| Using low-carbon and zero-carbon fuels such as natural gas,methanol and ammonia can be an effective solution to achieve the greenhouse gas(GHG)reduction targets of the shipping industry.In marine engines fueled with low-carbon and zerocarbon fuels,direct injection strategy is commonly adopted.Therefore,the injection of high-pressure fuels into the cylinder and subsequent mixing with air are of great significance.Laser-based diagnostic techniques are commonly-used ways to study the mixing characteristics of high-pressure fuel jets.Currently,most of the concentration measurement techniques are only applicable to single-component fuels,however,the actual low-carbon/zero-carbon fuels contain multiple components.In addition,due to the lack of quantitative measurement method of concentration in the evaporating spray of multi-component fuels,the current spray model is commonly established based on concentration information of single-component fuels,and therefore unable to accurately predict the mixture formation process of fuel jets in the actual engines.Consequently,the studies on quantitative measurement of fuel concentration and evaporation ratio in multi-component fuel jets are of great significance.The objective of this study is to establish a quantitative measuring method of local concentration and evaporation ratio in multi-component fuel jets based on the laser breakdown spectroscopy(LIBS)technique,to make clear of the mixing characteristics of multi-component fuel jets.Firstly,the theoretical basis of fuel concentration measurements by LIBS is elaborated and a set of LIBS measuring system is built up.The time and laser energy characteristics,as well as the pressure and temperature effects of LIBS measurement are in-depth investigated.Then,a method for component concentration and evaporation ratio of multi-component fuel jets is established,and the total measurement uncertainties are estimated in detail.Based on this method,the mixing effects of different gas jets are compared and the evaporation and mixing characteristics of n-hexane-methanol blend spray jets under different conditions are analyzed.The main research work and conclusions of this dissertation are summarized as follows:(1)Study on the theoretical basis of the LIBS technique and construction of LIBS measuring system.The theoretical basis of fuel concentration measurement by LIBS is elaborated in detail,and the key problems existing in concentration measurement by LIBS are pointed out.Then,a constant volume vessel(CVV)and an injection system are constructed to realize the high-pressure injection of gas and liquid jets.Besides,a complete measuring system is built up for the measurements of fuel concentration and evaporation ratio in multiple-component fuel sprays.(2)Study on the influencing factors of LIBS in fuel concentration measurement.The time characteristics of LIBS spectra show that the Hα/N(I)intensity ratio is more stable with time,and its relative fluctuations under different pressures are within 5%.According to the laser energy characteristics of the H656/N746 intensity ratio,the minimum stable laser energy is defined,which can achieve the lowest fluctuation of the peak intensity ratio.The pressure and temperature effects on LIBS are investigated in detail.The pressure effects on line intensity are mainly attributed to the increase of gas density and the enhancement of self-absorption effect.The temperature effects on H656/N746 are significant,especially at 0.1 MPa,mainly due to the decreased particle density and increased plasma temperature.(3)Study on quantitative measurements of fuel concentration in gas jets by LIBS.Because of the influences of gas temperature on LIBS at low pressures,a new approach of simultaneous measurement of fuel concentration and gas temperature is proposed.Results show that the fuel concentrations in methane jets after the temperature effects are significantly reduced.After the detailed analysis on the plasma characteristics at the higher pressures,a high-pressure one-dimensional LIBS quantitative measurement method is developed.The radial distributions of equivalence ratio and the ignitable mixture percentage are measured and analyzed in detail.Then,a quantitative measurement method for fuel concentration in multi-component gas jet is established by using multiple atomic lines and applied in different gas jets.The results show the stratification of different fuel components in ammonia-hydrogen jets.In addition,the equivalence ratio in methane jets is significantly higher,resulting in quite different distribution of combustible mixtures.Combining systematic error and random error,the measurement uncertainties of fuel concentration are estimated to be 5.6%.(4)Study on quantitative measurements of fuel concentration in liquid spray jets by LIBS.The spectra achieved in different n-hexane-methanol blends and nitrogen mixtures are analyzed.The H656/N746 and O777/N746 intensity ratios are linearly correlated with the corresponding atomic concentration ratios,indicating the LIBS is independent on fuel types.Based on the calibration results,quantitative measurements of the component concentrations in n-hexane-methanol blend jets are performed.In M15 jets,the blending fraction of methanol is gradually increased with the radial distance increasing,due to the slightly lower boiling point and the higher vapor diffusion coefficient of methanol.In addition,the radial equivalence ratios in M50 jets are significantly lower than those in M15 jets,which can be attributed to that M50 blend contains a higher proportion of methanol and the theoretical fuel-to-air ratio of methanol is much larger than that of n-hexane.(5)Study on quantitative measurements of local evaporation ratio in liquid spray jets by LIBS.A high-speed droplet microphotography approach is proposed to capture the image of droplet distribution before breakdown.The local concentration of fuel droplets can be determined by combining the droplet image and the depth of field of the microscope lens.C2 line is correlated with droplets,therefore C2-516/base is employed to characterize the liquid fuel concentration.For different n-hexane-methanol blends,the slopes of calibration curve are proportional to the mole fraction of n-hexane in total fuel.As a result,a calibration and measurement strategy for the liquid fuel concentration in n-hexane-methanol jets is established.The measurement uncertainties of evaporation ratio are within 19% and decrease with the increased radial distance.As the injection pressure increases,both fuel vapor concentrations and local evaporation ratios increase in M15 jets,indicating that increasing the injection pressure can improve the evaporation of spray.The local evaporation ratios in M50 jets are significantly lower than those in M15 jets.Since the latent heat of vaporization of methanol is much higher than that of n-hexane,the vaporization effects of M50 jets are diminished.In this study,a complete set of quantitative measuring method and procedures for fuel concentration and evaporation ratio in multi-component fuel jets were established.By using this method,the jet mixing characteristics of different marine lowcarbon/zero-carbon fuels are compared and analyzed.These results are of great significance for the development of novel measuring methods,the construction of simulation models for low-carbon/zero-carbon fuels,and even the development and optimization of the combustion systems of marine engine using low-carbon/zerocarbon fuels. |
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