Low-temperature ignition of droplets of suspension organic water–carbon fuels

The conditions and integral characteristics of stable low-temperature ignition of droplets of suspension organic water–coal fuels are established by using the means of high-speed video-detection. The oxidizer temperature for these conditions varies in the range of 600–900 K (much below the temperatures in furnace chambers of modern power installations exceeding 1200 K). The velocity of the oxidizer-flow motion varies from 0.5 to 5 m/s, and the sizes (conditional radii) of droplets vary from 0.5 to 1.5 mm. The times of initiation of burning under conditions of low-temperature heating and the duration of the complete combustion of droplets of the corresponding compositions of organic water-coal fuels are determined. The necessary and sufficient conditions of stable ignition are singled out for promising organic water–coal fuels.     

Ignition of a floating droplet of organic coal–water fuel

The results of experimental investigations are presented for the ignition of droplets (particles) of organic coal–water fuels (OCWFs) floating in a flow of an oxidizer using a special combustion chamber from high-temperature quartz glass. The temperature and the velocity of motion of the oxidizer vary in the ranges of 500–900 K and 0.5–3 m/s. The initial sizes (radii) of fuel droplets amounted to 0.3–1.5 mm. As the basic OCWF components, particles (of 80–100 µm in size) of brown coal “B2,” water, mazut, and waste castor and compressor oils are used. With use of the system of high-velocity video registration, the conditions providing for floating of OCWF particles without initiation of burning and with the subsequent steady ignition are established. Four modes of OCWF-droplet ignition with different trajectories of their motion in the combustion chamber are singled out. The times of the OCWF-ignition delay in dependence on the size of fuel particles and oxidizer temperatures are determined. The deviations of the OCWF-ignition-delay times obtained under conditions of suspension of a droplet on the thermocouple junction and while floating in the oxidizer flow are established.     

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.     

Visualization of aggregation process of dispersed water droplets and the effect of aggregation on secondary atomization of emulsified fuel droplets

The effect of aggregation of dispersed water droplets on secondary atomization of emulsified fuel droplets in a heating process was investigated. Secondary atomization was observed using a single droplet experiment in which a water-in-oil (W/O) emulsified fuel droplet prepared using colored water was heated by a halogen heater. The initial diameter of dispersed water droplets before heating was controlled, and the change in the diameter of dispersed water droplets was measured by image analysis. As a result, the aggregation process of dispersed water droplets in the heating process was successfully visualized. The dispersed water droplet diameter increased with an increase in W/O emulsified fuel droplet temperature. The occurrence probability of micro-explosion increased with an increase in the dispersed water droplet diameter in emulsified fuel droplets. It is suggested that the occurrence probability of micro-explosion can be increased by accelerating the aggregation and coalescence of dispersed water droplets below 430 K, which is the average temperature of the starting point of puffing.     

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.     

Application of the planar laser-induced fluorescence method to determine the temperature field ofwater droplets under intensive heating

Presented are results of experimental investigations concerned with formation of a nonstationary and essentially nonuniform temperature field of a water droplet (initial radius of 1 mm to 2 mm) under intensive heating in a flow of heated air (from 50?C to 1000?C). The method used for this purpose was a noncontact optical planar laser-induced fluorescence (PLIF) method. It is shown that temperature distribution in a water droplet is essentially inhomogeneous even under prolonged heating (to several tens of seconds). Reliability of the results of measurements by the noncontact PLIF method was analyzed by applying a group of fast miniature thermocouples. Restrictions of using the PLIF method for studying temperatures fields of evaporating droplets under high-temperature heating (over 800?C) were marked out. Characteristic times of droplet existence (complete evaporation) were determined. It was analyzed how the temperature difference in a water droplet affects this parameter during heating and intensive phase transitions. It was substantiated that it is expedient to consider essentially inhomogeneous and nonstationary temperature field of a water droplet inmathematical modeling of the heat andmass transfer processes in high-temperature gas–vapor-droplet systems (corresponding, e.g., to burning or heat cleaning of liquids, firefighting, production of composite and gaseous fuels, their combustion, etc.).     

