Decan, Gilles

Li, Haohan

Verhelst, Sebastian

Verschaeren, Roel

English

Li, Haohan000171344739802002748267976214150103E6B07828-B2BD-11E7-A167-CF81AD28A064

Verschaeren, Roel00007038794780200128205297152520344507FA8B6E-F0EE-11E1-A9DE-61C894A0A6B4

Decan, GillesUGent0001102257468020024010879735925333250000-0003-1130-447536ADAB94-F0EE-11E1-A9DE-61C894A0A6B4

Verhelst, SebastianTW080019941650028010013470440000-0003-2421-580XF5C38C8E-F0ED-11E1-A9DE-61C894A0A6B4

Archivsystem Ask23

Evaluation of breakup models for marine diesel spray simulations

Ghent University Academic Bibliography

ILASS–Europe 2019, 29th Conference on Liquid Atomization and Spray Systems, 2-4 September 2019, Paris, France   This work is licensed under a Creative Commons 4.0 International License (CC BY-NC-ND 4.0). Evaluation of breakup models for marine diesel spray simulations Haohan Li*1, Roel Verschaeren1, Gilles Decan1, Tarek Beji1, Sebastian Verhelst1,2 1Ghent University, Department of Flow, Heat and Combustion Mechanics, B-9000 Ghent, Belgium 2Department of Energy Sciences, Lund University, Sweden *Corresponding author: Haohan.Li@UGent.be   Abstract  The open-source CFD software OpenFOAM was used to simulate evaporating, high pressure marine diesel sprays. The most frequently used  models, based on Kelvin-Helmholtz and Rayleigh-Taylor instabilities, are adopted to describe the breakup of liquid fuel. The proposed approach was first validated using experimental data from the Engine Combustion Network for heavy-duty diesel engine sprays. A satisfactory agreement with SANDIA’s experimental data demonstrated that the simulations can correctly capture the spray processes. 2D simulations were then performed using the same approach and compared with the optical spray measurements obtained from the GUCCI (Ghent University Combustion Chamber I) setup. Significantly higher penetration predicted by simulation implied that the current approach cannot reproduce the marine diesel spray processes and thus needs to be improved. A quantitative analysis of the different cases is made to seek possibilities for improvement.   Keywords Marine diesel spray, CFD, Breakup models  Introduction Pollutant emission standards in the shipping sector have been lagging behind those in for instance the automotive sector, as a result of which international shipping is now responsible for an emission of harmful NOx that is similar to the NOx emission of the whole of Europe [1]. In diesel engines the spray characteristics have a direct influence on the fuel conversion and formation of harmful substances. The fuel injection and atomization processes are extremely complex involving transient two-phase, turbulent flows at high pressures, with a wide range of temporal and spatial scales [2]. Pollutant formation is strongly affected by the state of mixing during the combustion process. The  spray characteristics have a direct influence on the mixture formation and fuel conversion since they determine droplet size and positions. Studying diesel spray characteristics is therefore essential to optimize combustion in diesel engines.   Many experimental investigations of fuel spray characteristics are carried out in optically accessible constant volume combustion chambers, like the Sandia Combustion Vessel (SCV) [3], the Spray Combustion Chamber (SCC) of  Wärtsilä Switzerland [4] and the Ghent University Combustion Chamber I (GUCCI) [5, 6]. Using Computational Fluid Dynamics (CFD) is today becoming increasingly necessary in the optimization of the in-cylinder processes of  internal combustion engines. The detailed modelling of the spray is a prerequisite for the accurate calculation of ignition, combustion and pollutant formation.  