| China's oil and natural gas resources are so scarce that about 70% of the oil is imported.The widely of internal combustion engine in agriculture,manufacturing and transportation has sped up the development of human society and led to energy shortage and environmental pollution such as the emissions of HC,PM,CO and NOx caused by overconsuming of fossil fuel have become two major problems that need to be solved urgently.Biodiesel is renewable and has similar physicochemical properties with diesel when they blended with diesel.However,the low energy density and high contaminant have not been solved.The nanoadditives can promote energy density and reduce harmful emissions,nevertheless how the nanoadditives effect the evaporation process of fuel droplet have not been studied.Firstly,the single droplet evaporation experimental apparatus has been optimized by adding droplet protection device,which can promote the temperature of fuel droplet will not exceed 413 K when the ambient of temperature is 1273 K.The preparation process of jatropha oil and jatropha biodiesel was introduced,20% jatropha biodiesel and 80% diesel are mixed into based oil(BD),and a new method of blending nanoadditives(cerium oxide nanoparticles and multi-wall carbon nanotubes)with fuel oil was proposed.The process of data processing is introduced,and the uncertainty caused by microinjector,pre-evaporation and data processing is analyzed.Finally,the united uncertainty is 0.053 mm,which has negligible influence on the experimental process.Secondly,the evaporation characteristics of four concentrations of nanoadditives blended with basic fuel and basic fuel were studied by single droplet evaporation experimental.The normalized squared diameter curve and transient evaporation rate curve of five kinds fuel at the temperature of 673 K and 873 K were studied.The concentration range of nanoadditives will not inhibited evaporation process at the temperature of 673 K and the law that the evaporation promotion rate changes with the concentration of nanoadditive at 873 K temperature were found.The equation of computing evaporation time variation ratio was defined.It is found that at different ambient temperature and different concentration,the nanoadditives can inhibit and promote the evaporation of fuel droplets by comparing this study with previous research data through evaporation time variation ratio.The critical concentration point is defined that the maximum concentration of nanoadditives which will not inhibit the evaporation of fuel droplets at low temperature,and summarized that the "dual effect" model of droplet evaporation influenced by nanoadditives.Thirdly,a numerical model for droplet evaporation of fuel droplet blended with nanoadditives based on VOF free level tracing method was established.Based on the one-dimensional droplet evaporation model,the phase equilibrium equation,momentum equation and energy equation are established.Considering the influence of interface Stephen flow on gas-liquid interface mass transfer,the thermal conductivity of unit volume and the thermal conductivity of nanoadditives were deduced by adding the thermal conductivity source term of nanoadditives,and the thermal conductivity effect of nanoadditives on fuel droplet evaporation was simulated.Finally,the above model was implemented in Open Foam,a kind of open source CFD software.The grid independence and model accuracy were to verified,and the influence law of nanoadditives on the evaporation process of fuel droplets was studied.The results show that the nanoadditives can effectively enhance the internal heat transfer of droplets,improve the temperature at the gas-liquid interface,promote the increase of near-field velocity and velocity,and accelerate the evaporation at the gasliquid interface.The promoting effect of nanoadditives on fuel droplet evaporation is mainly reflected in the initial stage of droplet evaporation.The concentration of nanoadditives increases and the internal temperature of droplet increases more rapidly. |
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