مجله مهندسی مکانیک امیرکبیر
سال چهل و پنجم شماره 1 (تابستان 1392)

The main objective of this paper is the accurate interface capturing and study on the shock-bubble interaction in gas-gas and gas-liquid two-phase flows. For this aim¡ the HLLC Riemann solver and Godunov numerical method were used for the first time for the 5-equation Kapila model. Transient shock wave interaction with Helium-Air and Air-Water bubbles were simulated. Numerical results were compared with available experimental results. The results have excellent agreement with experimental and previous published numerical results obtained by methods that are more sophisticated. Results show that the present method is able to capture transient shock waves and materials discontinuities and interfaces instabilities accurately and without any diffusion and oscillation.

In this work, the numerical solution for compressible, unsteady, inviscid, two-phase and transonic liquid-vapor mixture flow is investigated using Ros’s FDS time marching method. For space discretization, the fluid properties are extrapolated to the cell faces with the third order MUSCL algorithm of van Leer, and the time integration is done with the explicit two-step Lax-Wendroff method. In this study, the continuity, momentum and energy equations have been written in the fully conservative form and the properties of two-phase flow mixture in the quasi one-dimensional convergence-divergence nozzle have been investigated using the equilibrium thermodynamic model. The paper follows our earlier work, in which condensing transonic two-phase flow in a shock-free (no shock) converging-diverging nozzle was studied. The main goal of this article is to exhibit the two-phase flow with normal shock and to show the related physics (e.g. liquid phase evaporation via the shock) of the problem completely.

Although embolism is important as a major cause of brain infarction, little information is available about the hemodynamic factors governing the path large emboli tend to follow. In this research, we simulated embolus movement in three carotid artery bifurcations, each of them having different dividing angles. Y-shaped geometries were investigated. The governing equations for blood flow are the Navier-Stokes formulations. In this paper, the phenomenon was modeled under laminar and Newtonian flow conditions. The measured stress-strain curve obtained from Ultrasound elasticity imaging of thrombus was set to the Sussman-Bathe material model for embolus material properties. Shear stresses in the inner wall of the internal carotid artery (ICA) were measured. High magnitude of wall shear stress (WSS) in the areas in which embolus and artery are in contact with each other was observed. Stress in the embolus was also calculated and areas prone to rapture were identified.

Heat transfer enhancement of Al2O3-water nanofluid with variable properties in a square cavity is investigated by solving the boundary layer equation and the Boussinesq-approximation form of the boundary layer equation. The governing equations are solved using the finite volume method and the SIMPLER algorithm. Temperature difference between the hot and cold walls of the cavity is considered to be 50°C, and the results are presented for Rayleigh numbers from 103 to105 and volume fraction of the nanoparticles from 0.0 to 0.09. The results show that the average Nusselt number and the maximum absolute value of the stream function for the case with variable density are greater than the corresponding value for the constant density case. Moreover, for the variable density case, the maximum heat transfer enhancement for the nanofluid occurs at Ra=103. However for φ=0.09, the average Nusselt number of the nanofluid for Ra=104 and 105 are lower than that of the base fluid. For φ=0.01, the maximum relative enhancement of the Nusselt number for Ra=103, Ra=104, and 105 are %26, %24 and %23, respectively.

In this paper, a two-dimensional axisymmetric model for premixed methane/air combustion in porous media has been developed. For this aim, a numerical model with chemkinII library is used along with its database. This code solves the continuity, navier stokes, the solid and gas energy and the chemical species transport equations using the finite volume method. The pressure and velocity have been coupled with the simple algorithm. In this paper, an attempt is made to apply the profile of porosity instead of constant porosity for two zones of the burner. The results showed that by applying the varying porosity along the burner, the peak temperature can be decreased by about 4.5%, and subsequently, the amount of exhaust pollutants such as NOx can also be decreased while increase in pressure loss along the burner is negligible. The effects of increasing the inlet velocity and the wall temperature are also investigated. The results showed that by increasing the inlet velocity, the peak temperature and the emission of NOx will be decreased and also by increasing wall temperature, the peak temperature and the emission of NOx will be increased.

