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

Abstract

Constant volume combustion (CVC) 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 these cycles, at least for their ideal versions. Shockless explosion combustion (SEC) is a method to approximate CVC. 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 auto-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 computational fluid dynamics 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.

Part Load Control for a Shockless Explosion Combustion Cycle

“…The goal is to adjust both values, T Air,k and T F,k , so that, at the moment of the first ignition, an ignitable fuel-air mixture lies exactly between the inlet and a given position of the tube. This was not possible with the previously proposed controller [3]. The desired position r P is the reference that can be changed to decrease or increase the main control variable for part load control, i.e., the length of the combustion region.…”

“…Regarding the cyclic characteristic of SEC, it seems reasonable to use an iterative learning controller (ILC), which improves fuel stratification from cycle to cycle. Such an ILC setup for SEC simulation has been introduced in a former study of full load operation [3]. As the scope of this work is focused on control of the length of the combustion region, the ILC system and the applied model will only be summarized shortly in the next two sections, giving further details not presented previously [3].…”

“…Thus, all parameters of the PI controllers are set with regard to the ignition time controller, which is designed first. Since previously [3] we were able to produce adequate results with an ILC system for full load operation, the same parameter settings are adopted for this work as well. This includes the weighting of the absolute control error W r as well as the penalization of its variance with W V within the cost function, see Eq.…”

“…However, due to the intricate relation between chemical and acoustic timescales, as proposed in the original idea of SEC [1], it is not obvious how this could be achieved. Moreover, the controller that has been proposed so far [3] has resulted in a homogeneous auto-ignition within a combustion region that did not start at the inlet of the tube. The scope of this work therefore will be the introduction of a control system that provides the capability of driving the SEC process to requested part load points of operation, i.e., set-points holding these states, and achieving ignition from the start of the tube.…”

Comprehensive Thermodynamic Analysis of the Humphrey Cycle for Gas Turbines with Pressure Gain Combustion

“…They used a typical operational map of a turbine expander and computed the work output with a quasi steady-state model. This approach has been adopted in the work of Stathopoulos [12] and Rähse [13,14] to compute the thermal efficiency of the pulsed detonation and the shockless explosion cycles. In this case, the processes in the combustor are resolved in time by solving the 1-D time dependent Euler equations with source terms for the chemical reaction.…”