共查询到20条相似文献,搜索用时 125 毫秒
1.
Pavan B. Govindaraju Thomas Jaravel Matthias Ihme 《Proceedings of the Combustion Institute》2019,37(3):3295-3302
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. 相似文献
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This paper presents a numerical model of micro-explosion for multicomponent droplets. The first part of the model addresses the mass and temperature distribution inside the droplet and the bubble growth within the droplet. The bubble generation is described by a homogeneous nucleation theory, and the subsequent bubble growth leads to the final explosion (i.e., breakup). The second part of the model determines when and how the breakup process proceeds. Unlike ad hoc/empirical approaches reported in the literature, the size and velocity of sibling droplets were determined by a linear instability analysis. After validated against available experimental data for bubble growth and homogeneous nucleation, the developed model was first used to study the effects of various parameters on the onset of micro-explosion. It was found that, optimum composition and high ambient pressure favor micro-explosion; however, extremely high pressures suppress micro-explosion because the volatility differential decreases. The vaporization behavior of an oxygenate diesel blend was analyzed at the end. It was found that micro-explosion is possible under typical diesel engine environments for this type of fuel. Occurrence of micro-explosion shortens the droplet lifetime, and this effect is stronger for droplets with larger sizes or a near 50/50 composition. 相似文献
4.
D. O. Glushkov A. V. Zakharevich P. A. Strizhak S. V. Syrodoy 《Thermophysics and Aeromechanics》2016,23(6):887-898
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. 相似文献
5.
The results of study of evaporation of water droplets and NaCl salt solution from a solid substrate made of anodized aluminum are presented in this paper. The experiment provides the parameters describing the droplet profile: contact spot diameter, contact angle, and droplet height. The specific rate of evaporation was calculated from the experimental data. The water droplets or brine droplets with concentration up to 9.1 % demonstrate evaporation with the pinning mode for the contact line. When the salt concentration in the brine is taken up to 16.7 %, the droplet spreading mode was observed. Two stages of droplet evaporation are distinguished as a function of phase transition rate. 相似文献
6.
Laser-induced fluorescence (LIF)-based spray volume and droplet-size measurements rely on assumptions about the evaporation or accumulation of fluorescent tracers during the evaporation of the droplets. We investigate the time-dependent variation of droplet-size and LIF signal intensity of CO2-laser-heated evaporating water droplets doped with rhodamine 6G. After an initial decrease of fluorescence intensity by 30% due to temperature-dependent diffusion of oxygen into the droplets, the LIF signal remains constant, indicating that the tracers have fully accumulated in the droplet. This evaporation-independent signal can be used as a reference for Mie-scattering-based droplet surface-area measurements that will allow the sensitive observation of spray evaporation and droplet breakup. PACS 42.62.Fi; 32.50.+d; 42.62.Cf 相似文献
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喷雾蒸发燃烧的研究对指导发动机燃烧系统设计具有重要意义。本文搭建了高速数字全息系统,在线测量乙醇喷雾火焰中液滴的粒径、三维位置、速度及蒸发率。对喷雾火焰中的液滴进行了统计分析,得到液滴粒径及三维空间分布。燃烧喷雾场液滴的平均粒径为68μm;非燃烧火焰测试区液滴数量多且较密集,燃烧火焰测试区液滴数量少且稀疏.追踪单液滴并处理得到湍流火焰中液滴的运动轨迹及速度。通过研究粒径的平方D2随停留时间ts的变化,测得液滴平均蒸发率为-3.343×10-7 m2/s. 相似文献
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The recently reported, experimentally observed, unusual behaviour of organic gellant-based fuel droplets which, under appropriate ambient thermal conditions, evaporate and burn in an oscillatory fashion is incorporated in a phenomenological manner in a model of a two-dimensional arbitrary multi-size spray diffusion flame. Non-unity Lewis numbers are permitted for the fuel vapour and oxidant. A combined analytical/numerical solution of the governing equations is presented and used to investigate how a spray's initial polydispersity and the frequency of oscillatory evaporation influence the combustion field. It is demonstrated that the initial droplet size distribution and the frequency of evaporation of the burning gel droplets can have an acute impact both on the homogeneous diffusion flame shape, height and width and on the thermal field downstream of the flame front. Hot spots of individual (or clusters of) burning droplets can be created and under certain operating conditions can lead to hotter temperatures than experienced in the main homogeneous flame. The intensity of these hotspots, their number and location are sensitive to spray related parameters. In realistic combustion chambers there is a danger inherent in the existence of hotspots in undesirable regions as they can damage the structural integrity. Other computed results demonstrate that, in relation to the spray diffusion flames obtained using an equivalent purely liquid fuel spray, the use of a gel fuel spray can lead, under certain operating conditions, to a reduction in flame height and temperature. The latter effect is critical when considering flame extinction. 相似文献
9.
