mehrdad raisee
University of Tehran, Mechanical Engineering Dept., Faculty Member
Le comportement de l'écoulement turbulent instationnaire en conduite résultant d'un coup de bélier est étudié ici numériquement. Un modèle précis de turbulence k-ω pour les couches limites bidimensionnelles sous des gradients de... more
Le comportement de l'écoulement turbulent instationnaire en conduite résultant d'un coup de bélier est étudié ici numériquement. Un modèle précis de turbulence k-ω pour les couches limites bidimensionnelles sous des gradients de pression contraires et favorables (Wilcox 1994) ...
Research Interests:
ABSTRACT Purpose – This paper aims to compute flow and heat transfer through a straight, orthogonally rotating duct, with ribs along the leading and trailing walls, in a staggered arrangement and at an angle of 45° to the main flow... more
ABSTRACT Purpose – This paper aims to compute flow and heat transfer through a straight, orthogonally rotating duct, with ribs along the leading and trailing walls, in a staggered arrangement and at an angle of 45° to the main flow direction. Design/methodology/approach – Flow computations have been produced using a 3D non-orthogonal flow solver, with two two-layer models of turbulence (an effective-viscosity model and a second-moment closure), in which across the near-wall regions the dissipation rate of turbulence is obtained from the wall distance. Flow comparisons have been carried out for a Reynolds number of 100,000 and for rotation numbers of 0 (stationary) and 0.1. Temperature comparisons have been obtained for a Reynolds number of 36,000, a Prandtl number of 5.9 (water) and rotation numbers of 0 and 0.2 and also at a Prandtl number of 0.7 (air) and a rotation number of 0. Findings – It was found that both two-layer models returned similar flow and thermal predictions which are also in close agreement with the flow and thermal measurements. The flow and thermal developments are found to be dominated by the rib-induced secondary motion, which leads to strong span-wise variations in the mean flow and the local Nusselt number and to a uniform distribution of turbulence intensities across the duct. Rotation causes the development of stronger secondary motion along the pressure side of the duct and also the transfer of the faster fluid to this side. The thermal predictions, especially those of the second-moment closure, reproduce the levels and most of the local features of the measured Nusselt number, but over the second half of the rib interval over-predict the local Nusselt number. Originality/value – The work contributes to the understanding of the flow and thermal development in cooling passages of gas turbine blades, and to the validation of turbulence models that can be used for their prediction, at both effective viscosity and second-moment closure levels.
Research Interests:
Research Interests:
This article reports the outcome of our recent effort in prediction of gas slip flow through micro-and nanochannels. Slip nitrogen flow through short (length to height ratio of 20) and long (length to height ratio of 2,500) microchannels... more
This article reports the outcome of our recent effort in prediction of gas slip flow through micro-and nanochannels. Slip nitrogen flow through short (length to height ratio of 20) and long (length to height ratio of 2,500) microchannels is analyzed and discussed using the ...
Research Interests:
Abstract. This paper discusses the abilities of two different low-Reynolds-number ε− k eddy-viscosity models in resolving the complex physical features that arises in turbulent flows around a square cylinder at Re= 22000. For the modeling... more
Abstract. This paper discusses the abilities of two different low-Reynolds-number ε− k eddy-viscosity models in resolving the complex physical features that arises in turbulent flows around a square cylinder at Re= 22000. For the modeling of turbulence, the Launder & ...
Research Interests:
Abstract In this paper the influence of surge tanks on transient flow generated by changing turbine gate opening has been numerically studied. For this purpose, the governing equations of transient flow are solved using the method of... more
Abstract In this paper the influence of surge tanks on transient flow generated by changing turbine gate opening has been numerically studied. For this purpose, the governing equations of transient flow are solved using the method of characteristics and considering ...
