An electrohydrodynamic (EHD) method, which is based on glow discharge plasma, is presented for flow control in an S-shaped duct. The research subject is an expanding channel with a constant width and a rectangular c...An electrohydrodynamic (EHD) method, which is based on glow discharge plasma, is presented for flow control in an S-shaped duct. The research subject is an expanding channel with a constant width and a rectangular cross section. An equivalent divergence angle and basic function are introduced to build the three-dimensional model. Subsequently, the plasma physical models are simplified as the effects of electrical body force and work (done by the force) on the fluid near the wall. With the aid of FLUENT software, the source terms of momentum and energy are added to the Navier-Stokes equation. Finally, the original performance of three models (A, B and C) is studied, in which model A demonstrates better performance. Then EHD control based on model A is discussed. The results show that the EHD method is an effective way of reducing flow loss and improving uniformity at the duct exit. The innovation in this study is the assessment of the EHD control effect on the flow in an S-shaped duct. Both the parametric modeling of the S-shaped duct and the simplified models of plasma provide valuable information for future research on aircraft inlet ducts.展开更多
A numerical analysis is presented for the oscillatory flow of Maxwell fluid in a rectangular straight duct subjected to a simple harmonic periodic pressure gradient.The numerical solutions are obtained by a finite dif...A numerical analysis is presented for the oscillatory flow of Maxwell fluid in a rectangular straight duct subjected to a simple harmonic periodic pressure gradient.The numerical solutions are obtained by a finite difference scheme method.The stability of this finite difference scheme method is discussed.The distributions of the velocity and phase difference are given numerically and graphically.The effects of the Reynolds number,relaxation time,and aspect ratio of the cross section on the oscillatory flow are investigated.The results show that when the relaxation time of the Maxwell model and the Reynolds number increase,the resonance phenomena for the distributions of the velocity and phase difference enhance.展开更多
We show existence of time-periodic supersonic solutions in a finite interval, after certain start-up time depending on the length of the interval, to the one space-dimensional isentropic compressible Euler equations, ...We show existence of time-periodic supersonic solutions in a finite interval, after certain start-up time depending on the length of the interval, to the one space-dimensional isentropic compressible Euler equations, subjected to periodic boundary conditions. Both classical solutions and weak entropy solutions, as well as high-frequency limiting behavior are considered. The proofs depend on the theory of Cauchy problems of genuinely nonlinear hyperbolic systems of conservation laws.展开更多
The flow field in a semi-circular duct is simulated by Large Eddy Simulation(LES)and its particle field is simulated by Lagrange particle tracking method.Reynolds number Reb(based on bulk velocity and hydraulic diamet...The flow field in a semi-circular duct is simulated by Large Eddy Simulation(LES)and its particle field is simulated by Lagrange particle tracking method.Reynolds number Reb(based on bulk velocity and hydraulic diameter)is 80,000 and Ret(based on friction velocity and hydraulic diameter)is 3528.Particle diameter dpis chosen as 10,50,100,500 mm corresponding to St as 0.10,2.43,9.72,243.05.The results show that the intensity of the secondary flow near the ceiling is less than that near the floor because the ceiling is curved and able to inhibit the secondary flow.It is found that the difference between the semicircular duct and the square duct is that the secondary flow in a corner of the semi-circular duct is not symmetrical along the diagonal although they have the same generation mechanism.Regarding the particles,small particles(dp≤10 mm)are found to uniformly distribute in the duct,while large particles(dp≥50 mm)preferentially distribute in the corner and floor center.The maximum particles(dp=500mm)fall on the floor quickly and their dispersion mainly depends on the secondary flow near the floor.Particle deposition in the corner depends on particle size due to the effect of secondary flow and gravity.The effect of lift force on particles becomes more significant for 50 and 100 mm particles in comparison with other smaller particles.In the end,the effect of secondary flow is found to be more significant to dominate particle behavior than that of flow fluctuation.展开更多
