Combustion characteristics and liquid-phase visualisation of single isolated diesel droplet with surface contaminated by soot particles

The combustion generated soot contamination effect on a single diesel droplet ignition and burning was investigated experimentally for the first time. Diesel droplet flame was used to contaminate the droplet to be investigated prior to ignition. Distinct differences in lifetime and stability of the burning of the neat and contaminated droplet samples were observed in their heating, boiling and disruptive phases. For a soot-contaminated droplet surface, the evaporation rate became weaker as a result of slower mass transfer thus contracted the flame formation. Contrary to the burning rate enhancement of droplet with stable and uniform suspension of particles observed by other researchers, the slightest contamination of soot particles in a fuel droplet surface can significantly reduce the burning rate. Denser agglomeration of soot can form a shell on the droplet surface which blocks the flow of gas escaping through the surface thus distort the droplet even further. At late combustion stage, bubbles are observed to rapture on the surface of the soot-contaminated droplet. Strong ejections of volatile liquid and vapour that would explode shortly after parting from the droplet are observed. It seems that the explosion and burning of ejected mixture have little interactions with the enveloped flame surrounding the primary droplet. Enhanced visualisation of droplet liquid-phase has clearly indicated the cause of declining trend in the burning rate and flame stand-off ratio of soot-contaminated diesel droplet. These insights are of significance for understanding the effect of fuel droplet contamination by combustion generated soot particles.     

Evolution of temperature of a droplet of liquid composite fuel interacting with heated airflow

The macroscopic patterns of a temperature change at the center of a droplet of three-component (coal, water, petroleum) composite liquid fuel (CLF) were studied using a low-inertia thermoelectric converter and system of high-speed (up to 10⁵ frames per second) video recording during the induction period at different heating intensity by the air flow with variable parameters: temperature of 670?870 K and motion velocity of 1?4 m/s. The studies were carried out for two groups of CLF compositions: fuel based on brown coal and coal cleaning rejects (filter cake). To assess the effect of liquid combustible component of CLF on characteristics of the ignition process, the corresponding composition of two-component coal-water fuel (CWF) was studied. The stages of inert heating of CLF and CWF droplets with characteristic size corresponding to radius of 0.75?1.5 mm, evaporation of moisture and liquid oil (for CLF), thermal decomposition of the organic part of coal, gas mixture ignition, and carbon burnout were identified. Regularities of changes in the temperature of CLF and CWF droplets at each of identified stages were identified for the cooccurrence of phase transitions and chemical reactions. Comparative analysis of the times of ignition delay and complete combustion of the droplets of examined fuel compositions was performed with varying droplet dimensions, temperatures, and oxidant flow velocity.     

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 < d_o < 400 µm), into methane/air mixtures with varying equivalence ratios (0 < ϕ < 1.2) over a range of compressed temperatures (700 K < T_c < 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.     

Coupling of turbulence on the ignition of multicomponent sprays

This study examines the effect of turbulence on the ignition of multicomponent surrogate fuels and its role in modifying preferential evaporation in multiphase turbulent spray environments. To this end, two zero-dimensional droplet models are considered that are representative of asymptotic conditions of diffusion limit and the distillation limit are considered. The coupling between diffusion, evaporation and combustion is first identified using a scale analysis of 0D homogeneous batch reactor simulations. Subsequently, direct numerical simulations of homogeneously dispersed multicomponent droplets are performed for both droplet models, in decaying isotropic turbulence and at quiescent conditions to examine competing time scale effects arising from evaporation, ignition and turbulence. Results related to intra-droplet transport and effects of turbulence on autoignition and overall combustion are studied using an aviation fuel surrogate. Depending on the characteristic scale, it is shown that turbulence can couple through modulation of evaporation time or defer the ignition phase as a result of droplet cooling or gas-phase homogenization. Both preferential evaporation and turbulence are found to modify the ignition delay time, up to a factor of two. More importantly, identical droplet ignition behavior in homogeneous gas phase can imply fundamentally different combustion modes in heterogeneous environments.     

Ignition and combustion of a falling, single sodium droplet

Ignition and combustion of a single sodium droplet has been studied experimentally, by use of a falling droplet. It is found that the ignition delay time increases first gradually and then rapidly, with decreasing initial temperature and/or oxygen concentration, and reaches the limit of ignitability, because of the suppression of surface reaction in the ignition stage. It is also found that with decreasing droplet diameter, the ignition delay time first decreases gradually, because of the decrease in the droplet mass to be heated, and then increases steeply, because of the enhancement of heat loss from the droplet surface. As for the effect of the relative speed, it is found that the ignition delay time increases with increasing relative speed, because of the enhanced heat loss. Experimental comparisons with the analytical results have also been conducted to elucidate dominant parameters, and it is confirmed that a set of comprehensive parameters in the literature can be useful in correlating dominant parameters that influence the ignition delay and/or the limit of ignitability. Furthermore, the analysis has been extended to determine the critical size for the ignition and that for the minimum ignition delay time. Combustion behavior after the ignition has also been examined, and it is found that d²-law can hold for the sodium droplet combustion. In addition, it is found that the burning rate-constant without forced convection has nearly the same value as those for usual hydrocarbon droplets, while the sodium combustion in air is quite similar to that of the usual hydrocarbon fuel in an oxidizer-rich environment.     

