Experiments And Analysis Of Effects Of Mixing Process On Swirling Premixed Combustion Stabilization

Lean premixed combustion technology is widely used to reduce NO_xemission in gas turbines.But it is facing the problem of combustion oscillation,which is accompanied by noise and pressure fluctuations in combustors and can do harm to structure components of combustors.The occurrence of combustion oscillation is significantly influenced by the mixing process of fuel and air.Previous researchers have conducted some investigations on this topic,but the mechanism is not so clear yet.Therefore,the effect of the mixing process of fuel and air on combustion stability is investigated in this thesis through a series of experiments in a swirling premixed model combustor.In addition,three-dimension numerical simulation of cold-state flow and mixing process in the model combustor is conducted to support the analysis of experimental results.Mixing time is an important variable to describe the mixing process.In the exper-iments premixing length and air flow velocity is changed to change mixing time.Under some certain premixing length and flow velocity,a number of cases with different equiv-alence ratios are conducted to obtain information of combustion stability and combustion oscillation modes to analyze the effect of mixing time on combustion stability.The fuel used in the experiments is natural gas.Air is used at ambient temperature and the pres-sure at the combustor exit is atmospheric pressure.Corresponding to experiment cases,swirling flow characteristics and the mixing process of fuel and air are computed with various premixing length,air flow velocity and equivalence ratio.Experimental results indicate that combustion oscillation in the combustor is longi-tudinal and the oscillation frequency is about 500 Hz.Combustion stability is influenced by equivalence ratio,air flow velocity and premixing length.Lean combustion oscillation only occurs when equivalence ratio is larger than 0.8 in the model combustor.At different premixing length,the range of air flow velocity where combustion oscillation occurs is obviously different.Combustion oscillation is more prone to occur at higher flow velocity with longer premixing length but more prone to occur at lower flow velocity with shorter premixing length.The disagreement can be unified via mixing time:combustion oscilla-tion can only occur when mixing time is in some range out of which combustion is always stable.The regularity indicates that mixing time has significant influence on swirling premixed combustion stability.It is possible to avoid or weaken combustion oscillation through appropriate configuration design.In experimental investigation of the thesis,the mean flow velocity of air flow is used to approximately calculate mixing time instead of the flow velocity of fuel flow.Results of numerical computation indicate that the approximate mixing time is a little shorter than practical mixing time.But the deviation induced by approximation has little influence on the length of the range of mixing time in which combustion oscillation occurs.The spatial mixedness of fuel and air has important effect on combustion characteristics.Results of numerical simulation indicate that when air flow velocity and equivalence ratio is constant,the increasing of premixing length improves the mixedness of fuel and air obviously when premixing length is relatively short.When premixing length is long enough,the mixedness keeps almost unchanged with premixing length increasing.If equivalence ratio and premixing length is kept constant,air flow velocity has little influence on the mixedness of fuel and air.The investigation in this thesis reveals the mixing process of fuel and air in swirling premixed combustors and its effects on combustion stability.The results are meaningful for the design of industrial burners and for further academic research on the topic.