Evaluation of a new computational fluid dynamics model for internal combustion engines using hydrogen under motoring conditions

The present work conducts a preliminary evaluation of a new CFD (computational fluid dynamics) model, which is under development at the authors' laboratory. Using this model, it is feasible to understand how the intake manifold and in-cylinder geometry affect the in-cylinder flow field and the mixing processes taking place in an Otto (spark-ignition) engine. The model is applied on a high-swirl, two-valve, four-stroke, transparent combustion chamber engine running under motoring conditions. To investigate the fuel–air mixing process, hydrogen is injected in the intake manifold. To evaluate the model three case studies are examined. First, the model is applied to simulate the external mixing in the intake manifold with a tee-mixer injection system. Secondly, the transient gas flow field in the intake manifold and engine cylinder is examined over the complete engine cycle. Finally, the transient mixing process in the intake manifold and the spatial and temporal distribution of species concentrations inside the cylinder are numerically computed using the developed model. To validate the model, the results obtained through the test cases examined are compared either with available experimental data or with simulated results, which are obtained using a commercially available CFD code applied under the same conditions.     

CFD modeling and experimental study of combustion and nitric oxide emissions in hydrogen-fueled spark-ignition engine operating in a very wide range of EGR rates

In the current work, the variation of EGR rates is investigated in a hydrogen-fueled, spark-ignition engine. This technique is followed in order to control the engine load and decrease the exhaust nitrogen oxides emissions. The external EGR is varied in the very wide range of 12% up to 47% (by mass), where in each test case the in-cylinder mixture is stoichiometric, diluted with the appropriate EGR rate. The operation of this engine is explored using measured data with the aid of a validated CFD code. Moreover, a new residual gas term existing in the expression of the hydrogen laminar flame speed, which has been derived from a one-dimensional chemical kinetics code, is tested in a real application for appraising its capabilities. The investigation conducted provides insight on the performance and indicated efficiency of the engine, the combustion processes, and the emissions of nitrogen oxides. More precisely, an experimental study has been deployed with the aim to identify the characteristics of such a technique, using very high EGR rates, focusing on the combustion phenomena. At the same time, the CFD results are compared with the corresponding measured ones, in order to evaluate the CFD code under such non-conventional operating conditions and to test a recent expression for the residual gas term included in the hydrogen laminar flame speed expression. It is revealed that the combustion takes place in few degrees of crank angle, especially at high engine loads (low EGR rates), whereas the exhaust nitrogen oxides emissions are significantly decreased in comparison to the use of lean mixtures for controlling the engine load. Additionally, the recent expression of the residual gas term, which has been tested and incorporated in the CFD code, seems to be adequate for the calculation of combustion phenomena in highly diluted, with EGR, hydrogen-fueled spark-ignition engines, as for every EGR rate tested (even for the higher ones) the computational results are compared in good terms with the measured data.     

A simulation of the combustion of hydrogen in HCCI engines using a 3D model with detailed chemical kinetics

The influence of changes in the swirl velocity of the intake mixture on the combustion processes within a homogeneous charge compression ignition (HCCI) engine fueled with hydrogen were investigated analytically. A turbulent transient 3D predictive computational model which was developed and applied to the HCCI engine combustion system, incorporated detailed chemical kinetics for the oxidation of hydrogen. The effects of changes in the initial intake swirl, temperature and pressure, engine speed and compression and equivalence ratios on the combustion characteristics of a hydrogen fuelled HCCI engine were also examined. It is shown that an increase in the initial flow swirl ratio or speed lengthens the delay period for autoignition and extends the combustion period while reducing NO_x emissions. There are optimum values of the initial swirl ratio and engine speed for a certain mixture intake temperature, pressure, compression and equivalence ratios operational conditions that can achieve high thermal efficiencies and low NO_x emissions while reducing the tendency to knock     

Development and validation of a laminar flame speed correlation for the CFD simulation of hydrogen-enriched gasoline engines

In this paper, a laminar flame speed correlation was developed and validated for the computational fluid dynamics (CFD) simulation of hydrogen-enriched gasoline engines. This correlation was derived through the tabulated data which was determined by a self-developed calculation program according to the flame temperature-based mixing rule. Wide ranges of hydrogen volume fractions (0–10%), equivalence ratios (0.6–1.5), unburned gas temperatures (300–2500 K), pressures (1–50 bar) and residual gas mass fractions (0–20%) were simultaneously considered in this correlation to cover the burning conditions encountered in SI engines. The estimated values of the new correlation were found to be in satisfying agreement with the experimental data under normal burning conditions. Moreover, the new correlation was implemented in the extended coherent flame model to evaluate its suitability for CFD simulation. Satisfying agreement between the experimental and calculated results was observed under all examined hydrogen addition levels. This indicated that the new correlation was suitable for the CFD simulation of hydrogen-enriched gasoline engines.     

