Multi-dimensional Computational Fluid Dynamics Simulations Research Articles

Adaptive physics-informed neural operator for coarse-grained non-equilibrium flows

This work proposes a new machine learning (ML)-based paradigm aiming to enhance the computational efficiency of non-equilibrium reacting flow simulations while ensuring compliance with the underlying physics. The framework combines dimensionality reduction and neural operators through a hierarchical and adaptive deep learning strategy to learn the solution of multi-scale coarse-grained governing equations for chemical kinetics. The proposed surrogate’s architecture is structured as a tree, with leaf nodes representing separate neural operator blocks where physics is embedded in the form of multiple soft and hard constraints. The hierarchical attribute has two advantages: (i) It allows the simplification of the training phase via transfer learning, starting from the slowest temporal scales; (ii) It accelerates the prediction step by enabling adaptivity as the surrogate’s evaluation is limited to the necessary leaf nodes based on the local degree of non-equilibrium of the gas. The model is applied to the study of chemical kinetics relevant for application to hypersonic flight, and it is tested here on pure oxygen gas mixtures. In 0-textrm{D} scenarios, the proposed ML framework can adaptively predict the dynamics of almost thirty species with a maximum relative error of 4.5% for a wide range of initial conditions. Furthermore, when employed in 1-textrm{D} shock simulations, the approach shows accuracy ranging from 1% to 4.5% and a speedup of one order of magnitude compared to conventional implicit schemes employed in an operator-splitting integration framework. Given the results presented in the paper, this work lays the foundation for constructing an efficient ML-based surrogate coupled with reactive Navier-Stokes solvers for accurately characterizing non-equilibrium phenomena in multi-dimensional computational fluid dynamics simulations.

Diesel combustion in heavy-duty engine with single- and double-injection strategies

Split injection strategies are attractive combustion strategies due to their potential in satisfying strict emission standards. In this study, a multidimensional computational fluid dynamics simulation was used to validate the numerical results for single- and double-injection strategies with experimental data. Thereafter, simultaneous effects of converting a single-injection strategy to a double-injection strategy and retarding the injection timing on the combustion parameters, pollutant emission, performance, and exergy terms were investigated. Results showed that double-injection strategies and retarding the start of injection could significantly affect the performance and engineout emissions, which could reduce the pollutant emission with a slight decrease in engine performance. Furthermore, results of exergy study confirm that double-injection strategies have a better exergy efficiency due to its lower irreversibility than the single-injection strategy.

Development and validation of a reduced reaction mechanism with a focus on diesel fuel/syngas co-oxidation

A reduced reaction mechanism has been developed for modeling Reactivity Controlled Compression Ignition (RCCI) engine combustion with diesel fuel and syngas. Employing n-heptane as a single representative chemical surrogate for diesel fuel, a comprehensive mechanism was reduced such that the reaction mechanism of syngas oxidation is included as a sub-mechanism with emphasis on the interaction of oxidation reaction pathways of syngas and n-heptane. Important reaction pathways to be kept in the reduced mechanism to maintain the performance of the detailed mechanism were identified through ignition delay curve sensitivity analysis. In addition, reaction steps to which ignition delays of multi-fuel blends are sensitive were identified and the importance of predictability of co-oxidation of multi-fuels in the mechanism reduction process was demonstrated.The reduced mechanism, with 81 species and 312 reactions, was validated against experimental ignition delay times available in the literature. The reduced mechanism was also applied to simulate diesel-syngas RCCI engine combustion, employing a multi-component surrogate model to accurately model the physical properties of diesel fuel sprays. Predicted pressure and heat release rate results were compared with engine experimental data and good agreement was observed. The present reduced mechanism gave reliable performance for combustion predictions of RCCI engine operation to help further the fundamental understanding of the process occurring in reformed-fuel RCCI combustion and increase the computational efficiency of multi-dimensional computational fluid dynamics simulations. The reaction mechanism development outlined highlights the importance of considering co-oxidation in generation of reduced reaction mechanisms for applications to dual-fuel combustion.

