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Ignition phenomena in shock tubes and the influence of non-ideal effects on measurements of combustion chemistry: Experiments, modeling, and simulations

Subject Area Fluid Mechanics
Chemical and Thermal Process Engineering
Term from 2016 to 2025
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 279056804
 
Final Report Year 2025

Final Report Abstract

Shock tubes provide an excellent platform for the development of chemical reaction mechanisms as they allow the isolated study of high-temperature reactions without interference from mixing and transport processes. For conditions relevant to practically important low-temperature combustion processes, several inherent phenomena can cause spatial and temporal inhomogeneities that lead to early local ignition and prevent simple data interpretation. Such ‘non-ideal’ processes include the interaction of the shock wave with boundary layers, delayed membrane opening, shock-wave bifurcation, early heat release and presence of hot particles. The overall goal of the project is to study inhomogeneous ignition processes in a high-pressure shock tube to determine the most suitable experimental configurations that enable direct or model-based evaluation. The combination of experiment and simulation also helps to exclude those reaction conditions, where a concatenation of several effects makes data interpretation impossible. In the first project period, non-ideal facility-dependent effects were investigated. By combining experimental data from different shock tubes and the support of simulation, an understanding of these effects was achieved and correlations between measured variables such as the gradual increase in pressure as a function of the experimental conditions were derived. The experiments in the first project period used ethanol as a model fuel for inhomogeneous ignition. The numerical simulations motivated the development and implementation of a method portfolio for the numerical simulation of chemically reacting compressible flows with compression shocks in three spatial dimensions (3D). The main challenge was on the implementation of high-order discretization schemes, necessary for the high-fidelity large-eddy simulations or direct numerical simulations of the ignition event and of the shock-boundarylayer interaction. The high-resolution 3D simulations were enabled by limiting the simulation domain to the end-section of the shock tube, which in turn required the development of suitable strategies for the boundary condition at the inlet. The second phase was focused on the very profound study of different sources of inhomogeneous ignition and the possibilities to suppress and avoid this phenomenon. The experiments were focused on propane as fuel which shows a great affinity to inhomogeneous ignition shown by the significant differences of shock-tube ignition delay time (IDT) measurements and simulations at low temperatures and long IDTs. The influence of a great variance of experimental parameters on the premature ignition, such as equivalence ratio, inert gas species and concentration, and facility-dependent effects were determined. The cooperation with the group of Prof. Eric Petersen at TAMU (J. Mike Walker ’66 Department of Mechanical Engineering, Texas A&M University, College Station, TX, United States) allowed us to determine the facilitydependent effects of different shock tubes. The TAMU group also performed high-speed camera measurements of the inhomogeneous ignition of propane providing deeper insight into this phenomenon. In the last phase of the second period, we used a new test section that provides extended accessibility for spatially and temporally resolved detection of pressure, temperature and local ignition. 0D, 2D, and 3D simulations were performed to achieve a fundamental understanding of the underlying effects. The new test section also allowed constrained-reactionvolume (CRV) measurements that we used as a second method to suppress premature ignition besides adding helium to dissipate ignition far away from the end flange. The flow solvers and simulation strategies developed in the first period enabled highly resolved simulations of the remote ignition in the shock tube, leading to valuable insights about the process, and helping to understand the limits of simplified (1D) models for prediction of remote ignition. Furthermore, the developed solver was used in cooperation with the group of Prof. Hai Wang (Stanford University, United States) to generate a set of direct simulations of a detonation structure and provided a unique, high value dataset.

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