The preparation and shock tube investigation of comparative ignition delays using blends of diesel fuel with bio-diesel of cottonseed oil

This paper describes an experimental effort for the production of cotton methyl ester (CME), cotton ethyl ester (CEE) and CEE-diesel blends from neat cottonseed oil (CSO) for use as a bio-diesel fuel and the investigation of the ignition delay times of these fuels using the shock tube. The transesterification of the neat CSO with methanol or ethanol has been performed to determine the optimum conditions for the preparation. The optimum parameters were cottonseed oil/alcohol molar ratio, 1:6; NaOH amount, 1% by the weight of the oil and reaction time, 75 min. The physical properties of all the tested fuels are measured. 89% of the neat CSO was converted into CME or CEE and the use of different alcohols (methanol or ethanol) presents few differences with regards to the kinetics of reaction but the final yield of esters remains almost unchanged. The ignition delay times were measured using a piezo-electric pressure transducer, charge amplifier, data acquisition card, IBM computer and LabVIEW program. Effects of equivalence ratio, initial charge temperature and initial charge pressure on the ignition delay times are discussed. The results show that the minimum ignition delay time was observed at an equivalence ratio of 1.05 for all the tested fuels. The ignition delay can be reduced considerably together with an increase of the initial charge temperatures and pressures. Also, the ignition delays of the tested fuels are compared with the diesel fuel.     

Droplet ignition of coal–water slurries prepared from typical coal- and oil-processing wastes

The requirements for stable ignition (and subsequent combustion) of fuel suspensions prepared from typical coal- and oil-processing wastes are studied experimentally. Attention focuses on the differences between the ignition characteristics of coal–water slurries (containing petrochemicals) obtained on the basis of filter cakes containing T, K, SS, Zh, D, and G coal. To eliminate the influence of the droplet holder (traditionally, thermocouple junctions, ceramic rods, or metal wire) on the ignition characteristics of the fuel droplet, the experiments employ a special model combustion chamber and a device for introducing a single drop of suspension. The ignition time and the minimum temperature of stable ignition of a droplet of coal–water slurries suspended in an oxidant flux are established. The influence of the following factors on the initiation of fuel combustion is determined: the oxidant temperature, the droplet size, the size of the coal dust, and the properties and concentrations of the components. The compositions of the coal–water slurries corresponding to optimal ignition (minimum inertia) are identified.     

IGNITION DELAY OF DROPLET CLOUDS: RESULTS FROM GROUP COMBUSTION THEORY

The ignition and evaporation of spherical cloud of droplets in a hot quiescent atmosphere is examined numerically using transient group combustion analysis. Ignition delay times are calculated as a function of cloud radius, ambient temperature, drop size and droplet number density. The ignition temperature for a cloud of drops was found to be less than that obtained from a single drop. The results indicated an interaction between chemical and physical effects resulting in the possibility of an optimal interdrop spacing for ignition of a fuel with a high boiling point. The model results indicate that for interdrop spacing to radius ratio of less than 5, the ignition and evaporation of a cloud of drops is confined to a thin layer at the surface of the cloud. For drops spaced farther apart thermal penetration from the hot ambient is possible resulting in vaporization within the cloud.     

浓度和点火位置对氢气-空气预混气爆燃特性影响

开展了氢气-空气预混气在透明方管内的爆燃实验研究,分析在一端开口一端封闭的狭长空间内,浓度和点火位置对氢气-空气预混气爆燃特性的影响。实验结果表明：氢气浓度和点火位置对火焰锋面结构以及发展有重要影响;各当量比条件下,均在距封闭端100 mm位置点火时反应最为迅速;在极贫燃或极富燃条件下,点火位置对火焰发展影响更大。氢气浓度与点火位置共同作用于压力波形,以距封闭端300 mm点火位置为界,分别在管道前后两段点火时,不同当量比条件下超压波形呈现复杂变化。超压峰值对氢气浓度具有极强依赖性,并且浓度对爆燃超压的影响程度远大于点火位置;在各点火位置下,均在Φ = 1.25时获得最大超压;最大超压对应的点火位置取决于当量比。     

Ignition kernel development studies relevant to lean-burn natural-gas engines

In lean-burn premixed natural-gas engines, ignition and combustion can be accelerated by burning a small fraction of the mixture in a pre-chamber. High pressure generated in the pre-chamber results in the discharge of burned products into the main chamber. This creates multiple ignition kernels along the surface of the resulting jet. In this work, lean-burn characteristics of methane under the high pressure and high temperature conditions of a hot-jet ignited combustion chamber are investigated numerically by initializing a kernel of specified composition, temperature and size in a lean premixed gas, and following the development of the flame. In the case of hot-jet ignition the kernel temperature is limited by the temperature of the hot products. The influence of variations in ignition energy, affected by both temperature and size, and equivalence ratio, on the flame development is studied in an initially quiescent gas. It is shown that as long as the available ignition energy is greater than a minimum, the duration in which a steady flame speed is achieved is a strong function of kernel temperature; it is not a function of kernel size.     

