Recently, plasma-assisted combustion attracts much attention of many researchers as a new interdisciplinary field between combustion and plasma sciences. In this article, we describe a brief explanation of various plasmas, concepts of plasma-assisted combustion, and the research trend of plasma-assisted combustion. We also report examples of experimental investigation on plasma-assisted combustion, which have been carried out in authors' research groups.

The direct and indirect plasma aided combustion technology is presented. First, the direct plasma aided combustion technology is introduced based on the review article by Starikovskii et al. They used ultrashort pulsed discharge as a non-thermal plasma source to generate various reactive species in the combustible gas mixtures such as (CH₄~C₅H₁₂)/O₂/Ar(~90%). Among the reactive species, atomic oxygen plays the principle role that greatly shorten the ignition delay time. Heat generated by non-thermal plasma also contributes to enhance ignition performance; however, gas heating is less important than radical production as far as gas heating is lower than 100℃. Finally, indirect plasma aided combustion technology is presented. The principle is based on pre-reforming of low-calorific fuels such as biogas where CH₄ content is approximately 60% or even less. We proposed plasma-catalyst hybrid reaction where catalyst is activated by low-temperature (400-500℃) exhaust gas from internal combustion engine; whereas CH₄ steam reforming is enhanced in a large degree by applying non-thermal plasma. As a result, combustibility of hydrogen enriched low-calorific fuel is greatly improved.

Research of a plasma jet (PJ) torch igniter is reviewed as a typical ignition system using thermal (equilibrium) plasma. Advantages of the PJ torch are high temperature and its role as a source of active species such as radicals (O,H,N,..), NOx (NO and NO₂), and fuel fragments (CH, CH₂, ….) which drastically accelerate chemical reactions. The effect of existence of a small amount of such active species in a combustible mixture was numerically investigated by calculation of ignition delay times with full chemical kinetics. Moreover, experimental results of ignition tests by the PJ torch in a supersonic airflow conducted by the author was also introduced.

The mechanism of air breakdown for engine ignition was investigated with the high-brightness pulse lasers. After the conformation of double pulses and ultrafast short pulse effects for laser induced breakdown, we have designed a compact (spark plug size), diode-pumped, passively Q-switched Nd:YAG/Cr⁴⁺:YAG micro-laser for ignition of engines. The output energy of 2.7mJ per pulse and totally 11.7mJ (sum of 4 pulses) was obtained at the pump duration of 500μs with the optical-to-optical conversion efficiency of 19%. The pulse width and M² value were 600ps and 1.2, respectively. The brightness of the micro-laser was calculated to be 0.3PW/sr-cm² and the optical power intensity of 5TW/cm² was estimated at the focal point of ignition. The enhanced combustion by the micro-laser ignition was successfully demonstrated in a thermostated constant-volume chamber with room temperature and with atmospheric pressure. The cross-section area of the flame kernel generated by the laser ignition is 3-times larger than the spark plug at 6ms after ignition in a stoichiometric mixture (A/F 15.2) of C₃H₈/air, even though the ignition energy of the laser is 1/3 of that of a spark plug. The real automobile engine ignition has been also successfully demonstrated by using this high-brightness, low energy micro-laser.

Turbulent premixed combustion has been discussed focusing on theoretical expectation of the flame structure in turbulence. To investigate combustion regimes of the turbulent premixed flames, clear definitions of reference length and velocity scales are introduced both for a laminar premixed flame and fluid turbulence. Based on the relevant reference scales, derivation of the combustion diagram for turbulent premixed flames is introduced by treating turbulent premixed combustion as an interaction between two physical phenomena: laminar premixed flame and turbulence. The characteristics of combustion diagrams by Borghi [2] and Peters [3,4] and their differences are presented, and drawback and advantage of each diagram are shown based on the physical insights obtained from recent progresses in turbulence and turbulent combustion researches.

A survey of published papers was carried out related to R&D of woody bio-fuels for industrial and civil usage. 45 domestic papers and 33 overseas papers were analyzed covering three categories ― solid bio-fuels, liquid bio-fuels and gas bio-fuels. For the research and development of woody solid-biofuel, research on utilization of wood pellets is predominant in Japan, and current R&D is mainly on the development of combustion devices. There appears to be little fundamental research in this area. For woody bio-liquid utilization, fundamental research seems to predominate. In this category, two types of processes are receiving attention. One is direct liquefaction with a liquid base and high-pressure treatment in autoclaves, or more recently by supercritical fluid technology. The second type of process leads to bio-oil or bio-oil/char-slurry which is appropriate for dry-biomass feedstocks such as woody biomass. The bio-oil-char/slurry route has an advantage in its high energy density and ease of storage and transport. Research and development appears to be very active in northern America and northern Europe, but there are few research reports from Japan. The most active R&D is carried out in the third category of bio-gas utilization. Various energy conversion process, from conventional fixed beds to fluidized bed and spouted bed systems are being investigated in terms of combustion characteristics and conversion system analysis, including cost estimation. Most systems are based on conventional coal gasification, and therefore they all suffer from wood-tar generation which is peculiar to biomass gasification. A number of methods for overcoming tars are being studied, typically using catalysts, but the efficacy of these processes is unclear at this time. The major trend of R&D in this category is oriented to reduced tar gasification process. Device and system development focused on Japanese utilization (locally scattered and small scale) combined with cost simulation are especially important.

