One of the greatest challenges in combustion today is to reduce carbon emissions by increasing energy conversion efficiency and using low carbon alternative fuels. Reduction of thermal heat loss and control of ignition timing using low temperature combustion at higher pressure and lean burn conditions are critical to the development of future advanced engines and low carbon fuels. In this paper, an overview of the recent progress in understanding low temperature flames such as the cool flames, warm flames, and hot flames as well as the double flames, multi-stage flames, and auto-ignition assisted flames will be presented. Special focus will be placed in the dynamics and burning limits of these low temperature flames. The phenomenological definition of cool flame, warm flame, and hot flame will be described. Three temperature dependent chain-branching pathways which correspond, respectively, to the cool, warm, and hot flame regimes will be summarized. The flammability limit diagrams for both premixed and non-premixed cool flames, warm flames, and hot flames will be presented. The effects of chemical sensitization and wall quenching on cool flames will be discussed. Furthermore, the auto-ignition assisted cool flame propagation in auto-igniting mixtures at engine conditions will be presented. The impact of cool flames on turbulent combustion and engine knock formation will be briefly discussed. A brief summary and future research of low temperature combustion under engine conditions will be made.
The new generation 2.0L gasoline engine utilizes extra high compression ratio compared with conventional gasoline fueled engine. Under part load condition, with the assistance of compression ignition combustion, highly diluted/lean mixture combustion were realized, resulting a large improvement in vehicle fuel economy. In this paper, the combustion technology of new generation gasoline engine with spark controlled compression ignition is described in detail.
The aspects of triple (or tribrachial) flames are briefly surveyed by showing our studies and reports. The triple flames consist of a lean and a rich premixed flame branches with a trailing diffusion flame branch. The three branches intersect at the triple points. The flames could play important role of lifted flames, local extinction and so on. First, the flame structure is explained using our results of numerical simulation. Next, experimental results of a lifted flame in a triple port burner are examined. Especially, a unique behavior of flip-flop between inner and outer flames is introduced.
Flame ball is a steady spherical flame observed under microgravity in a quiescent premixture. Flame ball does not propagate and is stabilized in extremely lean conditions. Molecular diffusions play essential roles for the stabilization of flame ball since no convection flow exists in a quiescent mixture. Flame ball was predicted analytically by Zeldovich in 1940’s while he himself concluded that it is unstable. Approximately forty years since its prediction, Dr. Ronney accidentally observed flame cells with slow propagation speed through short-duration microgravity experiments in U.S. Until early 2000’s, analytical studies with consideration of radiative heat loss, drop tower experiments at JAMIC and the subsequent Space-Shuttle experiments suggested that the existence of the ideal flame ball. Since 2010, our group has been focusing on the understandings of comprehensive combustion limits including relations between general propagating flame represented by low-speed counterflow premixed flames and flame ball. Through recent studies, some peculiar flame behaviors were confirmed experimentally and numerically. In this report, the recent studies related to flame ball as well as the history of related studies were summarized.
This paper reviews the features and hazards of large-scale fireball generated by accidental explosions. A catastrophic failure of storage vessel of flammable materials in a fire can lead to a fireball. The estimation of diameter and duration of fireball based on various experiments are discussed. The results calculated by empirical equations agreed reasonably well with the experimental data. The results demonstrated that the size and duration of the fireball depend on the mass of fuel. Additionally, in this article, the experimental investigations on fireball dynamics after rupture of 35 MPa and 70 MPa high-pressure hydrogen tank in a fire are described, and the results were used to validate the development of CFD model.
Accurate and reliable temperature measurements are crucial to improve the thermal efficiency and optimize the design of products because temperature is an important parameter to understand heat conduction, convention, radiation, and chemical reactions. The emission properties of phosphors are sensitive to temperature, hence, phosphor thermometry can make remote temperature sensing. Accordingly, phosphor thermometry has received considerable attention as a versatile alternative to conventional temperature measurement techniques. This article describes the principles and fundamental aspects of phosphor thermometry, that is, principles of luminescence, temperature dependences of luminescence characteristics and how to use those dependence characteristics such as life time method and spectral intensity ratio method.
The simultaneous temperature and velocity measurement method is introduced in this article. This method uses temperature sensitive luminescence particles as tracer particles of the PIV. There are many kinds of temperature sensitive particles, such as phosphor particles and porphyrin complex materials. Temperature is evaluated based on the ratiometric intensity ratio (spectral shape), lifetime or mono-color intensity. This method is applicable to a fluid flow regardless of kinds of working fluid. This article introduces applications of the simultaneous measurement to an air flow along a heated plate, a multiphase flow of phase-change-material, a high temperature air jet over 1000K.
The feasibility of a newly developed particle size distribution, complex refractive index and soot volume fraction determination method in a butane laminar coflow diffusion flame at atmospheric pressure is evaluated based on Mie scattering theory. The polarization measurements were performed using multi-wavelength light sources. The scattered light intensities were obtained by analyzing the soot particles images taken by polarization CCD cameras under a scattering angle of 60°. The flame height is set at z = 30 mm. Through calculation, information regarding the particle number, geometric mean diameter, geometric standard deviation and complex refractive index can be determined simultaneously. The experimental results show that the geometric mean diameters increase, and the particle numbers decrease as moving downstream of flame. Polystyrene standard particles of 46 nm and 269 nm in pure water under five different number densities are used to validate the accuracy of MPR method. The soot volume fraction was calculated and compared with the data obtained using the light extinction method (LEM). At z = 20 mm, the two results showed good agreement. At z = 25 mm, the soot volume fraction obtained by two methods had a difference. The reason is considered due to coagulation and aggregation of soot particles. The particle size distribution is also compared with the results obtained using a portable aerosol mobility spectrometer (PAMS). The particle size measurement obtained from the MPR method is underestimated in comparison with that of the PAMS. The possible reason considered that PAMS system is a sampling method to measure particle size distribution and it may influence the soot formation.
The effects of lateral acoustic forcing on a jet diffusion flame is investigated in this work. An H2-CO2 mixture is used as the fuel and its mixing ratio is varied. The mixture is ejected upward from a vertical tube of 1.25 mm in inner diameter and acoustic forcing is applied from its side with a sinusoidal sound wave of 7 kHz in frequency and 115 dB in pressure level. An effect of the forcing shows up in shorter flame height, which varies in accordance with the mixing ratio, or the mean molecular weight of the fuel, and the ejection velocity. The intensity of the flame excitation is evaluated by means of the ratio between the heights of the excited flame and the normal one; higher the intensity of the excitation is, lower the ratio is. It is found that the flame height ratio decreases as the ejection velocity increases until the flow reaches the critical Reynolds number, over which the flow becomes turbulent and the effect of the forcing becomes indistinguishable. It is also found that the ratio decreases as the mean molecular weight decreases while the Reynolds number is kept constant. Relative amplitude of the baroclinic torque is also evaluated on the basis of the experimental results and simple assumptions, and its relation to the flame height ratio is examined; the results infer that the baroclinic torque is not the factor directly causing the decrease of the flame height.