SIMULATION OF DIESEL SPRAY IN A CONSTANT VOLUME COMBUSTION CHAMBER USING OPEN SOURCE CFD (OPEN FOAM

International Conference on Advances in Materials and Mechanical Engineering (ICAMME-2015) SIMULATION OF DIESEL SPRAY IN A CONSTANT VOLUME COMBUSTION CHAMBER USING OPEN SOURCE CFD (OPEN FOAM) K. Senthilnathan1 R.Sivakumar2,C. Mariyappan3 1Mechanical, Egs pillai Engg.College, India E-mail: senthilnathan1979@gmail,com 2Mechanical, Karaikal polytechnic, India E-mail: divyamariyappan@gmail,com 3Mechanical, Karaikal polytechnic, India E-mail: yazgansiv2013@gmail,com Abstract: Modeling is a very important tool connect the theoretical approach to real time situation CFD(Computational Fluid Dynamics) techniques are more popular now a days to analyze the various fluid flow problems, using the latest developments on the various numerical equation solving methods like Eulerien grid, lagrangian Parcels etc.,In this paper , deals with various diesel spray modeling concepts and the spray model was developed using C++ for various process parameters like, Diesel fuel chemistry spray angle , nozzle geometry, modeling chemistry. Finally the diesel spray code was run using Open Foam and the results were discussed and shown. a deeper understanding of these complex flows. In the next stage, the spray model was integrated into the in-cylinder flow to observe the outcome of the interaction between the spray and the flow. The motivations of this project is to understand the functionality and utility of the CFD code OpenFoam in spray simulations . II . SPRAY MODELING CONCEPTS The different types of modeling are shown below and the system running time required for simulations . Keywords: IC Engines, CFD, Diesel spray , OpenFoam I . INTRODUCTION The Computational fluid dynamics (CFD) has developed to the point where the complete three dimensional flow field over the vehicle and through engines can be computed expeditiously with accuracy and reliability. To achieve this requirement, the industry must investigate a range of factors that can potentially reduce engine emissions, including fuels, combustion process, chemical-kinetics, in-cylinder flow, fuelair mixture formation, sprays, engine geometries, etc. Nowadays, computational simulations have been adopted as a key analysis tool in engine research to establish correlation with experimental studies and provide new information for designers. A significant advantage of using Computational Fluid Dynamics (CFD) is the flexibility of simulation setups and the time and cost efficiencies compared to experiments. Major CFD commercial codes that are available in the market currently include ANSYS CFX, ANSYS FLUENT,AVL FIRE and CD-adapco STAR-CD and STAR-CCM+, whereas KIVA and OpenFOAM are becoming popular as open source codes. OpenFOAM has been chosen in this study to take advantage of its ability to simulate general flow problems. It offers different kinds of models to evaluate engine characteristics. At the first stages of this work, different sub models of the engine, especially turbulence and spray models, were configured and the flow field was simulated in an effort to gain Fig. 1 Different types of models While the thermodynamic combustion models are relatively easy to handle and are characterized by a low computational effort, they are lacking the ability to make predictions of the effects of important engine parameters on combustion without prior measurements. The main reasons for this deficiency are that major sub processes are either not modeled at all or described by solely empirical correlations and that the assumption of an ideally mixed combustion chamber makes it impossible to estimate pollutant formation rates that are strongly affected by local temperatures and mixture E.G.S Pillay Engineering College, Nagapattinam K. Senthilnathan1 R.Sivakumar2, C. Mariyappan3 compositions. On the other hand, the multidimensional CFD models that are based on the locally resolved solutions of mass, energy, and momentum, conservation and that include detailed sub models for spray and combustion phenomena, are computationally expensive, and they demand that the user has a much deeper understanding of the governing physical and chemical processes in order to correctly interpret the simulation results. Moreover, the predictive quality with respect to global quantities such as pressure traces and apparent heat release rates is not necessarily better than with simpler models. Fig. 2 . Computation time for Different models A . The Spray Equation In typical diesel sprays the liquid fuel is atomized into a number of up to 108 droplets with average diameters in the ten-micrometer range. These numbers make it prohibitive to resolve each single droplet in numerical simulations. Instead, some kind of statistical averaging technique becomes necessary