In the present work, 2D simulations are performed using the OpenFOAM code together with Lib-Engine, which is a set of libraries and solvers developed by the Internal Combustion Engine group of the Politecnico di Milano [7]. In the simulations, the blob method [8] is used as the injection model, and the Kelvin-Helmholtz and Rayleigh-Taylor (KH/RT) [9] model is adopted for the breakup of liquid fuel. The capabilities of the proposed method are first validated by simulating experimental studies in the Sandia Combustion Vessel [10, 11]. Then, the same method is employed to simulate the conditions typically for medium speed marine diesel engines. Experimental measurements obtained on the GUCCI setup are used to evaluate the computational results.  Computational Models  The Eulerian-Lagrangian approach is employed to model the fuel spray. In this approach, the conservation equations for the continuous gas phase are solved in a Eulerian way, and for the description of the disperse phase (droplets) the Lagrangian methodology is used. Additional phenomenological sub-models are then needed to describe the processes occurring with the droplets. The computational sub-model for droplet break-up in this work is the blob-KH/RT model. The blob method was developed by Reitz and Diwakar [8], and assumes that the injected liquid jet is represented by large spherical blobs. The diameter of these blobs equals the nozzle hole diameter and ILASS – Europe 2019, 2-4 Sep. 2019, Paris, France  This work is licensed under a Creative Commons 4.0 International License (CC BY-NC-ND 4.0). their number is determined from the mass flow rate. The Kelvin-Helmholtz instability model is used to predict the primary break-up of the intact liquid core of the diesel jet, while the secondary break-up of individual drops is modelled with the KH model in conjunction with the Rayleigh-Taylor accelerative instability model [4]. According to the stability analysis of the Kelvin-Helmholtz model, the injected liquid jet breaks up due to its interaction with the gas as it penetrates, yielding a core which contains relatively large drops. The drop radius of the newly formed droplet, rnew , is assumed to be proportional to the wavelength ΛKH of unstable waves. The radius of the new droplets and the wavelength can be obtained from: rnew= B0∙ΛKH (1) 𝛬𝐾𝐻𝑟0=9.02(1 + 0.45√Z)(1 + 0.4 𝑇0.7)(1 + 0.865𝑊𝑒𝑔1.67)0.6 (2) where Z=(Wel)0.5Rel,  T=Z (Weg)0.5 , Weg=ρgr0urel2σ, Wel=ρlr0urel2σ, Rel=ρlr0urelμl (3)  B0= 0.61 is a constant. Z ,We, Re, 𝑇 , σ , urel , ρ, μl and r0 are respectively the Ohnesorge number, the Weber number, the Reynolds number, the Taylor number, the surface tension, the relative velocity magnitude between the two-phases, the density, the liquid viscosity and the radius of the undisturbed liquid jet. More details about the KH/RT model can be found in the work by Beale et al. [9] and Patterson et al. [12]. It can be seen from equations (1)-(3) that the breakup process is strongly influenced by the Reynolds and Weber numbers of the injected liquid jet.  The reduction rate of the liquid jet radius and the breakup time (𝜏bu) are expressed as drdt= -  r0 - rnewτbu (4) τbu=3.788∙B1rΛ∙Ω (5) Ω=0.34+0.38Weg1.5(1 + Z)(1 + 1.4∙T0.6)(σρlr03)0.5 (6)  Where Ω is the growth rate of the most unstable surface wave and B1 is an adjustable model constant including the influence of the nozzle hole flow on spray break-up. The value of B1 =10 is chosen for the present study. For the gas phase, the mass, momentum and energy equations are solved for a compressible, multi-component gas flow using a RANS (Reynolds Average Navier-Stokes) approach. For this work, the standard k − ϵ model was chosen for the modelling of turbulence in the spray simulation.  Test cases  Validation of the computational models is performed using three sets of measurement data. Case 1 and Case 2, measured in the Sandia Combustion Vessel (SCV) within the ECN (Engine Combustion Network), correspond to heavy-duty engines. Case 3 is representative of medium-speed marine engines and was measured [13] in the GUCCI setup. Due to its physical properties and similar reaction chemistry to diesel fuel, n-dodecane was chosen as reference fuel. In order to provide more accurate boundary conditions, a computer model [14] of the pump-line-nozzle (PLN) injection system was developed using Simcenter Amesim and validated using the GUCCI setup. Figure 1 shows the injection velocity profile of Case 2 and Case 3. A summary of the conditions used in this work is listed in Table 1. Lucchini et al.