Experimental And Modeling Study Of Auto-ignition Characteristics Of Butanol-Isomers/diesel Blends

Faced with increasingly urgent energy demand and environment pollution,reseachers are now making their efforts to find clean and renewable alternative fuels in the transportation field.Among the alternatives,butanol attracts much attention in terms of its favorable physochemical characteristics.Studies have shown that the form of butanol/diesel binary fuel is very promising in engine combustion due to its excellent combustion and pollution performance.Meanwhile,the performance optimization of new fuel engines requires fundamental research of autoignition characteristics of fuels.Based on this,this paper investigated the ignition delay times?IDTs?of three groups of fuel:butanol isomers,butanol-isomers/n-heptane blends and butanol-isomers/diesel blends,in a heated rapid compression machine?RCM?and a heated shock tube?ST?.The ignition properties of the fuels were investigated and factors affecting ignition were explored.Kinetic analysis based on different models were conducted to shed light on the oxidation mechanism and duel fuel interaction.For the butanol isomers,IDTs were measured at a pressure of 15 bar,an equivalence ratio of 1,and within a temperature range of 700-1000 K.Results reveal that n-butanol exhibits the highest reactivity,followed by s-butanol,while the IDT curves of t-butanol and i-butanol cross in the investigated range.Weak Negative Temperature Coefficient?NTC?behavior was observed in n-butanol combustion,while arrhenius-like dependence of IDT on the temperature were observed for other three fuels.Simulation results found that LLNL model predicts better IDTs than CRECK model does,and the latter needs further optimization in the sub-mechanisms of i-butanol,s-butanol and t-butanol.Reaction pathway analysis based on LLNL model showed that different reaction pathways lead to the discrepancy of the reactivity of four butanol isomers.For the butanol-isomer/n-heptane blends,IDTs were measured at a pressure of 10 bar,an equivalence of 1,and within a temperature range of680-1300 K.Different with single butanol isomers,the blends exhibit distinct two-stage ignition and Negative Temperature Coefficient?NTC?behavior?750-850 K?.The IDTs of the four blends are almost the same in the high-T region,while the IDTs of t-butanol/n-heptane blend is significantly lower than those of other three blends in the low-T and NTC region.The interaction of butanol isomers and n-heptane lies in that the butanol isomers will consume OH radicals generated by the low-T brainching sequence of n-heptane.The C-H bond dissociation energy differs in the four isomers,which leads to the difference in the IDTs of the blends.Furthermore,the pressure,temperature and main species evolution of the four blends were simulated and compared with CRECK model and LLNL model.It was revealed that fuel ignition in the system is the combined result of temperature,reactant reactivity and reactant concentration,among which temperature is of significant importance.The first-stage ignition occurs when ketohydroperoxide species generated in the low-T chain branching sequence rapidly decompose at⁸00 K,producing OH radicals to enlarge the free radical followed by a temperature and pressure surge.The hot ignition occurs when H2O2 decomposes into two OH radicals at¹000 K,leading to the exponential growth of the free radical pool and a more rapid increase of temperature and pressure of the system.Based on the understanding of autoignition of butanol isomers and butanol-isomer/n-heptane blends,the more complex mechanism of autoignition of butanol-isomer/diesel blends were investigated.For the n-butanol/diesel blends,IDTs were measured at pressures of 6/10/15 bar,equivalence ratios of 0.5/1,and oxygen contents of 14.8%/9.8%/7.4%.It was found that n-butanol/diesel blends exhibit apparent two stage ignition and NTC behavior similar to n-butanol/n-heptane blends,whilst NTC behavior was tempered for the former.The 1^st IDT of the blends decreases as the temperature rises with no observation of NTC behavior in the experiments.The total IDTs of the blends decrease with the increase of pressure,equivalence ratio?fuel content?and oxygen content,and increases with the increase of blending ratio.Compared with the total IDT,the 1^st IDT is less sensitive to the above factors,while more sensitive to the tempature.The IDTs of the four butanol-isomer/diesel blends at a pressure of 10bar,an equivalence of 1,and within a temperature range of 670-1300 K were studied and compared.In the high-T region,little difference was observed among the four blends,while in low-T and NTC region,t-butanol/diesel blends exhibited somewhat lower IDTs than other three blends.The 1^st IDT of the four blends behaves similarly.The CRECK model can capture well the NTC characteristic of the blends,yet there is still some discrepancy in the predicted NTC temperature region and the high-T activation energy,as well as the order of reactivity of the four blends.Brute force sensitivity analysis of the four blends under the temperatures of 685 K and 770 K revealed that the controlling reactions in the four blends are basically the same,including low-T chain branching of alkanes,OH/HO2 elimination reaction in NTC region,and H-abstraction reaction of fuels reacting with OH radicals.Besides,the sensitivity coefficient of the above controlling reaction increases with the rise of temperature.It is worth noting that the sensitivity coefficients of reactions controlling the ignition of t-butanol/diesel blends distinguish from other three blends,which needs further discussion.The effects of the butanol isomers on blends'IDT mainly lie in those reactions involving H-atom abstraction of butanol fuels with OH radicals,the rate constant of which can be optimized to improve the IDT predictability of CRECK model.