Spray combustion efficiency

Some concerns directly related to a tomizer operation include inadequate mixing of Hquid and gas, incomplete droplet evaporation, hydrodynamic instabiHty, formation of nonuniform sprays, uneven deposition of Hquid particles on soHd surfaces, and drifting of small droplets. Other possible problems include difficulty in achieving ignition, poor combustion efficiency, and incorrect rates of evaporation, chemical reaction, solidification, or deposition. Atomizers must also provide the desired spray angle and pattern, penetration, concentration, and particle size distribution. In certain appHcations, they must handle high viscosity or non-Newtonian fluids, or provide extremely fine sprays for rapid cooling. 

These results show that droplet vaporization must be different between the three flames. Droplet and fuel vapor transport must be significantly different for these flames and must affect combustion efficiency. The solid-cone nature of the spray flame was found to be preserved irrespective of the atomization gas. 

A number of systems have been devised to meet these two competing objectives. In one approach, water is mixed with oil before it is sprayed into a combustion chamber. The presence of water reduces combustion temperatures enough to reduce nitrogen oxide emissions by up to 15 percent without significantly reducing combustion efficiency. Another approach is to reduce combustion temperatures by... 

When diesel fuel is injected into the high-pressure environment of a combustion chamber, it must fully penetrate into the pressurized air in order to completely combust. Fuel with a low viscosity does not have enough thrust to penetrate effectively through the pressurized air and disperse completely. The result is a soft, nonpenetrating spray which does not spread throughout the chamber. Poor combustion efficiency and engine power result. 

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. 

As regards actual combustion of jet fuels, the two critical combustion factors are fuel volatility and hydrogen/carbon ratio. As might be expected, fuels that are too heavy for the spray system and for the combustor design do not burn as well as more volatile fuels. Low hydrogen/carbon ratios also interfere with combustion efficiency, even though straight aromatics have been handled in specially adapted burners (5). 

By assuming the Langmuir expression for the evaporation of a droplet with the Rosin-Rammler size distribution law, Sacks (74) found that the theoretical evaporation rate of a kerosine spray was about 100 times the experimentally observed values. He concluded that the Langmuir expression is based on the single drop and neglects the vapor pressure of the surrounding air, which would tend to inhibit vaporization in a spray. Consideration of the effects of dissociation of combustion products plus the effects of thermal conductivity for the vapors enabled Graves (33) to derive a theoretical curve for combustion rate which compared favorably with experimental data. However, the use of Probert s analysis to determine combustion efficiency, yielded efficiencies which were much lower than experimentally observed results. 

Reasonable correlations of combustion efficiency with fuel spray momentum and spray energy in two different combustors have been shown to hold over a range of altitude-engine idling conditions 133). As different curves were obtained with different injector nozzles, spray-cone angle was thought to be a factor. Further work showed that efficiency did correlate closely with expressions representing the spray momentum or... 

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... 

The objective of the analysis given in this section is to illustrate the use of equation (2) by considering the problem of determining the combustion efficiency of a variable-area, quasi-one-dimensional rocket chamber such as that illustrated in Figure 11.1, in which M different kinds of liquid droplets are present. In order to avoid considering the behavior of the gas, we must assume that the material burns to completion as soon as it evaporates. The amount of heat released will then be proportional to the mass evaporated, thus making it possible to relate the combustion efficiency to the mass of the spray present. Even in this case, the equations contain parameters, such as Rj, which depend on the local gas properties. However, estimates of these parameters are often obtainable without solving for the gas flow, so that, while the theory is essentially incomplete, it is not entirely useless. 

The final step in the analysis is to obtain the combustion efficiency for a chamber of length x from the size distribution at position x. Let Qj denote the heat released per unit mass of material evaporated from a droplet of kind j, and let Pi j represent the density of the liquid in droplets of kind j. The mass of the spray of kind j per unit volume of space is therefore lo corresponding mass flow rate (mass per second) is... 

For initial droplet-size distributions given by equation (22), the combustion efficiency and other spray properties may be expressed in terms of the function... 

The property of the spray that is of greatest practical importance is the combustion efficiency, which is given by equation (18). When Gj q is of the generalized Rosin-Rammler form [equation (22)], by using equations (12) and (23) in equation (18), we find that... 

The size distribution of particles in a wide variety of particulate systems is of paramount importance in the chemical processing industries. For example, the compacting and sintering behavior of metallurgical powders, the flow characteristics of granular material, the hiding power of paint (qv) pigments (qv), and the combustion efficiency of powdered coal (qv) and sprayed fluids, are all heavily influenced by the size of the constituent particles (see Flow... 

Hydro-carbon-based spray combusticni is associated with pollutant formation. These pollutants include the NO and NO2 (referred to as NOx), carbon monoxide (CO), particulate matter (soot), unbumed hydrocarbons (HC), and others such as sulfur oxides. Although pollutants form a small part of the overall exhaust gas composition, they are produced in such large quantities that they have become considerable environmental and health hazards. Therefore, poUutimi reductimi has become one of the most important aspects of spray combustimi research. A very effective approach to achieve this objective is to reduce pollutants at their source, and thereby contribute directly to the production of cleaner and more efficient spray combustion devices. 

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