An experimental study was performed on the behavior and extinction characteristics of the cylindrical diffusion flame affected by both factors of stretch and curvature. The cylindrical diffusion flame treated here has the convex curvature toward the air stream. The fuels used were propane and methane, and they were diluted with two kinds of inert gas, nitrogen and helium. The obtained results are described as follows. (1) The burner used in this study can form the cylindrical flame with good circularity. The minimum flame diameter is approximately 2.5mm. (2) The flame radius increases (decreases) with the increase in the fuel (air) flow velocity. (3) Flame luminosity has a maximum value when the air flow velocity is varied. On the other hand, the luminosity decreases monotonously with the increase in the fuel flow velocity. (4) Extinction stretch rate of counterflow propane 20%/nitrogen 80% vs. air flame is lower than that of counterflow methane 50%/nitrogen 50% vs. air flame. However, this extinction stretch rate relation is reversed in the case of cylindrical diffusion flame owing to the Lewis number effect caused by the flame curvature. (5) When the Lewis number of fuel flow is considerably larger than unity, the cylindrical diffusion flame can be formed even at the dilution rate with which the counterflow diffusion flame cannot be formed.
Dissociative adsorption processes of hydrogen molecules on a platinum surface were simulated by a molecular dynamics (MD) method. The effect of the incidence angle of the impinging gas molecules on the dissociation probability was analyzed. The embedded atom method (EAM) was used to construct an interaction potential between the gas molecule and the Pt(111) surface. Initially, the effect of the motions of atoms or molecules (dynamic effects) on the dissociation probability was studied. It showed that dynamic effects on the dissociation phenomena are very large for adsorption at the top site. A number of MD simulations of H2 or D2 molecules impinging on a Pt(111) surface were also carried out, with varying initial orientations and rotational energy of the molecule, and differing collision locations on the surface. The results were averaged and the effects of the isotopic nature, incident angle, and translational energy of the adsorbate were analyzed. The results were also compared to those obtained from molecular beam experiments to check the validity of the simulations. This comparison showed that the dependences of the dissociation probability on translational energy and incident angle roughly agree with experiments.
This research studied a high performance, low temperature reforming system with a power supply input of 12V/150W. A methanol-water mixture was used as fuel and atomized into particles with a diameter of 2∼3 µm through the use of a high frequency supersonic oscillator. The atomized particles were then dissociated into concentrated hydrogen at a low temperature through the use of an inductively coupled high density plasma producer. The aim of this project was to reduce the operating temperature of a conventional reformer in order to significantly decrease power consumption and generate more concentrated hydrogen in a shorter time after a cold start. The reforming system in this study was proven to be capable of generating syngas with 12 vol% and 34 vol% hydrogen stably in 10 seconds and 20-25 seconds respectively after ignition with operating temperatures below 42°C. Therefore, this reforming system provided a low operating temperature, a short cold start time, low power consumption and still produced concentrated hydrogen. Hence, this reforming system would be suitable for installation in vehicles or mobile devices.
Pyrolysis of cylindrical woody biomass has been investigated both numerically and experimentally with emphasis on intra-particle heat transfer and tar decompostion. In experiment, wood cylinder of 8 mm diameter and 9 mm length was pyrolyzed in an infrared reactor exposed to both convective and radiative heat fluxes in argon environment. The final reactor temperature was 973 K, and heating rate was 5, 10 and 30 K/s. Three K-type thermocouples were located in the sample to measure intra-particle temperature history. The weight fraction history and intra-particle temperature profiles were measured at different runs. Tar was obtained at a cold trap. In calculation, a two-dimensional, unsteady state single particle model was developed and used to simulate the pyrolysis process. Wood cylinder was modeled as an isotropic porous solid. Solid mass conservation equations were solved by using first-order Euler Implicit Method. Gas phase mass conservation equations and energy conservation equation were discretised by finite volume method. In order to investigate the effect of intra-particle heat transfer, simulations were carried out, first, by considering temperature gradient and second, by assuming uniform temperature within the sample. When temperature gradient was considered, simulation results were in good agreement with experimental data. When uniform intra-particle temperature was used in the simulation, simulation results were quite different from experimental measurements, the degree of difference increasing with increase in heating rate. Both calculation and experiment showed tar yield decreased with increasing heating rate. This was because tar formation reaction and intra-particle tar decomposition reactions were enhanced by increase in heating rate but the latter was dominant. It was shown that intra-particle heat transfer and tar decomposition played an important role in pyrolysis characteristics of wood cylinder.