Heat and Mass Transfer Investigation of Hydrocarbon Droplet Evaporation under Rotatory Movement
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J. Dgheim B. Zeghmati 《中国物理快报》2005,22(11):2933-2935
The steady-state boundary layer equations of a rotatory movement around hydrocarbon droplets saturated of pure fuel are numerically solved. The transfer equations are schemed by using an implicit finite difference method. The system of algebraic equations is solved by using the Thomas algorithm. The model is compared to the Kreith one and a good agreement is observed between both models. The dimensionless physical parameters of the evaporation phenomena of droplet in rotation are calculated and presented under the effect of the convection. 相似文献
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In many applications, the shape of a droplet may be assumed to be an oblate spheroid. A theoretical study is conducted on the evaporation of an oblate spheroidal droplet under forced convection conditions. Closed-form analytical expressions of the mass evaporation rate for an oblate spheroid are derived, in the regime of controlled mass-transfer and heat-transfer, respectively. The variation of droplet size during the evaporation process is presented in the regime of shrinking dynamic model. Comparing with the droplets having the same surface area, an increase in the aspect ratio enhances the mass evaporation rate and prolongs the burnout time. 相似文献
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During a colloidal droplet evaporation, a sol–gel transition can be observed and is described by the desiccation time τD and the gelation time τG. These characteristic times, which can be linked to viscoelastic properties of the droplet and to its composition, are classically rated by analysis of mass droplet evolution during evaporation. Even if monitoring mass evolution versus time seems straightforward, this approach is very sensitive to environmental conditions (vibrations, air flow…) as mass has to be evaluated very accurately using ultra-sensitive weighing scales. In this study we investigated the potentialities of ultrasonic shear reflectometry to assess τD and τG in a simple and reliable manner. In order to validate this approach, our study has focused on blood droplets evaporation on which a great deal of work has recently been published. Desiccation and gelation times measured with shear ultrasonic reflectometry have been perfectly correlated to values obtained from mass versus time analysis. This ultrasonic method which is not very sensitive to environmental perturbations is therefore very interesting to monitor the drying of blood droplets in a simple manner and is more generally suitable for complex fluid droplets evaporation investigation. 相似文献
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Time-dependent temperatures and compositions within individual fiber-supported droplets initially from about 2–3 mm in diameter were investigated. In the experiments, droplets were composed of mixtures of 1-propanol and acetone. The droplets evaporated in room air, where the air was heated by placing an electrically heated coil underneath the droplets. The experiments employed thin optical fibers to carry light from a UV–vis light source into and out of a droplet. The time-dependent UV absorption spectrum of the liquid between the fiber ends was measured using a spectrometer coupled to one of the fibers. This spectrum yielded real-time information on the composition of the liquid. Droplet temperatures were simultaneously measured using a thermocouple that was immersed into the liquid. Results demonstrate that droplet evaporation follows a multi-stage process and that acetone is preferentially gasified from a droplet. 相似文献
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Yoshiyuki Suzuki Takuji Harada Hirotatsu Watanabe Masakazu Shoji Yohsuke Matsushita Hideyuki Aoki Takatoshi Miura 《Proceedings of the Combustion Institute》2011,33(2):2063-2070
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. 相似文献
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S. Semenov V. M. Starov M. G. Velarde R. G. Rubio 《The European physical journal. Special topics》2011,197(1):265-278
The evaporation of single droplets and sprays into gaseous atmosphere and the evaporation of sessile liquid droplets on solid
substrates are here considered. We argue that if thermodynamics is augmented with Derjaguin’s (disjoining/conjoining) pressure
to handle phenomena in a vicinity of the three-phase contact line, problems like the singularity of the evaporation flux and
of the viscous stress at the three-phase contact line of a sessile droplet are ruled out. 相似文献
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《Combustion Theory and Modelling》2013,17(1):163-174
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. 相似文献
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D. V. Antonov G. V. Kuznetsov P. A. Strizhak 《Journal of Engineering Thermophysics》2016,25(3):337-351
