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About this book

This book focuses on developing strategies for ultra-lean combustion of natural gas and hydrogen, and contributes to the research on extending the lean flammability limit of hydrogen and air using a hot supersonic jet. The author addresses experimental methods, data analysis techniques, and results throughout each chapter and:Explains the fundamental mechanisms behind turbulent hot jet ignition using non-dimensional analysis
Explores ignition characteristics by impinging hot jet and multiple jets in relation to better controllability and lean combustion
Explores how different instability modes interact with the acoustic modes of the combustion chamber.
This book provides a potential answer to some of the issues that arise from lean engine operation, such as poor ignition, engine misfire, cycle-to-cycle variability, combustion instability, reduction in efficiency, and an increase in unburned hydrocarbon emissions. This thesis was submitted to and approved by Purdue University.

Table of Contents

Frontmatter

Chapter 1. Introduction

Greenhouse gases trap heat and make the planet warmer. According to the Environmental Protection Agency (EPA), the four major greenhouse gases (GHG) are CO2, methane, oxides of nitrogen (NOx), and fluorinated gases such as hydrofluorocarbon, perfluorocarbon, sulfur hexafluoride, nitrogen trifluoride, etc. [1]. Figure 1.1 shows US greenhouse gas emissions in the years 1990–2015. Human activities are responsible for almost all the increase in greenhouse gases in the atmosphere over the last 150 years [2, 3]. Figure 1.2 shows the greenhouse gas emissions in the USA in 2015. One of the largest contributors toward greenhouse emission is carbon dioxide. Major production of CO2 is from burning fossil fuels such as coal, natural gas, and oil. Methane is emitted during the production and transport of coal, natural gas, and oil [4].

Sayan Biswas

Chapter 2. Ignition Mechanisms

As described in the literature review section in the previous chapter, there exists a knowledge gap how ignition initiates by a hot turbulent jet. What are the ignition mechanisms from a fundamental point of view? What are the nondimensional parameters governing the ignition mechanism? To explore the fundamental ignition mechanisms by a hot turbulent jet, an experimental setup was built that uses a dual-chamber design (a small pre-chamber resided within the big main chamber). Two fuels, methane and hydrogen, were studied. Simultaneous high-speed schlieren and OH* chemiluminescence imaging were applied to visualize the jet penetration and ignition processes. It was found there exist two ignition mechanisms – flame ignition and jet ignition. A parametric study was conducted to understand the effects of several parameters on the ignition mechanism and probability, including orifice diameter, initial temperature and pressure, fuel/air equivalence ratios in both chambers, and pre-chamber spark position. The mean and fluctuation velocities of the transient hot jet were calculated according to the measured pressure histories in the two chambers. A limiting global Damköhler number was found for each fuel, under which the ignition probability is nearly zero. Lastly, the ignition outcome of all tests (no ignition, flame ignition, and jet ignition) was marked on the classical turbulent combustion regime diagram. These results provide important guidelines for design and optimization of efficient and reliable pre-chambers for natural gas engines.

Sayan Biswas

Chapter 3. Schlieren Image Velocimetry (SIV)

Particle image velocimetry (PIV) is a quantitative optical method used in experimental fluid dynamics that captures entire 2D/3D velocity field by measuring the displacements of numerous small particles that follow the motion of the fluid. In its simplest form, PIV acquires two consecutive images (with a very small time delay) of flow field seeded by these tracer particles, and the particle images are then cross-correlated to yield the instantaneous fluid velocity field. The nature of PIV measurement is rather indirect as it determines the particle velocity instead of the fluid velocity. It is assumed in PIV that tracer particles “faithfully” follow the flow field without changing the flow dynamics. To achieve this, the particle response time should be faster than the smallest time scale in the flow. The flow tracer fidelity in PIV is characterized using Stokes number, S k , where a smaller Stokes number (S k < 0.1) represents excellent tracking accuracy. Conversely, schlieren and shadowgraph are truly nonintrusive techniques that rely on the fact that the change in refractive index causes light to deviate due to optical inhomogeneities present in the medium. Schlieren methods can be used for a broad range of high-speed turbulent flows containing refractive index gradients in the form of identifiable and distinguishable flow structures. In schlieren image velocimetry (SIV) techniques, the eddies in a turbulent flow field serve as PIV “particles.” Unlike PIV, there are no seeding particles in SIV. To avoid confusion, a quotation mark is used for “particles” when describing the SIV techniques. As the eddy length scale decreases with the increasing Reynolds number, the length scales of the turbulent eddies become exceptionally important. These self-seeded successive schlieren images with a small time delay between them can be correlated to find velocity field information. Thus, the analysis of schlieren and shadowgraph images is of great importance in the field of fluid mechanics since this system enables the visualization and flow field calculation of unseeded flow.

Sayan Biswas

Chapter 4. Supersonic Jet Ignition

Motivated by the fact that turbulent jets from straight nozzles could ignite a lean (0.5 < ϕ < 0.9) main chamber reliably as discussed in Chap. 2, we wanted to explore the possibility to reach ultra-lean limit using supersonic jets. The same experimental setup that uses a dual-chamber design (a small pre-chamber resided within the big main chamber) was used except the straight nozzles were replaced by converging or converging-diverging (C-D) nozzles. The primary focus was to reveal the characteristics of supersonic jet ignition, in comparison to subsonic jet ignition. Another intention behind supersonic jets was from ignition delay standpoint; a high-speed jet could well reduce the ignition delay. Simultaneous high-speed schlieren photography and OH* chemiluminescence were applied to visualize the supersonic jet penetration and ignition processes in the main chamber. Infrared imaging was used to characterize the thermal field of the hot jet. Numerical simulations were carried out using the commercial CFD code, Fluent 15.0, to characterize the transient supersonic jet, including spatial and temporal distribution of species, temperature and turbulence parameters, velocity, Mach number, turbulent intensity, and so on. The present work focuses on the effect of supersonic jets on lean flammability limits.

