Study On Premixed Flame And Stability Of Engine Combustion Process Fueled With Shale Gas

Shale gas is a kind of unconventional gas existing in the dense fine clastic rocks, with the main component of CH4, and it is a promising alternative fuel for engines. There are abundant shale gas resources in China, but the compositions of shale gas in different places are greatly different, thus affecting the premixed combustion rate, leading to different combustion processes of engine, and restricting the promotion and application of shale gas in the engines. Therefore, it is necessary to conduct researches around the influence of shale gas components on the stability of the premixed flame and engine combustion process.The thesis is divided into 7 chapters, mainly conducting researches around the premixed flame of shale gas, shale gas engine combustion process and the changing rule of the cyclic variations, and the improvement measures to the engine combustion stability, etc. The method of constant volume combustion bomb test combined with numerical simulation is adopted to measure the propagation speed of the premixed flame of shale gas, and study the influence of free radical in reaction on the propagation process of the flame front surface. Through engine bench test, the thesis discusses the influence of shale gas components on the engine combustion process, analyzes the relationship among the combustion characteristic parameters, studies the dynamic characteristics of engine combustion cyclic variations with the methods of phase space reconstruction, poincare mapping and return mapping, and explores the change rules of the stability of engine combustion process along with the ignition moment, the number of spark plugs, and the HHO content.The constant volume combustion bomb test system is adopted to study the propagation speed of shale gas’ s premixed flame, analyze the stability of the flame front surface, and discuss the relationship between the premixed combustion speed and the stability of the flame; the change law of substance concentration is studied by the flame burning velocity model, and the CH4 consumption process in shale gas is studied using generating rate and sensitivity analysis method, to analyze the influence of adiabatic flame temperature on chemical reaction rate. Studies have shown that the influence of initial temperature on the chemical reaction rate is small, and the increase of H+OH mole fraction peak is the main cause of faster premixed combustion; when the initial pressure increases, the chemical reaction rate increases significantly, leading to the reduced concentration of H, O, and OH bases, the slowing of flame propagation speed, and the decline of flame front surface stability; when the equivalent ratio is 1.0, the propagation speed of premixed flame is the fastest, the inhibition of weak mixture on flame propagation velocity is stronger than the rich mixture, and the flame front surface becomes gradually stabile with the increase of equivalent ratio; R1 reaction has the greatest influence on CH4 consumption rate, CH3 base is one of the important free radicals to evaluate CH4 consumption rate, and with the increment of CH4 content, the CH3 concentration is reduced, and the consumption rate of CH4 is lowered, causing the flame propagation speed decreased; the inert gas inhibits the adiabatic flame temperature, chemical reaction rate decreases, flame propagation speed is reduced, the stability of flame front surface decreases, and the inhibitory effect of CO2 on premixed combustion rate is stronger than N2.With the method of premixed inlet, the single cylinder test engine is transformed into shale gas engine. Then through the engine bench test, the influences of different gas compositions on cylinder pressure and heat release process are studied, the cyclical fluctuation law of pressure in the cylinder is analyzed, and the relationship between the crank angle corresponded with the maximum explosion pressure and the maximum explosion pressure and mean indicated pressure is explored; and through the fluctuations of pressure at the various crank corners, the influences of engine on cyclic variations under different working stages are studied. Research has shown that, with the increase of engine speed, the stability of the combustion process decreases; when the CH4 content increases, the maximum explosion pressure drops and the cyclical variation of corresponding crank angle; such inert gases as N2 and CO2 inhibit the combustion velocity in the cylinder and the engine power performance declines, and with the increase of contents of such inert gases as N2 and CO2 in the shale gas, the cyclic variations of combustion increase, the relevance between the maximum explosion pressure and the corresponding crank angle decreases, the partial combustion cycles and misfiring cycles increase and the working stability of engine declines; the combustion phase is the main cause of cyclic variation production in shale gas engine, the influence on the cyclic variations is large in inlet stage, and the pressure fluctuation inside the cylinder is small in exhaust stage.Based on the phase space reconstruction theory, the thesis analyzes the motion law of the phase space trajectory, investigates the influence of cyclic variations on the phase space trajectory, reduces the dimensions of phase space and studies the distribution law of phase space trajectory and intersection points of poincare section using the method of poincare mapping, then analyzes the change law of the certainty of combustion