Combustion Characteristics Of Premixed Hydrogen/Air In Micro Catalytic Combustors

The micro power system has the advantages of low cost and high energy density.Now the system has wide application prospects.The development of the micro-combustor technology has an important influence on the improvement of the overall performance of the micro-power system since the micro-combustor is the core component of the micro-power system.As the characteristic size of the combustion chamber decreases,the face-to-face ratio of the combustion chamber increases,so that heat loss increases,and free radicals are more likely to be deactivated on the wall surface,resulting in insufficient and unstable combustion.However,catalytic combustion,due to its advantages of low pollution,high combustion efficiency,wide flammable range and stable combustion,can effectively solve the problems of micro combustion process.The Hydrogen/Air premixed combustion process in the micro catalytic combustor was studied based on the analysis of the research status at home and abroad.On aspect of experiments,the visual micro catalytic combustor was processed firstly.Secondly,the transformation process of the reaction type in the micro catalytic combustor was observed.Finally,the influence of the equivalent ratio,channel height and mixed gas flow rate on the reaction type transition process in the combustor was analyzed.On aspect of simulation,the numerical simulation model of the combustion process in the micro catalytic combustor was established and the experimental verification was carried out.The effects of the inlet velocity and the equivalent ratio on the combustion characteristics were calculated.The main contents and achievements of this paper were as follows:（1）Premixed combustion of Hydrogen/Air in a rectangular micro-catalytic combustor was investigated experimentally.The distribution of OH radicals in the combustor was observed by Plane Laser-Induced Fluorescence（PLIF）.The effects of combustor height and mixed gas flow rate on the transition characteristics of reaction type in the combustor were analyzed and the results demonstrated that the reaction type would transform with the change of equivalence ratio between the coupled hetero-/homogeneous reaction and the pure heterogeneous reaction.The coupled hetero-/homogeneous reaction was initiated firstly in the catalytic combustor,when the equivalence ratio decreased to the critical equivalence ratioФ_A,the flame was quenched and the heterogeneous reaction was still sustained.Then the flame was self-ignited and meant the pure heterogeneous reaction would transform into the coupled hetero-/homogeneous reaction when the equivalence ratio increased to the critical equivalence ratioФ_B.In addition,Ф_A was always less thanФ_B.The wall temperature of the pure heterogeneous reaction was always lower than that of the coupled hetero-/homogeneous reaction under the same conditions.When the height of the combustor increased,the critical equivalence ratio of the coupled hetero-/homogeneous reaction transformed into the pure heterogeneous reaction decreased.On the other hand the critical equivalence ratio of the pure heterogeneous reaction transformed into the coupled hetero-/homogeneous reaction increased.The critical equivalence ratio of the coupled hetero-/homogeneous reaction transformed into the pure heterogeneous reaction decreased with the increase of mixed gas flow rate,so as the critical equivalence ratio of the pure heterogeneous reaction transform into the coupled hetero-/homogeneous reaction.（2）The combustion characteristics of different types of reactions in the micro catalytic combustor were analyzed based on the experiments.The results showed that the wall temperature difference increased with the increase of the equivalent ratio when the coupled hetero-/homogeneous reaction maintained in the combustor,while the wall temperature difference decreased with the increase of the equivalent ratio when the pure heterogeneous reaction maintained in the combustor.The variation rule of components in different types of reactions in micro catalytic combustor was analyzed by numerical simulation.The decrease rate of the H₂ mass fraction near the wall when the pure heterogeneous reaction maintained in the combustor was greater than that of the coupled hetero-/homogeneous reaction in the combustor,while the decrease rate of the H₂ mass fraction at the center of the channel when the pure heterogeneous reaction maintained in the combustor was less than that of the coupled hetero-/homogeneous reaction in the combustor.With the increase of the equivalent ratio,the homogeneous reaction intensity became more intense and the effects on the heterogeneous reaction became more obvious when the coupled hetero-/homogeneous reaction maintained in the combustor.The reaction area of the pure heterogeneous reaction in the combustor increased with the increase of the equivalence ratio.And the reaction region of the pure heterogeneous reaction was much greater than that of the coupled hetero-/homogeneous reaction.（3）The influence of mixed gas velocity on the combustion characteristics of micro catalytic combustor was obtained by experiment and numerical simulation.The results showed that the equivalence ratio is more sensitive to the transition characteristics of the reaction type in the catalytic combustor than the flow rate.When the pure heterogeneous reaction maintained in the combustor,with the increase of the mixture gas inlet velocity,the intense reaction region gradually moves towards the outlet direction and the temperature difference on the outer wall became smaller.When the coupled hetero-/homogeneous reaction maintained in the combustor,with the increase of the mixture gas inlet velocity,the intense reaction region always located at the entrance of the combustor and the temperature difference on the wall of the combustor increased.When the pure heterogeneous reaction maintained in the combustor,the net consumption reaction rate of H₂ was always greater than0,which indicated that H₂ is continuously consumed during the whole reaction process.And the net consumption reaction rate of H₂O was always less than 0,which indicated that H₂O is continuously produced during the whole reaction process.With the increase of the mixture gas inlet velocity,the reaction area of the pure heterogeneous reaction gradually expanded when the pure heterogeneous reaction maintained in the combustor.The combustion process in the micro catalytic combustor was studied in this paper.The critical point of combustion type transformation was defined based on the experiment,and a method to determine the type of reaction in micro catalytic combustor by temperature was proposed.The influence of mixing gas flow rate,equivalence ratio and channel height on combustion characteristics in micro catalytic combustor was systematically discussed.The relevant conclusions are of great significance to enrich the basic research data of micro combustion,and also provide theoretical guidance for the design and optimization of micro catalytic combustor.