Radiative Intensity Solution With High Directional Resolution Based On Radiation Distribution Factor And Its Application

As one of the three basic forms of heat transfer,radiation heat transfer involves many fields,such as energy science,aerospace,material engineering,biomedicine and so on.The accurate simulation of energy transfer in the process of radiation transfer is very important for the development of related heat transfer technologies in these fields.At the same time,thermal radiation is also the carrier of photothermal information transmission.The accurate and effective solution of high directional resolution radiative intensity signal is of great significance for the development of high and new technologies such as target detection,infrared imaging,combustion diagnosis,temperature measurement and atmospheric remote sensing.It can provide rich measurement information for the reconstruction of multi parameter information such as radiation physical parameter field and temperature field.The accurate and efficient solution of radiative transfer equation is the key to obtain the information of radiative intensity,and it is also the basis of analyzing the problems related to thermal radiation.Due to the complex spectral,directional,long-range and full space effects of photothermal radiation transmission,it is very difficult to solve it accurately.At present,there are different defects in the solution methods of various radiative transfer equations,including the difficulty of both calculation accuracy and calculation efficiency,or only suitable for dealing with special radiation problems,or unable to obtain radiative intensity information in any direction.It is imperative to develop a solution method which can effectively obtain high-precision and high-directional resolution radiative intensity information from multi-dimensional complex participating media.Based on the idea of radiation distribution factor and Green function,this paper develops three efficient and accurate radiative transfer equation solving methods for solving radiative intensity in any direction,namely radiation distribution factor reverse Monte Carlo method(RDFRMC),radiation distribution factor integral equation method(RDFIEM)and equation-solving radiation distribution factor integral equation method(ES-RDFIEM).The above methods are applied to the non-contact temperature field reconstruction of high-temperature flame.The main researches are as follows:Based on the idea of reversibility of photon tracking trajectory of radiation transmission,the concepts of global RDF and directional RDF are proposed to characterize the distribution proportion of radiation energy in medium directional transmission.The spectral radiation properties are separated from the blackbody radiation source term of the medium by the two RDFs,and a calculation model of radiative intensity in any direction based on the product of radiation distribution factor and blackbody radiation source term is established.In this model,the complex physical process of photothermal radiation transmission is directly associated with the medium blackbody radiation source term(temperature)through RDF.Therefore,the calculation of radiation intensity in any direction becomes very simple,efficient and accurate when the RDFs is accurately obtained.The standard RMC probability statistical method for obtaining the medium RDF database is developed,and the accurate simulation of global RDF and directional RDF is realized.On this basis,the photon tracking trajectory is improved,and the null-collision RMC with higher RDF sampling efficiency is proposed,which overcomes the low efficiency of standard RMC in simulating the radiation transmission of inhomogeneous media.The accuracy evaluation system and convergence criterion of RDF are established,which provides theoretical guidance for evaluating the uncertainty of RDF and the photon sampling cut-off criterion of RDFRMC.According to the characteristics that spectral RDF is dependent only on the geometry and radiation physical properties of medium radiation system,it is introduced into the radiative transfer equation in integral form,and the analytical expressions of medium scattering enhancement source term and boundary reflection enhancement source term represented by RDF are derived.Combined with integral equation method(IEM)along the detection path,an RDFIEM method for solving the high-resolution radiative intensity in complex multi-dimensional participating media is developed.The effects of the number of photons tracked by the RMC and the SP_N solid angle discrete scheme on the memory requirements,calculation accuracy and efficiency of RDFIEM calculation are investigated,and the optimal combination of photon number and SP_N rule is given.The applicability and accuracy of RDFIEM in solving three-dimensional non-uniform participating medium radiation transmission problems with different medium radiation characteristics and wall reflection characteristics are further verified.In addition,because the RDF database only needs to be calculated once in the determined medium radiation system,RDFIEM has very high efficiency in the calculation of multi-directional radiative intensity signals.Based on the idea of Green function,an ES-RDFIEM method for solving RDF based on radiative transfer matrix equation is proposed,which opens up a new idea for calculating RDF database.The physical process of radiative transfer in complex participating media is analyzed as the superposition of radiative transfer effect under the excitation of point-by-point unit blackbody radiation source similar to point source Green function.According to the radiative transfer equation,the analytical calculation formulas of radiation parameters such as radiation source term,incident radiation and spatial radiation heat flux based on RDF are established.The RDF database is constructed by solving the linear matrix equations,which fundamentally avoids the serious time-consuming problem of traditional RMC photon tracking process and the problem of random statistical error,and improves the calculation speed and accuracy of RDF.The ES-RDFIEM method is applied to solve the radiative transfer problems under one-dimensional isotropic scattering,anisotropic scattering and diffuse boundary conditions,and gradually extended to three-dimensional non-uniform anisotropic scattering media.The effects of radiative physical properties,scattering characteristics and boundary conditions on the computational accuracy and efficiency of ES-RDFIEM are deeply analyzed.The results show that ES-RDFIEM has the same computational accuracy as RMC,and its computational efficiency can be improved by about 2-3 orders of magnitude for one-dimensional medium radiation transfer problem.Finally,the above radiation transfer method based on RDF is applied to the non-contact temperature measurement of three-dimensional high-temperature flame,and the fast and accurate acquisition models of high directional resolution photothermal information matching RDFIEM with multi camera imaging system and ES-RDFIEM with light field camera imaging system are established,respectively.The solution technology of inverse radiation problem based on L_p-norm regularization method and gradient descent method based on non-negative a priori information is developed,and the accurate reconstruction of flame temperature field of three-dimensional complex radiation system is realized.Through the study of various conditions including radiation physical properties,scattering characteristics,reflection characteristics and flame mode under the interference of measurement noise,the applicability and effectiveness of RDFIEM and ES-RDFIEM in flame non-contact temperature measurement are proved.The results also show that the orientation of the camera,the sampling way of pixels,the measurement noise level and the irregularity of the flame need to be fully considered in the process of flame 3D temperature field reconstruction.