Sharif Digital Repository

The governing equations of two-dimensional steady density currents are solved numerically using a finite volume method. The v2̄-f turbulence model, based on standard k - s model, is used for the turbulence closure. In this method, all Reynolds stress equations are replaced with both a transport equation for v2̄ and an elliptic relaxation equation for f, a parameter closely related to the pressure strain redistribution term. The Simple-C procedure is used for pressure-velocity coupling. In addition, Boussinesq's approximation is used to obtain the momentum equation. The computed height of the progressive density current is compared to the measured data in the literature, resulting in good... 

The deposition behavior of fine sediment is an important phenomenon, and yet unclear to engineers concerned about reservoir sedimentation. An elliptic relaxation turbulence model (v2̄ - f model) has been used to simulate the motion of turbid density currents laden whit fine solid particles. During the last few years, the v2̄ - f turbulence model has become increasingly popular due to its ability to account for near-wall damping without use of damping functions. In addition, it has been proved that the v2̄ - f model to be superior to other RANS methods in many fluid flows where complex flow features are present. Due to low Reynolds number turbulence of turbidity current,(its critical Reynolds... 

In this research, the developing turbulent swirling flow in the entrance region of a pipe is investigated analytically by using the boundary layer integral method. The governing equations are integrated through the boundary layer and obtained differential equations are solved with forth-order Adams predictor-corrector method. The general tangential velocity is applied at the inlet region to consider both free and forced vortex velocity profiles. The comparison between present model and available experimental data demonstrates the capability of the model in predicting boundary layer parameters (e.g. boundary layer growth, shear rate and swirl intensity decay rate). Analytical results showed... 

Fire events and the related toxicants such as CO are responsible for many fatalities in the current century. These hazardous events are much more dangerous when they occur in enclosed spaces. In the present study, a theoretical relation is developed for horizontal distribution of CO in a large tunnel fire. Then, the developed criterion is used to study the effect of some rudimentary parameters such as the heat release rate (HRR) of fire and tunnel's aspect ratio (AR) on CO and temperature stratification. Theoretical results of various heat release rates and aspect ratios for horizontal distribution of CO are compared with numerical results using fire dynamics simulator (FDS5.5). It is found... 

Three-dimensional simulation of a multiphase flow is performed using the Eulerian-Eulerian finite volume method in order to evaluate the separation efficiency and velocity field of deoiling hydrocyclones. The solution is developed using a mass conservation-based algorithm (MCBA) with collocated grid arrangement. The mixture approach of the Reynolds stress model is also employed in order to capture features of turbulent multiphase swirling flow. The velocity field and separation efficiency of two different configurations of deoiling hydrocyclones are compared with available experimental data. The comparison shows that the separation efficiency can be predicted with high accuracy using... 

In this study, the effect of cone angle on the flow field and separation efficiency of deoiling hydrocyclones is investigated taking advantage of large eddy simulation. The dynamic Smagorinsky is employed to determine the residual stress tensor of the continuous phase. The method of Lagrangian particle tracking with an optimized search algorithm (closest cell) is applied to evaluate the separation efficiency of deoiling hydrocyclone. Simulations are performed on a 35-mm deoiling hydrocyclone with the three different cone angles of 6, 10 and 20 degree. The numerical results revealed that the changes in the cone angle would affect the velocity and pressure distribution inside hydrocyclone, and... 

Longitudinal ventilation is a common technique for smoke management during a fire accident within tunnels. In this paper, fire and smoke movement behaviour in longitudinally ventilated tunnels is investigated focusing on critical ventilation velocity. Firstly, critical velocity (VC) is evaluated for different heat release rates and results are compared and verified with model scale experimental data. Secondly, two fire scenarios inside tunnels with different cross sections are arranged and the reliability of some existing correlations for VC is explored. Results illustrate that using a combined geometric scale rather than the traditional single length-scale ones results in better estimation... 

In this study, the optimal location of baffle in the header of air-to-air plate heat exchangers and the geometry of the header and the effects of triangular fins arrays on the effectiveness were numerically investigated for different flow rates and Reynolds numbers on different models. The main purpose of inlet baffle and geometry changes is to improve the pattern of flow distribution and assist flow uniformity. The investigations show that the optimal location of the inlet baffle with high Reynolds numbers has an insignificant effect on effectiveness, while with Reynolds numbers 250-3000, the effectiveness is affected significantly by baffle location. On the other hand, the reduction in... 

'Three-link-model' Exergoeconomic methodology optimises the design and operability of a system. Contrary to traditional iterative Exergoeconomic optimisation methods, a reversed method is used, since assumptions considered by Tsatsaronis for calculating cost-optimal Exergetic efficiency and relative cost difference, were not applicable, new assumptions have been adopted. In a case study applied to Mazandaran paper industry, iterative reversed optimisation results have been compared with evolutionary programming results. Replacement of Pressure Valve and Direct Cyclone Contact Evaporation is proposed, while by selection of the optimum decision variable, recoverable black liquor is increased... 

Longitudinal ventilation system is a frequent method of tunnel ventilation which is commonly implemented by use of jet fans. In most studies on longitudinal ventilation application, the effect of fire on its upstream is neglected and a constant-velocity flow is considered in the tunnel entrance. In this paper, fire consequences on ventilation system performance are investigated. It is noticed that airflow induced in tunnel declines as the fire intensifies. High reductions up to about 50% are observed and are attributed to two main phenomena; elevated downstream resistance and fire plume blockage. These effects are combined to form a parameter named fire pressure loss (FPL). An analytical... 

