As an alternative to the commonly used swirl burners in micro gas turbines (MGT), the FLOX®-based combustion concept promises great potential for the nitric oxide emission reduction and increased fuel flexibility. Previous research on FLOX®-based MGT combustors mainly addressed gaseous fuels and there is less experience available on liquid fuel FLOX®-based MGT combustors. A FLOX®-based liquid fuel burner is developed to fit into a newly designed combustor for the Capstone C30 MGT. The studied FLOX®-based burners consist of an air nozzle with a coaxially arranged fuel pressure atomizer. The combustion chamber walls are made of quartz glass to enable optical accessibility for analyzing the structural properties of the flame. Furthermore, a diagonal hot cross-flow is arranged to emulate the annular hot gas flow of the other two burners in the MGT. The cross-flow is realized by utilizing a 20-nozzle FLOX®-based natural gas combustor. Measurements include visualization of the reaction zone and analysis of the exhaust gas emissions. By detecting the hydroxyl radical chemiluminescence (OH*-CL) emissions, the position of the heat release zone within the combustion chamber is attained. Correspondingly, the flame lift-off height and flame length are calculated. The investigated design parameters include air preheat temperature up to 733 K, equivalence ratio, burner geometry, and thermal power. The work presented in this paper aims to deepen the understanding of the design parameter interactions involved within the single-nozzle liquid-FLOX®-based burners.
Computational Fluid Dynamics are widely used as a design tool for a variety of thermo-fluid systems. Advantages of those numerical approaches are clearly the fairly detailed degree of valuable data at low computational costs, when RANS (Reynolds Averaged Navier Stokes) methods are used in the system design process. In this work, a combustion system operated at elevated pressure conditions is re-designed with CFD RANS methods. The combustor is operated with liquid fuel and is positioned between an upstream recuperation and a downstream turbine section. System design is carried out on the basis of a commercially available C30 configuration from the Capstone® Turbine Corporation. The micro turbine produces 30kW of electrical power and is therefore highly suited for micro gas turbine related applications. In the design process, as presented in the paper, several modifications are carried out. The system recuperation is changed, thus inflow modifications are given. Recuperation was explicitly simulated and is used as a combustor inflow boundary condition. The system is then analyzed and modified in terms of air splits in order to achieve certain combustion characteristics. Optimization is carried out for combustor air splits and turbine inlet temperature profile conditions are significantly improved. Reacting multi-phase simulations are used in order to characterize flow field and combustion. Further on, conjugated heat transfer is taken into account in order to characterize temperature distribution in the combustor. Additionally, combustor residence times are determined. It is demonstrated that the pursued methods and procedures are computationally cheap but at the same time highly suited and sufficient for thorough combustion system development.
Increased global demand for cleaner energy production and growing concern about using fossil fuels have urged many researchers to focus their work on developing more efficient and flexible combustion processes. In this regard, a FLOX®-based liquid fuel single-nozzle burner is investigated for use in a Capstone C30 micro gas turbine (MGT). The main advantages of FLOX®-based combustor systems are their decreased NOx emissions and increased fuel flexibility. An atmospheric test rig is set up to investigate the behavior of the FLOX®-based liquid fuel burner under the influence of the hot gas. The circulating gas in the C30 annular combustion chamber is emulated by hot cross-flow gas generated by a 20-nozzle FLOX®-based natural gas burner operated on a separate horizontal test rig. The variation and combination of the process parameters of both burners are done systematically according to Design of Experiments (DOE) as a statistical design methodology. DOE methodology is adopted rather than the conventional one-factor-at-a-time (OFAT) strategy, as DOE considers any possible interaction between the factors and reduces the number of experiments. Employing statistical design of experiments allows determining which input variables are responsible for the observed changes in the response, developing a model relating the response to the important input variables, and using this model for improving the combustor system. The results are subsequently run through the Analysis of Variance (ANOVA) in order to allow for an objective conclusion about the effect of the factors on the selected responses, which include mass flow rate (·fuel) and global air equivalence ratio (λ) of both of the liquid and natural gas burners. The hot gas cross-flow interaction with the liquid fuel burner is assessed through analyzing exhaust gas emissions and averaged flame OH*-chemiluminescence images. The models developed by the DOE method can be used to estimate the emissions and the flame geometrical properties of any other operating points that are not explicitly tested.
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