Mehdi Jangi

Comparison of well-mixed and multiple representative interactive flamelet approaches for diesel spray combustion modelling

The application of detailed chemistry to the computational fluid dynamics simulation of combustion process in diesel engines has many potentials, including the possibility to predict auto-ignition, diffusion flame structure, stabilisation and soot formation in a wide range of operating conditions, also taking into account the effects of different fuel types. Among the approaches that were proposed over the years, the ones that are mostly used in practical calculations can be divided into two main categories: the first assumes each cell to be a well-stirred reactor, while the second employs the flamelet assumption to describe both auto-ignition and turbulent diffusion flame propagation. Despite the fact that both types of model have been widely validated over the years, a detailed comparison between them appears to be very useful in order to understand better the relevant parameters governing auto-ignition, flame stabilisation and the formation of pollutant emissions. This work is focused on a comparison of two different combustion models that were recently implemented by the authors in an open-source code. The first assumes each cell to be a homogeneous reactor and neglects interaction between turbulence and chemistry, while in the second, multiple laminar flamelets are used to represent the structure of a turbulent diffusion flame. Suitable techniques for online reaction rate tabulation and chemical mechanism reduction are also incorporated, to make the use of bigger mechanisms possible (up to 150 species). The two models are compared and validated by simulating constant-volume diesel combustion in a wide range of operating conditions, including variations of ambient temperature and oxygen concentration. Comparison between the computed and experimental data on flame structure, auto-ignition and flame lift-off enables an understanding of the main relevant differences between the models in the way both auto-ignition and flame stabilisation processes are predicted.

A multi-zone chemistry mapping approach for direct numerical simulation of auto-ignition and flame propagation in a constant volume enclosure

A direct numerical simulation (DNS) coupling with multi-zone chemistry mapping (MZCM) is presented to simulate flame propagation and auto-ignition in premixed fuel/air mixtures. In the MZCM approach, the physical domain is mapped into a low-dimensional phase domain with a few thermodynamic variables as the independent variables. The approach is based on the fractional step method, in which the flow and transport are solved in the flow time steps whereas the integration of the chemical reaction rates and heat release rate is performed in much finer time steps to accommodate the small time scales in the chemical reactions. It is shown that for premixed mixtures, two independent variables can be sufficient to construct the phase space to achieve a satisfactory mapping. The two variables can be the temperature of the mixture and the specific element mass ratio of H atom for fuels containing hydrogen atoms. An aliasing error in the MZCM is investigated. It is shown that if the element mass ratio is based on the element involved in the most diffusive molecules, the aliasing error of the model can approach zero when the grid in the phase space is refined. The results of DNS coupled with MZCM (DNS-MZCM) are compared with full DNS that integrates the chemical reaction rates and heat release rate directly in physical space. Application of the MZCM to different mixtures of fuel and air is presented to demonstrate the performance of the method for combustion processes with different complexity in the chemical kinetics, transport and flame–turbulence interaction. Good agreement between the results from DNS and DNS-MZCM is obtained for different fuel/air mixtures, including H2/air, CO/H2/air and methane/air, while the computational time is reduced by nearly 70%. It is shown that the MZCM model can properly address important phenomena such as differential diffusion, local extinction and re-ignition in premixed combustion.

Multidimensional chemistry coordinate mapping approach for combustion modelling with finite-rate chemistry

A multidimensional chemistry coordinate mapping (CCM) approach is presented for efficient integration of chemical kinetics in numerical simulations of turbulent reactive flows. In CCM the flow transport is integrated in the computational cells in physical space, whereas the integration chemical reactions are carried out in a phase space made up of a few principal variables. Each cell in the phase space corresponds to several computational cells in the physical space, resulting in a speedup of the numerical integration. In reactive flows with small hydrocarbon fuels two principal variables have been shown to be satisfactory to construct the phase space. The two principal variables are the temperature (T) and the specific element mass ratio of the H atom (J H). A third principal variable, σ=∇J H·∇J H, which is related to the dissipation rate of J H, is required to construct the phase space for combustion processes with an initially non-premixed mixture. For complex higher hydrocarbon fuels, e.g. n-heptane, care has to be taken in selecting the phase space in order to model the low-temperature chemistry and ignition process. In this article, a multidimensional CCM algorithm is described for a systematic selection of the principal variables. The method is evaluated by simulating a laminar partially remixed pre-vaporised n-heptane jet ignition process. The CCM approach is then extended to simulate n-heptane spray combustion by coupling the CCM and Reynolds averaged Navier–Stokes (RANS) code. It is shown that the computational time for the integration of chemical reactions can be reduced to only 3–7%, while the result from the CCM method is identical to that of direct integration of the chemistry in the computational cells.

