Microexplosion and ignition of biodiesel/ethanol blends droplets in oxygenated hot co-flow

Microexplosion and ignition characteristics of biodiesel/ethanol blends were studied by suspending single droplets on fibers in a tubular oven at different ambient conditions. The range of ethanol content in initial fuel, as an important parameter, was changed from 0% to 100%.Heating temperature and gas flow were also reference variables, which were respectively changed from 300 °C to 500 °C and from 0 L/min to 10 L/min. Behaviors of droplet microexplosion and ignition were recorded by high speed camera. Temporal variation of droplet size, time to microexplosion and ignition delay time were all analyzed for every cases. The results showed that microexplosion could be found in most cases due to high volatility distinction of biodiesel and ethanol and be expedited and intensified at fuel for nearly equivalent volume mixing, or at enough high temperature and in a certain gas flow. However, ignition was achieved only in some situations which biodiesel content in initial fuel and temperature was relatively high and gas flow rate was moderate. Further, under the condition that the temperature and flow rate remain unchanged, when the ethanol content reached 50%, the micro-explosion intensity which was named the normalized diameter reaches the maximum (1.3). At the same time, the delay time of micro-explosion is also reduced to the minimum (0.23 s). Under the condition that the ratio and flow rate remained the same, when the ambient temperature rose to 400 °C, the micro-explosion intensity reached the maximum. At this time, the normalized diameter of the mixed droplets approached 1.5. Under the condition that the temperature and ratio remained the same, when the flow rate was 5 L/min, the ignition delay time was the lowest (about 2.8 s), when the flow rate was 10 L/min, the micro-explosion intensity was the largest, and the normalized diameter reached 1.8. Beyond this, it was found that raising the ratio of ethanol in the blended fuel could increase the burning rate but lower the ignitability, and the ignition delay time could be shortened when the droplet exhibited microexplosion and fuel of near equi-volume blends, experiencing the most violent microexplosion, optimized the improvement, which could be found more obviously at high temperature and high speed airflow.     

Observations on the combustion of boron slurry droplets in air

Single fiber-supported slurry droplets composed of boron in JP-10 were ignited and burned in room-temperature air. Initial droplet diameters ranged from 1.2 to 3.0 mm and initial boron weight fractions f from 0 to 0.7. It was observed that although the liquid fuel apparently burns completely the boron does not ignite under these experimental conditions. For the pure liquid the combustion is smooth with a measured burning-rate constant of 0.43 mm²/s. At low f there is periodic swelling of the droplet with mildly disruptive emission of gas from the interior; the severity of this irregularity is greatest for f ≈ 0.1 and negligible for f ≳ 0.2. For f ≲ 0.4 a reduction in droplet diameter, according to a d² law, is observed for a period of time, followed by a burning period of essentially constant diameter. For f ≳ 0.5, the droplet diameter remains practically constant during combustion, although the measured burning time conforms to a d² law. These observations are compared quantitatively with theoretical predictions and are found to agree within accuracies ranging from 10% to 25%.     

Combustion characteristics of fuel droplets with addition of nano and micron-sized aluminum particles

The burning characteristics of fuel droplets containing nano and micron-sized aluminum particles were investigated. Particle size, surfactant concentration, and the type of base fluid were varied. In general, nanosuspensions can last much longer than micron suspensions, and ethanol-based fuels were found to achieve much better suspension than n-decane-based fuels. Five distinctive stages (preheating and ignition, classical combustion, microexplosion, surfactant flame, and aluminum droplet flame) were identified for an n-decane/nano-Al droplet, while only the first three stages occurred for an n-decane/micron-Al droplet. For the same solid loading rate and surfactant concentration, the disruption and microexplosion behavior of the micron suspension occurred later with much stronger intensity. The intense droplet fragmentation was accompanied by shell rupture, which caused a massive explosion of particles, and most of them were burned during this event. On the contrary, for the nanosuspension, combustion of the large agglomerate at the later stage requires a longer time and is less complete because of formation of an oxide shell on the surface. This difference is mainly due to the different structure and characteristics of particle agglomerates formed during the early stage, which is a spherical, porous, and more-uniformly distributed aggregate for the nanosuspension, but it is a densely packed and impermeable shell for the micron suspension. A theoretical analysis was then conducted to understand the effect of particle size on particle collision mechanism and aggregation rate. The results show that for nanosuspensions, particle collision and aggregation are dominated by the random Brownian motion. For micron suspensions, however, they are dominated by fluid motion such as droplet surface regression, droplet expansion resulting from bubble formation, and internal circulation. And the Brownian motion is the least important. This theoretical analysis explains the different characteristics of the particle agglomerates, which are responsible for the different microexplosion behaviors that were observed in the experiments.     

