Mixing, transport and combustion in sprays

Abstract

Al~traet-Recent advances concerning analysis of sprays and drop/turbulence interactions are reviewed. Consideration is given to dilute sprays and related dilute dispersed flows, which contain well-defined dispersed-phase elements (e.g. spherical drops) and have dispersed-phase volume fractions less than 1%; and to the near-injector, dense spray region, having irregularly-shaped liquid elements and relatively-high liquid fractions. Early analysis of dilute sprays and other dispersed flows assumed either locally-homogeneous flow (LHF), implying infinitely-fast interphase transport rates, or deterministic separated flow (DSF) where finite interphase transport rates are considered, but interactions between dispersed-phase elements and turbulence are ignored. These limits are useful in some instances; however, recent evidence shows that both methods are deficient for quantitative estimates of the structure of most practical dispersed flows, including sprays. As a result, stochastic separated flow (SSF) methods have been developed, which treat both finite interphase transport rates and dispersed phase (drop)/turbulence interactions using random-walk computations for the dispersed phase. Evaluation of SSF methods for particle-laden jets; nonevaporating, evaporating and combusting sprays; and noncondensing and condensing bubbly jets has been encouraging, suggesting capabilities of current SSF methods to treat a variety of interphase processes. However, current methods are relatively ad hoc and many fundamental problems must still be resolved for dilute flows, e.g. effects of anisotropic turbulence, modification of continuous-phase turbulence properties by the dispersed phase (turbulence modulation), effects of turbulence on interphase transport rates, and drop shattering, among others. Dense sprays have received less attention and are poorly understood due to substantial theoretical and experimental difficulties, e.g. the idealization of spherical drops is not realistic, effects of liquid breakup and collisions are difficult to describe, spatial resolution is limited and the flow is opaque to optical diagnostics which have been helpful for studies of dilute sprays. Limited progress thus far, however, suggests that LHF analysis may provide a useful first-approximation of the structure and mixing properties of dense sprays near pressure-atomizing injectors. Since dense-spray processes fix initial conditions needed to rationally analyze dilute sprays, more research is this area is clearly warranted. CONTENTS Nomenclature 294 Yuu et al. 25 : round 0.0008~0.004t 11,000-56,000 Air particle-laden jet McComb and Salih 2~' 27 : Small* 5,000-15,000 Air round particle-laden jet Laats and Frishman28'29: 0.3-1.4' 66,000-137,000 Air round particle-laden jet Levy and Lockwood3°: 1.14 3.50* 20,000 Air round particle-laden jet Shuen et a/.31:

Structure of particle-laden turbulent water jets in still water

“…Downstream of this position, particle volume fractions were <3%: therefore, particle collisions and effects of adjacent particles on particle transport properties are negligible (Faeth 1987). Particle dimensions were < 10% of the flow half-width; therefore, particles were assumed to be in a locally uniform environment, based on liquid properties at their center.…”

“…The procedure follows the conserved-scalar formalism which is widely used for flows having variable scalar properties (Bilger 1976;Faeth 1987). …”

“…Earlier studies by A1 Taweel & Landau (1977), Elghobashi & Abou-Arab (1983) and others (cf. Faeth 1987), have sought to include the effects of scales on turbulence modulation. Based on the present findings, additional theoretical and experimental work along these lines is clearly needed.…”

“…A complete discussion of earlier studies of particle-laden jets will not be undertaken since reviews of these flows have recently appeared (Faeth 1983(Faeth , 1987. The present study extends earlier work on multiphase jets in this laboratory, which included: particles in gases (Shuen et al 1983a(Shuen et al , b, 1985; drops and sprays in gases (Solomon et al 1985a,b;Shuen et al, 1986); and bubbles in liquids (Sun & Faeth 1986;Sun et al 1986).…”

Dynamic modeling of micro- and nano-sized particles impinging on the substrate during suspension plasma spraying

“…The dynamic behaviors of the particles are dominated by the forces exerted on them. These forces have been studied using various models (Faeth, 1987;Pfender, 1989;Huang et al, 1995;Jen et al, 2005;Pateyron et al, 2013), mainly using the drag force. Among these models, Jen et al (2005) considered the Brownian force and the Saffman lift force but only in a special flow domain, which is not universal and which introduces errors when used for the spray distribution with sub-micro-sized or nanosized particles.…”