甲烷部分氧化制乙炔过程研究

The partial oxidation of hydrocarbons is an important technical route to produce acetylene for chemical industry.The partial oxidation reactor is the key to high acetylene yields.This work is an experimental and numerical study on the use of a methane flame to produce acetylene.A lab scale partial oxidation reactor was used to produce ultra fuel-rich premixed jet flames.The axial temperature and species concentration profiles were measured for different equivalence ratios and preheating temperatures,and these were compared to numerical results from Computational Fluid Dynamics(CFD)simulations that used the Reynolds Averaged Navier-Stokes Probability Density Function(RANS-PDF)approach coupled with detailed chemical mechanisms.The Leeds 1.5,GRI 3.0 and San Diego mechanisms were used to investigate the effect of the detailed chemical mechanisms.The effects of equivalence ratio and preheating temperature on acetylene production were experimentally and numerically studied.The experimental validations indicated that the present numerical simulation provided reliable prediction on the partial oxidation of methane.Using this simulation method the optimal equivalence ratio for acetylene production was determined to be 3.6.Increasing preheating temperature improved acetylene production and shortened greatly the ignition delay time.So the increase of preheating temperature had to be limited to avoid uncontrolled ignition in the mixing chamber and the pyrolysis of methane in the preheater.     

高浓度丙酮VOCs高温预热近极限贫燃预混火焰特性的数值分析

为进一步对一种丙酮挥发性有机化合物（VOCs）焚烧炉进行设计优化和运行参数调节,本文对其在不同的燃料当量比、预热温度下的火焰特性进行了数值模拟,分析了其绝热火焰温度、着火延迟时间、火焰传播速度和一维火焰产物分布特性。研究结果表明：典型当量比（约0.113）下的绝热火焰温度为850~900℃,属于中低温燃烧,绝热火焰温度随预热温度和当量比（0.06~0.4）的升高均线性升高。预热温度和化学当量比对着火延迟时间的影响十分敏感。在其典型贫燃条件下,层流火焰传播速度随预热温度升高呈指数函数关系增大,随化学当量比增大而缓慢升高,且其层流火焰传播速度不超过150cm/s。反应过程首先发生丙酮的分解和部分氧化,并持续时间较长,仅当混合物的温度升高一定程度后才发生较剧烈的CO氧化。     

Enhanced Droplet Size Control in Liquid‐Liquid Emulsions Obtained in a Wire‐Guided X‐Mixer

The droplet size in a liquid‐liquid emulsion can be controlled by placing a metal wire along the centerline of an X‐mixer. Droplets gradually form when flowing along the wire, with droplet separation occurring at the tip of the wire rather than at the channel intersection in the X‐mixer. The droplet size is now defined by the Plateau‐Rayleigh instability developing in the axisymmetric annular flow region rather than by a sophisticated and hardly predictable three‐dimensional flow at the channel intersection. The wire‐guided droplet formation allows for fine control of the droplet size by changing the wire diameter, the position of the wire tip, and the flow rates. Further control of the droplet size can be achieved by adjusting the surface tension by adding a surfactant.     

A chemical kinetics model of iso-octane oxidation for HCCI engines

The necessity of developing a practical iso-octane mechanism for homogeneous charge compression ignition (HCCI) engines is presented after various different experiments and currently available mechanisms for iso-octane oxidation being reviewed and the performance of these mechanisms applied to experiments relevant to HCCI engines being analyzed. A skeletal mechanism including 38 species and 69 reactions is developed, which could predict satisfactorily ignition timing, burn rate and the emissions of HC, CO and NO_x for HCCI multi-dimensional modeling. Comparisons with various experiment data including shock tube, rapid compression machine, jet stirred reactor and HCCI engine indicate good performance of this mechanism over wide ranges of temperature, pressure and equivalence ratio, especially at high pressure and lean equivalence ratio conditions. By applying the skeletal mechanism to a single-zone model of HCCI engine, we found out that the results were substantially identical with those from the detailed mechanism developed by Curran et al. but the computing time was reduced greatly.     

