| How to improve the accuracy of radiative temperature measurement for high-temperature surface interfered with participating media,which refer to absorbing,emitting,and scattering media,is a common problem widely existing in metal smelting,petrochemical industry,aerospace,fire fighting,military blasting and other processes involving high-temperature combustion.Due to the absorption,emission and scattering of radiation energy by participating media,the measurement accuracy of radiative temperature is greatly reduced.The key to solve these problems is to understand the influence mechanism of participating media on radiation energy transfer,to make clear the influence of participating media on radiative temperature measurement method,so as to find out the effective method to eliminate its interference.Slab surface temperature is a very important control parameter in steel continuous casting.At present,non-contact method is mainly used to measure slab surface temperature.However,large amount of water vapor,water mist,dust and oil in the secondary cooling zone and the iron scale and water film distributed intermittently on the slab surface seriously affect the accuracy and reliability of the radiative temperature measurement for slab surface in the secondary cooling zone.For this reason,based on the mechanism of radiation energy transfer in participating media,this paper theoretically and experimentally investigated and summarized the effects of water vapor,water mist and iron oxide on the accuracy of radiative temperature measurement for slab surface.The main results are summarized as follows:(1)Combined with the theory of radiation energy transfer in participating media,this paper established a mathematical model of surface radiation temperature measurement in participating media,verified the accuracy of the model calculation by comparing with other results from literature.This model includes three sub-models:radiative transfer model in participating media,radiative properties of participating media and Measuring Principles of Radiation Thermometry.By using the model,the radiative properties of gases and particles are obtained by using statistical narrow band model and global,local grey band approximation and Mie scattering theory respectively;spatial radiation intensity and heat flux distribution are obtained by using discrete coordinate method and Monte Carlo method to solve the radiative transfer equation;theoretical measurement temperature is obtained by solving the radiation temperature measurement model with the mentioned heat flux substituted.This model builds a bridge between real target temperature and measured radiation temperature.Theoretical radiative measurement temperature can be obtained by solving the model with target real temperature,meanwhile,the modified target temperature can also be obtained by solving the model iteratively with actual radiometric temperature measurement.In terms of radiative properties of participating media,heat flux distribution,theoretical radiometric temperature measurement and modified target temperature,the model prediction results were compared with the experimental measurement results and theoretical calculation results in the literature to verify the accuracy of model calculation.(2)Based on the established radiative temperature measurement model,this paper quantitatively analyzed the individual and coupling effect of environmental radiation and absorptive gas on the accuracy of radiative temperature measurement using every infrared band;analyzed the cause of measurement error,explained the oscillation behavior of two-color temperature measurement data in different working bands;presented The methods of reducing and eliminating the effects of environmental radiation and gaseous media.The study found that in low temperature environment,short-wavelength two-color pyrometer can avoid the influence of environmental radiation more effectively than single-color pyrometer;high-temperature ambient temperature and high-temperature gas temperature are the main factors affecting the accuracy of radiation temperature measurement;due to low-temperature intervening gas,single-color pyrometer reading temperature is lower than true temperature,while two-color pyrometer reading temperature is lower or higher than true temperature depending on the working wavelength;the influence of low-concentration gas media on the temperature measurement can be effectively reduced by selecting working bands around 1.0 um,1.6 um,1.25 um and 2.2 um;when the measurement distance is short,the single and two-color temperature measurement error varies linearly with the distance,and this variation trend provides a linear extrapolation method to eliminate measurement errors.These results can be used to explain experimental phenomena and help to understand,recognize and avoid the influence of participating media.(3)To reduce or eliminate the influence of participating media in measuring environment,this paper proposed a concept named spectral energy decay rate and established the criteria for selecting the optimal wavelength of single and two-color pyrometer interfered with participating media on the basis of Planck's law and Wien's law.The applicability of the criteria is simulated and studied for the measuring environment of gas medium with strong spectral selectivity and particle medium with strong spectral scattering.The study found that the value of C_S and C_D can correctly reflect the single and two-color temperature measurement errors respectively;The closer the C_S value of single-color pyrometer or the C_D value of two-color pyrometer is to zero,the smaller their measurement error value is,respectively,which means that their working wavelength is more suitable for current working environment.The wavelength selection criteria of the radiation thermometers proposed in this paper have good scientific value and applicability,thus providing a necessary basis for better selection of the working wavelength of the single and two-color thermometers interfered with participating media.(4)In this paper,an experimental device for measuring the surface temperature of high temperature objects by radiation is constructed to simulate the secondary cooling zone environment of continuous casting;the experiments of radiation temperature measurement under different water mist thickness and different target temperature are carried out;Combined with thermocouple temperature measurement,the effect of surface oxide scale and water mist on single and two-color radiation temperature measurement error is analyzed.For the radiation thermometer containing single-color mode(0.75?1.1um or 0.95?1.1um)and two-color mode(0.75?1.1um/0.95?1.1um),when there is water mist in the measuring path,two-color temperature is higher than target real temperature,and single-color temperature is lower than target real temperature,and when the water mist concentration changes,the trend of two-color temperature is opposite to that of single-color temperature.Due to water mist,the two-color temperature error is less than the single-color temperature error.With the increase of thickness of water mist,the measured two-color temperature increases,while the measured single-color temperature decreases.Due to surface oxide scale,the two-color temperature error is smaller than the single-color temperature error,and the variation trend of the single-color temperature is the same as that of the two-color temperature.The mentioned experimental results show that two-color pyrometry can overcome the influence of participating media more effectively than single-color pyrometry.In the case of unknown water mist concentration,the true temperature range of the target can be obtained from single and two-color temperature measurement data.(5)Combined with theoretical calculation and experimental results,an online surface temperature measurement system for continuous casting secondary cooling environment,which is resistant to water vapor,fog and water film interference,has been developed.The accuracy of the measurement results is verified by the comparison between the on-line surface temperature measurement results and on-line solidification mathematical model calculation results.The stability of the online temperature measurement system is demonstrated by comparing the historical temperature measurement results and calculating the fluctuation variance and standard deviation of measured temperature.The system adopts a temperature band near 1.0um with high transmittance of water vapor and water film medium.The online temperature measurement results of casting slab surface show that the slab surface temperature decreases steadily in the direction of casting,and the surface temperature at 500mm to the right of straightening inlet center is usually higher than the surface temperature at the straightening inlet center.The influence of production process parameters,such as casting speed,on slab surface temperature was analyzed to guide the distribution of secondary cooling water in the production process. |
PDF Full Text Request