Shinji Kojima

Detailed modeling of n-butane autoignition chemistry

Abstract Two versions of a detailed chemical kinetic model of n-butane autoignition are proposed. The key distinctions between the versions are the exclusion (Model I) or inclusion (Model II) of the direct abstraction or apparently bimolecular path of the reaction HO2 + HO2 → H2O2 + O2, and the selection of rate parameters for the reaction C2H5 + O2 → C2H4 + HO2. Both models (versions) were evaluated over 1200–1400 K for a stoichiometric mixture and over 720–830 K for lean to rich mixtures by comparing the computed autoignition delays with those of two experiments: (1) a shock-tube experiment (representative of high-temperature chemistry) and (2) a rapid compression experiment (representative of low-temperature chemistry). The experiments demonstrate the ablity of the models to predict autoignition delays at high pressures typical of automobile engines. Four interpretations of the rapid compression data using the present model are also described. The two models provide the same behavior of autoignition delay (a macroscopic phenomenon), but the sensitivities of the delays to reaction rate constants (a microscopic aspect of the autoignition mechanism) are remarkably different. Therefore, while the two models are shown to reproduce the macroscopic experimental data, more research is needed to determine which model is valid at the microscopic level.

Uniform deposition of diamond films using a flat flame stabilized in the stagnation‐point flow

For uniform deposition of diamond films by acetylene/oxygen combustion flames, the flat flame stabilized in the stagnation‐point flow in front of the substrate has been realized by adding hydrogen to reduce the burning velocity. The flat flame stability was examined for the hydrogen/acetylene/oxygen ratio and the flow velocity of the reactant gas. In the flat flame, fields of temperature and concentrations of radical species which take part in diamond deposition, are homogeneous on the surface of the substrate. It has been confirmed by scanning electron microscopy and Raman spectroscopy that diamond film deposited by the round flat flame with a diameter of about 7 mm, are uniform over area with nearly the same diameter of the flame.

Photographic observation of knock with a rapid compression and expansion machine

A new type of rapid compression and expansion machine (RCEM) has been developed, and typical knock scenes were clearly recorded with a high speed laser shadowgraph at a speed of 100,000 frames per second. The RCEM is intended to simulate combustion in an automotive engine. Its piston is driven by an electrohydraulic servo system and is allowed to execute continuous reciprocations up to five times. The combustion chamber is a simple pancake type with an ignition plug on its side and the whole inner view is observable through a glass window on the top.

Autoignition-delay measurement over lean to rich mixtures of n-butane/air under swirl conditions

Abstract The autoignition delays over lean to rich mixtures of butane/air under various speeds of swirl flow were measured in a rapid compression experiment, together with photographic observation at 3000 frames per second. A lot of interesting features of the autoignition delay and the autoignition process were observed. Especially, it has been found that the swirl preferentially lengthens the autoignition delays of richer mixtures while lean mixtures with equivalence ratios less than about 0.6 are affected only little. With the results of computer simulation, it is concluded that this preferential effect indicates that the adiabatic core, in which the autoignition chemistry proceeds adiabatically, breaks more easily in richer mixtures at higher swirl speeds. Although the detailed mechanism of this preferential breakdown could not be clarified, this and other features of the autoignition under the swirl were fully discussed and interpreted.

Theoretical Evaluation of the Maximum Work of Free-Piston Engine Generators

Abstract Utilizing the adjoint equations that originate from the calculus of variations, we have calculated the maximum thermal efficiency that is theoretically attainable by free-piston engine generators considering the work loss due to friction and Joule heat. Based on the adjoint equations with seven dimensionless parameters, the trajectory of the piston, the histories of the electric current, the work done, and the two kinds of losses have been derived in analytic forms. Using these we have conducted parametric studies for the optimized Otto and Brayton cycles. The smallness of the pressure ratio of the Brayton cycle makes the net work done negative even when the duration of heat addition is optimized to give the maximum amount of heat addition. For the Otto cycle, the net work done is positive, and both types of losses relative to the gross work done become smaller with the larger compression ratio. Another remarkable feature of the optimized Brayton cycle is that the piston trajectory of the heat addition/disposal process is expressed by the same equation as that of an adiabatic process. The maximum thermal efficiency of any combination of isochoric and isobaric heat addition/disposal processes, such as the Sabathe cycle, may be deduced by applying the methods described here.

Maximum Work of Free-Piston Stirling Engine Generators

Abstract Using the method of adjoint equations described in Ref. [1], we have calculated the maximum thermal efficiencies that are theoretically attainable by free-piston Stirling and Carnot engine generators by considering the work loss due to friction and Joule heat. The net work done by the Carnot cycle is negative even when the duration of heat addition is optimized to give the maximum amount of heat addition, which is the same situation for the Brayton cycle described in our previous paper. For the Stirling cycle, the net work done is positive, and the thermal efficiency is greater than that of the Otto cycle described in our previous paper by a factor of about 2.7–1.4 for compression ratios of 5–30. The Stirling cycle is much better than the Otto, Brayton, and Carnot cycles. We have found that the optimized piston trajectories of the isothermal, isobaric, and adiabatic processes are the same when the compression ratio and the maximum volume of the same working fluid of the three processes are the same, which has facilitated the present analysis because the optimized piston trajectories of the Carnot and Stirling cycles are the same as those of the Brayton and Otto cycles, respectively.

Numerical calculation of the two-dimensional unsteady laminar flame propagation in a confined H2/air system

Abstract To ascertain the feasibility of multidimensional flame propagation computations with detailed chemical kinetics, a two-dimensional unsteady laminar H2/air flame in a disk-shaped chamber was calculated numerically. The flame stretching technique originally developed by Butler et al. was applied in order to overcome the computational difficulty inherent in multidimensional laminar flame propagation problems. The technique was modified to introduce a cutoff value of the temperature gradient below which the stretching procedure is not applied. Also the inverse transformation was devised to unstretch the results. To obtain the quantitative accuracy and reliability of the calculation, a simple and efficient formulation of the transport coefficients was determined from the sensitivity studies. By these studies it was clarified that the flame propagation is sensitive to the collision integrals, but not sensitive to the deviation of the Lennard-Jones potential parameters, and the correction for the polarity of H2O molecule is unnecessary. Using the deduced formulation of the transport coefficients and the flame stretching procedure, the calculation results agree well with the experiment and the computation time is acceptable.

Reaction Kinetics Path Based on Entropy Production Rate and Its Relevance to Low-Dimensional Manifolds

The equation that approximately traces the trajectory in the concentration phase space of chemical kinetics is derived based on the rate of entropy production. The equation coincides with the true chemical kinetics equation to first order in a variable that characterizes the degree of quasi-equilibrium for each reaction, and the equation approximates the trajectory along at least final part of one-dimensional (1-D) manifold of true chemical kinetics that reaches equilibrium in concentration phase space. Besides the 1-D manifold, each higher dimensional manifold of the trajectories given by the equation is an approximation to that of true chemical kinetics when the contour of the entropy production rate in the concentration phase space is not highly distorted, because the Jacobian and its eigenvectors for the equation are exactly the same as those of true chemical kinetics at equilibrium; however, the path or trajectory itself is not necessarily an approximation to that of true chemical kinetics in manifolds higher than 1-D. The equation is for the path of steepest descent that sufficiently accounts for the constraints inherent in chemical kinetics such as element conservation, whereas the simple steepest-descent-path formulation whose Jacobian is the Hessian of the entropy production rate cannot even approximately reproduce any part of the 1-D manifold of true chemical kinetics except for the special case where the eigenvector of the Hessian is nearly identical to that of the Jacobian of chemical kinetics.