Reaction Mechanism Reduction for Ozone-Enhanced CH4/Air Combustion by a Combination of Directed Relation Graph with Error Propagation, Sensitivity Analysis and Quasi-Steady State Assumption

In this study, an 18-steps, 22-species reduced global mechanism for ozone-enhanced CH4/air combustion processes was derived by coupling GRI-Mech 3.0 and a sub-mechanism for ozone decomposition. Three methods, namely, direct relation graphics with error propagation, (DRGRP), sensitivity analysis (SA), and quasi-steady-state assumption (QSSA), were used to downsize the detailed mechanism to the global mechanism. The verification of the accuracy of the skeletal mechanism in predicting the laminar flame speeds and distribution of the critical components showed that that the major species and the laminar flame speeds are well predicted by the skeletal mechanism. However, the pollutant NO was predicated inaccurately due to the precursors for generating NO were removed as redundant components. The laminar flame speeds calculated by the global mechanism fit the experimental data well. The comparisons of simulated results between the detailed mechanism and global mechanism were investigated and showed that the global mechanism could accurately predict the major and intermediate species and significantly reduced the time cost by 72%Peer reviewe

Transatmospheric vehicle research

Research was conducted into the alternatives to the supersonic combustion ramjet (scramjet) engine for hypersonic flight. A new engine concept, the Oblique Detonation Wave Engine (ODWE) was proposed and explored analytically and experimentally. Codes were developed which can couple the fluid dynamics of supersonic flow with strong shock waves, with the finite rate chemistry necessary to model the detonation process. An additional study was conducted which compared the performance of a hypersonic vehicle powered by a scramjet or an ODWE. Engineering models of the overall performances of the two engines are included. This information was fed into a trajectory program which optimized the flight path to orbit. A third code calculated the vehicle size, weight, and aerodynamic characteristics. The experimental work was carried out in the Ames 20MW arc-jet wind tunnel, focusing on mixing and combustion of fuel injected into a supersonic airstream. Several injector designs were evaluated by sampling the stream behind the injectors and analyzing the mixture with an on-line mass spectrometer. In addition, an attempt was made to create a standing oblique detonation wave in the wind tunnel using hydrogen fuel. It appeared that the conditions in the test chamber were marginal for the generation of oblique detonation waves

Numerical Investigation of Laser-Induced Ignition Phenomena

This thesis investigates various aspects of laser-ignition. Laser ignition is a form of combustion initiation by means of a focused laser pulse in a combustible mixture. The ignition process consists of a chain of processes with varying degrees of importance to the prediction of a successful flame propagation. Some of these processes include plasma formation, induced shock wave, emergence of a flame kernel, and successful transition to a self-sustained flame or flame quenching. This thesis will explore various aspects of this process using computational fluid dynamics and model analysis with the aim of identifying the controlling processes and simplified ways of capturing successful or failed ignition based on the injected laser energy, focusing optics and combustible gas compositions. The problem is motivated by practical considerations. Combustion systems are still the main energy conversion technologies and it appears that they will continue to be dominant in the near future. To address environmental pollution and sustainability concerns, clean and efficient systems are being explored. One of the key challenges encountered is the problem of assuring dependable ignition in these emerging technologies. Laser ignition is considered to be a promising technology which would guarantee smooth functioning of advanced clean and efficient engines. Benefits include its non-intrusive nature and the easy control of the spark location, timing, and energy deposition. For laser ignition systems to be useful, a good understanding of the process is needed. Understanding the degree to which each of the associated processes contributes to the development of a flame can lead to cost-effective models of ignition. This would align with current trends in computer aided engineering where simulations with physics-based models drastically reduce product development cycles. A perceived weakness in the laser ignition literature is the lack of simulations that compare models of different complexity in predicting the ensuing chemically reacting flows. The proposed research will focus on the laser ignition of methane and biogas from the perspective of numerical simulations. Experimental results will be used as validation targets for these simulations. The flow field and thermochemical features controlling the emergence of flame kernels will be determined. Explanations of possible quenching of the flame kernel will be sought. The problems addressed include numerical simulations of the laser-induced shock wave propagation, the transition of the laser-spark to a self-sustained flame with the help of chemical reactions, and the quenching of lean biogas flames. The shock wave study is found to accord with the blast wave theory, wherein the outward propagation can be predicted based on absorbed energy. Plasma kinetics is found to be unnecessary for the shock wave propagation. Using a compact or more detailed chemical scheme enables the prediction or the emergence of the flame. For prediction of the observed flame quenching behavior, however, the detailed scheme is necessary since the compact chemical scheme fails to capture the quenching event. Characteristic flow features are observed and explained in a manner that accords with experimental observations of global ignition features