Investigating the effect of crevice flow on internal combustion engines using a new simple crevice model implemented in a CFD code

A theoretical investigation is conducted to examine the way the crevice regions affect the mean cylinder pressure, the in-cylinder temperature, and the velocity field of internal combustion engines running at motoring conditions. For the calculation of the wall heat flux, a wall heat transfer formulation developed by the authors is used, while for the simulation of the crevices and the blow-by a newly developed simplified simulation model is presented herein. These sub-models are incorporated into an in-house Computational Fluid Dynamics (CFD) code. The main advantage of the new crevice model is that it can be applied in cases where no detailed information of the ring-pack configuration is available, which is important as this information is rarely known or may have been altered during the engine’s life. Thus, an adequate estimation of the blow-by effect on the cylinder pressure can be drawn. To validate the new model, the measured in-cylinder pressure traces of a diesel engine, located at the authors’ laboratory, running under motoring conditions at four engine speeds were used as reference, together with measured velocity profiles and turbulence data of a motored spark-ignition engine. Comparing the predicted and measured cylinder pressure traces of the diesel engine for all cases examined, it is observed that by incorporating the new crevice sub-model into the in-house CFD code, significant improvements on the predictive accuracy of the model is obtained. The calculated cylinder pressure traces almost coincide with the measured ones, thus avoiding the use of any calibration constants as would have been the case with the crevice effect omitted. Concerning the radial and swirl velocity profiles and the turbulent kinetic energy measured in the spark-ignition engine, the validation process revealed that the developed crevice model has a minor influence on the aforementioned parameters. The theoretical study has been extended by investigating in the same spark-ignition engine, during the induction and compression strokes, the way crevice flow affects the thermodynamic properties of the air trapped in the cylinder.     

CFD simulations of premixed hydrogen combustion using the Eddy Dissipation and the Turbulent Flame Closure models

This paper presents a CFD simulation of premixed combustion tests, and centers around a comparison between the classical Eddy Dissipation Model (EDM) and the more sophisticated Turbulent Flame Closure (TFC) model. The chosen tests relate to hydrogen-air deflagration experiments in the THAI and ENACCEF facilities, featuring respectively slow and fast dynamics.Validation of the models is accomplished by comparing model predictions against important measured combustion parameters (flame velocity and spatial propagation, pressure history, spectra, etc.). We follow CFD Best Practice Guidelines, in particular by conducting systematic mesh and time-step sensitivity studies.Both default models predict combustion evolution reasonably well in all tests studied. For the ENACCEF dual compartment experiments, the flame propagation features several dynamical phases, and the TFC model using the progress variable approach reproduces better than the EDM the flame velocity evolution, which leads to better estimation of the temporal gradient of pressure. The better performance of the TFC model comes however at the expense of a larger computational effort, i.e. larger meshes and smaller time steps. This observed trend in 2D geometries is likely to be enhanced in 3D settings.     

An experimental study of combustion and emissions in a spark-ignition engine fueled with coal-bed gas

Coal-bed gas has been considered an attractive alternative fuel for internal combustion engines due to its abundant source and low emissions. In the present study, a combustion system with a swirl chamber has been developed for a spark-ignition engine using coal-bed gas. Detailed experiments have been carried out to investigate the combustion and emission characteristics of the engine operating with three different grades of coal-bed gas. The results have shown that this combustion system allows satisfactory operation of the engine with a wide range of methane content in the supplied coal-bed gas. For all tested conditions, the CO emission has a maximum value of 0.062%, and the HC emission is less than 380 ppm. The NO emission increases with the engine load but is less than 1800 ppm, demonstrating a great advantage of coal-bed gas as a relatively clean engine fuel.     