Hydrogen generation via the reaction of an activated aluminum slurry with water

Presented here is the formulation and characterization of a stable aluminum slurry fuel that reacts readily and exothermically with liquid water to produce hydrogen gas and AlOOH (boehmite). Bulk pure aluminum is first surface-treated with a gallium-indium eutectic, then ground into a powder and suspended in mineral oil containing 4–8 wt% fumed silica as a shear-thinning agent. Aluminum mass fractions of up to 65 wt% are shown here. This formulation results in a slurry fuel that can be pumped continuously at low power while remaining in suspension for over 2 months. The fuel is also shown here to exhibit a high degree of reaction completion (93.4%) with a total measured energy density of 28.7 MJ/L and specific energy of 17.5 MJ/kg. Finally, to show the feasibility of using this fuel for on-demand hydrogen production in a representative power system, a prototype continuous-flow reactor was developed, and validation experiments were performed, the results of which are presented here.     

生物柴油和乙醇混合燃料的微爆特性实验研究

为了探究乙醇和生物柴油混合燃料的液滴微爆特性,设计并建立了悬挂液滴燃烧的实验装置和实验系统,在管式加热炉内用高速摄影拍摄并记录液滴的变化过程,以此得到了液滴的直径变化和微爆延迟,实验结果表明燃料的组分变化对液滴的微爆表现和性质有显著的影响,在混合燃料中乙醇和生物柴油的含量接近相等时液滴的微爆表现最好。     

Evaporation characteristics of kerosene droplets with dilute concentrations of ligand-protected aluminum nanoparticles at elevated temperatures

The evaporation characteristics of kerosene droplets containing dilute concentrations (0.1%, 0.5%, and 1.0% by weight) of ligand-protected aluminum (Al) nanoparticles (NPs) suspended on silicon carbide fiber were studied experimentally at different ambient temperatures (400–800 °C) under normal gravity. The evaporation behavior of pure and stabilized kerosene droplets was also examined for comparison. The results show that at relatively low temperatures (400–600 °C), the evaporation behavior of suspended kerosene droplets containing dilute concentrations of Al NPs was similar to that of pure kerosene droplets and exhibited two-stage evaporation following the classical d²-law. However, at relatively high temperatures (700–800 °C), bubble formation and micro-explosions were observed, which were not detected in pure or stabilized kerosene droplets. For all Al NP suspensions, regardless of the concentration, the evaporation rate remained higher than that of pure and stabilized kerosene droplets in the range 400–800 °C. At relatively low temperatures, the evaporation rate increased slightly. However, at relatively high temperatures (700–800 °C), the melting of Al NPs led to substantial enhancement of evaporation. The maximum increase in the evaporation rate (56.7%) was observed for the 0.5% Al NP suspension at 800 °C.     

Boiling of small droplets

Analysis is reported of the boiling of small diameter suspended droplets, such as found in emulsions. A one-dimensional model considers both the energy and momentum equations, and effects of differences in fluid and thermodynamic properties between the droplet and the surrounding liquid are examined. Results are presented in the form of time varying bubble radius and temperature. The initial boiling of the droplet is insensitive to the surrounding liquid and droplet diameter, while the final evaporation rate is strongly affected by the properties of the surrounding liquid. After a droplet has completely evaporated, the vapor bubble expands and contracts via radial oscillations near the Minnaert frequency for isothermal bubbles. Thermal damping is observed but the model does not capture acoustic damping.     

