21th International Multidimensional Engine User s Meeting at the SAE Congress 2011, April, 11, 2011 DETOIT, MI

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1 21th Internationa Mutidimensiona Engine User s Meeting at the SAE Congress 2011, Apri, 11, 2011 DETOIT, MI "A combined Euerian Lagrangian spray amortization (ELSA) in DI Diese combustion: Fuy couped Euerian / Lagrangian spray with ECFM-CLEH Combustion mode» A.Desportes, M. Zeat, G. Desoutter, D.Abouri, Y. Liang CD-adapco F. Ravet RENAULT Abstract. The spray atomization mechanisms under the conditions common for the Diese engines are very compex and not competey understood. At the same time the detaied spray information is necessary to provide a better contro of the combustion process. The principa difficuty of numerica spray simuation is the correct representation of the both characteristic zones of spray: dense near the nozze and diute downstream. Combining the advantages of Euerian and Lagrangian approaches, ELSA mode is abe to predict continuousy the whoe spray evoution. In the dense zone the spray and its gaseous environment are presented as an effective singe-phase fuid with a highy variabe density. To describe the dispersion of the iquid, the transport equations for the iquid mass fraction and for the iquid/gas interface density are soved. The transition to the Lagrangian cacuation is appied when the spray is considered to be diuted enough. This compete Euerian-Lagrangian spray atomization mode has been impemented into the computationa fuid dynamics code STAR-CD. The mode impementation is vaidated by comparing predicted iquid and vapour penetrations with experimenta data reported in iterature. Once vaidated, the ELSA mode is couped with an inside nozze simuation to study the impact of interna nozze fow (geometry, cavitation formation) on the spray and its characteristics. Finay, the couping with the ECFM-CLEH combustion mode has been competed in this work and appied to a fu DI Diese combustion process for a typica automotive DI Diese engine operating a fu oad. 1. Introduction The main difficuty of numerica spray simuation is the correct representation of the both characteristic zones of spray: dense near the nozze and diute downstream. The Euerian approach seems to be more appropriate to describe the dense zone because no dropet is formed and even a continuous medium of iquid can exist. So, the description of the spray inside this zone as a set of dropets is not the most suitabe. On the other hand, the use of the cassica Lagrangian approach inside the diuted zone coud permit to benefit of the important background accumuated for this kind of spray representation. Combining the advantages of Euerian and Lagrangian approaches, ELSA (Euerian-Lagrangian Spray Atomization) mode is abe to predict continuousy the whoe spray evoution from the inector nozze to the fina diute spray region. Moreover it is we known that the detais of the interna nozze fow induce consideraby the spray formation and its characteristics. It is especiay true when cavitation occurs inside the nozze. The numerica simuation of this fow coud provide the reaistic conditions to initiaize ELSA simuation and so, take into account the veocity and turbuence distribution over the nozze exit section as we as their tempora evoution during inection. The vaidations of the ELSA mode impemented into the commercia code STAR-CD were composed of 2 experimenta data bases [8, 11]. The present paper contains ony the vaidation based on the experimenta data base of Sandia [8]. This data base incudes the measurements of iquid and vapour penetrations obtained for non-vaporizing and vaporizing sprays under in-cyinder thermodynamic conditions around inection timing. The second phase of this work is the couping of the inside nozze fow with ELSA mode. We aso, present the first appication to a compete DI Diese engine (Sector mesh) operating at fu oad. Resuts of the detai around the spray and the combustion history are aso presented.

