Conditional Moment Closure Modelling for Dual-Fuel Combustion Engines with Pilot-Assisted Compression Ignition

Soriano, Bruno Souza; Richardson, E.S.; Schlatter, Stéphanie; Wright, Yuri M.

doi:10.4271/2017-01-2188

Cited by 11 publications

(9 citation statements)

References 32 publications

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“…Schlatter et al [17] used a two-dimension Conditional Moment Closure (CMC) formulation and defined the hot spots predicted from the CMC model as the initial kernels of a premixed flame. Soriano et al [18] developed a hybrid two-dimensional CMC and level-set G-equation model, to account for the autoignition and the subsequent flame propagation, respectively. The flame structures were explored in a conditioningvariable space, while the transient development and the underlying mechanism inducing the premixed flame initiation needs further investigations.…”

Section: Introductionmentioning

confidence: 99%

Dynamics of triple-flames in ignition of turbulent dual fuel mixture: A direct numerical simulation study

Jin

Luo

Wang

et al. 2019

Proceedings of the Combustion Institute

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Pilot-ignited dual fuel combustion involves a complex transition between the pilot fuel autoignition and the premixed-like phase of combustion, which is challenging for experimental measurement and numerical modelling, and not sufficiently explored. To further understand the fundamentals of the dual fuel ignition processes, the transient ignition and subsequent flame development in a turbulent dimethyl ether (DME)/methane-air mixing layer under diesel enginerelevant conditions are studied by direct numerical simulations (DNS). Results indicate that combustion is initiated by a two-stage autoignition that involves both low-temperature and hightemperature chemistry. The first stage autoignition is initiated at the stoichiometric mixture, and then the ignition front propagates against the mixture fraction gradient into rich mixtures and eventually forms a diffusively-supported cool flame. The second stage ignition kernels are spatially distributed around the most reactive mixture fraction with a low scalar dissipation rate. Multiple triple flames are established and propagate along the stoichiometric mixture, which is proven to play an essential role in the flame developing process. The edge flames gradually get close to each other with their branches eventually connected. It is the leading lean premixed branch that initiates the steady propagating methane-air flame. The time required for steady flame propagation is substantially shorter than the autoignition delay time of the methane-air mixture under the same thermochemical condition.Temporal evolution of the displacement speed at the flame front is also investigated to clarify the propagation characteristics of the combustion waves. Cool flame and propagation of triple flames are also identified in this study, which are novel features of the pilot-ignited dual fuel combustion.

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Section: Introductionmentioning

confidence: 99%

Dynamics of triple-flames in ignition of turbulent dual fuel mixture: A direct numerical simulation study

Jin

Luo

Wang

et al. 2019

Proceedings of the Combustion Institute

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“…The timing, size, and evolution of the initiated flame kernels strongly influence the flame front evolution and were shown to influence the UHC emissions as well as efficiency [4,[17][18][19]21]. Generally, the process is understood as a competition between an auto-igniting flame-front and premixed flame propagation [22][23][24]. Also in other combustion concepts like RCCI and partially premixed combustion studies the pilot-fuel injection strategy was shown to be the main parameter governing the engine efficiency and HRR [25][26][27].…”

Section: Introductionmentioning

confidence: 99%

“…Both the 0D/1D as well as CFD models have to be able to reproduce such characteristics for accurate HRR predictions. Many dual-fuel CFD models switch between an auto-ignition and a flame-propagation model, using different approaches to distinguish when a premixed flame has been established and to switch the model accordingly [24]. Also, the 0D-models rely on different assumptions when to change the HRR prediction model, for example by assuming conical ignition kernels and the pilot-fuel combustion duration to be proportional to the ignition delay [20].…”

Section: Introductionmentioning

confidence: 99%

Experimental investigation of pilot-fuel combustion in dual-fuel engines, Part 2: Understanding the underlying mechanisms by means of optical diagnostics