Research Interests:
Research Interests:
ABSTRACT The present study reports numerical analysis for mixed electroosmotic/pressure driven flow of Newtonian fluid in slit microchannels. Two dimensional Laplace, Poisson–Boltzmann, and momentum equations are solved numerically in a... more
ABSTRACT The present study reports numerical analysis for mixed electroosmotic/pressure driven flow of Newtonian fluid in slit microchannels. Two dimensional Laplace, Poisson–Boltzmann, and momentum equations are solved numerically in a rectangular microchannel using finite volume method and SIMPLE algorithm. Electroosmotic body force is considered in x and y-momentum equations. The effect of considering y-component electroosmotic body force in velocity distribution is investigated. Moreover, the effects of external applied electric field, zeta potential, Debye length, and the channel height on y-velocity distribution are studied. The numerical results show importance of considering y-component electroosmotic body force by reduction of zeta potential and Debye length.
Research Interests:
Fluid flow and heat transfer in a vertical tube under constant heat flux have been studied and the effect of buoyancy force on the heat transfer coefficient is investigated. The finite volume method is used to study turbulent flow in both... more
Fluid flow and heat transfer in a vertical tube under constant heat flux have been studied and the effect of buoyancy force on the heat transfer coefficient is investigated. The finite volume method is used to study turbulent flow in both upward and downward directions. For the ...
Research Interests: Mechanical Engineering, Aerospace Engineering, Turbulence, Modeling, Heat Transfer, and 12 moreNumerical Simulation, Air flow, Air, Mixed Convection, Cooling System, Turbulent Flow, Heat transfer coefficient, Heat Flux, Fluid flow, Interdisciplinary Engineering, Finite Volume Method, and Nusselt Number
ABSTRACT In this paper, a non-intrusive stochastic model reduction scheme is developed for polynomial chaos representation using proper orthogonal decomposition. The main idea is to extract the optimal orthogonal basis via inexpensive... more
ABSTRACT In this paper, a non-intrusive stochastic model reduction scheme is developed for polynomial chaos representation using proper orthogonal decomposition. The main idea is to extract the optimal orthogonal basis via inexpensive calculations on a coarse mesh and then use them for the fine-scale analysis. To validate the developed reduced-order model, the method is implemented to: (1) the stochastic steady-state heat diffusion in a square slab; (2) the incompressible, two-dimensional laminar boundary-layer over a flat plate with uncertainties in free-stream velocity and physical properties; and (3) the highly nonlinear Ackley function with uncertain coefficients. For the heat diffusion problem, the thermal conductivity of the slab is assumed to be a stochastic field with known exponential covariance function and approximated via the Karhunen–Loève expansion. In all three test cases, the input random parameters are assumed to be uniformly distributed, and a polynomial chaos expansion is found using the regression method. The Sobol's quasi-random sequence is used to generate the sample points. The numerical results of the three test cases show that the non-intrusive model reduction scheme is able to produce satisfactory results for the statistical quantities of interest. It is found that the developed non-intrusive model reduction scheme is computationally more efficient than the classical polynomial chaos expansion for uncertainty quantification of stochastic problems. The performance of the developed scheme becomes more apparent for the problems with larger stochastic dimensions and those requiring higher polynomial order for the stochastic discretization. Copyright © 2015 John Wiley & Sons, Ltd.
Research Interests:
... Raisee, M., 1999, "Computation of flow and heat transfer through two-and three-dimensionalrib-roughened passages ... Medwell, JO, Morris, WD, 1991, "Numerical simulation of three-dimensional turbulent flow... more
... Raisee, M., 1999, "Computation of flow and heat transfer through two-and three-dimensionalrib-roughened passages ... Medwell, JO, Morris, WD, 1991, "Numerical simulation of three-dimensional turbulent flow and heat transfer within a multi-ribbed cylindrical duct", Orlando, ...
