Spray combustion turbulence

The phase Doppler method utilizes the wavelength of light as the basis of measurement. Hence, performance is not vulnerable to fluctuations in light intensity. The technique has been successfully appHed to dense sprays, highly turbulent flows, and combustion systems. It is capable of making simultaneous measurements of droplet size, velocity, number density, and volume flux. 

Pulsation in a spray is generated by hydrodynamic instabilities and waves on liquid surfaces, even for continuous supply of liquid and air to the atomizer. Dense clusters of droplets are projected into spray chamber at frequencies very similar to those of the liquid surface waves. The clusters interact with small-scale turbulent structures of the air in the core of the spray, and with large-scale structures of the air in the shear and entrainment layers of outer regions of the spray. The phenomenon of cluster formation accounts for the observation of many flame surfaces rather than a single flame in spray combustion. Each flame surrounds a cluster of droplets, and ignition and combustion appear to occur in configurations of flames surrounding droplet clusters rather than individual droplets. 

I he experimental investigation of the combustion of sprays is complicated by the many variables involved. Common sprays are composed of a wide range of droplet sizes distributed unevenly in the spray cone. Turbulence of the air and the relative motion of the droplets through the air are poorly defined. The burning of an isolated droplet itself presents a difficult problem, although much progress has been made in this field in the past few years. To study the effect of any single variable on the combustion characteristics of a spray, other variables must be held constant. This paper reviews those fields of effort in which work has been done to simplify the complex physical aspects of this problem. 

Of major interest concerning these problems are influences of turbulence in spray combustion [5]. The turbulent flows that are present in the vast majority of applications cause a number of types of complexities that we are ill-equipped to handle for two-phase systems (as we saw in Section 10.2.1). For nonpremixed combustion in two-phase systems that can reasonably be treated as a single fluid through the introduction of approximations of full dynamic (no-slip), chemical and interphase equilibria, termed a locally homogeneous flow model by Faeth [5], the methods of Section 10.2 can be introduced reasonably successfully [5], but for most sprays these approximations are poor. Because of the absence of suitable theoretical methods that are well founded, we shall not discuss the effects of turbulence in spray combustion here. Instead, attention will be restricted to formulations of conservation equations and to laminar examples. If desired, the conservation equations to be developed can be considered to describe the underlying dynamics on which turbulence theories may be erected—a highly ambitious task. 

Onuma (12) studied the stmcture of a spray combustion flame. The results are qualitatively similar to the findings of Mizutani (II). Onuma, furthermore, showed that a good similarity exists between the spray combustion fiame and a turbulent gas diffusion flame. 

De Chaisemarttn, S., Freret, L Kah, D. et al. 2009 Eulerian models for turbulent spray combustion with polydispersity and droplet crossing. Comptes Rendus Mecanique 337, 438 48. 

Kah, D., Laurent, F., Freret, L. et al. 2010 Eulerian quadrature-based moment models for dilute polydisperse evaporating sprays. Flow, Turbulence, and Combustion 85, 649-676. 

Combustion processes are determined by the interaction of flow properties and chemistry and can be subdivided into premixed and non-premixed regimes for either laminar or turbulent flows. In premixed combustion, the fuel and the oxidizer are fully mixed at the time the chemical reactions take place. In contrast, in non-premixed combustion processes, the fuel and the oxidizer are separated until mixing and chemical reactions take place almost simultaneously. Non-premixed combustion is also referred to as diffusion combustion and is typically encountered in sprays. Note that spray combustion is almost always turbulent because the interaction between the spray droplets and the surrounding air induces a turbulent flow. Also, as discussed below in more detail, in transient spray processes such as those encountered in diesel engines, both premixed and non-premixed combustion take place. 

This implies that the spray tends to approach a saturation state unless additional heat and oxygen are supplied from the outside of the spray stream. It also implies that the behavior of the outer diffusion flame dominates the subsequent evolution of spray combustion from the spray boundary side. In real spray combustion, this boundary-layer type of change occurs dynamically because the boundary of the spray stream is located in the coherent vertical structure of the shear layer. In addition, turbulent effects are inevitable. However, such fluid dynamic effects have not yet been well characterized. Therefore, we focus on the behavior of the outer diffusion flame based on a quasi-steady continuum spray model. Chiu s theory is developed on this basis to classify the combustion modes excited by the penetration of the outer diffusion flame into the spray region. 

J. Reveillon, L. Vervisch Analysis of weakly turbulent dilute-spray flames and spray combustion regimes, J. Fluid Mech., 537, 317-347 (2005). 