The effects of the unburned-gas temperature and Lewis number on the intrinsic instability of high-temperature premixed flames under the constant-enthalpy conditions were investigated by two-dimensional unsteady calculations of reactive flows. A sinusoidal disturbance with sufficiently small amplitude was superimposed on a planar flame to obtain the relation between the growth rate and wave number, i.e. the dispersion relation. As the unburned-gas temperature became higher, the growth rate increased and the unstable range widened. This was due to the increase of the burning velocity of a planar flame. In addition, the obtained numerical results were consistent with the theoretical solutions in small wave-number region. As the Lewis number became smaller ( larger ), the growth rate increased ( decreased ) and the unstable range widened ( narrowed ), which was due to diffusive-thermal effects. The dispersion relation yielded the linearly most unstable wave number, i.e. the critical wave number. The critical wave number increased as the unburned-gas temperature became higher. Thus, the critical wavelength shrank, and then the cell size shrank. To clarify the characteristics of cellular flames induced by intrinsic instability, a finite disturbance with the critical wavelength was superimposed. The superimposed disturbance evolved, and a cellular-shaped front formed. In all Lewis numbers, the behavior of cellular flames became milder as the unburned-gas temperature became higher, even though the growth rate increased. The normalized burning velocities of cellular flames decreased monotonously. This was because that the thermal-expansion effects became weaker owing to the decrease of the difference in temperature between the burned and unburned gases, which was generated by the conditions of constant enthalpy, i.e. constant burned-gas temperature.
The characteristics of energy conversion from thermal energy into near-infrared radiant energy in a superadiabatic system with stacked porous quartz glass plates were investigated using one dimensional numerical simulation to optimize the parameters of the quartz media. 56% of the input thermal energy could be converted into radiant energy in the wavelength range of less than 2.2 µm using a number of thin porous quartz glass plates with low porosity. In addition, the efficiency of energy conversion could be expected to be 63% when a non-gray foam ceramics was used as the selective emitter. Radiant energy can be converted into electric power by spectral control using low bandgap photovoltaic cells. The superadiabatic system was demonstrated to be effective for a thermophotovoltaic generation system.
The purpose of this study is to develop an ultra-micro combustor that uses two types of coaxial cylindrical flames, rich premixed flame and diffusion flame. The combustor consists of inner and outer porous tubes, and rich propane-air mixture and air issued, respectively, through the inner tube outwardly and through the outer tube inwardly, forming a cylindrical stagnation plane sandwiched by the inner rich premixed flame and the outer diffusion flame. Petal type flame was also observed in the downstream of the cylindrical flames. Keeping the equivalence ratio φi and flow rate qi of the rich mixture constant, air flow rate qa was varied. The O2 and CO concentrations and temperature of the burnt gas were measured, and heat loss rate ηhl and combustion intensity L were evaluated. The obtained results are described as follows. (1)The relation curve of ηhl with the overall equivalence ratio φall, which is evaluated from the total flow rate of the fuel and the air, has a minimum value. (2)The relation curve of the minimum value of ηhl with L has a minimum value. (3) The CO concentration of the burnt gas increases as qa is increased because of local extinction of the petal type flame. (4)When qa is increased further, petal type flame is also extinguished. After that, the O2 concentration increases and the CO concentration decreases.
A standing-wave thermoacoustic engine is essentially composed of a stack, two heat exchangers, and tubes; the stack has many narrow flow channels and is located inside one of the tubes. One end and the other end of the stack are thermally connected to hot and cold heat sources via the heat exchangers, respectively. When the ratio of temperatures of the heat sources exceeds a critical value, the gas inside the tubes spontaneously oscillates and the stack generates acoustical energy using heat from the hot heat source. In this study, the critical temperature ratio needed for exciting the spontaneous gas oscillation was numerically calculated by changing the stack's length, flow channel radius, and position. Further, the thermal efficiency with the critical temperature ratio was calculated. These calculations allowed us to design and construct the engine in accordance with the heat source temperatures. The constructed engine worked with the heat sources having the temperature ratio 1.7 and achieved 8% of the Carnot efficiency. These obtained values quantitatively agreed with the design ones.
