| In today’s world,environmental pollution is becoming more and more serious,and hydrogen is better than traditional fuels in terms of clean combustion and energy saving.In addition,hydrogen has an important function to improve the combustion characteristics of traditional fuels.Therefore,it is important to study the combustion characteristics of hydrogen-added fuel.In this paper,a systematic LES（Large Eddy Simulation）study of partially premixed flame and diffusion flame of hydrogen-added fuel is carried out by using DTF（Dynamic Thickened Flame）combustion model.In this paper,the specific characteristics of the two types of flames are analyzed in terms of flame structure,flameout characteristics,and flow field evolution.Not only the combustion characteristics of hydrogen-added fuel are obtained,but also the applicability of DTF model to calculate partially premixed flame and diffusion flame is verified.Firstly,the GRI3.0 detailed reaction mechanism and the 28-step and 19-step simplified reaction mechanisms were used to compare the calculation of 1D laminar premixed flames under different hydrogen-added fuel states,and the calculation accuracy and efficiency of the three mechanisms were evaluated.Therefore,the 19-step simplified mechanism is chosen as the chemical reaction mechanism for subsequent calculations.Then the LES calculation was performed for the partially premixed flame of Sydney university with hydrogen-added fuel（H2:CH4:CO:CO2,0.2:0.2:0.27:0.33 in volume）as the fuel.The calculated blow-off velocities for the FA and FJ layouts of hydrogen-added fuel are 90 m/s and 109 m/s,respectively,while the corresponding flame blow-off velocities for pure methane are 74 m/s and 128 m/s,respectively.By comparing the plots of mixture fraction and temperature of the two fuels under FA/FJ layout,it is found that the blow-off speed under FA layout is smaller than that under FJ layout mainly due to the different mixing effect of fuel and air at the entrance of the combustion chamber,while the blow-off speed of methane under FA layout is smaller than that of hydrogen-added fuel mainly due to the fact that the methane flame temperature in the flow field is lower than that of hydrogen-added fuel.The blow-off speed of methane fuel under FJ layout is larger than that of hydrogen-added fuel mainly because the potential heat release of methane flame is larger than that of hydrogen-added fuel flame.The scatter plot analysis of the FA/FJ layout for both fuels shows that the flames in the FJ layout gradually change from the upstream partially premixed combustion mode to the downstream diffusion combustion mode,but the transition speed of hydrogen-added fuel lags behind that of methane fuel;the flames in the FA layout are in the diffusion combustion mode,and the temperature gradient in the rich premixed zone of hydrogen-added fuel is larger.Finally,LES calculation were performed for the bluff-body jet diffusion flame of Sydney university fueled by hydrogen-added fuel（H2:CH4,1:1 in volume）,and the HM1 flame and HM3 flame with blow-off ratios of 0.5 and 0.9,respectively,were calculated.The analysis of the temperature plots of HM1 and HM3 flames revealed that both flames showed the typical combustion characteristics of diffusion flames,but the flame surface of HM3 is more broken,and severe local flameout occurred in the intense mixing zone.The uniformly distributed mixture fraction upstream of the recirculation zone and the low scalar dissipation rate in the recirculation zone enable the recirculation zone to stabilize the flame.The flame combustion and the vortex structure of the recirculation zone have an interactive relationship.The statistical data analysis of the flow fields and components of HM1 and HM3 flames found that good agreement was obtained between the experimental and calculated values,demonstrating the ability of the DTF model to reasonably predict local flameout and large-scale mixing of the flow fields in diffusion flames. |
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