Numerical analysis of hydrogen release,dispersion and combustion in a tunnel with fuel cell vehicles using all-speed CFD code GASFLOW-MPI

Hydrogen energy is expanding world-widely in recent years, while hydrogen safety issues have drawn considerable attention. It is widely accepted that accidental hydrogen release in an open-air environment will disperse quickly, hence not causing significant hydrogen hazards. A hydrogen hazard is more likely to occur when hydrogen is accidentally released in a confined place, i.e. parking garages and tunnels. Prediction the main accident process, including the hydrogen release, dispersion, and combustion, is important for hydrogen safety assessment, and ensuring the safety installations during accidents. Hence, a postulated accident scenario induced by the operation of Thermal Pressure Relief Device in a tunnel is analysed for hydrogen fuel cell vehicles with GASFLOW-MPI in this study. GASFLOW-MPI is a well validated parallel CFD code focusing on the transport, combustion, and detonation of hydrogen. It solves compressible Navier-Stokes equations with a powerful all-speed Arbitrary-Lagrangian-Eulerian (ALE) method; hence can cover both the non-compressible flow during the hydrogen release and dispersion phases, and the compressible flow during deflagration and detonation. In this study, a 3D model of real-scaled tunnel is modelled, firstly. Then the hydrogen dispersion in the tunnel is calculated to evaluate the risk of Flame acceleration and the Deflagration-Detonation Transient (DDT). The case with jet fire is analysed with assuming that the hydrogen is ignited right after being injected forming a jet fire in the tunnel, the consequence of this case is limited considering the small hydrogen inventory. The detonation in the tunnel is calculated by assuming a strong ignition at the top of the tunnel at an unfavourable time and location. The pressure loads are calculated to evaluate the consequence of the hazard. The analysis shows that the GASFLOW-MPI is applicable at a widely range for tunnel accidents, meanwhile, the safety issues related to tunnel accidents is worthy further study considering the complexity of tunnels.     

Modeling and simulation of hydrogen combustion in engines

Hydrogen being an ecological fuel is very attractive now for engines designers. It is already actively used in rocket engines. There exist plans to use hydrogen in pulse detonation engines. However, peculiarities of hydrogen combustion kinetics, the presence of zones of inverse dependence of reaction rate on pressure, etc. prevent from wide use of hydrogen engines. Computer aided design of new effective and clean hydrogen engines needs mathematical tools for supercomputer modeling of hydrogen–oxygen components mixing and combustion gas dynamics.     

Fire dynamics simulation of a turbulent buoyant flame using a mixture-fraction-based combustion model

Fire dynamics simulations of a 7.1-cm buoyant turbulent diffusion flame were performed using a mixture-fraction-based combustion model. In our previous work, good agreement between the measured and the calculated fire flow field was achieved with carefully selected domain and grid sizes using a Lagrangian thermal-element combustion model. The Lagrangian thermal-element model exhibits qualitative as well as quantitative differences in the measured and calculated temperature profiles in the flame zone. The number of Lagrangian thermal elements must be carefully selected and the model is not designed to provide insights into the species distributions in the fire. To address these issues, a mixture-fraction-based combustion model was used in the present work. The domain and grid size dependence using this model are documented. Comparisons between the measured and the calculated velocities, mixture fractions and temperatures show that the mixture-fraction-based combustion model captures the qualitative and quantitative fire behavior very well.     

A review on the technical adaptations for internal combustion engines to operate with gas/hydrogen mixtures

The use of the hydrogen as fuel in the internal combustion engine represents an alternative use to replace the hydrocarbons fuels, which produce during the combustion reaction a pollutes gases. The hydrogen is the most abundant material in the universe and during its combustion with air only produces nitrous oxides (NO_x) gas, which can collect and avoid their emission to the atmosphere. In this paper we can present the most significant advances and developments made on the technical adaptations in the internal combustion engines which operate with mixtures of gas/hydrogen, doing more emphasis in the fuel injection and cooling systems. To understand such technical adaptations, it is necessary to know the chemical and physical characteristics of the hydrogen, and the processes relate with the chemical reaction between air and hydrogen, from a point of view of the thermo-chemistry and the chemical kinetics, as well as the ratios of the mixtures in the combustion process. Also, it mentions the advantages and disadvantages of the integration of hydrogen as a fuel, such as the pre-ignition, spontaneous ignition, knocking and backfire, also the advances in the research to avoid these phenomena during the combustion. Finally, it describes the best conditions of the ratio-mixtures in the internal combustion engines when they are fed with hydrogen. Also, it describes the perspectives and the futures fields on the future investigation.     