Influence of the Strain Rate,Particle Size,and Equivalence Ratio on the Combustion of the Premixed Air–Aluminum Microparticle Mixture with a Counterflow Structure

The effects of the strain rate, equivalence ratio, and particle diameter on the combustion of a mixture of aluminum microparticles with air under fuel-lean conditions are studied in the counterflow configuration with an approximate analytical perturbation method. The flame structure is assumed to consist of three zones: preheating, flame, and post-flame zones. Reasonable agreement between the current results and experimental data is obtained in terms of the flame temperature. The dimensionless ignition and ultimate flame temperatures, place of the flame starting point, and flame thickness are obtained as functions of the strain rate for different particle diameters and equivalent ratios. The results indicate that the ignition and ultimate flame temperatures and also the flame thickness decrease with increasing strain rate. With a decrease in the strain rate, the length of the preheating zone increases. With increasing particle diameter, the flame thickness increases, whereas the ignition and ultimate flame temperatures decrease. An increase in the equivalence ratio causes an increase in the ultimate flame temperature and reduction of the preheating zone and flame thickness.     

甲醇对汽油燃料均质压燃燃烧和排放机理的影响

为了探讨甲醇对汽油均质压燃(HCCI)燃烧和排放机理的影响,利用CHEMKIN软件从理论上研究了甲醇对汽油HCCI燃烧反应动力学机理的影响。结果表明∶甲醇抑制了汽油(甲苯参比燃料)各组分的二次加氧过程,异辛烷基和正庚烷基继续脱氢,再氧化分解产生甲醛;甲苯基则直接氧化分解产生甲醛。随着甲醇体积分数增大,放热开始时刻提前,高温反应阶段的放热率峰值呈先增大后减小趋势。因此,可以通过调整反应中燃料的比例来控制着火时刻和放热峰值。CO和HC摩尔分数随甲醇比例增大逐步减小。随着燃空当量比升高,主燃烧峰值升高,主燃烧持续期延长,OH基生成速率和摩尔分数峰值增大,但放热开始时刻和OH开始生成时刻几乎不变。所以改变当量比可以改变燃烧反应中自由基摩尔分数和反应持续时间,但不能控制着火时刻。CO和HC摩尔分数峰值随当量比增大逐步增大,当量比过小时,大量CO和HC未被氧化。     

铝/乙醇基纳米浆体燃料的着火燃烧特性研究

为了明确纳米铝粉从低浓度到高浓度变化对液体碳氢燃料着火燃烧特性的影响,采用液滴悬挂法研究了不同温度下(700~800℃)乙醇液滴和添加不同浓度(2.5wt%, 10wt%, 15wt%和20wt%)纳米铝粉的铝/乙醇基纳米浆体燃料液滴的着火燃烧特性。利用高速摄影系统捕捉了液滴整个燃烧过程,分析了其液滴寿命。通过热电偶对液滴附近气相温度的测量,获得了其着火性能参数。结果表明,添加纳米铝粉可以改善乙醇液滴的着火性能。不同铝粉浓度改善效果不同,低浓度时效果较好,着火延迟时间显著缩短,点火温度明显降低。随温度升高,乙醇及添加纳米铝粉的铝/乙醇基纳米浆体燃料液滴着火延迟时间及着火温度均明显降低。纳米铝粉(S2)对乙醇(S1)着火延迟时间和液滴寿命的降幅在750℃最大,其降幅分别达42.20%和18.43%。纳米铝粉(S3)着火温度降低,其最大降低幅度也出现在750℃,相对于乙醇(S1)降低幅度达28.57%。一定铝粉浓度范围内,液滴微爆炸程度和微爆炸时长随铝粉浓度升高而增大,但铝粉浓度超过10wt%后趋势变得平稳。     

Shock tube investigation of propane-air mixtures with a pilot diesel fuel or cotton methyl ester

The propane (or LPG) is one of the best candidates as an alternative fuel in dual-fuel engines which operate primarily on any type of gaseous fuel using pilot injection of diesel to achieve ignition. The ignition delay has received considerable attention in the published literature for various gaseous fuels using different dual-fuel engines which showed that the ignition delay in a dual-fuel engine is different from that in a diesel engine especially at low loads. In this research, the measurement of ignition delay of propane-air mixtures with a pilot diesel fuel or cotton methyl ester (CME) similar to mixtures used in dual-fuel engines have been performed in a shock tube. The operating conditions were the equivalence ratio ranging from 0.3 to 1.2, the initial pressure varied from 0.4 to 1.0 bar, the initial temperature varied from 423 to 673 K, the relative mass of pilot liquid fuel and the type of liquid fuel. The ignition-delay times were measured using a piezo-electric pressure transducer, charge amplifier, data acquisition card, PC computer and LabVIEW program. From the results, it is shown that, the minimum ignition-delay time for the dual-fuel combustion was observed at stoichiometric equivalence ratio for propane-air mixtures with a pilot diesel fuel or CME. Higher initial temperatures and pressures reduced the ignition delay. Also, the ignition delays of propane-air mixtures are affected by changes in pilot fuel quantities and properties.     