[7] investigated the spray-mesh-turbulence interactions for evaporating sprays at engine conditions and concluded that the best results are provided by a mesh size which is five times greater than the nozzle diameter. A grid independence test has been conducted for the 2D simulations in this work according to the conclusion above. Accounting for the size of the combustion chamber and the nozzle diameter, a mesh with 11664 cells was chosen for Case 1 and Case 2, and a mesh with 2592 cells was used for Case 3.  ILASS – Europe 2019, 2-4 Sep. 2019, Paris, France  This work is licensed under a Creative Commons 4.0 International License (CC BY-NC-ND 4.0).  Figure 1. Injection velocity profile of Case2 and Case 3  Table 1. Conditions for test cases  Case 1 Case 2 Case 3 Facility  SCV SCV GUCCI Injection system Common rail Common rail Pump-Line-Nozzle Application  Automotive  Automotive  Marine  Fuel  n-dodecane n-dodecane n-dodecane Nozzle diameter [mm] 0.0837 0.0894 0.4400 Injection duration [ms] 1.5 1.5 4.8 Injected fuel mass [mg] 3.33 3.83 111.90 Ambient Temperature (K) 900 700 700 Ambient density (kg/m3) 23.01 22.80 22.50  Results and discussion  Figure 2 presents the comparison of numerical results against experimental data in terms of vapor and liquid penetration for Case 1. The employed spray model is able to describe the spray penetration history, though the numerical result of liquid length is about 10% higher than the measurement.   Figure 2. Numerical and measured penetrations for Case 1.  Since the vapor penetration measurement for Case 2 is not available in the ECN database, Figure 3 only compares the liquid penetration. There is generally good agreement between simulations and measurements, except for an underestimation in the early stage (0 – 0.2 ms). As seen in Figure 4, the simulation for Case 3 overestimates both the vapor and liquid penetration significantly. This also means that the simulation underestimates the spray disintegration process.   ILASS – Europe 2019, 2-4 Sep. 2019, Paris, France  This work is licensed under a Creative Commons 4.0 International License (CC BY-NC-ND 4.0).  Figure 3. Numerical and measured liquid penetrations for Case 2.    Figure 4. Numerical and measured penetrations for Case 3.   Figure 5. Effect of B1 on penetrations for Case 1.  The effect of model constant B1  in equation (5) is investigated. Figure 5 illustrates the effect of the break-up constant B1 for Case 1. The liquid phase shows a significant sensitivity to the break-up constant, while the vapor penetration is slightly different. A higher value of B1 results in reduced break-up and increased penetration, while a smaller value leads to increased spray disintegration and reduced penetration. The result here is consistent with the work by Patterson et al.[12]. However, the comparison for Case 3 from Figure 6 shows that the penetration length is less sensitive to this constant than in Case 1.  ILASS – Europe 2019, 2-4 Sep. 2019, Paris, France  This work is licensed under a Creative Commons 4.0 International License (CC BY-NC-ND 4.0).  Figure 6. Effect of B1 on penetrations for Case 3.  In the following part of this section, the applicability of the blob-KH/RT model to marine diesel spray simulation is discussed. As seen in Table 1, the ambient temperatures of Case 2 and Case 3 are the same, and the ambient gas densities for the two cases are similar. The main differences between Case 2 and Case 3 are the initial injection conditions, which represent automotive engines and medium speed marine engines, respectively. Therefore, the discussion below focuses on Case 2 and Case 3. In order to make quantitative comparisons of the two cases above, some reasonable assumptions are made:  Fluid properties (density, viscosity) of n-dodecane at the nozzle exit are constant values (713 kg/m3 for density and 0.001 kg∙m-1∙s-1 for viscosity) during the injection process.   