Ignition delay of fatty acid methyl ester fuel droplets: Microgravity experiments and detailed numerical modeling

Recent optical engine studies have linked increases in NO_x emissions from fatty acid methyl ester combustion to differences in the premixed autoignition zone of the diesel fuel jet. In this study, ignition of single, isolated liquid droplets in quiescent, high temperature air was considered as a means of gaining insight into the transient, partially premixed ignition conditions that exist in the autoignition zone of a fatty acid methyl ester fuel jet. Normal gravity and microgravity (10⁻⁴ m/s²) droplet ignition delay experiments were conducted by use of a variety of neat methyl esters and commercial soy methyl ester. Droplet ignition experiments were chosen because spherically symmetric droplet combustion represents the simplest two-phase, time-dependent chemically reacting flow system permitting a numerical solution with complex physical submodels. To create spherically symmetric conditions for direct comparison with a detailed numerical model, experiments were conducted in microgravity by use of a 1.1 s drop tower. In the experiments, droplets were grown and deployed onto 14 μm silicon carbide fibers and injected into a tube furnace containing atmospheric pressure air at temperatures up to 1300 K. The ignition event was characterized by measurement of UV emission from hydroxyl radical (OH*) chemiluminescence. The experimental results were compared against predictions from a time-dependent, spherically symmetric droplet combustion simulation with detailed gas phase chemical kinetics, spectrally resolved radiative heat transfer and multi-component transport. By use of a skeletal chemical kinetic mechanism (125 species, 713 reactions), the computed ignition delay period for methyl decanoate (C₁₁H₂₂O₂) showed excellent agreement with experimental results at furnace temperatures greater than 1200 K.     

LES-CMC of high-altitude relight in an RQL aeronautical combustor

Large-Eddy Simulations with the Conditional Moment Closure sub-grid combustion model and detailed chemistry for kerosene were performed for the ignition process in an Rich-Quench-Lean aviation gas turbine combustor at high-altitude conditions. The simulations used realistic boundary conditions for the flow inlet and spray droplet size distributions and velocity. Due to the large droplets, the Central Recirculation Zone (CRZ) is filled with fuel, mostly in liquid form. The first phase of the ignition process is critical and the results show that the spark kernel must provide enough energy to evaporate the spray and pyrolyse the fuel for the flame to grow and establish in the corner of the combustor. The second phase is characterised by the flame burning the mixture in the scorner and propagating around the Inner Shear Layer. This phase is also critical, as the flame needs the prevaporised fuel and smaller droplets in the corner to sufficiently increase the temperature and be able to propagate inside the CRZ, filled with liquid fuel and cold air. If this propagation inside the CRZ is achieved, phase three is accomplished and the burner is fully ignited. The simulations demonstrate the particular importance of detailed chemistry and proper boundary conditions for flame ignition simulations in high-altitude relight conditions.     

Auto-ignition of a polydisperse fuel spray

In the present paper, the effect of fuel spray polydispersity on the auto-ignition process in a fuel cloud is considered. In many engineering applications it is common practice to relate to the actual polydisperse spray as being equivalent to a monodisperse spray with all droplets therein having some average diameter. In combustion systems, the Sauter mean diameter (SMD) is frequently used for this purpose; it is based on the ratio between the total droplet volume and the total droplet surface area of all the droplets in the polydisperse spray. The main purpose of the current work is to examine qualitatively the dynamics of ignition of a truly polydisperse spray in a combustible gas medium and compare it with the dynamics of an equivalent monodisperse spray based on the SMD. Since the system of governing equations represents a multi-scale problem the method of integral manifolds is applied in order to extract the dynamical behavior. Preliminary computed results suggest that the use of the usual SMD-based monodisperse spray leads to quite a significant over-estimate of the ignition time. An alternative modified definition of the SMD, in which the overall liquid fuel volume is also conserved in the averaging process, reduces the discrepancy between the ignition time for the polydisperse spray and that of the equivalent monodisperse spray. However, it seems that some other sort of average droplet size needs to be determined to minimize the aforementioned discrepancy. These results highlight the care that must be exercised before dispensing with the behavior of the actual polydisperse spray in favor of that of an equivalent monodisperse spray, even at the expense of complexity.     