Assessing the cyclic-variability of spark-ignition engine running on methane-hydrogen blends with high hydrogen contents of up to 50%

The cyclic variability in a spark-ignition (SI) engine is examined fueled with methane/hydrogen blends with the use of an in-house computational fluid dynamics (CFD) code. A recent methodology is followed, which has been developed with the main aim at providing accurate predictions of the coefficient of variation (COV) of the indicated mean effective pressure (IMEP) in a fraction of time. Instead of simulating several tens of engine cycles, the methodology is based on the numerical results obtained from just 5 cycles, which are then processed for developing suitable fitted correlations of the main parameters as a function of a normalized distance. The latter expresses the distance of the spheres of the initial flame within the computational cell at the spark-plug region with the local turbulent eddy, and provides a smooth transition from the laminar burning regime to the fully turbulent one. This sub-model is included in the ignition numerical approach and is applied here in a SI engine with 3 different hydrogen contents, 10%, 30% and 50%, and three equivalence ratios, 1, 0.8 and 0.7, showing that the COV of IMEP is well predicted compared to the available measured data. Other parameters of engine cycle variations are also examined, such as the distribution of the IMEP. The variability of NO (nitric oxide) emissions is also examined, showing that for the stoichiometric cases it follows a distribution similar to a normal (Gaussian) one, while for lower ratios it is positively skewed. Overall, the methodology seems to provide reliable results for the whole range of the operating conditions examined, while the next steps of this activity will focus on similar cases for engine with variable speed and load, with the final goal to include additional mechanisms that contribute to the engine cycle variations.     

The ADREA-HF CFD code for consequence assessment of hydrogen applications

Application of the CFD methodology for risk assessment of hydrogen applications and associated support of regulation, codes and standards has been growing its momentum during the last years. The CFD tools applied should prove to be “adequately” validated for hydrogen applications. This contribution focuses on the hydrogen related validation work performed with the CFD code ADREA-HF. The code is a three dimensional transient fully compressible flow and dispersion CFD solver, able to treat highly complex geometries using the porosity formulation on Cartesian grids. The ADREA-HF validation effort was performed within various EC co-funded projects (EIHP, EIHP-2, HyApproval, HyPer, HySafe). Various types of hydrogen release scenarios were considered, including gaseous and liquefied releases, open, semi-confined and confined environments, sonic (under-expanded) and low momentum releases. In parallel to its validation the ADREA-HF code has been extensively used for regulations, codes and standards support.     

Consequence analysis and safety verification of hydrogen fueling stations using CFD simulation

Now that environmental awareness is enhanced on a global basis, great hopes are placed on the expanded use of hydrogen stations and fuel-cell vehicles (FCVs) that economize hydrogen energy. Hydrogen stations must be safe and secure because they store large quantities of hydrogen under higher pressure than the hydrogen actually consumed by FCVs. Thus, multiple safety measures are taken to ensure that hydrogen does not leak from the stations. Furthermore, in the unlikely event of leakage, the damage needs to be kept on an allowable level. For this reason, it is necessary to understand the behavior of hydrogen gas leaking from the stations.     

Hydrogen fuel rocket engines simulation using LOGOS code

Computer aided design of new effective and clean hydrogen rocket engines needs mathematical tools for supercomputer modeling of hydrogen–oxygen components mixing and chemically reacting in rocket combustion chambers. The paper presents the results of computer code developing, verification and validation, making it possible to simulate unsteady processes of ignition and combustion of hydrogen fuel in rocket engines. Restrictions on unsteady gas dynamics working cycles supercomputer simulations due to accumulations of errors are developed.     

CFD modeling of heat transfer and fluid flow inside a pent-roof type combustion chamber using dynamic model

A numerical work has been performed to analyze the heat transfer and fluid flow in a pent-roof type combustion chamber. Dynamic mesh model was used to simulation piston intake stroke. Revolution of piston (1000 ≤ n ≤ 5000) is the main governing parameter on heat and fluid flow. k–ε turbulence model was used to predict the flow in the cylinder of a non-compressing fluid. They were solved with finite volume method and FLUENT 12.0 commercial code. Velocity profiles, temperature distribution, pressure distribution and velocity vectors are presented. It is found that the inclined surface of pent-roof type of combustion chamber reduces the swirl effect and it can be a control parameter for heat and fluid flow.     

Modeling pressure loads during a premixed hydrogen combustion in the presence of water spray

This paper proposes a method for pressure evolution modeling during combustion process in presence of water spray. A simplified model based on empirical correlations is developed, which allows the estimation of the main factors influencing the pressure evolution, such as the combustion rate, the convective heat loss and the droplet evaporation rate. The results are then used as a guideline to adjust the parameters of a three-dimensional hydrodynamic code based on CREBCOM combustion model developed and validated for large-scale hydrogen combustion. This methodology provides an approach to estimate the important parameters for the determination of the pressure loads. Simulation results for hydrogen-air combustion in presence of water spray using the present model compare favorably to the experimental data of Carlson et al. [1].     

