Three-dimensional Temperature Field Measurement For Gradient Refractive Index Flame

Temperature is one of the most important thermal parameters in high temperature industrial equipment.Through real-time measurement of flame temperature distribution,the concentration and density distribution of the flame can be further calculated.Physical processes and chemical mechanisms such as combustion reaction rate,combustion product formation and flame structure can be explored.An all-round understanding of the combustion process can be achieved and an accurate assessment of the flame combustion efficiency can be made.Current flame temperature measurement is usually based on the assumption of uniform medium in the calculation of radiation transmission in a participatory medium.However,the refractive index of flame internal medium in industrial production often presents non-uniform distribution due to factors such as temperature,pressure or composition.Therefore,based on the existing flame image temperature measurement technology,a three-dimensional temperature field measurement method of flame considering the refractive index gradient of the medium is proposed.Firstly,the theoretical process of solving the radiation transfer equation of gradient refractive index medium based on the finite volume method of Cartesian coordinate system is described in detail.Then,combined with the effective medium theory and the principle of flame radiation ray tracing,the flame radiation transmission model under gradient refractive index medium is proposed.For the radiation balance problem and heat transfer balance problem,the temperature field and the radiant heat flux density in the one-dimensional and two-dimensional gradient refractive index media are calculated by numerical simulation respectively.The correctness of the solution of radiation transmission model in gradient refractive index media is verified by comparing with the results in literature.The method was applied to the three-dimensional flame radiation model to study the effects of flame size and carbon black particle concentration on the flame imaging and flame reconstruction results.The results show that during the flame imaging process,the larger the size of the flame and the higher the concentration of the internal carbon black particles is,the larger the imaging difference between the conventional uniform medium radiation model and the gradient refractive index medium radiation model is.Similarly,the larger the flame size is,the more obvious the difference in the reconstruction effect between the two models is.The concentration of carbon black particles also has a certain effect on the flame reconstruction effect.In the approximate range of the hydrocarbon flame,as the carbon black concentration increases,The reconstruction accuracy of the gradient medium model increases first and then decreases.The experimental study on the influence of refractive index on light was carried out.The necessity of considering the refractive index gradient in the reconstruction of three-dimensional temperature field of flame was verified by the change of the center displacement of the laser spot and the size of the spot area.Then the experimental system for measuring the three-dimensional temperature field of flame radiation was built,and the flame radiation transmission model under gradient refractive index medium was used to calculate the three-dimensional temperature field inversion of the flame image collected by the camera.The reconstructed three-dimensional temperature field distribution results and the temperature results detected by the thermocouple in the experiment and the reference.The reconstructed three-dimensional temperature distribution is compared with the temperature detected by the thermocouple and the three-dimensional temperature field of the uniform medium model in the literature.Compared with the three-dimensional results of the uniform medium model,the re-connection results of the gradient medium model are more accurate.Finally,an on-line measurement system for three-dimensional temperature field of flame was developed based on the algorithm of degree field reconstruction with the system test was completed.