Nobuyuki Araki

Modeling of graphite oxidation in a stagnation-point flow field using detailed homogeneous and semiglobal heterogeneous mechanisms with comparisons to experiments

Abstract Numerical simulation results are presented on the mass burning rate and the gas-phase flame structure of a heated cylindrical graphite rod in a stagnation-point flow field, using detailed homogeneous chemical kinetics and semiglobal heterogeneous chemical kinetics. Extensive comparisons with new experimental data and data from the literature are shown for various oxidizer compositions, pressures and strain rates. The relative importance of the carbon-radical reactions in the two semiglobal heterogeneous mechanisms employed is demonstrated, while the deficiencies and limitations of applying semiglobal heterogeneous mechanisms for graphite rod oxidations are identified. Under simplifying assumptions, a method for including the graphite porosity in the present quasi-one dimensional formulation is described. The need to develop elementary reaction mechanisms for the heterogeneous kinetics and the importance of accurate estimation of effective surface are is stressed.

Measurements of thermophysical properties of sodium acetate hydrate

Methods to measure the thermal conductivity, the specific heat capacity, and the heat of fusion of sodium acetate hydrate have been developed and the measured results have been reported for various concentrations and especially for various supercooling temperatures. Thermal conductivity was measured by using a probe method with a thermistor. The sensor element is very small, with a diameter of 0.5 mm and a length of 1.5 mm. Data for both the ordinary liquid and the supercooled liquid are smoothly connected to each other.

The wave characteristics of thermal conduction in metallic films irradiated by ultra-short laser pulses

The behaviour of conduction heat transfer in a metallic film irradiated by a laser pulse with Gaussian temporal profile is investigated by using the thermal wave model. Calculations are performed to describe the propagation and reflection of a temperature wave and heat flux through the film. The temperature response at the front surface obtained from the thermal wave model is compared with that from the microscopic two-step conduction model by considering a real situation that a Cu film is irradiated by a picosecond laser pulse.

Analytical solution of non-Fourier temperature response in a finite medium under laser-pulse heating

A temperature wave solution predicted by the hyperbolic heat equation is developed for a finite medium exposed to a surface heat flux of laser with an actual temporal profile. By using the analytical solution, the temperature response, the propagation and the reflection of the temperature wave due to such heat pulse are investigated for different pulse duration, thickness of the medium, and energy absorption depth.ZusammenfassungFür ein begrenztes Medium, dessen Oberfläche durch einen Laserpuls mit vorgegebenem zeitlichen Intensitätsverlauf beaufschlagt wird, konnte die aus der hyperbolischen Wärmeleitungsgleichung deduzierbare Temperaturwellenlösung gefunden werden. Mit Hilfe dieser analytischen Lösung läßt sich die Temperaturantwort, die Ausbreitung und Reflexion der durch einen solchen Wärmeimpuls ausgelösten Temperaturwelle für verschiedene Pulsdauern, Mediumdicken und Energieabsorptionstiefen untersuchen.

Combustion of artificial graphite in stagnation flow: Estimation of global kinetic parameters from experimental results

Abstract The combustion of graphite rod in the stagnation flow of an oxidizer is studied experimentally in order to investigate the coupled nature caused by the interaction between the gas-phase and surface reactions. Not only the combustion rate of the graphite rod but also the surface temperature at which the CO flame is established in the forward stagnation region of the burning graphite is measured. By investigating effects of ambient pressure, oxidizer, and concentrations of oxygen and water vapor as well as graphite density. It is experimentally observed that the establishment of the CO flame yields the reduction of the combustion rate. By estimating kinetic parameters for the surface and gas-phase reactions with the combustion rate and the surface temperature at the ignition of CO flame, respectively, and by comparing the predicted results with experimental data, it has turned out that the appearance of the CO flame can be prediceted when kinetic parameters for the surface and gas-phase reactions are known.

An analytical solution of temperature response in multilayered materials for transient methods

Transient methods, such as those with pulse- or stepwise heating, have often been used to measure thermal diffusivities of various materials including layered materials. The objective of the present study is to derive an analytical solution of the temperature rise in a multilayered material, the front surface of which is subjected to pulse- or stepwise heating. The Laplace transformation has been used to obtain the analytical solution. This solution will enable us to establish the appropriate measurement method for thermophysical properties of the multilayered material. It is also shown that the present solution can be extended to functionally gradient materials (FGM), in which thermophysical properties as well as compositions change continuously.

Transient temperature response in functionally gradient materials

The temperature response in functionally gradient materials (FGM), subjected to pulseor stepwise heating at the front surface, is evaluated. Applicability of the approximate solution for the temperature response is investigated by comparing it with an exact analytical solution for the FGM in which thermophysical properties have certain profiles. When the FGM is composed of conventional solid materials, appropriateness of the approximate solution for the FGM is demonstrated as far as the temperature response near the rear surface is concerned. The approximate solution is also compared with the solution for the multilayered material. It is shown that an eight-layered material can be regarded as an FGM, as far as the temperature response at the rear surface is concerned, and that the approximate solution can predict the temperature response within 6% error. Because of its simplicity and fair degree of agreement, the approximate solution is anticipated to be used not only for qualitative but also for quantitative prediction of the temperature response near the rear surface of the FGM in engineering applications.

On non-fourier temperature wave and thermal relaxation time

In this paper, the non-Fourier effects in a material under heating flux with an actual pulse and periodic temporal profile are investigated after introducing the physical mechanism of the thermal relaxation model. By using the analytical solution of the non-Fourier hyperbolic conduction equation, a discussion about the wave characteristics of non-Fourier conduction is given, and the manner in which relaxation time affects the temperature behavior is discussed. Then a measuring method for the relaxation time is suggested for these two kinds of heating flux.

Transient Characteristics of Thermal Conduction in Dispersed Composites

The effective thermal conductivity of dispersed composites with a hot-melt-adhesive matrix, measured using the steady-state method, is compared with the apparent thermal conductivity calculated from the average heat capacity and from the thermal diffusivity measured by the laser-flash method. The transient effect has been observed obviously at higher volume percentages for various dispersed particle sizes and ratios of the thermal conductivity values of dispersed and continuous phases. All of the experimental results are compared with those calculated by existing models and by the finite element method (FEM). An attempt has been made to show how the criterion for the homogeneity of dispersed composites under transient conditions is affected by the percentages of dispersed phase, dispersed particle size, and ratio of the thermal conductivity values of dispersed and continuous phases.

An Inverse Analysis to Estimate Relaxation Parameters and Thermal Diffusivity with a Universal Heat Conduction Equation1

This paper presents an inverse analysis for simultaneous estimation of relaxation parameters and thermal diffusivity with a universal heat conduction model by using temperature responses measured at the surface of a finite medium subjected to pulse heat fluxes. In the direct analysis, the temperature responses in a finite medium subjected to a pulse heat flux are derived by solving the universal heat conduction equation. The inverse analysis is performed by a nonlinear least-squares method for determining the two relaxation parameters and thermal diffusivity. Here, the nonlinear system of algebraic equations resulting from the sensitivity matrix is solved by the Levenberg–Marquardt iterative algorithm. The inverse analysis is utilized to estimate the relaxation parameters and the thermal diffusivity from the simulated experimental non-Fourier temperature response obtained by direct calculation.