Research Interests:
ABSTRACT A numerical investigation has been undertaken to study fluid flow and heat transfer through artificially rib-roughened channels. Such flows are of particular interest in internal cooling of advanced gas turbine blades. The main... more
ABSTRACT A numerical investigation has been undertaken to study fluid flow and heat transfer through artificially rib-roughened channels. Such flows are of particular interest in internal cooling of advanced gas turbine blades. The main objective is to test the suitability of recently developed variants of the cubic non-linear k- model for the prediction of cooling flows through ribbed passages. The numerical approach used in this study is the finite-volume method together with the SIMPLE algorithm. For the modelling of turbulence, the Launder and Sharma low-Re k- model and a new version of the non-linear low-Re two equation model that have been recently shown to produce reliable thermal predictions in impinging jet flows and also flows through pipe expansions, have been employed. Both models have been used with the form of the length-scale correction term to the dissipation rate originally proposed by Yap and also more recently developed differential version, NYap. The numerical results over a range of flow parameters have been compared with the reported experimental data. The mean flow predictions show that both linear and non-linear k- models with NYap can successfully reproduce the distribution of the measured streamwise velocity component, including the length and width of the separation bubble, formed downstream of each rib. As far as heat transfer predictions are concerned, the recent variant of the non-linear k- leads to marked improvements in comparison to the original version of Craft et al. Further improvements in the thermal prediction result through the introduction of a differential form of the turbulent length scale correction term to the dissipation rate equation. The version of the non-linear k- that has been shown in earlier studies to improve thermal predictions in pipe expansions and impinging jets; it is thus found to also produce reasonable heat transfer predictions in ribbed passages.
Research Interests:
ABSTRACT In this study, we investigated the effects of volute tongue geometry variation on the head, efficiency, and radial force of a centrifugal pump. Numerical simulation modeling based on turbulence with automatic near wall treatments... more
ABSTRACT In this study, we investigated the effects of volute tongue geometry variation on the head, efficiency, and radial force of a centrifugal pump. Numerical simulation modeling based on turbulence with automatic near wall treatments was used to simulate the turbulent flow. The effect of blade position with respect to the volute tongue on instantaneous pump characteristics was investigated. The parametric studies were done for cutwater gap, tongue shape, and volute tongue angle. Numerical results showed that the large cutwater gap caused lower radial force, especially at high flow rates. Investigations using various volute tongue shapes indicated that the short tongue volute decreased the radial force at design and low flow rates. Considering all aspects, the most satisfactory volute tongue angle was found to be 5° less than the outlet velocity angle of the impeller; yielding about 40% lower radial force than others at the design point.
Research Interests:
ABSTRACT Measurements were performed on pulsating fully turbulent flows in a pipe test rig with a diameter of 100 mm. Sinusoidal oscillatory flow at different frequencies was superimposed on a mean flow of averaged Reynolds number... more
ABSTRACT Measurements were performed on pulsating fully turbulent flows in a pipe test rig with a diameter of 100 mm. Sinusoidal oscillatory flow at different frequencies was superimposed on a mean flow of averaged Reynolds number Re=20000 based on the pipe diameter. The measurements have been performed at different forcing frequencies (0.001 < ω+ < 0.08) covering all the oscillatory regimes; quasi-steady, relaxation, quasi laminar and high frequency. The amplitude of the flow oscillation was small enough to allow a linear response in the measurements, i.e., all flow parameters showed an oscillatory behavior at the frequency of the flow. The amplitude of the oscillatory flow was about 10% of the mean velocity in all cases. The results include mean and phase averaged values of different parameters. The centerline velocity was measured by a 2D LDA system. Hot film and constant temperature anemometry system was used to determine the wall shear stress. Bulk velocity and pressure gradient along the pipe were also acquired. The results showed a good agreement with the previous analytical, experimental and numerical results available in the literature.
ABSTRACT This paper discusses laminar mixed convection of air flow through vertical tubes. Calculations were performed by solving the Navier-Stokes and energy equations for a number of heating lengths. The Reynolds number based on the... more
ABSTRACT This paper discusses laminar mixed convection of air flow through vertical tubes. Calculations were performed by solving the Navier-Stokes and energy equations for a number of heating lengths. The Reynolds number based on the fluid bulk velocity and diameter of the tube is Re = 500 and Grashof number based on wall heat flux is Gr = 106 . The numerical results have been obtained using a finite-volume code which solves the governing equations in axi-symmetric coordinate system. The pressure field is obtained with the SIMPLE algorithm. The HYBRID scheme is used for the convective terms. The computer code was validated by comparing its predictions with the reported analytical, numerical and experimental results. For various heating lengths the values of Nusselt number and friction coefficient are presented and the effects of heating length on these parameters are studied. It was found that for the buoyancy-aided convection, the velocity in the vicinity of the wall increases while decreases in the core region. These result in an enhancement of wall heat transfer coefficient.
