An overview of fundamental and principal mechanism of high-pressure combustion and some examples of combustion phenomena in a high-pressure environment were presented. High pressure combustion is effective for improving thermal efficiency of internal combustion engines, enhancing heat release in a unit volume of high-load combustors, as well as increasing product yields from chemical reactors. The effects of pressure concern detail chemistry, gas-solid reaction, boundary layer thickness, structure of turbulence, decreasing laminar burning velocity, enhancing soot formation, etc. Combustion diagnostics for high pressure flames is also challenging theme. High pressure combustion gives us various subjects not only of scientific interests but also development of advanced and efficient combustion systems.
The effects of pressure on spherically propagating laminar and turbulent flames are described in this article for various fuels, such as methane, propane and iso-octane. Spherically propagating flames have not only the properties of common premixed flames but also have unique properties not seen in steady premixed burner flames. Such natures of spherically propagating flames arise as a consequence of variations of their size and flame stretch during the flame propagation. The Markstein number is employed to quantify the effects of pressure on the laminar and turbulent flames. The burning velocities and the Markstein numbers of the laminar flames decreased with the increase in the mixture pressure. The laminar flames were more unstable at elevated mixture pressures. The ratio of turbulent and unsteretched laminar burning velocities increased with the mixture pressure. The burning velocity of the turbulent flames kept increasing during the flame propagation. This increase in the turbulent burning velocity during the flame propagation was attributed to the progress in the effective turbulence intensity.
Evaporation and self-ignition experiments of a single suspended droplet of palm methyl ester (PME) were conducted at high pressures to obtain fundamental data related to spray combustion. Ambient temperature and pressure were varied from 473 to 873 K and from 0.10 to 4.0 MPa, respectively. Combustion experiments were also performed at room temperature and atmospheric pressure. The initial droplet diameter was regulated between 0.5 and 0.6 mm. In order to obtain reference data, fatty methyl esters (FAME) of PME components, light oil, n-decane, and n-hexadecane were employed as a test fuel. Temporal variation of droplet diameter was measured from sequential back-lit images recorded with a high-speed video camera. PME droplet evaporation at relatively-low temperatures slows down drastically on the latest stage of evaporation. It is supposed to be due to alteration of the unsaturated FAMEs in PME during evaporation. As the increase in the ambient pressure, the corrected evaporation lifetime of PEM droplet increases below 673 K in the ambient temperature, and decreases above 773 K. The slow evaporation does not occur during droplet combustion. The corrected combustion lifetime of PME droplet is almost equal to that of light oil droplet at atmospheric pressure. The self-ignition delay time of PME droplet is 1.5 times as long as that of light oil droplet at the ambient pressure of 2.0 MPa.
Spray combustion is utilized in many industrial devises such as gas turbine engine and diesel engine. Therefore, the spray combustion behavior has been studied by many researchers experimentally and numerically. However, the mechanism of spray combustion has not been fully understood yet. In particular, the effects of ambient pressure on the spray combustion behavior have not been well clarified mainly because the combustion conditions and the acquired properties are extremely limited due to the difficulty of the measurements. In this article, authors' recent numerical works on the effects of ambient pressure on evaporation and combustion of fuel droplets are introduced. The characteristics of (1) evaporation of a single droplet in a quiescent flow, (2) evaporation and combustion of multiple fuel droplets in an initially-quiescent flow, and (3) a turbulent spray jet flame in high pressure environments obtained by Direct Numerical Simulation (DNS) or Large-Eddy Simulation (LES) are summarized.
IGCC is one of the most promising technologies to reduce the environmental impact of coal power generation, and gasification is a key process for IGCC system. Since entrained-flow gasifiers are operated at high temperature and elevated pressure, it is important to clarify coal reactivity in such high-temperature and elevated-pressure conditions. Coal is pyrolyzed rapidly in a gasifier and devolatilization is suppressed by furnace pressure. The changes in char structure because of elevated pressure have been found in a pressurized drop tube reactor (PDTF), and an extended chemical percolation model has been proposed as a useful devolatilization model which is applicable to elevated pressure. Several mechanisms and kinetic models of char gasification with carbon dioxide and/or steam have been proposed, and L-H type reaction rate equations and a random pore model are also useful for char gasification. These kinetic models are applicable to char gasification in Zone II where pore diffusion controlles the observed reaction rate of char gasificaiton. The gasification reaction models discussed in this paper would be useful to evaluate gasification performances in pressurized entrained-flow gasifiers.
