Numerical Researching Of Combustion Process Of Micro HCCI Free-Piston Engine

With the development of nanometer,Power MEMS(Micro Electro Mechanical System) has appeared.It has small volume but high energy-flux density.Power MEMS is a very new technique which has military,civil value and a widely outlook of application. With the development of MEMS,micro combustion is studied to meet the demand of the portable electronic equipment and the need of efficient drive andelectric power for the micro system in military field.Based the internal and overseas investigation of micro engine,the micro HCCI free-piston engine was researched.The structure of free-Piston Engine is simple,which is better suited to the development for micro.The method combinated HCCI(Homogeneous Charge Compress Ignition) with micro free-piston engine can not only avoid flame quenching near the combustion chamber wall,but also solve the problem of ignition control of HCCI at the same time.Moreover,the efficiency of the energy of micro free-piston engine conversing to power is high.Therefore,the research based on HCCI combustion micro-free piston engine are expected to exceed restrictions by the smaller Combustor,and impact on the development of Power-Micro Electro Mechanical greatly.(1) The developed single-zone model of operation process of the micro HCCI free-piston engine contains compression and ignition model,fixed-volume batch reactor model and expansion model.The paper coupled the detailed chemical kinetic model of propane with numerical models of operation process to simulate the operation process of the micro HCCI free-piston engine.The characteristic of ignition and combustion of the micro HCCI free-piston engine were obtained.The results contain the determination of igniting time,combustion pressure,kinetic characteristic of free-piston,changeable of temperature,concentration of intermediate mass and the product.(2) By coupling the detailed chemical kinetic model of propane and the numerical models of operation process,the operation process of the micro HCCI free-piston engine was simulated.The characteristic of ignition and the combustion of the micro HCCI free-piston engine were obtained.The results contain the determination of igniting time, combustion pressure,kinetic characteristic of free-piston,changeable of temperature, concentration of intermediate mass and the product.For the work characteristics of the micro HCCI free-piston engine,the two-dimensional models of the engine combustion under ideal conditions(adiabatic,no leakage),and considering the influence of leakage and heat transfer respectively are established.According to relationship between the pressure on the piston and its velocity,the moving mesh procedures for simulating the piston work process is established to achieve the coupling gas chemical kinetics with the kinematics of piston.In this paper,the author make use of the means of the chemical reaction coupled with the flow computing to model the whole working process of the HCCI free-piston engine single shot,using the software of STAR/Kinetics.(3) By using the method of varying the influencing parameters,the change of igniting time and combustion characteristic were obtained under the conditions of different dimensions,different initial temperature and pressure of fuel,different initial equivalence ratio,different initial piston velocities(compression ratio) and different stroke of the micro HCCI free-piston engine.Further,the results provided the theory for controlling the combustion process.The influence of heat transfer and Leakage for the micro HCCI free-piston engine single shot process is researched respectively by the model contained the leakage or heat transfer,and got the conclusions that heat transfer impact on the entire operation process relatively small,and the influence of leakage is relatively larger.(4) Finally,the combustion characteristic and dynamic performance of the micro HCCI free-piston engine operation process were discussed,the results provided the theory for development the micro HCCI free-piston engine.(5) Catalytic combustion of hydrogen/air mixture inside micro-tube was numerically investigated by using the commercial fluid dynamics code FLUENT coupled with the chemical reaction dynamics software CHEMKIN.Combustion characteristics for different reaction models and the influence of inlet velocity,equivalence ratio of hydrogen/air mixture,tube diameters,conducting wall and wall material on surface catalytic combustion reaction were discussed.The Computational results indicate that the surface catalytic combustion restrains the gas phase combustion.Inlet velocity,equivalence ratio of hydrogen/air mixture and tube diameters has important influence on catalytic combustion of hydrogen.For smaller inlet velocities,reaction is dominated by the gas phase combustion.As the inlet velocity increases,the existence of the surface catalytic combustion and the gas phase combustion has two controlling factors on combustion.For bigger inlet velocities,reaction is dominated by the surface catalytic combustion.The temperature of the flame core decreases with the decrease of tube diameter.Equivalence ratio of hydrogen/air mixture is the important influence factor of hydrogen conversion rate. The OH concentration shifts upstream and becomes weaker with the decrease of tube diameter.The conducting wall in micro-tube preheats the inlet mixture by axial heat transferring.The existence of conducting wall will help to reach complete reaction in the micro-tube.The higher wall temperature gradient for low wall thermal conductivity will promote the gas phase combustion shift upstream and will result in a higher temperature distribution.Numerical simulations of catalytic combustion of methane inside micro-tube whose surface has been coated with platinum or rhodium catalyst are conducted at low intake gas pressures.The influences of the kind of catalysts on catalytic combustion process were discussed.(6) The effects of catalytic combustion on the ignition timing of the micro HCCI free-piston engine is analyzed and a comparative study is carried out to clarify the influences of intake temperature,compression ratio and equivalence ratio on combustion process of the micro HCCI free-piston engine with and without catalytic coatings.