Stagnation-point polydisperse spray flame ignition

A theory of stagnation-point flow polydisperse spray flame ignition by an isothermal hot surface is presented for the first time. The configuration investigated consists of a mixture of fuel droplets and air flowing against an isothermal hot surface (such as a hot ignition probe). The polydisperse spray of droplets is modelled using the sectional approach. A single global chemical reaction is assumed for the case when ignition occurs. The mathematical analysis makes use of a small parameter that is exploited for an asymptotic approach. An analytical criterion for ignition is derived which includes effects of the flow field, the reactants and all the fuel spray-related parameters, including the initial size distribution of the spray's droplets. Numerical calculations disclose how the actual droplet size distribution impacts on the critical stagnation point temperature necessary to promote ignition. Additionally, the analytical estimates are compared with predictions of a numerical finite difference code with very satisfactory agreement.     

Ignition of a vapor-gas mixture by a moving small-size source

A theoretical analysis of the ignition of a liquid fuel vapor-air mixture by a moving small source of heating was performed. A gas-phase model of the ignition with consideration given to heat transfer, liquid fuel evaporation, diffusion and convective motion of fuel vapor in the oxidizer medium, crystallization of the heating source, kinetics of the vaporization and ignition processes, temperature dependence of the thermophysical characteristics of the interacting substances, and character of motion of the heating source in the vapor-gas mixture was developed. The values of the ignition delay time τ _d , the main characteristic of the process, were determined. It was established how τ _d depends on the initial temperature, heating source sizes, velocity and trajectory of the heating source, and ambient air temperature.     

Heat and mass transfer at ignition of liquid fuel droplets spreading over the surface of massive hot bodies

Processes of heat and mass transfer with phase transitions and chemical reactions at the ignition of a liquid fuel droplet colliding with the surface of a hot metal substrate are numerically investigated. The droplet ignition delay times are found. The scale of the influence of the temperature of the substrate, droplet, and oxidizer, and also the droplet size and spreading rate on the ignition inertia is determined. Conditions in which the liquid fuel droplet spread plays an important role in the ignition process are found.     

双液滴撞击平面液膜的流动与传热特性

采用耦合的水平集-体积分数法(CLSVOF)对双液滴连续撞击恒定壁温壁面上的热液膜的流动和换热特性进行了数值模拟及分析,得到了双液滴撞击热液膜后形态演变的过程.分析了液滴垂直间距、撞击速度、液膜厚度以及液滴直径对双液滴撞击液膜后的流动与传热特性的影响,结果显示,壁面平均热流密度随液滴撞击速度的增大而增大,液滴垂直间距、液膜厚度和液滴直径对平均热流密度的影响较小,但会对热流密度在撞击区域和交界区的分布产生重要影响.     

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.     

Influence of the degree of coal metamorphism on characteristics and conditions of ignition of coal-water fuel drops

The results of theoretical studies of the processes of ignition of water-coal fuel droplets based on brown coal, semi-anthracite, anthracite, long-flame and fat coal under the conditions corresponding to the combustion spaces of typical modern boilers are presented. The influence of the degree of metamorphism (structural-molecular transformation of organic matter of coal) and concentration of the organic component of the base fuel (coal) on the conditions of ignition of water-coal fuel particles is analyzed. It is determined that the type and grade of coal have a significant impact on the dynamics of fuel ignition. It was shown that in the case of ignition of coal-water fuel made of mineral coal, the ignition of particles based on semi-anthracite and anthracite is the fastest (by 20%), and ignition of coal-water fuels of fat coal is the slowest. The latter is explained by the lower heat capacity and thermal effect of pyrolysis of this fuel, as well as the relatively high heat conductivity of anthracite coal as compared to fat coal. It has been determined that drops of coal-water fuel made of brown coal ignite substantially (2 times) faster than drops prepared from coal of coal-water particles. This is due to the high content of volatiles in the composition of brown coal.Comparative analysis of the main characteristics of the process: ignition delay times (t_ign) obtained by mathematical modeling and experiments showed a satisfactory agreement between the theoretical and experimental values of t_ign.     

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.     

