Large outdoor fires have been a significant problem all over the world for some time. This ‘FEATURE’ introduces s the role of combustion research in large outdoor fire problems. Although the fires themselves are large, the fundamental of fire spread remain the same as smaller fires; flame contact, radiation and firebrands. The attempts to understand large outdoor fires are introduced from four aspects; the firebrands, fire whirl, flame spread, and wildfire modeling from the combustion prospective.
Firebrands are produced from combustion of both vegetative and structural fuels in large outdoor fires. It is well known that firebrand generation, transport, and ignition mechanisms result in rapid and potentially devastating fire spread processes in large outdoor fires. In this article, the basic mechanisms of firebrand generation, transport, and ignition are discussed with an emphasis on how fundamental combustion knowledge may play an important role in this complex problem.
The formation of a fire whirl in a large-scale urban or wildland fire leads to a significant increase in the fire spread rate, and its strong whirlwind (whose maximum speed may reach 60-80 m/s) also causes a serious damage. It is therefore important from a fire safety point of view to understand when and how fire whirls are generated. This article reviews previous fire whirl studies with particular focus on the mechanism of flame-height increase and the condition of fire whirl generation. Studies that used rotating or fixed-frame fire-whirl generators are first reviewed to discuss mechanisms of flameheight increase. Enhanced heat transfer to the combustible material within the Ekman layer and flow laminarization in the rotating fluid are identified as factors that increase the height of a fire whirl. The critical crosswind velocity that leads to the formation of strong fire whirls and its scaling law are then discussed by reviewing studies on fire whirls in crossflows.
Flame spread is the process by which a flame propagates over the surface of a solid fuel. Its practical interest is that is a primary mechanism for a fire to develop and grow. However, it is also of fundamental interest because the spread of a flame is a complex process involving the interaction between solid thermo-physical processes (heat transfer, thermal decomposition, gasification) and gas thermo-chemical processes (transport, mixing, chemical kinetics). As these processes are complex, simplifications are often made to gain some traction. One approach that is discussed here is to non-dimensionalize the conservation equations, evaluate the dominant terms while neglecting those less important for the situation at hand. Using this approach, simplified models for both opposed and concurrent flame spread over the surface of solid combustible materials in laboratory scale environments are derived from the concept of flame spread as a series of ignitions where the flame is both the source of heating and ignition. Our treatise concludes by discussing the application of these concepts to spread in both microgravity and porous fuel beds (wildland fires). The insight gained from such a fundamental understanding of the flame spread process can inform many aspects of large outdoor fires, be they through structures or wildland fuels.
Since the end of 90', the mechanical engineering and combustion communities have manifested a growing interest in wildfires physics with a significant effort in the development of new approaches in wildfires modelling on more physical bases. Wildfires physics is certainly one of the more complex problem in fire engineering and also in combustion science. The spectra of length scales (idem for the time scales) associated to wildfires is very large (ranged between 100 μm for the flame depth to several 100 kms for the plume trajectory), coupling many non-linear physical phenomena such as the atmospheric turbulence in interaction with the vegetation and the flame, the degradation of the vegetation, the radiation heat transfer associated to the production and the transport of soot particles from the burning zone, and so on ... For all these reasons the intima understanding of wildfires behavior is very challenging, justifying easily in classifying this problem as a multiscale complex problem. The aim of this short review is to identify some progress done these last years in the understanding of the physics of wildfires and to elaborate some strategies for the future in order to propose some new approaches and engineering tools for the reduction of fire risk in impacted zones by this natural disaster, with a particular interest to the wildland urban interfaces (WUI).
Absorption spectroscopy uses the absorption phenomena to measure concentration and temperature. The strength of the permeated light is related to the absorber concentration according to Lambert-Beer's law. Atomic or molecular concentration and temperature are determined by the line shape functions and the Boltzmann equation. There are several methods which uses the principle of absorption phenomena. These include Tunable Diode Laser Absorption Spectroscopy (TDLAS), Fourier transform infrared spectroscopy (FTIR), non-dispersive infrared spectroscopy (NDIR), photoacoustic spectroscopy (PAS), and cavity ring-down spectroscopy (CRDS). This paper explains the basics of absorption spectroscopy for the combustion application. The basics and application of TDLAS using Computer Tomography (CT) is also introduced as a 2D measurement method which can attain the time-series imaging in measured fields.
Laser absorption spectroscopy (LAS) plays an important role in diagnosing a wide range of combustion systems. Among various laser diagnostic techniques, LAS is relatively simple, quantitative, and versatile; therefore, it has been widely used to measure gas composition, concentration, temperature, etc. in both laboratory and industrial systems. Recently, convenient access to the entire spectrum of the species is possible because a wide range of light sources are available. Especially, mid-infrared lasers, such as quantum-cascade lasers or interband cascade lasers, have become advanced, and provide highly sensitive measuring capabilities for gas species in combustion. In this article, applications of LAS to chemical kinetics studies on the low-temperature oxidation of hydrocarbons and studies on the measurements of trace gas emitted from combustion systems (e.g. internal combustion engines) are introduced. First, fundamentals of commonly used highly sensitive measurement techniques, such as frequency modulation spectroscopy (FMS) and cavity ring-down spectroscopy (CRDS), are introduced. Next, some recent chemical kinetic studies using highly sensitive LAS techniques and emission measurements are introduced. Finally, our latest studies on HCHO measurements in the low-temperature oxidation of isooctane using a rapid compression machine and a mid-infrared LAS, and trace gas measurements of ethylene in the gasoline engine emissions have been described.
In order to ignite fuel spray directly by a laser beam, it is necessary to investigate the characteristics of laser-induced breakdown ignition and generation of plasma as ignition sources in fuel spray. This study conducted experiments of laser-induced breakdown ignition in an steady ethanol/air premixture and ethanol spray flow by using third harmonic of the Q-switched Nd:YAG laser. For evaluation of breakdown occurrence and ignition probability, the flame and the plasma images were obtained. And laser beam incident energy was also measured. Although probability of the breakdown occurrence and the ignition increased with the increase of incident energy, there are great difference between the breakdown threshold and the minimum ignition energy in the case of spray flow. Also, in spray flow, plasma was generated at multiple points and its number, size and position are different from pulse to pulse. The initial flame kernels generated by these plasmas were growing while being affected by each other. It is suggested that the behavior of the fuel droplets around the initial flame kernel affect the ignition probability since the ignition probability with increasing number density of droplets is decreased.
In this study, we have investigated the combustion characteristics of premixed mixtures supported by pilot flames with different equivalence ratio. The flame behavior was measured by OH-PLIF imaging. The turbulent flames were stabilized in a cyclone-jet combustor. The fuel was propane. The main jet velocity and the cyclone jet velocity were fixed at 10 or 30 m/s. On the other hand, these equivalence ratios were widely changed. When the extinction limit was measured, the equivalence ratio of the main jet was decreased by keeping that of the cyclone jet in the range of φp = 0 - 5.0. The overall equivalence ratio was one parameter for discussing the flame stability and the emission characteristics. Results showed that the equivalence ratio of the cyclone jet greatly affected the flame extinction limit. The soot concentration was almost zero for all conditions in this experimental condition. EINOx was smaller as the overall equivalence ratio decreased.