GASFLOW-MPI: A new 3-D parallel all-speed CFD code for turbulent dispersion and combustion simulations: Part I: Models,verification and validation

The objective of the presented work is to develop an efficient and validated approach based on a multi-dimensional computational fluid dynamics (CFD) code for predicting turbulent gaseous dispersion, conjugated heat and mass transfer, multi-phase flow, and combustion of hydrogen mixtures. Applications of interest are accident scenarios relevant to nuclear power plant safety, renewable energy systems involved in hydrogen transport, hydrogen storage, facilities operating with hydrogen, as well as conventional large scale energy systems involving combustible gases. All model development is conducted within the framework of the high-performance scientific computing software GASFLOW-Multi-Physics-Integration (MPI). GASFLOW-MPI is the advanced parallel version of the GASFLOW sequential code with many newly developed and validated models and features. The code provides reliability, robustness and excellent parallel scalability in predicting all-speed flow-fields associated with hydrogen safety, including distribution, turbulent combustion and detonation. In the meanwhile, it has been well verified and validated by many international blind and open benchmarks.The recently developed combustion models in GASFLOW-MPI code are based on the transport equation of a reaction progress variable. The sources consist of turbulence dominated and chemistry kinetics dominated terms. Models have been implemented to compute the turbulent burning velocity for the turbulence controlled combustion rate. One-step and two-step models are included to obtain the chemical kinetics controlled reaction rate. These models, combined with the efficient and verified all-speed solver of the GASFLOW-MPI code, can be used for simulations of deflagration, detonation and the important transition processes like flame acceleration (FA) and deflagration-to-detonation-transition (DDT), without additional need for expert judgment and intervention. It should be noted that the major goal is to develop a reliable and efficient numerical tool for large-scale engineering analysis, instead of resolving the extremely complex physical phenomena and detailed chemistry kinetics on microscopic scales. During the course of this development, new verification and validation studies were completed for phenomena relevant to hydrogen-fueled combustion, such as shock wave capturing, premixed and non-premixed turbulent combustion with convective, conductive and radiation heat losses, detonation of unconfined hydrogen–air mixtures, and confined detonation waves in tubes. Excellent agreements between test data and model predictions support the predictive capabilities of the combustion models in GASFLOW-MPI code. In Part II of the paper, the newly developed CFD methodology has been successfully applied to a first analysis of hydrogen distribution and explosion in the Fukushi Daicchi Unit 1 accident.The major advantage of GASFLOW-MPI code is the all-speed capability of simulating laminar and turbulent distribution processes, slow deflagration, transition to fast hydrogen combustion modes including detonation, within a single scientific software framework without the need of transforming data between different solvers or codes. Since the code can model the detailed heat transfer mechanisms, including convective heat transfer, thermal radiation, steam condensation and heat conduction, the effects of heat losses on hydrogen deflagrations or detonations can also be taken into account. Consequently, the code provides more accurate and reliable mechanical and thermal loads to the confining structures, compared to the overly conservative results from numerical simulations with the adiabatic assumptions.Predictions of flame acceleration mechanisms associated with turbulent flames and flow obstacles, as well as DDT modeling and their comparisons to available data will be presented in future papers. A structural analysis module will be further developed. The ultimate goal is to expand the GASFLOW-MPI code into an integral high-performance multi-physics simulation tool to cover the entire spectrum of phenomena involved in the mechanistic hydrogen safety analysis of large scale industrial facilities.     

Enhancing the performance of a spark-ignition methanol engine with hydrogen addition

This paper investigated the effect of hydrogen addition on enhancing the performance of a methanol engine at part load and lean conditions. The experiment was conducted on a modified spark-ignited engine equipped with an adjustable dual-fuel injection system. The engine was run at an engine speed of 1400 rpm with two hydrogen volume fractions in the intake of 0% and 3%. The test results illustrated that the engine cyclic variation was eased and the brake thermal efficiency was enhanced after the hydrogen blending. Besides, the hydrogen enrichment was effective on reducing the flame development and propagation periods. HC and CO emissions were generally reduced after the hydrogen blending. NOx emissions from the hydrogen-blended methanol engine could be dropped to a low level when the engine was run under high excess air ratios.