Quantitative Study Of Flow Field Distribution Based On Background Oriented Schlieren Technique

Quantitative measurement of the temperature distribution in the flow field is a key step in the research process in the fields of aerodynamics,fluid mechanics and combustion.The study of the external flow field of combustible gas flame is helpful to deeply understand the characteristics of laminar premixed combustion and further improve the combustion efficiency.The background schlieren technology combines the schlieren technology with the particle tracking image processing technology.By calculating the background pixel displacement caused by the refraction of the flow field,the refractive index distribution of the light in the observation area is obtained,so as to obtain the quantitative information of the temperature distribution of the flow field.It makes up for the shortcomings of traditional schlieren technology in flow field observation.This technology has the advantages of convenient system construction,convenient debugging and maintenance,strong adaptability,and large observation range.It is a flow display technology for non-contact quantitative measurement of transparent media.In this paper,the background schlieren system is used to analyze and reconstruct the natural thermal convection field above the flame.The main research contents are as follows:In terms of system construction,by improving the structure of the existing background schlieren display device,the brightness of the background display component is greatly improved,and the visible light interference of the flame is avoided.The drawing type and arrangement of the background patterns are determined through experiments,and diagonal stripes arranged alternately in color are used to achieve a coverage rate of more than 90%for the natural thermal convection field moving vertically upward.Through the sensitivity test,determine the parameter settings of each part of the system.In the aspect of algorithm research,inspired by the color schlieren technology,the background schlieren image is separated by computer image processing technology,and the data separated from the same group of images is solved by numerical method to obtain more abundant image information..The displacement vector value of the corresponding pixel point of the background schlieren image is calculated by the cross-correlation algorithm,and the calculation efficiency is improved by the fast Fourier transform.The iterative reconstruction method and the Radon inverse transform are analyzed and compared in the background schlieren technology,and the Radon inverse transform is finally selected.In terms of experimental research,a safe and reliable background schlieren experimental platform was built to collect images of the natural thermal convection field above the butane flame,and the finite element simulation technology was used to simulate the flame temperature of different air-fuel ratios.The results show that the air-fuel ratio is between 0.80— In the range of 1.00,the flame temperature shows an upward trend;when the air-fuel ratio is in the range of 1.10-1.50,the flame temperature shows a downward trend.The background schlieren technique and the measurement results of the thin-film platinum thermal resistance on the flow field were compared and analyzed,and the infrared thermal imager was used to verify that the thin-film platinum thermal resistance would interfere with the flow field.Using the background schlieren technique to quantitatively analyze the flow field above the laminar premixed flame,it can be seen that when the air-fuel ratio is in the range of 0.80-0.95,the shape of the flow field gradually changes from chaos to stability.The point cannot be stabilized in the lower center region of the flow field.In the range of air-fuel ratio from 1.00 to 1.30,the shape of the flow field is gradually stable.With the increase of air-fuel ratio,the number of irregularities in the flow field changes from three times to one time,and the highest temperature point appears in the middle of the bottom of the flow field.When the air-fuel ratio is in the range of 1.40-1.50,the flow field gradually becomes two parallel and vertical stable laminar flow fields,and the highest temperature point appears at the bottom two sides of the flow field.