Mingfa Yao

Development of an n-heptane/toluene/polyaromatic hydrocarbon mechanism and its application for combustion and soot prediction

A chemical reaction mechanism has been developed for modeling the combustion process and polyaromatic hydrocarbon formation of diesel and n-heptane/toluene fuels. A reduced n-heptane/polyaromatic hydrocarbon mechanism was applied and updated to better predict the formation of polyaromatic hydrocarbon up to four rings (A4) in ethylene and n-heptane premixed flames. In addition, a reduced toluene mechanism was updated and combined with the n-heptane/polyaromatic hydrocarbon mechanism to predict the combustion and polyaromatic hydrocarbon formation of diesel and n-heptane/toluene fuels. The final mechanism consists of 71 species and 360 reactions. This mechanism was validated with experimental ignition delay data in shock tubes, premixed flame species concentration profiles, homogeneous charge compression ignition combustion and direct injection spray combustion data. A practical multistep soot model was integrated with the polyaromatic hydrocarbon kinetic model to predict soot emissions of diesel and n-heptane/toluene direct injection engine data. Constant-volume combustion vessel simulations were also conducted and the effects of combustion parameters, such as temperature and equivalence ratio, together with the n-heptane/toluene ratio on polyaromatic hydrocarbon and soot formation are discussed. The results show that the present mechanism provides promising agreement in terms of polyaromatic hydrocarbon prediction for various fuels in premixed flames and highlights the importance of aromatics on the polyaromatic hydrocarbon formation and soot emissions. Homogeneous charge compression ignition combustion and direct injection spray combustion simulation results confirm that the present mechanism gives reliable predictions of combustion and soot emissions for both diesel and n-heptane/toluene fuels under various conditions.

EXPERIMENTAL STUDY ON HOMOGENEOUS CHARGE COMPRESSION IGNITION COMBUSTION WITH PRIMARY REFERENCE FUEL

Abstract By mixing iso-octane with octane number 100 and normal heptane with octane number 0, it was possible to obtain a primary reference fuel (PRF) with octane rating between 0 and 100. The influences of PRF fuels octane number on the combustion characteristics, operation range, performance, and emissions characteristics of homogeneous charge compression ignition (HCCI) engine were investigated. The experiments were carried out on a modified single cylinder direct injection diesel engine. The test results show that, with the increase of the octane number, the ignition timing delays, the combustion rate decreases, the combustion duration prolongs, and the cylinder pressure decreases. The HCCI combustion can be controlled, then the HCCI operating range can be extended by burning different octane number fuel at different engine modes, in which engine burns low octane number fuel at small load mode and large octane number fuel at high load mode. There exists an optimum octane number that achieves the highest indicated thermal efficiency at different engine load. With the increase of the PRF fuel octane number, NOx, HC and CO emissions increase, especially for HC emissions.

An Experimental and Numerical Study on the Effects of Fuel Properties on the Combustion and Emissions of Low-Temperature Combustion Diesel Engines

Experimental and numerical investigations on the effects of fuel properties on combustion and soot emissions under both conventional and premixed low-temperature combustion (LTC) conditions have been conducted. Three different fuels, diesel, gasoline, and n-butanol, were used to formulate five fuels with different fuel properties. Computational fluid dynamic (CFD) simulations were conducted to predict the combustion processes and soot emissions of the various fuels. The results show that under both conventional and premixed combustion conditions, the cetane number (CN) has the dominant effect on the ignition delay; the volatility, aromatic, and oxygen contents only have a minor influence on ignition delay. High CN fuels need much higher exhaust gas recirculation (EGR) to provide a sufficiently long ignition delay compared to the fuels with lower CN. As a result, the carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions of the high CN fuels are higher than the low CN fuels due to the lower intake oxygen concentrations. The volatility can be important under high, mixing-controlled, conventional combustion conditions, and the high volatility and oxygen content are also beneficial for CO and UHC reduction under high EGR premixed LTC conditions. The CN plays a dominant role in soot emissions, followed by the oxygen content and the volatility under low oxygen concentration conditions. The experiments show that the trade-off between NOx and soot can be totally eliminated by optimizing the diesel/gasoline/butanol blended fuel. A reduced primary reference fuel (PRF)-n-butanol-polycyclic aromatic hydrocarbon (PAH) mechanism was formulated to predict the combustion and soot emissions of the tested fuels, and the effects of fuel chemistry properties on the combustion processes and soot emissions were well predicted. The simulation results show that the mixing process can be greatly improved by adjusting the fuel chemistry properties, which leads to improved combustion and low soot emissions.

