Cool flames, which are governed by the low-temperature chemistry, are responsible for multistage ignitions and knocking phenomenon of internal combustion (IC) engines. Recently, it has been shown that there exists more significant wall chemical effects on cool flames than on hot flames. Therefore, elucidation of such phenomenon is extremely important for further improvements in the thermal efficiency and reduction in pollutant emissions of IC engines. In this paper, we introduce recent progresses in the cool flame research focused on wall chemical effects. The cool flame was ignited and stabilized in the vicinity of the wall by impinging a mixture of Dimethyl Ether (DME) and oxygen on a heated wall. The wall was coated with a thin film of specific materials, so that the wall chemical boundary condition can be modified while keeping the thermal boundary condition identical. Distributions of the HCHO, CO, and gas temperature on different wall materials were measured through Planar Laser-Induced Fluorescence (PLIF), Gas Chromatography (GC), and thermocouple scanning, respectively. Furthermore, the cool flame ignition process was observed through HCHO-PLIF with ramping up the wall temperature. The results show that the wall chemical effect can significantly modify not only cool flame structures but also ignition profiles.
Wall chemical effect caused by radical adsorption and recombination on wall surfaces under flame-wall interactions is investigated by using two different types of plasma techniques as well as combustion diagnostics. In this paper, the main focus is placed on surface reaction of H-atoms, which play a key role in the wall chemical effect on hot flames. Firstly, H adsorption is directly measured through molecular beam scattering technique using a non-equilibrium plasma-driven beam source with an ultra-high vacuum chamber. The initial sticking coefficient S0 of H-atom on stainless-steel (SUS) wall is 0.49~0.53 at the wall temperature of 30~800 ℃. This result is in good agreement with S0 estimated in our previous combustion experiments as 0.1~1, and more precise value can be obtained here. Secondary, non-equilibrium plasma jets are employed to examine H-atom adsorption at atmospheric-pressure. H-atoms are successfully generated by the addition of appropriate amount of water vapor into the plasma feed gas. The produced H-atoms are issued onto quartz and alumina walls, and it is found that H-atoms could be adsorbed on the quartz wall. Finally, H-atom recombination on SUS wall is evaluated using a premixed flame formed in a stagnation flow near a heated SUS wall. In order to estimate the recombination rate, the near-wall H2 concentration is analyzed by gas-chromatography and compared with numerical results. The H-atom recombination rate constant is larger than 104 s-1, showing that the wall chemical effect is adsorption-limited.
Flame quenching resulting from flame-wall interactions (FWIs) is important in several thermochemical processes of practical relevance, such as internal combustion engines. Even though FWIs are restricted to regions close to walls of a combustion chamber, they are crucial for wall heat fluxes and unburned hydrocarbon emissions. This review starts with the discussion about the role of laser diagnostics in FWIs. Following a description of current understanding of FWIs, the focus is on applying optical diagnostics to investigate parametric sensitivities that influence flame quenching at walls and to better understand the influence of non-adiabaticity upon the thermo-chemical states in FWIs. The review closes with a discussion of issues and implications for future experimental researches.
The role of catalytic combustion in conventional power devices has been limited in reducing emission or treatment of off-gas, and is not a major part of energy. However, if the size of the device is reduced to fingertips, the influence of surface on combustion becomes significant and catalytic combustion can be used effectively for power generation. In the present article, a micro-combustor of 10W class and its characteristics are introduced. The combustor has sintered catalyst layer, which sustains combustion in a narrow tube of sub-millimeter diameter and yields wide flammability. Using the micro-combustor, we developed a micro-power-unit which supplies heat and electricity. The generator part consists of thermoelectric conversion modules and the micro-combustor. The fuel, propane/butane, is supplied by its vapor pressure and the air is pumped into the combustor by a micro-blower driven by a part of the generated electricity. The developed micro-power-unit generated electricity of about 300mW with heat input of 15W. The final conversion efficiency reached 2%, which is state-of-the-art among these small power devices.