2 2. Euerian Lagrangian Spray Atomization mode ELSA mode was proposed in 2001 by R.Borghi and A.Vaet [10] and since has been under deveopment [, 4, 6, 7 and 9]. This mode is intended for appication to turbuent spray with both high Reynods and high Weber numbers. The mode version impemented into STAR CD code and used for a presented cacuations is issued from the Ph.D thesis of P.A. Beau []. ELSA mode combines the Euerian approach in the near-nozze region and Lagrangian approach in the region downstream of the nozze. In dense region of the spray, iquid and its gaseous environment are represented as an effective singe-phase fuid with a highy variabe density. To describe the dispersion of the iquid, the transport equation of the iquid mass fraction is soved with a gradient aw cosure appied to mode the turbuent iquid fux. A iquid/gas interface density is aso transported in order to determine the mean size of the iquid fragments. The switch to the Lagrangian cacuation is appied when the spray is considered to be diuted enough. The initiaization of the dropet characteristics is determined directy from the Euerian description and not set a priori as in the cassica Lagrangian approach. 2.1 Liquid dispersion modeing In dense region of the spray, the iquid and its gaseous environment is considered as an effective singe phase of iquid and gas mixture. The mean properties of this effective fuid or mixture (ike mean density or Favre averaged mean veocityu ~ i ) are defined with the foowing reationships: ~ ~ ~ 1 Y 1Y ~ ~ ~ ; 1 Y R gtg U i YU, i 1 Y U g, i ; P ~ (1) 1 Y g Here Y ~ is iquid mass fraction and and g stand respectivey for the iquid and gas densities. In the equation of state (ast equation in (1)), we take into account the voume occupied by iquid. The cassica transport equations are soved for these mean variabes. It shoud be noticed that the momentum conservation equation does not contain any momentum exchange terms between iquid and gaseous phases. In order to mode the iquid dispersion, this set of equations is competed by the transport equation for the iquid mass fraction: ~ ~ ~ Y U Y u y (2) t x x In the transport equations, there are 2 turbuent fuxes to be cosed. The turbuent stress tensor in momentum conservation equation is modeed with a cassica k mode cosure. Concerning the iquid turbuent diffusion fux in mass conservation equation, the gradient aw approximation is appied: ~ Y t u y () Sct x 2.2 Liquid/gas interface density In order to characterize the size of the iquid fragments resuted from the et atomization, the notion of iquid surface density is introduced. This variabe is defined as the quantity of iquid/gas m 1 interface per unit of voume ( ). Using this new variabe, one can obtain the Sauter mean diameter of dropet: D 6 Y ~ 2. A transport equation for iquid surface density is postuated by anaogy with the fame surface density [10]: ~ ~ ~ U ~ t ~ ~ ~ ~ ~ init mean turb co coa (4) t x x Sct x 2

3 Here, Beau [] introduced the other notion of iquid/gas interface per unity of mass that is defined as ~ ( m 2 kg ). The production and destruction of iquid surface are accounted for with source terms detaied beow. More detais about the mode cosure and parameters can be found in Ph.D. thesis of P.A. Beau []. 2. Transition from Euerian to Lagrangian formuation As mentioned before, Euerian part of the mode is couped with the cassica Lagrangian formuation. So, there are characteristic zones of spray: dense zone, transition zone and diuted one. The Euerian formuation is appied into the whoe numerica domain. Once the spray is considered to be diuted enough the Lagrangian dropets are generated. The criterion switch is based on the vaue of iquid voume fraction that is inked to the ratio of mean free path between two dropets and mean equivaent radius of the dropets in the ce. In this paper, the transition is done when the iquid voume fraction becomes ower than 0.01.The transition zone is composed of the computationa ces that form the border with the dense zone (i.e. zone where the iquid voume fraction is greater than 0.01) and ony one parce is generated per transition ce and per time step. The