Experimental And Numerical Study On Mild Combustion In A Serial Staged Combustor

Mild combustion is characterized by homogenous temperature field, low combustion noise and low pollution emission. All of these features make mild combustion is promising in being utilized in gas turbine combustor. In present study, mild combustion focusing on gas turbine combustor is mainly achieved by mixing fresh fuel/air with flue gas. Therefore, a better understand of the interaction between fresh fuel/air and flue gas is meaning for promoting the application of mild combustion. Besides of this, among all of the combustor structure used to achieve mild combustion, staged combustion, which can supply flue gas in a convenient way and does not depend on complicated structure to achieve flue gas recirculation, is a promising method. While with the present serial staged combustor both topics mentioned above can be investigated.First stage of this combustor is used to produce axial flow high temperature flue gas, which is used to assist to achieve mild combustion. In the mixing part, fresh CH4 and air mixture mix with flue gas in the way of crossflow injecting. In the result, mild combustion happens in the second stage. In the experiment, two types second stage, one with divergent part and the other's shape is cylinder, are utilized based on different research purposes. For the one with divergent part, flame structure and phenomena of ignition delay are studied. While cylinder shape second stage is mainly used for investigating the properties of pollution emissions. In the research, four variables, which include first stage equivalence ratio, first stage mass flow rate, second stage equivalence ratio and second stage mass flow rate, are considered.For combustor with divergent part, OH-PLIF images and CFD simulation are used to analysis the flow field, composition profile and temperature field of second stage. Based on the results, flame characteristics and ignition delay features are investigated. Results show that mixing characteristics and initial temperature are the most important factors. When keeping other parameters unchanged, better mixing and higher initial temperature will make shorter ignition delay time and more intense combustion region.Pollution emissions, which include NO and CO, of cylinder shape combustor are measured. Together with chemical reaction network simulation, emission properties of the combustor are investigated. In the simulation, the mild stage is simplified to Plug Flow reactor. Results indicate that NO formation mainly influenced by combustion temperature and residence time. Higher combustion temperature and longer residence time will result in more NO formation. While for CO emission, the simulation results are much higher than measurement results. This is because simulation employs adiabatic model and theoretical temperature is much higher than real one. In this condition some CO2 will dissociate to CO, as a result, CO concentration become much higher.Based on the present study, the flame feature, ignition delay and properties of pollution emission of mild combustion are better understood.