Research On Combustion Flame Characteristics Of Blending Biodiesel Based On Digital Image Processing Technology

Combustion flame can directly reflect the speed of fuel combustion and combustion quality,so it is important to research the flame characteristics of fuel to improve combustion efficiency and reduce pollutant emission.Biodiesel is a potential alternative fuel,but there are few reports on its combustion flame characteristics in domestic and foreign literature.In this paper,based on digital image processing technology,the combustion flame images are collected,processed and analyzed,and the fractal characteristic,temperature characteristic and stability influencing factors and influencing rules of the flame with different blending ratios of biodiesel and diesel fuel were systematically analyzed.The research provides a theoretical basis for improving the combustion process of biodiesel and inhibiting the formation of pollutants.(1)A combustion test device in atmospheric environment was set up,and the combustion flame images of biodiesel and diesel fuel with different blending ratios were collected by CCD camera under different air flow rates.Using digital image processing technology,the flame images collected in the atmospheric environment and the in-cylinder flame images of biodiesel obtained in the early stage were preprocessed.Using non-linear median filter method,histogram equalization method and maximum between-class variance method to remove the image noise,enhance the image contrast and divided the flame image,the flame image pixel sizes were also adjusted.The image quality after preprocess is obviously improved,and the flame structure is clearer,which lays a foundation for the extraction of flame characteristic parameters.(2)The fractal characteristic of biodiesel combustion flame in cylinder and atmosphere was researched.The fractal dimension(D)of flame images was calculated by using the difference fraction box method and numerical box method.The variations of fractal dimension with blending ratio of biodiesel,rotational speed,air flow rate and fuel temperature were analyzed.The results show that the fractal dimension decreases first and then increases with the increase of the blending ratio of biodiesel in the two environments.The fractal dimension increases with the increase of rotating speed and air flow rate.With the increase of fuel temperature,fractal dimension increases gradually.Because the "self-oxygenation capacity" of biodiesel can promote combustion.Stronger airflow motion and higher rotating speed can promote the mixing of oil and gas at the flame edge,and increase the disturbance of the flame edge profile,so that the flame edge combustion is more intense,resulting in an increase in the degree of fold,which is manifested as the increase of fractal dimension.(3)The evaluation indexes and calculation schemes of flame temperature characteristic were proposed,and the influencing factors and changing rules of flame temperature characteristics in two environments were also analyzed.Based on the full radiation law of combustion flame,the equation between the grayscale of combustion flame image in cylinder and the temperature theoretical relationship was established by using the linear relationship among the full radiation temperature,pixel grayscale and the full radiation brightness.The Radial Basis Function(RBF)prediction model of combustion flame temperature under the atmospheric environment was constructed,and the RBF model was trained and verified by taking the brightness value of the three primary colors from the pixels of the flame image as the input and the temperature value measured by the thermocouple as the output.Two kinds of flame temperature calculation models were used to calculate and analyze the change rules of evaluation indexes,such as high temperature area ratio,effective flame average temperature,high temperature pixel quantity and maximum flame temperature.The results show that with the increase of the biodiesel blending ratio in the cylinder,the high temperature area ratio decreases gradually,and the effective flame average temperature decreases first and then increases.As the rotating speed increases,the high temperature area ratio and the effective flame average temperature also increase.In the atmospheric environment,with the increase of air flow rate,the high temperature pixel quantity and the maximum flame temperature show a trend of increasing first and then decreasing.With the increase of biodiesel blending ratio,the high temperature pixel quantity decreases first and then increases,and the maximum flame temperature gradually increases.This is mainly caused by the combustion promotion of the "self-oxygenation capacity" of biodiesel.The lower air flow rate and higher rotating speed will make the oil and gas mixture more homogeneous,and the heat released by combustion will increase,making the average flame temperature rise and the area of high-temperature area increase.Also,lower calorific value of biodiesel and higher air flow rate will reduce the heat released by combustion,resulting in different degrees of decrease in average flame temperature and high-temperature area.(4)The flame stability was carried out under two kinds of environment,and six evaluation indexes(the abundance,propagation rate,circularity,height,width,effective area of flame)were put forward.Based on the algorithms of image characteristic value,the linear relationship between the image pixels and the actual area was determined,the change rule and the influence factors of the index were calculated and analyzed.The results show that,with the increase of the blending ratio of biodiesel,the flame abundance and flame propagation rate decreased first and then increased,and the flame circularity first increased and then decreased.With the increase of rotating speed,the flame propagation rate decreased,the flame abundance and flame circularity obviously increased.In the atmospheric environment,with the increase of air flow rate,the height and effective area of the flame first increase and then decrease,and the flame width gradually decrease.The height,width and effective area of the flame decrease with the increase of biodiesel blending ratio.Because the larger viscosity of biodiesel makes it less effective in evaporation and atomization.When the mixing uniformity of oil and gas is reduced,the heat and the kinetic energy of the flame are unstable,and the flame stability is weakened.Higher rotating speed and lower air flow rate can cause the gas mixed homogeneously,the heat released by combustion and the kinetic energy of flame are changed and stabilized,and the flame stability is enhanced.