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Constant volume combustion (CVC) in gas turbines is a promising way to achieve a step change in the efficiency of such systems. The most widely investigated technique to implement CVC in gas turbine systems is pulsed detonation combustion (PDC). Unfortunately, the PDC is associated with several disadvantages, such as sharp pressure transitions, entropy generation due to shock waves, and exergy losses due to kinetic energy. This work proposes a new way to implement CVC in a gas turbine combustion system: shockless explosion combustion (SEC). This technique utilizes acoustic waves inside the combustor to fill and purge the combustion tube. The combustion itself is controlled via the ignition delay time of the fuel-air mixture. By adjusting the ignition delay in a way such that the entire fuel-air volume undergoes homogeneous auto-ignition, no shock waves occur. Accordingly, the losses associated with a detonation wave are not present in the proposed system. Instead, a smooth pressure rise is created due to the heat release of the homogeneous combustion. The current paper explains the SEC process in detail, and presents the identified challenges. Solutions to these challenges and the numerical and experimental approach are presented subsequently alongside with first preliminary results of the numerical studies.

With the ongoing stagnation of the progress towards higher efficiency gas turbines, alternative approaches in combustion receive more attention than ever before. Besides, increasing efficiency and reducing emissions at the same time has become a first priority of the industry in the last few decades. Constant volume combustion is considered a technology capable of achieving a significant increase in thermal efficiency when applied in gas turbines. In this work, models of gas turbine cycles with two different combustion methods, being a shockless explosion combustion and an isobaric homogeneous combustion, will be simulated and compared. A code based on the one dimensional Euler equations is utilized to calculate the exhaust gas outlet parameters of the shockless explosion combustion chamber, while taking into account all the gas dynamic phenomena in it. The efficiency of the turbine is computed by steady state operational maps. The simulations provide numerous detailed results with a focus on the dependency of the SEC cycle’s thermal efficiency to the compressor pressure ratio and the turbine inlet temperature. Evaluating the kinetic energy in the total enthalpy of the turbine inlet flow is also an essential investigation.

Abstract. Fuel-air mixing is a crucial process in low emission combustion systems. A higher mixing quality leads to lower emissions and higher combustion efficiencies. Especially for the innovative constant volume combustion processes "Shockless Explosion Combustion" (SEC) the mixing of fuel and air is an important parameter, since the whole combustion process is triggered and controlled via the equivalence ratio. To enhance the passive scalar mixing, fluidic oscillators are investigated and compared to the standard jet in crossflow fuel injection configurations. The mixing quality of the different geometries is assessed in a water test-rig by making use of planar laser induced fluorescence. After a short introduction to the SEC-process, the test-rig and the different injection configurations are introduced. To verify whether the mixing quality is sufficient for the SEC-process, a numerical investigation using the experimentally determined unmixedness is conducted. It is not only shown that the fluidic oscillators are able to enhance the mixing quality and create an independence of the mixing quality from the jet in crossflow momentum, but it is also verified in a first numerical calculation that the achieved mixing quality might be good enough for the Shockless Explosion Combustion process.

A change in the combustion concept of gas turbines from conventional isobaric to constant volume combustion, such as in pulse detonation combustion (PDC), promises a significant increase in gas turbine efficiency. Current research focuses on the realization of reliable PDC operation and its challenging integration into a gas turbine. The topic of pollutant emissions from such systems has so far received very little attention. Few rare studies indicate that the extreme combustion conditions in PDC systems can lead to high emissions of nitrogen oxides (NOx). Therefore, it is essential already at this stage of development to begin working on primary measures for NOx emissions reduction if commercialization is to be feasible. The present study evaluates the potential of different primary methods for reducing NOx emissions produced during PDC of hydrogen. The considered primary methods involve utilization of lean combustion mixtures or its dilution by steam injection or exhaust gas recirculation. The influence of such measures on the detonability of the combustion mixture has been evaluated based on detonation cell sizes modeled with detailed chemistry. For the mixtures and operating conditions featuring promising detonability, NOx formation in the detonation wave has been simulated by solving the one-dimensional (1D) reacting Euler equations. The study enables an insight into the potential and limitations of considered measures for NOx emissions reduction and lays the groundwork for optimized operation of PDC systems.