In this study, three dimensional numerical simulations of fluid and heat transfer on the transonic vane have been performed using v2-f turbulence model. The objective of this paper is to evaluate the v2-f model’s ability to predict the heat transfer characters on the 3D showerhead film cooled vane. Exit Mach number of Mex=0.76, corresponding to exit Reynolds number based on vane chord of 1.1×106, is tested with an inlet free stream turbulence intensity (Tu) of 16% and integral length scale normalized by vane pitch (Λx/P) of 0.23. For the showerhead film cooling simulations, a new three simulations technique is discussed to calculate the recovery temperature, adiabatic wall temperature, and surface Nusselt number. CFD predictions show an overall good agreement with experimental results.
We propose a novel patterning method for self-assembled monolayers (SAMs) using near-field photothermal desorption (NPTD). This paper reports the patterning principle and the study on optimum heating conditions. The proposed method utilizes the thermal desorption of constituent molecules of a SAM through the irradiation with near-field light, which can make noncontact and noncontaminating patterning of the SAMs at the nanoscale. In order to verify the validity of the proposed patterning method for SAMs, the preliminary patterning of a SAM by irradiating with a laser beam was performed. The results suggested that the constituent molecules were thermally desorbed and the subsequent modification of another kind of a SAM was successfully carried out. The numerical analysis of the temperature distribution after heating with near-field light was demonstrated by using the finite element method to investigate the heating condition of NPTD. The simulation results of laser heating well agreed with the preliminary experimental results, therefore, the applicability of the proposed analytical model was confirmed. Furthermore, the analytical results of the temperature distribution indicated that the local heating at the nanoscale with sufficient temperature increase for NPTD could be induced by the irradiation with near-field light generated by using an apertured fiber probe coated with Ag. As a result, the validity of the patterning principle and the optimal heating conditions were verified, and therefore, the possibility of the nanoscale patterning of SAMs using NPTD was confirmed.
Thermal radiation in a turbulent natural convection plays an important role in a wide area of engineering and nature. The purposes of this study are to investigate the effects of turbulent fluctuation on radiative heat transfer, and to evaluate radiative heat transfer models applied to turbulent natural convection. The present radiative heat transfer analysis of a turbulent natural convection using direct numerical simulation (DNS) provides a useful fundamental data for the complete coupling simulation in the future.
The effects of compression ratio and simulated exhaust-gas recirculation (EGR) on combustion characteristics and exhaust emissions of a diesel PCCI engine were investigated using a single-cylinder test engine. Tests were carried out under constant speed with various compression ratios and EGR rates. Exhaust emissions and in-cylinder pressure were measured for all experimental conditions. Analyses based on engine performance and exhaust emissions were conducted. An optimum compression ratio that provided better indicated thermal efficiency and IMEP while yielding lower emissions of smoke, HC, and CO, and reasonable NOx without EGR was identified. High rates of EGR led to the simultaneous reduction of NOx and soot emissions due to a lower combustion temperature compared with conventional diesel combustion, with a slight penalty in HC and CO.
We investigated the effects of the microporous layer (MPL) and the gas flow configuration on water transport in an operating polymer electrolyte membrane fuel cell by means of tunable diode-laser absorption spectroscopy (TDLAS). TDLAS, which permits optical remote sensing of the distribution of the water vapor concentration along a channel, was used to examine local through-plane water transport across the membrane electrode assembly (MEA). TDLAS observations showed that an MPL at the cathode side enhances back-transport of water generated at the cathode to the anode under conditions of low relative humidity in both co-flow and counter-flow configurations, because it acts as a mass-transport resistance in the MEA. We also showed that the counter-flow configuration established an internal circulation of water in the fuel cell.
Edited and published by : The Japan Society of Mechanical Engineers and The Heat Transfer Society of Japan Produced and listed by : Showa Joho Process Co., Ltd.(Vol.8 No.3-) Sanbi Printing Co., Ltd.(Vol.1 No.1-Vol.8 No.2)