A Methodology To Measure Velocity And Size Simultaneously Based On Gray Autocorrelation And Difference

The characteristics of fuel spray atomization are an important fact that influences the combustion process of the internal combustion engine. The measurement of droplet sizes and velocities simultaneously and instantaneously is of great significance on the micro fuel atomization mechanism. Therefore, this paper proposed a new technique based on gray autocorrelation and difference for measuring particle velocities and diameters simultaneously. This method uses two planar laser pulses with different widths to illuminate a flow field and a digital camera takes a photo, the gray-sums of two images of one particle are different too, and two particle images can be matched with the relationship between the ratio of two different gray-sums and that of two different laser pulse widths. Then the coordinates of two matched particle images are used to calculate the particle velocity, two different gray-sums are used for particle size calculation and direction definition of the particle velocity.This paper introduces as follows: theoretical models based on Mie scattering theory, hardware compositon of the imaging system, the process introduction of the related software and detailed algorithms of all image processes, experimental validation of theoretical models, simultaneously velocity and size measurement of an actual low-speed flow field and the analysis of the factors influencing measurement results.The validation results show that the relationships between particle image gray-sum and exposure time, or particle diameter are consistent with the theoretical models. 35μm and 75μm standard particles were used separately in the low-speed flow field, with stable velocity 1.98mm/s and fixed direction. The velocity measurement result shows that the maximum error related to the default velocity is about 10%, and equals to±2 image pixels. The measured velocity distribution can indicate the flow characteristic of the low-speed flow field, and almost all velocity directions are measured correctly. The maximum error of the diameter measurement result is about 15%, a little bigger than that of two standard particles. They are 7.8% and 5.8%.Overall, this new approach, whose measurement objects are the particles in a flow field and measurement basis is the lateral scattered light of the particles, has a certain viability and credibility of measurement results, can be developed for fuel spray measurement.