Sharif Digital Repository

Valveless piezoelectric micropumps are in wide practical use due to their ability to conduct particles with absence of interior moving mechanical parts. In this paper, using finite element method, an extended numerical study on fluid flow through micropump chamber and diffuser valves is conducted to find out the optimum working conditions of micropump. In order to obtain maximum generality of the reported results, an analytical study along with a dimensional analysis is presented primarily, to investigate the main dimensionless groups of parameters affecting the micropump net flux. Consequently, the parameters appeared in the main dimensionless groups have been changed in order to understand... 

Two-dimensional stagnation-point flow of viscoelastic fluids is studied theoretically assuming that the fluid obeys the upper-convected Maxwell (UCM) model. Boundary-layer theory is used to simplify the equations of motion which are further reduced to a single non-linear third-order ODE using the concept of stream function coupled with the technique of the similarity solution. The equation so obtained was solved using Chebyshev pseudo-spectral collocation-point method. Based on the results obtained in the present work, it is concluded that the well-established but controversial prediction that in stagnation-point flows of viscoelastic fluids the velocity inside the boundary layer may exceed... 

This paper deals with the solution of two-dimensional fluid flow problems using the truly meshless Local Petrov-Galerkin (MLPG) method. The present method is a truly meshless method based only on a number of randomly located nodes. Radial basis functions (RBF) are employed for constructing trial functions in the local weighted meshless local Petrov-Galerkin method for two-dimensional transient viscous fluid flow problems. No boundary integration is needed, no element matrix assembly is required and no special treatment is needed to impose the essential boundary conditions due to satisfaction of kronecker delta property in RBFs. Three different radial basis functions (RBFs), i.e.... 

Free surface flows are frequently encountered in hydraulic engineering problems including water jets, weirs and around gates. An iterative solution to the incompressible two-dimensional vertical steady Navier-Stokes equations, comprising momentum and continuity equations, is used to solve for the priori unknown free surface, the velocity and the pressure fields. The entire water body is covered by a unstructured finite element grid which is locally refined. The dynamic boundary condition is imposed for the free surface where the pressure vanishes. This procedure is done continuously until the normal velocities components vanish. To overcome numerical errors and oscillations encountering in... 

A two impinging-jets contacting device (TIJCD) for liquid-liquid extraction processes was proposed and tested through the standard test system recommended by the European Federation of Chemical Engineering (EFCE), toluene-acetone-water, as a typical example of liquid-liquid extraction processes. The results obtained for the overall volumetric mass transfer coefficient (capacity coefficient) were more than three times higher than those reported in the conventional contactors and in an air-driven two impinging-streams contactor. These experimental results clearly indicate the greater performance capability of TIJCD relative to those of conventional contactors. To have a perfect study on the... 

Adomian decomposition method was employed to obtain an approximate solution to two-dimensional and axisymmetric jet impinging flows in this work. Assumptions have been made to reduce the related full Navier-Stokes equations to a non-linear ordinary differential equation. A trial and error strategy has been used to obtain the constant coefficient in the approximated solution. Velocity Profiles, Shear stresses and displacement thickness were chosen to be compared with accurate numerical data. Sensitivity of the results to the number of terms have been discussed. The results showed for open boundary problems having at least one boundary condition at infinity, the applicable range of the... 

Based on MRI diffusion and perfusion, a new criterion for detection and the healing progress of damaged tissue is suggested. The study is based on the ratio of capillary radii in symmetrical damaged and normal tissue neighboring spaces. The Apparent Diffusion Coefficient (ADC) and Cerebral Blood Flow (CBF) were measured in the brain tissues of six male Wistar rats utilizing suggested MRI measurement techniques. The ADC values of damaged and normal regions were (392 ± 34.1) × 10-6 mm2s-1 and (659 ± 40.7) × 10-6 mm2s-1, respectively. The CBF values of damaged and normal regions were 14.5 ± 10.13 ml/min/ 100 g and 125 ± 41.03 ml/min/100 g, respectively. The geometrical parameters of the... 

In the present study criterion for local thermal equilibrium assumption is studied. It concerns with the fluid flow and heat transfer between two parallel plates filled with a saturated porous medium under non-equilibrium condition. A two-equation model is utilized to represent the fluid and solid energy transport. Numerical Finite Volume Method has been developed for solving coupled energy equations and the Non-Darcian effects are considered for description of momentum equation. The effects of suitable non dimensional parameters as Peclet number and conductivity ratio has been studied thoroughly. A suitable non dimensional equation proposed in wide range of Peclet number and conductivity... 

Gas turbine performances are directly related to site conditions. The use of gas turbines in combined gas-steam power plants, also applied to cogeneration, increases such dependence. In recent years, inlet air cooling systems have been introduced to control air temperature at compressor inlet, resulting in an increase in plant power and efficiency. In this paper, the dependence of outside conditions for a simple gas turbine and a combined cycle plant is studied, using absorption chiller as inlet air cooling system. We used, as case study, a simple plant equipped with one frame E gas turbine and a combined cycle with a two pressure level heat recovery steam generator (HRSG). It was found that... 

In the present research a numerical investigation has been made to evaluate the results of analytical approach in the analysis of micro channel heat sink (MCHS) which is increasingly used in the cooling of high heat dissipating electronic devices. In this regard the effects of geometrical and flow parameters affecting the absolute thermal resistance of MCHS have been analyzed. The effect of using different fluids in the value of the thermal resistance is considered as well. The accuracy of analytical method in predicting heat transfer and fluid flow regimes in micro channel is checked in comparison with numerical method. The effects of hydraulic and thermal entrance lengths of these systems... 

