Taro Hirasawa

Determination of laminar flame speeds using digital particle image velocimetry: Binary Fuel blends of ethylene, n-Butane, and toluene

The atmospheric laminar flame speeds of mixtures of air with ethylene, n -butane, toluene, ethylene- n -butane, ethylene-toluene, and n -butane-toluene were experimentally and computationally investigated over an extended range of equivalence ratios. Binary fuel blends with 1:1, 1:2, and 2:1 molar ratios were examined. Experimentally, the laminar flame speeds were determined using digital particle image velocimetry (DPIV). Since the use of DPIV enables the mapping of the two-dimensional flow field adhead of the flame, the reference speed based on the minimum axial velocity point as well as the imposed strain rate can be identified simultaneously. The latter can now be unambiguously determined by the radial velocity gradient at the minimum velocity point. By systematically varying the imposed strain rate, the corresponding laminar flame speed was obtained through nonlinear extrapolation to zero strain rate. The associated experimental accuracy of the DPIV measurements was also assessed and discussed. Computationally, the laminar flame speeds were simulated for all single-component fuel/air and binary fuel blend/air mixtures with a detailed kinetic model. Comparison of experimental and computed flame speeds shows generally good agreement. A semiempirical mixing rule was developed. The mixing rule which requires only the knowledge of the flame speeds and flame temperatures of the individual fuel constituents, is shown to provide acurate estimates for the laminar flame speeds of binary fuel blends under the conditions tested.

Temperature Dependence of Intensities of Laser-Induced Fluorescences of Ethylbenzene and Naphthalene Seeded in Gas Flow at Atmospheric Pressure: Implications for Quantitative Visualization of Gas Temperature

The present study has been carried out with the aim of developing a technique for measuring two-dimensional gas temperature profiles based on two-color fluorescence induced by a one-color laser. The laser sheet of the fourth harmonic (266 nm) from a Nd:YAG laser induced fluorescence in species doped in a nitrogen gas flow. The LIF spectra of seven fluorescent species, namely acetone, methylethylketone, acetaldehyde ethylbenzene, anisole, aniline, and naphthalene, were measured to select the best prospective pair of fluorescent species for this technique. Ethylbenzene and naphthalene show relatively high LIF intensities and their LIF spectra overlap less with each other than with other species. Also, ethylbenzene has a high temperature dependence while naphthalene has a low temperature dependence. Thus by selecting one portion of wavelengths in the range where ethylbenzene or naphthalene is dominant, the temperature of the gas can be determined using the ratio of LIF intensities of the mixture at the two wavelengths with good temperature sensitivity. In addition, a general principle is presented for finding out an optimum pair of wavelengths to obtain a good temperature sensitivity in those LIF spectra.

Response of flame displacement speeds to oscillatory stretch in wall-stagnating flow

Abstract The response of the flame displacement speeds of stagnating flat premixed flames to the periodical fluctuation of stretch is investigated experimentally, regarding two mixtures with different Lewis numbers: lean C 3 H 8 /air and lean CH 4 /air in the range of the Strouhal number 2.3 to 4.8 (24 to 51 Hz). The stagnation wall is oscillated sinusoidally along the stagnation streamline. The oscillation of the stagnation wall induces the periodic fluctuations of flow velocity and flame stretch, and hence the fluctuations of flame displacement speed. The amplitude of the flame stretch fluctuation increases with increasing frequency of the wall in the Strouhal number greater than unity owing to the increase in the amplitude of the oscillatory wall velocity, when the stagnation wall is oscillated along the stagnation streamline at constant amplitude. The displacement speed is measured as the propagating velocity of the flame relative to the unburned gas velocity at the cold edge of the flame by using the history of flow velocity fluctuation. The response of the flame displacement speed is discussed from two viewpoints: its amplitude response and phase response. The significant dependence of the flame displacement speed on the flame stretch has been shown by the amplitude response. It is shown that the sinusoidal fluctuations of flame displacement speeds for both mixtures follow the sinusoidal fluctuation of flame stretch with a phase delay from about 20° to 90° in the present frequency range. Hence, the displacement speeds for both mixtures, although these mixtures have the contrary ratio of thermal to mass diffusivity against unity, show the same increasing dependence on unsteady stretch because the flow divergence affects the displacement speed with unsteady stretch as well as steady stretch.

