Kenji Amagai

Diffusion flames and their flickering motions related with Froude numbers under various gravity levels

Abstract Combustion phenomena related to Froude number and Strouhal number were investigated in diffusion flames under various gravity levels. The Froude number of the fuel jet was controlled by gravity level, diameter of the nozzle, fuel properties, and fuel flow rate. Artificial gravity was created by a centrifuge and its level was varied from 1 G to 14 G. On the other hand, microgravity condition was formed by a drop tower. CH4, C3H8 and CH4 + C3H8 (54:46 mol %) were used as the test fuels. Many kinds of diffusion flames were observed on nozzles ranging in diameter from 0.5 to 10.0 mm. Flame lengths were measured and correlated by a dimensionless parameter which was proposed by Altenkirch et al. Using their parameter, an empirical equation was obtained as follows: Lf/d = 0.58 Re0.667 Fr0.089, where Lf was the average flame length and d was nozzle diameter, where Re is the Reynolds number (5 ≦ Re ≦ 570) and Fr is the Froude number (4.2 × 10−4 ≦ Fr ≦ 5.3 × 103). Since gravity levels of experiments were accounted for via the Froude number, this equation could express the flame lengths at various gravity levels (low- and high-gravity conditions). Also, flickering frequencies under various gravity levels (1.0–5.0 G) were measured. Over a wide range of fuel jet velocity (from 0.03 m/s to 36.2 m/s), flickering frequencies were correlated by two relations coupled through the Froude number. One relation was St ∝ Fr−0.50 for low Froude number conditions, and the other was St ∝ Fr−0.41 for high Froude number conditions. It meant that the Froude numbers of the low and high levels were dividing criteria for the flickering frequencies, and that flickering motions expressed by these two equations were affected by buoyancy in different ways.

Gravity effects on stability and flickering motion of diffusion flames

Abstract Effects of gravity level on the flame stability and flickering motion were experimentally investigated for the propane jet diffusion flames. A spin tester was used to form the high gravity fields. Flames were observed under the various conditions of injection Reynolds number of the fuel jet and gravity level which were controlled independently. Regions of stable flame, flickering flame, and lifting flame were mapped with the gravity level and injection Reynolds number. Then, it could be pointed out that the onset of flame lifting was strongly affected by the gravity compared with the onset of flickering. Flickering frequency increased with an increase of gravity level. Gravity effect on the wavelength and wave velocity were also investigated to clarify the reason for the change of frequency. It was found that the increase of flickering frequency with the rise of gravity level was caused by the increase of wave velocity and the decrease of wavelength. Flickering frequencies obtained here could be summarized using Strouhal number and Froude number, and in a relation of St ∝ Fr-0.57. This result showed a good agreement with the previous results of Hamins obtained from normal gravity experiments.

Flickering frequencies of diffusion flames observed under various gravity fields

Flame tip fluctuations observed in diffusion flames were investigated using thermal boundary layers. Particularly, flickering motion, which meant a periodic flame oscillation with low frequency (ranging from 10 to 20 Hz), was studied under various gravity levels. The artificial gravity level was changed from 1 G to 11 G with a centrifuge, where G was defined by a combined acceleration of the centrifuge and standard gravity. From the experimental results for wide conditions regarding the Reynolds number and gravity levels, it was found that there were two different modes of fluctuation. One was a “tip flickering,” in which the top of a flame was merely oscillating or elongating periodically. The other was a “bulk flickering,” which was accompanied by a fire plume separated from the top of the main flame. These two types of flickering were characterized by the Froude number coupling with the buoyant force. A relation for the Strouhal number, representing the dimensionless frequency St =0.056 Fr −0.41 , was obtained for tip flickering in high Froude number conditions and St =0.29 Fr −0.50 was obtained for bulk flickering in low Froude number conditions. In schlieren images of thermal boundaries around the flames, it was observed that the thermal boundaries were also fluctuating with the flames, but the fluctuation scales differed between the two flickering modes. It seemed that bulk flickering was caused by a large-scale vortex which was driven by buoyancy, and tip flickering was caused by flame stretch due to the strong shear force around the fuel jet.

Scale Modeling of Puffing Frequencies in Pool Fires Related with Froude Number

Quantitative investigation of the gravity effect was performed for small-sized acetone and kerosene pool fires. Several investigations on the behavior of pool fires have been conducted to understand the physical model. In their primary papers, it was mentioned that the buoyancy effect on the motion of pool fires was significant to understand the characteristics such as flame height, oscillatory frequency, and so on. Under these circumstances, in this investigation, a centrifuge was used to create elevated gravity fields and to examine the gravity effect. Small-scale pool fires were observed under various high gravity fields. Regions of stable flame, puffing flame and irregular oscillatory flame were categorized to make a map related with the gravity level and pool diameter. Flame height decreased and oscillatory frequency increased with an increase in the gravity level. Behavior of the flame height was agreed quantitatively with the scaling prediction presented by Orloff and Heskestad. Puffing phenomena observed under various gravity fields were summarized with the relationship between Strouhal and Froude number. As the result, an empirical equation expressed by St = 0.517Fr -0.502 could be obtained. From this equation, puffing frequency could be estimated for a flame of various pool diameters varying from 0.01 to 50m.