Effects of fuel cetane number on the structure of diesel spray combustion: An accelerated Eulerian stochastic fields method

An Eulerian stochastic fields (ESF) method accelerated with the chemistry coordinate mapping (CCM) approach for modelling spray combustion is formulated, and applied to model diesel combustion in a constant volume vessel. In ESF-CCM, the thermodynamic states of the discretised stochastic fields are mapped into a low-dimensional phase space. Integration of the chemical stiff ODEs is performed in the phase space and the results are mapped back to the physical domain. After validating the ESF-CCM, the method is used to investigate the effects of fuel cetane number on the structure of diesel spray combustion. It is shown that, depending of the fuel cetane number, liftoff length is varied, which can lead to a change in combustion mode from classical diesel spray combustion to fuel-lean premixed burned combustion. Spray combustion with a shorter liftoff length exhibits the characteristics of the classical conceptual diesel combustion model proposed by Dec in 1997 (http://dx.doi.org/10.4271/970873), whereas in a case with a lower cetane number the liftoff length is much larger and the spray combustion probably occurs in a fuel-lean-premixed mode of combustion. Nevertheless, the transport budget at the liftoff location shows that stabilisation at all cetane numbers is governed primarily by the auto-ignition process.

Evaluation and optimisation of phenomenological multi-step soot model for spray combustion under diesel engine-like operating conditions

In this work, a two-dimensional computational fluid dynamics study is reported of an n-heptane combustion event and the associated soot formation process in a constant volume combustion chamber. The key interest here is to evaluate the sensitivity of the chemical kinetics and submodels of a semi-empirical soot model in predicting the associated events. Numerical computation is performed using an open-source code and a chemistry coordinate mapping approach is used to expedite the calculation. A library consisting of various phenomenological multi-step soot models is constructed and integrated with the spray combustion solver. Prior to the soot modelling, combustion simulations are carried out. Numerical results show that the ignition delay times and lift-off lengths exhibit good agreement with the experimental measurements across a wide range of operating conditions, apart from those in the cases with ambient temperature lower than 850 K. The variation of the soot precursor production with respect to the change of ambient oxygen levels qualitatively agrees with that of the conceptual models when the skeletal n-heptane mechanism is integrated with a reduced pyrene chemistry. Subsequently, a comprehensive sensitivity analysis is carried out to appraise the existing soot formation and oxidation submodels. It is revealed that the soot formation is captured when the surface growth rate is calculated using a square root function of the soot specific surface area and when a pressure-dependent model constant is considered. An optimised soot model is then proposed based on the knowledge gained through this exercise. With the implementation of optimised model, the simulated soot onset and transport phenomena before reaching quasi-steady state agree reasonably well with the experimental observation. Also, variation of spatial soot distribution and soot mass produced at oxygen molar fractions ranging from 10.0 to 21.0% for both low and high density conditions are reproduced.

Investigation of Chemical Kinetics on Soot Formation Event of n-Heptane Spray Combustion

In this reported work, 2-dimsensional computational fluid dynamics studies of n-heptane combustion and soot formation processes in the Sandia constant-volume vessel are carried out. The key interest here is to elucidate how the chemical kinetics affects the combustion and soot formation events. Numerical computation is performed using OpenFOAM and chemistry coordinate mapping (CCM) approach is used to expedite the calculation. Three n-heptane kinetic mechanisms with different chemistry sizes and comprehensiveness in oxidation pathways and soot precursor formation are adopted. The three examined chemical models use acetylene (C2H2), benzene ring (A1) and pyrene (A4) as soot precursor. They are henceforth addressed as nhepC2H2, nhepA1 and nhepA4, respectively for brevity. Here, a multistep soot model is coupled with the spray combustion solver to simulate the soot formation/oxidation processes. Comparison of the results shows that the simulated ignition delay times and liftoff lengths have good agreements with the experimental measurements across wide range of operating conditions when the nhepC2H2 model is implemented. The performance of this mechanism however drops in cases with low ambient temperatures. Besides, the overall soot precursor and particle distribution prediction is found to be improved with the use of A4 as soot precursor. The variation of the soot precursor production with respect to the change of ambient temperature and oxygen levels qualitatively agrees with that of the conceptual models. Also, the revised nhepC2H2 model replicates the experimental spatial soot distribution reasonably well, although the absolute soot volume fraction values are not reproduced with the default soot model constant values.

Numerical Simulation of the ECN Spray A Using Multidimensional Chemistry Coordinate Mapping: n-Dodecane Diesel Combustion

