Computational study of auto ignition, spark ignition and dual fuel droplet ignition in a rapid compression machine, A

2017 Summer.Includes bibliographical references.A series of computational modeling studies were performed using the CONVERGETM computational fluid dynamics (CFD) platform to gain in-depth understanding of the chemically reacting flow field, ignition and combustion phenomena in a various rapid compression machine (RCM) experiments conducted at CSU including homogeneous autoignition, laser ignition and droplet ignition experiments. A three-dimensional, transient computational modeling study was initially performed to examine premixed, homogeneous autoignition of isooctane/air and methane/air mixtures. A reduced chemical kinetic mechanism for isooctane comprising of 159 species and 805 reactions was developed using direct relation graph error propagation and sensitivity analysis (DRGEPSA) method. Computational results showed good agreement with experimental results capturing the negative temperature coefficient (NTC) behavior of isooctane. The premixed computations also revealed the importance of the piston crevice design for maintaining a homogenous flow field inside a RCM. The result showed that, as the volume of the piston crevices is increased, the roll up vortices are eliminated, which reduces the mixing of the lower temperature boundary layer gases with the higher temperature core gases, thereby maintaining the homogeneity of the flow field. Next, three-dimensional computational modeling laser-ignited premixed fuel/air mixtures at elevated temperatures and pressures in the RCM was performed with detailed chemical kinetics (86 species, 393 reactions). For methane/air mixtures, the computational results were compared against previously reported RCM experiments. Computations were also performed on laser-ignited n-heptane/isooctane/air mixtures under similar simulated conditions in the RCM. In the computations, a simulated spark modeled as a localized hotspot was introduced in the center of the combustion chamber resulting in an outwardly propagating flame, which, depending on the fuel reactivity, produced ignition in the end gas upstream of the flame. Methane/air computations were performed at equivalence ratio of 0.4 ≤ Ф ≤ 1.0 for direct comparisons with experimental measurements of instantaneous pressure, flame propagation rate, and lean limit. For compressed temperature of 782 K, a methane/air lean limit of Ф = 0.38 was predicted computationally (combustion efficiency, χ = 0.8), which was in good agreement with the experimental measurement of Ф = 0.43. For n-heptane/isooctane/air computations, auto-ignition of the end gas was predicted depending on the compressed temperature and Octane Number, which suggests the use of the laser ignition/RCM system as a means to quantify fuel reactivity for spark ignited engines. Lastly, RCM experiments in which single n-heptane droplets are suspended and ignited via compression-ignition in a quiescent, high-pressure, high-temperature, lean methane/air environments were simulated using the 86-species dual-fuel chemical kinetic mechanism developed previously. The simulations capture the ignition event in the vicinity of a spherical n-heptane droplet, which bifurcates into a propagating, premixed methane/air flame and stationary n-heptane/air diffusion flame. Comparisons against experimental measurements of droplet gasification rate, premixed flame propagation speed, and non-premixed flame position will be used to develop revised dual-fuel chemical kinetic mechanisms

Ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures at elevated pressures and temperatures

