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A theoretical and experimental study of phenomena related to dense sprays is described. Two aspects of dense sprays are being considered: effects of turbulence modulation, which is the direct effect of particle (drop) motion on the turbulence properties of multiphase flows; and the structure and mixing properties of the dense-spray region of pressure atomized sprays. Turbulence modulation is being studied by considering spherical monodisperse glass particles falling in a stagnant water bath, where effects of turbulence modulation are responsible for the entire turbulence field. Measurements involve phase velocities and temporal and spatial correlations and spectra of the continuous phase velocities using a two-point phase-discriminating laser Doppler anemometer. Flow properties are being analyzed using stochastic methods: assuming linear superposition of randomly arriving particle wakes (Poisson statistics) for liquid phase properties; and random-walk calculations based on statistical time-series methods for particle properties. Multiphase flow, Sprays, Particle-laden flow. (jes).
A theoretical and experimental study of aspects of dense sprays is described, considering: the structure and mixing properties of the near-injector, dense-spray region of pressure-atomized sprays; and the direct effect of particle (drop) motion on the turbulence properties of multiphase flows (which is often called turbulence modulation). The structure of dense sprays was studied using large-scale (9.5 and 19.1 mm jet exit diameters) water sprays in still air. Measurements included: flow visualization using flash photography; liquid volume fractions using gamma-ray absorption; streamwise mean and fluctuating velocities at the injector exit, and entrainment velocities, using laser velocimetry; and dispersed-phase properties using single- and double-flash holography. Predictions based on the locally-homogeneous flow (LHF) approximation of multiphase flow theory were evaluated using the measurements. Measurements showed that mixing was strongly influenced by the degree of flow development at the injector exit and the breakup regime: fully-developed flow and atomization breakup yielded the fastest mixing rates. Turbulence modulation was studied by considering nearly monodisperse spherical glass particles falling in a stagnant water bath, where effects of turbulence modulation were responsible for the entire turbulence field. Measurements included phase velocities, as well as the temporal and spatial correlations of continuous-phase velocities, using a two-point phase-discriminating laser velocimeter; and calibration of particle motion properties using motion-picture shadowgraphs. (JHD).
This report describes one aspect of an investigation of dense-spray processes: namely turbulence/dispersed-phase interactions. The work was divided into two phases: (1) measurements of particle-laden jets injected into a still liquid; and homogenous particle flows, consisting of particles falling in a still (in the mean) liquid bath. The structure of turbulent, dilute, particle-laden water jets, submerged in still water, was studied both experimentally and theoretically. Nonintrusive measurements were made of mean and fluctuating phase velocities and particle number fluxes. Analysis was used to help interpret the measurements, considering three limiting cases, as follows: (1) locally-homogenous flow, where relative velocities between the phases are ignored; (2) deterministic separated flow, where relative velocities are considered, but particle/turbulence interactions are ignored; and (3) stochastic separated flow, where both phenomena are considered using random-walk methods. The locally-homogenous flow approximation was more effective than for past work involving larger density ratios between the phases; nevertheless, stochastic analysis yielded best agreement with measurements. Effects of enhanced drag (due to high relative turbulent intensities of particle motion) and effects of particles on liquid turbulence properties (turbulence modulation), were observed. Several recent proposals for treating these phenomena were examined; however, none appears to be adequate for reliable general use.
Turbulent reactive flows are of common occurrance in combustion engineering, chemical reactor technology and various types of engines producing power and thrust utilizing chemical and nuclear fuels. Pollutant formation and dispersion in the atmospheric environment and in rivers, lakes and ocean also involve interactions between turbulence, chemical reactivity and heat and mass transfer processes. Considerable advances have occurred over the past twenty years in the understanding, analysis, measurement, prediction and control of turbulent reactive flows. Two main contributors to such advances are improvements in instrumentation and spectacular growth in computation: hardware, sciences and skills and data processing software, each leading to developments in others. Turbulence presents several features that are situation-specific. Both for that reason and a number of others, it is yet difficult to visualize a so-called solution of the turbulence problem or even a generalized approach to the problem. It appears that recognition of patterns and structures in turbulent flow and their study based on considerations of stability, interactions, chaos and fractal character may be opening up an avenue of research that may be leading to a generalized approach to classification and analysis and, possibly, prediction of specific processes in the flowfield. Predictions for engineering use, on the other hand, can be foreseen for sometime to come to depend upon modeling of selected features of turbulence at various levels of sophistication dictated by perceived need and available capability.
Controlling turbulence is an important issue for a number of technological applications. Several methods to modulate turbulence are currently being investigated. This book describes various aspects of turbulence structure and modulation, and explains and discusses the most promising techniques in detail.
This book reflects the outcome of the 1st International Workshop on Turbulent Spray Combustion held in 2009 in Corsica (France). The focus is on reporting the progress of experimental and numerical techniques in two-phase flows, with emphasis on spray combustion. The motivation for studies in this area is that knowledge of the dominant phenomena and their interactions in such flow systems is essential for the development of predictive models and their use in combustor and gas turbine design. This necessitates the development of accurate experimental methods and numerical modelling techniques. The workshop aimed at providing an opportunity for experts and young researchers to present the state-of-the-art, discuss new developments or techniques and exchange ideas in the areas of experimentations, modelling and simulation of reactive multiphase flows. The first two papers reflect the contents of the invited lectures, given by experts in the field of turbulent spray combustion. The first concerns computational issues, while the second deals with experiments. These lectures initiated very interesting and interactive discussions among the researchers, further pursued in contributed poster presentations. Contributions 3 and 4 focus on some aspects of the impact of the interaction between fuel evaporation and combustion on spray combustion in the context of gas turbines, while the final article deals with the interaction between evaporation and turbulence.
Explore a thorough and up to date overview of the current knowledge, developments and outstanding challenges in turbulent combustion and application. The balance among various renewable and combustion technologies are surveyed, and numerical and experimental tools are discussed along with recent advances. Covers combustion of gaseous, liquid and solid fuels and subsonic and supersonic flows. This detailed insight into the turbulence-combustion coupling with turbulence and other physical aspects, shared by a number of the world leading experts in the field, makes this an excellent reference for graduate students, researchers and practitioners in the field.