Eran Sher

Experimental optimization of an electric blower by corona wind

The effect of corona wind produced by stretched steel wire and two copper wings on the heat transfer from a heated horizontal plate was investigated experimentally. Although in such an arrangement the heat transfer augmentation is expected to be lower, some advantages may be postulated. In such a construction, the plate to be cooled is not a part of the wind generation system, it is not charged, the electrical field next to it is negligible, and it may be constructed from non-metallic materials. In the course of the study, optimal geometric parameters of the electric blower together with optimal value of high voltage supply have been established. Under these optimal conditions, augmentation by three times of the heat transfer coefficient over that for the natural convection has been achieved.

Numerical modeling of spark ignition and flame initiation in a quiescent methane-air mixture

Abstract The initiation of a spark kernel and the subsequent propagation of a self-sustained flame in an internal combustion engine have been investigated numerically. A theoretical model which employs a two-dimensional cylindrical coordinate system and assumes axial symmetry has been developed. It considers the various physical and chemical phenomena associated with the ignition process and employs a detailed chemical reaction scheme for a methane-air mixture which contains 29 chemical species and 97 reaction steps. The thermodynamics and transport properties of the plasma at high temperatures are evaluated by a statistical thermodynamics approach, while assuming local thermodynamic equilibrium. Using the PHOENICS and the CHEMKIN codes, the appropriate conservation equations are solved in the domain of solution. It was concluded that the kernel growth can be described as a two-step process. In the early short stage (1–5 μs) the mass and energy transfer processes are very much dominated by the pressure wave and the violently expanding plasma kernel, while the contribution of the chemical reactions is negligible. This stage is followed by a much longer period in which diffusion and thermal conduction control mass and energy transfer as the flame becomes gradually self-sustained. Owing to the heat release by chemical reactions, the expansion of the combustible mixture is accelerated at the beginning of the diffusive stage.

The effect of an electric field on the shape of co-flowing and candle-type methane–air flames

Abstract The effects, which an electric field exerts on flames, have been observed and reported in the literature for a long time. Burning velocity, flame stability, flame shape, flame luminosity, extinction limit, and soot formation, are among the effects that have been observed. Most of the studies in this field were experimental observations. There is fairly limited information in the literature on numerical studies in the area of electric field and flame interaction. Therefore, our fundamental understanding of the process and our ability to use electric field as a means to control the combustion process, are restricted. In the present work, co-flowing diffusion methane/air flames and candle-type methane/air flames under the electric field effect have been observed experimentally. A numerical model, which considers the more important physical and chemical phenomena associated with the flame–field interaction process, has been developed to explain the experimental observations. The model employs a two-dimensional cylindrical coordinate system and assumes axial symmetry. A simplified chemical reaction scheme for a methane–air mixture, which contains 19 chemical species and 31 reactions is employed. It combines existing methane oxidation mechanisms with a series of chemiionization, ion-molecule, and dissociative-recombination reactions, which are important for the ionic species. The mass, momentum, species and energy conservation equations are solved numerically by an integrated version of the PHOENICS and CHEMKIN computer codes. It is concluded that the effects of an electric field on the flame behavior are mainly due to ionic wind effects.

Optimization of variable valve timing for maximizing performance of an unthrottled SI engine—a theoretical study

Previous investigations have demonstrated that improvements in gasoline engine performance can be accomplished if the valve timing is variable. In this work valve timing strategies for maximizing engine torque and minimizing bsfc in terms of the exhaust opening (EO), intake opening (IO) and intake closing (IC) timings of a commercial SI engine are studied. The MICE (Modeling Internal Combustion Engines) computer program, which simulates an actual SI cycle, has been used. Overall performance characteristics such as the cycle efficiency, engine power, and exhaust gas composition are calculated. The model has been calibrated with data obtained from a measured indicator diagram, and validated against the overall performances of the engine. It is concluded that when both valves and spark timings are optimized, the optimal timing of each valve, depends apparently linearly on the engine load, linearly (in a good approximation) on the engine speed, while the slope depends in a weak manner on the engine load. When VVT is employed, the maximum engine power has been increased by 6%, and the engine bsfc has been decreased by 13%. The maximum torque has been shifted towards a lower engine speed. The present results are summarized as working maps for the engine designer. These show the influence of the intake and exhaust valve timing on the engine performance at the entire range of operation conditions (engine load and speed).

