Numerical Investigation On The Effect Of Hydrogen Addition On Micro Combustion And Extinction Characteristics Of Methane-air

Micro devices, such as sensors, MAV, micro medical devices, micro-pumps, micro-motors, have been playing a key role on human life for aviation, spaceflight, automobile, biomedicine, environmental regulation, military affair and so on. Its matching energy device has become the bottleneck of restricting its development, therefore the micro combustor is considered as the ideal device for it. With the advantages of high power density, longevity, small volume and light weight, microcombustor draws more and more attentions of researchers. But the size of the combustor from millimeter to centimeter scale is a strong result for a sharp increase in the rate of heat loss and a significant shortening of time during which fuel traverses through the channel. In many cases, the characteristics size of the combustor is less than the flame quenching distance. Thus, combustion in micro reactor is hard to maintain stability and high efficiency. Currently, catalytic reaction has been usually adopted by loading the catalyst to transform conventional combustion into wall combustion for the promotion of combustion stability and the increase of methane conversion in the micro-reactor. Because of a higher sticking probability of O2 in comparison to that of CH4 for the noble metal which is usually used for catalyst, the surface sites Pt(s) are covered by O2, inhibiting the adsorption of CH4 and the proceeding of reaction. Addition hydrogen is adopted to restrain a great quantity of O(s) produced on the wall, but there is no unambiguous answer to the specific impact of hydrogen addition on catalytic microcombustion of methane. Hence, further studies of extinction characteristics of methane combustion and the effect of hydrogen addition in micro scale are very important.In this paper the catalytic combustion of methane on platinum is numerically modeled in steady condition. The effect of operating conditions such as equivalence ratio, inlet velocity and surface site density of catalyst on methane combustion are studied firstly. In addition, the effects of wall materials, thickness and heat emission on stability of methane combustion are analyzed. At last, hydrogen addition on catalytic micro-combustion characteristics of methane is inspected. The thermal effect and chemical effect of hydrogen for methane combustion are verified.The numerical results show that the conversion rate of methane increases with decreasing inlet velocity. But when the inlet velocity is small, there is a large reaction velocity of methane combustion at the inlet of combustor. This will result in the temperature here flying up to generate hot spot. In addition, when the surface site density of Pt catalyst is bigger than 2.72×10-9 mol/cm2, increase surface site density merely increases the cost of combustion, and there is not effects on reaction velocity and conversion rate of methane. So appropriate inlet velocity is choice and the surface site density of Pt catalyst is near 2.72×10-9 mol/cm2 according to various practical conditions to improve conversion rate of methane.The smaller heat conductivity coefficient is, the bigger gradient of wall temperature appears, so that thermal stress will produce. This leads to the crack on combustor. Hence, the bigger heat conductivity coefficient is adopted for delay lifetime of combustor and preheating the upstream gas on the choice of wall materials. The heat conductivity coefficient and wall thickness are closely bound up with stability of methane combustion. When outer wall surface is thermal isolation and inlet velocity is 0.35 m/s, combustion will steadily take place if only /109.3KWb-3l′3×.Although hydrogen addition will expand the limit of burnable, conversion rate of methane reduces with the increase of hydrogen fraction when the inlet velocity is smaller than 0.36 m/s. Increasing hydrogen addition fraction boosts methane conversion rate as inlet velocity is lager. The effect of hydrogen on the methane conversion rate is more significant in the higher inlet velocity. This is mainly because the short retention time of methane in combustor let methane whose combustion is insufficient discharge to outlet. Hydrogen addition brings ignition location forward to prolong time of methane combustion and this improves methane conversion rate.Hydrogen is not merely thermal effect but also chemical effect for methane combustion so that O(s) surface coverage drops and Pt(s) surface coverage increases. It can promote methane combustion. But the effect of hydrogen addition on methane combustion becomes more and weaker as hydrogen addition fraction is larger. The more hydrogen is added, the thermal effect of hydrogen is more obvious. But if hydrogen addition fraction is oversize, the chemical effect drop slightly. So the effect of hydrogen on methane combustion is weaker when hydrogen addition fraction is larger.