Masaya Nakahara

Influence of local flame displacement velocity on turbulent burning velocity

In our previous works, the mean local burning velocity turned out to be changed from the originallaminar burning velocity due to the preferential diffusion effect, and it was found to be an important factor dominating the turbulent burning velocity. The present study investigates directly the local flame propagation properties of methane, propane, and hydrogen premixed turbulent flames in the weak turbulence region. A laser tomography technique is used to obtain the temporal local flame configuration and movement in a constant-volume vessel, and quantitative analyses are performed. The local flame-front curvature and the local flame displacement velocity S F are quantitatively obtained as the key parameters of the turbulent combustion. First, the observation of sequential flame tomograms shows that the turbulent flame front seems to be classified into active and inactive parts depending on its geometric configuration. Next, the values of S F are found to be distributed over a wide range and to be dependent on the local curvature of the turbulent flame. In addition, the values of S F for the methane and hydrogen mixtures have a tendency to become larger with decreasing equivalence ratio, whereas those for the propane mixture become smaller. We discuss also the influence of preferential diffusion and Markstein number on the local flame displacement velocity.

Study on the Turbulent Burning Velocity of Hydrogen Mixtures Including Hydrocarbons

To clarify the turbulent burning velocity of hydrogen in the presence of hydrocarbons, a two-component fuel mixture of hydrogen, methane, and propane was considered. Both hydrocarbon fuel-lean and fuel-rich mixtures were prepared while keeping the laminar burning velocity approximately constant. Even though the laminar burning velocities were approximately the same, a distinct difference in the measured turbulent burning velocity at the same turbulence intensity is observed, depending on the addition of hydrocarbon, the equivalence ratio, and the kind of hydrocarbon. The burning velocities of lean mixtures changed almost monotonically as the rate of addition changed, whereas the burning velocities of the rich mixtures showed no such tendency. This trend can be explained qualitatively based on the mean local burning velocity, which is estimated by taking into account the preferential diffusion effect for each fuel component.

Experimental study of the turbulent combustion mechanism of non-stoichiometric mixtures

Abstract Recently, in order to establish the lean-bum technique, the structure of non-stoichiometric mixture flames has been studied extensively, as well as that of stoichiometric mixture flames. In those studies, emphasis was placed not only on macroscopic behavior of the turbulent flame but also on microscopic behavior of reactants. The purpose of this paper is to estimate the change in the local burning velocity in turbulent flame. In addition, it is suggested that the turbulent burning velocity and quenching mechanism may be explained regardless of the kind of fuel, when the estimated local burning velocity is used as a reference instead of the laminar burning velocity.

Experimental study on the configuration and propagation characteristics of premixed turbulent flame

Abstract An attempt is made to examine the configuration and propagation characteristics of premixed turbulent flame in a weak turbulence region. A laser tomography technique is used to obtain the flame shape, and quantitative analysis is performed. It is found that the turbulent flame front can be classified into the active and the inactive part for the flame propagation, where the former is the convex part of flame toward the unburned mixture and the latter is toward the burned gas. In this paper, the local burning velocity and the local flame front curvature are determined directly by sequential flame tomograms.

An Experimental Study of Constructing Turbulent Burning Velocity Model for Hydrogen Mixtures

This study is attempted to establish a prediction model of turbulent burning velocity for hydrogen mixtures based on the local burning velocity of turbulent flames instead of laminar burning velocity. The turbulent burning velocity characteristics of the specific mixtures having the same laminar burning velocity SL0 with different equivalence ratio φ are examined experimentally at a constant volume vessel (φ=mainly 0.3~1.4, SL0=10~50 cm/s). The mean local burning velocity SL of turbulent flames is also estimated by using our proposed method which takes the preferential diffusion effect into consideration. It is found that the lower the equivalence ratio is, the higher the turbulent burning velocity becomes and the more extended the quenching limit is. SL/SL0 for each φ of hydrogen mixture has a tendency to increase or decrease with u’/SL0, as compared with hydrocarbon mixtures. Therefore, the SL for hydrogen mixtures needs to take account of u’ as well as φ and SL0.The quantitative accuracy of our proposed model for hydrogen mixtures is improved by using SL based on φ and u’/SL0.

A Study on Premixed Turbulent Combustion of Hydrogen Including Hydrocarbon

In order to clarify the turbulent burning velocity of hydrogen mixtures including hydrocarbons as a multicomponent fuel mixture, both lean and rich hydrocarbon(methane and propane)-added hydrogen mixtures as two-component fuel mixtures were prepared while maintaining the laminar burning velocity approximately constant. A distinct difference in the measured turbulent burning velocity at the same turbulence intensity is observed depending on the additional rate of hydrocarbon, the equivalence ratio and the kind of hydrocarbon, even the laminar burning velocities are approximately the same. The burning velocities of lean mixtures change almost monotonously as the rate of addition changes, whereas the burning velocities of the rich mixtures show no such tendency. This trend can be explained qualitatively based on the mean local burning velocity, which is estimated by taking into account the preferential diffusion effect for each fuel component. In addition, a model of turbulent burning velocity proposed for single-component fuel mixtures can be applied to hydrocarbon-added lean hydrogen mixtures by considering the estimated mean local burning velocity of each fuel.