initiaization of the characteristics (such as diameter or veocity) and ocation of the generated dropets are determined directy from the Euerian description and not set a priori as in the cassica Lagrangian approach. The mean number of dropets per generated parce is obtained from the mass conservation.. Vaidation tests: SANDIA resuts The ELSA approach described above was impemented into STAR CD code. In order to vaidate its impementation, comparisons with two experimenta series were reaized [8, 11]. The ony resuts of comparison with Sandia s data base provided by Naber and Siebers [8] are presented. The inector and fue used by Naber and Siebers [8] are a high-pressure, common rai diese fue inector and a Phiips research grade diese fue. The fue density is 705 kg.m at the considered inection temperature of 440 K. The inector has a fast opening time, i.e. time required to reach the fu inection rate, and a constant inection rate. Ambient gas density effects on sprays are examined by comparing inections over a wide range of ambient gas density ( to 124 kg.m ). Fue vaporization effects are examined by comparing inections into non-vaporizing (ambient temperature of 450 K) and vaporizing (ambient temperature of 1000 K) inert environments at the same density. The experimenta conditions for the non-vaporizing and vaporizing experiments are shown in Tabe 1. A the non-vaporizing experiments are conducted with N2 as ambient gas. The vaporizing experiments are conducted in the burnt gases issued from the premixed combustion and composed of 90.% N 2, 6.11% CO 2 and.56% H 2 O. Ambient gas density (kg.m - ) Tabe 1. SANDIA experimenta conditions Ambient gas temperature (K) Fue pressure (MPa) Time to fu inection rate (μs ) Bernoui s veocity (m.s -1 ) A the cacuations are performed using a 2D axis-symmetric mesh composed of ces. The ce size of 10 to 20 microns across the nozze diameter is needed to perform an accurate modeing of the near inector zone.

4 Figures 1 and 2 show the comparison of vapor and iquid penetrations predicted using the present ELSA mode and experimenta data of Naber and Siebers [8]. The experimenta vaues are given with an uncertainty of 7% [8] (see error bars on figures). The numerica penetration is defined as the distance D from the inector for which 90% of the tota vapor or iquid mass is ocated between the inector and D. Figure 1 shows the iquid penetrations obtained under the non-vaporizing conditions (at six ambient gas densities). It is observed that the iquid penetrations predicted by the ELSA mode are in good agreement with the experimenta ones. It can be concuded that ELSA mode reproduces the main trend shown by the experimenta penetration data, i.e. a decrease in penetration with an increase in ambient density under non-vaporizing conditions. Figure 2 shows the vapor numerica penetrations obtained for 5 vaporizing inection conditions. This figure aso compares non-vaporizing iquid penetrations with vaporizing penetration data for ambient density conditions (right part). Naber and Siebers [8] observed that the vapor penetrations are smaer than the iquid ones. The right part of Fig.2 shows that ELSA mode is abe to predict this trend. 1.00E E-02 penetration (m) 6.00E E E E E E E E E E E-0 time (s) ELSA : 124 SANDIA : 124 ELSA : 60 SANDIA : 60.6 ELSA : 0 SANDIA : 0.2 ELSA : 14 SANDIA : 14.8 ELSA : 7 SANDIA : 7. ELSA : SANDIA :.6 Fig. 1. Sandia: iquid penetration versus time for non-vaporizing sprays 1.20E E E E-01 penetration (m) 8.00E E-02 ELSA : 58.4 SANDIA : E-02 ELSA : 28.6 SANDIA : 28.6 ELSA : E-02 SANDIA : 1.9 ELSA : E+00 SANDIA : E E E E E E E-0 ELSA :. time (s) SANDIA :. penetration (m) 8.00E-02 ELSA : vapour E-02 SANDIA : vapour 58.4 ELSA : iquid 60.6 SANDIA : iquid E-02 ELSA : vapour 1.9 SANDIA : vapour E-02 ELSA : iquid 14.8 SANDIA : iquid 14 ELSA : vapour. 