Research Interests:
... Arman Rokhzadi, Department of Mechanical Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran. ... The turbulence models employed for computation are the zonal k − ε/one-equation model, the Launder and Sharma (1974)... more
... Arman Rokhzadi, Department of Mechanical Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran. ... The turbulence models employed for computation are the zonal k − ε/one-equation model, the Launder and Sharma (1974) low-Re k − ε model, and a recently ...
Research Interests: Engineering, Turbulence, Heat Transfer, GAS TURBINE, Flow, and 12 moreMathematical Sciences, Design Methodology, Turbulent Flow, Fluid flow, Heat transfer enhancement, Cross Section, Three Dimensional, Finite Volume Method, Nusselt Number, Turbulence Model, Forced Convection, and Low Reynolds Number
ABSTRACT In the present paper the ability of an analytical and a numerical wall function proposed by Grasimove (2003) and Gant (2002) respectively in prediction of separated flow from a three-dimensional hill in a duct is examined. A... more
ABSTRACT In the present paper the ability of an analytical and a numerical wall function proposed by Grasimove (2003) and Gant (2002) respectively in prediction of separated flow from a three-dimensional hill in a duct is examined. A modified non-linear eddy-viscosity ...
ABSTRACT Fluid flow and heat transfer in a vertical tube under constant heat flux have been studied and effect of buoyancy force on the heat transfer coefficient is investigated. The finite volume method is used to study turbulent flow in... more
ABSTRACT Fluid flow and heat transfer in a vertical tube under constant heat flux have been studied and effect of buoyancy force on the heat transfer coefficient is investigated. The finite volume method is used to study turbulent flow in both upward and downward directions. For the turbulence modeling, a zonal k-e model is employed and the numerical results are compared with available experimental data. The results of the simulation show that for the downward flow, heat transfer is enhanced and for strong buoyancy force, flow reversal is observed. In contrast, for the heated upward flow, heat transfer can be either impaired or enhanced by the buoyancy force depending on its strength. Partial laminarization is caused by the buoyancy in the case of modest buoyancy force. For the condition of stronger buoyancy force, a sudden decrease in the fully-developed Nusselt number is evident in the experimental data and well predicted by the numerical solution. In general, the quantitative agreement between the numerical results and the experimental data is satisfactory.
Research Interests:
The behaviour of unsteady turbulent pipe flow resulting from water hammer is herein numerically studied. An accurate k–θ turbulence model for two-dimensional boundary layers under adverse and favorable pressure gradients (Wilcox 1994) was... more
The behaviour of unsteady turbulent pipe flow resulting from water hammer is herein numerically studied. An accurate k–θ turbulence model for two-dimensional boundary layers under adverse and favorable pressure gradients (Wilcox 1994) was applied. The results of ...
Research Interests:
In the present paper, the effects of adding nanoscale metallic particles 3 2O Al γ on the hydrodynamic and thermal characteristics of laminar liquid flow (water and ethylene glycol) through two-dimensional and axi-symmetric passages are... more
In the present paper, the effects of adding nanoscale metallic particles 3 2O Al γ on the hydrodynamic and thermal characteristics of laminar liquid flow (water and ethylene glycol) through two-dimensional and axi-symmetric passages are numerically investigated. The present numerical results are obtained using a 2D finite-volume code which solves the governing equations in polar and Cartesian coordinate systems. The pressure field is obtained with the well-known SIMPLE algorithm. Advective volume-face fluxes are approximated using the upstream quadratic interpolation scheme, QUICK. It is assumed that the passages examined in this investigation are under constant wall heat flux boundary condition. For each passage, numerical results are obtained at two Reynolds numbers of 100 and 250. The nanoparticle volume concentration is varied between 1 to 10%. It is found that the addition of nanoparticles increases both the wall heat transfer coefficient and wall shear stress. In general, the ...
