Simulation On Transient Ignition Of High-metal Magnesium-based Fuel Water Ramjet

The water ramjet is a new kind of propulsion system with high energy density.The increase of the content of metal in the fuel is beneficial to enhance the steady state of engine performance,but also put forward more challenges for the ignition start-up process.There are few theoretical researches on ignition transient of water ramjet with high metal content at present.The cost of engine test is too high and success rate of ignition is low.For this status,this paper studied on magnesium based water ramjet whose mass fraction of magnesium is 73%.By combining theoretical analysis and numerical simulation methods,ignition transient process and performance of water ramjet with high metal content was studied in this paper.Decomposition characteristics and the structure of components in different stages of ignition process of water ramjet is studied based on present test results and thermodynamic software analysis.Piecewise self-sustaining combustion model responding to surface temperature of ignition process is established using solid phase ignition theory、Arrhenius equation and energy conservation law.Compared with test results,the rationality of the mathematical model is verified.The influence of magnesium/water reaction on self-sustaining combustion is studied based on that model.The increase of water inflow can help to raise equilibrium surface temperature of engine during self-sustaining combustion.At the same time,the need for energy increases,which is released by gunpowder during initial stage of igniting start-up.When the water inflow is given,energy released by gunpowder has a minimum value to maintain self-sustaining combustion of engine after flooding.Multiphase turbulent flow calculation model of water ramjet during ignition process and simulation model of ignition engine is established.Combined with self-sustaining combustion model established in the second chapter,a coupled calculation method based on commercial CFD software for ignition heat transfer/ fuel self-sustaining combustion multiphase flow combustion of flow field is realized through programming user-defined functions.We compared the results with experimental results,and analyzed the basic characteristics of parameter distribution of flow field in engine combustion chamber after ignition.The performance evaluation index of water ramjet ignition is established,and according to ignition timing of water ramjet,analyze different types of powder when there is no water、flow characteristics of ignition transient process and the influence on ignition performance of the engine.An optimal design case of powder type and igniter quantity is proposed,and based on this,we analyzed the characteristics flow field and the influence on ignition performance under the moment and amount of the first water injection.The results showed that,compared with B/KN03 ignition composition,the average equilibrium surface temperature of the black gunpowder is lower,but it still meets the condition for fuel burning.Ignition delay time is short during ignition process.The igniter quantity does not much affect the equilibrium surface temperature of fuel,but when the igniter quantity is too much,the peak pressure during ignition may exceed the engine load.When we use black powder whose quantity is 40 g as ignition powder,after working for 100 ms,we can find that the injection time of water has little effect on state parameters of fuel combustion equilibrium state.But when the time of the first water injection is before peak pressure of ignition,peak pressure of ignition will be larger and the time takes for equilibrium state will be less.According to the cases in this paper,magnesium/water reaction has little influence on equilibrium surface temperature.When the inflow gets larger,mass fraction of magnesium vapor is lesser,and equilibrium pressure of combustion chamber is lesser.The research results in this paper will promote the development of the application technology of water ramjet with magnesium based fuel.It is of significance in theoretic research and engineering practice for the future research on power system of high speed underwater weapon.