摘要:

AbstractMany commercially important pigments such as titanium dioxide and carbonblack are produced as fumes from a combustion process. The fuming process generates open structured agglomerates conforming to a morphology predicated by physical considerations. For this reason widely different chemical pigments often manifest common physical structure. This structure can be characterized using the concepts of fractal geometry. In this communication the potential usefulness and physical significance of descriptive parameters of the agglomerate structure of fumed pigments based on the concepts of fractal geometry are explored. Similar structures present in high energy ceramic powders are discussed and the potential importance of fractal dimension parameters for the prediction of the physical behaviour of powders is outlined.

ISSN:0934-0866    

DOI:10.1002/ppsc.19910080112

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractA combined experimental and theoretical study of the droplet deposition and mist supercooling in a turbulent channel flow has been performed. The measurement of droplet deposition on the unheated and heated channel wall was carried out using a particle‐sizing two‐dimensional reference‐mode laser‐Doppler anemomentry technique in a 41 mm × 41 mm vertical square channel atRe= 1.0 × 104− 6.05 × 104. The dimensionless deposition coefficientkd/uZ. H. Yang Prof. S. L. Lee Department of Mechanical Engineering, State University of New York at Stony Brook, Stony Brook, NY 11794 (USA)is analyzed theoretically based on a new unified theoretical approach for the entire transverse flow region from the turbulent core to the quasi‐laminar region next to the boundary wall. The proposed analytical model shows satisfactory agreement with the results of present experimental measurement. In the mist supercooling, the heated surface is cooled by the evaporation on the outside surface of an extra thin continuous liquid film which is maintained by the continuous deposition of extremely small droplets of an optimal size determined by a selection process on the droplets transverse migration due to the dynamic interaction between the phases in a parallel turbulent mist flow. The heat transfer enhancement coefficient depends on droplet deposition rate, droplet size, concentration and the R

ISSN:0934-0866    

DOI:10.1002/ppsc.19910080113

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractThe evaporation rates of particles of solid KBr or NH4Cl suspended in argon were studied under high‐temperature conditions behind incident shock waves. The mass‐transfer process during particle evaporation was observed by two optical techniques allowing the measurement of scattered light from a particle ensemble and also from individual particles. The scattered light flux signals were interpreted on the basis of the Mie theory, resulting in values for the time‐dependent particle size and for the refractive index of particle materials. The experiments were performed in the gas‐phase temperature rangesTG= 1070–1300K (KBr) and 525–650K (NH4Cl). The initial size of the suspended particles ranged between 0.6 and 1.0 μm. From the measured decrease in the particle size during the mass‐transfer process the evaporation coefficients of both materials were determined as a function of the gas‐phase temperature. They depend on the vapour pressure and the diffusion coefficient for vapour into gas. Typical parameters (àKBr,m) describing the diffusion coefficient of KBr vapour in argon and the vapour pressure of NH4Cl co

ISSN:0934-0866    

DOI:10.1002/ppsc.19910080114

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractA dispersed system is considered which will form its condensed phase at high temperatures and is characterized by a high density of very small supercritical nuclei. It is assumed that particle growth is determined by coagulation and (viscous flow) coalescence. The morphology of the final particles depends on the temperature history of the formation process. Compact spherical particles and agglomerates may be formed. Agglomerates result when the process temperature decreases to an extent that coalescence is quenched in the course of the growth process. The average size of the primary particles in the agglomerates is calculated by using the analysis and the approximations described in a previous paper. The growth kinetics of the primary particles are presented in a dimensionless form. The results show that the dimensionless size of the primary particles depends only on the ratio of two characteristic lengths of the system. For small values of this ratio, the size of the primary particles composing the agglomerates, although much greater than the initially formed nuclei, is independent of the volume fraction of the condensed aerosol. For large values of this parameter, the theory merges into the classical theory of perfectly coalescing spheres.

ISSN:0934-0866    

DOI:10.1002/ppsc.19910080115

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractSome parameters which affect the explosibility of dusts have been investigated. Important parameters are particle size, dust concentration, pore size and chemical composition. The dust explosion index measurements illustrate the importance of pore size and particle size. The reaction mechanism occurring is also important. Particles which burn at the solid surface and form gaseous products tend to react completely whereas other materials show a limited transport of the reactants.

ISSN:0934-0866    

DOI:10.1002/ppsc.19910080116

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractA statistical analysis is made for the extinction of radiations in suspensions, whereby the whole range of possible particle concentrations is covered. The extinction signals are characterized by their average value and by a characteristic length of fluctuation. For simplicity, the considerations are restricted to geometric ray propagation in dispersions of perfectly absorbent, monosized spherical particles. An extinction equation is derived, which reduces to Lambert‐Beer's law in the limit of low particle concentrations. For higher concentrations, significant deviations from Lamberg‐Beer's law due to steric particle‐particle interactions are predicted. The characteristic length of the fluctuations is a function of particle diameter and transmission and has a broad minimum with medium transmissions, allowing for maximum resolution with measurements in this

ISSN:0934-0866    

DOI:10.1002/ppsc.19910080117

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

ISSN:0934-0866    

DOI:10.1002/ppsc.19910080118

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractSome results are presented of investigations on the flow behaviour of very fine α‐alumina powders having different particle size distributions. Shear tests were performed in a translation shear cell. The humidity was in the range 0.3

ISSN:0934-0866    

DOI:10.1002/ppsc.19910080119

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractThe operational characteristics of cake‐forming filter apparatus are strongly influenced by the structure of the dust cake formed. The quantitative evaluation of sectional images of loosely bound filter cake deposits is introduced. First the techniques with which the dust cake is prepared and with which scanning electron micrographs may be obtained are described, together with the subsequent digitization by video and computer equipment. This is followed by a comprehensive description of the evaluation of the digitized images, with special emphasis on the determination of the deposit's porosity and the computation of the particle and pore‐size distributions at selected locations. Finally, results derived from a true filter cake are presen

ISSN:0934-0866    

DOI:10.1002/ppsc.19910080120

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY

摘要:

AbstractAn image shape analysis technique which measured the angles between adjacent vectors on the image edge was previously proposed byPiper, but the accompanying data was scant. This method is shown to have great promise when applied to several images obtained using a video camera with a computer. Better data representation is obtained using a single plot of the average measured angle at each step length versus the step length. Both scale and degree of maximum roughness are immediately evident on this plot. The measured angles may be corrected to remove image circularity effects from the angles, in which case some macroscopic shape effect is also described by the plot. Similarity with the Richardson (fractal dimension) plot is discussed.

ISSN:0934-0866    

DOI:10.1002/ppsc.19910080121

出版商:WILEY‐VCH Verlag GmbH    

年代:1991

数据来源: WILEY