Sharif Digital Repository

The effects of particle size on the solid flow pattern in gas-solid bubbling fluidized beds were investigated numerically using two-fluid model based on the kinetic theory of granular flow. In this regard, the set of governing equations was solved using finite volume method in two-dimensional Cartesian coordinate system. Glass bead particles with mean sizes of 880. μm, 500. μm, and 351. μm were fluidized by air flow at excess gas velocities of 0.2. m/s and 0.4. m/s. For particle diameters of 880 and 351. μm, the predicted characteristic times for solid dispersion were 0.14. s and 0.15. s, respectively, while characteristic times for solid diffusivity were 1.68. ms and 0.75. ms in the same... 

Effects of sprue base size and design on flow pattern during aluminum gravity casting have been investigated by employing different sprue base sizes and using computational fluid dynamics (CFD). Calculations was carried out using SUTCAST simulation software based on solving Navier-Stokes equation and tracing the free surface using SOLA-VOF algorithm. Flow pattern was analyzed with focusing on streamlines and velocity distribution in sprue base, runner and in-gate. Increasing well size was produced a vortex flow at the bottom of sprue base which increased the surface velocity of liquid metal in runner. Using a rather big sprue well could eliminate vena contracta, but in-gate velocity was... 

We investigate the stability of the pressure-driven, low-Reynolds-number flow of Brownian suspensions with spherical particles in microchannels. We find two general families of stable/unstable modes: (i) degenerate modes with symmetric and antisymmetric patterns; (ii) single modes that are either symmetric or antisymmetric. The concentration profiles of degenerate modes have strong peaks near the channel walls, while single modes diminish there. Once excited, both families would be detectable through high-speed imaging. We find that unstable modes occur in concentrated suspensions whose velocity profiles are sufficiently flattened near the channel centreline. The patterns of growing unstable... 

Identification of different flow regimes in industrial systems operating under two-phase flow conditions is necessary in order to safely design and optimize their performance. In the present work, experiments on two-phase flow have been performed in a large scale test facility with the length of 6 m and diameter of 5 cm. Four main flow regimes have been observed in vertical air-water two-phase flow at moderate superficial velocities of gas and water namely: Bubbly, Slug, Churn, and Annular. An image processing technique was used to extract information from each picture. This information includes the number of bubbles or objects, area, perimeter, as well as the height and width of objects... 

In this study, the flow of a fiber filled viscoelastic matrix through planar contractions is investigated. It was found that by adding fiber to the matrix vortex, the intensity increases. Fiber orientation along "x" and "y" axes was studied too. It was found that fiber orientation could be used for determining the flow regime through the contraction geometry. The rigidity condition of fibers, which needs the trace of the orientation tensor to be unity everywhere in the domain, is correct except near walls and the reentrant corner, which is slightly less than one. In these regions, the stress magnitude is higher, which results in more numerical errors, and which further leads to some error in... 

In this article, mixed-convective heat transfer of nanofluids in a vertical channel partially filled with highly porous medium was studied. In the porous region, the Brinkman-Forchheimer extended Darcy model was used to describe the fluid flow pattern. Different viscous dissipation models were also applied to account for viscous heating. At the porous medium-fluid interface, interfacial coupling conditions for the fluid velocity and temperature were used to derive the analytical solution using a two-parameter perturbation method. The model used for the nanofluids incorporates the effects of Brownian motion and thermophoresis. With constant wall temperature, velocity and temperature profiles... 

The response of a rod-stabilized, V-shaped, premixed flame to upstream velocity and equivalence ratio perturbations was characterized as a function of excitation frequency. The response of the flame to equivalence ratio perturbations was calculated, assuming that the heat release response is controlled by contributions from three disturbances. These disturbances include flame speed, heat of reaction and flame area. Using an analytical model, based on linearization of the front tracking equation for inclined flames, the kinematics of a V-flame anchored on a central obstacle was investigated and its response was compared with that of a conical flame. The results suggest that the phase response... 

In this paper, a 3-dimensional mathematical model for one cell of an anode-supported SOFC (solid oxide fuel cells) is presented. The model is derived from the partial differential equations representing the conservation laws of ionic and electronic charges, mass, energy, and momentum. The model is implemented to fully characterize the steady state operation of the cell with countercurrent flow pattern of fuel and air. The model is also used for the comparison of countercurrent with concurrent flow patterns in terms of thermal stress (temperature distribution) and quality of operation (current density). Results reveal that the steady-state cell performance curve and output of simulations... 

In order to safe design and optimize performance of industrial systems which work under two phase flow conditions, it's often needed to categorize flow into different regimes. In present work the experiments of two phase flow were done in a large scale test facility with length of 6m and 5cm diameter. Four main flow regimes were observed in vertical air-water two phase flows at moderate superficial velocities of gas and water: Bubbly, Slug, Churn and Annular. Some image processing techniques were used to extract information from each picture. This information include number of bubbles or objects, area, perimeter, height and width of objects (second phase).Also a texture feature extraction... 

