فصلنامه مهندسی و مدیریت انرژی
پیاپی 2 (زمستان 1390)

This study addresses the problem of controlling a variable- speed wind turbine with a doubly fed ‎induction generator (DFIG) by control System that called One-Cycle control (OCC). Generator-side converter regulator speed and Grid-side converter controls reactive power and voltage of the DC-Link. Proposed method under different conditions such as load change, fault and nonlinear loads are studied. The whole DFIG wind turbine model is set up in Matlab/Simulink and simulation has been done for a sample network in different conditions. Comparisons of OCC and PWM control methods have been presented through simulations where OCC has demonstrated better features compared to PWM control.

This paper proposes a new direct power control (DPC) for wind turbine system driven by doubly fed induction generator (DFIG) in order to track maximum absorbable power in different wind speeds. New DPC employs input output linearization and sliding mode nonlinear controller for robust control of active and reactive power and obtaining maximum power from wind turbine. As the three phases voltage with desired frequency and amplitude is injected to rotor windings and constant switching frequency in rotor side converter is achieved by using space vector modulation. The conventional look up table method and also a new online method for determining maximum absorbable power from wind turbine has been used every instant. Simulation results on 660kw wind turbine show that the proposed algorithm has low error for tracking maximum power in compression with the existing method in presence machine parameters variation.

In this paper, the theoretical and experimental performance of the ejector refrigeration cycle is investigated. Making use of EES software, laws of mass and energy conservation as well as the exergy balance equations, a one-dimensional thermodynamic model is developed and validated using experimental results obtained in the laboratory. It is assumed that mixing takes place at a constant pressure and a constant cross section part of the ejector. Two factors are considered as the most important effects in the modeling: the frictional loss due to the mixture of the flows and the efficiency at the different parts of the ejector. Also a parametric study is carried out to study the effects of evaporator and generator temperatures on the coefficient of performance and second law efficiency of the cycle. The results indicate an increase of COP with increasing evaporator temperature. The maximum exergy losses occur in the ejector, generator and condenser respectively. Also at any pressure in the generator, an optimum value exists for the second law efficiency. Also the exergy destruction distribution analysis indicate that the highest exergy destruction occur in the ejector, condenser and generator.

Control of nitric oxides (NOx) is one of the key problems in designing combustion systems because of important role on ozone destruction and smog formation. Because of sensitivity of thermal NO mechanism to temperature, a practical and scientific method for thermal NO suppressing that is temperature reduction. Dilution is one way to reduce maximum temperature in the combustion chamber which in turn leads to a reduction in the formation of thermal NO. In the present paper, the aim is to examine the experimental studies of the effect of CO2 dilution on the formation of the pollutant NOx in the non-premixed flame of propane-air in the combustion chamber. The experimental results were obtained by designing a furnace with a cylindrical combustion chamber which was axisymmetrical. The experiments were conducted with the equivalence ratios 0.7 to 1.3 and various dilution ratios. The experimental results indicated that by increasing the extent of dilution, maximum temperature of the flame as well as emission of the pollutant NOx from the combustion chamber decrease.

In this research, the second Law for free convection of air flow in L-shaped enclosure with a heat generating body, is numerically analyzed. In this analysis, the continuity, momentum and energy equations are solved, using a finite volume method and SIMPLER algorithm. By positioning the heat generating body at three various locations inside the enclosure for three aspect ratio of enclosure in various Rayleigh numbers between 103 and 105 have been studied. Results show that, increasing the Rayleigh number increases the value of mean Nusselt number over the hot step. Heat is transferred mostly by the conduction for lower Ra numbers, for higher Ra numbers convection mechanism becomes dominant. When the heat generating body is at the center of enclosure, heat transfer rate and EG (Entropy Generation) have their maximum values.

In this paper, the effect of a constant magnetic field on flow field, temperature field, and heat transfer in steady state turbulent natural convection in a square enclosure filled with liquid metals has been numerically investigated. The left and right walls of enclosure are hot and cold respectively and horizontal walls are adiabatic. The effect of variations of Rayleigh number between 107 and 109 and Hartmann number between 0 and 600 on natural convection of liquid metals with Prandtl numbers 0.011, 0.022 and 0.054 has been investigated using the standard k-ε turbulence model. According to the results, when a magnetic field is introduced, the convection strength is suppressed and by increasing Hartmann number, temperature gradient near the vertical walls diminishes. Also by increasing Prandtl number, the maximum absolute value of stream function as a criterion of flow strength is increased.