Study On Direct Injection Combustion And Laminar Premixed Flame Characteristics Fueled With Natural Gas-hydrogen Blends

With increasing concern about fossil fuel shortage and stringent emission regulations, researches on high efficiency combustion technology fueled with clean alternative fuel has become major research aspects in the combustion community and engine development society. Natural gas and hydrogen are regarded as the most promising clean alternative fuels in the near and long term future. One of the main shortcomings of natural gas and hydrogen used as engine fuel is that the relatively lower energy density compared to that of gasoline leading to the lower power output. Direct injection combustion has potential to increase thermal efficiency and decrease pollutions. Gas fuel direct injection combustion can increase the power output by introducing more air to the cylinder. The key problem of direct injection combustion is the formation of appropriate fuel-air charge stratified distribution under wide operating condition. The improper charge distribution will lead to misfire during the flame propagation from the relatively richer zone near the spark plug to the leaner zone on the periphery of the cylinder. Meanwhile, the NOx formation will be increased on the richer zone. The NOx conversion is very difficult on the oxygen richer environment for direct injection overall lean combustion. Hydrogen possesses high burning velocity under wide equivalence ratio and turbulent condition. The flame propagating velocity and lean burn stability of natural gas will be improved with hydrogen addition. Thus the flame propagation stability of direct injection stratified charge combustion will be improved as hydrogen is added to natural gas. The combination of natural gas-hydrogen fuel blends with direct injection is a feasible way to achieve high efficiency and clean combustion. The fundamental research was conducted on the direct injection combustion fueled with natural gas-hydrogen fuel blends in this dissertation. The main results are summarized as follows,(1) Combustion and emissions characteristics of a direct injection engine fueled with natural gas-hydrogen blends were investigated. Brake effective thermal efficiency increased with the increase of hydrogen fraction at low and medium engine loads and high thermal efficiency is maintained at high engine load. The beginning of the heat release advanced with the increase of hydrogen fraction. The rapid combustion duration decreased and the maximum heat release rate increased with the increase of hydrogen fraction. This phenomenon was more obvious at low engine speed. This indicated that the effect of hydrogen addition on natural gas combustion is more obvious at low turbulence intensity condition. The maximum mean gas temperature and the maximum rate of pressure rise increased with the increase of hydrogen fraction. Exhaust HC and CO2 concentrations decreased with the increase of the hydrogen fraction. Exhaust NOx concentration increased with the increase of hydrogen fraction at high engine load, and it increased remarkably when the hydrogen volumetric fraction exceeds 20%. From comprehensive evaluation of engine performance and emissions, the study suggests that 20% of hydrogen fraction in natural gas can get the optimum results.(2) Experimental study of the characteristics of high pressure injected natural gas jets was investigated in a constant volume vessel using high speed photograph method. Jet development process can be divided into two parts, they are the initial part and the main part. At the initial part, the jet penetration increased rapidly with the increase of time, while in the main part, the jet penetration increased linearly with the increase of time. Jet penetration, jet volume and jet overall excessive air ratio increased with the increase of time. Jet dispersion angle kept almost invariable during jet developing process. Jet penetration, jet volume and jet overall excessive air ratio increased with the increase of injection pressure, while jet dispersion angle decreased with the increase of injection pressure. Jet penetration and jet volume decreased with the increase of environmental pressure, while jet dispersion angle and jet overall excessive air ratio increased with the increase of environmental pressure.