Plasma Ignition and Stabilization of Flames

General Features of Plasma-Assisted Ignition and Combustion

Spark ignition is one of the oldest applications of plasma, known and successfully applied for thousands of years. Even in the automotive industry, spark ignition has been applied for more than a hundred years. Nevertheless, other plasma discharges, especially, non-thermal discharges, have been attracting more and more attention for use in ignition and stabilization of flames. An example, in this regard, is the non-thermal plasma ignition of fuel–air mixtures at moderate pressures and high velocities, including ignition in supersonic flows, plasma enhancement of combustion at atmospheric pressure, and stimulation of combustion of lean mixtures (Anikin et al., 2005; Starikovskaia, 2006). Numerous investigations have been focused on plasma ignition and stabilization of flames. The effectiveness of spark ignition relies on the essential non-uniformity of the thermal plasma of spark discharges and, therefore, restrictions of the system geometry (see, for example, Thiele, Warnatz, & Maas, 2000). Relevant application of thermal arc discharges, related in particular to hypersonic flows, has been analyzed, for example, by Takita (2002) and Matveev et al. (2005). Initiation of flame by a short-pulse thermal discharge and a conventional arc has been investigated in CH4–air mixtures using the time-resolved interferometry by Maly and Vogel (1979). The ignition effect of gliding arc discharges, which generates non-thermal plasma but also can result in some controlled heating, has been analyzed by Ombrello et al. (2006a, b).

The yield of a total plasma-chemical process is due to synergistic contributions of numerous different elementary reactions taking place simultaneously in a discharge system. The sequence of transformations of initial chemical substances and electric energy into products and thermal energy is usually referred to as the mechanism of the plasma-chemical process. Elementary reaction rates are determined by the micro-kinetic characteristics of individual reactive collisions (like, for example, reaction cross sections or elementary reaction probabilities) as well as by relevant kinetic distribution functions (like the electron energy distribution function [EEDF], or population function of excited molecular states). The elementary reaction rate is actually a result of integration of the reaction cross section or probability over the relevant distribution function and characterizes the energy or excitation state of reactants. We will focus in this chapter mostly on the micro-kinetics of the elementary reactions – on their cross sections and probabilities – assuming, if necessary, conventional Maxwellian or Boltzmann distribution functions. More sophisticated non-Maxwellian and non-Boltzmann kinetic distribution functions typical for strongly non-equilibrium discharge conditions, like the Druyvesteyn EEDF for electrons or Treanor distribution for vibrationally excited molecules, are to be considered in the next chapter.

Ionization Processes

Plasma is an ionized gas. The key process in plasma is ionization, which means conversion of neutral atoms or molecules into electrons and positive ions. Thus, ionization is the first elementary plasma-chemical processes to be considered.

Although the public understanding of plasmas may be limited to plasma TVs, low-temperature plasma processes are beginning to enter into a higher level of consciousness due to the importance of plasma in many aspects of technological developments. The use of plasma for industrial purposes began more than 100 years ago with plasma sources used to produce light. Since then, plasma processes have emerged in transforming wide-ranging technologies, including microelectronics, gas lasers, polymers and novel materials, protective coatings, and water purification, and finally found their ubiquitous place in our homes. Plasma systems or plasma-treated materials are now commonly used and can be found in air-cleaning systems; food containers; fruit, meat, and vegetable treatment; fabrics; and medical devices.

In recent years, new application areas of plasma chemistry and plasma processing have been established, such as plasma nanotechnology with the continuous growth of the “dusty plasmas” domain, plasma production and modification of nanotubes, plasma aerodynamics, and plasma ignition and stabilization of flames. With the recent emphasis on alternative energy and environmental concerns, plasma chemistry has revolutionized hydrogen production, biomass conversion, and fuel-cell technology. In the same manner, the use of non-thermal plasmas in biology and medicine will likely “explode” in the coming years for various applications. Plasma is expected to soon be widely used in surgery, decontamination and sterilization of surfaces and devices, and air and water streams, as well as in tissue engineering and direct treatment of skin diseases.

As noted in §1.5.1, a reference frame consists of a spatial coordinate system and clock. The principle of material frame indifference requires constitutive equations to be invariant or unaffected by an arbitrary time-dependent translation, rotation, and reflection of the coordinate axes and by an arbitrary translation of the time axis. In contrast, constitutive equations expressed in tensor form are invariant under time-independent transformations of the coordinate axes, but they are not necessarily invariant under time-dependent transformations.

The basis for frame indifference is the intuitive idea that the response of a material should be independent of the motion of the observer (Oldroyd, 1950; Noll, 1955, p. 45; Noll, 1959). The principle cannot be proved, but simple examples have been given (Truesdell and Noll, 1965; Hunter, 1983, p. 123) where its validity appears plausible. An example taken from Hunter (1983, p. 123) is discussed below.

A pilot bails out of an aircraft and opens his parachute. Suppose that the force P exerted by the parachute on the pilot is measured by the extension of a spring attached to the harness of the parachute. Consider two reference frames, one fixed to the pilot and the other to an observer standing on the ground. Suppose that the spring is visible in both the frames. When relativistic effects are ignored, it is usually implicitly assumed that at any time t, the distance between two points in space has the same value regardless of the motion of the reference frame (see, e.g., Resnick, 1968, p. 5). This assumption implies that the pilot and the observer on the ground will record the same extension for the spring.

A granular material is a collection of solid particles or grains, such that most of the particles are in contact with at least some of their neighboring particles. The terms “granular materials,” “bulk solids,” “particulate solids,” and “powders” are often used interchangeably in the literature. Common examples of granular materials are sand, gravel, food grains, seeds, sugar, coal, and cement. Figure 1.1 shows the typical size ranges for some of these materials.

Granular materials are commonly encountered in nature and in various industries. For example, with reference to the chemical industry, Ennis et al. (1994) note that about 40% of the value added is linked to particle technology. Similarly, Bates (2006) notes that more than 50% of all products sold are either granular in form or involve granular materials in their production. In spite of the importance of granular materials, their mechanics is not well understood at present. Nevertheless, some progress has been made during the past few decades. The goal of this book is to describe some of the experimental observations and models related to the mechanical behavior of flowing granular materials. As studies in this area are increasing rapidly, our account is necessarily incomplete. However, it is hoped that the book will provide a useful starting point for the beginning student or researcher.

A material is called a dry granular material if the fluid in the interstices or voids between the grains is a gas, which is usually air. On the other hand, if the voids are completely filled with a liquid such as water, the material is called a saturated granular material.