The purpose of this work is to develop a physical simulation model for the purpose of studying the effect of various design parameters on the performance of a packaged liquid chiller. A computer model which simulates the steady-state cyclic performance of a vapor compression chiller is developed for the purpose of performing detailed physical design analysis of actual industrial chillers. The model can be used for optimizing design and for detailed energy efficiency analysis of packaged liquid chillers. The simulation model takes into account presence of all chiller components such as compressor, shell-and-tube condenser and evaporator heat exchangers, thermostatic expansion valve and... 

Improvements in sealing mechanism between the rotating and the stationary parts of a rurbomachine can extensively reduce the endwall leakage flow. In this regard, abradable seals are incorporated into compressor and turbine blade-tip region. In a gas turbine, equipped with abradable seals, tip of the rotor blade is designed to cut into the material coating of the casing and to form a close fitted circumferential groove for the movement of the blade tip. As a result, the resistance to the leakage flow in the tip gap region increases due to smaller tip clearances (available without any rub-induced damages). Minimizing the tip clearance size can lead to an increase in performance and stability.... 

A numerical model has been developed for studying the flow and heat transfer characteristics of single phase liquid flow through a microchannel. In this work the heat transfer characteristics of pressure driven and electroosmotic flow through microchannels have been studied. The governing equations are the Poisson-Boltzmann and Navier-Stokes equations which have been solved numerically using the standard Galerkin and the Mixed 4-1 finite element methods, respectively. Finally the energy equation is solved numerically using the Stream-wise Upwind Petrov Galerkin (SUPG) method. Two dimensional Poisson-Boltzmann equation was first solved to find the electric potential field and net charge... 

The casing treatment and flow injection upstream of the rotor tip are two effective approaches in suppressing instabilities or recovering from a fully developed stall. This paper presents numerical simulations for a high-speed transonic compressor rotor, NASA Rotor 37, applying a state-of-the-art design for the blade tip injection. This is characterized by introducing a jet flow directly into the casing treatment machined into the shroud. The casing treatment is positioned over the blade tip region and exceeds the impeller axially by ∼30 per cent of the tip chord both in the upstream and in the downstream directions. To numerically solve the governing equations, the three-dimensional finite... 

Most available numerical methods face problems, in the presence of variable topographies, due to the imbalance between the source and flux terms. Treatments for this problem generally work well for structured grids, but most of them are not directly applicable for unstructured grids. On the other hand, despite of their good performance for discontinuous flows, most available numerical schemes (such as HLL flux and ENO schemes) induce a high level of numerical diffusion in simulating recirculating flows. A numerical method for simulating shallow recirculating flows over a variable topography on unstructured grids is presented. This mass conservative approach can simulate different flow... 

Pushing a fluid with a less viscous one gives rise to the well known Saffman-Taylor instability. This instability is important in a wide variety of applications involving strongly non-Newtonian fluids that often exhibit a yield stress. Here we investigate the Saffmann-Taylor instability in this type of fluid, in longitudinal flows in Hele-Shaw cells. In particular, we study Darcy's law for yield stress fluids. The dispersion equation for the flow is similar to the equations obtained for ordinary viscous fluids but the viscous terms in the dimensionless numbers conditioning the instability now contain the yield stress. This also has repercussions on the wavelength of the instability as it... 

Patterns formed by the flow of an inhomogeneous fluid (suspension) over a smooth inclined surface were studied. It was observed that fractal patterns are formed. There exists a threshold angle for the inclination above which global fractal patterns are formed. This angle depends on the particle size of the suspension. We observed that there are two fractal dimensions for these patterns, depending on the area from which the pattern is extracted. If the pattern is taken from the top which only consists of the beginning steps of the pattern forming, one finds two fractal dimensions, i.e. 1.35-1.45 and 1.6-1.7, in which the first one is dominant. And if the entire pattern is taken, then fractal... 

One main challenge in numerical treatment of incompressible fluid flow problems is to suppress the decoupling of pressure and velocity fields. The challenge has prompted research toward suggesting and implementing various coupling strategies. In this work, a novel strategy which suitably couples pressure and velocity in a collocated grid arrangement is presented. The current strategy develops a unique cell-face velocity expression which provides infinite cell-face velocity magnitudes in the algorithm. A smoothing factor is incorporated in the cell-face velocity expression in order to produce a wide range of velocity magnitudes. The smoothing factor provides a smooth transition from an unreal... 

Despite great advancement in the lattice Boltzmann method and its application in fluid flow problems, there are still major restrictions in treating either the solution domains with complex boundaries or buoyant flow problems. The past experience shows that the heat equation is a source for instabilities which jeopardizes the stable solution of the lattice Boltzmann method in solving fluid flow problems with heat transfer. The instabilities Increase with increasing buoyant force strength. In this work, we suggest a new approach to overcome the restrictions through implementing the advantages of finite volume method in LBM. In this regard, the lattice Boltzznann equation is incorporated with... 

In this work, an efficient bi-implicit strategy is suitably developed within the context of a finite volume element approach in order to solve turbulent reactive flow governing equations. Based on the essence of control-volume-based finite-element methods, the formulation retains the geometrical flexibility of the pure finite element methods while derives the discrete algebraic governing equations through using the conservation balance applied to discrete control volumes distributed all over the solution domain. The physical influence upwinding scheme is used to approximate the advection fluxes at all cell faces. While respecting the physics of flow, this scheme also provides the necessary...