process through the return mapping of combustion characteristic parameters, and explores the influence of gas composition on the correlation dimension, maximum Lyapunov index and other chaotic characteristic parameters. The research has shown that, during the combustion process of shale gas engine, the similar quasi-periodic state is reflected in the pressure inside cylinder during the whole phase space, and with the increase of CH4, N2 and CO2 contents in the shale gas, the circular arc section trajectory gradually disperses, and the randomness of the engine working process is enhanced; the poincare mapping points of trajectory in phase space in ∑XY+ cross section are in the monotone increasing distribution, and when the inert gas content increases, the linear relationship among scattered points is gradually enhanced, and the intersections between trajectory and ∑XY- and ∑XZ- cross section are distributed in monotone decreasing; the return mapping points of combustion characteristic parameters are distributed in reunion, and when the N2 and CO2 contents increase, some scattered points are distributed in the left and lower sides of the dense regions of mapping points, as some combustion cycle or misfiring cycle; some chaos characteristics are presented in the shale gas engine working process, and with the increase of combustion cyclic variations, the correlation dimension and maximum Lyapunov index increase, and the chaotic characteristic of engine’s working process becomes obvious.Through shale gas engine bench test, the thesis analyzes the influence of ignition location and ignition time on the combustion process, discusses the change law of the engine’s working process stability with the ignition advance angle, studies the influences of synchronization double-point and asynchronous double-point on the combustion process and the cyclic variations, analyzes the nonlinear dynamic characteristics of cyclic variations, and explores the influence of dual-spark plug on the propagation process of flame inside the cylinder. Research has shown that, when the ignition position is relatively close from the center of the combustion chamber, the change of flame development period is small, the rapid combustion period is shortened, the combustion rate increases, the maximum explosion pressure rises, and the corresponding crank angle is advanced; with the decrease of the ignition advance angle, the combustion of part shale gas is postponed to the expanding and acting period, the turbulent kinetic energy in cylinder is reduced, the flame propagation speed is reduced, the maximum explosion pressure and the peak heat release rate drop, the premix time is prolonged leading to the flame development period shortened, the cyclic variations of combustion enlarge, partial combustion and misfiring cycle also increase, the phase space trajectory tracking gradually disperses, and the working stability of engine declines; when the shale gas engine adopts double spark plug, the turbulence intensity inside the cylinder increases obviously, the propagation distance of flame is reduced, the flame surface density increases so that the flame development period and rapid combustion period are shortened, the combustion temperature in cylinder rises, the promotion of synchronization double-point to the flame propagation speed is stronger than that of asynchronous double-point, the cyclic variations of combustion are small, the certainty of the engine working process is strong, and the chaotic characteristic is weakened.The composition characteristics of HHO and the law to distinguish free radicals are analyzed with the gas chromatography- mass spectrometry and electron paramagnetic resonance spectrometer; through bench test, the influence of HHO on the combustion process of shale gas engine is studied, the change rule of cyclic variations is analyzed, the relationship among the combustion characteristic parameters is explored, the distribution law of phase space trajectory in the combustion process is studied, and the changing characteristics of pressure in cylinder under different frequency bands are analyzed; the propagation process of flame in cylinder is analyzed through the numerical simulation method. The research has shown that, HHO contains a certain amount of H2 and O2, oxygen(·O) and hydroxyl(·OH), and with the increase of standing time, the number of free radicals is gradually reduced, and HHO reactivity decreases; during the combustion process of shale gas engine mixed with HHO, the pressure in cylinder is increased, the distribution of pressure rise rate increases, the peak rate of heat release increases, and the flame development period and rapid combustion period are shortened; along with the increase of the content of HHO, the cyclic variations of combustion become small, the stability of the engine’s combustion process increases, the relevance between the maximum explosion pressure and the corresponding crank angle increases, partial combustion cycle disappears, the phase space trajectory distribution gradually becomes intensive, the cyclicity of the engine’s working process increases, and the chaotic characteristic is weakened; when the HHO content increases, the pressure inside the cylinder increases under various frequency bands, the combustion temperature inside the cylinder rises, the flame propagation speed is quickened, the peak of flame surface density rises, and the flame at the bottom of the combustion chamber gradually moves to the left and right sides in the middle of the rapid combustion period.