Research Interests:
This paper discusses the abilities of two different low-Reynolds-number ε − k eddy-viscosity models in resolving the complex physical features that arises in turbulent flows around a square cylinder at Re=22000. For the modeling of... more
This paper discusses the abilities of two different low-Reynolds-number ε − k eddy-viscosity models in resolving the complex physical features that arises in turbulent flows around a square cylinder at Re=22000. For the modeling of turbulence, the Launder & Sharma (LS) (1) and Kawamura & Kawashima (KK) (2) low-Re ε − k models have been employed. The present numerical results were obtained using a two-dimensional finite- volume code. The pressure field is obtained with the well-known SIMPLE algorithm. Advective volume-face fluxes are approximated using a bounded version of the upstream quadratic interpolation scheme, QUICK. Comparisons of the numerical results with the experimental data indicate that the steady computations, as expected, cannot produce reliable flow field predictions in the wake region downstream of the square cylinder. Consequently, the time derivatives of dependent variables are included in the transport equations and are approximated using the second-order Crank-N...
Research Interests:
This paper deals with the computation of unsteady laminar flow between co-rotating disks with a stationary outer casing. The geometry examined is a disk configuration with inner to outer radii a/b = 0.5, gap ratio s/b = 0.3 and two... more
This paper deals with the computation of unsteady laminar flow between co-rotating disks with a stationary outer casing. The geometry examined is a disk configuration with inner to outer radii a/b = 0.5, gap ratio s/b = 0.3 and two Reynolds numbers (Re = Omega b(2)/v) of 10(4) and 1.46 x 10(5). The nature of flow in this range of Reynolds number is so that steady computations can lead to unrealistic results. At Reynolds number of 10(4), the unsteady computations predict periodic counter-rotating vortices in the outer region and a solid body rotation motion in the inner region while at Re = 1.46 x 10(5) an unsteady non-periodic flow structure is predicted. Computed velocity components at Re = 1.46 x 10(5) compared with the measurements of Gan et al. [Flow in a rotating cavity with a peripheral inlet and outlet of cooling air, Int. Gas Turbine and Aero-engine Cong., Paper 96-GT-309, 1996] where reasonable agreement is found.
Research Interests:
Wastewater disinfection processes are typically designed according to heuristics derived from batch experiments where the interaction among wastewater quality, reactor hydraulics and inactivation kinetics is often neglected. In this... more
Wastewater disinfection processes are typically designed according to heuristics derived from batch experiments where the interaction among wastewater quality, reactor hydraulics and inactivation kinetics is often neglected. In this paper, a CFD study was conducted in a non-deterministic (ND) modeling framework to predict the Escherichia coli inactivation by peracetic acid (PAA) in municipal contact tanks fed by secondary settled wastewater effluent. The extent and the variability associated with the observed inactivation kinetics were both satisfactorily predicted by the stochastic inactivation model at a 95% confidence level. Moreover, it was found that: (a) the process variability induced by reactor hydraulics is negligible when compared to the process variability one caused by inactivation kinetics; (b) the PAA dose required for meeting regulations is dictated equally by the fixed limit of the microbial concentration as well as its probability of occurrence; (c) neglecting the p...
Research Interests:
ABSTRACT Large radial force causes several issues in pumps, such as noise, vibration, and extra load on the bearings. To reduce the radial force, the effects of concentric volute and multivolute geometry on the head, efficiency, and... more
ABSTRACT Large radial force causes several issues in pumps, such as noise, vibration, and extra load on the bearings. To reduce the radial force, the effects of concentric volute and multivolute geometry on the head, efficiency, and radial force of a low speed centrifugal pump at off-design conditions were investigated. Commercial software with the k - ω turbulence model and automatic near wall treatment was employed for the prediction of fluid flow inside the pump. Flow simulations for three casings concentric at 180°, 270°, and 360° from the tongue showed that the 270° concentric volute generates the lowest radial force at throughout the entire range of flow rate. The triple-volute and tetravolute casings are also proposed as new volute geometries. The flow analysis of a double-volute, triple-volute, and tetravolute show that the triple-volute is the most appropriate volute geometry at off-design conditions.