Surface tension at the liquid-gas interface at the nozzle is 0.02 kg/s2 according to Liu and Reitz [15].  The velocity of  the ambient gas is much smaller than the fuel injection velocity , so Uinj ≈ urel . Based on these assumptions, the liquid Weber number (Wel), Reynolds number (Rel) and Ohnesorge number Z at the nozzle hole exit can be calculated. Using the injection velocity profile of Case 2 and Case 3, the liquid Weber number and Reynolds number are obtained and shown in Figure 7 and Figure 8, respectively. Compared to Case 2, the Reynolds number of Case 3 is much higher, while the Weber number is lower at the onset of the injection.    Figure 7. Reynolds number for Case 2 and Case 3.    Figure 8. Liquid Weber number for Case 2 and Case 3.  The Ohnesorge numbers, which are not related to the injection velocity, are 0.0396 (Case 2) and 0.0178 (Case 3). In marine diesel engines, the nozzle diameter of injectors is normally of the order of 0.1 mm and larger, which is ILASS – Europe 2019, 2-4 Sep. 2019, Paris, France  This work is licensed under a Creative Commons 4.0 International License (CC BY-NC-ND 4.0). about an order of magnitude larger than in automotive applications. From equation (3), a lower value of the Ohnesorge number leads to a smaller value of the RHS (right-hand side) term of equation (2), which results in reduced breakup. Meanwhile, the initial droplets (undisturbed liquid jet) diameter of Case 3 is rather large, so the newly formed droplets evaporate very slowly. Moreover, a lower injection velocity leads to a smaller value of gas Weber number (equation 3), which results in a slow growth rate Ω of the most unstable surface wave according to the theoretical analysis given by Reitz et al. [16, 17]. The discussion above reveals that at relatively low speeds the liquid fuel does not break up easily. Figure 4 also shows that the simulation predicts a faster penetration than the measurement, which is mainly caused by the primary breakup being too slow in the simulation. Baumgarten [18] summarized four possible mechanisms of primary break-up: aerodynamics induced (KH model) break-up, turbulence induced break-up, cavitation induced break-up and velocity relaxation induced break-up. Usually, more than one mechanism occurs simultaneously and cannot be clearly separated from each other. The simulation and measurement for Case 3 demonstrate that the aerodynamics induced break-up is not the dominating mechanism in medium speed marine engines. The model should be improved to include other possible breakup mechanisms.   Conclusions  In this work simulations based on the blob-KH/RT model, representative of heavy-duty engines and medium-speed marine engines, have been performed. A satisfactory agreement with SANDIA’s experimental data demonstrated that this approach can correctly capture the spray processes in automotive diesel engines. However, the over-prediction of penetration by the simulation implies that the blob-KH/RT is incapable of modelling marine diesel sprays with a relatively low Ohnesorge number. It is concluded that a modification of the current model is needed,  accounting for the high Reynolds and low Weber numbers for accurate prediction of diesel sprays in marine engines.   Acknowledgements  The authors would like to acknowledge Professor Tommaso Lucchini from the Politecnico di Milano for sharing the LibICE codes. The authors would like to thank former master students Alexander Jan De Santis and Thibo Moerman for their work on Amesim. Haohan Li thanks the Chinese Scholarship Council (CSC) for his Ph.D. grant (CSC No.201704910932).   