Experimental And Numerical Simulation Research On Combustion Process In Micro Free-piston Power Device

The development of Micro Electro Mechanic System (MEMS) brings profound iMPact not only on daily life but also on national military. However, with the development of technology, these devices gradually tend to miniaturization and function integration, and require a power source to have characteristics of small size, high energy density, long life span and quick charge technology. The power supply section has become the bottleneck of MEMS further development. In recent years, due to the advantage of high energy density of hydrocarbon fuels, micro power system based on burning hydrocarbon fuels has aroused public concern of scholars.The combustion condition and environment change with the rapidly reducing in size of micro power device, residence time of mixture gas in micro scale space is shortened and the reaction time increases, resulting in combustion instability. It also will cause flame quenching phenomenon, and heat transfer loss will happen because of large area to volume ratio, it will reduce the energy conversion efficiency of micro power device. For micro combustion difficulties and challenges, micro free-piston power device is selected as the research object in this paper. Mainly because that homogeneous charge compression ignition (HCCI) combustion mode is used in micro free-piston power device, the combustion is rapid and without flame spread phenomenon. The compression ratio is variable, and this power device is easy to miniaturization because of its simple structure. At the same time, the micro combustion problems mentioned above can effectively be resolved through preheating and catalytic method.At present, research about micro free-piston power device is still in the initial theoretical research stage, and there is no mature theory for reference, so this paper firstly focuses on the research of micro free-piston power device core part, micro compression combustion process. By establishing single compression combustion model, numerical simulation and visualization experimental study on basic theory research were carried out, and some achievements with academic significance and practical value have been acquired: Firstly, the micro free-piston power device is conceptual designed, in order to improve its combustion stability and power output performance. The intake pipe is arranged in the exhaust pipe, using the exhaust gas heat to preheat the intake pipe, aiming to increase the initial temperature of homogeneous gas. Air sweeping plate and exhaust gas chamber are designed, and the special structures can make the burned gas exhaust much more effectively, more fresh homogeneous gas into micro combustion chamber by utilizing pressure difference. Adding catalyst coating at the bottom of micro combustion chamber, micro combustion will be more complete and stable and the combustion limit could be broaden through HCCI catalytic combustion, and catalytic combustion can also expand the scope of fuel application.A visual experimental platform was set up according to the single compression combustion process, and the pictures of micro compression combustion process were captured by means of high speed digital camera, the result proved that homogeneous gas could be compressed combustion in micro space. According to the analysis of experimental images, four typical types of HCCI combustion process in micro-combustor were defined: compression process without ignition, critical compression ignition process, complete compression combustion process and high compression ratio combustion process. Variable parameter experiments were carried out under different conditions of piston initial velocities, piston mass and micro combustion chamber length, and the experimental results show that piston initial velocity is an important factor deciding whether the homogeneous gas could be compressed ignition or not. Heavier the piston is, stronger the combustion is, and longer the micro combustion chamber is, more difficult to be compressed ignition when other conditions remain unchanged. And a conclusion could be obtained through a large number of experimental data that the homogeneous gas could not be compressed ignition until the compression ratio is greater than18when using dimethyl ether.Mathematical model of single compression combustion process was established according to experiment principle, coupling the chemical reaction process and the motion of piston, and the micro HCCI combustion process was simulated. CoMParing to the experimental results, the calculation model was verified. Subsequently, variable parameter simulation for single compression combustion process were conducted to analyze the iMPact factors on micro combustion and piston motion characteristics, such as piston compression initial velocity, piston mass, homogeneous gas equivalence ratio, initial temperature and pressure, micro combustion chamber length and diameter, heat transfer model, leakage model, catalytic combustion and so on.Finally, by summing up the calculation results, principle of parameter design of micro free-piston power device was obtained. The gap between micro combustion chamber inner surface and free piston should be less than8μm, improving the sealing performance benefits the power output of the micro power device. Formula of critical compression ignition initial kinetic energy value is obtained by statistical calculation, homogeneous gas could not be compressed combustion completely until the compression initial kinetic energy obtained by piston is greater than critical compression ignition initial kinetic energy, which has a certain guiding significance to the study on start-up condition of micro free-piston power device. The piston mass and compression initial velocity selection principle was analyzed, the piston with great mass and low compression initial velocity suits for low speed engine, and the piston with light mass and high compression initial velocity suits for high speed engine under the same condition of compression initial kinetic energy, but the micro combustion is not stable when the working frequency is too high. The structure design of micro combustion chamber was also studied, and the results show that slender combustion chamber is more suitable for micro free-piston power device when other parameters are the same.This work could enrich the micro combustion theory and also could be the basic theories for designing the micro free-piston power device, and all the research result could also be applied to other micro power systems.