Srna

Rotz

Bolla

et al. 2019

Fuel

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The pilot-fuel auto-ignition and combustion under engine like conditions in compressed methane/air mixtures have been investigated. Experiments were performed in an optically accessible Rapid Compression-Expansion Machine featuring quiescent charge conditions and a single-hole coaxial diesel injector mounted on the cylinder periphery. In Part 1, the phenomenology of the pilot-fuel combustion has been studied based on the thermodynamic analysis. With the addition of methane, a strongly prolonged pilot-fuel combustion duration was observed, especially at increased EGR rates. The aim of Part 2 is to improve the understanding of the underlying processes governing the pilot-fuel burning and premixed flame initiation in dual-fuel engines. For this purpose, the thermodynamic analysis was supplemented by the application of optical diagnostics including high-speed CH 2 O-PLIF, Schlieren and OH* chemiluminescence, and corroborated with homogeneous reactor and laminar flame speed calculations. The investigations focused on determining the role of (a) ignition location and number of ignition kernels, (b) stratification of the ignition delay due to the chemical influence of methane and (c) the role of flame propagation during the pilot-fuel burning. In the initial phase, the combustion was found to propagate through an auto-igniting front. When combustion reaches the lean zones with high ignition delay spatial stratification, the premixed flame propagation becomes the dominant mechanism owing to its higher spreading rate. Both processes influence the pilotfuel combustion duration. With increasing methane concentration the simulations predict increasing stratification of the ignition delay in lean regions, and a moderately increased laminar flame speed in the pilot-fuel lean regions. Overall, this explains the observed trends of longer pilot-fuel burning in the dual-fuel cases and indicates an increasing role of flame-propagation in the dual-fuel combustion pilot-fuel burning.

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“…In order to apply the flame speed model in engine simulations, the progress variable in the mixture ahead of the flame then needs to be modelled, either by simulating the evolution of the chemical composition during ignition as in Ref. [34], or potentially by modelling the progress variable as a function of the Livengood-Wu integral [35].…”

Section: Effects Of Pre-ignition Chemistrymentioning

confidence: 99%

Investigation of flame propagation in autoignitive blends of n-heptane and methane fuel

Soriano

Richardson

2019

Combustion Theory and Modelling

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The effects of pre-ignition chemistry on laminar flame speed in methane/n-heptane fuel blends are investigated numerically, leading to flame speed modelling accounting for these effects. The laminar flame speeds of fuel blends are important input parameters for turbulent combustion models needed to support design of dual-fuel engines. At the autoignitive conditions found in engines, pre-ignition reactions cause the speed of the reaction front to increase. Fuels that exhibit two-stage ignition behaviour, such as n-heptane, also exhibit a two-stage increase in the speed of the reaction front as the reactant residence time increases. There is a corresponding reduction in the flame thickness until the residence time approaches the ignition delay time, whereupon the deflagrative scaling of flame thickness breaks down. The analysis shows that the increase in flame speed is due to distinct contributions of heat release, reactant consumption, and enhanced reactivity ahead of the flame. Addition of methane to n-heptane-air mixtures retards and reduces the first-stage increase in flame speed, in part due to dilution of the more-reactive n-heptane fuel, and in part due to consumption of radical species by the methane chemistry. The effect of methane/n-heptane fuel blending on flame speed is described adequately by a linear mixing rule. The effect of pre-ignition chemistry can then be modelled as a linear function of the progress variable ahead of the flame-accounting for heat release, reactant consumption, and enhanced reactivity ahead of the flame. The flame speed model accurately describes the variation of flame speed across the full range of methane/n-heptane blends at engine-relevant conditions, up to the deflagration/ignition transition.

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Conditional Moment Closure Modelling for Dual-Fuel Combustion Engines with Pilot-Assisted Compression Ignition

Cited by 11 publications

References 32 publications

Dynamics of triple-flames in ignition of turbulent dual fuel mixture: A direct numerical simulation study

Dynamics of triple-flames in ignition of turbulent dual fuel mixture: A direct numerical simulation study

Experimental investigation of pilot-fuel combustion in dual-fuel engines, Part 2: Understanding the underlying mechanisms by means of optical diagnostics

Investigation of flame propagation in autoignitive blends of n-heptane and methane fuel

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