Characteristic temperatures and concentrations of a vapor–gas mixture in a wake of water droplets moving through combustion products (initial temperature 1170 K) were determined using the Ansys Fluent mathematical modeling package. We investigated two variants of motion: motion of two droplets (with sizes from 1 mm to 3 mm), consecutive and parallel, and motion of five staggered droplets. The influence of the relative position of droplets and also of distances between them (varied from 0.01 mm to 5 mm) on temperatures and concentrations of water vapor was established. The distances determine the relation between the evaporation areas and the total volume occupied by a droplet aggregate in the gas medium. The results of modeling for conditions that take into account vaporization on the droplet surface at average constant values of evaporation rate and also with consideration of the change in the latter, depending on the droplet temperature field, are compared. We determined conditions under which the modeling results are comparable for the assumption of a constant vaporization rate and with regard to the dependence of the latter on temperature. The earlier hypothesis on formation of a buffer vapor layer (“thermal protection”) around a droplet, which decreases the thermal flow from the external gas medium, was validated. 相似文献
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Vanteru M. Reddy Mustafa M. Rahman Appala N. Gandi Ayman M. Elbaz Robert A. Schrecengost 《Combustion Theory and Modelling》2016,20(1):154-172
Heavy fuel oil (HFO) as a fuel in industrial and power generation plants ensures the availability of energy at economy. Coke and cenosphere emissions from HFO combustion need to be controlled by particulate control equipment such as electrostatic precipitators, and collection effectiveness is impacted by the properties of these particulates. The cenosphere formation is a function of HFO composition, which varies depending on the source of the HFO. Numerical modelling of the cenosphere formation mechanism presented in this paper is an economical method of characterising cenosphere formation potential for HFO in comparison to experimental analysis of individual HFO samples, leading to better control and collection. In the present work, a novel numerical model is developed for understanding the global cenosphere formation mechanism. The critical diameter of the cenosphere is modelled based on the balance between two pressures developed in an HFO droplet. First is the pressure (Prpf) developed at the interface of the liquid surface and the inner surface of the accumulated coke due to the flow restriction of volatile components from the interior of the droplet. Second is the pressure due to the outer shell strength (PrC) gained from van der Walls energy of the coke layers and surface energy. In this present study it is considered that when PrC ≥ Prpf the outer shell starts to harden. The internal motion in the shell layer ceases and the outer diameter (DSOut) of the shell is then fixed. The entire process of cenosphere formation in this study is analysed in three phases: regression, shell formation and hardening, and post shell hardening. Variations in pressures during shell formation are analysed. Shell (cenosphere) dimensions are evaluated at the completion of droplet evaporation. The rate of fuel evaporation, rate of coke formation and coke accumulation are analysed. The model predicts shell outer diameters of 650, 860 and 1040 µm, and inner diameters are 360, 410 and 430 µm respectively, for 700, 900 and 1100 µm HFO droplets. The present numerical model is validated with experimental results available from the literature. Total variation between computational and experimental results is in the range of 3–7%. 相似文献
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The complex nature of multicomponent aviation fuels presents a daunting task for accurately simulating combustion behavior without incurring impractical computational costs. To reduce computation time, chemical fuel surrogates comprised of only a few species are used to emulate the combustion of complex pre-vaporized fuels. These surrogates are often unable to match the vaporization behavior and physical properties of the real fuel and fail to capture the effect of preferential vaporization on combustion behavior. Therefore, a computationally efficient, hybrid droplet vaporization-chemical surrogate approach has been developed which emulates both the physical and chemical properties of a multicomponent kerosene fuel. The droplet vaporization/physical portion of the hybrid uses the Coupled Algebraic–Direct Quadrature Method of Moments with delumping to accurately solve for the evolution of every discrete species in a vaporizing fuel droplet with the computational efficiency of a continuous thermodynamic model. The chemical surrogate portion of the hybrid is linked to the vaporization model using a functional group matching method, which creates an instantaneous surrogate composition to match the distribution of chemical functional groups (CH2, (CH2)n, CH3 and Benzyl-type) in the vaporization flux of the full fuel. The result is a hybrid method which can accurately and efficiently predict time-dependent, distillation-resolved combustion property targets of the vaporizing fuel and can be used to investigate the effects of preferential vaporization on combustion behavior. 相似文献
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