illumination, it is centered at 390nm. The absorbance o the Uvitex is oor at this wave length comared to the one at 365nm. That's the reason why additional care is needed while using LEDs illumination and additional image treatments are required. - 6 -
2.3.2.Validation tests The method has been develoed making numerous validation test cases. The inal test case was the successive deosition o 20µl dros. The drolets were deosit with a calibrated iette. Figure 5 resents various tests which have been conducted. For each case, it lots the measured volume against the injected one. I a linear regression is done indeendently on each test, the mean sloe is 0.97 with 10% error. The absolute error is located at the initial ordinate. This is due to the background image which leads to a non zero volume when there is no water on the image. Convenient method to take into account the background has not been ound yet. The best and more eicient way is to remove the volume calculated without water on the image. But the urose o those measurements is the evolution o the volume along the time, so a 10% on the sloe is accetable. 2.3.3. Wall deosition rate measurements The method is alied to the case o the sudden exansion low. It is a comlex case rom the oint o view o the deosition rocess. Figure 6 resents the assembly which links the LEDs and the camera and which was used to insect the bottom o the exansion low. The average image size was 70mm along the x axis er 95mm along the z axis. They were divided in 4 arts along the z axis and the volume was monitored in each art o the image along the time. Images were taken every three seconds in coniguration 1 and every second in coniguration 2. Figure 7 resents a tyical evolution o the measured volume along the time. Snashots are also included to icture the deosition igure o the drolets at the wall. One can consider three arts in the lot: a irst art where the volume increases raidly, a second where the rate is constant or a long time, and a third art where the volume sky rockets. This last art corresonds to the deosited water rom the side walls which slis to the bottom. The second art is where the deosition rate is evaluated. Actually, the deosition rate is the sloe o this art. It corresonds to the deosition o thousands o little drolets which create water agglomerates to the wall. With the drolet diameter chosen and the velocities o drolets evaluated close to the wall, the criteria rom [Mundo et al(1995)] shows that there's no slashing or bouncing, drolets simly deosit at the wall. The irst art o the curve is an establishing time. It has not been ound whether this time corresonds to a hysical establishing time o the deosition rocess (establishing the articles build u in the boundary layer) or i it corresonds to the limitation o the measurement method as the volume measured in this art is very low and diicult to obtain in a validation test cases. However the length o the second art is suicient enough to have a good evaluation o the sloe, i.e. the deosition rateφ d Figure -6 Assembly or deosition measurements Figure -7 Examle o the evolution o water volume deosited along the time, grey levels o images are extended or better visualization - 7 -
3. Results Figure 8- (a) Fluid turbulent kinetic energy k, white line is the zero axial mean velocity contour, (b) Dimensionless deosition rate, (c) Wall normal luid luctuation roile at 6mm rom the wall 3.1. Drolets disersion Figure 8 (a) shows the turbulent kinetic energy k measured via PIV in the x=0 lane or the 2 2 20m/s coniguration. k is calculated as k = 1 2( 2 u' y u' z ) assuming that u ' x u' y and which has been conirmed via additional three comonents hot wire measurements. One can see the shear layer turbulence develoment in the sudden exansion low. The shear layer exands to the wall and creates a recirculation zone. It has been outlined by lotting the zero axial velocity contours. The reattachment oint is located at z r / h = 9. 2 or coniguration 1 and z r / h =10. 1 or coniguration 2, Szczeura gives z r / h = 9.46 and Hardaluas z r / h = 11. Figure 9 shows an axial roile located at y=-0.035m, inside the shear layer, it resents comarisons between mean and luctuating velocities o the two hases. The drolets velocities have been extracted rom PIV ields taken in the same conditions than the luid one. One can see the rogressive drolets acceleration rom their release oint. The luid led the drolets till z 1 h = 0.5 i.e. z1 50mm. So, drolets reach the luid velocity within a distance o 52mm at an average velocity o 5m/s (rom 0 to luid velocity 10m/s at y=-0.035m) so within a time t 10ms, which nearly equals twice the drolets resonse timeτ 5.10-3 s. The settling velocity is given by τ g 0.05m s and is low comared to drolets velocity. So, the inluence o gravity is low at this / location. Figure 9 also shows the anisotroy o the luid turbulence as the axial luctuation is twice the radial one. The anisotroy o articles is even more ronounced as the axial luctuation is more imortant than the luid one, and less imortant or the radial one. - 8 -