Effects of pressure on flame propagation in a premixture containing fine fuel droplets

Combustion experiments on fuel droplet–vapor–air mixtures have been performed with a rapid expansion apparatus which generates monodispersed droplet clouds with narrow diameter distribution using the condensation method. The effects of fine fuel droplets on flame propagation were investigated for ethanol droplet–vapor–air mixtures at various pressures from 0.2 to 1.0 MPa. A stagnant fuel droplet–vapor–air mixture, generated in a rapid expansion chamber, was ignited at the center of the chamber using an ignition wire. Spherical flame propagation under constant-pressure conditions was observed with a high-speed video camera and flame speed was measured. Total equivalence ratio, and the ratio of liquid fuel mass to total fuel mass, was varied from 0.6 to 1.4 and from zero to 56%, respectively. The mean droplet diameter of fuel droplet–vapor–air mixtures was set at 8.5 and 11 μm. It was found that the flame speed of droplet–vapor–air mixtures less than 0.9 in the total equivalence ratio exceeds that of premixed gases of the same total equivalence ratio at all pressures. The flame speed of fuel droplet–vapor–air mixtures decreases as the pressure increases in all total equivalence ratios. At large ratios of liquid fuel mass to total fuel mass, the normalized flame speed (the flame speed of droplet–vapor–air mixtures divided by the flame speed of the premixed gas with the same total equivalence ratio), increases with the increase in pressure for fuel-lean mixtures, and it decreases for fuel-rich mixtures. The outcome is reversed at small ratios of liquid fuel mass to total fuel mass; the normalized flame speed decreases with the increase in pressure for fuel-lean mixtures, and increases for fuel-rich mixtures. The results suggest that the increase in pressure promotes droplet evaporation in the preheat zone.     

多组份混合燃料滴着火规律的研究

1前言多组份燃料滴的着火研究有很重要的实际与理论意义。人们在实际中使用的传统液体燃料通常都是由多种分子结构、沸点等技术指标相差较大的组份混合而成的，另外，人们在实践中发现使用一些混合燃料如油一酒精，油一甲醇，重油一轻油以及油水乳化液等，可以改善蒸发过程，改善气相反应的化学特性。这就使得混合燃料具有改善内燃机工况及减少污染物排放的潜在优点。所以多组份燃料滴着火燃烧的研究更有意义。多种组份液滴的蒸发、着火比单组份液滴更为复杂，不同点主要集中在以下三个方面：（1）液滴内热量与组份的输运过程更为复杂；（2…     

Ignition of fuel issuing from a porous cylinder located adjacent to a heated wall: A numerical study

This work deals with the numerical simulation on an unstructured mesh of the ignition and burning in an oxidizing atmosphere of a fuel droplet heated on one side. This is relevant for studying the ignition of droplets in a spray when they are crossing a flame zone stabilized in it. The droplet here is replaced by a porous cylinder, and the flame by a hot solid wall. The reaction is assumed to be described by a single step, A?+?νB?→?P. The cell-centred finite volume scheme considered here uses a generalized Roe's approximate Riemann solver with the monotonic upwind scheme for conservative laws (MUSCL) technique for the convective part and Green–Gauss type interpolation for the viscous part. The thinness of the reaction zone is taken into account by using an adaptive refinement–unrefinement procedure. It has been found that the process of droplet ignition takes place by means of a propagation of a triple flame around the ‘droplet’ when the chemical reaction is sufficiently fast with respect to the molecular heat and mass diffusion process.     

Simulation of the ignition of liquid fuel with a local source of heating under conditions of fuel burnout

The processes of heat and mass transfer with phase transitions and chemical reactions in the ignition of liquid fuel by a local source of heating, a hot metal particle, under conditions of fuel burnout are studied. The influence of liquid fuel burnout on the ignition characteristics is analyzed, and the results of investigation of the extent of influence of this factor for solid and liquid condensed materials under conditions of local heating are compared.     

Comprehensive ignition characterization of a non-toxic hypergolic hybrid rocket propellant