Numerical Simulation of a Nano-pulsed High-voltage Discharge and Impact on Low-temperature Plasma Ignition Processes for Automotive Applications

Spark-ignition (SI) processes are facing some challenges with the SI engine research continuing to move towards extremely dilute operation. Typical response from the automotive OEMs is to increase the spark energy to hundreds of mJs. However, this approach reduces the spark-plug lifetime due to erosion. In recent years, several lowtemperature plasma (LTP) technologies (e.g. microwave, nanosecond pulsed discharge, Corona discharge) have been proposed for automotive applications as a substitute for the conventional SI process, yet no LTP ignition models are available for commercial computational fluid dynamics (CFD) codes for the evaluation and optimization of these advanced ignition systems. This paper summarizes recent efforts to model LTP generated by a nano-pulsed highvoltage discharge in a multi-dimensional fashion. Streamer discharges between two pin electrodes are modeled through 2-D computations using the non-equilibrium plasma commercial solver VizGlow. The impact of key parameters such as peak voltage and gas density on the characteristics of the streamers is evaluated. The experimental dataset is used to validate the numerical predictions in terms of thermal and chemical properties of the generated plasma at the end of the discharge. Then, the impact of the post-discharge characteristics on the LTP ignition process is evaluated in combustion simulations performed using the CFD code CONVERGE

Modeling Ignition and Premixed Combustion Including Flame Stretch Effects

Objective of this work is the incorporation of the flame stretch effects in an Eulerian-Lagrangian model for premixed SI combustion in order to describe ignition and flame propagation under highly inhomogeneous flow conditions. To this end, effects of energy transfer from electrical circuit and turbulent flame propagation were fully decoupled. The first ones are taken into account by Lagrangian particles whose main purpose is to generate an initial burned field in the computational domain. Turbulent flame development is instead considered only in the Eulerian gas phase for a better description of the local flow effects. To improve the model predictive capabilities, flame stretch effects were introduced in the turbulent combustion model by using formulations coming from the asymptotic theory and recently verified by means of DNS studies. Experiments carried out at Michigan Tech University in a pressurized, constant-volume vessel were used to validate the proposed approach. In the vessel, a shrouded fan blows fresh mixture directly at the spark-gap generating highly inhomogeneous flow and turbulence conditions close to the ignition zone. Experimental and computed data of gas flow velocity profiles and flame radius were compared under different turbulence, air/fuel ratio and pressure conditions

The inception of pulsed discharges in air: simulations in background fields above and below breakdown

We investigate discharge inception in air, in uniform background electric fields above and below the breakdown threshold. We perform 3D particle simulations that include a natural level of background ionization in the form of positive and O$_{2}^-$ ions. When the electric field rises above the breakdown and the detachment threshold, which are similar in air, electrons can detach from O$_{2}^-$ and start ionization avalanches. These avalanches together create one large discharge, in contrast to the `double-headed' streamers found in many fluid simulations. On the other hand, in background fields below breakdown, something must enhance the field sufficiently for a streamer to form. We use a strongly ionized seed of electrons and positive ions for this, with which we observe the growth of positive streamers. Negative streamers were not observed. Below breakdown, the inclusion of electron detachment does not change the results much, and we observe similar discharge development as in fluid simulations