Validation of unsteady flamelet / progress variable methodology for non-premixed turbulent partially premixed flames

This paper highlights the modeling capabilities of UFPV approach for the modeling of turbulent partially premixed lifted flames to capture the extinction and re-ignition phenomena. Large eddy simulation (LES) with the probability density function (PDF) approach provides the turbulence-chemistry interaction. All scalars are represented as a function of mean mixture fraction, mixture fraction variance, mean progress variable and scalar dissipation rate. Mixture fraction is assumed to follow a β-PDF distribution. Progress variable and scalar dissipation rate distributions are assumed to be a δ-PDF. Results are compared with experimental data of a vitiated co-flow burner with fuels like CH4/Air and H2/N2. Results of radial plots for temperature, mixture fraction and scattered data of temperature with mixture fraction at various axial locations are compared. Lift-off height for a CH4/Air flame appears to be over-predicted while the predicted lift-off height for a H2/N2 flame shows an under-prediction

Numerical study of propane and hydrogen turbulent premixed flames in a small scale obstructed chamber

This paper presents numerical study of turbulent premixed flames characteristics of two different fuel/air mixtures, namely propane and hydrogen. The flames under study are propagating past solid baffle plate(s) in a small-scale combustion chamber. The chamber design allows for up to three baffle plates to be inserted followed by a square obstacle to promote the generation of turbulence. The test cases considered in this paper examine various configurations of the baffles and one central obstacle at a fixed equivalence ratio of 0.8. An in-house computational fluid dynamics (CFD) model is used to numerically evaluate the characteristics of the flame propagation. The large eddy simulation (LES) technique is used for turbulence flow modelling. Three different flow configurations with various obstacles positioning are used to highlight the generated overpressure and flame speed. The numerical results are then validated against published experimental data to confirm the capability of computational models in capturing the features of hydrogen and propane flames. A conclusion is drawn that different configurations affect the generated peak overpressure as well as the flame structure. It was also concluded that hydrogen flames generated a significantly greater peak overpressure inside the combustion chamber when compared to propane

Numerical study of vented hydrogen explosions in a small scale obstructed chamber

There is a growing need to understand and estimate the explosion hazards associated with hydrogen storage and utilisation. This paper presents a comprehensive numerical study on the explosion characteristics of a lean hydrogen-air mixture in a small-scale obstructed vented chamber. The large eddy simulation (LES) technique is employed to study the highly unsteady turbulence-driven explosion when the flame propagates past successive obstructions. A dynamic flame surface density (DFSD) model is applied to the filtered chemical source term in the LES to account for the progressive wrinkling of the deflagrating flame. The driving mechanism of pressure rise and the underlying physics of flame-obstacle interactions are illustrated using the detailed LES results. The paper considers 11 individual flow experimental configurations of various obstacle number, size and location. They are further classified into six groups to investigate the influence of the level of blockage and the separation distance between adjacent obstructions. Critical safety-related parameters including the maximum overpressure and its incidence time are analysed. A comparison with propane is also made to highlight the substantial overpressure and flame acceleration of hydrogen deflagrations. Satisfactory agreements have been obtained between the LES and the experimental data, and this confirms the capability of the developed computational models in capturing essential explosion features and information for the study of vented hydrogen explosions

A numerical study of intake valve jet flapping in a GDI engine

This paper presents findings from a numerical study of intake valve jet flapping within a gasoline direct injection engine, using a large eddy simulation turbulence modelling approach. The experimental test case and computational setup, including choice of sub-grid scale turbulence model, are presented and discussed. An example cycle where intake valve jet flapping is seen to be prominent is discussed in detail. It was found to be initiated as a consequence of turbulent fluctuations in the intake valve curtains. Cycle-by-cycle variations in valve curtain flux and subsequent jet flapping events are investigated and significant cyclic variability is found. Finally, it was observed that due to the highly transient nature of this flow phenomenon, the typical ensemble-averaging procedure used in LES simulations causes most information related to this process to be lost

A numerical study of the effects of IEGR on the set-off auto-ignition in an HCCI engine

The shortcomings of traditional combustion techniques are being continually evaluated and alternate combustion modes are being sought. One such combustion mode that is receiving a lot of attention is Homogenous Charge Compression Ignition (HCCI). HCCI is a combustion process that has the potential to be highly efficient and produces low emissions. It can provide high, diesel-like efficiencies using gasoline, diesel, and most alternative fuels. The major drawback with HCCI is controlling the ignition timing over a wide range of load and speed. The local chemical and thermal conditions of the charge mixture, towards the end of the compression stroke, have significant influences on the set-off auto-ignition. In this paper, numerical study has been carried out to examine the effects of mixture quality on the occurrence of auto-ignition at the end of the compression stroke inside a pentroof combustion chamber. The effect of different Internal Exhaust Gas Re-circulation (IEGR) are investigated. The use of IEGR acts as an indirect control method, the rate of combustion can be slowed down; however the percentage of IEGR retained in the cylinder affects the onset of auto-ignition. The calculated results have been validated against published experimental data, so that the correlation between the two can be discussed. It is found that the inhomogeneity of the air, fuel and the IEGR mixing, presented here in terms of temperature distribution, plays an important role in initiating, and potentially further controlling the HCCI combustion. During the compression process, certain parts of the engine charge are found to reach a higher temperature which auto-ignited depending on the percentages of IEGR used

