| Summary: | 博士 === 國立成功大學 === 航空太空工程學系 === 107 === This study is to investigate the effect of different CH4/AIR mixing ratios on ignition delay time. Using the gas/liquid as fuel (CH4/n-heptane), CH4 for pre-introduction into the combustion chamber to form a gaseous fuel/air premixed environment, and then spray igniting by quantitative n-heptane (n-heptane). There are four case for comparative analysis. CASE1: CH4/AIR premixed equivalence ratioφ is 0.0, and auto-ignition delay times (IDT) for inner chamber temperatures are 673K, 700K, 773K, 873K, 973K and 1073K. IDT, as the temperature inside the chamber is higher, the liquid fuel (n-heptane) will be shortened in the nozzle spray to the ignition time. CASE2: The pre-mixing equivalence ratio of the fixed chamber temperature 773K and φ= 0.0, but the effect of different chamber pressures (Pa = 15 bar, 20 bar, 25 bar, 30 bar and 35 bar) on the automatic point delay time. As the Pressure inside the chamber is higher, the spray fuel auto-ignition delay time (IDT) will be shortened. CASE3: With fixed chamber temperature 773K and pressure 25bar, The equivalence ratio (φ) is under from 0.0 to 1.0. It can be seen that in the case of pure spray combustion (φ = 0.0), the fuel is a diffusion combustion of a typical diesel engine. The combustion of air premixed methane (φ = 0.2, 0.4, 0.6, 0.8 and 1.0) is not as fast as the initial ignition of fuel injected into the combustion chamber, but the temperature inside the chamber rises above the auto-Ignition temperature of methane. After the point, the ignition of methane is a large range of multi-point combustion. With the increase of the equivalent ratio, the chemical reaction rate of the initial ignition is suppressed. Thus it, caused an increase in the overall automatic delay time. CASE4: The pressure curve of the spontaneous process in the fixed combustion chamber pressure (25 bar) andφ=0.0 and 0.6 under different chamber temperatures (673K, 773K and 873K). At 673K, the spontaneous ignition temperature of methane (923K) is higher than that of liquid fuel, thus inhibiting the reaction of liquid fuel spray combustion, so that the automatic ignition mechanism cannot be completed under the temperature condition of 673K at φ=0.6. the higher chamber temperature same CH4/AIR premixed equivalence ratio, that accelerates the methane participation in the n-heptane auto-combustion process and shortens the overall auto-ignition delay time (IDT).
In addition, chemical mechanism sensitivity and reaction rate analysis are performed. under the conditions that are constant chamber temperature is 773K, pressure is 25bar and various CH4/AIR premixed equivalence ratio (φ=0.0, 0.2, 0.4, 0.6, 0.8 and 1.0), it is known that the sensitivities of R99 (CH4+H〈=〉CH3+ H2), R100 (CH4+OH〈=〉CH3+H2O) and R101 (CH4+O〈=〉CH3+OH) are increased with the increase of φ. In the reaction rate analysis, it was also found that the reaction rates of R100 and R101 are increased with φ increasing, which also showed that H, OH and O were absorbed by R99, R100 and R101. Therefore, the product OH of •OOC7H14OOH→•OC7H13OOH +•OH is absorbed by R100, so that the free radicals having not immediately enough OH radicals to cause a divergent chain reaction, and it slow down the chemical reactions in the following stages the auto-ignition delay time extended.
|
|---|