Includes bibliographical references.2022 Fall.The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has the potential to increase fuel efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual-fuel engines are presently constrained by uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion and a better understanding of dual-fuel combustion processes is necessary to overcome these challenges. In addition to dual-fuel engines, this work is also motivated by abnormal combustion phenomenon that has been observed in highly boosted, spark ignited, natural gas engines, which is caused by engine lubricant oil droplets entering the cylinder and serving as unwanted ignition sources for the natural gas/air mixture. To study the fundamental combustion processes of ignition and flame propagation in dual-fuel engines and abnormal combustion triggered by lubricant oil droplets, single isolated liquid hydrocarbon droplets were injected into premixed CH4/O2/Inert mixtures at elevated temperatures and pressures. In this research, a rapid compression machine (RCM) was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets of 40 to 500 μm along the stagnation plane of the RCM combustion chamber. A high-speed Schlieren optical setup was used for imaging the combustion events in the chamber. Experiments were conducted for diesel fuel and lubricant oil droplets at various initial diameters (50 μm < do < 500 μm), various CH4/O2/Inert equivalence ratios (0 < ϕ < 1.2) and various compressed temperatures (740 K < Tc < 1000 K). Dual fuel experiments revealed multiple modes of droplet ignition, droplet combustion, and premixed flame propagation, which depend on the initial droplet temperature, droplet diameter, droplet velocity, and stoichiometry of the CH4/O2/N2/Ar mixture. In the case of small droplets, spherical ignition events were observed that transition into spherical non-premixed flames that envelope the droplet, producing an outwardly propagating spherical premixed flame. For larger droplet diameters moving at moderate velocity, the ignition event occurs near the droplet surface on the leeward side of the droplet and subsequently creates a non-premixed flame that envelopes the droplet and a non-spherical premixed flame. For droplets moving with high velocities, the ignition event occurs in the wake of the droplet, multiple diameters from the droplet surface, and creates a flame that propagates toward the droplet. Spherical, outwardly propagating premixed flames were observed for diesel droplet ignition in stoichiometric CH4/O2/N2/Ar mixtures, whereas elongated premixed flames were observed in lean CH4/O2/N2/Ar mixtures. The experiments conducted to understand abnormal combustion caused by lubricant oil droplets provided a valuable dataset of ignition delay periods of various petroleum and ester-based lubricant oils at a wide range of thermodynamic and mixture conditions. In concert with the experiments, a combined analytical droplet evaporation model and computational combustion model were developed that simulate the evaporation, ignition, and combustion processes observed in the experiments. The ignition delay dataset was used to successfully develop and validate a surrogate chemical kinetic mechanism suitable for mimicking the ignition characteristics of different lubricant oils. The experiments also revealed the different thermodynamic and mixture conditions at which the lubricant oil droplets did not show ignition. At compressed pressure of 24 bar and varied compressed temperatures of 740 K < Tc < 900 K in CH4/O2/N2/Ar mixture of ϕ = 0.6, two ester-based oils (EBO3 and EBO4) showed no ignition. The experiments and modeling indicate the minimum and maximum droplet sizes for which ignition will occur, the location and mode of ignition in the vicinity of the liquid droplet and the conditions under which the ignition event will transition into a propagating premixed flame. These experimental observations further enhance our understanding of lubricant oil combustion and provide qualitative information of engine operating conditions which can lower the abnormal combustion occurrence in natural gas engines. The results of this study advance the fundamental understanding of dual fuel combustion and provide the practical knowledge to inform which lubricant oil types and droplet sizes promote or inhibit abnormal combustion in natural gas engines

A study of ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures in a rapid compression machine

The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has been demonstrated as a means to increase efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual fuel engines are constrained by the onset of uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion, which can result in high unburned hydrocarbon emissions. To study the fundamental combustion processes of ignition and flame propagation in dual fuel engines, a new method has been developed to inject single isolated liquid hydrocarbon droplets into premixed methane/air mixtures at elevated temperatures and pressures. An opposed-piston rapid compression machine was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets along the stagnation plane of the combustion chamber. A high-speed Schlieren optical system was used for imaging the combustion process in the chamber. Experiments were conducted by injecting diesel droplet of various diameters (50 µm \u3c do \u3c 400 µm), into methane/air mixtures with varying equivalence ratios (0 \u3c ϕ \u3c 1.2) over a range of compressed temperatures (700 K \u3c Tc \u3c 940 K). Multiple autoignition modes was observed in the vicinity of the liquid droplets, which were followed by transition to propagating premixed flames. A computational model was developed with CONVERGE™, which uses a 141 species dual-fuel chemical kinetic mechanism for the gas phase along with a transient, analytical droplet evaporation model to define the boundary conditions at the droplet surface. The simulations capture each of the different ignition modes in the vicinity of the injected spherical diesel droplet, along with bifurcation of the ignition event into a propagating, premixed methane/air flame and a stationary diesel/air diffusion flame