High-pressure fuel spray ignition behavior with hot surface interaction

Fuel-flexible aircraft propulsion systems using compression ignition engines will require novel strategies for reducing the ignition delay of low-reactivity fuels to feasible timescales. Hot surface ignition of fuel sprays has been implemented in some practical situations, but the complex nature of flame formation within the spray structure poses significant challenges. In order to design next-generation ignition devices, the capacity of hot surface heating elements to promote fuel spray ignition must be investigated. In this study, a rapid compression machine (RCM) was used to examine the ignition process of a single kerosene-based F-24 jet fuel spray with a cylindrical heating element inserted into the spray periphery. The experiments, performed with moderately high injection pressures of 40 MPa, have demonstrated two modes of ignition governed by surface temperature and insertion depth of the heating element. There exists an optimal position where the heating element tip is located in the fuel vapor cone around the liquid spray. For this configuration, a critical surface temperature was identified (～1250 K), above which short ignition delays associated with a “spray ignition” mode are consistently achieved. In this case, a local ignition flame kernel propagates downstream to the flame lift-off length before full ignition of the spray. In comparison, below the critical temperature a slower “volumetric” mode results. The extended ignition delays associated with this mode may be impractical for compression ignition engines operating at high speeds and increased altitude.     

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.     

Expansion,injection and micro-explosion phenomena during the full combustion cycle of mixed fuel droplets of ethyl alcohol and biodiesel

In this study, mixed fuel droplets of ethyl alcohol and biodiesel with different volumes or ratios were prepared and the related expansion, injection and micro-explosion phenomena were investigated. During the tests, a mixed-fuel droplet was hung on a high-temperature-resistant Cr-Ni wire, which was transported to the preheating piston for heating and then was ignited automatically. Micro-needles and an infusion set also were employed to prepare mixed droplets with identical ratios but different volumes. Using a single-lens-reflex (SLR) camera, colour videos that captured the combustion process of the mixed droplets were taken and were processed into colour images using Adobe Premiere Pro CC. The colour image series were analysed to determine the ignition delay time, the burnout time and the flame characteristics during combustion of mixed droplets with different ratios. Instantaneous grey images were obtained by means of high-speed photography during combustion of the mixed droplets with different volumes and different ratios, and of droplets of E40 (in which the volume ratio of ethyl alcohol was 40%). Using Image pro plus and Origin software, the variation of normalised mean droplet diameter with time over the whole combustion cycle was calculated and the underlying reasons for various combustion phenomena were analysed.     

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.     

Flame structure and stabilization of lean-premixed sprays in a counterflow with low-volatility fuel

An experimental study was performed on the combustion of lean-premixed spays in a counterflow. n-Decane was used as a liquid fuel with low volatility. The flame structure and stabilization were discussed based on the flame-spread mechanism of a droplet array with a low-volatility fuel. The spray flame consisted of a blue region and a yellow luminous region. The flame spread among droplets and group-flame formation through the droplet interaction were observed on the premixed spray side, while envelope flames were also observed on the opposing airflow side. The blue-flame region consisted of premixed flames propagating in the mixture layer around each droplet, the envelope diffusion flames around each droplet, the lower parts of the group diffusion flame surrounding each droplet cluster, and the envelope flame around droplets passing through the group flame. The flame was stabilized within a specific range of the mean droplet diameter via a balance between the droplet velocity and the flame-spread rate of the premixed spray.     

Investigation of droplet combustion in strained counterflow diffusion flames using planar laser-induced fluorescence

Experimental investigation of an isolated droplet burning in a convective flow is reported. Acetone droplets were injected in a steady laminar diffusion counterflow flame operating with methane. Planar laser-induced fluorescence measurements applied to OH radical and acetone was used to measure the spatial distribution of fuel vapour and the structure of the flame front around the droplet. High-magnification optics was used in order to image flow areas with a ratio of 1:1.2. The different combustion regimes of an isolated droplet could be observed from the configuration of the envelope flame to that of the boundary-layer flame, and occurrence of these regimes was found to depend on the droplet Reynolds number. Experimental results were compared with 1D numerical simulations using detailed chemistry for the configuration of the envelope flame. Good agreement was obtained for the radial profile of both OH radical and fuel vapour. Influence of droplet dynamics on the counterflow flame front was also investigated. Results show that the flame front could be strongly distorted by the droplet crossing. In particular, droplets with high velocity led to local extinction of the flame front whereas droplets with low velocity could ignite within the flame front and burn on the oxidiser side. PACS 33.50.-j; 42.62.-b; 47.55.D-; 47.70.Pq; 47.80.Jk