with additional sub models in order to describe the subscale processes. B . Spray breakup regimes Fig. 3 Various Breakup Models C. Atomization model A hybrid model for the atomization of the liquid fuel jet was developed and tested in previous work . It distinguishes between jet primary breakup and droplet secondary breakup. For the latter different models are used as the droplet Weber number changes. For the low pressure case the Wave breakup model was adopted to simulate liquid core atomization. For high-pressure sprays the effects of jet turbulence and cavitation on liquid core primary breakup are considered. In order to distinguish between primary and secondary breakup the following assumptions have been made: • primary blobs may undergo primary breakup; • the secondary droplets formed may undergo secondary breakup; • the primary blobs that have lost the 80% of the initial mass may undergo secondary breakup. The flow model was used to evaluate the occurrence of cavitations and the level of turbulence in the injector. Initial blob size, velocity and related k and E are estimated. Then Gosman's approach is followed, but if cavitation does occur it is taken into account since it affects turbulence and related values of k and E in the injector. The model also takes into account exponential growth by K-H instabilities due to interaction with air, so the three main mechanisms are considered. 1) The WAVE model is based on a stability analysis of liquid jets. It can be used to simulate the primary atomization of the liquid core in the regimes in which jet breakup is governed by aerodynamic interaction with air (low-medium injection pressure case). 2) The TAB and DDB models are based on the dynamic of a single droplet and they can be therefore considered as secondary breakup models. Since in the first the breakup is due to the amplification of droplet deformation resulting from vibrational resonance of the surface, this model has been chosen to predict the droplet breakup in the Vibrational regime. The latter is a deformation-induced secondary breakup model and it is used in the Bag regime. 3) The R-T model considers Rayleigh-Taylor instabilities that arise on very high-speed droplet surfaces and therefore it can be adopted to model droplet secondary breakup in the catastrophic regime in competition with a K-H instability based model (WAVE). The WAVE model is based on the physics of a liquid column (primary breakup) but, since it considers K-H instability effects, it can be also used to simulate the breakup of secondary droplets in those regimes in which it may be ascribed to the shear forces at the interface. A variety of mathematical models for drop breakup have been proposed in the literature. Most of these models have been established in order to describe one particular of the above breakup mechanisms. Nevertheless, in engine spray simulations they are - for the sake of simplicity - often applied to the entire spectrum of breakup regimes. This is not entirely true though. In recent years it has become more and more standard to determine the governing breakup mechanism for a droplet class and then apply the more appropriate of at least two breakup models, e.g. a combination of the Kelvin-Helrnholtz and the Rayleigh-Taylor model. In the subsequent sections the secondary breakup models that are applied most often in engine spray simulations will be discussed International Conference on Advances in Materials and Mechanical Engineering (ICAMME-2015) III. VARIOUS MODELS APPLIED IN OPEN FOAM IV. MODELING OF DIESEL SPRAY USING OPENFOAM A.The Reitz-Diwakar Model The overall structure of OpenFOAM is shown in Figure In a first attempt to include secondary drop let breakup in CFD spray calculations Reitz and Diwakar Whenever one of the breakup criteria is satisfied for a drop let dass for longer than the respective breakup time, it is assumed that the original droplet is disintegrated into a number of smaller drop lets. All child droplets are of equal size. It is determined from equating the respective breakup criterion to its critical value and by solving it for the droplet diameter. Thus, it is assumed that the new child droplets are initially in a state that is just stable. The below equation depicts the relationship of the child and parent droplets formed. A.Pre-processing B.The Rayleigh-Taylor Breakup Model The following steps describes how to pre-process, run and post-process a case involving compressible re-acting flow with Lagrangian evaporating particles in a threedimensional domain. It also describes how to copy the solver, copy an evaporation model and how to add a second material to the discrete particles Fig.4. Structure of openfoam The Rayleigh-Taylor (RT) breakup model is based on theoretical considerations of Taylor , who investigated the stability