Sayan Biswas

Chapter 5. Combustion Instability at Lean Limit

In recent years gas engine manufacturers have faced stringent emission regulations on oxides of nitrogen (NOx) and unburned hydrocarbons (UHC) [1, 2]. Operating internal combustion engines at ultra-lean conditions can reduce NOx emissions and also improve thermal efficiency [3, 4]. An approach that can potentially solve the challenge of igniting ultra-lean mixtures is to use a reacting/reacted hot turbulent jet to ignite the ultra-lean mixture instead of a conventional electric spark [5–10]. The hot turbulent jet is produced by burning a small amount of stoichiometric or near-stoichiometric fuel/air mixture in a small volume separated from the main combustion chamber called the pre-chamber. The higher pressure resulting from pre-chamber combustion pushes combustion products into the main combustion chamber in the form of a hot reacting/reacted turbulent jet, which then ignites the ultra-lean mixture in the main combustion chamber. Compared to conventional spark ignition, the hot turbulent jet has a much larger surface area containing numerous ignition kernels over which ignition can occur. Hot jet ignition has the potential to enable the combustion system to operate near the fuel’s lean flammability limit, leading to ultralow emissions.

Sayan Biswas

Chapter 6. Ignition by Multiple Jets

The main reason that hot turbulent jet ignition has become attractive to gas engine manufacturers is that hot jet ignition can achieve faster burn rates due to the ignition system producing multiple, distributed ignition sites, which has greater likelihood igniting a lean mixture compared to spark ignition. This leads to better thermal efficiency and low NOx production. Compared to conventional spark ignition, a hot jet has a much larger surface area leading to multiple ignition sites on its surface which can enhance the probability of successful ignition and cause faster flame propagation and heat release. Over the last few decades, pre-chamber jet ignition had technologically advanced from conceptual design phase to actual engines. The early designs developed by Gussak [1–4], Oppenheim [5, 6], Wolfhard [7], and Murase [8] showed the promise of lean ignition by a hot turbulent jet. Later, Ghoneim and Chen [9], Pitt [10], Yamaguchi [11], Elhsnawi [12], Sadanandan [13], Toulson [14, 15], Gholamisheeri [16], Attard [17], Perera [18], Carpio [19], Shah [20], Karimi [21], Thelen [22], and Biswas [23, 24] further investigated in detail the parametric effects and fundamental physics of turbulent jet ignition in laboratory scale prototype combustors and at engine-relevant conditions. All these studies support that turbulent jet ignition possesses several advantages over traditional spark ignition during ultra-lean combustion such as higher ignition probability, faster burn rates, and multiple ignition kernels.

Sayan Biswas

Chapter 7. Impinging Jet Ignition

Turbulent jet ignition can reliably be used to ignite an ultra-lean fuel/air mixture as illustrated in previous chapters. This ignition technique can be utilized in various applications ranging from pulse detonation engines, wave rotor combustor explosions, to supersonic combustors and natural gas engines. Compared to a conventional spark plug, the hot jet has a much larger surface area leading to multiple ignition sites on its surface which can enhance the probability of successful ignition and cause faster flame propagation and heat release. In short, turbulent jet ignition has many advantages over conventional ignition system.

Sayan Biswas

Chapter 8. Flame Propagation in Microchannels

Combustion at small scales (micro- and mesoscales) is gaining increasing attention these days due to the wide spectrum of potential applications in sensors, actuators, portable electronic devices, rovers, robots, unmanned air vehicles, thrusters, industrial heating devices, and, furthermore, heat and mechanical backup power sources for air-conditioning equipment in hybrid vehicles and direct ignition (DI) engines as well [1–3]. Combustion of hydrocarbon fuels is more attractive to manufacturers of miniature power devices because the energy density of hydrocarbons is several times higher than modern batteries [4]. Microscale combustion physics is quite different from those at larger length scales. For example, flame propagation through narrow channels has unique characteristics, e.g., the increasing effects of flame–wall interaction and molecular diffusion [5–10]. In small-scale combustion systems, the surface-to-volume (S/V) ratio is large, which leads to more heat loss and thus causes flame extinction more easily.

Sayan Biswas

Chapter 9. Summary and Outlook

The ignition characteristics of methane/air and hydrogen/air mixtures using a hot turbulent jet generated by pre-chamber combustion were studied from a fundamental point of view. The existence of two different ignition mechanisms, namely, jet ignition and flame ignition, was found. Jet ignition produced a jet of hot combustion products (which means the pre-chamber flame is quenched when passing through the orifice); flame ignition produced a jet of wrinkled turbulent flames (the composition of the jet is incomplete combustion products containing flames). As the orifice diameter increased, the ignition mechanism switched to flame ignition, from jet ignition. With the increase in pressure, flame ignition became more prevalent. The ignition took place on the side surface of the hot jet during the jet deceleration process for both mixtures. A critical global Damköhler number, Dacrit, defined as the limiting parameter that separated ignition from no ignition, was found to be 140 for methane/air and 40 for hydrogen/air. All possible ignition outcomes were compared on the turbulent combustion regime diagram. Nearly all no-ignition cases fell into the broken reaction zone, and jet and flame ignition cases mostly fell within the thin reaction zones.

Sayan Biswas

Backmatter

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