Kazunori Wakai

Acoustic computer tomographic pyrometry for two-dimensional measurement of gases taking into account the effect of refraction of sound wave paths

The algorithm which takes into account the effect of refraction of sound wave paths for acoustic computer tomography (CT) is developed. Incorporating the algorithm of refraction into ordinary CT algorithms which are based on Fourier transformation is very difficult. In this paper, the least-squares method, which is capable of considering the refraction effect, is employed to reconstruct the two-dimensional temperature distribution. The refraction effect is solved by writing a set of differential equations which is derived from Fermats theorem and the calculus of variations. It is impossible to carry out refraction analysis and the reconstruction of temperature distribution simultaneously, so the problem is solved using the iteration method. The measurement field is assumed to take the shape of a circle and 16 speakers, also serving as the receivers, are set around it isometrically. The algorithm is checked through computer simulation with various kinds of temperature distributions. It is shown that the present method which takes into account the algorithm of the refraction effect can reconstruct temperature distributions with much greater accuracy than can methods which do not include the refraction effect.

Downward flame spread over poly(methyl)methacrylate

The thermal regime of downward flame spread over poly(methyl)methacrylate (PMMA) in an oxygen/nitrogen environment in normal gravity has been revisited experimentally, computationally, and analytically. Although spread rate data is plentiful in the literature, different fuels, usually PMMA and cellulose, have been used in thermally thick and thin limits. Moreover, most of these experiments, conducted at atmospheric oxygen levels, belong to the kinetic regime where finite-rate kinetics plays a dominant role. As a result, there remain some unanswered questions about the thermal regime, which constitutes the backbone of downward flame spread. We present spread rate data for the first time in the thermal regime where the fuel thickness is changed from the thin to the thick limit. A comprehensive computational model, verified through comparison with all relevant analytical solutions, is used to establish the transition criterion between the chemical and thermal regimes, and between the thin and thick fuel regimes. An earlier extension of the de Ris formula, capable of predicting the spread rate accurately for forced opposed flow, has been extended to downward configuration by proposing an empirically determined equivalent buoyant convection velocity. The predictions from the resulting closed form spread rate formulas, which retain the functional form of the de Ris formula, are shown to compare well with computational and experimental results, including all previously published data in the thermal regime. An alternative to traditional Damkohler number correlation, in which an equivalent oxygen level is used in conjunction with thermal-regime spread rate formulas, is proposed to capture the effect of finite-rate chemistry. A simple formula for the transition thickness between the thin and thick fuel regime is proposed as τ crit = 2 τ V f,Thin V f,Thick where the spread rates can be obtained from experiments or the proposed formulas.

Effect of radiation loss on flame spread over a thin PMMA sheet in microgravity

Flame spread over a thin polymethylmethacrylate sheet in microgravity is investigated experimentally and analytically. A scale analysis yields a simple equation, η+ R /ζ=1, which states the radiation loss is a function of relative flow velocity and the fuel thickness. The prediction with the scale analysis states that the reduction in the relative flow velocity enlarges the size of the preheat zone, which increases radiation loss, and that the presence of radiation loss reduces the spread rate and also may cause extinction. To confirm the prediction, drop experiments are carried out with a 4.5 s drop tower in MGLAB, in Gifu, Japan. Flame-spread rates are measured with varying ambient flow velocity, fuel thickness, and oxygen level. Additionally, the temperature fields near the flame front are measured in quiescent conditions with a Michelson interferometer system. The experimental result shows that the spread rate is actually a function of sample thickness and ambient flow velocity, and that the spread rate becomes the minimum when the relative flow velocity is close to zero. It is also found that steady flame spread is established even in quiescent conditions if R is sufficiently low. The interferometer measurement shows the enlargement of the preheat zone in quiescent microgravity conditions as predicted by the scale analysis. It also shows that the steady heat balance is established when R is small, whereas it is not established in near extinction conditions. These results support the scale analysis and clarify the extinction mechanism via radiation loss.

Predictions of a critical fuel thickness for flame extinction in a quiescent microgravity environment

Abstract A simplified analysis and data acquired in the 4.5 s drop tower in MGLAB, Japan in a quiescent oxygen/nitrogen environment are presented for the prediction of the flammability limit in a quiescent microgravity environment. In the experimental matrix the oxygen level and thickness of PMMA are treated as control parameters. Published data from quiescent microgravity experiments on thin ashless filter paper and thick PMMA are also compared with the prediction of the analysis. Based on scale analysis, it is hypothesized that all fuels—from PMMA to cellulose—behave as thermally thin fuels during steady spread of flames in a quiescent environment. An expression for the spread rate that includes radiative effects is proposed for the first time: η0 ∼ 1/2 + 1/2 1−4 R 0 , where η0 is the spread rate non-dimensionalized by its thermal limit and R 0 is the non-dimensional radiation number. For R 0 > 1/4, which in dimensional terms translates to a critical thickness criterion τ > (F2/4)(ρgcg/ρscs)(λg/eσ)[(Tv − T∞)/(Tv4 − T∞4)], flame extinction occurs irrespective of all other environmental conditions. Based on this prediction, an extinction thickness can be calculated even at 100% oxygen level. The experimental data from the MGLAB agree reasonably well with this prediction. Flammability maps with fuel half-thickness and oxygen level as coordinates are developed for PMMA and cellulosic fuels, which are shown to be consistent with the current and published data.