Research Interests:
ABSTRACT Turbulent flow in curved ducts occurs in many engineering applications such as centrifugal pumps, aircraft intake and in the cooling passages of gas turbine blades. An important characteristic which distinguishes such flows from... more
ABSTRACT Turbulent flow in curved ducts occurs in many engineering applications such as centrifugal pumps, aircraft intake and in the cooling passages of gas turbine blades. An important characteristic which distinguishes such flows from those in straight ducts is the generation of strong secondary motions. In this work, a numerical investigation has been undertaken to study incompressible, developing turbulent flow through a 90° duct of rectangular cross-section [1]. The main objective has been to assess the effectiveness of the Launder and Sharma [2] low-Re k − ε model and the nonlinear low-Re k − ε model of Craft et al. [3] in predicting turbulent flow in curved ducts. In this study, the governing equations of mean flow are solved using an extended version of STREAM-3D [4] finite volume code. The pressure field is obtained using the SIMPLE algorithm and the convection terms in all transport equations are approximated using the QUICK scheme. The predicted velocity vectors, in Figure 1(a), indicates that the duct curvature causes flow acceleration and deceleration along the convex surface, and reverse along the concave surface. Furthermore, Figure 1(b) shows that the curvature induces a strong secondary motion which as a result of that the core fluid displaces towards the convex surface. It is noted that both the linear and nonlinear k − ε models return similar axial velocity contours and secondary flow motions. Comparisons of the predicted axial velocity, turbulent kinetic energy, and turbulent shear-stress with measurements of Kim and Patel [1], shown in Figure 2, indicates that while both turbulence models successfullly reproduce the measured axial velocity, the nonlinear k − ε model returns more accurate turbulence field predictions. As shown in Figure 3(a), both turbulent models correctly reproduce the variation of pressure coefficient. Note, however, that as shown in Figure 3(b) the distribution of friction coefficient is more faithfully reproduced by the nonlinear k − ε model. In general, it appears that the nonlinear low-Re k − ε model does offer the better route for the reliable computation of turbulent flow in curved ducts.
Research Interests:
ABSTRACT Solar thermal collectors are applicable in the water heating or space conditioning systems. Due to the low efficiency of the conventional collectors, some suggestions have been presented for improvement in the collector... more
ABSTRACT Solar thermal collectors are applicable in the water heating or space conditioning systems. Due to the low efficiency of the conventional collectors, some suggestions have been presented for improvement in the collector efficiency. Adding nanoparticles to the working fluid in direct absorption solar collector, which has been recently proposed, leads to improvement in the working fluid thermal and optical properties such as thermal conductivity and absorption coefficient. This results certainly in collector efficiency enhancement. In this paper, the radiative transfer and energy equations are numerically solved. Due to laminar and fully developed flow in the collector, the velocity profile is assumed to be parabolic. As can be observed from the results, outlet temperature of collector is lower than that obtained using uniform velocity profile. Furthermore, a suspension of carbon nanohorns in the water is used as the working fluid in the model and its effect on the collector efficiency is investigated. It was found that the presence of carbon nanohorns increases the collector efficiency by about 17% compared to a conventional flat-plate collector. In comparison with the mixture of water and aluminium nanoparticles, a quite similar efficiency is obtained using very lower concentration of carbon nanohorns in the water.
Research Interests:
Abstract: The present study reports numerical analysis of micromixing for mixed electroosmotic/pressure driven flow of Newtonian fluid in microchannels. Two dimensional Laplace, Poisson-Boltzmann, momentum, and species concentration... more
Abstract: The present study reports numerical analysis of micromixing for mixed electroosmotic/pressure driven flow of Newtonian fluid in microchannels. Two dimensional Laplace, Poisson-Boltzmann, momentum, and species concentration equations are solved numerically using finite volume method and SIMPLE algorithm. The equations are solved for rectangular microchannels with heterogeneous zeta potential distribution along the channel walls. Flow streamlines are presented for microchannels with and without electroosmotic ...