0.00E+00 SANDIA : vapour E E E E E E E-0 ELSA : iquid.6 time (s) SANDIA : iquid Fig. 2. Sandia: vapour penetrations versus time for vaporizing sprays (eft) and comparison with iquid penetrations for non-vaporizing sprays (right) 4. ELSA mode couped with nozze fow simuation In every cacuations presented previousy, the inector s outet section has been considered as an inet boundary for ELSA mode with homogenous conditions on its surface. In this section, more reaistic inection conditions with non-uniform distributions at the inector outet have been tested. These distributions are determined through hydrauic cacuations inside the inector s nozze. These cacuations have been performed independenty from ELSA cacuations using STAR-CD with Rayeigh mode activated in order to take into account the cavitation effect. 4.1 Boundary conditions couping with hydrauic cacuation Two Bosch inection nozzes with six hoes are being considered: (1) a cavitating nozze (nozze 195 Kf0) with cyinder-shaped and non hydro-eroded (sharp) hoes and an outet diameter 147 m ; D in 4

5 (2) a non-cavitating nozze (nozze 194 Ks1.5) with cone-shaped (ratio of 1.5) and hydro-eroded (rounded-inet) hoes and an outet diameter D in 18 m. A series of resuts obtained from stationary cacuations carried out at severa neede ifts for each considered nozze (provided by Renaut) is avaiabe. The 2D fieds of ongitudina veocity, kinetic turbuent energy, its dissipation rate and voume fraction of vapor at the inector s outet section can be determined through these resuts. The experimenta inection rate is used to define the moment when each neede ift shoud take pace. ELSA cacuations are performed for a singe hoe of each nozze in the conditions of experimenta measurements of iquid penetrations: ambient gas pressure p ch 10 bar, fue inection pressure p in 800 bar, inection duration T in 500μs and iquid density 750 kg m. 4.2 Numerica characteristics of ELSA cacuations The structure of the D mesh used for ELSA cacuations is represented in Fig., showing severa views of this mesh in the pane perpendicuar to the inection s direction: zones 1 and 2 are the usua zones of an O-mesh whereas zones and 4 (the ast one representing the nozze outet) are integrated in order to have a thinner mesh in the area cose to the inector. In z direction, the mesh is non uniform with the stretch ratio of Fig.. Mesh structure used for ELSA anaysis 4. Comparisons The foowing experimenta resuts provided by Renaut and CORIA are avaiabe for comparisons [5]: engths of the iquid penetration and dropets veocities. Figure 4 shows the comparison of the iquid penetration for two nozzes. Experimenta penetrations are obtained at the same inection conditions as the ones appied in the ELSA cacuations: p in 800bars, p ch 10bars and T in 500 s. The uncertainties of experimenta are % [5] Liquid penetration (m) Exp. : conica nozze Exp.: cyindrica nozze ELSA : cyindrica nozze ELSA : conica nozze time (s) Fig. 4. Liquid penetration evoution: cyindrica and conica nozzes 5

6 A deay between numerica and experimenta penetrations is observed. Indeed, it can be seen that numerica spray penetration is composed of two distinct periods: an initia sow penetration period foowed by a fast penetration period. The observed experimenta penetrations are more inear. After 0.25 ms of inection, numerica and experimenta rates of penetration are amost the same. It can aso be noticed that numerica penetrations are simiar for the two nozzes, cavitating or non-cavitating, which is aso the case for measured penetrations (in the imits of experimenta uncertainties). It must aso be noticed that for a other conditions considered experimentay [5] (different fue inection pressures, ambient gas pressures and inection durations) the penetration for the conica nozze is sighty higher than that for the cyindrica nozze. Unfortunatey, the inside nozze cacuations (that we needed for ELSA inet conditions) are ony avaiabe for the case for which this experimenta tendency is reversed. 