Constant-volume (pressure-gain) combustion cycles show much promise for further increasing the efficiency of modern gas turbines, which in the last decades have begun to reach the boundaries of modern technology in terms of pressure and temperature, as well as the ever more stringent demands on reducing exhaust gas emissions. The thermodynamic model of the gas turbine consists of a compressor with a polytropic efficiency of 90%, a combustor modeled as either a pulse detonation combustor (PDC) or as an isobaric homogeneous reactor, and a turbine, the efficiency of which is calculated using suitable turbine operational maps. A simulation is conducted using the one-dimensional reacting Euler equations to obtain the unsteady PDC outlet parameters for use as turbine inlet conditions. The efficiencies for the Fickett–Jacobs and Joule cycles are then compared. The Fickett–Jacobs cycle shows promise at relatively low compressor pressure ratios, whereas the importance of the harvesting of exhaust gas kinetic energy for the cycle performance is highlighted.

The shockless explosion combustion (SEC) is a novel approach to constant volume combustion in gas turbines. It promises an efficiency gain comparable to that of pulse detonation combustion (PDC), but without the drawbacks associated with detonations. It utilizes homogeneous combustion of a volume of fuel/air to avoid strong shock waves, similar to the RCCI process in internal combustion engines. Recharging is handled analogous to a pulse jet through the pressure waves in the combustion chamber. To achieve homogeneous auto ignition, the process involves setting up a stratified layer of fuel/air such that it homogeneously auto ignites in an approximately constant volume combustion (CVC). This becomes feasible by using fuels with small dependence of their auto ignition time on temperature, e.g. blends involving fuels with negative temperature coefficient (NTC) behaviour. The ignition process of such fuels is complex and often involves multi-stage ignition on time scales comparable to the acoustic time scale. It is hence expected that even though a SEC effectively is CVC, the ignition can not be modeled in 0D, but that it instead involves complex interaction between gas dynamics and chemical kinetics. The stratification process therefore has to be numerically optimized in CFD calculations. Optimization, especially if whole cycles are to be simulated, requires small kinetics models, even if restricted to one dimension, to be computationally feasible. On the other hand, the interaction of kinetics and gas dynamics at ignition rules out an easy to evaluate optimization goal for reduction of the chemical kinetics using offline methods like directed relation graphs and the techniques based on sensitivity analysis introduced by Williams/Peters. Even if sufficient computation power was invested, the accuracy constraints on the auto ignition times severely limit the usability of a conventional reduced mechanism. Online tools like CSP or ILDM would be an option for practical purposes, but do not provide insight into the ignition process and are therefore of little help for fundamental research. For a CFD simulation of a SEC, a new ansatz has therefore been developed. We exploit the constraint of a small temperature dependence of the auto ignition time on temperature to introduce a model specialized for SEC simulation that is sufficiently small to allow optimization of a fuel/air stratification, yet features correct auto ignition delay times for each ignition stage to the accuracy of experimental measurements. We then proceed to present simulation results which a posteriori justify our approach and demonstrate that shockless explosion combustion is feasible.

Recently, the focus has been laid on the characteristics of pollutant emissions from pulse detonation combustion (PDC). Initial studies indicate possibly high nitrogen oxides (NOx) emissions, so the assessment of potential primary reduction methods is advisable. The present work considers the following reduction methods: lean combustion, nitrogen and steam dilution, as well as flue gas recirculation. Since such changes in the combustion mixture reduce its reactivity and thus detonability, they can impair a reliable operation in technical systems. In order to explore the potential and limitations of each of these reduction methods, they are compared for mixtures featuring an identical characteristic detonation cell size at given initial conditions. Furthermore, building upon the use of steam dilution, a modified method to add steam to the combustible mixture is investigated. In order to avoid the strong reduction of mixture detonability by steam addition and ensure a robust detonation formation, steam is injected into the already developed detonation front. It was found that, for sufficiently even steam distribution, NOx reduction comparable to a premixed dilution could be achieved. This approach enables the realization of NOx reduction in PDC also for such conditions, for which premix dilution is not feasible. Therefore, combining the premix dilution with postshock injection offers a promising strategy to substantially reduce NOx emissions from PDC, while at the same time ensuring its reliable operation.