Computational Studies of Reactive Flow
In view of the shortage of fossil fuel supply and the environmental impact arising from carbon emission and greenhouse gas effects of these fuels, there is an urgent need to explore alternative sources. This dissertation mainly addresses three topics related to alternative fuels: investigation of dimethyl ether (DME) combustion properties, development of skeletal mechanism for methane-hydrogen-air combustion, and modification of nitrogen chemistry of the San Diego mechanism.
Experimental and computational studies are carried out to elucidate the structure and extinction of laminar partially-premixed flames employing the counterflow configuration, where fuel-rich stream is made up of DME, and nitrogen (N2) with small amounts of oxygen (O2) and the fuel-lean stream is made up of O2, and N2 with small amounts of DME. To clarify the chemical influences of partial premixing on extinction, studies are carried at values of stoichiometric mixture fraction and temperature for various values of equivalence ratio at fuel-lean and fuel-rich sides. The experiments for two cases, addition of DME to fuel-lean stream and addition of oxygen to the fuel-rich stream, are conducted. The key observation is that the former case enhance the overall reactivity while latter case has little influence on the overall reactivity.
The second contribution of this dissertation is to eliminate unimportant steps from a detailed chemical-kinetic mechanism in order to identify a skeletal kinetic mechanism that can predict with sufficient accuracy ignition delay times and laminar premixed-flame velocities for hydrogen-methane mixtures under conditions of practical interest in gas-turbine applications, which pertain to high pressure, high reactant temperature, and primarily lean-to-stoichiometric mixture compositions. Thirty nine reversible elementary steps involving eighteen species are found to describe with sufficient accuracy.
The third contribution of this dissertation is to update a chemical-kinetic combustion mechanism for employing ammonia-hydrogen mixtures in gas turbines as drop-in fuel to replace the use of natural gas. This mechanism encompasses 60 elementary steps among 19 reactive chemical species which can be efficiently utilized in large-scale CFD calculations. The performance of the new mechanism is discussed through comparisons of its predictions with experimental data.