Haolin Yang

A Surface Analysis-Based Investigation of the Effect of Wall Materials on Flame Quenching

An investigation of a premixed methane-air flame was conducted with a slit burner between two parallel walls to examine the effects of wall material and temperature on flame quenching. Three different materials (i.e., zirconia ceramics, stainless steel 304, and Si) were tested at wall temperatures of 100–700°C. The quenching distances for the three wall materials at the same temperature decrease in the order stainless steel 304 > Si > zirconia ceramics. For all materials, the quenching distance shortens with increasing wall temperature. To clarify the differences among the materials, the surface structure and composition of the raw and used walls were characterized by X-ray diffraction and X-ray photoelectron spectroscopy. The results show that the percentages of chemisorbed oxygen on the surfaces decrease in the order zirconia ceramics > Si > stainless steel 304—a behavior that can be correlated with the order of quenching distances of the three wall materials, which moves from small to large. In short, the higher the percentage of chemisorbed oxygen, the shorter the quenching distances.

Effect of surface reactions on ignition delay of methanol/air mixture

A surface reaction kinetic model for the combustion of methanol/air mixture was developed in order to investigate the ignition inhibitory mechanism of wall on the premixed gas in a micro closed volume. In this model, except for H, O, OH and CH3 radicals, the absorption of hydrogen peroxide and hydroperoxyl on the surface were also considered. By applying CHEMKIN-Pro software, the model was integrated into the calculation of homogeneous combustion process of gas mixture. Surface reactions were found resulting in the increase of ignition delay time. The sensitivity analysis showed that the loss of hydrogen peroxide on the wall was the main reason, due to the direct suppression effect on the generation and accumulation of OH in the radical pool. However, the loss of hydroperoxyl would take the place of hydrogen peroxide as the main inhibitory factor when the sticking coefficient became as large as the order of 10−3. In addition, the ignition delay time increased with sticking coefficient or surface-area-to-volume ratio. Enhancing the initial temperature of premixed gas was able to reduce the inhibitory effect of surface reactions.

Numerical Study On Combustion Characteristics Of Partially Premixed Tubular Flame Burner For DME

ABSTRACT The combustion characteristics of dimethyl ether (DME) in a partially premixed tubular flame burner have been investigated numerically. The species distribution, stability limit of tubular flame, wall temperature and burn-off rate have been studied at different equivalence ratios and inlet velocities. Meanwhile, the corresponding combustion characteristics in the rapidly mixed tubular flame burner have been simulated and compared. The results show that the partially premixed burner forms a unique species distribution and the fuel and the oxidant are not mixed uniformly. This unique species distribution will promote the chemical enthalpy supply to the flame front by recirculation reverse flow in the center, and thus strengthen the combustion process. Based on this unique species distribution, the partially premixed burner has larger tubular flame stability limit, lower heat loss, and higher burn-off rate, especially for lean operating conditions. Comparing the formation mechanisms of the tubular flames in two types of burners, it is found that the partially premixed burner forms partially premixed tubular flame, which is no need to create a uniform premixed gas zone, thereby suppressing the influence of velocity difference between fuel and oxidant present in the rapidly mixed burner.

Experimental Study on Propane/Air Flame Propagation Characteristics in a Disc-Like Gap Chamber

ABSTRACT A disc-like gap chamber was developed to investigate the microscale effects on flame propagation characteristics in closed chamber, which is very important to understand the combustion process in micro internal combustion engines. Under initially normal pressure and temperature, the morphology and evolution of outwardly propagating flame, the flammable limit, flame speed and flame instability of propane-air flames influenced by gap width and equivalence ratio were experimentally studied. The results show that, as the gap width nearly equals to the quenching distance, the sustained propagation was possible only with rich flames. The smooth flames and wrinkled flames were observed. It clearly shows that the flame wrinkles developed gradually instead of instantaneously. The flame speed was slower in the 2.0 mm gap chamber than in a conventional combustion bomb, and the flame speed decreased with the increase of flame radius. As a result, the flame acceleration was not observed in present experiments. With the smaller gap width, the wrinkled flame occurred early, and the flame speed was slower. Due to the geometry of disc-like gap chamber, simple theoretical analysis indicates that the heat loss and elevated pressure play important roles in reducing flame speed.

Ignition Characteristics of Premixed Methane/air in Micro Chamber

The ignition process of premixed methane/air in a micro-scale chamber is simulated with the combination of surface chemistry and gas-phase reactions. The effect of different parameters on the ignition characteristics are analyzed in detail and the sensitive analysis is performed on main elementary reactions. It is found that the ignition of the mixture is inhibited by surface reaction, which mainly depends on the sticking ability of the surface on CH3 radical. The ignition inhibition can be reduced by increasing the initial temperature and pressure. The increase of equivalence ratio has small impact on ignition delay time when it is larger than stoichiometric ratio. The ignition delay time would increase significantly with surface-area-to- volume ratio. The product of sticking coefficient and surface-area-to-volume ratio can be used to measure the intensity of surface reaction. The larger the product is, the stronger the inhibition of the surface reaction on ignition has.