فصلنامه مکانیک هوافضا
سال ششم شماره 2 (پیاپی 20، تابستان 1389)

Internal ballistics of solid rocket motor is simulated by solving the axisymmetric Navier-Stokes equations on a moving unstructured grid. Then, the flow velocity, pressure, temperature, and other variables are computed for problems with complex geometries and boundary conditions. The dynamic (moving) mesh is used to model flows where the shape of the domain changes with time due to motion of the domain boundaries (burning surfaces).Updating of the mesh is handled automatically at each time step, based on the new position of the boundaries. This study showed that CFD with unstructured dynamic meshing can be a powerful tool for simulating internal ballistics of solid rocket motors.

In this paper the performance of a solid oxide fuel cell (SOFC) in the anode-supported structure is numerically investigated. The mass, momentum, and energy equations, as well as the electrochemical and fuel reaction kinetic equations, are solved. To improve the accuracy of the results, complete electrochemical analysis, temperature dependent form of the thermodynamical and physical parameters, radiation model between the SOFC solid surfaces,and five layers of temperature are considered. The results are validated against the available experimental and numerical data and show that the cathode activation, anode activation, and ohmic polarizations have major effects on the cell polarization, respectively (in the order given). The molar fraction of the gas components in the fuel channel shows that the fuel internal reforming, which continues up to about 80% of the cell length, is very important in temperature distribution in the cell solid parts.

In this study a modified numerical scheme using meshless procedure is proposed to solve the inverse heat conduction boundary value problems. Homotopy perturbation method (HPM) is adopted for trial functions of the collocation method used. The resultant functions are used as basis functions and an ill-conditioned system of equations is obtained by a simple collocation. To regularize the resultant ill-conditioned system of linear equations, the Tikhonov regularization technique and the L-curve were applied. Note, the approach is readily extendable to solve inhomogenous problems. The results indicate that the approach used provides a relatively accurate scheme.

The aim of this paper is to investigate the possibility of simultaneous reduction of soot and NOx emissions in a direct injection diesel engine, using split injection method. For showing the effect of applying this injection system on pollutants of MT-4.244 DI diesel engine, the three-dimensional AVL_FIRE_CFD software was used. It was assumed that the engine was working under full-load condition and engine speed was 2000 rpm. First, the results of the simulation in single injection mode were validated, using the base state experimental data to find the base single injection mode. Then, different split injection modes were considered, in which the effects of pre-injection and postinjection were investigated by changing the mass of fuel in each pulse of injection. The effects of varying dwell time between injection pulses was analyzed as well. Then, the results of different split injection modes were compared with the base single injection one and finally the optimum mode of injection and optimum dwell time between injection pulses were selected. According to the results, the optimum injection mode for this engine for simultaneous reduction of soot and NOx emissions was achieved by injecting 75 percent of fuel in first pulse and 25 percent in the second with dwell time of 25 crank angle degrees between injection pulses.

In this study pressure drop of R-134a refrigerator under convective boiling conditions in horizontal flattened tubes was investigated. Round copper tubes with outer diameter of 9.52 mm are flattened into oblong shape with different internal height of 6.6 mm, 5.5 mm, 3.8 mm, and 2.8 mm. The experimental set-up used is a well-instrumented vapor compression refrigeration cycle. It includes three evaporators named pre-evaporator, test evaporator, and after evaporator. The results show that pressure drop increases as the tube profile is flattened. The obtained experimental data are compared with the results of existing correlations for estimation of pressure drop inside flattened tubes. In worst condition, the maximum boiling flow pressure drop is increased to 600% for the horizontal flattened tube relative to the round tube values. Also, a correlation is developed to predict the boiling flow pressure drop inside flattened tubes. This correlation predicts the experimental results within an error band of ±20%.

In this work a rectangular micro-heat exchanger with constant hydraulic diameter at various aspect ratios was considered and its pressure drop and thermal parameters versus pumping power in laminar flow at constant heat flux of 0.3 MW/m2 were analyzed and compared. Also, the effect of temperature-dependent fluid properties (viscosity and conductivity coefficient) on the pressure drop and thermal resistance of micro heat exchanger were investigated.Finally, a new criterion, based on pumping power and transmitted heat per temperature gradient, was proposed as a tool to optimize the aforementioned micro-heat exchanger.

In this study, the SI engine piston heat transfer is calculated. In order to simulate the combustion chamber heat transfer and the transient temperature of the piston, a resistor-capacitor model for wall temperature prediction is used. Twenty four heat transfer equations coupled with two-zonal combustion model were solved simultaneously,based on the resistor-capacitor thermal network model. The model predicts the wall temperature well. For each model parameter, an expression as a function of the engine geometry, operational parameters, and material properties were derived to make the model applicable to other similar engines. The simulations were performed writing a computer program on MATLAB and the results were validated using the experimental data of the EF7.TC engine.