Research Interests:
In the present paper a new model is proposed for electric double layer (EDL) overlapped in nanochannels. The model aimed to obtain a deeper insight of transport phenomena in nanoscale. Two-dimensional Nernst and ionic conservation... more
In the present paper a new model is proposed for electric double layer (EDL) overlapped in nanochannels. The model aimed to obtain a deeper insight of transport phenomena in nanoscale. Two-dimensional Nernst and ionic conservation equations are used to obtain electroosmotic potential distribution in flow field. In the proposed study, transport equations for flow, ionic concentration and electroosmotic potential are solved numerically via finite volume method. Moreover, Debye-Hückle (DH) approximation and symmetry condition, ...
Research Interests:
ABSTRACT The effects of the volute geometry on the head, efficiency, and radial force of a low specific-speed centrifugal pump were investigated focusing on off-design conditions. This paper is divided into three parts. In the first part,... more
ABSTRACT The effects of the volute geometry on the head, efficiency, and radial force of a low specific-speed centrifugal pump were investigated focusing on off-design conditions. This paper is divided into three parts. In the first part, the three-dimensional flow inside the pump with rectangular volute was simulated using three well-known turbulence models. Simulation results were compared with the available experimental data, and an acceptable agreement was obtained. In the second part, two volute design methods, namely, the constant velocity and the constant angular momentum were investigated. Obtained results showed that in general the constant velocity method gives more satisfactory performance. In the third part, three volutes with different cross section and diffuser shape were designed. In general, it was found that circular cross section volute with radial diffuser provides higher head and efficiency. Moreover, the minimum radial force occurs at higher flowrate in circular volute geometry comparing to rectangular cross section volute.
Research Interests:
ABSTRACT In this paper, the fluid flow and heat transfer characteristics of two-dimensional micro/nanochannel flows are examined. The Direct Simulation Monte Carlo (DSMC) method for molecular gas dynamics is utilized to simulate the gas... more
ABSTRACT In this paper, the fluid flow and heat transfer characteristics of two-dimensional micro/nanochannel flows are examined. The Direct Simulation Monte Carlo (DSMC) method for molecular gas dynamics is utilized to simulate the gas flows through two-dimensional micro/nanochannel. The collision process has been treated in a statistical way using the no-time-counter (NTC) scheme and the variable hard sphere (VHS) model has been employed to simulate the collision from the kinetic viewpoint. Results revealed that the temperature and velocity distributions have an unequalled behavior and may have a significant effect on the advancement and the design of MEMS and NEMS.
In this paper, the physico-chemical treatment of municipal wastewater for the simultaneous removal of pollutant indicators (chemical oxygen demand (COD) and total coliforms) and organic contaminants (total phenols) was investigated and... more
In this paper, the physico-chemical treatment of municipal wastewater for the simultaneous removal of pollutant indicators (chemical oxygen demand (COD) and total coliforms) and organic contaminants (total phenols) was investigated and assessed. A secondary settled effluent was subjected to coagulation, disinfection and absorption in a multifunctional reactor by dosing, simultaneously, aluminum polychloride (dose range: 0-150 μL/L), natural zeolites (dose range: 0-150 mg/L), sodium hypochlorite (dose range: 0-7.5 mg/L) and powder activated carbon (dose range: 0-30 mg/L). The treatment process was optimized using computational fluid dynamics (CFD) and response surface methodology. Specifically, a Latin square technique was employed to generate 16 combinations of treating agent types and concentrations which were pilot tested on an 8 m(3)/h multifunctional reactor fed by a secondary effluent with COD and total coliform concentrations ranging from ≈20 to 120 mg/L and from 10(5) to 10(6) CFU/100 mL, respectively. Results were promising, indicating that removal yields up to 71% in COD and 5.4 log in total coliforms were obtained using an optimal combination of aluminum polychloride (dose range ≈ 84-106 μL/L), powder activated carbon ≈ 5 mg/L, natural zeolite (dose range ≈ 34-70 mg/L) and sodium hypochlorite (dose range ≈ 3.4-5.6 mg/L), with all treating agents playing a statistically significant role in determining the overall treatment performance. Remarkably, the combined process was also able to remove ≈ 50% of total phenols, a micropollutant known to be recalcitrant to conventional wastewater treatments.
