On the Influence of Fuel Stratification and its Control on the Efficiency of the Shockless Explosion Combustion Cycle

Abstract

Constant volume combustion cycles for gas turbines are considered a very promising alternative to the conventional Joule cycle and its variations. The reason is the considerably higher thermal efficiency of theses cycles, at least for their ideal versions. Shockless explosion combustion is a method to approximate constant volume combustion. It is a cyclic process that consists of four stages, namely wave propagation, fuel injection, homogeneous auto-ignition and exhaust. A pressure wave in the combustion chamber is used to realize the filling and exhaust phases. During the fuel injection stage, the equivalence ratio is controlled in such a way that the ignition delay time of the mixture matches its residence time in the chamber before self ignition. This means that the fuel injected first must have the longest ignition delay time and thus forms the leanest mixture with air. By the same token, fuel injected last must form the richest mixture with air (assuming that a rich mixture leads to a small ignition delay). The total injection time is equal to the time that the wave needs to reach the open combustor end and return as a pressure wave to the closed end. Up to date, fuel stratification has been neglected in thermodynamic simulations of the SEC cycle. The current work presents its effect on the thermal efficiency of the cycle and on the exhaust conditions (pressure, temperature and Mach number) of shockless explosion combustion chambers. This is done by integrating a fuel injection control algorithm in an existing CFD code. The capability of this algorithm to homogenize the auto-ignition process by improving the injection process has been demonstrated in past experimental studies of the SEC. The numerical code used for the simulation of the combustion process is based on the time-resolved 1D-Euler equations with source terms obtained from a detailed chemistry model.