Vegetation acts as a momentum and thermal sink, affecting the mixing of species and temperature-dependent constants of reaction rates. Numerical simulations were performed to investigate the effects of vegetation on the dispersion of reactive pollutants using a computational fluid dynamic (CFD) model coupled with NO-NO2-O3 photochemistry. Moreover, characteristics of temperature and flow fields were analyzed for different aspect ratios and leaf area densities. The results showed that flow is reversed in the presence of trees, and it enhances as leaf area density (LAD) increases; additionally, vegetation creates downward and vortex flows. The results also revealed that the dispersion of... 

Linear stability analysis is used to characterize the dynamics of a plane jet by incorporating non-modal stability analysis besides classical global temporal stability analysis. It is explained that similar shapes of different global modes are the result of non-normal characteristics of linearized Navier Stokes equations. Optimal initial disturbances and their eigenfunctions together with transient energy growth are obtained for different time horizons and Reynolds numbers of the jet in the linear unstable configuration. These structures are localized at the upstream of the jet nozzle at the boundary layer. The transient growth of the inlet perturbation in limited time bounds is found in the... 

Because reversible fans must be able to provide similar performance in either rotation direction, they need to have completely symmetrical blades. In the present study, an elliptic profile with itsmaximum thickness of 8%chord length at themiddle of the chord line is studied. For verification, theNACA0012 airfoilwas once analyzed two-dimensionally in the isolatedformationandonce in cascade formation with0.55 and 0.83 solidities, andthe obtainedresultswere comparedwith numerical and experimental results. Three well-known Reynolds-averaged Navier-Stokes turbulence models including Spalart-Allmaras, realizable κ-ϵ, and shear-stress transport κ-ω were used to find the suitable turbulence method.... 

Roof greening is a new technique for improvement of outdoor thermal environment which influences air quality through its impacts on thermal and flow field. In order to examine effects of green roofs on reactive pollutant dispersion within urban street canyons, a computational fluid dynamics (CFD) model was employed which contained NO-NO2-O3 photochemistry and energy balance models. Simulations were performed for street canyons with different aspect ratios (H/W) of 0.5, 1.0, and 2.0 such that leaf area density (LAD) of green roofs changed. It was found that roof greening led distribution of pollutants to alter for H/W = 0.5 and 1.0 cases in such a manner that their averaged concentrations had... 

Because reversible fans must be able to provide similar performance in either rotation direction, they need to have completely symmetrical blades. In the present study, an elliptic profile with itsmaximum thickness of 8%chord length at themiddle of the chord line is studied. For verification, theNACA0012 airfoilwas once analyzed two-dimensionally in the isolatedformationandonce in cascade formation with0.55 and 0.83 solidities, andthe obtainedresultswere comparedwith numerical and experimental results. Three well-known Reynolds-averaged Navier-Stokes turbulence models including Spalart-Allmaras, realizable κ-ϵ, and shear-stress transport κ-ω were used to find the suitable turbulence method.... 

In the present study, the fluid characteristics of triangular turbulent jet flow are considered experimentally and numericafly. The results of spatially developed three- dimensional jet, issued from an equilateral triangular nozzle are presented. The jet is discharged to both bounded and unbounded domains. Because of the wind tunnel set up restrictions, the experimental study has done just for the bounded domain. The hot-wire anemometry is used for experimental study. A numerical method employing control volume approach with collocated grid arrangement which couples the velocity and pressure fields with SIMPLEC algorithm is introduced to discrete the governing equations of fluid flow. The... 

The three-dimensional supersonic turbulent flows over wrap-around fin missiles have been computed using the Thin Layer Navier-Stokes (TLNS) equations to reduce the computational efforts compared to those of the Full Navier-Stokes (FNS) equations. In this research, the missile configuration is divided into multi regions to enable fluid flow simulation using Personal Computers (PC). It also makes it possible to use a different number of nodes and distribution of grids in each region to enhance the accuracy. The Thin Layer Navier-Stokes equations in the generalized coordinate system were solved using an efficient, implicit, finite-difference factored algorithm of the Beam and Warming. For the... 

In this paper, the motion of density currents, released on sloping beds and under still bodies of clear water, is numerically investigated. The turbulent flow equations of mass, momentum and diffusion are solved simultaneously in the fixed Cartesian directions, on a non-staggered grid using finite-volume scheme. The velocity-pressure coupling is handled by SIMPLEC method. A modified k - ε model is used to account for the influence of Reynolds stresses in the turbulent momentum equations. Density currents with uniform velocity and concentration enter the channel via a sluice gate into a lighter ambient fluid and move forward down the slope. Comparison of the computed velocity, concentration... 

A numerical simulation has been carried out to investigate the heat transfer enhancement in a shell-and-tube heat exchanger using a porous medium inside its shell and tubes, separately. A three-dimensional geometry with k-ε turbulent model is used to predict the heat transfer and pressure drop characteristics of the flow. The effects of porosity and dimensions of these media on the heat exchanger's thermal performance and pressure drop are analyzed. Inside the shell, the entire tube bundle is wrapped by the porous medium, whereas inside the tubes the porous media are located in two different ways: (1) at the center of the tubes, and (2) attached to the inner wall of the tubes. The results... 

By simultaneously applying pinch technology and the exergoeconomic method to a complex process system, beneficial and energy-efficient measures are identified. The "three-link-model" exergoeconomic methodology optimizes the design and operability of a system. In this work, contrary to traditional exergoeconomic methods, a reversed method is used. The approach proposed for the optimization of such a complex system is to iteratively optimize subsystems. Since the reversed exergoeconomic method is used, assumptions considered by Tsatsaronis (based on four assumptions for calculating the cost-optimal exergetic efficiency and relative cost difference) are not applicable and new assumptions are to...