Surrogate Fuel Formulated By Emulating Real-fuel Physical-chemical Properties And The Investigation Of Its Auto-ignition Characteristics

The problems of energy shortage and environmental pollution are becoming more and more serious in the worldwide.It is imminent to develop advanced engines with high efficiency and ultra-low emissions.At present,Computational Fluid Dynamics（CFD）coupled with fuel chemical dynamics provides a short-cycle,low-cost and high-accuracy method for the design and optimization of future engines.Especially,the chemical kinetic mechanism of real fuel is essential to the numerical simulation method.However,real fuels typically consist of thousands of molecular compounds.As a result,simulations directed toward these complex fuels are beyond the computational limitation of current engine-level CFD applications.Therefore,it is necessary to formulate surrogate fuels with significantly fewer components which can emulate the key properties of real fuels.So far,the research on surrogate fuels for RP-3 aviation kerosene and heavy fuel oils is still insufficient.In this paper,comprehensive and reliable surrogate fuels are constructed for these two real-world fuels by using experimental and simulating methods.At first,the study builds surrogate fuels for RP-3 kerosene.In the first step,various standard test methods were applied to measure the physical and chemical properties of the target fuel,especially,two-dimensional gas chromatography with time-of-flight mass spectrometry（GC×GC-TOFMS）and ¹³C and ¹H nuclear magnetic resonance（NMR）spectroscopy are developed to characterize the compositional characteristics of RP-3.In the next step,two surrogate fuels（K1,a mixture of five components and K2,a mixture of seven components）were optimally determined through a multi-property regression algorithm by matching carbon types（CTs）,distillation curve,cetane number（CN）,density and threshold sooting index（TSI）.In the third step,the measured and estimated values of both target properties and non-target properties of surrogates were validated against the experimental data of RP-3 kerosene.Furthermore,auto-ignition characteristics of both surrogates were investigated in a heated shock tube（ST）and a heated rapid compression machine（RCM）under engine relevant conditions,and then validated against the measured results of RP-3.Overall,K1 and K2 both exhibited good matching on the compositional characteristics,physical-chemical properties,and gas phase ignition behaviors with the target fuel.In contrast,the seven-component K2 was more competitive and more comprehensive.Then,based on the analysis results of GC×GC-TOFMS and a series of property-measurement methods for the heavy fuel oil,a six-component heavy surrogate M1 and a four-component light surrogate M2 were proposed using a multi-property regression algorithm.Among them,M1 is constructed by matching the distillation curve,cetane number and density,and M2 is constructed by matching the cetane number,hydrogen-carbon ratio and density.In order to match the distillation characteristics of the target fuel,M1 contains some macromolecular components with extremely high boiling points,however,the kinetic models of which are still unavailable.Thus,four light compounds which have relatively mature chemical kinetic models are selected for M2,in order to meet the needs for the development of available models in the near future.For validation,both surrogate fuels were produced on a lab scale and tested under the same conditions as that of the target fuel.The experimental results of the surrogates were compared with both the experimental values of the target fuel and the predicted values of the surrogate fuels based on the thermodynamic models of various properties.The results show that the two surrogate fuels generally achieved their respective matching targets.In order to further explore the auto-ignition characteristics of the four-component surrogate M2,and finally develop a chemical kinetic mechanism,the research investigates the ignition delay times（IDTs）of M2 on a heated RCM and improves the prediction quality of the existing mechanism.Firstly,IDTs for surrogate M2 were measured at pressures of 7,10,15 and 20 bar over a range of temperatures from 675 to 915 K and for equivalence ratios from 0.5 to 1.5.And then the two-stage ignition behavior and NTC characteristics of IDTs were experimentally confirmed.The effects of pressure,equivalence ratio,and oxygen mole fraction on IDTs were systematically investigated.Based on sensitivity analyses and high reaction uncertainty,the original mechanism was tuned in order to improve the performance on predicting the IDTs.Finally,the prediction results show that the tuned mechanism can better predict the auto-ignition characteristics of M2 in a wide range of conditions,but the ability to predicting IDTs in intermediate temperature range still needs to be improved.Finally,sensitivity analysis is performed to identify the critical reactions,which provides important information for further optimization of the kinetic model.