Tadao Takeno

An Excess Enthalpy Flame Theory

Abstract A simple idea is proposed to produce an excess enthalpy flame by inserting a porous solid of high thermal conductivity into the one-dimensional flame zone. Heat is recirculated internally through the solid from the downstream high temperature region to the upstream low temperature region and large excess enthalpy is produced at the head of the reaction zone. The potentiality of the proposed artificially modified flame is analyzed on the basis of the simplified one-dimensional flame theory. It is found that in this flame the mass flow rate is not the eigenvalue but becomes a mere parameter. The heat transfer coefficient between the solid and the reacting gas is another parameter. With these additional two parameters the controllability over the flame characteristics increases remarkably. The analysis reveals several attractive characteristics of the flame and that the proposed idea is promising to burn mixtures of low heat content in a simple combustion system.

Effects of temperature and pressure on burning velocity

Abstract An experimental study on effects of temperature and pressure on the burning velocity of methaneair and hydrogenair mixtures has been conducted by using the spherical bomb technique proposed by the present authors. The ranges of measurement covered the equivalence ratio from 0.8 to 1.3 for methaneair, and from 0.5 to 4.0 for hydrogenair mixtures. The mixture temperature was varied from 291 to 500K, while the pressure from 0.5 to 30 atm for methaneair and from 0.5 to 25 atm for hydrogenair mixtures. The empirical equations for the burning velocities have been obtained. The obtained burning velocities were correlated by the Arrhenius form expression, which is based on the flame theory with a one-step kinetics, to yield the apparent order and activation energy of the overall reaction. The derived values were compared with those obtained by other investigators.

A flame-controlling continuation method for generating S-curve responses with detailed chemistry

Abstract A flame-controlling continuation method is formulated for the generation of the ignition-extinction S-curve characteristic of quasi-one-dimensional flames as well as the investigation of the associated flame structure and response, especially for states near the turning points. Using the counterflow premixed and diffusion flames as examples, the method capitalizes on the distinct nature of the profile and location of the scalars of the flame properties, such as the temperature and species concentrations, in response to changes in the flow strain rate. Thus instead of using the strain rate as an imposed parameter and the scalars as the flame responses, the value of a flame scalar at a given location y ∗ is used as an internal boundary condition while the strain rate becomes the flame response. Consequently, by fixing y ∗ and incrementing the value of the flame scalar, continuous mapping of the relation between the flame response and strain rate is accomplished. Sample calculations were performed for the premixed twin flame and for diffusion flames with equal and unequal exit velocities from the opposing nozzles. Continuations using one-point temperature controlling, two-point temperature controlling, and one-point hydrogen radical concentration controlling were demonstrated. The method appears to be fairly expedient in implementation.

NO emission characteristics of methane-air double flame

Abstract NO emission characteristics of methane-air Bunsen-type burner flames were studied numerically in terms of counterflow flame. The flames have the well-known double flame structure: the rich premixed flame to produce CO and H 2 as the main intermediate products and the diffusion flame where the intermediate products burn with surrounding air, and the structure can be simulated by using rich counterflow flame with air. The similarity solution was adopted to describe the flow, temperature, and concentration fields and the detailed kinetics calculation was made by using C 2 chemistry with the all mechanisms leading to NO formation, including thermal and prompt NO mechanisms. The calculation was made as well for thermal mechanism alone, so as to distinguish contribution of the respective NO formation mechanisms in total NO production. The emission characteristics were evaluated quantitatively in terms of the emission index, as compared to the normal premixed and pure diffusion flames. The effects of equivalence ratio and the velocity gradient on the emission index of these flames were studied.

A theoretical study on an excess enthalpy flame

A further theoretical study was performed on the excess enthalpy flame system proposed by Takeno and Sato to burn mixtures of low heat content. The previous analyses were extended to include effects of the finite length of the porous solid inserted for internal heat recirculation, so as to predict the flammability limit. In the analysis the temperature of the solid is an eigenvalue of the system, while mass flow rate remains a controllable parameter. Numerical calculations have revealed the existence of a critical mass flow rate above which combustion cannot be sustained. The critical flow rate is more than ten times the burning velocity for a thin porous disk of the order of the flame thickness. Below the critical flow rate, the system has two combustion states with the distinct solid temperatures. As the flow rate is decreased asymptotically to the burning velocity, the inserted disk comes to play the role of a downstream flame holder for the state with the higher solid temperature, whereas it plays the role of an upstream flame holder for the state with the lower solid temperature.

Species conservation and emission indices for flames described by similarity solutions

The objective of the present study is to derive the species conservation relation in the counterflow flames and the tubular flames described by similarity solutions and to make it possible to evaluate NO x emission indices

An experimental study on the stability of jet diffusion flame

Abstract The stability behavior of jet diffusion flame developing in a coflowing high temperature air stream was studied experimentally, using city gas and hydrogen as fuel gases. The primary variables studied were temperature and velocity of the air stream. Two distinct types of stability limits were observed. The first is the blow-off of the rim-stabilized flame, which is familiar and has been studied by a number of investigators. The second is the break-off or extinction of the turbulent portion of the flame at the transition point from laminar to turbulent flow. The second limit is quite unfamiliar and is interesting in relation to the structure of turbulent diffusion flames. The important implications of the experimental findings concerning the stability and the structure of turbulent diffusion flames is discussed.

A tubular flame theory

Abstract The axisymmetric tubular flame established in a rotating flow field was studied theoretically to explain the effects of flow rate and equivalence ratio observed in the previous experiment. The incompressible viscous flow field was solved exactly in terms of an axisymmetric similar solution. The predicted flame structure and stability behavior correlate satisfactory with those of the experiment.

Fractal-like character of flamelets in turbulent premixed combustion

The fractal-like character of the laminar flamelet surface in turbulent premixed combustion was studied to determine if the surface is truly a self-similar fractal or not. The turbulent burner flame of a lean methane-air mixture stabilized in an isotropic homogeneous turbulent flow was used, and the laser tomography technique was adopted to visualize the instantaneous image of two-dimensional sections of the surface. The analysis of the images in vertical and horizontal sections revealed that the surface actually exhibits fractal behavior in a narrow range of scale. The inner cutoff scale was the laminar flame thickness, while the outer cutoff scale was the burner size. The derived fractal-like character, represented in terms of the fractal dimension and the fractal degree, was found to depend on the orientation and position of the section. The fractal dimension was much smaller than those measured for nonreacting various turbulent flows, and was larger for the vertical section than the horizontal sections, and increased towards downstream. It increased with the turbulence intensity of the approach flow. However, it is suggested that the flamelet surface does not behave as a passive surface in the given turbulent flow, and hence the observed fractal-like character is not directly connected to the turbulence characteristics of the approach flow. It should rather represent certain aspects of the flamelet itself.

Ignition of Magnesium and Magnesium-Aluminum Alloy by Impinging Hot-Air Stream

Abstract An experimental study on the nonsteady ignition process of magnesium and 50-50 magnesium-aluminum alloy was made by using the stagnation region of an impinging hot air stream. The study has revealed that the ignition of magnesium was caused to occur through a four-stage surface oxidation process and finally through a homogeneous exothermic reaction in the gaseous phase. The ignition mechanism of the magnesium-aluminum alloy was essentially identical with that of magnesium, although the observed ignition behavior was more complicated. The difference could be explained in terms of the aluminum ingredient contained in the oxide film formed on the sample surface.