Research On Methane Jet Combustion Optical Testing Technology And Image Processing Method

The study of methane jet combustion characteristics is a key factor supporting the future development of natural gas direct injection engines,and experimental research is the most basic method to explore the combustion characteristics.Different from the previous combustion test,in this methane jet combustion test,due to the density difference between the methane jet,the combustion flame,the background gas,and the thermal movement caused by the high temperature environment,a low-sensitivity schlieren system must be used to capture the jet and the flame at the same time.The application of low-sensitivity schlieren system increases the difficulty of image processing.Therefore,according to the optical test images captured by the low-sensitivity schlieren system,this article judges jet combustion,extracts characteristic parameters through image processing,and summarizes the relevant methane jet combustion characteristics.First of all,the constant volume bomb（CVB）is an important test device for studying jet combustion theory.In this paper,a constant-volume combustion bomb optical test platform is built,which mainly includes injection and exhaust system,temperature control system,thermocouple cooling system,pressure measurement system,ignition system,optical schlieren system,etc.Bade on this test platform,the methane jet combustion tests have been carried out under condition of different interval between ignition and jet（injection delay）,and the optical images are obtained.This test platform succeeds to realize the visualization of methane jet combustion.Secondly,image processing is the main way to solve the problem of schlieren test image recognition and flame feature extraction under low sensitivity.This article innovatively proposes a methane jet combustion judgment method,which is different from the method of comparing different time series of a set of images.It is based on the spatial difference of the same frame of images of different image groups,and the judgment of methane jet combustion is realized based on the change of the parameters that characterize the image properties caused by the jet combustion.Judging whether jet combustion occurs is the prerequisite for image processing,and extracting characteristic parameters is the basis for quantitatively studying of methane jet combustion characteristic.Based on MATLAB,a combustion flame image processing program is developed,which can extract three characteristic parameters such as the flame equivalent radius,the flame propagation distance,and the flame propagation speed.Further,the quantitative analysis of methane jet combustion characteristic has been carried out based on these characteristic parameters.Finally,using the developed device and software,the methane jet combustion characteristics under conditions of different injection delay are obtained.The results show that the methane injection delay determines the development of the equivalent radius of the premixed laminar flame.The smaller the methane injection delay,1)the larger the equivalent radius of the premixed laminar flame,2)the higher the flame intensity,3)the harder it is to be blown off when impacted by the methane jet,and 4)the methane jet is more likely to be ignited by the premixed laminar flame to form jet combustion.The formation of jet combustion will promote the development and propagation of the flame,resulting in a rapid increase in flame propagation distance and flame propagation speed.The optical experiments of methane jet combustion demonstrates that this methane jet combustion optical test system can meet the requirements of the combustion test with strong practicability,can support the research on the methane jet combustion characteristics,and can solve the problem of flame recognition and feature extraction of low sensitivity schlieren images.