Directly heated supercritical oxy-fuel power cycles have potential to offer a higher thermal efficiency and lower pollutant emissions compared to existing power cycles. Due to the fact that combustion occurs at the supercritical phase, usually at or above 30MPa, conventional gas turbine combustors cannot be used to produce electricity using this concept. Besides, oxy-fuel combustion produces relatively higher temperatures than the air-fuel combustion process, which introduces material limitations particularly at the high pressures. Motivated by the advantages of the directly heated supercritical phase combustion technique, the authors present a design of an injector inside a combustion chamber. The combustor presented in this paper is designed to operate at a 30MPa pressure. The current study incorporates methane as the fuel and oxygen as the oxidizer. The design process of such a combustor incorporates an immense amount of challenges since the fluid properties as well as combustion behavior at the supercritical phase are still largely unknown. However, in order to design this type of system a reasonable understanding of fluid behavior in the combustion environment is essential.
A commercial computational fluid dynamics simulation tool, ANSYS Fluent, is used to simulate the fluid flow and combustion inside the combustor. The real gas effect is added into the code by coupling equation of state with the fluid flow computation. The Lee Kesler equation, which is found in literature and in this paper to be the most relevant equation of state for this case, is implemented via user defined function along with Plocker Knapp mixing rule. The system presented in this study is intended to operate at a 1 MW power input. Carbon dioxide is delivered axially into the combustion chamber separate from fuel and oxidizer. The study shows more than 90% carbon dioxide needs to be recirculated to keep the combustor exit temperature below material operation limit of approximately 1450K. Thus, it is presumable that the combustion occurs in a carbon dioxide filled environment. The investigation has been performed by varying the carbon dioxide mass flow rate splitting between two inlets. It is observed that the 30%–70% split displays the optimum performance among all three cases. This condition offers wall temperature reduction, uniform exhaust products and an even temperature profile at the outlet of the combustor.