Combustion Mechanism Of Hydrocarbon Continuous Rotating Detonation

For the engineering application of hydrocarbon continuous rotating detonation(CRD)engine,the combustion mechanism of hydrocarbon CRD is investigated through the combination of experimental research,numerical simulation and theoretical analysis in this thesis.The enriched understanding of combustion mechanism can provide the theoretical guideline and technical support for the practical application of CRD.Firstly,the contribution of the hollow chamber to the combustion organization of hydrocarbon CRD is verified.In the laboratory-scale hollow chamber,ethylene-air and methane-air CRD are achieved with high propagation performance.Based on the abundant experimental data,the time difference calculation method,integral of chemiluminescence intensity,and axial distribution of chemiluminescence intensity ratio are proposed for quantitative analyses on the reconstruction of CRD flowfield,the propagation characteristics of reaction zone,and the distribution of heat release,respectively.With the time difference calculation method,the angle of upstream oblique shock wave is defined for representing the influence of CRD on the air inflow.The increases of equivalence ratio and contraction ratio make the angle of upstream oblique shock wave decrease,indicating that the impact on air inflow strengthens.Through the integral of chemiluminescence intensity,the propagation frequency of reaction zone obtained by high-speed photography matches that of pressure wave obtained by PCB sensors,indicating that the reaction zone is coupled with the pressure wave.Among the three fuels,the combustion of hydrogen is finished quickly within the shortest distance;and combustion of ethylene follows;and the combustion of methane is finished relatively slowly within the longest distance.Secondly,the promotion mechanism of the hollow chamber to the combustion organization of CRD is clarified.Combustor width plays a key role in the realization of hydrocarbon CRD,which is verified directly through optical observation.In the hollow chamber,the expansion effect of CRD wave in the combustor center increases rapidly,and thus wave front is bended distinctly in the axial cross section making the length of wave prolong.It is exactly suitable to satisfy the combustor width criterion.The deflagration in the upstream annulus combined with the deflagration prior to CRD wave in the hollow chamber part is supposed to be parasitic combustion.The excessively parasitic combustion destroys the accumulation of combustible mixture,which is negative for the combustion organization of CRD.Therefore,it is suggested that CRD should be quickly initiated within the short distance to reduce the impact of parasitic combustion.A re-circulation zone is formed in the forepart of the hollow chamber,in which the high-temperature combustion products are aggregated.There is an added circumferential contact surface between deflagration flame in the re-circulation zone and combustible mixture,which can pre-heat the combustible mixture appropriately in advance and effectively reduce the inducing time and length,through the exchange of matter and heat.The added circumferential contact surface makes the detonability of combustible mixture enhance distinctly,which is a key factor of enhancement mechanism to the combustion organization of CRD.Thirdly,the combustion characteristics of hydrocarbon CRD around the boundary of operating range are investigated.The essence of sawtooth wave is deflagration coupled with weak pressure wave,which is the critical mode between CRD and deflagration.The time occupying proportion of pressure rise in a cycle and the increasing rate of pressure rise are proposed to distinguish the mode.For sawtooth wave,the time occupying proportion of pressure rise in a cycle is distincly higher,and the rate of pressure rise is evidently lower,which are different from these of CRD wave.The reaction zone of sawtooth wave is stabilized without periodic rotating motion.Through the combination of the integral of chemiluminescence intensity and high-frequency pressure,the transformation processes between CRD and sawtooth wave are captured.For sawtooth wave,the pressure wave and combustion wave are decoupled,which is verified through the short-time fourier transform.The distinction between CRD and sawtooth wave is observed directly through the especial combustor inlet-optically accessible combustor.Compared with the intrinsic frequency of combustor,the frequencies of CRD wave and sawtooth wave both show a frequency agreement with the first tangential mode.The manifestation of high-frequency tangential instability in detonation mode is CRD wave.The manifestation of high-frequency tangential instability in deflagration mode is sawtooth wave.Lastly,the annular cavity-based combustor is proposed,and the impacts of cavity length-to-depth ratio and cavity location are investigated.When the cavity is arranged in the zone of detonation combustion,the cavity-stabilized flame is coordinated with the reaction zone of detonation,providing intensified combustion organization.With the intensified impact,CRD waves propagate in co-rotating two-wave mode with higher outlet pressure,higher frequency,and higher stability.With the proper cavity,the combustor can obtain an additional specific impulse increase of approximate 10%.Length-to-depth ratio is a key parameter of cavity,and three operating zones,I:Excessive Impact(deflagration,sawtooth wave);II: Proper Impact(single wave,co-rotating two-wave);III: Deficient Impact(counter-rotating two-waves).The impact of cavity to CRD combustion organization decreases with the cavity moving downstream.When cavity is downstream arranged out the detonation zone,the majority of detonation concentrates on the annular part.The cavity-stabilized flame is separated from the reaction zone of detonation,and the impact of cavity cannot reach the reaction zone of detonation in the upstream annular part.