B URNING V ELOCITY
3.5. CONCLUSION
The analyses reported so far have demonstrated that MILD combustion differs from deflagration, detonation, and explosion in their classical forms, as well as processes which generate diffusion flames. Therefore, MILD combustion deserves both to be analyzed independently and to be given a specific name. This name was proposed and used in recent literature (Cavaliere and de Joannon, 2004), and occurs when the process refers to maximum, frozen, and autoignition temperatures, which combine in such a way that the mixture can be autoignited and can yield a relatively small temperature increase comparable to the difference between the ignition and the standard tempera- tures. Such a broad definition can be made more specific in the case of certain processes in homogeneous mixtures (HBBI, HCCI, and HFFI) and in separated reactants (HDDI).
This is a field that must be further studied in order to provide a clearer and narrower definition of MILD combustion, but the main characteristics are clear. It is a process that does not have a double state (frozen and burnt) and develops for a wide range of mixture fractions when it is restricted to the HDDI case. These characteristics are of interest in comparison to the diffusion flame structure where the mixture fraction range in which the reaction takes place is quite narrow and changes slightly with dissipation rate. Here, the quenching dissipation rate characterizes the whole reacting system when the fuel and oxidant are fixed. In contrast, the oxidation structure of MILD combustion shifts along the mixture fraction phase according to different boundary conditions (tempera- ture and dilution). Therefore, it cannot be easily parameterized, at least under the present state of technology.
The processes described in the previous section have been used in some configur- ations of interest in gas turbine applications because of potential benefits with regard to low levels of pulsations in the combustion chamber as well as very low NOxemission.
It is also possible to obtain the same high temperature, high diluted conditions with external recirculation of either the flue gas itself or some of its parts with the use of combustion products from a companion unit (sequential combustion), from a reservoir fed by an external combustion unit, or from inert species like water vapor or carbon dioxide. In this field, such technologies seem to be very promising in terms of efficiency increase and pollutant reduction, although the complexity of power plants is increased.
From this point of view, Rankine, Brayton, and Hirn cycles can be combined in different ways so that MILD combustion conditions are created in the combustion process.
The final type of MILD combustion application mentioned here is the most attractive in terms of potential, but has not been pursued in a systematic way even though there are exceptional examples. The application is based on the fact that MILD combustion has been recognized as a kind of clean combustion, particularly in terms of soot and nitrogen oxide suppression (Cavaliere and de Joannon, 2004). It is still not clear whether this is true for nanoparticle particulates or for oxygenated compounds, but it is likely that selection of appropriate working conditions could ensure a minimization of these products. In particular, the choice of diluent flow may be decisive in the inhibition of these pollutant categories, since it is possible that there is a reactive species that favors complete oxidation.
This last property further pushes the process toward clean combustion in the sense that the pollutants are not just suppressed in their formation, but are also destroyed if they are present individually (Sabia et al., 2007). This can hold true for species containing carbon, hydrogen, and nitrogen atoms because they can be oxidized or reduced to carbon dioxide, water, or molecular nitrogen. Additionally, inorganic atoms different from nitrogen cannot be transformed in the aforementioned species, but can be released in the form of species that are less noxious or easier to split as a result of the chemical reaction they undergo, or when they interact positively with other diluent species or combustion products. Thus, it is suitable to name the process that is capable of such a transformation as clearing combustion. In this case,“to clear”is used in the same sense as that used in the separation of particulate in liquid streams. The fact that MILD combustion can lead to clearing combustion is based on properties that develop over a narrow temperature range in inert diluted flow.
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Lean-Burn Spark-Ignited
Internal Combustion Engines
Robert L. Evans
Nomenclature
BMEP Brake mean effective pressure BOI Beginning of pilot injection BSFC Brake specific fuel consumption BSNOx Brake specific nitrogen oxides BSTHC Brake specific total hydrocarbons COV Coefficient of variation
Cp Specific heat at constant pressure Cv Specific heat at constant volume
d Bowl diameter
D Cylinder bore
DD Detroit Diesel EOI End of injection
HC Hydrocarbon
HCCI Homogeneous charge compression ignition IMEP Indicated mean effective pressure
MBT Minimum advance for best torque PSC Partially stratified-charge
r Compression ratio rc Cutoff ratio
rv Volumetric compression ratio TDC Top-dead-center
tHC Total hydrocarbon WOT Wide-open throttle
g Ratio of specific heats,Cp=Cv DyC Combustion duration
Efficiency
l Relative air–fuel ratio f Equivalence ratio
Lean Combustion: Technology and Control
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