By using the sooted plate technique, a detonation cellular pattern is recorded. The cell width λ is a parameter that depends on the chemical reaction rate of the mixture. Therefore, many studies have been carried out to measure the cell width under various mixture conditions. Hence, it can be expected that the cell width is a function of the local thermodynamic properties of the mixture. In the present study, which is a fundamental study on detonation cellular structure, we focused on the change in the detonation cellular structure with change in the local thermodynamic properties. In this study, the change is made by a head-on collision with a shock wave. A detonation wave collides with shock waves of the different strength in stoichiometric hydrogen-oxygen mixtures. The change in the cell width is measured on the basis of the soot track record. It is clearly observed that the cell width changes after the collision. The cell width decreases with increasing shock wave strength. The relation between the cell width and pressure and temperature of the stoichiometric oxyhydrogen mixture is obtained under the present experimental conditions.

Ignition by lasers is studied to apply for rocket (reaction) engines for attitude-control of spacecrafts. To optimize the ignition devices, the minimum ignition energy (E_min) of propellant (H₂-O₂) must be evaluated. There are no experimental data for H₂ with pure O₂ since the H₂ flame tends to transit to detonation. The E_min can be evaluated as the enthalpy carried with a minimum flame. The enthalpy density, the flame thickness and activation energy (β) of the plane-adiabatic flame were evaluated by numerical experiments using a flame code, CHEMKIN. The minimum flame governed by heat loss and selective diffusion in curved H₂ flames was investigated by an activation energy asymptotics by using the β. The E_min of H₂-O₂ and H₂-air flames was estimated from the quenching diameter propagating in narrow tubes. The E_min of the H₂-air flame was evaluated to be about 10 μJ, which can be compared with the experimental data of 20 μJ. The quenching diameter and the E_min of the H₂-O₂ stoichiometric flame were predicted to be approximately 500 μm and 2 μJ, respectively. Conditions necessary to achieve the laser-ablation ignition were discussed.

Detonation cell width and velocity deficit, both implicitly include the wall effects, are investigated to obtain insight on propagating detonation waves in narrow channels. The total length of the 50.5 mm i.d. detonation tube is 5350 mm, which comprises of several sections including a 850 mm long driver section and a 1500 mm long test section. The detonation in the test section is initiated via a detonating driver gas, which bursts a thin diaphragm separating these two sections. The velocity deficits predicted by the boundary layer displacement theory developed by Murray were compared to those obtained experimentally. Good agreements are found between the theoretical predictions and the experimentally obtained velocity deficits for all the channel dimensions studied for the present gaseous systems. However, the experimentally measured cell widths are found to be larger than those evaluated from the induction zone length calculated by the ZND model. This gives a basis to conclude that the extinction of some of the triple points, which leads to form larger cellular structures inside the channel as compared to the original one measured before the channel entrance, may be made by the boundary layer developed at the wall surfaces. To approximate the experimental cell width (λ_m) in the channel through the predicted cell width (λ_P), a relation λ_m=Bλ_P was found in this study where the value of B is slightly dependent on the reaction model. B was evaluated to be 1.2 using Lutz et al.'s reaction model and 1.3 using GRI Mech 3.0.

The spray combustion is applied to various practical combustors such as a diesel engine, a gas turbine, etc. However, the spray combustion is the very complicated, unsteady two-phase reactive physicochemical phenomenon composed of preheating, evaporation, mixing, ignition, combustion and quenching, etc. Chiu et al. have considered the spray combustion field with the concept of group combustion, and proposed that the group combustion can be classified into four modes in terms of group combustion number G. In this paper, using the numerical analysis with PSI-CELL model, we examined combustion process in three-dimensional heterogenous combustion field of fuel droplets and air, so as to improve Chiu's group combustion mode. We arranged many fuel droplets using random variables, and changed the size and the number density of droplets. We obtained the following results; 1) The group combustion is classified into six modes. 2) The initial equivalence ratio ϕ is needed to consider the influence of oxygen existing initially in the combustion field. 3) The group combustion mode can be organized by the initial equivalence ratio ϕ when the size of fuel droplet is small, while it can be organized by group combustion number G when the size of fuel droplet is large.