Investigation On The Characteristics Of Combustion Induced Vortex Breakdown Flashback Of Swirl-premixed Hydrogen Flame

To achieve near-zero emission, integrated gasification combined cycle (IGCC) is considered as one of the most promising technologies. Recently, interests in gas turbine engines burning hydrogen have increased as pre-combustion capture and sequestration of CO2 has become a viable option for IGCC power plants. However, flashback phenomena, especially CFVB (Combustion Induced Vortex Breakdown) flashback, hampers the development of hydrogen fired gas turbine.Given this, this dissertation describes the numerical and experimental investigations of CIVB flashback carried-out on a swirl-premixed burner prototype supplied by hydrogen-nitrogen mixture. Firstly, the flame property data of hydrogen were collected and the appropriate chemical mechanism and CFD models were screened out for the CIVB flashback simulation. Secondly, to study the effects of multi-factors, including combustor geometry, preheating temperature, fuel composition and mass flow rate, design of experiments (DoE) method was used to arrange numerical trials. Through this kind of analysis, the importance sequencing of all factors and the factors that have significant effects on CIVB flashback have been determined. To validate the reliability of numerical investigations on the effects of multi-factors, experimental activities were also performed. Then, the generating process and mechanism of CIVB flashback were explored by monitoring the variation of physical quantities, including axial velocity, pressure, temperature and azimuthal vorticity, of two points inside the mixing zone. Finally, the conclusions of CIVB flashback analysis were put into application. The conclusions can be briefly summarized as below:1. Numerical investigation on the effects of multi-factors on CIVB flashbackVia DoE analysis, the factors are listed in descending order of the importance as swirler geometry, mixing zone geometry, preheating temperature, fuel composition, chamber geometry and mass flow rate. Wherein, the first four factors have highly significant effect on CIVB flashback which should be considered carefully during the combustor design. The factors take effect on CIVB flashback by changing flow distribution and flame property. Based on the effects of multi-factors on flashback, scope of the time scale model limited to specific combustor geometry previously is extended to various combustor geometries. At this time, once the quenching constant Cq of the modified time scale model is determined experimentally or numerically, the flashback limits for various operating conditions and geometries can be predicted. This is of great significance for assisting the design of upgraded combustor quantitatively.2. Experimental investigation on the effects of multi-factors on CIVB flashbackThrough experimental analysis, the results about the effects of mixing zone geometry, mass flow rate and fuel composition are according with the numerical results, which confirms the reliability of numerical simulations. In addition, the modified time scale model is also confirmed. Furthermore, the proportional relation between mass flow rate and Cq is complemented in the modified time scale model.3. Numerical investigation on the generating process and mechanism of CIVB flashbackThe whole flashback process could be divided into three phases, including flame propagation upstream, vortex breakdown and flame stabilization, which are described by one-dimensional conservation model, vorticity transport equation model and time scale model correspondingly. The division of the phases, one-dimensional conservation model and time scale model are universal, and has nothing to do with fuel composition or combustor geometry. However, the occurence of vortex breakdown or the vorticity transport equation model shows a relation with centerbody. For burner with centerbody, adverse pressure gradient causes deceleration of the axial velocity and starts the whole flashback process. Subsequently, azimuthal vorticity works and induces axial velocity opposite to local axial velocity, which firstly reinforces and then weakens the deceleration of axial velocity. At the same time, the value of axial velocity affects azimuthal vorticity in turn. Azimuthal vorticity and axial velocity counteracts each other until an equilibrium state is established. Under the equilibrium state, axial velocity, azimuthal vorticity and pressure difference reach individual extremum value and vortex breakdown occurs. For burner without centerbody, the vortex breakdown process is almost the same with the burner with centerbody, except that azimuthal vorticity reinforces the deceleration of axial velocity continually until vortex breakdown occurs. Vortex breakdown represents the flow route of CIVB flashback while flame propagation upstream and stabilization inside the vortex represents the flame route. Nevertheless, the occurrence of vortex breakdown causes irreversible pressure difference loss, which restricts the further change of flow route and flame route. This suppressive effect represents the feedback route of CIVB flashback. Only the three routes work together, does CIVB flashback occur.4. Investigation of CIVB flashback controlAccording to the results about the effects of multi-factors on CIVB flashback, the passive control of the CIVB flashback could be implemented by adjusting swirler geometry and mixing zone geometry. For the original geometry, once Cq is determined, the upgraded geometry parameter could be deduced via Cq and the expectant flashback limit. In addition, according to the CIVB flashback mechanism, active control could be implemented by injecting an additional air stream before mixing zone and near centerbody to weaken pressure gradient. Through comparion, the passive control method is recommended in the current application, while the active control method could be considered as the further goal.5. Improvements of design method for swirl-premixed combustor fired with hydrogenCompared to natural gas, there are some differences in designing hydrogen combustor:(1) need to check both the iso-thermal and the reactional flow fields, (2) consider the design of swirler geometry and mixing zone geometry carefully and adopt the swirl intensity as low as possible, and (3) avoid formating wake vortex behind fuel injecting or at the edge of swirler vanes.Overall, through the numerical and experimental studies, the effects of combustor geometry parameters and operating parameters on CIVB flashback have been illustrated, the generating process and mechanism of CIVB flashback have been uncovered, and the obtained conclusions have been applied to presenting flashback control methods and improving the design method of swirl-premixed combustor fired with hydrogen.