| Characteristic dimension of the chamber of miniature internal combustion engine is in millimeter scale. It is close to the thickness of microscale flame. The release of chemical energy of the fuel have to be completed in a very small space with a very short period of time. Therefore, the study of the flame propagation characteristics in micro-scale space has a great deal of significance on the development of miniature internal combustion engine. In terms of micro-scale combustion, previous studies were mainly for combustion within a stable flow field.There were less study on flame propagation characteristics in enclosed space. And it is lack of experimental data, especially. This paper presents a new type of micro scale constant volume combustion facility. At room temperature and atmospheric pressure,it carried out research of flame propagation characteristics in a micro scale enclosed space with low-carbon hydrocarbon fuel/air premixed stationary gas. A high-speed camera was used to record the flame propagation in milliseconds. MATLAB programs were dealt images of flame propagation have been processed.At different equivalence ratio, scale and hydrogen fraction, flame shapes and characteristics of flame propagation speed of ethane/air mixture, propane/air mixture and n-butane/air mixture were analyzed. The working mechanism of flame propagation was obtained. The research results advised for the development of new micro internal combustion engine and provided theoretical guidance. The main contents and results are as follows:(1) In the microscale constant volume combustion chamber, range of combustion of carbon hydrocarbon fuel is decreased with the increase of the number of carbon atoms. For one fuel, the range of combustion increased with the increase of feature size in combustion chamber. For propane/air mixture, at the H=2mm, the range of combustion is 1.0 to 1.45. When H=5mm, the range of combustion is extended from0.9 to 2.2.(2) In the microscale constant volume combustion chamber, microscale flame front will show smooth, wrinkle and fracture. When the height of gap is constant, it is prone to wrinkle and fracture flame with the increase of equivalence ratio. Forexample, at the diameter of 150 mm, 2mm gap constant volume combustion chamber,propane/air mixture, flame wrinkles appeared earlier with increasing equivalence ratio. When the ratio increased to 1.45, fracture flame front occurred. At the same equivalence ratio, flame wrinkles and flame fracture prone to occur with the decrease of the height. For example, at φ=1.2, wrinkles occur earlier within 2mm gap than that in 3mm gap, and wrinkles occur within 3mm gap earlier than that in 5mm gap. It shows that uncoordinated level of the thermal diffusion and mass diffusion was increased with the increase of equivalence ratio. It leading to flame instability,wrinkles occur earlier, even fracture could happen. With the decrease of height, micro combustion chamber surface area to volume ratio increased which resulted in a large heat loss and a large of decrease of combustion intensity. In addition, with the decrease of the height, the influence of hydrodynamic is increased, which resulted in flame instability, wrinkles and fracture.(3) In the microscale constant volume combustion chamber, premixed flame propagation velocity decreases with the increase of radius. At the same height, with the increase of equivalence ratio, flame propagation velocity of ethane/air mixture,propane/air mixture and n-butane/air mixture increases first then decreases, and the highest are near φ=1.2. In addition, at the height of 2mm, the propagation speeds of three fuels premixed flame are similar. At the same equivalence ratio, with the decrease of the combustion chamber space, the flame propagation velocity increases first then decreases. In this study, due to the coupling effect of hydrodynamic and heat-recirculation, at φ=1.2 and H=3mm, flame propagation velocity of propane/air premixed and n-butane/air premixed reached the highest.(4) With the analysis of stretch rate of n-butane premixed flame, it shows that at the same height, the influence of equivalence ratio is small; at the same equivalence ratio, the influence of height is big. At the last phase, hydrodynamic instability is the main factor which leaded to instability of the flame. It is indicated that smaller gap of combustion chamber had bigger influence of hydrodynamic.(5) The propagation speed and characteristics of combustion were impacted by hydrogenation. With hydrogenation increase, the flame propagation speed propane /air mixtures increases generally. At H=2.5mm, with the fraction hydrogen is 13.2%,the propagation speed was up to 8m/s, and deflagration occurred. It showed that deflagration was possible in a micro space. At the same equivalence ratio and height,the reaction surface was increased with more wrinkles, so that the reaction of combustion enhanced. Experiments also found some special phenomenon of microscale combustion. For example, at the height of 5mm and φ=1.4, the thickness of the flame front of n-butane / air mixture had a jump change. When the ratio is 1.8,a combustion phenomenon occurred, the unburned gas flame pass through the flame front. At the height of 2.5mm and φ=1.5, flame propagation of n-butane/air mixture was very slow. Propagation velocity was lower than 0.28m/s; at the height of 5mm and φ=1.5, the propane/air mixture has a partial turbulence in the propagation. |
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