Reduced chemical reaction mechanisms: experimental and HCCI modelling investigations of autoignition processes of iso-octane in internal combustion engines

A semi-reduced (70 species, 210 reactions) and a skeletal (27 species, 29 reactions) chemical reaction mechanism for iso-octane are constructed from a semi-detailed iso-octane mechanism (84 species, 412 reactions) of the Chalmers University of Technology in Sweden. The construction of the reduced mechanisms is performed by using reduction methods such as the quasi-steady-state assumption and the partial equilibrium assumption. The obtained reduced iso-octane mechanisms show, at the mentioned conditions, a perfect coherence with another more detailed iso-octane mechanism of ENSIC-CNRS (2411 reactions and 473 species) and the semi-detailed iso-octane mechanism of Chalmers. The validity of this mechanism with regard to the ignition delay is determined for several engine parameters adhering to HCCI conditions: inlet temperature (303–363 K), equivalence ratio (0.2–0.7) and compression ratio (10–16). The ignition delay is found to be decreased by an increase in the inlet temperature, a decrease in the equivalence ratio and an increase in the compression ratio. In order to validate the effects of the inlet temperature, compression ratio on the auto-ignition delay, experiments are performed on a CFR engine. A good agreement is obtained between experimental results and calculations.     

Theory of competition between breakup and coalescence of droplets in flowing polymer blends

The Smoluchowski equation for the breakup and coalescence of dispersed droplets has been solved for flowing polymer blends. A scaling form for the distribution of droplet sized derived and published for a system of clusters with fragmentation and coagualation was used in our dervation. Equations are developed here for the average droplet size and for the characteristic time of transition to steady state flow of blends with a high content of the dispersed phase. Expressions reasonably describing the average size of droplets for all concentrations were obtained by a theory modification. Measured dependences of droplet size on the blend composition can be matched only if simultaneous collisions of three and more droplets are considered. The results of the theory indicate that the mechanism of droplet breakup (formation of pieces with the same or different volumes) has only a small effect on their average size in concentrated systems. The dependence of droplet size on the shear rate in flow is determined by properties of the blend components, and is generally nonmonotonic.     

Effects of NTO Oxidizer Temperature and Pressure on Hypergolic Ignition Delay and Life Time of UDMH Organic Gel Droplet

Organic gel propellants are promising candidates for a variety of rocket motor and scramjet applications, since they are intrinsically safe and provide high performance. It is well known that organic gel fuel droplets exhibit distinct combustion characteristics compared with conventional liquid fuel droplets, and furthermore an understanding of the ignition delay and lifetime of these droplets is critical to the improvement of combustor design. In this work, investigations of the combustion of unsymmetrical dimethylhydrazine (UDMH) organic gel droplets in different nitrogen tetroxide (NTO) oxidizing atmospheres were conducted using two sets of experimental apparatus. The combustion characteristics under different conditions of temperature and pressure were compared and analyzed based on the flame shapes observed during experimentation. From these trials, an unsteady combustion model was developed and used for the numerical simulation of spray‐sized UDMH organic gel droplet combustion in an NTO atmosphere. The hypergolic ignition and burning characteristics of the organic gel droplets under conditions simulating either engine startup or steady state combustion were compared, and changes in ignition delay and droplet lifetime with ambient temperature and pressure were analyzed. The experimental and numerical results show that the UDMH organic gel droplets exhibit periodic swell‐burst behavior following the formation of an elastic film at the droplet surface. Each droplet burst results in fuel vapor ejection and flame distortion, the intensity of which declines with increasing ambient pressure. However, the swell‐burst period is extended with increasing ambient pressure, which results in potential flameout. Under conditions of low temperature and pressure similar to those at engine startup, the ignition delay and lifetime of spray‐sized gel droplets decrease with increasing temperature or pressure, although there is a sharp increase in droplet lifetime when the ambient pressure reaches a critical value associated with flameout. The ignition delay was found to be a rate‐limited phenomenon linked to the droplet heating rate. The proportion of ignition delay and droplet lifetime due to droplet heating‐up decreased with increasing temperature or decreasing pressure. Conversely, at high temperatures and pressures simulating the engine’s steady state operating conditions, the droplets were observed to flameout after several swell‐burst periods and both ignition delay and lifetime decreased monotonically with increasing temperature or pressure. The ignition delay time was determined to be rate‐limited by gas phase chemical reactions and contributed very little to the overall droplet lifetime compared with the engine startup condition.     

