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MILD combustion in diffusion-controlled regimes of Hot Diluted Fuel
No full textReactive structures have been characterized in a steady laminar, unidimensional mixing layer on a dense grid of parameters in moderate or intense low-oxygen dilution (MILD) combustion conditions with hot and diluted fuel. The structures have been studied in terms of temperature and heat release profiles in a mixture fraction space for various ranges of stretch rates and for two reference pressures (1 and 10 bar) using a standard code and standard kinetic scheme. In the analysis of reactive structure three synthetic characteristics have been pointed out in previous works as discriminative for the occurrence of different combustion regimes. Such characteristics are the thickness of the oxidation structures, the presence/absence of a pyrolysis region and the correlation/no correlation of the regions of maximum heat release with those at which the mixture is stoichiometric. Following the same criteria, maps of regimes have been built up on a inlet fuel temperature - temperature increase plane for fixed stretch rates and different pressures. It has been pointed out that in diffusion controlled regimes of Hot Diluted Fuel, analysed in this paper, MILD combustion conditions are characterized by flame thickening and pyrolysis depression, which are also typical of flameless combustion. In addition, the region of maximum heat release has a generally high level of correlation with the stoichiometric regions. Thus, MILD combustion regime differs from MILD combustion regime found in other diffusion controlled regimes where the region of maximum heat release is generally not correlated with the stoichiometric regions. In the case presented in this paper, only a second solution, corresponding to a very low conversion occurring in a very wide stretch-rate range, shows no correlation between heat release and stoichiometric mixture fraction. This behavior has been attributed to the oxidative pyrolysis of methane. At atmospheric pressure, the nameless characteristics extend to a low level of preheating, provided that the fuel is diluted, with a consequently low level of temperature increase, thus confirming that MILD and flameless regimes are not coincident in all possible feeding conditions. In general, the results obtained in these Hot-Fuel-Diluted-Fuel conditions are consistent with and extend those reported in the literature for Hot-Oxidant-Diluted-Fuel, Hot-Oxidant and Diluted-Fuel conditions, supporting the assertion that these inlet parameters are a suitable choice for the definition of MILD combustion
Air dilution effects on tetradecane spray autoignition in transcritical and supercritical regimes
No full textAutoignition time delay of tetradecane spray injected in nearly quiescent, high-temperature, high-pressure, diluted and not diluted air has been measured by detecting spontaneous luminous emission. The photodiode location and its spectral sensitivity were carefully tested with preliminary comparison to spectrally resolved measurements hy using a spectrograph equipped with an intensified CCD camera. The measurements were performed starting from a temperature of 873 K, a pressure of 3.6 MPa, and all oxygen molar fraction of 0.21. One of these ambient conditions Ir?asfuced and the other two parameters, were lowered to 873 K ambient temperature, 1.1 MPa, and 0.06 oxygen molar fraction. The results were analyzed in relation to the expected ones, which were based on the available models relative to processes controlled by atomization, evaporation, quenching, and chemical kinetics. The main result of this analysis has been the characterization of the relationship between ignition delay time and oxygen molar fraction in the ambiance that has been found to be well described by a -2 power law. This behavior supports the low-temperature kinetic scheme for paraffin, which hypothesizes oxygen addition on fuel molecules. Moreover, transcritical and supercritical droplet evaporation regimes have been identified as functions of ambient temperature and pressure
Zero-dimensional analysis of diluted oxidation of methane in rich conditions
No full textClean combustion technologies, based on reactant dilution, have shown very peculiar and innovative characteristics. Reduction of light and noise emission and uniformity of temperature and composition distribution inside the combustion chamber, related to an extension of the reaction zone, make these processes very promising in several technological fields. Analysis of combustion in very diluted conditions is needed for a general understanding of these processes, which remain unknown from many aspects. The study is also useful for the identification of practical constraints which define the most suitable reactor configurations and working parameters for process reliability. The present work deals with a theoretical analysis of methane oxidation in diluted conditions using one of the detailed kinetic schemes available in the literature. A well-stirred reactor (WSR) configuration has been considered as a first attempt of process schematization. This choice is consistent with the experimental characterization of the flameless combustion processes. The influence of residence time, C/O ratio, and inlet temperature on the steady state was studied for an oxygen molar fraction (0.05), chosen as representative of flameless combustion processes. It has been pointed out that only rich conditions (C/O > 0.25) are possible for WSR creation, because they allow partial methane conversion. Three kinetic regimes, related to different temperature ranges, have been identified on the basis of product distribution analysis. Both the oxidation and pyrolitic regimes, occurring respectively in low- and high-temperature ranges, are the most interesting working conditions for diluted combustion. The former, leading to CO and H2O as main reaction products, is suitable for reburning technology. The latter, which corresponds to large production of CO and H-2, has been shown to be a reasonable explanation of flameless combustion. Advantages and potentials of this innovative process are analyzed, taking into account the kinetic pathway followed in the different working conditions
HDDI in Mild combustion
No full textMild combustion is an emerging technology in several fields of practical applications, from material treatment to energy conversion as well as to pollutant abatement [1]. In a survey of potential keys for the mitigation of environmental problem, it represents a flexible and clean process that is the result of a trade-off in the optimization of fuel conversion with respect to efficiency (in term of energy saving and pollutant abatement) while not requiring drastic changes in the configuration of traditional plants. In this chapter the attention will be focused on non-premixed combustion processes, thus on configurations where the fuel and the oxidant flows are fed separately, then mix and react. A combination of both heating and dilution of oxidant and/or fuel yields a not premixed combustion process which is named Hot Diluted Diffusion Ignition (HDDI) [2, 4] when
• heating contributes significantly to the creation of an oxidative structure in the sense that no combustion process occurs without it;
• dilution is so intense that the maximum temperature attainable inside this structure is so low that, in turn, it affects significantly its placement in the mixture fraction domain, the structure itself and the physical and chemical kinetics when compared to a diffusion flame process
Dependence of autoignition delay on oxygen concentration in mild combustion of high molecular weight paraffin
No full textNew combustion technologies aim to increase the conversion efficiency and reduce the pollutant formation of combustion processes by extending the operating reaction conditions to new domains. The mild combustion mode represents one of the most promising technologies in view of its many potential advantages and numerous fields of application. The large-scale diffusion of such systems in practical applications is limited because of the lack of confidence of the system reliability, due mainly to the lack of studies carried out on the fundamentals of mild combustion processes. This paper can be considered a contribution to set up tuning and control tools for the application of the mild operating mode in practical combustion devices. Based on the definition of the conditions required to operate a combustion system in a mild mode, the specification of some requirements for the operation of high-compression ignition engines, like the homogeneous charge compression ignition engine or mild gas turbine, in mild conditions are analyzed. The role of oxygen dilution on the autoignition delay (tau) of heavy paraffins in temperature (600-1200 K) and pressure (1.3-4.0 MPa) ranges of interest in such applications is discussed on the basis of both experimental and numerical approaches. The very good agreement of results obtained by means of the two methodologies leads to the identification of a power law dependence of tau on oxygen molar fraction in the environment (tau proportional to X-O2(-n)) where the exponent is a function of temperature. Three main regimes with respect to ignition delay dependence on oxygen concentration at high pressure have been identified. The different functional dependencies found both in the modeling and experimental results have been correlated to the different reaction pathways followed in the different temperature ranges
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