A Study On Flame Characteristics Of Non-premixed Combustion In Planar Micro-combustors

As the energy densities of hydrogen and hydrocarbon fuels are dozens of times higher than that of chemical batteries,there is great potential for micro power systems based on combustion to become the power source of micro devices in the future.However,there are also some special challenges in micro-combustion,such as short gas residence time,slow gas mixing caused by laminar flow,enhanced heat loss effect and radical quenching effect,which may cause flame instabilities and low combustion efficiency.Thus,it is of great theoretical significance and application value to study micro-combustion characteristics and flame stabilization methods.Non-premixed combustion has more advantages than premixed combustion in device miniaturization and safety.But at present,the research on non-premixed combustion in micro scale is far from enough,especially on non-premixed flame stabilization strategies.In the study of micro-combustion,combustion characteristics that are difficult to measure in experiments can be quantitatively analyzed by numerical simulations,so as to reveal the internal mechanisms leading to special flame characteristics and study the influences of various parameters systematically.Therefore,in the present thesis,the characteristics of non-premixed flame in planar micro combustors were investigated numerically and the effects of operation parameters,structural parameters and material properties were discussed.Meanwhile,the underlying mechanisms of the slotted bluff-body,porous media and catalytic wall on non-premixed flame stabilities were analyzed.The main findings are as follows:（1）In a planar micro combustor,the combustion efficiency is mainly affected by gas mixing,which primarily depends on molecular diffusion.Thus it is higher when the inlet velocity and the channel height are smaller.In addition,the flame blow-off limit is affected by gas mixing performance and the low velocity region downstream the splitter,which is decreased with the increase of channel height.Moreover,the flame blow-out limit is affected by the coupled influences of gas mixing and heat loss ratio,leading to a non-monotonic variation with the increase of channel height.Specially,when the channel height is 0.6 mm,1.0 mm and 1.4 mm,the flame blow-out limit is 11.1 m/s,17.4 m/s and 12.0 m/s,respectively.（2）In the planar micro combustor with a slotted bluff body,gas mixing is improved greatly by turbulent effect induced by the slotted bluff-body.With the increase of blockage ratio,the recirculation intensity is increased and gas mixing is improved,which is benefitcial to the flame stability.However,a much larger blockage ratio causing excessive stretch effect on flame,which may lead to local extinction and flame splitting.Thus,the combustor with a medium blockage ratio of the bluff-body can gain a tradeoff between the positive and negative aspects and achieve a largest blow-off limit.When the blockage ratio is 0.45,0.55and 0.64,the blow-off limit is 17.5 m/s,34 m/s and 32m/s,respectively.（3）In the micro combustor filled with porous media,gas mixing becomes greatly better due to the mass dispersion effect of porous media and thus the impact of channel height on gas mixing is weakened.The flame is located at upstream channel in the combustor filled with porous media compared with that in the combustor without porous media,indicating a wider flammable range.In addition,with the increase of channel height and nominal equivalence ratio,the flammable velocity range is increased.Moreover,the lower and upper velocity limits decrease with the increase of porous media porosity.（4）In the planar micro combustor with catalytic walls,stable CH₄/air combustion can occur in the channel whose characteristic scale is much smaller than the quenching diameter of CH₄/air gas-phase reactions,but the heterogeneous reactions play a major role in fuel conversion and heat release.Catalytic reactions mainly occur on the upper catalytic wall（fuel inlet side）.In addition,as the channel height increases,the maximum temperature and combustion efficiency increase first and then decrease.Moreover,both fuel-rich combustion and oxygen enrichment can significantly improve combustion efficiency.Fuel-rich combustion can increase catalytic reaction intensity and notable catalytic reactions occur on the lower catalytic wall（oxidant inlet side）.Oxygen enrichment can enhance both homogenous and heterogeneous reactions.（5）Inspired by the study on catalytic combustor above,an improved catalytic combustor with a symmetrical structure was proposed.In the symmetrical catalytic combustor,the inlet is divided into three passages with two splitters.As the upper and lower passages are for CH₄,catalytic walls are in a fuel-rich condition,which can enhance catalytic reaction intensity.Thus,wall temperature and combustion efficiency are significantly improved compared with that of the asymmetrical catalytic combustor.In addition,with the increase of inlet velocity,combustion efficiency decreases,while radiant energy output and radiant efficiency from outer wall surfaces increase first and then decrease.Moreover,fuel-lean combustion has negative impacts on catalytic reactions.When the nominal equivalence ratio is 0.9,excess oxygen in midstream combustor diffuses to the inner wall surfaces from channel central region.Thus the local equivalence ratio near catalytic walls decreases,resulting in a sudden increase in O（s）surface coverage and a precipitous decrease in catalytic reaction intensity on catalytic walls.Finally,the variation of thermal conductivity and emissivity of wall material has few effects on combustion efficiency but affects radiation efficiency significantly.As wall thermal conductivity increases,radiation efficiency increases first and then decreases.With the increase of wall emissivity,outer wall temperature decreases significantly,while the radiation efficiency increases.In conclusion,the flame stability of micro non-premixed combustion is affected by the combined effects of many factors.The variation of parameters may cause non-monotonic changes in flammable limit,combustion efficiency and thermal efficiency.Numerical results in this thesis demonstrated that using the slotted bluff-body,porous media and catalytic wall surface can improve the flame stability of non-premixed combustion through different physical and chemical mechanisms.The present thesis can enrich the understandings of non-premixed combustion in micro scale and guide the design and operation of non-premixed combustors.