Microstructure–performance relationships of hollow-fiber membranes with highly efficient separation of oil-in-water emulsions

The critical breakthrough pressure related to the membrane surface wettability and pore size is a key parameter determining membrane performance in particular applications, such as oil–water mixture separations. A series of hydrophilic polysulfone hollow-fiber membranes with different pore sizes were prepared and characterized to evaluate the separation performance of oil-in-water emulsions and to develop an optimum membrane for such emulsions. For the optimum membrane, the absolute value for the cosine of the surface oil droplet contact angle (0.72) was close to the ratio of the outer surface pore size to the oil droplet size (0.71); it was also similar to the absolute value of the cosine of the underwater oil contact angle on the polysulfone material (0.65). From the point of view of the surface wettability, theoretical calculations were performed to select a suitable membrane with the aim of reaching the maximum efficiency in practical oil–water mixture separation. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47615.     

Combustion of Nanoaluminum and Water Propellants: Effect of Equivalence Ratio and Safety/Aging Characterization

The deflagration and combustion efficiency of 80 nm aluminum/ice (ALICE) mixtures with equivalence ratios of ϕ=1.0, 0.75, and 0.67 were experimentally investigated. We find that pressure exponent and burning rate vary little between these three mixtures, with the exponent varying only from 0.42 to 0.50 and burning rate at 6.9 MPa varying from 2.05 to 2.10 cm s⁻¹. However, reducing the equivalence ratio from 1.0 to 0.67 surprisingly increases combustion efficiency from 70 % to 95 % with unburned aluminum agglomerates visible in electron microscopy photographs of 70 % combustion efficiency (ϕ=1.0) products. Our findings suggest that nanoaluminum/water combustion is diffusionally limited for all conditions considered. Aging tests on the propellant show that storage at −30 °C essentially stops the Al/H₂O reaction such that little nanoaluminum degradation occurs after 200 days. Electrostatic discharge (ESD), shock initiation, and impact sensitivity tests indicate that the propellant is insensitive to ignition by these stimuli. Specifically, while neat nanoaluminum powders are highly ESD sensitive (ignition threshold 0.3–14 mJ), nAl/H₂O mixtures are insensitive to ESD and have ignition thresholds in excess of 400 mJ. Likewise, nAl/H₂O mixtures are insensitive to impact ignition, having an ignition threshold in excess of 2.2 m. Propellants containing 80 nm or larger average particle size aluminum were also found to be insensitive to shock initiation.     

An experimental and numerical analysis of the influence of the inlet temperature,equivalence ratio and compression ratio on the HCCI auto-ignition process of Primary Reference Fuels in an engine

In order to understand better the auto-ignition process in an HCCI engine, the influence of some important parameters on the auto-ignition is investigated. The inlet temperature, the equivalence ratio and the compression ratio were varied and their influence on the pressure, the heat release and the ignition delays were measured. The inlet temperature was changed from 25 to 70 °C and the equivalence ratio from 0.18 to 0.41, while the compression ratio varied from 6 to 13.5. The fuels that were investigated were PRF40 and n-heptane. These three parameters appeared to decrease the ignition delays, with the inlet temperature having the least influence and the compression ratio the most. A previously experimentally validated reduced surrogate mechanism, for mixtures of n-heptane, iso-octane and toluene, has been used to explain observations of the auto-ignition process. The same kinetic mechanism is used to better understand the underlying chemical and physical phenomena that make the influence of a certain parameter change according to the operating conditions. This can be useful for the control of the auto-ignition process in an HCCI engine.     

Effect of fuel properties on biomass combustion: Part I. Experiments—fuel type,equivalence ratio and particle size

Moving bed combustion is commonly used for energy conversion of biomass. Conditions on the moving bed can be conveniently represented by a time dependent fixed bed. The present work experimentally investigates the combustion of four biomass materials having different fuel properties in a fixed bed under fuel-rich conditions. Temperature, gas composition and mass loss curves identified two distinct periods as the combustion progresses in the bed: the ignition propagation and char oxidation. The effects of bulk density, particle size and air flow rate on the combustion characteristics during the two periods are interpreted by using the ignition front speed, burning rate, percentage of mass loss, equivalence ratio and temperature gradient. Different channelling of air was observed for small miscanthus pellets and large wood particles due to the fast propagation of the ignition front around a channel. The elemental ash composition was also analysed, which explained the sintered agglomerates of miscanthus ashes in terms of alkali index.