Research On Flow Field Distribution Measurement Based On Schlieren Technique

The study of fluid flow is crucial to the development of aerodynamics and fluid mechanics,and the measurement of the combustion temperature field has always been the key to research in the fields of aerodynamics,fluid mechanics and combustion.Using schlieren technology,non-contact measurement of the temperature distribution of the combustion flame by the deflection of light in the flow field facilitates the analysis of the combustion of hydrocarbon gases and further improves the combustion efficiency.In this project,using methane as fuel,the flow field temperature distribution characteristics of methane-air/methane-air-nitrogen premixed combustion flame are used as the research goal,and a schlieren measurement experimental system is built to complete the non-contact measurement of premixed combustion flame.The specific work is as follows:In terms of experimental platform construction,the schlieren measurement experimental system is built with a split design,which consists of an optical part,a mechanical lifting part,a control part and a premixed combustion device.Through sensitivity test analysis,the appropriate system parameter values are determined,and the image acquisition and recording of the flow field above the premixed combustion flame is completed.In the field of experimental image processing,the flow field recognition and preprocessing system of the schlieren image developed to provide technical support for the subsequent further exploration and analysis of the flow field distribution characteristics.In order to obtain the flow field observation with a larger field of view,this paper analyzes and compares the two image stitching algorithms of SURF algorithm and SIFT algorithm,and the results show that there will be false matching points in the matching process of SURF algorithm,the success rate is slightly lower than that of SIFT algorithm,and the matching rate of SURF algorithm is faster than that of SIFT algorithm,and finally the SURF algorithm is selected as the image stitching algorithm for this topic.In the experimental research,the probe thermocouple convection field is used for contact information acquisition,and the schlieren experimental device is used for non-contact measurement of the flow field.After the flow field measurement is completed,the measurement method selected for this project is evaluated to verify the feasibility of the schlieren measurement experimental system.The experimental phenomena of nitrogen addition under different air-fuel ratio conditions and under the same air-fuel ratio condition were analyzed.The results show that when the temperature measurement of the flow field above the methane-air premix combustion flame,the methane and air under the air-fuel ratio of 1.00,1.05 and 1.10 are fully reacted and the heat released is the most.When the air-fuel ratio is in the range of 0.70～0.95,the methane does not have enough oxidant and cannot be fully combusted,and with the increase of the air-fuel ratio,the heat released by the combustion reaction is also increasing;When the air-fuel ratio is in the range of1.15～1.45,the volume fraction of methane becomes smaller and smaller with the increase of the air-fuel ratio,and the heat released by combustion is absorbed by the excess air.The temperature value trend under the 5 % nitrogen condition was consistent with that above the methane-air premix combustion flame,and the measured temperature value was basically lower than above the methane-air premix combustion flame under the same air-fuel ratio condition.Obviously,with the addition of nitrogen,the flow field temperature is reduced,because the mixing of 5 % nitrogen reduces the volume fraction of methane,which does not participate in the combustion reaction,but instead absorbs the heat of reaction during the experiment.Finally,the schlieren images taken in the vertical direction of the flow field are stitched together to achieve a large-field of view observation of the flow field.