Particles are transorted in the shear layer which exands. Turbulent structures become larger and the drolets turbulent Stokes number decreases. Downstream the otential core, the low is made o large eddies which transort articles to the near wall region as it can be seen in igure 2. The eddy u' y lie time o this large structures is given by te =. I the maximum o u' y ( 2.6m/s) is taken h τ into account or the calculation, that leads to a turbulent Stokes number S t = o 0.12 or te coniguration 1 and 0.02 or coniguration 2. So drolets are exected to ollow those eddies to the wall. 3.2. and deosition Figure 8 (b) shows the results o deosition rate measurements along a stri rom the ste to downstream the reattachment oint at the bottom o the sudden exansion, or coniguration 1 and 2 and both inlet velocities. The deosition rate is made dimensionless as ollowing k φd = S D 0 where S is the surace where φ d is measured and D 0 is the eeding rate o the ultrasonic atomizer (here D 0 =500ml/h). A suicient number o tests have been done so that there was an overla o the ields and every art o the sudden exansion stri has been measured. Figure 8 (b) shows sliding averages made on the whole measurements. Error bars set at 10% are also reresented. The evolution o this average deosition rate is very similar in both conigurations 1 and 2 but three times more imortant in coniguration 2. The irst oint is that the maximum deosition is located ustream the reattachment oint, inside the recirculation zone. For an inlet at 20m/s, the maximum is a lateau ranging rom z / h = 6 to z / h = 9 in coniguration 1, and rom z / h = 8 to z / h = 11 in coniguration 2. In 10m/s coniguration, the deosition is more imortant in the recirculation zone, a maximum eak is more ronounced and located at z / h = 6 or coniguration 1, and z / h = 8 or coniguration 2. Figure 8 (c) reresents the wall normal luctuation roile taken at 6mm rom the wall. It aears that the maximum o deosition rate corresonds to the maximum o wall normal turbulence. So the maximum o deosition is not linked to the mean recirculation coniguration but rather to the trace o the large eddies iminging the wall. Figure 9- Axial roile located at y=-0.035m (let) axial mean drolets and luid velocity (right) axial and radial luctuating drolets and luid velocity - 9 -
3.3 Wall normal PDF analysis Close to the wall the luid turbulence decreases raidly and drolets which are brought by large structures with suicient inertia will cross the boundary layer without interacting with it and deosit. On the other hand drolets which ollow the local turbulence o the boundary layer will have a comlex behavior and may not deosit. Figure 10 shows a comarison between the main hase wall normal velocity PDF and the drolets one in coniguration 1, 20m/s. The luid PDF has been extracted rom PIV instantaneous ields and the drolets PDF rom PTV instantaneous ields. They have been calculated in a bin centered at 6mm rom the wall (2mm wide along y) in the recirculation zone (4.8< z / h <5.25). v stands or the drolet wall normal velocity (v<0 drolet goes toward the wall). It shows that the luid PDF is nearly symmetric and close to a Gaussian distribution, which is not the case or the drolet PDF. Indeed, the shae o the drolet PDF is close to the luid one or v>0, but not or the v<0 art. Figure 10- (let) Comarison between luid and drolets wall normal velocity at 6mm rom the wall. (right) Drolets PDF artitioning. (Coniguration 1, 20m/s, 4.8< z / h <5.25) The idea here is to divide the PDF into the wall normal ositive art and the wall normal negative one ( =0 or v>0 and = ( =0 or v<0), ). At the wall, in the ull absortion case (no slashing or rebound) =0, as no article escae rom the wall. More, articles which go away rom the wall are driven by the turbulence and then are exected to have a 2 c ( v b) Gaussian distribution or v>0. I one interolates a Gaussian gauss ( v) = ex on 2π a 2a² (blue curve a, b and c are identiied with ), it doesn't match. It has been ound that minus the Gaussian interolation o (red symbols) can be interolated by a Gumbel c v a v a distribution gumb ( v) = ex ex ex which is a continuous orm o the b b b Poisson distribution (red curve a, b and c are identiied with ). The addition o the two models gauss gumb gauss matches remarkably the initial measured distribution (black curve). The hot oint is that this artitioning can be alied almost everywhere in the ields measured. The Gumbel distribution reresents the asymmetry o the PDF. This asymmetry may due to a oulation o articles which may not ollow the local turbulence. In our case, they come rom the large structures - 10 -
