Optical Research On Spray, Combustion And Soot Formation Process By Using Biodiesel Fuel

With the increase of industrialization of the whole world, energy shortage and environmental pollution become the most serious problems on the earth. As a clean and alternative energy, biodiesel can be produced from broad available feedstock, possess excellent combustion and emission characteristics, therefore more and more countries have deeply cognized that biodiesel using is important for reducing foreign dependence and saving domestic fuel market. At present, the study on biodiesel is considered prevalent and hot trend. Consequently, my works is mainly aimed at the optical spray, combustion and soot formation research by using biodiesel fuel. It will conduce to elevate fuel evaporation, improve combustion pattern and reduce exhaust emission preferably.In this thesis, high temperature and high pressure environment produced in an optical constant volume chamber simulates the real engine working conditions. Apply advanced laser testing and high speed imaging technology, biodiesel spray, combustion and soot formation process are investigated under various engine operation and different blend ratio conditions. As a new method measured soot mass, Forward Illumination Light Extinction (FILE) method records 2D soot distribution and instantaneous mass result during the whole soot formation process. Different from traditional soot measured method, FILE makes laser and high speed camera assembled on the same side, laser pass though soot could twice, and only one optical window required. So it is very fit for low soot emission fuels. Light scattering method is used to examine liquid jet penetration in order to investigate fuel evaporation, break up and mix with fresh air clearly. Auto ignition timing and location, flame propagation and natural flame luminosity are also observed by using high speed camera and 700 nm ND filter. The 2D time-resolved results of fuel spray, combustion and soot formation can be combined in temporal and spatial measurement simultaneously.A typical natural aspiration and no EGR operation condition for diesel engine is chosen: constant 21% oxygen content (by volume) and density of 14.8 kg/m3 in the chamber, ambient temperature varied from 800K to 1200K. Based on ultra sulfur diesel, the soybean biodiesel of 20%, 50% and 100% volume fraction are examined on spray, combustion and soot formation. Then, as compensate fuel properties difference from diesel, adds n-butanol to biodiesel fuel by low blend ratio, fuel combustion pressure and heat release rate, liquid jet penetration, natural flame luminosity and soot emission are researched. Measured results showed that sudden but repeatable drop of liquid jet penetration length by using B10D80N10 blend fuel, suppose micro- explosion exist. Therefore, utter fuel spray experiment by using biodiesel fuel with n-butanol additive implement in high temperature and pressure environment. In order to eliminate the effect of combustion on spray jet, amount of excess oxygen content is set to zero. This ensures that spray behavior is not distorted by the combustion process of the fuel. At last, a numerical model of micro-explosion in multi-component bio-fuel droplets is proposed by using KIVA-3V software. The onset of micro-explosion is determined by the homogeneous nucleation theory and characterized the effect of ambient temperature, fuel component and normalized onset radius. The numerical predictions match with experimental data very well.The experimental results show that the jet penetration is firstly detected when injection initiation, then reach the maximum penetration depth rapidly, and keep stable until injection end. It can be concluded that the fuel evaporation process mostly received the energy from hot ambient air and the influence from flame at the downstream is limited. Because biodiesel fuel has higher boiling point, kinematic viscosity and surface tension than diesel fuel, the liquid jet penetration length of biodiesel is 10%-12% longer than the ones of diesel at the same operation conditions.There is no obvious difference of fuel combustion pressure and heat release rate between biodiesel and diesel. With biodiesel blend ratio increasing, the combustion pressure and heat release rate decreases slightly resulting from lower heat value of biodiesel fuel. At relative lower ambient temperature condition, biodiesel fuel has longer ignition delay timing, but the premixed burn speed is higher than diesel fuel because of oxygen content in biodiesel fuel. With ambient temperature increasing, the ignition delay becomes unperceivable for all fuels, and biodiesel represents stronger diffusion combustion process.Due to 10% oxygen content in biodiesel, the weak natural flame luminosity and no soot formation shown at initial phase. At low ambient temperature condition, the auto ignition locate well premixed area near cylinder wall for both fuels, and a small quantity of soot generate at rich fuel region downstream of liquid spray when the combustion developed. With ambient temperature increasing, ignition delay of biodiesel advance and auto ignition move to jet upstream. Soot formation and natural flame luminosity enhance resulting from stronger diffusion combustion phase. At all engine operation conditions, biodiesel fuel burn completely, flame temperature uniformly distribute and no concentrated luminosity. Different from two soot high density area for diesel fuel: jet downstream of rich fuel and near wall region where low fresh air entrain, biodiesel flame is hard to reach cylinder wall and main soot distribution locate at jet downstream. So pure biodiesel fuel produces soot mass 30%-40% lower than diesel fuel. When biodiesel blend ratio is 20% by volume, natural flame luminosity distinctly reduce, and average soot mass is 100ug which is 50%-60% of diesel fuel probably. They don't go on lessening even biodiesel proportion heightening.Biodiesel with n-butanol additive, which not rather has the similar fuel properties to diesel, but also enhance stability of mixture fuel and keep homogeneous miscible liquid with no particles or crystals when atmospheric temperature fluctuate widely. Adding n-butanol to biodiesel fuel at low blend ratio, improve liquid atomization and evaporation, extend ignition delay and enhance premixed combustion process. B10D80N10 and B15D80N5 blend fuels demonstrate evident low soot formation rate and high soot oxygenation rate, average soot mass decrease 40% than B20D80 fuel consequently.Otherwise, Biodiesel with n-butanol additive possess enlarged difference of volatility between light component and heavy component in mixed fuel that induces micro-explosion at specifical conditions which promotes fresh air and fuel mix well. High n-butanol and biodiesel content bring out micro-explosion occur at relative low ambient temperature environment. The micro-explosion time advances with ambient temperature increasing. Numerical simulation results show that stronger micro-explosion exists at relative low ambient pressure environment, since on the one hand the less volatile biodiesel surface contributes to bubble expansion, on the other hand n-butanol has adjacent boiling point and super heat limit which is prone to nucleation. The optimal composition for micro-explosion is 30% n-butanol and 70% biodiesel.