Enhancement of heat transfer by means of a corona wind created by a wire electrode and confined wings assembly

An electrostatic blower, which is utilized to cool heat-generating bodies, such as typical power-unit chips, was studied. The device, employs a long stretched thin wire electrode, which is confined by two inclined wings. The latter provides a longitudinal nozzle for the air stream, and at the same time an electric shield. It is concluded that the heat transfer coefficient can easily be increased by a factor of more than two, as compared with a natural convection mechanism. A linear relationship was found between the Nusselt and the Reynolds number. This is explained by the complex structure of the boundary layer due to the air stream impingement effect.

From Spark Ignition to Flame Initiation

ABSTRACT The process of spark ignition and the subsequent flame propagation in an internal combustion engine have been investigated. A unique theoretical model which considers the various physical and chemical phenomena associated with the ignition process has been developed. It employs a two-dimensional cylindrical coordinate system and assumes axial and radial symmetry. The model employs also a detailed chemical reaction scheme for a methane-air mixture which contains 29 chemical species and 97 reactions. The thermodynamic and transport properties are evaluated by using statistical thermodynamics and molecular theory approach while including the various energy modes stored in the mixture particles. The appropriate conservation equations are solved numerically by using an integration of the PHOENICS and the CHEMKIN codes. It was concluded from the numerical results that the spark kernel growth can be described as a two-step process. The early short stage (1–5 μs), which involves a pressure wave emission,...

On the birth of spark channels

Abstract The breakdown stage of a spark discharge in air is analyzed. This short duration stage (

Reaction kinetics of hydrogen-enriched methaneair and propaneair flames

A computer program has been developed to calculate the burning velocity and intricate paths of hydrogen-enriched propaneair and hydrogen-enriched methaneair flames. A comprehensive reaction scheme and a rigorous method for the estimation of the transport coefficients have been adopted. The model predictions were examined against experimental results of other investigators, and good agreement has been generally found for a wide range of initial conditions. These include 80 KPa < P < 2 MPa, 280 K < T < 650 K, 0.4 < Φ < 1.5 and hydrogen to fuel molar ratio between 0 and ∞. The major chemical reaction channels by which these mixtures are burned have been analyzed by using the time-dependent kinetic flow charts technique. A possible explanation, which is based on the shortage of H atoms approach, for the complex behavior of the burning velocity of H2-enriched CH4air and C3H8air flames along an unburned gas isentrope as observed by Milton and Keck, has been proposed.

A theoretical study of the ignition of a reactive medium by means of an electrical discharge

Abstract A mathematical model is presented to simulate the evolution with time of a short segment of a spark channel in a methane-air mixture. The model assumes an axisymmetric cylindrical flame propagation and conducting column with moving boundaries in which local thermodynamic equilibrium exists at every point. The phenomena associated with the breakdown phase are considered as initial conditions. These are based on the experimental observations of other investigators. The radial profile of the time-dependent electrical energy input during the arc phase is determined by the computed plasma conductivity. The model employs a realistic equation of state, experimental transport coefficients at high temperatures, measured data for the mean emission coefficient for heat radiation, and a detailed chemical kinetics of a CH 4 -air system. The evolution with time of the conductivity channel and the associated flow, temperature, and concentration fields are calculated by numerical integration of the relevant conservation equations in the one-dimensional Lagrangian coordinates.

Spark ignition of combustible gas mixtures

Abstract A mathematical model is presented to simulate the evolution with time of a spark channel into a combustion wave. The model considers the phenomena associated with electrical breakdown and arc phases, the plasma conductivity and realistic transport coefficients at high temperatures, and a detailed chemical reaction scheme. The growth of the initial flame radius is calculated by a numerical integration of the model equations and compared with the experimental observation of Tagalian and Heywood. The time needed for the establishment of a flame propagation in the “like-laminar” regime was found to be strongly dependent on the breakdown energy and on the spark duration, and to a small extent on the initial pressure, temperature, and residual gas fraction. The model was used also to examine quantitatively the effect of some relevant parameters on the cycle-to-cycle variation in the steady-state burning velocity and it was concluded that the cycle-to-cycle variation is attributed mainly either to the inhomogeneity of the trapped mixture and/or to the cycle-to-cycle variation in trapped conditions; a variation of 5% of the volumetric efficiency affects the burning velocity by some ±13 %.