o the shear layer. At a distance o one mean ree ath l = τ, those articles will cross the boundary layer turbulence and deosit via the so-called ree light mechanism [Friedlander and Johnstone(1957)]. This analysis gave the direction o a new PDF model or modeling article disersion close to the wall and which is currently develoed in the rame o this study. Figure 11 resents PDF artitioning taken at 6mm rom the wall ustream the reattachment oint, or coniguration 1 (10m/s, 8.4< z / h <8.9) and conigution2 (10m/s and 20m/s, 8.6< z / h <9.4). For each artitioning, the ratio r = gumdv dv is calculated in order to v v< 0 v< 0 evaluate the asymmetry due inertial articles. One can see that the asymmetry is more imortant in coniguration 2 than in coniguration 1. A higher amount o articles centriuged by larges structures reaches the near wall region in coniguration 2. It is linked to higher turbulence level close to the wall in coniguration 2 and that could exlain that the deosition rate is three times higher. More, it aears that the asymmetry is less imortant in 20m/s coniguration than in 10m/s one. Residence time o articles in the large structures is higher or the 10m/s coniguration, and then more articles are robably centriuged toward the wall comared to the 20m/s coniguration. That could exlain the higher deosition rate in the recirculation zone. But the deosition rocess can t be resumed by inertial articles which cross the boundary layer turbulence thanks to a ree light rocess [Narayanan et al(2003)]. At a distance which is close to the mean ree ath o articles, PDF analysis shows that there is an imortant amount o drolets which are in local equilibrium with the main hase turbulence. The deosition rocess to the wall or such articles remains comlex to exlain. Figure 11 -PDF artitioning at 6mm rom the wall 4. Conclusions This study is about drolet disersion in a sudden exansion low. It resents in the same exeriment both articles disersion and wall deosition measurements. Classical PIV and PTV measurements were used and a new method or deosition rate measurements based on the UV luorescence o a dye is resented. It has been shown that drolets which are released in the shear layer rogressively interact with its turbulent structures. Downstream the otential core, the turbulent Stokes number o the drolets is less than one and drolets are thus transorted by the shear layer large eddies iminging the wall. Thanks to the new method or deosition rate measurements, it has been ound that the maximum o deosition was located inside the recirculation zone and was linked to the trace at the wall o those large eddies rather than to the mean reattachment oint. A new method or wall normal velocity PDF analysis has been resented. It allows quantiying - 11 -
the asymmetry o the PDF close to the wall. The asymmetry seems to be in accordance with the evolution o the deosition rate between each low coniguration and related to an amount o inertial articles brought by larges eddies rom the shear layer. Deosition mechanisms remain comlex and can t be entirely exlained with the data o the resent exerimental study. However, it constitutes an exerimental database in order to evaluate numerical models or drolets disersion and deosition in a comlex low coniguration. The authors wish to thank eole rom the LEA and CSTB or their hel and advices during measurements camaigns, esecially Patrick Braud and François Paillé rom the LEA and Dominique Lenoir and Patrick Guilbaud rom the CSTB. Reerences [Ahmadi and Chen(1998)] Ahmadi G, Chen Q (1998) Disersion and deosition o articles in a turbulent ie low with sudden exansion. Journal o Aerosol Science 29:1097 116 [Dumouchel et al(2003)] Dumouchel C, Sindayihebura D, Bolle L (2003) Alication o the Maximum Entroy Formalism on Srays Produced by Ultrasonic Atomizers. Part Part Syst Charact 20:1 1 2 [Friedlander and Johnstone(1957)] Friedlander S, Johnstone H (1957) Deosition o susended articles rom turbulent gas streams.ind Eng Chem 49:1151 [Hardaluas et al(1992)] Hardaluas Y, Taylor A, Whitelaw J (1992) Particle disersion in a vertical round suddenexansion. Phil Trans R Soc Lond A 341:411 442 [Kallio and Reeks(1989)] Kallio G, Reeks M (1989) A Numerical Simulation O Particle Deosition In Turbulent Boundary Layers. International Journal O Multihase Flow 15:433 446 [Liu and Agarwal(1974)] Liu BY, Agarwal JK (1974) Exerimental observation o aerosol deostion in turbulent low. Journal o Aerosol Science 5:144 155 [Marchioli et al(2003)] Marchioli C, Giusti A, Salvetti MV, Soldati A (2003) Direct numerical simulation o article wall transer and deosition in uward turbulent ie low. International Journal o Multihase Flow 29:1017 1038 [McCoy and Hanratty(1977)] McCoy D, Hanratty T (1977) Rate o deosition o drolets in annnular two-hase low. International Journal o Multihase Flow 3:319 331 [Mundo et al(1995)] Mundo C, Sommereld M, Troea C (1995) Drolet Wall-Collisions: exerimental studies o the deormation and break-u rocess. International Journal o Multihase Flow 21 No.2:151 173 [Narayanan et al(2003)] Narayanan C, Lakehal D, Botto L, Soldati A (2003) Mechanisms o article deosition in a ully develoed turbulent oen channel low. Physics o Fluids 15 issue 3:763 [Ribes et al(2005)] Ribes C, Bouchet JP, Borée J, Carrega S, Scotto d Aolonia A (2005) Mesure d éaisseur de ilm liquide, caractérisation des débordements d eau de luie sur les vitres latérales d un vehicule automobile en soulerie. In: Fluvisu 2005 [Sbrizzai et al(2004)] Sbrizzai F, Verzicco R, Pidria MF, Soldati A (2004) Mechanisms or selective radial disersion o microarticles in the transitional region o a conined turbulent round jet. International Journal on Multihase Flow 30:1389 1417 [Szczeura(1985)] Szczeura R (1985) Flow charateristics o an axisymmetric sudden exansion. Tech. re., ERCOFTAC Database [Vignal(2006)] Vignal L (2006) Chute d un nuage de articules dans une turbulence diusive. Etude des coulages entre hases ar diagnostics otiques. PhD thesis, Institut National Polytechnique de Toulouse - 12 -