Research On Measurement Method For Radiation Properties Of Liquid-particle Suspensions

Particulate material can be dispersed in gases,liquids and solids,and accumulated to form the aerosols,the particle suspensions and the solid composites.The radiative properties of the liquid-particle suspensions have important applications in the solar energy,microbial energy and detection of biological fluid.The knowledge of the radiative properties in the liquid-particle suspensions is fundamental to the analysis and optimization of the radiative transport process.Currently,some works have been devoted to electromagnetic theory and experimental measurement.However,due to the complexity and uncertainties of particle morphology and grain structure,theoretical analysis method is difficult to provide accurate result for the engineering problems.Becasuse of the effect of multiple scattering between the particles,the difference in refractive index between two neighboring media at interface and the coupling of particle scattering and the interface reflection,it is difficult to accucate measure the radiative properties in the liquid-particle suspensions via experimental measurement.At present,the experimental research method of radiative properties in the liquid-particle suspensions needs to be further studied.In this paper,high-precision experimental method for measuring radiative properties in the liquid-particle suspensions is investigated.According to analyze the influence factor on the precision of measurement of radiative properties in the liquid-particle suspensions,the high-precision experimental method of the particle-suspension system,including window material,base fluid and particle,is developed.Also,the experimental system was constructed for measuring the radiative properties of liquid-particle suspensions.The main work includes:The optical constants of the window materials can directly affect the measurement accuracy of the radiative properties in the liquid-particle suspensions.Generally,window materials are offering a wide range of high transparency,but it's difficult to measure their optical constants precisely,especially the absorption index in the transparent spectral region.A combination technique(DOPTM-EM)using both the double optical pathlength transmission method(DOPTM)and the ellipsometry method(EM)is presented to obtain the optical constants of highly transparent substrates,which overcomes the deficiencies of either of the two methods.The EM cannot give accurate result of optical constants when the absorption index is very weak.The DOPTM is suitable to retrieve the weak absorption index,however,two sets of solutions exist for the retrieved refractive index and absorption index,and only one of which is the true value that needed to be identified.In the DOPTM-EM,the optical constants are measured first by using the EM and set as initial value in the gradient-based inverse method used in the DOPTM,which ensures only the true optical constants are retrieved.BaF2 substrate was taken as an example to verify this method.The results show that the improved method can obtain reliable and highly precise values of optical constants.The optical constants of five example materials under different temperature(Ba F2,CaF2,MgF2,ZnSe and ZnS)were measured using the DOPTM-EM in spectral range from the ultraviolet to the infrared.The base fluid of the liquid-particle suspensions is generally a liquid medium whose optical constants are important parameters for analyzing radiative properties of the particle suspensions.It is noted that the measurement of absorption index in the visible spectral region is difficult due to the low absorption in this spectral range.The improved double optical pathlength transmission method(IDOPTM)is implemented to the base fluid.An inverse identification model using ray tracing approach based on the glass–sample–glass is proposed,which is more suitable to obtain the optical constants of liquid medium.Deionized water was taken as an example to verify this method.The IDOPTM provides accurate results of refractive index and absorption index of deionized water in the research spectral range,the relative difference of refractive index is less than 1%.The results show that the absorption index of sodium chloride solution increases with sodium chloride concentrations in the weak-absorption region,but varies little in the strongabsorption region at room temperature.The absorption index exhibits little difference from the different types of edible oils in the weak-absorption region.The variation of absorption index was found to be insensitive to the increasing of temperature in the spectral range from 300 to 500 nm and from 1100 to 2500 nm.In the traditional transmission method for measuring the extinction coefficient of particles in a cuvette,the multiple reflections and refractions between the air/vessel and liquid/vessel interfaces cannot be sufficiently eliminated by using the reference system,and the induced measurement error increases significantly with increasing difference in refractive index between two neighboring media at these interfaces.An improved transmission method(ITM)is proposed to measure the extinction coefficient of liquidparticle suspensions with various mean particle sizes.The problem of the traditional transmission method and the influence of forward scattering photons were investigated.The extinction coefficient of the particles is determined based on an optical model taking into account the multiple reflection and refraction at the glass/liquid interfaces.By considering the higher-order transmission terms,the improved transmission method generally achieved high accuracy improvement over the traditional transmission method for extinction coefficient measurement,especially for the case with small optical thickness of particle suspensions.An experimental validation was conducted for SiO2 suspensions with various mean particle sizes.The measured results obtained by the improved method agree well with the predicted values of Lorentz-Mie theory.The results show that the higher-order transmission terms caused by multiple reflections at medium interfaces cannot be ignored when the optical thickness of particles is small.This work provides an alternative and more accurate way for measuring the extinction characteristics of micro/nano particle suspensions.The extinction coefficient becomes the key parameter to be determined to solve the radiative transfer equation in photobioreactors,which are used for cultivating microalgae,to predict the local fluence rate.The presented methods will facilitate the measurement of radiative properties for microalgae suspensions.The optical extinction characteristics of one kind of freshwater algae(Chlorella sp.)and three kinds of microalgae(Nannochloropsis maritima,Ellipsoidion sp.(277.03)and Dunaliella tertiolecta)were experimentally determined using an improved transmission method in the near ultraviolet,visible and near-infrared spectral range.The measured extinction cross-sections of the microalgae cells are independent of cell concentrations and decreases with the increase of wavelength in the studied spectral range.Because the effect of multiple reflections at vessel/sample and vessel/air interfaces is not considered for the non-immersion method,the scattering phase function of particles is difficult to be accurately measured.An improved method is presented to obtain the scattering phase function of liquid-particle suspensions.The correction coefficient of the scattering light is obtained using standard particles with known optical constants and particle diameter distribution for eliminating the influence of the vessel.The correction coefficient is used to correct the scattering phase function of measured particles.The results obtained by using the correction coefficient agree well with the predicted values of Lorentz-Mie theory.The method is demonstrated to be able to obtain precise scattering phase function of particles.