Reaction Mechanisms and HCCI Combustion Processes of Mixtures of n-Heptane and the Butanols

A reduced primary reference fuel (PRF)-Alcohol-Di-tert-butyl Peroxide (DTBP) mechanism with 108 species and 435 reactions, including sub-mechanisms of PRF, methanol, ethanol, DTBP and the four butanol isomers, is proposed for homogeneous charge compression ignition (HCCI) engine combustion simulations of butanol isomers/n-heptane mixtures. HCCI experiments fuelled with butanol isomer/n-heptane mixtures on two different engines are conducted for the validation of proposed mechanism. The mechanism has been validated against shock tube ignition delays, laminar flame speeds, species profiles in premixed flames and engine HCCI combustion data, and good agreements with experimental results are demonstrated under various validation conditions. It is found that although the reactivity of neat tert-butanol is the lowest, mixtures of tert-butanol/n-heptane exhibit the highest reactivity among the butanol isomer/n-heptane mixtures if the n-heptane blending ratio exceeds 20% (mole). Kinetic analysis shows that the highest C-H bond energy in the tert-butanol molecule is partially responsible for this phenomenon. It is also found that the reaction tC4H9OH+CH3O2 =tC4H9O+CH3O2H plays important role and eventually produces the OH radical to promote the ignition and combustion. The proposed mechanism is able to capture HCCI combustion processes of the butanol/n-heptane mixtures under different operating conditions. In addition, the trend that tert-butanol /n-heptane has the highest reactivity is also captured in HCCI combustion simulations. The results indicate that the current mechanism can be used for HCCI engine predictions of PRF and alcohol fuels.

PRIMARY COMBUSTION INTERMEDIATES IN LOW-PRESSURE PREMIXED LAMINAR 2,5-DIMETHYLFURAN/OXYGEN/ARGON FLAMES

Primary combustion intermediates in low-pressure premixed laminar 2,5-dimethylfuran (DMF)/oxygen (O2)/argon (Ar) flames with equivalence ratios of 0.8 and 1.5 were investigated by using tunable synchrotron vacuum ultraviolet photoionization and molecular-beam mass spectrometry. DMF is a promising biofuel, with properties similar to those of gasoline. However, the combustion chemistry of DMF is not well-studied. Possible reaction pathways of DMF and its primary combustion derivatives were proposed based on the combustion intermediates identified in this work. Photoionization efficiency curves (PIEs) of the combustion intermediates in the DMF/O2/Ar flames were recorded. Ionization energies (IEs) were measured from the PIEs. The combustion intermediates were identified by the agreement of the measured IEs with those reported in the literatures or calculated at G3B3 level. H abstraction and the consecutive reaction products were identified, including 5-methylfurfural, (Z)-1-oxo-1,3,4-pentatriene, and 2-ethyl-5-methylfuran, etc. Furan was not observed in the low-pressure DMF flames. H and OH addition products were also identified, including 2-methylfuran, (2Z,3E)-1-oxo-1,3-pentadiene, and 2-oxo-2,3-dihydro-5-methylfuran. The extra methyl side chain may explain the lower laminar burning velocity of DMF relative to that of 2-methylfuran.

Experimental Study on Homogeneous Charge Compression Ignition Operation by Burning Dimethyl Ether and Methanol

In this paper, a new approach to burning methanol in engine is proposed, in which the engine burns dimethyl ether (DME) and methanol dual fuel in homogeneous charge compression ignition (HCCI) mode, and DME is converted from methanol. Combustion, engine performance, and pollutant emissions of the new HCCI combustion system were investigated. The results show that the stable HCCI operation of DME/methanol can be obtained over a quite broad speed and load region. Both DME and methanol affect HCCI combustion strongly, and by regulating DME/methanol proportions the HCCI combustion process could be controlled effectively. NOx emissions are, very low overall, while HC and CO emissions are much higher than that in conventional compression ignition engines. An appropriate optimal HCCI operation can be obtained by controlling the DME and methanol supply according to operating conditions.

Numerical Simulation on Combustion and Emission Processes of Premixed/Direct-Injected Fuel Stratification Combustion

A fully coupled multi-dimensional computational fluid mechanics and reduced chemical kinetics model is adopted to investigate the combustion and emission mechanisms of premixed/direct-injected fuel stratification combustion, stratification of which was achieved by most uniform fuel and other small partial direct-injected fuel. The results show that the low temperature reaction of the premixed/direct-injected fuel combustion first occurs in the regions with homogeneous fuel and the high temperature reaction begins from high fuel concentration regions involved in the spray process. At the end of the low temperature reaction, the concentrations of H2O2 and CH2O in the rich mixture regions are richer than other regions. There are two kinds of stratifications in this study: the premixed/direct-injected fuel stratification combustion injecting fuel near the start of the low temperature reaction and the premixed/direct-injected fuel stratification combustion with appropriate early fuel injection in the case of a small direct injection (DI)ratio. In the first case, with the increase of the ratio of the direct-injected fuel to the total fuel (DI ratio), the onset of the high temperature reaction advances, the maximum pressure rise rate decreases, high unburned hydrocarbon (UHC) emissions decrease, CO emissions increase, and NOx emissions rapidly increase. In the second case, with the advance in the injection timing, the UHC emissions increase, CO emissions decrease, and NOx emissions reduce. Such stratification combustion has the potential for reducing UHC emissions and CO emissions simultaneously, while NOx emissions are very low. The characteristics of the premixed/direct-injected fuel stratification combustion are consistent with the imposed stratification combustion in our previous work.