5. The spray structure The spray structure obtained under non-vaporizing conditions is presented and discussed in this section. Figure 5 shows some snapshots of the spray soution obtained at different portions of the spray tip. Circes are used to represent the iquid in the discrete Lagrangian part whie contours are used to represent the iquid in the continuous Euerian part. Fig. 5. The spray structure: time after inection = 0.5 ms. Contours for continuous part, circes (diameter proportiona to dropets diameter) for discrete part. In figure 5 the structure of the spray is represented at 0.5 ms highighting the different zone of the spray. The spray structure shown in this figure indicates ceary the presence of a continuous iquid region at the nozze exit and persist severa nozze diameter downstream the inection direction. It is aso shown dropet with reativey sma diameter around this continuous part. The deveopment of the discrete part with arge dropets diameter takes pace near the haf of the spray ength as it is expected. 6. Couping with ECFM-CLEH combustion mode The couping with the combustion is straightforward and standard approach has been made in this first impementation, as we made the reasonabe hypothesis that combustion occurs in the vapour phase. In this work ELSA is couped to one of advanced and detaied combustion for Diese Engine ECFM-CLEH. The combustion mode is described in detai in [1] and [2]. We present here a short resume and the concept. 6

7 6.1 The ECFM-CLEH combustion mode The approach ECFM CLE-H proposes to substitute the origina mode for burnt gas deveoped in ECFM-Z by two sub-modes eaborated on both the description of the premixed fame (ECFM / Extended Coherent Fame Mode formuation) and the diffusion fame (CLE-H / Combustion imited by thermodynamic equiibrium formuation). Thus, the fame front in each ce is spit into three zones (Figure 6): (i) Passive mixing zone; (ii) Premixed fame zone and (iii) Diffusion fame zone. ~ The coupe Z, Y ~ Fig. 6. Fame front modeing structure. governs each zone. A transport equation is used to determine the vaue of each of these two variabes in each zone. The formaism Z ~ refers to Favre mean vaue for the mixing variabe and Y ~ refers to the mean vaue for the associated reactive scaar. 7. Compete DI Diese Engine: Mode verification 7.1 The engine configuration The computed configuration is a typica 2 itters DI Engine-4 cyinders, under production. The symmetry of the geometry, the reative position of the inector into the combustion chamber and the nature of the swiring fow at Intake Vave Cosing make the choice of a sector mesh we enough for this evauation. The geometry and the mesh are shown in figure 7. As it was described in paragraph 4, to take a fu benefit from ELSA a good refinement around the inector is necessary. As we don t have the restriction of arge bubbe probem ike in the traditiona approach, the refinement shoud be in the order of 1/10th inector diameter. Fig. 7. Typica mesh for ELSA and combustion with adequate refinement around the inector to describe part of the region. 7

8 The boundary condition from the nozze anaysis are mapped and interpoated in the region (see Fig 8). The mapped quantities are obviousy the veocity profies, the iquid and vapor concentration (in case cavitations occur) as we as the turbuent quantities. The inection mass fow rate profie appied in ELSA simuation in shown in Fig 9. Boundary Inet exit of the inector nozze Wa inector surface Fig. 8. Boundary conditions at the nozze ocation. TDC Fig. 9. Inection profie for this specific operation condition. 7.2 Spray resuts and anaysis A fu oad engine operation condition has been chosen for mode vaidation. The fu oad may exhibit the existence of an intact core of dense iquid and in addition it pushes the mode to extreme imit to udge the robustness as high inection veocities are expected generating high mean fow shear stresses in the iquid part. The functionaity of the mode is demonstrated in this exampe (Figure 10). As a resut, it is cear that intact iquid core exits during the whoe inection process. Its penetration becomes stabe when the maximum of iquid inected mass fow rate is reached. Due the decay of the turbuence this iquid core in maintained and the diffused in the combustion. Due to the difference of the diffusion coefficient between the gas the Euerian part of the iquid, this iquid part burnt at a reativey ow speed compare to top diffusion around dropets. This is shown in Fig. 11 beow iustrating the concentration of intact iquid core, which (it is too eary to concude) correspond neary to the Un-burnt Hydrocarbon under these operating condition. We think that at east the presence of non break-up iquid participate to UHC. 8