This paper describes a comprehensive characterization of ignition properties of a metal-hydride based non-toxic hypergolic hybrid rocket propellant. The propellant consists of Rocket Grade Hydrogen Peroxide (RGHP) as oxidizer, high-density Polyethylene (HDPE) as fuel and sodium borohydride (NaBH₄) as the additive, embedded in the HDPE matrix. Ignition quality was characterized as ignition delay, ignition probability and flame spread. In a drop-test setup, ignition characteristics were determined as a function of seven parameters: RGHP concentration, additive loading, oxidizer droplet impact velocity, oxidizer droplet volume, pressure, diluent gas, and environmental exposure. The parameters encompass thermo-chemical, fluid/droplet dynamics and environmental factors affecting ignition. Ignition delays as low as 3 ms were observed, one of the lowest using non-toxic hypergolic hybrid propellants in open-air. An overwhelming majority of conditions tested yielded <10 ms ignition delays and 100% ignition success. All conditions tested affected ignition to varying degrees with RGHP concentration, NaBH₄ loading and drop impact velocity significantly affecting ignition. Further, contrary to expectations, exposing sanded fuel samples to humidity for a few h enhanced ignition instead of hampering it and exposure for 24 h did not lead to ignition degradation. Tests with diluent gases other than air (at atmospheric and elevated pressures) revealed that atmospheric oxygen played a negligible role in the reaction process. This proved that oxygen for the initial ignition event was obtained from RGHP decomposition, with atmospheric oxygen playing no role in ignition performance. Aside from demonstrating excellent ignition characteristics, our results further show a need to go beyond thermo-chemical properties and to consider aspects of ignition other than ignition delay in hypergolic propellant research to enable a complete understanding of the ignition processes. The comprehensive ignition characterization demonstrates the chosen propellant's ability to overcome ignition challenges in hybrid rockets and serves as a proof of concept for its further development.     

Numerical simulation of laser ignition of a liquid fuel film

Numerical simulations were used to examine a set of interrelated physicochemical processes involved in the ignition of a liquid fuel film by a low-power laser beam. The delay time of ignition of a liquid fuel film and the ignition zone location were determined. The scale of influence of the power and radius of the laser beam on the ignition characteristics was determined. The ignition criteria of ignition were identified.     

Numerical simulation of the ignition of liquid fuel with a limited-energy source under turbulent flow conditions

The ignition of a typical liquid fuel with a limited-energy source, a small metal particle heated to high temperature is numerically simulated with consideration given to the possible turbulization of the fuel vapor flow. The dependences of the integral ignition characteristics on the key parameters of the local heat source are established. The integral ignition characteristics, as well as the fields of fuel vapor concentrations and velocities predicted by models accounting for the laminar and turbulent modes of the vapor-oxidizer mixture flow are compared.     

Propagation and extinction of an unsteady spherical spray flame front

Experimental evidence seems to indicate that the life of a laminar spherical flame front propagating through a fresh mixture of air and liquid fuel droplets can be roughly split into three stages: (1) ignition, (2) radial propagation with a smooth flame front and (3) propagation with flame front cellularization and/or pulsation. In this work, the second stage is analysed using the slowly varying flame approach, for a fuel rich flame. The droplets are presumed to vaporize in a sharp front ahead of the reaction front. Evolution equations for the flame and evaporation fronts are derived. For the former the combined effect of heat loss due to droplet vaporization and radiation plays a dominant explicit role. In addition, the structure of the evaporation front is deduced using asymptotics based on a large parameter associated with spray vaporization. Numerical calculations based on the analysis point to the way in which the spray modifies conditions for flame front extinction. Within the framework of the present simplified model the main relevant parameters turn out to be the initial liquid fuel load in the fresh mixture and/or the latent heat of vaporization of the fuel.     

Analyzing the characteristic times of physical-chemical processes running at ignition of a liquid condensed substance under local heating

The ranges of times of heat and mass transfer processes, phase transitions, formation of a reactive vapor-gas mixture, and abruptly exponential acceleration of oxidation at ignition of a liquid condensed substance by a typical source with a limited heat content, that is, by a small hot metal particle, are found. Results of the performed numerical and experimental investigations are used to find the limiting values of the main parameters (temperature, sizes) of a local energy source, which are sufficient for ignition of a typical liquid fuel.     

Effect of the shape of a particle heated to a high temperature on the gas-phase ignition of a liquid film

The process of gas-phase ignition of a liquid fuel film with incandescent small metal particles in the form of a parallelepiped, disk, or hemisphere was numerically simulated. The magnitude of influence of the particle shape on the delay time of ignition of a liquid fuel was determined. The range of parameters of the particle at which the effect of its shape on the ignition delay time is unimportant was established.     

The influence of radiation heat exchange on characteristics of liquid fuel ignition by a heated metal particle

A complex of interrelated heat-mass transfer processes at gas-phase ignition of a typical liquid fuel by a hot metal particle immersed partially into a liquid is investigated numerically. The scale of influence of the radiation heat exchange at particle—liquid fuel and particle—gas—vapor mixture interfaces is found. Conditions under which the impact of this factor can be neglected are determined.     

Analysis of possible reasons for macroscopic differences in the characteristics of the ignition of a model liquid fuel by a local heat source and a massive heated body

A numerical study of the ignition of the liquid fuel drop-massive heat source-air and liquid fuel-small-size heat source-air systems was performed. It was established how the ignition delay times of single drops and large amounts of liquid fuel depend on the temperature of the heated body. Possible modes of ignition of a typical fuel by small and extensive heat sources were identified.