of liquid-gas interfaces when accelerated in a normal direction to the plane. Generally, it can be observed that the interface is stable when acceleration and density gradient point to the same direction, whereas Rayleigh-Taylor instabilities can develop if the fluid acceleration has an opposite direction to the density gradient. For a liquid droplet decelerated by drag forces in a gas phase this means, that instabilities may grow unstable at the trailing edge of the droplet. Fig.5 A Portion of Combustion Chamber C. CFD Formulations The below steps to be followed for implementing any kind of CFD simulations .Frame the governing equation and reduce to RANS equations , apply the assumptions for turbulence and kinetic energy, finally frame the model . 1) 2) 3) 4) 5) Governing Equations Reynolds Averaged Navier-Stokes (RANS) Equations Standard k-ε model Renormalization Group (RNG) k-ε model Standard k-ω model The geometry consists of a block filled with air, with a 0.01x0.01 meter base and a length of 0.1 meter. An injector is centrally placed on the top boundary where n-Heptane (C7 H16 ) is injected. When the discrete droplets enter the domain they evaporate and combustion takes place in the gas phase. There are several gas phase reaction schemes supplied with the case ranging from a reaction scheme with 5 species and one reaction up to a reaction scheme involving 300 reactions and 56 species. This section covers the necessary setup needed to get the diesel Foam case running with chemistry ,it also covers a brief introduction to reacting flows in numerical simulations. B.Running the code E.G.S Pillay Engineering College, Nagapattinam K. Senthilnathan1 R.Sivakumar2, C. Mariyappan3 First establish the required model, For evaporation , spray breakup, collision etc If required change the initial conditions like temperature and pressure etc. Change the boundary conditions After running the various models , The results are listed by the following figures. Remove ft and fu and in the aachenBomb/0 directory since these are not needed for this setup (keeping them will result in post-processing problems). cd $FOAM_RUN/aachenBomb/0 rm ft fu Turn chemistry on in the /constant/chemistryProperties file moodels Fig . 6 CASE1: Ignition off, Chemistry on, Temperature 800K,Spray cone angle 60o , ReitzKHRT model. chemistry on; and ignition on in the /constant/combustionProperties file. This step is not necessary, the mixture will still ignite when the species are properly mixed due to the high temperature. ignite on; Mesh the geometry using blockMesh, and start the dieselFoam solver. cd $FOAM_RUN/aachenBomb blockMesh dieselFoam Fig . 7 CASE2: Ignition off, Chemistry off Temperature 300K,Spray cone angle 40o , ETAB model C.Post-processing in ParaView Since paraFoam can not handle Lagrangian particles use foamToVTK and then ParaView. cd $FOAM_RUN/aachenBomb foamToVTK paraview In the /VTK directory open the case ( aachenBomb\1.vtk ) and also, open the particles in the file Fig. 8 CASE3: Ignition off, Chemistry off, Temperature 300K,Spray cone angle 20o , Rietzdiwakar model /Lagrangian/defaultCloud_2.vtk file. D .Spray breakup models Also in this project work the following cases are implemented the various spray characteristics as follows, 1) ETAB model 2) RietzKHRT Model 3) RietzDIWAKAR model 4) TAB model 5) Initial Tempreture 6) Initial Pressure 7) Spray cone angle ( 20o, 40o , 60o) Fig . 9 CASE4: Ignition off, Chemistry off, Temperature 300K,Spray cone angle 20o , TAB model International Conference on Advances in Materials and Mechanical Engineering (ICAMME-2015) V. CONCLUSION In this Paper ,presents the various applications of spray modeling in a constant volume combustion chamber using OPENFOAM .Improved sub models for turbulence and chemistry interactions using a detailed chemistry approach is presented. The latest developments on the various numerical equation solving methods like Eulerien grid (for gas Phase),lagrangian Parcels(liquid phase) are used for multiphase flow modeling. Also varying the following process parameters, Temperature, Pressure, and Spray breakup models (TAB, ETAB, REITZKHRT, REITZDIWAKAR), switching off the chemistry, switching off ignition, and Varying spray cone angles ( 20o , 40o, 60o) diesel foam solver was run and the results are analyzed REFERENCES [1] [2] [3] [4] [5] O'rourke, P.J., and Amsden, A.A. The Tab Method for Numerical Calculation ofSpray Droplet Breakup. SAE Technical Paper 872089, 1987. Reitz, R.D. Modeling Atomization Processes in High-Pressure Vaporizing Sprays.Atomisation and Spray Technology, 3, pp. 309337, 1987. Model with the Eulerian-Lagrangian Spray Atomization (ELSA) Model in DieselEngine Conditions. SAE Technical Paper 2005-010213, 2005. www.caelinux.org www.openfoam.org E.G.S Pillay Engineering College, Nagapattinam