The emission regulations for HC, NOx and CO emitted from automotive cars become increasingly stringent. Furthermore, due to the recent development of thermally efficient lean burn engines, average exhaust gas temperature is significantly decreased. In addition, test modes such as Real Driving Emissions and WLTP, in which gaseous compositions as well as temperatures tend to be drastically fluctuated are recently introduced. Therefore, catalysts are strongly required to be activated for the wide range of temperature or gaseous compositions of exhaust gas. The computer aided development, i.e., numerical simulation which is capable of quantitative predictions of TWC performance is desired. In this paper, development procedure of surface reaction mechanism with recently developed 1+1D honeycomb monolith reactor model is described. The surface reaction mechanisms and its development procedure can contribute to the drastic improvement of designing catalysts by numerical simulation.
NH3 has been recognized as a carbon-free and renewable energy source. However, NH3 fuel has a high ignition temperature and its use results in the production of N2O/NOx. Therefore, these issues must be overcome. To overcome these issues, novel NH3 combustion catalysts have been developed. Previously, combustion catalysts for hydrocarbon (HC)-based fuels were studied as a promising approach to decreasing NOx emissions. On the other hand, it is considered that the catalytic NH3 combustion system has advantages for decreasing not only NOx but also N2O emissions, and the low operating temperatures. In this articles, recent developments for NH3 combustion catalysts are covered. Firstly, the catalytic activity for NH3 combustion over metal oxides was investigated. According to the results, it is suggested that the NH3 combustion activities of metal oxides are correlated with the metal–oxygen bond energy. In addition, in comparison with the other metal oxides, CuO (CuOx) showed relatively high catalytic activity and low N2O and NOx product selectivity. Secondly, various CuOx-based supported catalysts were prepared. Because, among the various supported CuOx catalysts, CuOx/3Al2O3·2SiO2 (3A2S) catalysts exhibited the high performance for NH3 combustion, the local structures, NH3 combustion properties, and combustion mechanisms of CuOx/3A2S after thermal ageing at 900 ℃ were studied in detail. Finally, monolithic honeycomb catalysts for NH3 combustion were prepared, and the reaction properties were determined.
Laser induced incandescence (LII) has been the most commonly used technique for measurement of soot primary particle size and soot volume fraction in combustors, engine exhaust gases, and ambient atmospheric environments. Unlike other soot detection techniques that need soot sampling process, LII is in-situ measurement in which thermal radiation from soot particles heated by a pulsed laser is detected to evaluate soot particle temperature or soot volume fraction. In this article, fundamentals of LII measurement are described for better understanding of its principle, in particular with time-resolved two-color LII, highlighting recent progress in LII models. In addition, combination of LII and elastic laser scattering is introduced as a challenging technique providing information on aggregation of soot particles.
The essential setup for applying two-dimensional laser induced incandescence (LII) on the multi-phase combustion (spray flame and pulverized coal jet flame) is reported in this serial lecture. This paper includes the effects of the selection of incident laser wavelength, detection wavelength and detection timing as well as the effects of spatial profile of laser sheet and fluence on the LII signal intensity. Additionally, the example results of two-dimensional LII and time resolved LII (TiRe-LII) for spray flame, and simultaneous measurement of PAHs-LIF and LII for the pulverized coal jet flame were addressed.
In this study, based on the OH fluorescence signals, OH concentration of Bunsen flames for premixed mixtures of methane-hydrogen and air was investigated. In experiments, when the hydrogen ratio in the total fuel increased, a part of methane in the mixture was replaced by hydrogen. For discussion of the effect of hydrogen, a numerical simulation of one-dimensional flame was conducted. For the methane flame, as the equivalent ratio of the rich mixture increased, the burning velocity decreased and the flame became longer, with the decrease of OH fluorescence intensity. For the methane-hydrogen flame, as the hydrogen ratio increased, the burning velocity decreased, with the flame length longer. Simultaneously, the OH fluorescence intensity tended to decrease. From the numerical simulation, it was confirmed that, as the hydrogen ratio increased, the production rate of OH caused mainly of the R38 elementary reaction decreased, and the OH concentration decreased. Resultantly, the consumption rate of OH caused mainly by the R84 elementary reaction decreased, resulting in the smaller heat release rate of the premixed flame. These findings are reasonable, because hydrogen has lower heat of combustion than methane. For the methane flame, the good correlation between the OH concentration obtained by the simulation and the burning velocity was observed. On the other hand, for the methane-hydrogen flame, the burning velocity was different even when the OH concentration was the same.