Numerical evaluation of combustion regimes in a GDI engine

There is significant interest in the gasoline direct-injection engine due to its potential for improvements in fuel consumption but it still remains an area of active research due to a number of challenges including the effect of cycle-by-cycle variations. The current paper presents the use of a 3D-CFD model using both the RANS and LES turbulence modelling approaches, and a Lagrangian DDM to model an early fuel injection event, to evaluate the regimes of combustion in a gasoline direct-injection engine. The velocity fluctuations were investigated as an average value across the cylinder and in the region between the spark plug electrodes. The velocity fluctuations near the spark plug electrodes were seen to be of lower magnitude than the globally averaged fluctuations but exhibited higher levels of cyclic variation due to the influence of the spark plug electrode and the pent-roof geometry on the in-cylinder flow field. Differences in the predicted flame structure due to differences in the predicted velocity fluctuations between RANS and LES modelling approaches were seen as a consequence of the inherently higher dissipation levels present in the RANS methodology. The increased cyclic variation in velocity fluctuations near the spark plug electrodes in the LES predictions suggested significant variation in the relative strength of the in-cylinder turbulence and resultant thickening of the propagating flame front from cycle-to-cycle in this region. Throughout this paper, the numerical results were validated against published experimental data of the same engine geometry under investigation

A numerical study of intake valve jet flapping in a gasoline direct injection engine

This paper presents findings from a numerical study of intake valve jet flapping within a gasoline direct injection (GDI) engine, using a large eddy simulation (LES) turbulence modelling approach. The experimental test case and computational setup, including choice of sub-grid scale (SGS) turbulence model, are presented and discussed. An example cycle where intake valve jet flapping is seen to be prominent is discussed in detail. Intake valve jet flapping was found to be initiated as a consequence of turbulent fluctuations in the intake valve curtains. Cycle-by-cycle variations in valve curtain mass flux and the subsequent jet flapping events are investigated and significant cyclic variability is found. It was observed that when an ensemble-averaging procedure, typically used in LES simulations and experimental PIV data post-processing, is applied, due to the cyclic variability of the variations in valve curtain mass flux, most of the information related to this flow phenomenon is lost

Impingement characteristics of an early injection gasoline direct injection engine: A numerical study

This paper describes the use of a Lagrangian discrete droplet model to evaluate the liquid fuel impingement characteristics on the internal surfaces of an early injection gasoline direct injection (GDI) engine. The study focuses on fuel impingement on the intake valve and cylinder liner between start of injection (SOI) and 20° after SOI using both a single- and multi-component fuel. The single-component fuel used was iso-octane and the multi-component fuel contained fractions of iso-pentane, iso-octane and n-decane to represent the light, medium and heavy fuel fractions of gasoline, respectively. A detailed description of the impingement and liquid film modelling approach is also provided Fuel properties, wall surface temperature and droplet Weber number and Laplace number were used to quantify the impingement regime for different fuel fractions and correlated well with the predicted onset of liquid film formation. Evidence of film stripping was seen from the liquid film formed on the side of the intake valve head with subsequent ejected droplets being a likely source of unburned hydrocarbons and particulate matter emissions. Differences in impingement location and subsequent location of liquid film formation were also observed between single- and multi-component fuels. A qualitative comparison with experimental cylinder liner impingement data showed the model to well predict the timing and positioning of the liner fuel impingement

Computational study on the charge mixing of internal exhaust gas recirculation initiated controlled auto ignition

Controlled Auto Ignition (CAI) uses compression heat to auto ignite a homogeneous air/fuel mixture. Using internal exhaust gas re-circulation (IEGR) as an indirect control method, CAI offers potentially superior fuel economy and pollutant emission reductions. The local chemical and thermal conditions of the engine charge towards the end of the compression stroke have significant influences toward fuel auto ignition performance. In this study, KIVA-3V has been employed to investigate the mixing process involving the fuel, air and the IEGR inside a pentroof engine. The calculated results were compared with experimental data. A mixing index was formulated to show the level of homogeneity in the mixture during the compression process. Good correlations were obtained between the measured and calculated data. Results showed that the level of mixing between trapped burnt gas and the fresh mixture is enhanced by increasing the percentage of trapped IEGR

A DYNAMIC SGS MODEL FOR LES OF TURBULENT PREMIXED FLAMES

Numerical simulations are carried out for transient turbulent premixed flames using large eddy simulations (LES) technique. The sub-grid scale (SGS) mean chemical reaction rate is calculated using a simple algebraic relation for flame surface density (FSD), with a dynamic formulation for the model coefficient. The dynamic model is derived based on fractal theory and a flame wrinkling factor and implemented in compressible LES code. The developed model is used to simulate turbulent premixed flames of stoichiometric propane/air mixture in a vented combustion chamber, propagating over built-in solid obstacles. The fractal dimension is dynamically calculated by viewing the flame front as a fractal surface, using the SGS velocity fluctuations and the strained laminar burning velocity. The model predictions are validated against experimental measurements where good agreements are obtained