فهرست مطالب

Energy Equipment and Systems - Volume:1 Issue:1, 2013
  • Volume:1 Issue:1, 2013
  • تاریخ انتشار: 1392/08/10
  • تعداد عناوین: 6
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  • Farzad Bazdidi, Tehrani, H. Foroutan, M. Rajabi, Zargarabadi Pages 5-18
    The present article focuses on the evaluation of a first-moment closure model applicable to film cooling flow and heat transfer computations. The present first-moment closure model consists of a higher level of turbulent heat flux modeling in which two additional transport equations for temperature variance kθ and its dissipation rate εθ are considered. It not only employs a time scale that is characteristic of the turbulent momentum field, but also an additional time scale devoted to the turbulent thermal field. The low Reynolds number k — ε turbulence model is combined with a two-equation kθ — εθ heat flux model to simulate the flow and heat transfer in a three-dimensional single row of cylindrical holes film cooling application. Comparisons with available experimental data show that the two-equation heat flux model improves the over-predictions of center-line film cooling effectiveness caused by the standard simple eddy diffusivity (SED) model with a fixed value of turbulent Prandtl number. This is due to the enhancement of turbulent heat flux components in the first-moment closure simulations. Also, the span wise distributions of effectiveness are computed with more accuracy due to better predictions of coolant jet spreading. However, the limitations of first-moment closure due to its isotropic approach should be taken into consideration.
    Keywords: Film Cooling, First, Moment Closure, Two, Equation kθ εθ Heat Flux Model, Turbulent Heat Flux Modeling
  • Amir Nejat, Ali Izadi, Vahid Esfahanian Pages 19-34
    Tip leakage loss introduces major part of losses of the rotor in axial gas turbines. Therefore, the rotor blade tip has a considerable effect on rotor efficiency. To understand the flow physics of the rotor tip leakage, we solve the flow field for different tip platforms (passive flow control) and by considering coolant tip injection (active flow control). Various blade tip configurations such as squealers and extensions on both pressure and suction sides, partial PS-squealer and flat tip with various tip clearances are generated. The computational domains are generated using unstructured prism layers for boundary layer resolution and unstructured, tetrahedral mesh for main flow. By using a finite volume CFD solver capable of solving RANS equations in an unstructured domain, the transonic compressible flow in the domain is solved. To capture the turbulent field in blade tip, shear stress transport (SST) k-ω model is employed. By using mixing plane approach, it is possible to couple outlet boundary of stator and inlet boundary of rotor and investigate the stator-rotor interaction in the rotor flow field and its consequence tip leakage flow. To investigate the combined effects of active and passive flow control measures in blade tip region, we simulate baseline geometry with and without tip coolant to show the effects of geometrical features of the rotor tip as well as the effect of tip coolant mass flow rate. Taking into account various rotor tip configurations and their tip leakage losses, it is possible to propose an optimum configuration.
    Keywords: Active, Passive Flow Controls, CFD, Tip Leakage Flow
  • Pedram Hanafizadeh, Sina Karbalaee M., Behdad Sharbaf E., S. Ghanbarzadeh Pages 35-58
    In many industrial equipment such as boilers and heat exchangers, the cylindrical tubes are exposed to the gas- liquid two phase flow. For any immersed body in flow field vortex shedding is created with a frequency that may be constant or variable, according to conditions such as flow rates, geometry of body, and etc. The failure will happen in the equipment, when this frequency is close to one of the natural frequencies of them. This can cause noise and flow induced vibration problem which is one of the main defects in the heat exchangers. Therefore considering these flows can play a significant role in long-term reliability and safety of industrial and laboratory equipment. In this study Eulerian–Eulerian approach is employed to simulate two-phase flow around the cylindrical tube. Since the Reynolds Stress Model (RSM) accounts for the effects of streamline curvature, swirl, rotation, and rapid changes in strain rate in a more rigorous manner than other turbulence models, it has greater potential to give accurate predictions for complex flows. So in this study the RSM is used to recognize behavior of vortex shedding in the flow. Drag coefficient, Strouhal number, vortex shedding behind cylinder, void fraction and pressure coefficient distribution were investigated in air-water two phase flows. In order to verify validity of CFD model, inlet void fraction was set to zero, Strouhal number and it’s relation with low Reynolds number (100-3000) in single phase flow were compared with experimental and numerical result of available literatures. The results show a good agreement between them. Having reasonable judgment of fluctuating lift force frequency, fast Fourier transform (FFT) was applied to lift coefficient of cylinder. The FFT produces averaged spectral coefficients that are independent of time and are useful to identify dominant frequencies in a signal. Some remedies were introduced to reduce vortex frequency of cylindrical tube subjected in gas-liquid two phase flows and protect tube from hard vibrations. For this purpose, some geometrical modifications were applied and results showed that in all cases drag coefficient and Strouhal number reduce.
    Keywords: Fast Fourier Transform, Flow Induced Vibration, Reynolds Stress Model, Strouhal Number, Two Phase Flow, Vortex Frequency
  • Ahmad Sedaghat, Ahmad Tahmasebi, Rasool Kalbasi, Mahdi Moghimi Zand Pages 59-74
    In this paper, a hybrid solid oxide fuel cell (SOFC) and micro gas turbine (MGT) power system is parametrically studied to evaluate the effect of different operating conditions. The SOFC/MGT power system includes SOFC reactor, combustion chamber, compressor and turbine units, and two heat exchangers. The effects of fuel utilization, temperature, and pressure are assessed on performance of the hybrid SOFC/MGT power system using energy and exergy analyses. This study reveals that the main exergy loss occurs in the external reformer and the maximum achievable output power is about 7kW for the hybrid system. Finally, the promising first law thermal efficiency of up to 83% is achieved when the second law efficiency enhances to 65% for the hybrid system.
    Keywords: Energy, Exergy, Micro Gas Turbine, Solid Oxide Fuel Cell, Thermal Efficiency
  • Ali Behbahani, Nia, Mahsa Sayfikar Pages 75-90
    This article presents a transient model of a solar adsorption cooling system. A computer program has been developed to simulate the operation of a two bed silica gel- water adsorption cooling system as well as flat plate collectors and the hot water storage tank. This program is then utilized to simulate the performance of a sample solar adsorption cooling system used for cooling a set of rooms that comprises an area of 52 m2 located in Ahwaz city in Iran. The system has been simulated with typical weather data of solar radiation and ambient temperatures of Ahwaz. The results include the temperature profiles of hot, cooling and chilled water in addition to Temperature profiles of adsorption/desorption beds, evaporator and condenser. Also the effects of the cycle time, switching time and hot water temperature on COP and refrigeration capacity are studied. Furthermore, the effects of solar collectors’ surface area and the storage tank volume on total cost of the system are investigated in order to determine their optimum values able to maximize the overall thermo-economic performance of the system under analysis.
    Keywords: Adsorption Cooling, Solar Energy, Solar Collectors
  • Amir Nejat, Hossein Hamedi Pages 91-97
    Flow in the first two-stage of V 94.2 gas turbine is simulated numerically. In this turbine, the second stator is clocked relative to the first stator to different positions. Steady-state analysis was carried out by varying the circumferential relative position of the consecutive stator vanes to study the effects of the clocking on turbine performance. A density based compressible inviscid flow (Euler equations) solver is used for the flow simulation. The efficiency analysis and entropy generation investigation are performed and the appropriate position of the second stator is obtained. The attained efficiency improvement matches the results obtained by the entropy generation analysis.
    Keywords: Clocking, Entropy Generation, Gas Turbine, Stator Position