水煤浆

水煤浆是 70年代末 80年代初开发出的煤基代油流体燃料 ,由约 70 %的煤粉、约 30 %的水及少量的化学添加剂经物理加工制备而成。它是煤在水中的悬浮混合物 ,具有像燃料油一样的流动性 ,可以长距离管道输送 ,可以在电站锅炉和窑炉中直接燃用 ,还可以代油或代煤造气。作为燃料 ,水煤浆具有低污染、易泵送、燃烧效率高等优点。我国是以煤碳为主要能源的国家 ,低污染、高效率利用煤碳资源是国民经济发展中的重大课题。水煤浆技术是洁净煤技术的重要分支 ,水煤浆技术的开发和应用 ,对国民经济建设和环境保护等方面具有重大意义。我国 80年代初开…     

Evaporation of multicomponent liquid fuel droplets

A theoretical investigation on evaporation of a two-component liquid fuel droplet in high-temperature quiescent gaseous surroundings has been made from the numerical solution of conservation equations of heat, mass and momentum transports in the carrier and droplet phases. Liquid fuel droplets containing (i) components of widely varying volatilities, namely, n-hexane and n-hexadecane and (ii) components of closely spaced volatilities, namely, n-hexane and benzene, have been considered for the studies. The evaporation characteristics, namely mass depletion, droplet temperature and droplet composition histories with time, have been evaluated in terms of the pertinent input parameters, namely the initial composition of the drop constituents and the free stream temperature. The present studies have been made on the basis of both (i) the interdiffusion (finite diffusion in the droplet phase) model, and (ii) the rapid mixing (infinite diffusion in the droplet phase) model. The results from both the models have been compared to ascertain the accuracy of the rapid mixing model.     

Surface tension of evaporating nanofluid droplets

Measurements of nanofluid surface tension were made using the pendant droplet method. Three different types of nanoparticles were used – laponite, silver and Fe₂O₃ – with de-ionized water (DW) as the base fluid. The reported results focus on the following categories: (1) because some nanoparticles require surfactants to form stable colloids, the individual effects of the surfactant and the particles were investigated; (2) due to evaporation of the pendant droplet, the particle concentration increases, affecting the apparent surface tension; (3) because of the evaporation process, a hysteresis was found where the evaporating droplet can only achieve lower values of surface tension than that of nanofluids at the same prepared concentrations; and (4) the Stefan equation relating the apparent surface tension and heat of evaporation was found to be inapplicable for nanofluids investigated. Comparisons with findings for sessile droplets are also discussed, pointing to additional effects of nanoparticles other than the non-equilibrium evaporation process.     

正丁醇水溶液液滴的蒸发特性实验研究

自湿润流体是一种具有特殊的表面张力特性的二元流体,了解其蒸发传热特性对于揭示其强化传热机理十分重要.为了探究添加自湿润流体液滴的蒸发特性,采用液滴形状分析仪(DSA100)研究了不同温度(30、40、50、60℃)下铜底板上去离子水、正丁醇水溶液(质量分数为0.5％)液滴的蒸发特性.结果 表明:加入少量正丁醇溶液并不影...     

Evaporation of acoustically levitated multi-component liquid droplets

A theoretical model for the evaporation of multi-component liquid droplets based on the model by Abramzon and Sirignano is presented and applied to the evaporation of acoustically levitated droplets. The liquid phase is treated as a thermodynamically real fluid, using the UNIFAC method for calculating the component activities, and the gas phase as ideal. Computational results, which consist in the droplet surface and volume, temperature and composition as functions of time, are verified by experiments carried out with single droplets evaporating in an acoustic levitator. The results are in excellent agreement, suggesting that the model correctly captures the physico-chemical phenomena in multi-component liquid droplet evaporation.     

Modelling of biofuel droplets dispersion and evaporation

Alternative renewable fuels are more and more important due to increasing oil prices, environmental concerns and their potential to preserve the agricultural activity. For the case of biofuels derived from agricultural crops, several possibilities can be considered, such as raw oil, oil-derived methyl esters, bioethanol or mixtures with conventional fuels (diesel fuel or gasoline). The straight use of vegetable oils and their derivates poses some problems on the mixture formation process and droplet evaporation, that are essentially derived from their much higher viscosity (up to five times the viscosity of the diesel fuel) and higher boiling temperature. The present paper presents a numerical study on evaporating biofuel droplets injected through a turbulent cross-stream. This study uses an Eulerian/Lagragian approach to account for turbulent transport, dispersion, evaporation and coupling between both processes in practical spray injection systems, which usually include air flows in the combustion chamber-like swirl, tumble and squish in I.C. engines or cross-flow in boilers and gas turbines. An array of evaporating biofuel droplets through a cross-flow is studied, and a comparison of the droplet fuel dispersion and evaporation with conventional fuels is performed. The results obtained with DF-2 and RME showed that an homogeneous mixture can only be obtained with very high levels of pre-heating, and the use of ethanol (obtained from sugar or starch crops) may be a better alternative in ignition-assisted combustion systems, while RME can be successfully used as an alternative fuel in applications that utilize diffusion flames.     

