Experimental And Numerical Studies Of Humid Air Diffusion Flames

Humid air combustion is a key process of the advanced thermodynamical cycles such as HAT, IGCC and IGHAT, so its performance is directly related to the system performance of the thermodynamical cycles. Thus, according to the requirement of constructing the cycles, the diffusion combustion of natural gas and synthesis gas from coal gasification in humid air was studied experimentally and numerically. The purpose is to achieve more clear and deep understanding on the concept of humid air combustion and then help the operation on humid air combustion and humid air combustor design of gas turbines.First of all, the influence of humid air on flame shape, size and color was studied. It was found in the experiments that the size of natural gas flames significantly increased as the air became more humid under the condition the fuel flow rate was increased when increasing air humidity thus to keep the exit temperature of the combustor to be constant. While for the synthesis gas flames, the influence of air humidity on flame size is not so apparent. Via theoretical analysis and numerical simulation, it has been known that the change of flame size is induced by the comprehensive impact of the variation of oxygen concentration and flow field due to air humidification. On the basis of the previous study, a method for improving the condition that the size of methane-humid air flame is too large was proposed and the feasibility of the method was validated by experiments.Then the stability of diffusion flames in humid air was studied. The experimental results show that the stability of methane-humid air flames is much poor than that of synthesis gas-humid air flames. The reson of the poor stability of the methane flames was known via analysis and calculation. A method to improve the stability of the methaneflames was proposed and validated experimentally.Finally, the characteristics of pollutant generation and emission for the humid air combustion of different types of fuel were studied. The experimental results of a full-size combustor show that the influence of humid air on pollutant emission is different for hydro-carbon fuels and synthesis gas, especially on CO emission. Due to there exists some doubts on the influence of humid air on CO emission, the process was calculated and analyzed considering the chemical mechanism by using an opposed diffusion flame. Subsequently, the mechanism of CO emission of synthesis gas-humid air diffusion flames was studied by using a model combustor. It was found that the CO emission did not increase monotonically with air humidity. However, the CO emission would at first decrease and then increase. The results of numerical simulation clearly explained the reason of the exhibition of this trend.After the research work of the present study, the profile of the influence of humid air on diffusion flames and combustor performance becomes basically clear.