فصلنامه مکانیک هوافضا
سال هشتم شماره 2 (پیاپی 28، تابستان 1391)

Fuel–air explosive devices are used to disperse liquid fuels into open atmosphere and create detonation waves in the resultant cloud. To numerically study of flow field inside the cloud, the initial conditions of injection as well as the droplet initial size should be designated. In the present paper the primary break up of liquid fuel dispersed from a FAE device was simulated by using film instability model and a computer code was written to perform the calculations. Experimental results were used to validate the numerical one and the estimated droplet initial mean size was given to a far field simulation code as input data. Then the result of this latter simulation was compared with the related experimental results. With regard to numerical and experimental results, we proposed an interval to estimate the initial size of droplet dispersed from a FAE device. Generally, the results show mean droplet diameter is of centimeter order of magnitude.

Adding gas turbine units to steam power plants for enhancing steam cycle’s properties and regenerating heat from adjunct cycle(s) have been known as repowering. Repowering methods are divided into two main categories, partial and full repowering. One of the latest methods of partial repowering is parallel feed water heating repowering. In this paper the mentioned method has been applied to Shahid Rajaee power plant. Cost functions of generated-electricity and exergy efficiency are defined as objective functions in one and two objective optimizations. Finally, the most suitable scenario has been chosen to apply this method on, considering desirable technical and economic parameters for partial and nominal load. The results of the chosen design are: 3.4% and % 27 increases in exergy efficiency and steam power plant capacity, respectively.

In this study, an exact analytical solution for two-dimensional steady-state conductive heat transfer in cylindrical composite laminates made of graphite-epoxy is presented. These laminates are in cylindrical shape and fibers have been winded around the cylinder in each layer. First, the appropriate Fourier transformation has been obtained using Sturm-Liouville theorem. Coefficients of series are achieved by solving a set of equations related to the thermal boundary conditions at inside/outside of the cylinder and temperature/heat flux continuity at the boundaries located between the layers. The solution of this set of equations is calculated using Thomas algorithm. In this article, the effect of fibers’ angle on temperature distribution of composite laminate is investigated under general boundary conditions. Here, we show that the temperature distribution for any composite laminates is between temperature distribution for laminates with fiber angles equal to the θ = 0 andθ = 90.

This paper presents a numerical investigation of mixed convection heat transfer from an open cavity in a horizontal channel. Three heat sources are located at the bottom of the cavity and a baffle hanging from the top wall of the channel. A free flow at a relatively lower temperature enters the channel and is expected to remove heat from the heat sources. The governing equations are solved numerically with a finite volume approach using the SIMPLE algorithm. The results of this study show that due to the existence of the baffle, the ventilation performance of the cavity is improved at low Richardson numbers, while it is deteriorated at high Richardson numbers. The length and location of the baffle also show to have a significant impact on the ventilation performance of the cavity.

In this study a Plate-Fin Heat Exchanger (PFHE) with offset-strip fins has been optimized on air-air, water-water and air-water working fluids types. In the first step - designing the PFHE - the heat transfer and pressure drop analyses were done. Considering the geometrical parameters of fins (pitch, height and offset length of fins) and the relationship of the amount of heat transfer and pressure drops with the velocity of working fluids in each types, the volume of PFHE and its total annual cost based on the allowable pressure drop on both sides as the constraints were calculated. Then, Genetic Algorithm (GA) was used for searching and optimization of the structure sizes of the PFHE considering pressure drop constraints. Comparing the current results with the literature shows that the volume of optimized PFHE for all three types decreases from 10 to 15% and the total annual cost declines from 5 to 25%.

To investigate the mixed convection flows through nano-fluid in a square double lid-driven cavity with various inclination angles, a numerical method based on the finite volume approach is applied. In This paper Sio2/water nano-fluid have been used. The cavity is un-uniformly heated from the left, Th, and cooled from the opposite wall;the top and the bottom moving walls have been supposed to be insulated. The study has been executed for the Richardson number 0.1 to 10, Reynolds number 1 to 100 while solid volume fraction (φ) of nano-particles altered from 0 to 0.06. The impact of angle of inclination, solid volume fraction of nano-fluids, velocity of moving walls and Richardson number on hydrodynamic and thermal characteristics have been studied and discussed. As a result, it was found that the heat transfer increase with increase in solid volume fraction for a particular Reynolds.Also in higher Richardson, the effect of solid volume fraction increases on thermal behavior and flow pattern.

In this research, free convection heat transfer over vertical sinusoidal wall in constant temperature has been performed, using Navier-Stokes equations in laminar flow regime with finite volume method. Also, a new pressurebased model for the convective fluxes is offered, which minimizes the need for artificial dissipation. The first and second-order versions of the suggested model are unable to calculate the convective fluxes with a good accuracy.Numerical results are well adapted with the numerical or experimental results of other researchers. In this study, the variable parameters are Reyleigh number and the ratio of domain length to the sinusoidal wave length. The results show that the local heat transfer coefficient variation frequency for space is equal to the wall frequency. Meanwhile,with increasing the Reyleigh number, the average heat transfer coefficient increases.

This paper is concerned with the influence of blade angles on centrifugal impeller performance parameters such pressure ratio, diffusion ratio and efficiency. The other basic geometric parameters are held constant. The influence of the blade angles change on the observed values was determined from numerical solution of the flow in the impeller with help of the FLUENT software. The numerical simulation focused on the air flow from compressor impeller inlet to exit, and the performance of impeller is predicted. The numerical solution was performed for original impeller geometry and for two other cases, in which blade inlet angle and backward sweep was changed.The standard k −ε turbulence model was used to obtain the eddy viscosity. Performance of the code was verified using measured data for the Eckardt impeller.