A Reduced Chemical Kinetic Mechanism for Low Temperature Diesel Combustion and Soot Emissions

A reduced diesel surrogate fuel chemical kinetic mechanism of n-heptane/toluene/1-hexene including poly-aromatic hydrocarbons (PAHs) formation was developed for prediction of the diesel combustion process and soot emissions. The proposed mechanism, which includes 60 species and 123 reactions, agrees well with experimental ignition delays in shock tubes. The proposed mechanism was coupled with the KIVA-3V Release 2 computational fluid dynamics (CFD) code to predict the combustion process and soot emissions in constant volume spray chamber and diesel direct injection combustion cases. The simulation results predict the combustion processes for diesel fuel under various conditions well. However, the predicted soot emissions have notable deviations compared to the experimental data when C2H2 or phenanthrene (A3) were chosen as precursors in the soot model at lower oxygen concentration conditions. The predicted soot emissions at different oxygen concentrations have better agreement with the experimental data when pyrene (A4) was chosen as the soot precursor. Compared to C2H2 or A3, A4 is a more suitable precursor for soot predictions in the LTC simulations. The overall results show that the present mechanism can be used to predict the combustion process and soot emissions of low temperature diesel combustion.

Laser-Induced Fluorescence Measurements of Formaldehyde and OH Radicals in Dual-Fuel Combustion Process in Engine

Dual-fuel combustion is a promising method for achieving high-efficiency clean combustion in internal combustion engines.Most current research focuses on the effects of dual-fuel injection on engine performance and emissions.Our understanding of dual-fuel in-cylinder combustion processes needs further investigation.In this study,an optical diagnostic system was established to determine the intermediate products during in-cylinder combustion;the system enabled simultaneously qualitative two-dimensional measurements of formaldehyde and OH radicals.To confirm the feasibility of using this laser diagnostic system,laser-induced fluorescence(LIF) spectra and images of formaldehyde and OH radicals in a laminar premixed methane flame were acquired;the excitation laser wavelengths for formaldehyde and OH radicals were verified to be 355 and 292.85 nm,respectively.Non-simultaneous determination of formaldehyde and OH radicals in the combustion chamber was performed to analyze the two-stage heat release process and distribution regions of formaldehyde and OH radicals during dual-fuel combustion.In this investigation,the engine speed was kept at 1200 r·min~(-1) and the total equivalent fuel quality was 30 mg of n-heptane.Isooctane was injected in intake manifold at the beginning of the intake stroke and n-heptane(9 mg) was directly injected into the cylinder at 10° crank angle before compression top dead center.The results indicate that formaldehyde is formed in the low-temperature heat-release stage and is mainly located in the region near the spray jet;formaldehyde then fills most of the combustion chamber.When the high-temperature heat-release stage is initiated,formaldehyde located at the edge of the combustion chamber is consumed first,followed by consumption of formaldehyde in the center region.Accompanied with the disappearance of formaldehyde during the high-temperature heat-release stage,OH radicals first emerge at the edge of the combustion chamber and later fill the whole combustion chamber.Finally,simultaneous measurements of formaldehyde and OH radicals were conducted.Formaldehyde consumption is spatially accompanied by the formation of OH radicals.In general,the distributions of formaldehyde and OH radicals are separate spatially,but there are some regions where formaldehyde and OH radicals exist simultaneously.

Methyl Radical Imaging in Methane–Air Flames Using Laser Photofragmentation-Induced Fluorescence

Imaging detection of methyl radicals has been performed in laminar premixed methane–air flames at atmospheric pressure. A nanosecond Q-switched neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was employed to provide the fifth-harmonic-generated 212.8 nm laser beam. The intense ultraviolet (UV) laser pulse was sent through the flame front to photodissociate the methyl (CH3) radicals in the reaction zone of the flames stabilized on a piloted jet flame burner. The emission spectra from the photodissociated fragments were collected using an imaging spectrometer with the flame-front structure spatially resolved. Combining the spatial and spectral information, we recognized that the emission from the (A-X) methine (CH) transitions located at 431 nm was generated from the CH3 photolysis and could be used to visualize the distribution of CH3 radicals. With proper filtering, the high-power UV laser (around 15 mJ/pulse) provided by the compact Nd:YAG laser makes it possible to visualize CH3 distribution naturally generated in the reaction zone of laminar methane–air premixed flames.