Nomenclature rnew            newly formed droplet radius [ m ] r0               droplet radius of undisturbed liquid jet [ m ] ΛKH         wavelength [m] Z             non-dimensional Ohnesorge number [ / ] T             non-dimensional Taylor number [ / ] Rel          non-dimensional Reynolds number of liquid jet [ / ] Wel         non-dimensional Weber number of liquid jet [ / ] Weg        non-dimensional Weber number of gas [ / ] ρg           density of gas [ kg/m3 ] ρl            density of liquid jet [ kg/m3 ] σ             surface tension [ kg/s2 ] μl            dynamic viscosity [ N•s/m2 ] urel          relative velocity [ m/s ] Uinj          injection velocity [ m/s ] D             injector nozzle diameter [ m ] Ω             growth rate of most unstable wave [ s-1 ] τ              characteristic time scale [ s ]   References  [1] Andersson, K., and Winnes, H., 2011, "Environmental trade-offs in nitrogen oxide removal from ship engine exhausts," Proceedings of the Institution of Mechanical Engineers, Part M: Journal of Engineering for the Maritime Environment, 225(1), pp. 33-42. [2] Heywood, J. B., 1988, "Internal combustion engine fundamentals." [3] Idicheria, C. A., and Pickett, L. M., 2007, "Effect of EGR on diesel premixed-burn equivalence ratio," Proceedings of the Combustion Institute, 31(2), pp. 2931-2938. ILASS – Europe 2019, 2-4 Sep. 2019, Paris, France  This work is licensed under a Creative Commons 4.0 International License (CC BY-NC-ND 4.0). [4] Chryssakis, C., and Kaiktsis, L., "Evaluation of Fuel Spray Atomization Models for Conditions Applicable to Large Marine Diesel Engines‟," Proc. 22nd European Conference on Liquid Atomization and Spray Systems. [5] Galle, J., 2013, "Spray atomization of alternative fuels in medium speed diesel engines,"PhD thesis, Ghent University. [6] Galle, J., Van De Maele, C., Defruyt, S., Verhelst, S., and Verschaeren, R., 2013, "Evaluation of Some Important Boundary Conditions for Spray Measurements in a Constant Volume Combustion Chamber," No. 0148-7191, SAE Technical Paper. [7] Lucchini, T., D’Errico, G., and Ettorre, D., 2011, "Numerical investigation of the spray–mesh–turbulence interactions for high-pressure, evaporating sprays at engine conditions," International Journal of Heat and Fluid Flow, 32(1), pp. 285-297. [8] Reitz, R. D., and Diwakar, R., 1987, "Structure of high-pressure fuel sprays," SAE transactions, pp. 492-509. [9] Beale, J. C., and Reitz, R. D., 1999, "Modeling spray atomization with the Kelvin-Helmholtz/Rayleigh-Taylor hybrid model," Atomization and sprays, 9(6). [10] Bardi, M., Payri, R., Malbec, L. M. C., Bruneaux, G., Pickett, L. M., Manin, J., Bazyn, T., and Genzale, C. L., 2012, "Engine combustion network: comparison of spray development, vaporization, and combustion in different combustion vessels," Atomization and Sprays, 22(10). [11] Meijer, M., Somers, B., Johnson, J., Naber, J., Lee, S.-Y., Malbec, L. M. C., Bruneaux, G., Pickett, L. M., Bardi, M., and Payri, R., 2012, "Engine Combustion Network (ECN): Characterization and comparison of boundary conditions for different combustion vessels," Atomization and Sprays, 22(9). [12] Patterson, M. A., and Reitz, R. D., 1998, "Modeling the effects of fuel spray characteristics on diesel engine combustion and emission," SAE transactions, pp. 27-43. [13] Verschaeren, R., 2019, "An optical study of evaporating fuel sprays for a medium speed engine,"PhD thesis, Ghent University. [14] De Santis, A. J., 2017, "Development of an injection system model for the optimization of a dual fuel engine,"MSc thesis, Ghent University. [15] Liu, Z., and Reitz, R., 1997, "An analysis of the distortion and breakup mechanisms of high speed liquid drops," International journal of multiphase flow, 23(4), pp. 631-650. [16] REITZ, R., 1987, "Modeling atomization processes in high-pressure vaporizing sprays," Atomisation and Spray Technology, 3(4), pp. 309-337. [17] Reitz, R. D., 1982, "Mechanism of atomization of a liquid jet," Physics of Fluids, 25(10). [18] Baumgarten, C., 2006, Mixture formation in internal combustion engines, Springer Science & Business Media.  

https://biblio.ugent.be/publication/8645384/file/8645385

Evaluation of breakup models for marine diesel spray simulations

Abstract

Similar works

Full text

Available Versions

Archivsystem Ask23

Ghent University Academic Bibliography