| Powdered fuel ramjet, which holds the advantages of both the liquid fuel ramjet and the solid ducted rocket ramjet, has an excellent theoretical performance. However, mechanisms of the combustion in the powdered fuel ramjet are still unclear, combustion optimization design of the engine is very difficult and the combustion efficiency of the engine is very low due to the particularity of the powdered fuel and the complexity of the gas-solid two-phase flow combustion. Therefore, to improve the performance of powdered fuel ramjet, the combustion characteristics of the particle cloud should be investigated further in details.Base on the thermal explosion theory, the analytical model of both single magnesium particle ignition and pulverized magnesium particle cloud ignition have been established; the ignition processes of both single magnesium particle and pulverized magnesium particle cloud have been analyzed. The result shows that both of the processes are different from each other significantly. The ignition criterion for single magnesium particle has been established under different conditions, but the unambiguous formula of ignition criterion for the pulverized magnesium particle cloud can’t be proposed by analyzing the two models. Furthermore, ignition criteria of magnesium particle cloud under different conditions have been summarized. And the scopes of the applicability, advantages and disadvantages for these criteria have also been analyzed. Finally, the criterion suitable for the current study has been proposed.Focusing on the reliable ignition technique of powdered fuel ramjet, both zero-dimensional unsteady ignition model of pulverized magnesium particle cloud and one-dimensional unsteady ignition model of pulverized magnesium particle cloud with finite influencing sphere have been established based on the analysis of ignition mechanism of pulverized magnesium particle cloud. With the lumped parameter approach, the details of each single particle ignition and the coupled interaction between particle and gas phase have been included in the zero-dimensional ignition model, from which variations of both solid parameters and gas phase parameters over time have been obtained. Based on the idea of limited interaction between particles and gas, particle cloud ignition characteristics are related with that of finite influencing sphere in the one-dimensional model. This model not only takes the detailed particle ignition process into account, but also considers the diffusive process of gaseous species and condensed Mg O, condensed Mg O movement along with gas flow and heat transfer in gas phase. Furthermore, particle cloud ignition process has been simulated with the model, and distributions of the gas temperature and species concentration in the sphere have also been obtained during ignition period. Both models have been applied to simulate the ignition process of pulverized magnesium particle cloud in blackbody radiation environment. The transition phenomena of both particle and gas phase temperature have been found, and the variations of both particle parameters and gas parameters with time have been obtained during the ignition process of pulverized magnesium particle cloud. The effects of particle concentration, particle diameter, initial particle cloud temperature, oxygen concentration, environment pressure and radiation source temperature in the ignition process of pulverized magnesium particle cloud have been analyzed numerically. Computational results have a good agreement with the corresponding experimental data in literatures indicates the validity of the models.On the background of the phenomenon of non-uniform distribution of fuel particles in the combustion chamber of powdered fuel ramjet, ignition and combustion processes of pulverized magnesium in local region with dense particles have been analyzed, and a one-dimensional unsteady ignition and combustion model of magnesium spherical particle cloud has been proposed. In addition to the detail of the combustion processes for each magnesium particle and gas phase thermodynamics, this model also takes into account the attenuation of radiation intensity when passing through the particle cloud. Then ignition and combustion process of the pulverized magnesium particle cloud has been simulated numerically with the proposed model. The spatial and temporal behaviors of ignition-combustion processes, gas phase temperature and composition variations have been obtained. "Double flame peak" phenomenon during the ignition and combustion process of pulverized magnesium spherical particle cloud has been observed in the study. And the inner and outer flame propagation rules and the combustion characteristics have been achieved. Influences of various parameters, such as the particle concentration, particle diameter, oxygen concentration, initial temperature of the spherical particle cloud, environment temperature, temperature of radiation source and environment pressure on the ignition-combustion process have also been investigated numerically. The numerical results have been validated by the experimental data from literatures.Focusing on the self-sustained steady combustion technique for the powdered fuel ramjet, numerical and experimental researches on a one-dimensional laminar combustion of the pulverized magnesium particle cloud have been carried out. The one-dimensional premixed laminar combustion model for the pulverized magnesium particle cloud, which is an universal unsteady model for ignition from different ends, has been proposed. In this model, not only the details of the combustion processes for each magnesium particle and the coupling between the gas phase and the solid phase are considered, but also the radiative heat exchange between the flame and the unburned pulverized magnesium particle cloud is described in detail. The flame propagating process is obtained by solving governing equations of both the gas phase and the solid phase directly and accurately. The relevant experimental platform for investigating the laminar premixed flame propagating process of the pulverized magnesium particle cloud has been built. Premixed laminar combustion processes for many other kinds of pulverized solid fuel particle cloud can also be studied with the platform. With numerical and experimental investigations of the laminar flame propagating process of the pulverized magnesium particle cloud, the laminar flame propagation mechanisms and the fine structure of the flames have been obtained for ignition occurring at either the open end or the closed end. And the influencing factors, such as the particle concentration, the particle size, the oxygen concentration, the initial temperature of the pulverized magnesium particle cloud, the environmental pressure and so on, have been analyzed. The influence of different ignition ends in the flame propagation of the pulverized magnesium particle cloud has been compared. The accordance between the experimental and the numerical results indicates the validity of the proposed model. |
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