《外文文献和翻译(24页).doc》由会员分享，可在线阅读，更多相关《外文文献和翻译(24页).doc（24页珍藏版）》请在taowenge.com淘文阁网|工程机械CAD图纸|机械工程制图|CAD装配图下载|SolidWorks_CaTia_CAD_UG_PROE_设计图分享下载上搜索。
1、-外文文献和翻译-第 22 页南京工业大学化学工程与工艺专业本科毕业论文(设计)外文资料翻译原文名称 Effects of temporally varying liquid-phase mass diffusivity in multicomponent droplet gasification 原文作者 Huiqiang Zhang, Chung K. Law 原文出版 Combustion an Flame 翻译内容页码 全文 中文名称 在多元液体气化中改变液相大规模扩散的 暂时性影响 学生姓名唐柯楠 专业 化学工程与工艺 班级学号040126指导教师（签字） 对译文的评价 技术学院20
2、08年 6 月Effects of temporally varying liquid-phase mass diffusivityin multicomponent droplet gasificationAbstractThe relative roles of liquid-phase diffusional resistance and volatility differential in multicomponent droplet gasification are revisited, recognizing that liquid-phase mass diffusivities
3、 can be substantially increased as the droplet is progressively heated upon initiation of gasification, leading to a corresponding substantial weakening of the diffusional resistance. Calculations performed using realistic and temperature-dependent thermal and mass diffusivities indeed substantiate
4、this influence. In particular, the calculated results agree with the literature experimental data, indicating that the gasification mechanism of multicomponent fuels is intermediate between diffusion and distillation limits. Investigation was also performed on gasification at elevated pressures, rec
5、ognizing that the liquid boiling point and hence the attainable droplet temperature would increase with increasing pressure, causing further weakening of the liquid-phase diffusional resistance. This possibility was again verified through calculated results, suggesting further departure from diffusi
6、on limit toward distillation limit behavior for gasification at high pressures. The study also found that diffusional resistance is stronger for the lighter, gasoline-like fuels as compared to the heavier, diesel-like fuels because the former have overall lower boiling points, lower attainable dropl
7、et temperatures, and hence lower mass diffusivities in spite of their lower molecular weights. 2008 The Combustion Institute. Published by Elsevier Inc. All rights reserved.Keywords: Multicomponent droplet; Liquid mass diffusivity; Distillation1. IntroductionIt is well established that the gasificat
8、ion mechanism of a multicomponent droplet is controlled by three competing factors, namely the volatility differentials, the liquid-phase mass diffusivity, and the droplet surface regression rate 17. Consequently, for slow surface regression relative to mass diffusion, as in the case of vaporization
9、 in a low-temperature environment, the droplet composition tends to be perpetually uniformized and the fractional gasification rates would be closely controlled by the volatility differentials between the constituents. This leads to a gasification mode that is largely independent of liquid-phase mas
10、s diffusion, and as such resembles that of batch distillation 8. This is the formulation adopted in early studies of multicomponent droplet gasification. On the other hand, in the limit of very fast gasification relative to mass diffusion, the composition of the droplet is effectively frozen, so tha
11、t the fractional gasification rates of the individual constituents are equal to their respective fractions in the original liquid composition. This leads to an onionskin mode of gasification, which is independent of the volatility differentials and as such is similar to the gasification of a solid.S
12、ince mass diffusion does occur in the liquid, even for situations of slow diffusion and rapid gasification, the gasification mechanism that has emerged for such mass-diffusion-limited gasification is one that consists of three periods 3,4. Specifically, after initiation of gasification, most of the
13、volatile components in the surface layer are preferentially gasified, leaving this layer with a higher concentration of the less volatile, higher-boiling-point components. The droplet temperature subsequently increases, being largely dependent on the boiling points of the less volatile components. A
14、fter the concentration layer is established, the supply of species from the droplet interior to the surface is controlled by the slow diffusion and droplet surface regression, resulting in a prolonged period of steady-state gasification, with the diffusion rate balancing the surface regression rate.