A three dimensional numerical simulation of the ECN “Spray A” is presented. Both primary and secondary breakup of the spray are included. The fuel is n-Dodecane. The n-Dodecane kinetic mechanism is modeled using a skeletal mechanism that consists of 103 species and 370 reactions [9]. The kinetic mechanism is computationally heavy when coupled with three dimensional numerical simulations. Multidimensional chemistry coordinate mapping (CCM) approach is used to speedup the simulation. CCM involves two-way mapping between CFD cells and a discretized multidimensional thermodynamic space, the so called multidimensional chemistry coordinate space. In the text, the cells in the discretized multidimensional thermodynamic space are called zone to discriminate them from the CFD cells. In this way, the CFD cells which are at the similar thermodynamic state are identified and grouped into a unique zone. The stiff ODEs operates only on the zones containing at least one CFD cell. This zones are called “active zones”. Several CFD cells may fall into a unique zone. Thereby, speedup in the stiff ODEs integration is expected. In simulation of the “Spray A”, it is shown that the number of active zones is of the order of hundred times less than the total number of CFD cells. It is therefore a significant speedup in the simulation is expected. After the stiff ODEs integration the mean reaction rate computed in the zones are mapped back to the corresponding CFD cells. While the CCM approach significantly reduces the compositional time, the results are very promising. For example, the predicted spray penetration length, lift-off and the structure of the reaction zone agree well with the experimental measurements. The ignition delay time however was over-predicted. This may be attributed to the kinetic mechanism used in the simulation.

Large Eddy Simulation of Partially Premixed Combustion in an Internal Combustion Engine

This paper reports on numerical investigations of the mixing, ignition and combustion processes in a laboratory engine operating under partially premixed combustion (PPC) conditions. The engine is a modified version of a 13-liter Scania D13 engine. The fuel is injected at two different timings with different fuel mass portions at the two injections, with and without swirl. For comparison one single injection simulation with swirl is also performed. In literature it has been found that by optimizing the injection timing and amount of injected fuel at different injection timing, the heat release and combustion process can be optimized and thus high engine efficiency and low emission levels can be achieved. The goal of this study is to improve the understanding of the important phenomena involved. Large Eddy Simulation for the gas phase is coupled with Lagrangian Particle Tracking (LPT) for the liquid phase. Dynamic mesh motions and parallelized computations are taken into account using OpenFoam. It was shown that the approach used in this study could capture the important characteristics of the phenomena occurring in the engine. With sufficient time between the pilot injection and the main injection, fuel distribution becomes more homogeneous in the presence of swirl compared with that of non-swirling case. It was found that with 25 % less fuel the pressure rise rate for the two split injection case with swirl is almost identical to that of non-swirling case; indicating a higher efficiency with swirl. Also, the combustion fronts were found to be more uniformly spreading in the cylinder when swirl was present, while in the non-swirling case the combustion front is limited to the neighborhood of the spray fuel jet.

Diesel flame lift-off stabilization in the presence of laser-ignition: a numerical study

Diesel flame lift-off and stabilization in the presence of laser-ignition were numerically investigated with the method of Eulerian stochastic fields. The aim was to scrutinise the interaction between the lifted diesel flame and an ignition kernel upstream of the lifted flame. The numerical simulation was carried out in a constant-volume combustion vessel with n-heptane as fuel. The process was studied previously in an experiment employing Diesel #2 as the fuel in the same combustion vessel. In the experiment a lifted flame was first established at a position downstream of the nozzle. An ignition kernel was then initiated using a high-energy pulse laser at a position upstream of the natural lift-off position of the diesel flame. The laser-ignition kernel was modelled using a high-temperature (∼2000 K) hot spot. In both experiment and simulations the upstream front of the ignition kernel was shown to remain around the initial laser ignition site for a substantially long period of time, while the downstream front of the ignition kernel propagates rapidly towards the natural lift-off position downstream of the laser ignition site. The lift-off position oscillated before the final stabilization at the natural lift-off position. The structures and the propagation speed of the reaction fronts in the laser-ignition kernel and the main flame were analysed. Two different stabilization mechanisms, the auto-ignition mechanism and the flame propagation mechanism, were identified for the naturally lifted flame and the laser-induced reaction front, respectively. A mechanism was proposed to explain the oscillation of the lift-off position.

The Effect of Splitting Timing on Mixing in a Jet with Double Injections : A Large-Eddy Simulation Study

We present large-eddy simulation (LES) of a high-pressure gas jet that is injecting into a quiescent inert environment. The injection is through a nozzle with a diameter of 1.35 mm. Four injection strategies are considered in which the results of a single continuous injection case are compared with those of double injection cases with different injection splitting timing. In all double injection cases, the injection pulsing interval is kept the same, and the total injected mass is equal to that of the single injection case. On the other hand, the splitting timing is varied to investigate the effects of various injection splitting strategies on the mixture formation and the penetration length of the jet. Results show that the jet penetration length is not so sensitive to the splitting timing whereas the mixing quality can significantly change as a result of shifting the onset of injection splitting toward the end of injection. Especially, it is found that by adopting a post-injection strategy where a single injection splits into the main injection and late small injection near the end of injection period the mixing between the injected gas and ambient air is significantly improved. This trend is not as obvious when the injection splitting timing shifts toward the beginning or even in the middle of injection period. The increase of entrainment in the tail of each injection is one of the underlying physics in the mixing improvement in double injection cases. In addition to that, splitting a single injection into two smaller injections increases the surrounding area of the jet and also stretches it along the axial direction. It can potentially increase the mixing of injected gas with the ambient air.