喷淋室荷电液滴捕集颗粒物的数值模拟研究

以带有荷电装置的空调喷淋室为研究对象,运用Fluent软件对其进口风速和喷嘴出口液滴表面荷电量对细颗粒物捕集效率的影响进行了数值模拟研究。结果表明:荷电液滴对颗粒物的捕集效率明显高于液滴未荷电的情况;进口风速越小,捕集效率越高;液滴表面荷电量越高,对颗粒物的静电引力越大,捕集效率越高;粒径越小的颗粒物受静电引力的作用越大,越容易被捕集。因此,采用带有荷电装置的空调喷淋室来处理空气,不仅可以实现空调房间温湿度控制,而且能够对空调系统的送风进行颗粒物去除,具有一定的实际应用意义。     

Promising components of waste-derived slurry fuels

The paper presents the results of experimental studies of energy (calorific value, ignition delay times and threshold ignition temperatures, duration and temperature of combustion) and environmental (CO₂, NO_x and SO_x emission) characteristics of fuel slurries based on pulverized wood (sawdust), agricultural (straw), and household (cardboard) waste. Wastewater from a sewage treatment plant served as a liquid medium for fuels. Petrochemical waste and heavy oil were additives to slurries. The focus is on obtaining the maximum efficiency ratio of slurry fuel, calculated taking into account environmental, cost, energy and fire safety parameters. All slurry fuels were compared with typical coal-water slurry for all the parameters studied. A comparison was also made between slurries and traditional boiler fuels (coal, fuel oil). The relative efficiency indicator for waste-based mixtures was varied in the range of 0.93–10.92. The lowest ignition costs can be expected when burning a mixture based on straw, cardboard and oil additive (ignition temperature is about 330 °C). The volumes of potential energy generated with the active involvement of industrial waste instead of traditional coal and oil combustion are forecasted. It is predicted that with the widespread use of waste-derived slurries, about 43% of coal and oil can be saved.     

Prediction of micro-explosion delay of emulsified fuel droplets

Burning a water-in-oil emulsion enables reduction in solid and gaseous pollutants in comparison with neat oil. In the emulsion, Heavy Fuel-Oil and water lie in distinct phases, having a high difference in boiling point (up to 200 K). In an emulsion droplet injected and subsequently heated inside a flame, the internal water droplets are enclosed inside the emulsion and do not systematically vaporise at boiling point. They are known to reach a metastable state, breaking up at a temperature below the spinodal limit of water. From this moment, the surrounding Fuel-Oil is fragmented into numerous faster and smaller droplets by the suddenly expanding steam. This physical phenomenon is called “micro-explosion”. This work demonstrates a numerical modelisation of unsteady heat and mass transfer at the surface and inside of the emulsion droplet, and provides a prediction of its micro-explosion delay, using homogeneous nucleation hypothesis. This assumption of homogeneous nucleation for internal water droplets matches the use of a “drop tower” experimental facility. Finally, comparisons between predicted ranges for micro-explosion delays and experimental delays from literature are discussed, along with combustion parameters (ambient temperature, relative velocity) and combustible emulsion parameters. As a result, the experimental and numerical micro-explosion delays decrease with liquid or ambient temperature and relative velocity, and increase with water content and radius of emulsion droplet. Their low average deviation reveals the accuracy of the assumption of homogeneous nucleation in the considered situations.     

水煤浆技术与应用

水煤浆是一种煤基流体燃料，是煤炭深加工的新型产品之一。它是由65％～70％的煤粉和29％～34％的水及1％左右的微量化学添加剂制备而成的浆体。水煤浆是新型洁净环保燃料，它具有良好的流动性，既可以长距离管道输送，又可以用汽车槽车、铁路罐车及船舶运输。它的雾化性能好，可以稳定着火燃烧；和煤相比，