Studies On Flame Characteristics And Direct Moment Closure Model For Pulverized Coal Gas-Solid Turbulent Combustion

The theory of gas-solid turbulent combustion represented by coal combustion constitutes the basis for developing new combustion technologies with high efficiency and low pollutant emissions.Mastering the inherent characteristics of gas-solid combustion and their effective control and reasonable utilization is of significant importance to solve China's energy use and pollution control issues.With the development of computer technology and numerical calculation theory,computational fluid dynamics has become an important tool for studying gas-solid two-phase turbulent combustion.In order to reveal the characteristics of pulverized coal gas-solid turbulent combustion flame,based on the high-precision DNS database of pulverized coal combustion,the flame structure,combustion characteristics and stabilization mechanism of pulverized coal flame are studied in detail,which will provide more understandings of pulverized coal combustion.In addition,in order to accurately calculate the actual multi-regime combustion process of pulverized coal flames,this paper innovatively proposes a direct moment closure model,and applies it to pulverized coal combustion after careful validations,which has academic and engineering significance.First,based on the detailed analysis of high-precision DNS of pulverized coal flame,the flow characteristics,particle distribution,combustion characteristics,flame structure and flame stabilization mechanism of the pulverized coal flame are studied in detail.It is found the mean velocities for both gas-phase and coal particles can develop into self-similar profiles.The ignition and combustion process can significantly enhance the turbulence intensity.By studying the flame features,three typical flame structures of coal jet flames,namely interspersed flame,stripe flame and stable flame base,are identified.Though with different intuitive flame patterns,these three typical flame zones have similar gaseous reaction and the non-premixed flame dominates.By comparing the heat release proportion of the three zones,it is found that the contribution of upstream interspersed flame is less.The stripe flame contributes about 26%and is important to the whole stable flame formation downstream.The evolutions of flame base and stabilization point show that the autoignition of the volatile is the key mechanism responsible for the current pulverized coal jet flame.Because of the co-exist of premixed and non-premixed combustion in the DNS coal flame,a turbulent combustion model for all flame regimes is needed.Then a-priori validation using a DNS database is performed to test the unified second-order moment combustion model proposed by Prof.Zhou Lixing,which laid the foundation for the development of direct moment closure model.It is found that the overall idea and method are feasible,but there are some deficiencies,such as the neglecting of density fluctuation and the uncertainty of the model coefficient selection.Thus a new direct moment closure model considering the density fluctuation is proposed in this paper.Different from the traditional moment closure method,in order to avoid the Taylor expansion,the whole highly non-linear temperature exponential function term is treated as a single variable K in the direct moment closure method,and correlation moments up to third-order are used to close the averaged reaction rate.By further ignoring some moments,the second-order moment and first-order moment models can be obtained.In the direct moment closure method,temperature variance is used to close K,and the algebraic closure form is used to model the second-order moments,where the model coefficients are determined by dynamic procedure.Because there is no assumption on combustion regimes,this model can be used for the prediction of premixed flames,non-premixed flames,and partially premixed flames.Subsequently,in order to verify the applicability of the new model,a-priori and a-posteriori analysis are carried out by studying a DNS premixed flame and laboratory-scale Sydney partially premixed flames with inhomogeneous inlets.The a-priori results show that compared with the traditional moment closure method,the direct moment closure method can reduce the number of higher-order moments.When the reaction zone is larger,the second-order moment closure method can be used to close the averaged reaction rate by ignoring the third-order moment.The tests of K and second-order moments show that the proposed closure methods are feasible.By doing the a-posteriori studies using the low Mach large eddy simulations of these two flames,it is found that the second-order moment closure method can obtain the results consistent with the measurement.Meanwhile the current model can capture the transition from premixed combustion to non-premixed combustion modes,demonstrating that the second-order model can be applied to the prediction of multi-regime flames.Based on the LES results of the partially premixed flames with inhomogeneous inlets,the combustion regimes and extinction phenomena are analyzed in detail,and the related flame stabilization enhancement mechanism is deduced,which is important to actual gaseous combustion devices.Finally,the direct moment closure model is extended to gas-solid two-phase turbulent combustion.By performing the large eddy simulation to simulate the DNS pulverized coal flame,it is found that the current LES can also obtain the similar flame as the DNS.