In the present study, the heat transfer, moisture distribution, air flow and thermal comfort in a residual room which is warmed by convectional thermal panels under different environmental conditions was assessed numerically. The analyzed model is the real and dynamic 3D simulation, and a virtual thermal manikin with real physiological dimensions has been added to the model. The manikin is standing at the center of room. The room has dimensions of 3*4*4 m with an inlet device that has dimensions of 1.28*0.054 2 m and two outlet devices with dimensions of 0.054*1.02 2 m . The inlet and outlet devices have been modeled above and under the door. To determine the optimal arrangement and number of panels, three models have been studied. The models include one panel with real dimensions of 1.242*0.81 2 m , two panels with real dimensions 0.61*0.962 m and three panels with real dimensions 0.61*0.81 2 m , with a thickness of 20cm in all panels. The uniform distribution of temperature, velocity and moisture, and proper and symmetric arrangement of panels, inlets and outlets decrease the energy consumption and satisfy the thermal comfort of inhabitants. Thermal loss increases due to the panels under the window. It was concluded that using two panels with smaller dimensions at the left and right of the model instead of one panel under the window or three panels at the left, right and under the window significantly reduces the thermal loss.

In this study, a type of Integrated Collector Storage (ICS) solar system is constructed and experimental tests are performed. The ICS system is a type of solar water heater with simple construction. In ICS, the storage tank is used for both water heating and storage, and there is no separate collector. Therefore, a larger storage tank is used which is subjected to direct solar radiation. In the present studies, the experimental model was investigated with three different reflectors. The results showed that the system has the lowest thermal efficiency with an aluminum foil reflector.

Nowadays , the use of thermoelectric coolers and generators has greatly increased . Applications of this phenomenon include electronic cooling , portable refrigerator , air conditioning , high-precision temperature measurement and space applications . The phenomenon of thermoelectricity has outstanding features compared to other energy conversion methods , such as exclusion of moving parts , high reliability and long life span. The requirements for high temperature heat source and low efficiency are the remarkable challenges of this method . Today , the first challenge has been overcome, thanks to abundant wasted heat sources , elihw increasing the efficiency of thermoelectric devices are under extensive studies . Segmentation is a popular way for the increment of thermoelectric efficiency that is focused on in this paper.

Repowering as a tested and general method can be effective for repowering steam power plants. There are different repowering methods to choose from based on a plant‟s specifications. Repowering has been categorized into two main categories: Full repowering and partial repowering. Feedwater heating is one of the best options available for partial repowering. In the feedwater heating based on partial repowering which has been studied in this paper, two heat exchangers have been used in parallel to the existing feedwater heaters and the required heat is supplied by an appropriate gas turbine. Two options have been considered for the added gas turbine. In the first option, a 25 MW gas turbine has been considered while in the other scenario, the considered gas turbine power output has been optimized as a dependent variable. The final result using a 25MW gas turbine is a 1.55% increase in exergy efficiency and an electricity generation cost of 404.15 Rial/kWh. In the other scenario (dependent gas turbine), exergy efficiency is increased by 1.73% and electricity generation cost is 396.22 Rial/kWh.

In this study, diesel engine simulation is carried out with a new phenomenological multi-zone combustion model. This model is used to simulate the diesel cycle including fuel injection and combustion processes. The model has been developed using well-known combustion imaging studies of Dec, the spray penetration studies by Siebers, and assumptions of Maiboom. This model divides the combustion chamber into six zones and provides local information such as temperature, mean equivalence ratio and composition in each zone. In order to determine the amount of emissions, the Zeldovich model is used for the NO and NSC model for soot. A comparison of model predictions with available experimental data shows very good agreement for different main engine parameters such as engine speed and load, inlet air temperature and pressure, exhaust gas recirculation (EGR) rate, injection pressure and injection timing.