The study of air-water, two-phase flows in hydraulic structures such as pressurized flow tunnels, culverts, sewer pipes, junctions, and similar conduits is of great importance for design purposes. Air can be provided by vortices at water intakes, pumping stations, aerators, steep channels, etc. Under certain conditions, air may also be introduced into pressurized intake systems, which may form large bubbles in portions of the pipe. The bubbles may, in turn, cause an unstable slug flow, or other flow patterns, that leads to sever periodic transient pressure. In this paper, an experimental model (a circular and transparent pipeline, 90 mm in ID and 10 m in length) is used to predict pressure... 

A finite difference technique is developed to predict the second stage creep displacement rates and stress analysis of a short fiber metal matrix composite subjecting to a constant axial load through a micromechanical approach. The technique is capable to take into account the presence of interfacial debonding as one of the main factors affecting the creep performance of short fiber composites. The exponential law is adopted to describe the matrix creep behavior. Also, a model for prediction of interfacial debonding at fiber/matrix interface is developed using a stress based method. The obtained results could greatly help to better understand the flow pattern of matrix material and the load... 

Aerosol measurement is used in a variety of fields such as nanotechnology, materials science, pollution monitoring, air quality measurements, combustion and engine exhaust analysis, inhalation toxicology, and medical studies. One of the most prevalent methods for aerosol measurement is to use electrical mobility. An electrical mobility spectrometer (EMS) is used to measure aerosol particles size distribution ranging from 10-1000 nanometers, under the influence of an electric field. The accuracy of this distribution is influenced by flow conditions, the geometry of the EMS, the electric field, and the number of electrode rings. In this work, a multi-channel EMS is studied using computational... 

The secondary flow of PTT fluids in rectangular cross-sectional plane of microchannels under combined effects of electroosmotic and pressure driving forces is the subject of the present study. Employing second-order central finite difference method in a very refined grid network, we investigate the effect of electrokinetic and geometric parameters on the pattern, strength and the average of the secondary flow. In this regard, we try to illustrate the deformations of recirculating vortices due to change in the dimensionless Debye–Hückel and zeta potential parameters as well as channel aspect ratio. We demonstrate that, in the presence of thick electric double layers, significant alteration... 

In this paper, the effect of several different parameters on the thermal resistance of a Closed Loop Pulsating Heat Pipe (CLPHP) has been investigated. These parameters include the working fluid, the inclination angle, the filling ratio and the heat influx. Also, the impact of using nanofluids with different nano-particle concentrations has been analyzed. It was observed that a CLPHP can increase the heat transfer up to 11.5 times compared to an empty pipe. Optimum performance for a system with the water-silver nanofluid was achieved at conditions of 50% filling ratio and 0.9 K/W of thermal resistance, and for the water-titanium oxide system, these optimal conditions were found to be 40%... 

There is currently a significant focus on using boundary layer control (BLC) approach for controlling the flow around bodies, especially the foil sections. In marine engineering this is done with the hope of increasing the lift - to - drag ratio and efficiency of the hydrofoils. In this paper, effects of the method on hydrodynamic characteristics and tip vortex formation of a hydrofoil are studied. Steady water injection at the tip of the hydrofoil is simulated in different conditions by using ANSYS-CFX commercial software. Validity of the proposed simulations is verified by comparing the obtained results against available experimental data. Effects of the injection on the lift, drag, and... 

Flow around a 2D circular cylinder with attached swinging thin splitter plates is numerically investigated. The ratio of the plates’ length to the cylinder diameter is 1 ([Formula presented]=1) where L is the Plates’ length and D is the cylinder diameter. The plates are attached at ±55 degrees (trigonometric angle) downstream and are forced to oscillate at different ratios of the natural vortex shedding frequencies with magnitudes of FR=0.75,1,1.25,1.5 and 2. The oscillation amplitude “α” as the other main variable ranges from 10 to 18 degrees. Two-dimensional simulations are carried out at the Reynolds number 100, and then extended to higher Reynolds number of 200. The results show that in... 

Pulsating Heat Pipes (PHPs) are cooling devices that are compact in size and have an ability to transfer heat in low temperature differences. Working fluids strongly affect the thermal performance of PHPs. In this paper, effects of some thermophysical parameters relating to working fluids, such as boiling point, latent heat of vaporization, surface tension, thermal conductivity and dynamic viscosity, are presented based on experimental and numerical studies done in recent years. Addition of nanoparticles to fluids, or making nanofuild, is a new method of improving thermophysical properties of fluids. Recently, many studies are carried out on thermophysical properties of nano-fuild. Results... 

A fully coupled formulation of thermo-fluid shape design problems has recently been developed in which the unknown nodal coordinates appear explicitly in the formulation of the problem. This "direct design" approach is, in principle, generally applicable and has been successfully applied in the context of potential and Euler flow models. This paper focuses on the direct design of ducts using the ideal flow model and may be considered as an addendum to the paper entitled "Direct Design of Ducts" [1]. However, a cell-vertex finite volume method is used and a different boundary condition implementation technique is applied, as compared to the method presented in the previous paper. The other...