Sayan Biswas

A Numerical Investigation of Ignition of Ultra-Lean Premixed H 2 /Air Mixtures by Pre-Chamber Supersonic Hot Jet

Gas engines often utilize a small-volume pre-chamber in which fuel is injected at near stoichiometric condition to produce a hot turbulent jet which then ignites the lean mixture in the main chamber. Hot jet ignition has several advantages over traditional spark ignition, e.g., more reliable ignition of extra-lean mixtures and more surface area for ignition resulting in faster burning and improved combustion burn time. Our previous experimental results show that supersonic jets could extend the lean flammability limit of fuel/air mixtures in the main chamber in comparison to subsonic jets. The present paper investigated the characteristics of supersonic hot jets generated by combustion of stoichiometric H2/air in a pre-chamber to understand the ignition mechanism of ultra-lean mixtures by supersonic hot jets. Numerical simulations were carried out to examine the transient hot jets issued from six different nozzles (two straight nozzles, one converging nozzle, and three converging-diverging (C-D) nozzles) using a detailed H2/air chemistry. The detailed flame propagation process inside the pre-chamber was investigated. Then the characteristics of the hot jets from six nozzles were compared, including the spatial and temporal distribution of velocity, vorticity, pressure, turbulence quantities, temperature, shock structures, and species concentrations. The results show that supersonic jets exhibit shock diamond structures. The static temperature rises after each shock and a significant temperature rise occur after the final shock. The location of this high-temperature zone is consistent with the experimental observations where ignition was initiated. The profile of Damköhler numbers based on the local flow properties was determined. A critical Damköhler number was found to be 11, below which the main chamber ignition would unlikely to occur. Additionally, the Damköhler number profiles help to explain why the two C-D nozzles with an area ratio of 4 and 9 could extend the flammability limit, whereas the C-D nozzle with an area ratio of 16 failed to do so.

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.

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.

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.

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.

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.

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.