| Understanding the fuel atomization process is the prerequisite for designing and realizing the efficient and clean combustion mode of internal combustion engines.In order to perfect the fuel spray theory,it is important to accurately measure the characteristic parameters of fuel spray,study the characteristics of fuel spray under different experimental conditions,and establish mathematical models that can accurately describe the fuel spray process.In this paper,firstly,the precise extraction method of foreground spray from fuel spray schlieren image was studied.Subsequently,the image post-processing method of the particle image analysis(PIA)system used to measure the microscopic characteristics of the fuel spray was improved.Finally,the microscopic characteristics of fuel spray measured under different environments and experimental conditions were discussed and analyzed in depth.(1)An image post-processing method was designed and implemented for accurately measuring the macroscopic characteristic parameters of spray from the spray schlieren image.The method was mainly divided into four parts.The first part was the foreground possibility multiple estimation.By calculating the change of the pixel value of each pixel and its neighboring pixels,it can judge whether the pixel was a foreground pixel and preliminarily extract the foreground spray region.The second part was the gas forced flow noise elimination.This part was designed according to the spatial variation process of environmental density.It can eliminate the gas forced flow noise caused by spray development,and further refine the foreground spray region.The third part was the erroneous foreground region elimination.It can eliminate the error foreground pixels that have been transformed into the background,and finally complete the precise extraction of the foreground spray region.The fourth part was based on the extracted foreground spray region to measure the macroscopical characteristic parameters of spray.In this paper,the fuel spray schlieren images taken under different environmental conditions were used to test the foreground extraction method.The results showed that the extracted foreground spray region was consistent with the actual spray region,and the measured macroscopical characteristic parameters of the spray were highly agreement with the actual value.(2)A PIA system image post-processing method based on multiple droplet feature similarity evaluation for droplet pairing was designed.When using the PIA system to measure the microscopic characteristics of fuel spray,determining the spatial position of the same droplet in two consecutive droplet images was the key to accurately measuring the droplet characteristics.The commonly used PIA system image post-processing method was designed according to the morphological characteristics of the droplets.This method will produce a large number of incorrect droplet pairings.This paper summarized the shortcomings of the PIA system image post-processing method based on the droplet morphological characteristics for droplet pairing,and proposed a multiple characteristics similarity droplet pairing method.This method evaluated the basic characteristics similarity,motion characteristics similarity and neighborhood spatial characteristics similarity between droplets,and paired the droplets with the highest similarity in two frames of droplet images.The accuracy and applicability of the multiple characteristics similarity droplet pairing method were verified by using the spray microscopic droplet images taken by the PI A system in different environments and experimental conditions.The results showed that compared with the previous image postprocessing method,the multiple characteristics similarity droplet pairing method can greatly improve the droplet pairing accuracy and obtain more precise spray droplet characteristics.(3)This paper conducted an experimental study on the microscopic droplet characteristics of spray after ending of diesel injection.The droplet movement process was qualitatively analyzed.The droplet velocity,droplet size distribution,SMD and Weber number were quantitatively calculated.And the spray velocity formula on the spray axis after ending of fuel injection was obtained.The results showed that,after ending of fuel injection,the movement direction of droplet on the spray axis was not vertical downward,but in all directions.With the kinetic energy from the spray dissipating,the influence of ambient gas flow on the droplet movement increased.The ambience pressure had little effect on the droplet size outside the spray,but had a greater impact on the droplet size inside the spray.The droplet size increased with the increase of ambient pressure.After ending of fuel injection,the droplet size at both ends of the spray was smaller,while the droplet size at the middle of the spray was larger.The droplet velocity in each region from the spray tail to the spray tip was approximately linearly increased.(4)In this paper,the microscopic characteristics of fuel spray under cross-flow conditions were studied,and the droplet velocity and droplet size in different regions inside the spray were measured.Based on the measured results,the formulas for calculating between the fuel spray SMD and the droplet SMD of each spray region under different cross-flow conditions and the experimental conditions,fuel physical parameters and cross-flow speed was obtained.Under cross-flow conditions,the internal region of spray can be divided into two categories:one was the region dominated by spray,and the droplet movement was mainly affected by spray,and the droplets could break and collide.The other type was the region dominated by cross-flow,and the droplet movement was mainly affected by cross-flow,but the cross-flow only transported droplets. |