(3) The effect of hydrogen addition, turbulence intensity and fuel-air charge stratification on natural gas direct injection combustion were studied experimentally using a constant volume vessel. Turbulence was generated by injecting the high-pressure fuel into the vessel. Flame propagation images and combustion characteristics via pressure-derived parameters were analyzed at various hydrogen volumetric fractions (from 0% to 40%) and the overall equivalence ratios of 0.6, 0.8 and 1.0. The results showed that the flame kernel is strong wrinkled with random propagating direction for direct injection combustion. While, the laminar premixed flame kernel is smooth spherical. The flame kernel pattern is much less irregular and concentrate to the spark position with the increase of hydrogen fraction. The spark ignition is more stable and the flame kernel is concentrated to the spark position with the increase of premixed ratio. Combustion duration was shortened with the advance of ignition timing which corresponding to high turbulence intensity in the vessel at spark timing. Both the initial combustion duration and main combustion duration decreased with the increase of hydrogen fraction for laminar and direct injection combustion at lean mixture condition. However, slight influence on combustion characteristics was presented with variation of hydrogen fraction at the stoichiometric equivalence ratio with and without the turbulence in the vessel. The cycle-by-cycle variations were initiated at the early stage of flame development. Direct-injection natural gas combustion can achieve the stable lean combustion along with low cycle-by-cycle variations due to the mixture stratification in the vessel. The cycle-by-cycle variations decreased with the increase of hydrogen addition and this trend is more obvious at ultra-lean burn condition. Hydrogen addition weaken the effect from turbulent flow on flame propagating process, thus reduce the cycle-by-cycle variations related to the gas flow. There exists interdependency between the early combustion stage and the subsequent combustion process for direct-injection combustion due to the stratified mixture distribution in the vessel weaken the influence of the early combustion stage on the subsequent combustion process. Both the initial and main combustion durations are increased with the increase of premixed ratio, while they show the decreasing trend with hydrogen addition to natural gas. Partially premixed direct injection combustion combining with hydrogen addition can achieve the stable spark ignition and fast combustion rate at the lean mixture condition.(4) Laminar premixed stoichiometric methane-hydrogen-oxygen-argon flames were investigated with tunable synchrotron vacuum ultraviolet(VUV) photoionization and molecular-beam sampling mass spectrometry techniques. All observed flame species, including stable intermediates and radicals in the flames, were detected by measuring photoionization mass spectra and photoionization efficiency (PIE) spectra. Mole fraction profiles of major species and intermediates were derived by scanning burner at some selected photon energies near ionization thresholds. The influence of hydrogen addition on mole fraction of major species and intermediates were analyzed. The results showed that the major species mole fraction of CO, CO2 and CH4 decreases with the increase of hydrogen fraction. The mole fraction of intermediates measured in this experiment decreases remarkably with the increase of hydrogen fraction. This would be due to the increase of H and OH radicals by hydrogen addition where the high diffusivity and activity of H radical promotes the chemical reaction. In addition, the increase of H/C ratio with the increase of hydrogen fraction also leads to the decrease of the mole fraction of carbon related intermediates and contributes to the decrease of unburned and incomplete combustion products. The reaction rate is increased with hydrogen addition to methane, while, the reaction pathway of methane combustion is not changed with hydrogen fraction less than 40%.(5) The stoichiometric methane-hydrogen-air freely propagated laminar premixed flames at normal temperature and pressure were calculated by using PREMIX code of CHEMKIN program with GRI-Mech 3.0 mechanism. The mole fraction profiles and the rate of production of the dominant reactions contributing to the major species and some selected intermediate species in the flames of methane-hydrogen-air were obtained. The rate of production analysis was conducted and the effect of hydrogen addition on the reactions of methane-air mixtures combustion was analyzed by the dominant elementary reactions for specific species. The results showed that the mole fractions of major species CH4, CO and CO2 were decreased while their normalized values were increased as hydrogen is added. The rate of production of the dominant reactions contributing to CH4, CO and CO2 shows a remarkable increase as hydrogen is added. The role of H2 in the flame will change from an intermediate specie to a reactant when hydrogen fraction in the blends exceeds 20%. The enhancement of combustion with hydrogen addition can be ascribed to the significant increase of H, O and OH in the flame as hydrogen is presented. The decrease of the mole fractions of CH2O and CH3CHO with hydrogen addition suggests a potential in the reduction of aldehydes emissions of methane combustion as hydrogen is added. The methane oxidation reaction pathways will move toward the lower carbon reaction pathways when hydrogen is available and this has the potential in reducing the soot formation. Chemical kinetics effect of hydrogen addition has a little influence on NO formation for methane combustion with hydrogen addition.