Development and Validation of a Reduced Chemical Kinetic Mechanism for Computational Fluid Dynamics Simulations of Natural Gas/Diesel Dual-Fuel Engines

A reduced chemical kinetic mechanism consisting of 141 species and 709 reactions has been constructed to simulate the combustion of both natural gas and diesel fuels in a dual-fuel engine. Natural gas is modeled as a mixture of methane, ethane, and propane, while the diesel fuel is modeled as n-heptane. The new reduced mechanism combines reduced versions of a detailed n-heptane mechanism and a detailed methane through n-pentane mechanism, each of which was reduced using a direct relation graph method. The reduced dual-fuel mechanism is validated against ignition delay computations with full detailed mechanisms, adiabatic homogeneous charge compression ignition simulations with full detailed mechanisms, experimental premixed laminar flame speeds of CH4/O2/He mixtures at 40 and 60 atm, ignition delay and lift-off length from a diesel spray experiment in a constant-volume chamber, and finally against dual-fuel engine experiments using multidimensional computational fluid dynamics simulations. The engine simu...

A new improvement on a chemical kinetic model of primary reference fuel for multi-dimensional CFD simulation

In the present study, for the multi-dimensional CFD (computational fluid dynamics) combustion simulations of internal combustion engines, a new optimized chemical kinetic reaction mechanism for the oxidation of PRF (primary reference fuel) instead of gasoline has been developed. In order to carry out the in-depth research for combustion phenomenon of internal combustion engines, an optimized reduced PRF mechanism including more intermediate species and radicals was developed. The developed mechanism contains of iso-octane (C8H18) and n-heptane (C7H16) surrogates, which contains of 51-species and 193 reactions. Compared with many other mechanisms of PRF, more reactions of C0–C1 oxidation (100 reactions) are added in the present mechanism. In order to improve the performances of the model, the developed mechanism focused on the improvement through the prediction of the ignition delay time. The developed mechanism has been validated against various experimental and simulation data including shock tube data, laminar flame speed data and HCCI (homogeneous charge compression ignition) engine data. The results showed that the developed PRF mechanism was agreements with the experimental data and other approved reduced mechanisms, and it could be applied to the multi-dimensional CFD simulations for internal combustion engines.

Chemical Kinetic Mechanism of a Three-Component Fuel Composed of Iso-octane/n-Heptane/Ethanol

A chemical kinetic mechanism for a three-component fuel composed of iso-octane/n-heptane/ethanol has been developed. The mechanism consists of 51 species and 204 elementary reactions. Two elementary reactions that significantly affect laminar flame speed are subjected to sensitivity analysis. The validated results show that the predictions using the present mechanism are in good agreement with the experimental ignition delay time and laminar flame speed. Both the chemical reaction and the transportation process are accurately reproduced by the simulation using the present model in iso-octane/air premixed flames and in n-heptane/air counterflow partially premixed flames. The chemical kinetic mechanism is applicable to multidimensional computational fluid dynamics simulations of the combustion of gasoline surrogates or the cocombustion of primary reference fuels (PRF) with ethanol because of its smaller number of species and fewer reactions.

Multidimensional CFD simulation of syngas combustion in a micro-pilot-ignited dual-fuel engine using a constructed chemical kinetics mechanism

A multidimensional computational fluid dynamics (CFD) simulation of a constructed syngas chemical kinetic mechanism was performed to evaluate the combustion of syngas in a supercharged dual-fuel engine for various syngas initial compositions under lean conditions. The modelled results were validated by comparing predictions against corresponding experimental data for a supercharged dual-fuel engine. The predicted and measured in-cylinder pressure, temperature, and rate of heat release (ROHR) data were in good agreement. The effect of the hydrogen peroxide chain-propagation reaction on the progress of combustion under supercharged conditions was examined for different types of syngas using various initial H 2 concentrations. The effect of the main syngas kinetic mechanism reactions on the combustion progress was analysed in terms of their contribution to the total heat of the reaction. The best results compared with experimental data were obtained in the range of equivalence ratios below about 0.8 for all types of syngas considered in this paper. As the equivalence ratio increased above 0.8, the results deviated from the experiment data. The spatial distribution of the in-cylinder temperature and OH ∗ within this equivalence-ratio range showed the completeness of the combustion. The present CFD model captured the overall combustion process well and could be further developed into a useful tool for syngas-engine combustion simulations.