15、 Finally, toward the end of the droplet lifetime, mass diffusion becomes efficient again, resulting in a brief period during which the more volatile components are rapidly depleted from the droplet interior, with the concomitant increase of the droplet temperature to approach the effective boiling p
16、oint of the less volatile components. Thus the characteristic of mass-diffusion limited gasification is the attainment of a fairly steady state of gasification resembling the onionskin mode, except for the presence of the thin transition concentration layer at the surface.Experiments were conducted
17、9 in which freely falling bicomponent droplets undergoing either pure vaporization or burning were abstracted at various stages of their lifetime, and the spatially averaged composition was subsequently analyzed. Results show that, except for mixtures whose volatility differential is minimal, the av
18、erage molar fraction of the volatile component steadily decreases quite substantially, implying that a steady mode of gasification is not attained. Furthermore, since the gasification sequence also does not appear to conform to that of batch distillation, the experimental results seem to indicate mi
19、xed-mode behavior.The primary objective of the present study is to gain further understanding, particularly quantitatively, on the gasification mechanism of multicomponent droplets, with the following focuses. First, recognizing that diffusion-limited gasification is favored for rapid gasification r
20、ates and components with large volatility differentials, while distillationlike gasification is favored otherwise, we shall extend previous studies 4,7 to systematically demonstrate these distinguishing influences.Second, it is also recognized that liquid-phase mass diffusivity is a sensitive functi
21、on of temperature. Consequently it is reasonable to expect that the diffusivity could become progressively larger as the droplet is heated up upon the initiation of gasification, hence weakening the diffusional resistance responsible for the quasi-steady behavior. The extent of this sensitivity need
22、s to be assessed.Third, as a corollary to the temperature sensitivity, and recognizing that the droplet temperature would increase with ambient pressure because of the corresponding increase of the liquid boiling point, it is also of interest to assess the extent to which diffusional resistance is f
23、urther weakened as the ambient pressure is increased. This issue is of practical relevance because most internal combustion engines operate at elevated pressures.Fourth, we shall study mixtures that are representative of both diesel and gasoline fuels, noting that while the former have smaller diffu
24、sivities because of their higher molecular weights, the diffusivities can be enhanced to a greater extent because the droplet can attain higher temperatures on account of the higher boiling points of these fuels. In contrast, gasoline fuels have low molecular weights but also lower boiling points. I
25、t is therefore not clear a priori what are the relative gasification modes of diesel vs gasoline fuels.The structure of the paper is as follows. Since a satisfactory resolution of the above questions would require quantitatively realistic assessments, particularly in light of the sensitivity of the
26、liquid-phase mass diffusivity with temperature and the mixture constituents, we shall first extend, in Section 2, the constant (liquid-phase) property model 4 to variable properties. In Section 3 we shall study the response of bicomponent, diesel-like mixtures with a large volatility differential un
27、dergoing vaporization at moderate temperatures. The investigation, however, will still be performed in the context of constant liquidphase diffusivities in order to clearly bring out some of the underlying physics. In Section 4 we validate our variable property formulation by comparing the calculate
28、d results with literature experimental data, and subsequently we present, in Section 5, results and understanding gained on the various aspects of the gasification response discussed earlier.2. Variable property formulationThe problem of interest is the spherically symmetric gasification of a drople
29、t, initially of radiusand temperatureand consisting ofconstituents having similar liquid densities and characterized by their respective diffusive and thermodynamic properties. At timethis droplet is introduced, and ignited in the case of combustion, in a stagnant, unbounded atmosphere. The atmosphe
30、re is characterized by its temperatureits pressureand the concentrations of itsspecies consisting of thegasifying species, the oxidizer gasand a noncomdensible inert species such as nitrogen. In addition to the usual droplet combustion responses such as the droplet burning rate, the flame temperatur
31、e, and the flame radius, we are also interested in determining the temporal variations of the temperature and composition profiles within the droplet, and the fractional gasification rates of the individual components. The detailed formulation, including the numerical scheme for the solutions and it
