Combustion spray

Spray combustion is a complicated subject because it involves many different processes. A typical sequence of events would be the injection and 

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. 

In this chapter, the equation governing the statistical counting procedure for sprays is first derived (Section 11.1) and is applied (Section 11.2) to a very simplified model of rocket-chamber combustion in order to obtain an estimate of the combustion efficiency. This illustrative example and others 

Many practical, important devices, ranging from home oil heaters to chemical rocket motors, involve the burning of liquid (or solid) particles in a gas. The term spray will here be defined to include all such systems in which there are 

In practical applications the turbulence is especially important to the mixing of injected liquid fuel and sprays with the ambient oxidizing gas. 

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Vei y small solid fuel particles such as sawdust, agricultural grains, or coal dust can sustain flames when they are suspended in air. In fact, very serious fires have occurred in grain storage towers and coal mines because of the flammability of suspended dusts. The combustion of the individual particles follows the usual pattern of solid particle burning— devolatization and char burning. The combustion of the whole cloud of particles is similar to spray combustion and its characteristics depend on the nature of the fuel, size of the particles, and the number of particles in a given volume. 

Images of spray and combustion in the rapid compression machine obtained for conditions representative of typical HSCI-engine operation. The sequence of four images covers the period immediately after injection—far left, and until the full development of a reacting jet—far right. (From Lu, P.-H., Han, J.-S., Lai, M.-C., Henein, N., and Bryzik, W., Combustion Visualization of DI Diesel Spray Combustion inside a Small-Bore Cylinder under Different EGR and Swirl Ratios, SAE, 2001-01-2005, 2001. With permission.)... 

Spray Combustion - Diesel Engines - Spark Ignition Engines - Gas Turbines - Rocket Engines - Industrial Furnaces - Domestic Heating Boiler... 

Figure 1.1. Schematic of spray combustion process (a) annular combustion chamber in a single spool turbojet with an axial flow compressor (b) fuel injection and droplet formation in combustion chamber.

Figure 1.6. Size ranges of droplets/particles found in nature and generated by atomization of normal liquids and melts in aerosol spray, spray combustion, powder production, and spray forming processes.

Atomization of normal liquids has been long studied in the fields of spray combustion and spray drying. The most widespread application of the atomization of normal liquids is in spray... 

Emphasis is placed on the atomization processes used in spray combustion and spray drying from which many atomization processes have evolved. Advantages and limitations of the atomization systems are discussed along with typical ranges of operation conditions, design characteristics, and actual and potential applications. The physical properties of some normal liquids are listed in Table... 

In addition to the designs mentioned above, other atomizer configurations such as those used in spray combustion and spray drying processes may also be considered as an alternative in gas atomization of melts for special purposes after appropriate modifications. 

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. 

It should be noted that the dynamic conditions of droplet impact processes discussed above cover a large range of the actual conditions in many industrial processes, such as spray forming, thermal spray, spray combustion, spray cooling, and aircraft flight. Under these conditions, the spreading behavior of droplets on a flat surface is essentially governed by inertia and viscous effects (Fig. 

The experimental results of Lefebvre et al. 8 l also revealed that the mean droplet size is virtually independent of injector orifice diameter. This is a desirable feature for spray combustion applications. 

Many laser-based droplet diagnostic techniques have evolved from the fields such as spray combustion and spray drying. Phase-Doppler particle analyzer is now recognized as the most successful and advanced diagnostic instrument for spray characterization. Other proven diagnostic techniques include laser velocimetry and... 

Sirignano, W. A. 1983. Fuel droplet vaporization and spray combustion theory. Progress Energy Combustion Science 9 291-322. 

Continello, G., and W. A. Sirignano. 1990. Counterflow spray combustion modeling. Combustion Flame 81 325-40. 

Experiments were carried out in a spray combustion facility that can simulate the combustion behavior of many practical combustion systems. The facility... 

Ghaffarpour, M., and B. Chehroudi. 1993. Experiments on spray combustion in gas turbine model combustor. Combustion Flame 92 173-200. 

Stephens, J.R., S. Acharya, and E. J. Gutmark. 1997. Swirl-stabilized spray combustion with periodic heat addition. AIAA Paper No. 97-0464. 

Stephens, J. R., S. Acharya, E. J. Gntmark, and D. C. Allgood. 1997. Active forcing of air and fuel feed in swirl stabilized spray combustion. ISOABE Proceedings. Chattanooga, TN. 

Experiments were designed to study the effects of porous media on spray combustion and resulting emissions. A number of factors could affect combustion performance with the presence of porous inserts, including the location, thickness, and pore size of the porous insert and operating conditions such as firing rate and fuel-air ratio. For different operating conditions, the baseline tests without porous inserts were completed. After the baseline tests, the same operating conditions were repeated with porous layers installed at various locations. More tests were then completed with different porous material properties. 

Several papers give excellent introductions to the problems of spray combustion (5A, 9A, ISA, JHAy 19A). Lloyd (14A) includes a diagrammatic sketch of the physical and chemical factors influencing combustion. The fuel used, method, and degree of atomization, and the many combustion parameters all affect the final combustion efficiency. 

Problems pertinent to the combustion of sprays are discussed by Gerstein (5C). He places special emphasis on the distribution, dynamics, trajectories, and evaporation of droplets, and includes an additional section on drag coefficients. His list of references includes much of the latest work done in the field of spray combustion. 

Distribution of Sizes. Although special techniques can in some cases produce nearly uniform suspensions, most spray combustion research must be conducted on systems composed of a wide range of droplet sizes. A knowledge of the distribution of particle size is of great importance. 

Gerstein, M., Some Problems Pertinent to the Combustion of Sprays, Combustion... 

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