Research Interests:
Research Interests:
This paper assesses the effectiveness of a modified cubic low-Reynolds-number two-equation turbulence model in predicting a number of complex turbulent flows including a turbulent impinging jet, turbulent flow in a curved duct, turbulent... more
This paper assesses the effectiveness of a modified cubic low-Reynolds-number two-equation turbulence model in predicting a number of complex turbulent flows including a turbulent impinging jet, turbulent flow in a curved duct, turbulent heat transfer in a three-...
Research Interests:
ABSTRACT Solar thermal collectors are applicable in the water heating or space conditioning systems in which surface-based absorption of incident solar flux cause high heat losses. Therefore, an enhancement in the efficiency of solar... more
ABSTRACT Solar thermal collectors are applicable in the water heating or space conditioning systems in which surface-based absorption of incident solar flux cause high heat losses. Therefore, an enhancement in the efficiency of solar harvesting devices is a basic challenge which requires great effort. Adding nanoparticles to the working fluid in direct absorption solar collector, which has been recently proposed, leads to improvement in the working fluid thermal and optical properties such as thermal conductivity and absorption coefficient. This results certainly in collector efficiency enhancement. In this paper, the characteristics of nanofluids consisting carbon nanoball in water- and ethylene glycol-based suspensions in consideration of their use as sunlight absorber fluid in a DASC are investigated. It was found that by using of 300 ppm carbon nanoballs, the extinction coefficient of pure water and ethylene glycol is increased by about 3.9 cm−1 and 3.4 cm−1 in average, respectively. With these significantly promising optical properties, a direct absorption solar collector using carbon nanoball-based nanofluids can achieve relatively higher efficiencies, compared with a conventional solar collector.
Research Interests:
Oscillating structures and actuators can induce flow kinematics that enhances mixing. This approach is specifically effective for mixing enhancement in meso-scale channels, where the flow kinematics can be actively controlled using... more
Oscillating structures and actuators can induce flow kinematics that enhances mixing. This approach is specifically effective for mixing enhancement in meso-scale channels, where the flow kinematics can be actively controlled using micro-electro-mechanical-systems (MEMS). In this paper, numerical results for mixing of two incompressible ideal gas (Schmidt number of 1.0) streams through a 2D minichannel via a rotationally oscillating circular cylinder are presented and discussed. Simulations are performed for blockage ratio of D/H=l/3 and Reynolds number of 100 and oscillation amplitudes of π/3, 2π/3,π/2 and π for subharmonic (F < 1), harmonic (F = 1) and superharmonic (F > 1) regimes. Numerical results indicate that mixing performance is improved by about 70% compare to the plane channel at oscillation amplitude of π and excitation frequency of 25% higher than the natural frequency of vortex shedding of a stationary cylinder. It is shown that the mixing efficiency is increased by increasing of amplitude in all the cases except at very low excitation frequencies. This study also shows that when the excitation frequency is equal to the vortex shedding frequency the maximum power is required for mixing of two gases.
Le comportement de l'écoulement turbulent instationnaire en conduite résultant d'un coup de bélier est étudié ici numériquement. Un modèle précis de turbulence k-ω pour les couches limites bidimensionnelles sous des gradients de... more
Le comportement de l'écoulement turbulent instationnaire en conduite résultant d'un coup de bélier est étudié ici numériquement. Un modèle précis de turbulence k-ω pour les couches limites bidimensionnelles sous des gradients de pression contraires et favorables (Wilcox 1994) ...
Research Interests:
An effective numerical method to compute hydraulic resonance in pressurized piping of hydropower systems is presented. For this purpose, the impulse response method is used, ie, a unit pressure impulse is introduced at the downstream... more
An effective numerical method to compute hydraulic resonance in pressurized piping of hydropower systems is presented. For this purpose, the impulse response method is used, ie, a unit pressure impulse is introduced at the downstream waterway as an exciter. The method of ...