Impacts Of Interactions Between Multiple Effects On Flame Stability In Narrow Channels

The small size and large surface-area-to-volume ratio of the micro-or mesoscale combustor lead to a short residence time of fuel and large heat losses respectively, which easily cause an unstable flame and flame extinction. The conventional stabilization methods of fluid and heat recirculation were used to improve the micro-or mesoscale flame stability in present paper, and the combustors with a bluff body, cavities and a plane flame holder and preheating channels were developed. The impacts of interactions between multiple effects (heat transfer, species transport, flow field and combustion chemical reaction) on the flame stability were studied systematacially and deeply.At first, the combustion characteristics of hydrogen/air premixed mixture in a micro combustor with bluff body were studied. The results show that the flame in the micro combustor can be anchored well by the recirculation zone behind the bulff body, which will significantly improve the flame stabilization. As a result, the flame blow-off limit is extended. In addition, the effects of geometrical parameter of bluff body and solid material on the flame blow-off limit were investigated. It can be known that the effect of bluff body shape on blow-off limit is obvious. This is mainly because that the bluff body shape will significantly impact the size of recirculation zone behind the bluff body and shear stress on the flame front. Besides, as the recirculation zone under a bigger blockage ratio is larger, the flame blow-off limit increases with the increase of blockage ratio. Additionally, for the combustors made of quartz, stainless steel and SiC, the flame blow-off limit of the quartz combustor is the biggest, and that of the SiC combustor is the smallest among them, which are determinded by the coupling effect of the flow field and heat losses.Moreover, the combustion characteristics of hydrogen/air premixed mixture in a micro combustor with cavities were investigated. The results show that the flame stability is obviously improved by the recirculation zone and low velocity zone in cavity and boundary layer near the downstream channel. However, the phenomenon of flame tip-opening will occur in combustion chamber under a big inlet velocity, and a flame-splitting limit is defined in order to discuss quantitatively. The flame-splitting limit is bigger for a larger length-depth ratio of cavity due to a larger recirculation zone and low velocity zone. In addition, the effect of the thermal conductivity of solid material and channel gap on the flame-splitting limit are also significant, and the flame-splitting limit increases at first and then decreases with the increasing thermal conductivity of solid material and channel gap, which is mainly caused by the coupling effects of the heat transfer and flow field. In other words, a moderate thermal conductivity of solid material and channel gap are benefical for the flame stability.Furthermore, the combustion characteristics of methane/air premixed mixture in a meslscale combustor with or without a bluff body were investigated experimentally and numerically. Results demonstrate that only inclined flames were observed in the combustor without bluff body. On the contrary, the flame can keep stable and symmetrical under a big inlet velocity in the combustor with a bluff body, and its flame blow-off limit is approximatly two times bigger than that of the combustor without bluff body. These mean that the effect of the bluff body on stabilizing flame is prominent. However, when the inlet velocity is larger than the blow-off limit, the phenomenon of flame pinch-off will occur and then the flame will be totally blown out of the combuction chamber.Besides, the combustion characteristics of methane/air premixed mixture in a meslscale combustor with cavities were studied. The results show that the symmetric stable flames appear in the combustor with cavities under a big inlet velocity which is several times larger than the corresponding burning velocty. These indicate that the anchoring effect of cavity on flame is significant, which is mainly caused by the low velocity zone in cavity, preheating effect of upstream solid wall on incoming unburned mixture and a larger local equivalence ratio zone near the cavity. However, the excessive flame shear stress and heat losses rate which exist at the transition point between the ramped cavity wall and the downstream inner wall at high inlet velocity will lead to local extinction and flame blow-off. Additionally, the coupling effects of heat transfer, flow field and chemical reaction on the flame stability were investigated by changing the pressure, thermal conductivity of solid matrial and channel gap. The results indicate that the flame blow-off limit increases at first and then decreases with the increasing pressure, and this is mainly because that the anchoring ability of flame root is larger and shear stress on the flame front is smaller at a moderate pressure. However, the flame blow-off limit decreases firstly and then increases with an increasing thermal conductivity of solid material, which is the same as the changing rule of the distance between the flame front and transition point. The channel gap also has a significant effect on the flame blow-off limit. The heat recirculation effect is better and heat loss from the flame root is samller at a bigger channel gap, which lead to a larger flame blow-off limit.In the end, a mesoscale combustor with plane flame holder and preheating channels was developed to make full advantages of both the flow recirculation zone and heat recirculation effect. Results indicate that this special structure can significantly improve the flame stability. The flame stability in the combustor made of copper, stainless steel and SiC is mainly determined by the heat recirculation effect which is obviously affected by the thermal conductivity of the solid material, and it can be got that a bigger thermal conductivity is good for the flame stability in this kind of combustor. Additionally, the results show that the flame blow-off limit increases firstly and then decreases with the decreasing length of flame holder, which results from the coupling effects of the heat recirculation and flow field. The combustor with a moderate length of flame holder has a larger recircualtion zone behind the flame holder, a better heat recirculation effect and a samller shear stress on the flame front, which are beneficial for the flame stability.In summary, the flame stability in micro-or mesoscale combustor is mainly determined by the interactions between the flow field (such as recirculation zone and low velocity zone), heat transfer (such as heat loss and preheating effect), species transport (such as preferential diffusion effect) and combustion chemical reactions, and importance of the above four aspects on the flame stability may be different for the different structure and solid material type. The special structure proposed in present paper can significantly improve the micro-or mesoscale flame stability, which are beneficial for the improvement of relevant combustion theory and the development of micro-mesoscale combustor.