| In order to solve the problems of energy shortage and environmental pollution,as well as the problems of the existing combustion modes,such as many incomplete combustion products,poor combustion stability,narrow operating conditions and so on.Our reaearch group proposed a new combustion mode of "Reaction controlled homogeneous mixture cascade spontaneous combustion mode".The premise of realizing this combustion mode is that the fuels which participate in the reaction have enough difference in critical spontaneous combustion boundary threshold.Therefore,in this paper,we measured the auto-ignition of alcohol fuels(methanol,ethanol and n-butanol)and hydrocarbon fuel(n-heptane)at a stoichiometric ratio,the amount of fixed fuel and the amount of air on the constant volume burning bombs which was developed by our research group.Experimental results show:There is a sufficient difference between the threshold of auto-ignition boundary of alcohol fuel and hydrocarbon fuel.so it is feasible to achieve cascade combustion of two component fuels by controlling the thermochemical atmosphere in the cylinder combustion reaction.From the experimental results,it is also found that the critical autoignition temperature of alcohol fuel fluctuates greatly with the change of initial pressure and equivalence ratio while the critical autoignition temperature of hydrocarbon fuels keeps almost unchanged with the change of external conditions,In order to explain the cause of this phenomenon from the perspective of reaction mechanism to provide an important reference for the selection of strategies and methods for implementing cascade spontaneous combustion.In this paper,the closed homogeneous reactor model of chemkin software was used to simulate the combustion of alcohol fuel in a constant volume combustion bomb.The characteristics of self-ignition reaction and parameter sensitivity of alcohol fuel were analyzed and studied.The main reaction characteristics of spontaneous combustion of fuels are compared,the specific research contents and conclusions are as follows:(1)The leading reaction characteristic analysis of alcoholic fuel spontaneous combustion: The detailed reaction mechanism of coupled homogeneous methanol,ethanol and butanol using a closed-loop homogeneous reactor model chemkin software,at T = 800 K,P = 560 psi,? = 1 to ensure that several fuels can be reliably spontaneous combustion under certain initial boundary conditions,simulated fuel combustion in a constant volume vessel,find out the chain transfer relationship by induction yield analysis,the dominant reactions of alcoholic fuels are summarized as chain initiation,chain transfer and chain termination,then draw the reaction path diagram of methanol,ethanol and butanol(including isomers),in which complete combustion of methanol requires 5 steps involving 6 components,11 basic reactions;complete combustion of ethanol requires 9 steps involving 10 components,20 motifs.However,the butanol reaction pathway is relatively complex,but small molecules can still be formed after 3 or 4 reaction steps.The reaction path of n-heptane is more complicated,and when external conditions change elementary reaction is not unique,it's dominant reaction does not exist,This is also the reason why critical auto-ignition temperature of alcoholic fuels is more sensitive than hydrocarbon fuels as the boundary conditions change.(2)Parameter sensitivity analysis of alcoholic fuel spontaneous combustion: Using variance analysis method,three-factor orthogonal test table of three factors(T,P,?)was constructed to determine the temperature as the core factor which affected the spontaneous combustion of alcoholic fuel.Then,the single variable method was used to analyze the sensitivity of ignition marker OH with different equivalence ratios,pressures and temperatures.It was found that the equivalence ratio and pressure changes had little effect on the dominant reaction path,but only the dominant reaction rate.However,the change of temperature has a great influence on the dominant reaction path,and the reaction of small molecules with increasing temperature gradually increases.The influence of carbon number on spontaneous combustion of alcohol fuels is relatively complicated.When the temperature is less than 1000 K,the larger the number of carbon atoms,the shorter the flame retardation.This is because the more carbon atoms have the more reaction paths,Thus can quickly form a fire free radicals at low temperatures.when the temperature is greater than 1000 K,the fuel reaction thermochemical atmosphere conditions are available at this time,so the more carbon atoms means the fuel reaction path is more complicated,resulting in the longer the ignition delay,but the increase of the number of the carbon atoms leads to the increase of the dominant reaction type of the fuel,so that the retardation of the fuel is less affected by the external conditions.When the number of carbon atoms is the same,the number of branches and methyl groups in the fuel molecules are the main factors affecting the auto-ignition of alcoholic fuels.The more branches in the fuel molecules and the higher the degree of isomerization,the cetane number of the fuel is lower.and the more numbers of methyl groups in the fuel molecule,the higher the energy of the fuel itself,the more stable of the molecules,so the longer the retarding period.(3)The correlation analysis of alcoholic fuel spontaneous combustion boundary: the three factors influencing the spontaneous combustion of alcoholic fuel were established by correlation of temperature,pressure and equivalence ratio.The effects of these three factors on spontaneous combustion of alcoholic fuel were studied.The results show that,at the same temperature,the product of guaranteed equivalence ratio and pressure is unchanged,that is,the total number of C-H bonds in the fuel is constant,the fuel lag period basically unchanged.However,when the initial temperature increases,the energy of the reactive fuel molecules increases,although the number of C-H bonds in the fuel is guaranteed to be the same,However,due to the increase of the number of reactive molecules and the temperature rise has changed the reaction path,the small molecule reaction gradually increased,so the fuel will continue to retard the combustion period.In the same way,at the same temperature,guaranteed the number of C-H bonds in the three fuels of methanol,ethanol and n-butanol was same,and the study found that the retardation of the three fuels was still different at this temperature.The reason is that ethanol and butanol molecules also contain C-C bond,and C-C bond breaking will have an impact on the reaction rate,resulting in the retardation of the three fuels are still different,but for the same fuel,to ensure ? P is a certain value,that is guaranteed the number of C-H bonds in the fuel is constant,so the fuel retarding period is still basically unchanged. |
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