مجله مهندسی مکانیک امیرکبیر
سال چهل و دوم شماره 2 (پاییز 1389)

Continuous Variable Valve Timing System or CVVT has a duty of variation of continuous open timing of the valves. This system has been put on the intake camshaft in EF7 engine. In this article, thermodynamic analysis of variation for open selective angles of valve according to variation of engine speed and load has been investigated. Opening angles in two state of gas and gasoline fuel by selecting of best point on the basis of data results of engine test has been achieved and this article analyses of the cause of this selection from thermodynamic point of view.

Cycle to cycle combustion variability is a prominent characteristic of spark ignition engine that makes the engine very difficult to control. In this study, the combustion pressure fluctuations in a spark ignition engine fueled with natural gas were investigated. The results showed that the shape of coefficient of variation in the cylinder pressure versus crank angle is independent of the experimental conditions. Three points of this curve are important. They correspond to the combustion beginning, combustion end and 50% mass fraction burned. The results also showed that the cycle to cycle variations indicated mean effective pressure decrease as the equivalence ratio increased in the lean mixture region, and increase as the equivalence ratio increased in the rich mixture region. The cylinder pressure cyclic dispersion showed that the combustion stability is optimum for spark timings and equivalence ratio corresponding to maximum torques. The effect of retarding the spark timing from maximum torque timing and rich mixture on cycle to cycle variations are respectively higher than the effect of advancing the spark timing from maximum torque timing and lean mixture.

A new mathematical model is introduced to predict structure of premixed flame propagating in combustible systems, with uniformly distributed volatile fuel particles and air. In the present paper the flame structure is divided into four zones that consists of a preheat zone, an extensive particles vaporization zone, an asymptotically thin reaction zone, and finally a post flame zone. It is presumed that the fuel particles vaporize first to yield a gaseous fuel, which is subsequently oxidized. The study involves the Damkohler number , the ratio of chemical reaction rate to vaporization rate, and the Zeldovich number , the nondimensional form of activation energy, as essential parameters. Finally, with considering unity Lewis number and neglecting the latent heat of vaporization, for several equivalence ratios and several diameters of particles the analysis yields results for the mass fraction of the fine-solid particles, mass fraction of the fuel in the gaseous phase and nondimensional temperature. This prediction is in agreement with experimental observations of fine particles combustion.

Premixed combustion is widely used for simulation of combustion chambers of gas turbines, utilized for low NOx emission applications. However, this category of gas turbine is susceptible to combustion instability. The main aim of this investigation is to focus on thermo-acoustic instability modes in gas turbine combustion chambers. Both analytical and experimental methods are applied for this study. For this purpose, an experimental combustion chamber is designed and fabricated and various experiments are planned and performed in order to achieve the behavior of combustion chamber during stable and unstable conditions. In this research, gaseous propane is introduced as fuel and experiments are performed at nearly atmospheric pressure with equivalence ratios within the range of 0.7 to 1.2. To distinguish the frequency of combustion instability, through various operating conditions, probability density functions, spectral diagrams and the discretized fast Fourier transform of pressure wave oscillations are employed. Moreover, an analytical method is applied and a computer code is written and elaborated for this purpose. Instability frequencies are derived experimentally and analytically and the results are compared with each other. Accordingly, good agreements are observed between the results.

In this study, Artificial Neural Networks (ANNs) technique is applied for the determination of thermodynamic properties of the Lithium Bromide-Water solution, which is widely used in the thermodynamic simulations. For training of the ANNs the simulation results of a thermodynamic analysis are used. The presented ANN model provides simpler and faster results comparing to complex differential equations and exsiting limited experimental data. Using the ANN technique, the thermodynamic properties of LiBr-Water solution are derived as the mathematical relations. Simulation results show the effectiveness of ANN in identification of LiBr-Water solution thermodynamic properties.

Propeller is one of the important marine propulsors for generating thrust to overcome the ship resistance. This paper presents the anlaysis of the skew propeller and propulsion for a submarine by using the Boundary Element Method (BEM). This method is useful for analysis and design of lifting bodies like hydrofoils and propellers. A five-bladed Highly Skewed Propeller (HSP) has been chosend for a submarine of 120 [Tones] with forward constant speed 8 [knots] and the calculated results of the hydrodynamic performance have presented. The efficiency values have been obtained 0.65 and 0.68 at two surfaces and submerged conditions, respectively.

In this paper, by using implicit fourth order central difference method and TLNS equations, the numerical solution of the steady axisymmetric viscous supersonic flow is implemented over blunt cone with shock-fitting method. Because of using high order terms of Taylor series in discretization of derivatives, this method has high accuracy and low numerical error (dispersion error) compared with low order method. The boundary-closure scheme has an important role in stability of this method. By using a coarse grid in this method, the results of numerical solution are found to be very close to those obtained with a fine grid employing the second order (Beam-Warming) method. Higher accuracy of this method is identified relative to the second order method when the grid is being refined. The convergence of this method can be adjusted to accommodate the computational hardware capabilities.

In the present study, the interplaying effects of a pressure-driven flow and the induced electric potential, corresponding to the zero net electrical current, have been numerically investigated. The governing equations, which consist of the Poisson equation for the distribution of electric potential, the Nernst-Planck equation for the distribution of charge density, and the modified Navier-Stokes equations for the flow field are solved numerically for an incompressible steady flow of a Newtonian fluid using the finite-volume method. In the presence of electric double layer and the maximum induced voltage condition, the mass flow rate decreases negligibly with respect to the corresponding pure pressure-driven fellow. Surprisingly, the absolute value of induced voltage approaches a maximum value at zeta potentials smaller than 100 mV and then drops. The exponentially increase of the average electric conductivity coefficient beyond 100 mV is accounted for this behavior. Thus the common practice of assuming constant electric conductivity is justified at low zeta potentials.