| To change the long-standing dominance of coal in the energy mix,the development,and promotion of clean energy is a matter of urgency.Biodiesel,which has similar characteristics to diesel,has become a popular fuel with its green and renewable features and is one of the alternative fuels to help achieve the double carbon target in the 14th FiveYear Plan.However,the high kinematic viscosity and poor atomization and evaporation characteristics of biodiesel restrict its promotion and use.Biodiesel consists of saturated and unsaturated fatty acid methyl esters,of which methyl stearate and methyl oleate are typical representatives.The unsaturated bonds in unsaturated fatty acid methyl esters reduce the viscosity of the fuel,and in addition,they are less thermally stable than saturated fatty acid methyl esters at high temperatures and are more prone to thermal cracking and gas generation,which allows for micro-explosions when the fuel droplets evaporate and promotes secondary droplet atomisation,thereby enhancing the fuel this promotes secondary droplet atomisation,which in turn improves the evaporation and combustion efficiency of the fuel in the engine.The evaporation and combustion mechanisms of monounsaturated and saturated fatty acid methyl ester blends are not well understood,so the evaporation and combustion characteristics of methyl stearate/oleic acid methyl ester fuel droplets were investigated and the combustion results were simulated and calculated for different operating conditions in a YC6G270-30 engine.The following is the main work:Firstly,the evaporation characteristics of five ratios of methyl stearate/methyl ethyl oleate blends at different temperatures(673 K and 973 K)were experimentally investigated.The results showed that high temperatures promoted fuel droplet evaporation,with the longest droplet life at 10% methyl oleate content at the same temperature.When the experimental temperature was 673 K,no microbursts were observed during droplet evaporation for all fuel mix ratios,and the highest droplet evaporation rate was observed at70% methyl oleate content.When the experimental temperature was 973 K,droplets of mixed fuels with oleic acid methyl ester concentrations above 70% were observed to microburst during evaporation.This was due to gas generation from the cracking of unsaturated methyl ester,when enough gas was generated to cause the pressure inside the bubble to grow and break the limit of surface tension and the atmospheric pressure of the liquid film causing the droplets to a microburst.Secondly,the combustion characteristics of single droplets of mixed fuels with a methyl oleate ratio greater than 60% were experimentally investigated.It was found that bubble nucleation,steam injection,and micro-explosion occurred during combustion for all mixed proportions of fuel droplets,and the intensity and number of micro-explosions increased with the increase of unsaturated methyl oleate proportion.Both vapor injection and micro-explosions cause large distortions in the flame structure,manifested by irregular fluctuations in the flame area.Secondly,the content of methyl oleate in the fuel mix had an important influence on the ignition delay time,the double bonds in the molecular structure of methyl oleate were unstable and ignition was more likely to occur when the electrode was heated,but the relationship was not linear,the ignition delay of the mixed droplet was shortest at its content of 80%.Finally,the combustion process of the fuel mixture in the combustion chamber of the YC6G270-30 engine was studied in simulation.It was shown that as the initial pressure increased,carbon soot emissions decreased,but NOx emissions increased.Higher initial temperatures were detrimental to fuel economy,while lower initial temperatures achieved higher heat release rates and reduced carbon smoke emissions.In addition,the smaller the diameter of the spray hole,the greater the spray penetration distance of the fuel and the better the oil and gas mixing. |
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