Study On The Premixed Laminar Combustion Characteristics Of Methane–n-Heptane Mixtures

As a clean alternative fuel,natural gas has been drawing much attention.As to natural gas–diesel dual-fuel engine,the in-cylinder premixed combustion of the natural gas–diesel–air mixtures formed during the combustion delay period has a significant influence on the operation of the engine.At present,the insight to the characteristics for the premixed laminar combustion of the natural gas–diesel mixtures is not sufficient,and therefore a study on the characteristics is necessary to provide strong support for the validation and improvement of the chemical kinetic mechanism of the natural gas–diesel mixtures and the research on the in-cylinder turbulent combustion of the engine.Natural gas and diesel are both complex mixtures,and in this dissertation,methane and n-heptane are selected as the representatives of natural gas and diesel,respectively.Then via the experiments of the outwardly expanding spherical flames and the simulation of the one-dimensional flat flames,the characteristics for premixed laminar combustion of methane–n-heptane mixtures were investigated at different methane contents and initial conditions including equivalence ratio,initial pressure and temperature.Chemical kinetic analysis of the combustion processes of the mixtures was also conducted.In addition,the fractal features of the propagation processes for the spherical flames of the methane–n-heptane mixtures were studied based on the fractal theory.The detailed contents and the main conclusions of the present study are as follows.?1?During the process of the data processing for the methane–n-heptane flames,the Canny edge detection method was applied to the schlieren flame images to derive the edge pixels of the flame fronts,and the value of the flame radius is figured out through the circle fitting method.Both the linear and nonlinear relationships between the stretched flame speed and the stretch rate for the methane–n-heptane flames were investigated.The results show that the nonlinear fitting lines have better agreement with the experimental data points.Then the error for the laminar burning velocity through the spherical flames was mathematically derived.At last,to ensure the accuracy of the modeling of the one-dimensional flat flames of methane–n-heptane mixtures,the number of the grid points is controlled to be above 450 through changing the values of the gradient and curvature of the numerical solution.?2?The laminar burning velocity for the methane–n-heptane mixtures firstly increases and then decreases with the increasing equivalence ratio,and decreases continually when elevating the initial pressure.The increase of the initial temperature will lead to the rise of the laminar burning velocity.Based on the theory for the premixed laminar combustion,the details about the effect of the initial conditions on the laminar burning velocity were explored in terms of the parameters including the adiabatic flame temperature and the overall reaction order.Then the effective Lewis number,the thermal expansion ratio and the flame thickness were computed to analyze the thermodiffusive and hydrodynamic instabilities.The results show that within the ranges of the initial conditions studied,the change in the equivalence ratio will affect both thermodiffusive and hydrodynamic instabilities.When elevating the initial pressure,only the hydrodynamic instability is altered significantly.When the initial temperature increases,slight changes are observed in themodiffusive and hydrodynamic instabilities.?3?The variations of the laminar burning velocity and the flame instability?the Markstein length?for the methane–n-heptane mixtures with the increasing methane content are significantly nonlinear.The laminar burning velocity and the flame instability for the mixtures remain nearly unchanged when the methane content is below a certain value.Only at high methane content the laminar burning velocity and the flame instability start to change remarkably.When the methane content is weighed in terms with the mass fraction or energy fraction of methane in the methane–n-heptane mixtures,better linear relationships are observed between the laminar burning velocity and the flame instability and the methane content.?4?In the flat flames of methane–n-heptane mixtures,the temperature,mole number and flow velocity for the mixtures will increase rapidly after the mixtures run through the flame front,but the density behind the flame front reduces significantly.The amplitudes of changes in these parameters are related to the initial conditions and the methane content.The peak of the heat release rate firstly increases and then decreases with the equivalence ratio,and increases continually with the increasing pressure and temperature.When elevating the methane content,the peak of the heat release rate decreases gradually.The temperature at which the heat release rate reaches its maximum is within the high temperature range.Through the sensitivity analysis of the laminar burning velocity of the methane–n-heptane flames,it is seen that the elementary reactions which significantly affect the laminar burning velocity of the mixtures are the ones related to the small molecules.In the methane–n-heptane flames,peaks of the mole fractions of the main radicals and the reaction rates of the main elementary reactions firstly increase and then decrease with the equivalence ratio.When elevating the initial pressure,the former reduces continually but the later increases gradually.The increase of the initial temperature has a slight influence on the mole fractions of the radicals and the reaction rates of the elementary reactions.However,when elevating the methane content,complex changes are observed in the mole fractions of the radicals and the reaction rates of the elementary reactions.?5?A method for the global reaction path analysis was proposed based on the modeling results of the one-dimensional flat flames.Then the reaction path analysis of the combustion process of methane–n-heptane mixtures was conducted in terms of the reduced chemical mechanisms.It is shown that in methane flames,methane is mainly converted to CH₃,and the reaction between methane and O atom consumes the largest part of methane.There are four channels for the consumption of CH₃,and the most important one is that CH₃ reacts with O atom to form CH₂O,which then is converted to HCO.Finally HCO reacts with species such H atom to produce CO and CO₂.In n-heptane flames,reactions occur between n-heptane and H atom and OH to produce four different isomers of C₇H₁₅.Then through reaction processes such as thermal decomposition,multiple species,the carbon numbers of which range from one to three,are formed.During the oxidation processes of the species which have one or two carbon atoms,the importance of a reaction paths is enhanced compared to the oxidation process of methane.This reaction path is that C₂H₄ is converted to C₂H₃and C₂H₂ through the hydrogen abstraction,and then C₂H₂ reacts with other species to produce HCCO.During the combustion process of the mixture of methane and n-heptane,the existence of methane alters the decomposition processes of the four isomers(C₇H₁₅)produced from n-heptane.In addition,the reaction between C₂H₅ and itself to form C₄H₁₀ is suppressed,but the production of C₃H₈ from the reaction between C₂H₅ and CH₃ is promoted.?6?Three methods including box-counting method,circle method and caliper method were used to compute the fractal dimensions of methane–n-heptane flames,and a comparison was conducted.Based on the comparison it is seen that the box-counting is capable of computing fractal dimensions of spherical flames more accurately compared to the other two methods.On this basis,the propagation characteristics of methane–n-heptane flames at different initial pressures and equivalence ratios were investigated through the fractal dimensions.The results show that when the number of the crackles increases,the fractal dimension of the flame will rise,indicating the flame instability is enhanced gradually.In addition,according to the variations of the fractal dimensions with the flame radius,it is seen that the fractal dimension can also be used to quantitatively analyze the transition state of the flame from laminar to turbulent state.