32、s accuracy, is given in 4. Briefly, the formulation consists of the quasi-steadydescription of the gas-phase transport, with or without flame-sheet burning, which is supported by the release of thefuel species with different gasification rates. The gas-phase solution is coupled to an unsteady analys
33、is of the heat and mass transport processes within the quiescent droplet interior that is bounded by the regressing droplet surface, being governed bysubject to the initial and boundary conditionswhereis the droplet temperature, the liquid mass fraction of speciesthe radial coordinate, andthe drople
34、t radius. Furthermore, andare the nondimensional expressions for the total and fractional mass vaporization rates, respectively, andare the nondimensional heat transfer to the droplet from the gas and the latent heat of vaporization, respectively, andandwhereis the thermal conductivity, the specific
35、 heat, and the subscriptsandrespectively designate the gas and liquid phases. Expressions for some of these parameters are given in 4. It is also noted that the use of Ficks law of diffusion for the mass fractionsinstead of the molar fraction, holds rigorously for a bicomponent mixture, which is the
36、 case studied in the rest of this paper, and approximately for components with similar molecular weights.The above equations can be readily solved numerically, given the gas-phase conditions or solutions. The gas-phase properties are treated as constants while the liquid-phase properties are treated
37、 as variables. This is a reasonable approximation because the transient nature of the present problem is driven by the corresponding transient variation of the liquid-phase diffusivity. The gas-phase properties, while spatially varying, are not expected to be temporally varying to any great extent.I
38、t is also noted that this is a moving boundary problem because of the regressing droplet surface. In particular, although Eqs. (1) and (2) are the standard heat diffusion equations consisting of the transient and diffusion terms, the regressing surface imparts a convective influence to the transport
39、 processes within the droplet.The constituents of the mixtures studied are alkanes. In Appendix A we list the relations used in the evaluation of the various liquid-phase properties of these constituents and their mixtures.3. Constant-property resultsIt is useful to first specialize the variable liq
40、uidphase property formulation to that of constant properties in order to investigate the roles of volatility differential and surface regression rate in the gasification process. Two cases are considered: an equimolar hexadecanetetradecane droplet burning in 1300-K, 1-atm air, and an equimolar hexad
41、ecane decane droplet undergoing vaporization in 1020-K, 1-atm air. The former tends to promote quasi-steady diffusion-limited gasification behavior because of the small volatility differential and the high surface regression rate, while this tendency is weakened for the latter as the volatility diff
42、erential is widened and the surface gasification rate decreases.The effects of liquid-phase diffusional resistance are represented by a constant liquid-phase Lewis number, Le, defined as the ratio of a thermal diffusivity to a mass diffusivity. Various Lewis numbers are used to simulate the influenc
43、e of liquid-phase diffusional resistance: the larger the Le, the stronger the resistance. Since liquid-phase mass diffusivity is usually much smaller than thermal diffusivity, Le is a large number and we have adopted the value of 30, used in 4, to investigate the effects of strong diffusional resist
44、ance. In addition, we have also used the values of 5, 1, and 0.1 to characterize situations of weakened diffusional resistance. The cases Le =1 and 0.1 are both artificial, simulating the batch distillation mode of gasification, with the latter exaggerating the influence of mass diffusion.Fig. 1 sho
45、ws the surface and center values of the molar fraction of the more volatile component, tetradecane, in the hexadecanetetradecane droplet. The time used here is a nondimensionaltime, which is a normalized physical time when theholds rigorously. Results for the Le =30 case demonstrate that the gasific
46、ation process basically follows diffusion-limited behavior in the development of a surface concentration boundary layer that persists until almost the end of the droplet lifetime, as shown previously 4. On the other hand, for Le =1 ，the facilitated diffusion renders the droplet composition fairly un
47、iform throughout the droplet lifetime, with the volatile component steadily decreasing. This steady, instead of fairly rapid, reduction of the more volatile component is due to the small volatility differential between the two components. Thus the less volatile component, hexadecane, is gasified fai
48、rly efficiently even in the batch distillation limit.Fig. 2 shows the corresponding plot for the hexadecane decane droplet undergoing vaporization. It is seen that, for the Le =30 case, the strength of the diffusional resistance is mostly maintained, except that the diffusion wave does reach the droplet center earlier, hence slightly changing the composition there. The larger volatility differential also leads to a smaller volatile concentration at the surface, as compared to the tetradecane concentration in Fig. 1. For Le =1 and 0.1, the larger volatility differentia