A Study on Energy Conversion Efficiency of Direct Flame Fuel Cell Supported by Clustered Diffusion Microflames

The micro power generator based on clustered diffusion microflames and a direct flame fuel cell (DFFC) is proposed and its energy conversion efficiency is evaluated to investigate the optimum state of combustion. The clustered diffusion microflames are established on the 2.5 mm pitch 3-by-3 array of fuel jets with a diameter of 0.07 mm and a velocity of 24 m/s at the outlet by the interaction with other microflames around. The clustered diffusion microflames is more suitable for micro power generator than microflame array because of suppression of soot formation without air supply system. Although the conversion efficiency of the system with clustered microflames is quite sensitive to the separation distance between the burner and the cell, compared with the microflame array, it has attained the same level of conversion efficiency with microflame array, i.e. 0.45 %. The results of cell temperature suggest that the rapid decrease of conversion efficiency with increasing separation distance is caused by the decrease of species supply such as CO and H2 rather than the decrease of heat supply. The observation of the secondary flame suggests that the reason of high sensitivity to the separation distance is because the cell position changes the flowrate of entrained ambient air and hence the flame equivalence ratio.

Interaction of Two Micro-slot Flames: Heat Release Rate and Flame Shape

This paper studies the interaction between two identical micro-slot diffusion flames. Here, we define a micro-slot flame as a slot flame of which the slot width is less than about 1 mm. Because of its smallness, a micro-slot flame has a high heating density and can be used as a small heat source. However, the heat release rate of a single micro-slot flame is limited, and therefore, multiple micro-slot flames may be used to increase total heat release rate. As a first step, this paper considers a situation in which two micro-slot flames are used with certain burner spacing. When two diffusion flames are placed closely, flame shape changes from that of an isolated flame. Studying such flame shape change and resultant change in total heat release rate is the topic of this paper. Experiment is conducted and total heat release rate is measured by integrating CH* chemiluminescence recorded using a CCD camera and an optical filter of the wavelength of 430 nm. Two different burner materials, copper and glass, are tested to study the effect of heat loss to burners. An analytical model is applied to predict flame shape. In addition to the classical Burke-Schumann assumptions, two slot flames are modeled as line sources with zero width, enabling a simple analytical solution for the critical burner spacing at which two flames touch each other. The critical burner spacing is a key parameter that characterizes the interaction between two micro-slot flames. Computational fluid dynamics (CFD) simulations are then conducted to test the validity of the present theory. CFD results are favorably compared with the theoretical prediction.

Two-dimensional non-contact temperature measurement of high temperature air jet using phosphor thermometry

The two-dimensional non-contact temperature measurement of high temperature air jet was demonstrated by using the phosphor thermometry based on the two color intensity ratio method. The temperature could be measured up to 1000 K.

BURNING VELOCITY MEASUREMENTS OF SOOTING PREMIXED FLAMES

The propagation velocity of premixed sooting ethylene/air flames is studied in two separate experiments. First, the widely-used counterflow configuration is employed and it is demonstrated to be inadequate for burning velocity measurements of sooting premixed flames. Thus a spherical flame geometry, achieved by discharging gas mixture from a porous spherical burner into a quiescent ambience in microgravity, is instead utilized. These purely curved flames are quasi-1D in nature and allow the measurement of flame speeds without the complications of hydrodynamic strain, flame-flame interaction and heat loss to the burner. The experimental setup uses a compact Rainbow Schlieren optical system to accurately determine the flame radius and the flame temperature field. It is found that the experimentally- determined burning velocities of sooting ethylene/air flames are much lower than the predicted values considering gas-phase reactions only.