M. Boileau, S. Pascaud, E. Riber, B. Cuenot, L.Y.M. Gicquel, T.J. Poinsot, and M. Cazalens. Investigation of two-fluid methods for large eddy simulation of spray combustion in gas turbines. Flow, Turbulence and Combustion, 80 291-321, 2008. 

Because of a vivid interest in spray combustion, quite a few papers deal with the effect of finely dispersed particles or droplets on the turbulence characteristics of jets and sprays. The interaction of particle dynamics and turbulence in these flows has prompted very fundamental stochastic approaches involving filtering and averaging techniques, probability density functions, and quadrature-based moment methods (Fevrier et al, 2005 Fox, 2012 Labourasse et al, 2007) which are beyond the scope of this chapter. Riber et al (2009) and Senoner et al (2009) obtained LES results for recirculating and evaporating two-phase flows, respectively, by both Euler-Lagrange and Euler—Euler methods and compared them mutually and with experimental data. 

The vapor cloud of evaporated droplets bums like a diffusion flame in the turbulent state rather than as individual droplets. In the core of the spray, where droplets are evaporating, a rich mixture exists and soot formation occurs. Surrounding this core is a rich mixture zone where CO production is high and a flame front exists. Air entrainment completes the combustion, oxidizing CO to CO2 and burning the soot. Soot bumup releases radiant energy and controls flame emissivity. The relatively slow rate of soot burning compared with the rate of oxidation of CO and unbumed hydrocarbons leads to smoke formation. This model of a diffusion-controlled primary flame zone makes it possible to relate fuel chemistry to the behavior of fuels in combustors (7). 

In modem Hquid-fuel combustion equipment the fuel is usually injected into a high velocity turbulent gas flow. Consequently, the complex turbulent flow and spray stmcture make the analysis of heterogeneous flows difficult and a detailed analysis requires the use of numerical methods (9). 

This review paper is restricted to stirred vessels operated in the turbulent-flow regime and exploited for various physical operations and chemical processes. The developments in the field of computational simulations of stirred vessels, however, are not separated from similar developments in the fields of, e.g., turbulent combustion, flames, jets and sprays, tubular reactors, and multiphase reactors and separators. Fortunately, there is a strong degree of synergy and mutual cross-fertilization between these various fields. This review paper focuses on aspects specific to stirred vessels (such as the revolving impeller, the resulting strong spatial variations in turbulence properties, and the macroinstabilities) and on the processes carried out in them. 

Jet or Spray Fires - Are turbulent diffusion flames resulting from the combustion of a liquid or gas continuously released under pressure in a particular direction. 

Some early spray models were based on the combination of a discrete droplet model with a multidimensional gas flow model for the prediction of turbulent combustion of liquid fuels in steady flow combustors and in direct injection engines. In an improved spray model,[438] the full Reynolds-averaged Navier-Stokes equations were... 

In the development of the Diesel engine (117) the design of the combustion system has played an important role. Several combustion chamber shapes are presently in use. These include the open chamber (38, 54, 68, 82), the precombustion chamber (24, 32, 92, 108), the turbulence or air-swirl chamber (32), the air cell, and the energy cell (32). In all of these fuel is vaporized by transferring heat from the air charge to the fuel. In a recently developed combustion system (87) the fuel is sprayed on a hot surface which... 

The HiSmelt process being developed jointly by CRA of Australia and Midrex Direct Reduction Corp. uses a horizontal vessel, relying on turbulence in the bath to spray particles of slag and iron into the atmosphere above the bath, where heat is transferred from the post-combustion flame to the particles. Here, air is used instead of oxygen, thus removing the requirement of an oxygen plant. This technology emphasizes bottom injection of coal and dust into the iron bath. 

Liquid fuel sprays are not yet fullj understood [310]. The atomization process of a liquid fuel jet [376 332 345 293 309], the turbulent dispersion of the resulting droplets [256 253 262 333 319], their interaction with walls [259 365], their evaporation and combustion [290] are phenomena occurring in LES at the subgrid scale and therefore require accurate modeling. 

While it is tempting, it would be premature to apply these equations and findings directly to more complex spray combustor situations. Apart from obvious differences in overall geometry, in practical sprays three effects are superimposed transients associated with oxidizer entrained in the fuel injector region, droplet-size-dependent relative motion between the fuel droplets and the surrounding gas, and oxidizer and product transport by turbulence and convection. Rather, our present QS and future transient studies of the behavior of quiescent fuel droplet clouds should be viewed as necessary first steps in the qualitative and quantitative theoretical understanding of fuel droplet sprays. Future work should be concerned not only with the conditions under which theoretical group combustion occurs in fuel sprays but also with the implications of such cooperative phenomena for combustion eflBciency in volume-limited systems, and pollutant emissions. 

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