9 Fig. 10. Liquid core iustrated by the Euerian iquid concentration (eft) Dropets and gas coored by temperature (right). Intact Liquid: 1.5% of tota inected mass Fig. 11. Existing Euerian Liquid versus Inected quantity. 7. Goba thermodynamic resuts To compete the vaidation, the thermodynamic pressure and the apparent rate of heat reease are compared to measurement as show the figure beow. This comparison for thermodynamic as shown in Fig. 12 is quite encouraging and iustrate the potentia of ELSA (fine description of the inector) and the ECFM-CLEH combustion mode. Concusions An Euerian Lagrangian Spray Atomization mode has been impemented in the STAR-CD commercia code. The mode vaidation was carried out using experimenta data of vaporizing and non-vaporizing sprays under engine ike conditions in a constant-voume bomb. It was found that ELSA cacuations are abe to capture the effects of gas density on iquid and vapour penetrations. Subsequenty, ELSA simuations with infow boundary conditions obtained from inside nozze fow modeing were performed for two non-cavitating and cavitating nozzes. The comparison with the measurements was based on the data for iquid penetration and dropet veocity. For the inection conditions considered (ambient gas pressure and fue inection pressure), a very simiar behaviour was observed for the sprays formed by two different nozzes as expected by the measurements. 9

10 The spray structure indicates ceary the presence of a continuous iquid region at the nozze exit and persist severa nozze diameter downstream the inection direction. It is aso shown dropet with reativey sma diameter around this continuous part. The deveopment of the discrete part with arge dropets diameter takes pace near the haf of the spray ength as it is expected. A first appication for a compete DI Diese engine (Sector mesh) operating at fu oad is aso presented. The presented resuts are extremey encouraging and demonstrate the potentia of such approach, which in principe doesn t require any tuning parameters, especiay for the spray. Fig. 12. Comparison between experimenta and predicted thermodynamic quantities: in-cyinder pressure (eft); apparent rate of heat reease (right). Red experiment, back predicted vaues. References [1] Abouri D, Zeat M, Duranti S and Ravet F (2008) Advances in combustion modeing in STAR-CD: Vaidation of ECFM-CLEH Mode to engine anaysis. 18ith IMEM SAE Congress [2] Abouri D, Zeat M, Desoutter G, Cano A and Ravet F (2009) Advances in combustion modeing in STAR-CD: Vaidation of ECFM-CLEH Mode to engine anaysis. 19ith IMEM at the SAE Congress [] Beau PA (2006) Modeisation de atomisation d un et iquide Appication aux sprays diese. Ph.D. Thesis, University of Rouen. [4] Bokkee G, Barbeau B and Borghi R (200) A D Euerian mode to improve the primary breakup of atomizing et. SAE Technica Paper [5] Hespe C, Baisot JB and Margot X (2010) Infuence of nozze geometry on spray shape, partice size, spray veocity and the air entrainment of high pressure Diese spray. To be pubished. [6] Lebas R (2007) Modeisation Euerienne de Atomisaton haute pression Infuences sur a vaporisation et a combustion induite. Ph.D. Thesis, University of Rouen. [7] De Lucas M (2007) Contribution a a modeisation de a puverisation d un iquide phytosanitaire en vue de reduire es poutions. Ph.D. Thesis, University of Aix-Marseie II. [8] Naber JD, Siebers DL (1996) Effects of gas density and vaporization on penetration and dispersion of Diese sprays. SAE Technica Paper [9] Ning W, Reitz RD, Lippert AM and Diwakar R (2007) Deveopment of a next-generation spray and atomization mode using an Euerian-Lagrangian methodoogy. 17 th Int. Mutidimensiona Engine Modeing User s Group Meeting, Detroit, MI. [10] Vaet A, Buruka AA and Borghi R (2001) Deveopment of a Euerian mode for the atomization of a iquid et. Atomization and sprays, vo. 11, pp [11] Verhoeven D, Vanhemeryck JL and Baritaud T (1998) Macroscopic and ignition characteristics of high-pressure sprays of singe-component fues. SAE Technica Paper