Better understanding on material flammability to prevent the fire accident is a vital issue. In this article, representative material flammability tests (LOI, UL94, Cone calorimeter, NASA-STD-6001B) for fire safety evaluation are briefly reviewed and classification of the fire-safeness in each test is presented. It is important to stress out that the classification strongly depends on the test method because the test is designed to judge the specific aspects of the fire; e.g., ignitability, flammability, toxicity etc. For ensuring the fire safety in space, the “worst” case scenario is always taken into account to evaluate the material flammability since the space habitants shall be highly-secured from any fire accident, which potentially causes catastrophic disaster. However, it is suspected that the “worst” scenario considered so far can be even worse depending on gravity, so that the new regulation is needed to establish. As summarized, current evaluation method can ensure specific aspect of fire safety so that systematic understanding and evaluation is necessary. It is believed that the systematic development of new test and evaluation method for material flammability and introduction of new guideline to cover the any potential causes of fire is mandatory task.
A newly developed small-sized IES (inductive energy storage) circuit with a semiconductor switch at turn-off action was successfully applied to an ignition system in the previous papers [1,2]. In this ignition system, both thermal and non-thermal plasmas are utilized actively. In this paper, the focus is placed on the clarification of ignition characteristics of non-thermal plasma. For this purpose, the ignition and combustion characteristics of non-thermal plasma are examined and compared with those of a conventional spark ignition. As a result, it is found that streamer discharge characterized by non-thermal plasma cannot only ignite combustible mixtures as well as conventional thermal plasma, but there are also some advantages, such as volumetric ignition. In addition, OH LIF measurement is carried out to probe the OH time history induced by the non-thermal plasma. In conclusions, a number of OH radical can accumulate from pulse to pulse during a train of repetitive pulses, and the created radicals can initiate chemical chain reaction, which result in ignition finally.
Thermophoretic velocities in surrounding gas of argon, nitrogen, carbon dioxide, methane, or nitrous oxide are measured by means of microgravity experiments. Adopted particles are PMMA spheres with mean diameter of 2.91 µm. The temperature gradient is set at 10 K/mm while the pressure is set at several conditions in the range of 20 kPa to 100 kPa. Terminal velocities of particles suspended in a gas are individually measured. The residue values for all combination of accommodation coefficients are calculated from the obtained experimental results. Studies are made to determine the tangential momentum accommodation coefficient by assuming the thermal accommodation coefficient to be unity. Dependence of the coefficient on gas properties is investigated.
In the present study, the acceleration phenomena of expanding spherical hydrogen/air flames during large-scale unconfined hydrogen explosions have been investigated experimentally. Large-scale experiments, in which the hydrogen/air mixture of a prescribed concentration was filled in a plastic tent of thin vinyl sheet of 1 or 27 m3 and ignited by an electric spark at the center, at atmospheric pressure have been conducted. The propagation behaviors of expanding spherical flames were recorded by using an infrared photography and a visible photography. Results demonstrate the flame wrinkling on the flame surface progressively developed owing to the flame instabilities and the flame front area increased and thereby the speed also accelerated. The critical flame radius rc associated with the onset of the self-acceleration due to the diffusional-thermal and hydrodynamic instabilities was evaluated by the plotting the measured flame speed versus different flame stretch rate and the critical Peclet number associated with the onset of the self-acceleration, Pec = rc/δ (δ: Flame thickness), increases with the mixture equivalence ratio. Such acceleration has been evaluated using fitting formula associated with the fractal theory. Results demonstrate that the onset of the flame instabilities, when the flame radius is small, thus occurs and thereby the flame propagates with self-acceleration for Pe > Pec. Experimental acceleration exponent α increased with the flame propagation and eventually the power law exponent reached the limited values associated with the fractal dimension. Such results illustrate that the self-acceleration (α > 1) and the self-similarity (α=constant) regimes during the flame propagation definitely existed.
To give an insight into wall material effect on the chemical quenching phenomena, a methane-air premixed flame formed in narrow quartz channels with metal wall surface is investigated. In the present study, stainless-steel321 (SUS321) and Inconel600 are chosen as the surface materials for high oxidation/heat resistivity. SUS321 and Inconel600 thin films ∼150 nm in thickness are deposited on polished quartz substrates by using vacuum arc plasma gun to establish equivalent thermal boundary condition for different wall surface reactions. OH-PLIF and numerical simulation with detailed reaction mechanisms are employed to examine interaction between the gas-phase and surface reactions. When the wall temperature is higher than 1073 K, the wall chemical effect starts to take over the thermal effect. It is shown through the PLIF measurements that OH* mole fraction near the SUS321 and Inconel600 surfaces becomes significantly lower than that near the quartz surface. By using a radical quenching model, the initial sticking coefficient S0 associated with radical adsorption is evaluated at the metal and the quartz surfaces. It is found that S0 are estimated to be 0.1 and 0.01 for the metal and the quartz surfaces. A series of numerical simulation is also made to examine the effect of S0 on the methane flame in micro channels. It is found that the wall chemical effect becomes of great importance for the gross flame characteristics such as the initiation temperature of the chain reaction and the heat release rate.