Iranian Journal of science and Technology (B: Engineering)
Volume:33 Issue: 5, Oct2009

Investigations into boiling, the generation of vapor and the prediction of its behavior are important in the stability of boiling water reactors (BWR). The present models are limited to simplifications made to draw governing equations or lack of closure framework of the constitutive relations. The commercial codes fall into this category as well. Consequently, researchers cannot simply find the comprehensive updated relations before simplification in order to simplify them for their own works. This study offers a state of the art, phase-weighted, ensemble-averaged, two-phase flow, two-fluid model for the simulation of two-phase flow with heat and mass transfer. This approach is then used for modeling the bulk boiling (thermal-hydraulic modeling) in BWR. The resultant approach is based on using the energy balance equation to find a relation for quality of vapor at any point. The equations are solved using SIMPLE algorithm in the finite volume method and the results compared with real BWR (PB2 BWR/4 NPP) and the boiling data. Comparison shows that the present model is satisfactorily improved in accuracy.

The purpose of the present study is to simulate a co-current pilot plant spray dryer with cooling air jacket, air distributor and pressure nozzle, using a numerical technique. For the spray dryer, cooled air enters its secondary wall which has a helical passage around the drum and the main process fluid enters in the axial flow direction from an air distributor which is located on the top center position of the dryer. Fluid flow and heat transfer from the inner fluid to the surrounding air jacket is studied with a special treatment to predict convection heat transfer through the helix. By knowing the air temperature and flow velocity entering the jacket and the thermal condition of the process fluid entering the drum in a counter-current direction, combined free and forced turbulent convection heat transfer in the drum are investigated using the RNG k-ε model of a turbulent model. Analyses were carried out using the SIMPLEC scheme and Euler-Lagrange model with and without spray water and milk to determine the pressure field, velocity distribution, temperature field of the process fluid throughout the drum and its wall, as well as the particles temperature and humidity, time of evaporation and diameter. Comparison of transient simulation of the particle size and time of evaporation with empirical relations is made for certain conditions. Validation of the numerical results with the experimental measurements shows good agreement.

Falling film evaporators have been given great attention due to their wide applications in industrial processes. In these processes, due to large time delay and disturbances the tight exact and proper control of product concentration is difficult. In this study, a nonlinear dynamic model has been considered for modeling and simulation of a three-effect falling film evaporator. The basic structure of the model is composed of heat and mass conservation equations coupled with thermodynamics as well as auxiliary correlations for prediction of physical properties. Also, a triple loop cascade control is proposed and tuned for control of this process. The proposed algorithm consists of three conventional loops. Results obtained from this proposed algorithm are compared with results of conventional PID control. It is shown that the regulatory and servo responses can be significantly improved by the proposed control algorithm.

Welding induces residual stresses in structures which may cause buckling distortion, if they exceed the critical buckling stress of the structure. This paper presents a predictive distortion analysis approach for a welded structure. In addition, the effect of external constraint on buckling behavior is studied, as well. A 3-D thermo-mechanical welding process simulation is performed to determine the residual stresses and deformation. Moreover, the critical buckling stress along with the buckling mode is computed through 3-D eigenvalue analysis. Also, the model is validated against the experimental work. The present results clearly demostrate that the proposed 3D welding analysis can predict not only the time, but also the shape of buckling during welding. Finally, this study also reveals that external restraint effect on buckling mode as well as a suitable combination of thickness and external restraint can eliminate buckling.

In this research the thermodynamic of mechanochemical activation by high energy milling of hematite has been studied. It has been shown that transformation reaction due to ball milling of α-Fe2O3 has been started after 15 hours milling. As the sum of amorphization and dislocation energies is a suitable approximation of total stored energy, the contribution of these two energies has been identified. The contribution of amorphization and dislocation energies after 15 hours milling have been calculated at about 76 and 9 kJ/mol, respectively. Thus, the increase in molar enthalpy of milled hematite is about 85 kJ/mol. The total Gibbs free energy of mechanochemical transformation of hematite to magnetite after 15 hours milling has also been determined. The temperature for transformation of activated hematite to magnetite has been calculated at about 300 K. The bulk temperature of powder after 15 hours milling has been measured at about 325 K. Therefore, at this temperature hematite cannot be stable and will transform to magnetite during milling.

Flight dynamics of the guided vehicle is modelled by the aid of linear and angular equations of motions using Lagrange''s approach in this paper. Governing equations of the control system and the elastic behavior of the vehicle are added to the equations of dynamic states. Flexibility effect is modelled using the normal modes, generalized coordinates and forces. For validation of modified FORTRAN simulation code, the stability of specific vehicles is determined and compared with the same results in the literature. Using this code and by solving the governing equations for the desired flight vehicle, the aeroservoelasticity is analyzed and then the results are compared with the rigid cases and with the flight test data. Errors induced to control system sensors are shown in the figures. Good compatibility is achieved between the simulation and the experimental results. Fast Fourier transformations (FFT) is used for extracting the structural flexibility frequencies from the elastic simulation and flight test data.

Three-dimensional numerical simulation of the cross-stream migration of a drop in simple shear flow at finite Reynolds numbers neglecting the gravity influence is presented. In this study the full Navier-Stokes equations are solved by a finite difference/front tracking method. A drop is shown to migrate to the centreline of the channel in a shear flow. In other words, the centre of the channel is a global attractor of trajectories of a drop, regardless of the initial position and velocity. The migration velocity of the drop depends on surface tension and fluid velocity. Increasing the Weber number and decreasing the Reynolds number decrease the time of the migration to the centreline. The study showed that after an initial transient period the drop leads the local undisturbed velocity, for all cases. While time progresses, the velocity of the drop along the flow direction (x) decreases, whereas the velocity of drop along the flow velocity gradient increases. When the x-velocity reaches the local undisturbed velocity, the slip velocity tends to zero, except during a short initial transient. The slip velocity is the difference between the drop velocity and the ambient fluid velocity at the centre of the drop for the undisturbed flow. To validate the present calculations, some typical results are compared with the available experimental and theoretical data, which confirm that the present approach is qualitatively reliable in predicting the drop migration.

In this study, Al-ZrO2 composites were produced by Vortex method using ZrO2 powder with 1 micron average diameter as reinforce particles and Al-356 as the matrix metal. The melt composites were stirred for 13 minutes, then casted into a metallic mold. Different samples of 5, 10 and 15 volume percent of ZrO2 in different casting temperatures of 750, 850 and 950oC were produced. The latter 2 casting temperatures are not a common practice but were chosen to enhance fluidity. Effects of volume percent of ZrO2 particles and casting temperature on tensile strength, microstructure, and fracture surfaces of Al-ZrO2 composites have been investigated. The highest tensile strength was achieved in the specimen containing 15 vol. % ZrO2 produced at 750°C which shows an increase of 60% in comparison to the Al-356 non-reinforced alloy. Microscopic investigations of fracture surfaces revealed that fracture in a brittle manner with little or no necking happening. By increasing ZrO2 content and casting temperature, the composites fracture goes in a more severely brittle manner.