NEW CORRELATIONS OF WEIGHTED SUM OF GREY GASES MODEL APPLICABLE TO COMPUTATIONAL FLUID DYNAMICS FOR OXY-FUEL COMBUSTION AND IMPLEMENTATION

摘要

Radiation heat transfer is the dominant model of heat transfer in the large scale industry boiler, especially in oxy-fuel combustion condition. Radiative properties of combustion gases and char oxidation in the oxy-fuel condition are obviously different from the air-fuel combustion, due to the N_2 replaced by CO_2. Through researchers proposed many helpful correlations based on the air-fuel Weighted-Sum-of-Grey-Gases-Model (WSGGM), the absorption coefficients were commonly constant or correlations were discrete by the classical molar ratio of H_2O to CO_2 (MR), which were mismatching the continuous value of MR in the real furnace. Meanwhile, the discrete MR is also not applied to the computational fluid dynamics (CFD). In this paper, new correlations for the WSGGM are determined as polynomial function of MR and temperature, which can be conveniently employed in Fluent by the form of user-defined-functions in C language. Parameters of model are fitted by total emittances calculated based on the timely HITEMP 2010 database. New correlations are validated by comparing the emittances with line-by-line calculations and other classical models. New correlations are employed in the CFD for the real industrial oxy-fuel combustion with the temperature range of 400-2600K, pressure path-length between 0.01 and 60 bar m. Several assumed test cases have been investigated to evaluate the accuracy of the models. Modified correlations for WSGGM give a better accuracy of the total emittances for the mixed combustion gases in the real furnace. New models including radiative and chemical reaction mechanisms have been employed to CFD modeling of combustion process for a tangentially fired 300MWe utility boiler. The industrial boiler is modeled by a partition meshing method with the hexahedral structured mesh. Due to the atmosphere shift from N_2 to CO_2, three aspects are essential to be modified for oxy-fuel: radiation model, char oxidation model and homogeneous volatile oxidation model. To investigate the performance of the furnace, air-fuel combustion selected as the conference, three other cases employed are defined as Oxy21 (vol21%, O_2), Oxy26 (vol26, O_2) and Oxy29 (vol29%, O_2), respectively. Temperature profile and heat transfer are investigated for the different test cases. Meanwhile, the simulation and calculation heat transfer in the furnace are also compared. The results show the new modified simulation has an approximate 4-11% lower than the thermodynamic calculation. To achieve an identical heat flux and temperature distributions with the air-fuel case, the molar fraction 29% of O_2 is essential for the selected implementation. (CSPE)