Ignition limit of lean mixture by hydrogen flame jet ignition

Abstract A hydrogen flame jet ignition method is proposed to achieve the secure ignition and stable combustion of an ultra lean mixture. The mechanism of enhancement by the jet ignition method, especially the role of radicals in the flame jet, is analyzed first and the lean limit of ignition is investigated by the hydrogen flame jet ignition method. This proved that the effect of radicals on the enhancement of lean mixture combustion is smaller than the turbulent effect. The lean limit of stable ignition at atmospheric pressure is therefore almost the same as that of the inflammability limit, namely around φ =0.5. When gas temperature and pressure are increased to actual engine conditions, the lean limit is widened to around 0.3 which is much lower than conventional lean burn engine conditions.

Resistance wire CT pyrometry used to analyse the knocking problem in direct injection engines

To study the effect of the fuel concentration gradient on the auto-ignition phenomenon in a cylinder, a rapid compression machine (RCM), whose axial direction is set horizontally, is employed. The fuel concentration gradient is controlled by adjusting the duration of evaporation and diffusion from the time of injection of the fuel into the bottom of the cylinder. To achieve the desired concentration distribution, the history of the fuel concentration distribution in the RCM is measured by an infrared laser absorption method. There is a possibility that the initial concentration distribution will be altered by gas movement such as roll-up vortices, so the resistance wire CT method is proposed and applied. This method can measure a two-dimensional temperature distribution generated by a fuel concentration gradient during the compression process with the histories of wire resistance variations and computer tomography (CT) algorithm. The auto-ignitions took place with various concentration distributions in the cylinder. Various patterns of pressure history and direct photographs of the auto-ignition process are recorded. The Livengood–Wu integral method is applied using two-dimensional temperature and concentration history and the results of knocking intensity and tendency coincide qualitatively well with the experimental data.

Instantaneous measurement of two-dimensional temperature and density distributions of flames by a two-band-emission-CT pyrometer

Because temperature is one of the most important factors influencing combustion reactions, a variety of temperature measurement methods have been developed for burnt gas. Infrared radiation pyrometry using water vapor or carbon dioxide, which are present in high density in a burnt gas, has a long history. However, these classical methods can measure only a mean temperature or step-wise temperature distribution of several segments along an optical path. Due to the severe demand for cleaner and more efficient combustion, more detailed temperature information is required. Computed tomography (CT) applied to radiation methods (same as X-Ray CT in medical use) enables measurement of a two-dimensional temperature distribution. The authors have developed several types of infrared CT pyrometers. Because CT methods generally take a long time to obtain projection data, it is thought that they are not applicable for high speed unsteady combustion. In this report, a two-band-emission-CT pyrometer, which was developed by the authors, is further developed to enable time-resolved measurement. An algorithm and optical configuration is introduced for fan-beam scanning. The accuracy is then investigated. The experiment was performed using only one optical unit as a preliminary investigation using a jet flame with good reproducibility.

Effects of combustion on flowfield in a model scramjet combustor

The effects of combustion on the change of the flowfield in a model SCRamjet combustor with a backward step is investigated experimentally and numerically. The main airflow has a Mach number of 2.0. The total temperature is 1000 K for cold flow and 1800 K for hot flow. Hydrogen fuel is injected parallel to the main airflow through a slit on the backward face of the step. The combustion mode is categorized in two modes. One is a weak combustion (WC) mode that is not accompanied by a shock wave, where the flowfield is similar to that in cold flow. Another is an intensive combustion (IC) mode that is accompanied by a shock wave, where the flowfield is much different from that in cold flow. In IC mode, a large separation region is generated behind the step by the shock wave, and the vortex generated at the region rolls the fuel up. The main reacting region is in the shear layer just behind the shock wave, where the main airflow bumps the rolled-up fuel, and the temperature is relatively high due to the shock wave. The flowfield is then controlled by the rate of mixing, leading to fast heat release, which raises the pressure level in the combustor and supports the shock wave. This passive feedback works, and both the mixing efficiency and the combustion efficiency become high. On the other hand, in WC mode, the reacting region spreads over the shear layer downstream of the step, and its heat release rate is lower than that in IC mode. The flowfield is then controlled by the rate of chemical reaction, and the combustion efficiency remains low.

Microcombustor with a sintered porous catalytic layer

The 5-Watt class microcombustor which has a porous catalytic layer inside a narrow ceramics tube whose inner diameter is 0.8mm has been developed. In order to obtain monolithic porous fine structure, catalyst colloidal paste of Pt or Pd is painted inside the tube and then it is sintered with rich methane-air mixture. By adjusting the sintering temperature on the surface, the size of pores can be controlled. The great merit of this method is that it can be applied to complicated combustor configuration with very low cost. This combustor with the present catalytic later can work in wide ranges of equivalence ratio (0.8-8.0) and mixture flow rate (20cm 3 /min-200cm 3 /min at 293K) with methane fuel. The continuous combustion time has recorded more than 1000 hours. The exhaust gas temperature is about 1100K, and the combustion efficiency reaches 95%. By coupling with Bi-Te thermoelectric modules, the combustor can convert a part of thermal output to electricity as a micro-cogenerator. This device generates 161.3mW electricity with the fuel supply of 5.37W, which corresponds to final conversion efficiency of 3.00%.

Investigation of two-phase flow mixing between two subchannels

The cross flow between subchannels in a BWR fuel assembly has been typically analyzed using three types of mixing models, namely, pressure difference, turbulent mixing, and void drift which are expressed by time-averaged flow parameters. However, in our previous paper, we expressed the above cross flow phenomenon simply by a fluctuating pressure model and confirmed its validity experimentally. In this present study, we examine the relationship between the fluctuating pressure difference and the cross flow rate more precisely by using a short mixing zone with no steady pressure difference. Results show that the experimental cross flow data agree well with the calculations using this model. Furthermore, we tried to express the fluctuating pressure difference by using a sinusoidal wave as a new cross flow model. This model is shown to have no dependence on frequency. We verify that the cross flow can be analyzed using only the pressure difference amplitude.