Numerical analysis of the gravitational effect on the buoyancy-driven fluctuations in diffusion flames

Unsteady behavior of buoyant diffusion flames which were observed at various gravity fields was studiednumerically by a finite volume method using the low-Mach number approximation and the temperaturedependent physical properties such as viscosity and diffusivity. The numerical model was a time-dependent and axisymmetric flow including a single-step chemical reaction. The objective was to clarify a periodic instability mechanism of buoyant diffusion flames such as flickering flame. The parameters of the jet injection velocity, V f , and dimensionless gravity level, G , were varied to clarify the effects of shear force and buoyant force on the unsteady motion of the flame. The gravitational effect was evaluated by varying the gravity acceleration ranging from 0.5G to 5G. An oscillatory frequency increased with an increase in gravity level. Convective flow in the flame caused by the vortical roll-up motion was enhanced in a highgravity condition. To elucidate the mechanism of the instability motion, axial velocity of convection, oxygen mass fraction, vorticity, and the distribution of radial density gradient were derived from the calculated results. It was found that the convective velocity and the oxygen mass fraction played a very important role in flame instability, And it seemed that the origin of unsteady motion was the generation of the vorticity. The maximum value of the vorticity estimated in high-gravity field was greater than that of the normalgravity field. Furthermore, the unsteady behavior caused by the buoyancy was correlated to the radial density gradient.

Catalytic Combustion of Pre-Vaporized Liquid Fuel

Fundamental characteristics of the catalytic combustion of vaporized kerosene spray were experimentally investigated. This study is a part of the development of a ceramic gas turbine engine for automobiles. Kerosene was used as a test fuel and its spray was injected from a swirl atomizer into a hot air stream. The inlet air temperature was elevated up to 900 K to vaporize the kerosene spray. Premixed gas of air and kerosene vapor was introduced into the catalyst. The total equivalence ratio was controlled from Φ=0.18-0.32. The palladium catalyst was supported on a cordierite honeycomb monolith. Catalytic combustion phenomena were categorized in three typical states: (a) state of partial reaction in the catalytic monolith, (b) state of homogeneous reaction in the monolith, (c) state of homogeneous reaction with a blue flame supposed on the monolith. A parabolic shape blue flame in the state of (c) appeared downstream of the monolith. This flame was very stable and its temperature was relatively low compared with conventional premixed flames of hydrocarbon fuel because the equivalence ratio was much lower than those of premixed flames. The distance from the monolith to the ignition point of this flame became short with a rise of the inlet air temperature, even if the volumetric airflow rate increased with the air temperature. Spontaneous emission spectra of radiation from the blue flame were measured. Strong spectral peaks of OH, CH, and CO + radicals were observed in the spectra. This spectral structure was quite different from that of a blue flame of premixed propane.

Fabrication of Two-Layered Aluminum Foam Having Layers with Closed-Cell and Open-Cell Pores

Two-layered aluminum foam having layers with both closed-cell and open-cell pores was fabricated using the precursor foaming process and the sintering dissolution process. It was found that a two-layered Al foam with different pore structures but similar compression properties in each layer can be obtained. This foam is expected to have a region with superior thermal insulation and a region with superior heat release properties.

Measurement of Interfacial Profiles of Wavy Film Flow on Inclined Wall

Falling liquid films on inclined wall present in many industrial processes such as in food processing, seawater desalination and electronic devices manufacturing industries. In order to ensure an optimal efficiency of the operation in these industries, a fundamental study on the interfacial flow profiles of the liquid film is of great importance. However, it is generally difficult to experimentally predict the interfacial profiles of liquid film flow on inclined wall due to the instable wavy flow that usually formed on the liquid film surface. In this paper, the liquid film surface velocity was measured by using a non-intrusive technique called as photochromic dye marking method. This technique utilizes the color change of liquid containing the photochromic dye when exposed to the UV light source. The movement of liquid film surface marked by the UV light was analyzed together with the wave passing over the liquid. As a result, the liquid film surface was found to slightly shrink its gradual movement when approached by the wave before gradually move again after the intersection with the wave.

Fundamental study of air flow effects on liquid removal from wafer surface

Effects of air flow on droplet removal from wafer were investigated as the fundamental study of CMP cleaning. Deformation and movement of droplet on wafer by air flow were visualized by a high speed camera, and characteristics of droplet behavior were evaluated. Effects of wafer film types (Two types of wafer, Type-A and Type-B were used in this study) on droplet behavior were also investigated. Droplet on Type-A wafer was moved by the air flow with relatively large deformation. However in Type-B wafer, the droplet was moved with almost no deformation. Critical air velocity of droplet movement for Type-B wafer was lower than that of Type-A wafer. From these results, it was confirmed that the liquid removal processes by the air flow was strongly influenced by the property of the wafer surface.

Ignition Position of a Diesel Spray Impinging on a Wall : Effect of Wall Angle, Injection Pressure and Fuel Quantity

Ignition and combustion characteristics of diesel spray impinging on a wall were experimentally investigated. Ignition position and appearance position of the luminous flame kernel were stereoscopically observed using a two-way fiber optical system. Flat impingement wall was fixed in a high temperature, high pressure combustion chamber. Inclined angle of the flat wall was set at 30 degees or normal against the center axis of the injection spray. Distance from nozzle tip to the impingement point on the wall was set at 50 mm. Effects of injection pressure and fuel quantity on ignition position were investigated. As the result, ignition positions were observed near the spray periphery. And the radial position of ignition was shifted to the larger radial position when the injection pressure increased. Luminous flame appeared near the wall surface in the both cases of 30 degrees and normal impingements. And the region of luminous flame appearance was not changed significantly with an increase of injection pressure.