The objective of this work is to estimate the accuracy of a predicted velocity profile which can be gained from experimental results, in comparison with the exact ones by the methodology of entropy generation. The ana...The objective of this work is to estimate the accuracy of a predicted velocity profile which can be gained from experimental results, in comparison with the exact ones by the methodology of entropy generation. The analysis is concerned with the entropy generation rate in hydrodynamic, steady, laminar, and incompressible flow for Newtonian fluids in the insulated channels of arbitrary cross section. The entropy generation can be calculated from two local and overall techniques. Adaptation of the results of these techniques depends on the used velocity profile. Results express that in experimental works, whatever the values of local and overall entropy generation rates are close to each other, the results are more accuracy. In order to extent the subject, different geometries have been investigated. Also, the influence studied, and the distribution of volumetric geometries is drawn. of geometry on the entropy generation rate is local entropy generation rate for the selected geometries is drawn.展开更多
The effects of the aspect ratio on unsteady solutions through the curved duct flow are studied numerically by a spectral based computational procedure with a temperature gradient between the vertical sidewalls for the...The effects of the aspect ratio on unsteady solutions through the curved duct flow are studied numerically by a spectral based computational procedure with a temperature gradient between the vertical sidewalls for the Grashof number 100 ≤ Gr ≤ 2 000. The outer wall of the duct is heated while the inner wall is cooled and the top and bottom walls are adiabatic. In this paper, unsteady solutions are calculated by the time history analysis of the Nusselt number for the Dean numbers Dn = 100 and Dn = 500 and the aspect ratios 1≤γ≤ 3. Water is taken as a working fluid (Pr =7.0). It is found that at Dn = 100, there appears a steady-state solution for small or large Gr. For moderate Gr, however, the steady-state solution turns into the periodic solution if γ is increased. For Dn = 500, on the other hand, it is analyzed that the steady-state solution turns into the chaotic solution for small and large Gr for any γ lying in the range. For moderate Gr at Dn = 500, however, the steady-state flow turns into the chaotic flow through the periodic oscillating flow if the aspect ratio is increased.展开更多
In this paper,to simulate the three dimensional turbulent flow in suddenly expanded rectangular duct numerically,the SIMPLEC algorithm is employed to solve the incompressible Navier-Stckes equation with k-εturbulenc...In this paper,to simulate the three dimensional turbulent flow in suddenly expanded rectangular duct numerically,the SIMPLEC algorithm is employed to solve the incompressible Navier-Stckes equation with k-εturbulence model.The numerical resulis show well the three dimensional turbulent flow field in the rectangular duct behind the sudden expansion cross-section. and agree.fairly well with the experimental result of the length of the main circumfluence.The numerical method of this paper can be applied to numerical analysis of this kind of turbulent flow.展开更多
One-dimensional analytical theory is developed for supersonic duct flow with variation of cross section, wall friction, heat addition, and relations between the inlet and outlet flow parameters are obtained. By introd...One-dimensional analytical theory is developed for supersonic duct flow with variation of cross section, wall friction, heat addition, and relations between the inlet and outlet flow parameters are obtained. By introducing a self- similar parameter, effects of heat releasing, wall friction, and change in cross section area on the flow can be normalized and a self-similar solution of the flow equations can be found. Based on the result of self-similar solution, the sufficient and necessary condition for the occurrence of thermal choking is derived. A re- lation of the maximum heat addition leading to thermal choking of the duct flow is derived as functions of area ratio, wall friction, and mass addition, which is an extension of the classic Rayleigh flow theory, where the effects of wall friction and mass addition are not considered. The present work is expected to provide fundamentals for developing an integral analytical theory for ramjets and scramjets.展开更多
The fully developed slip flow in an annular sector duct is solved by expansions of eigenfunctions in the radial direction and boundary collocation on the straight sides. The method is efficient and accurate. The flow ...The fully developed slip flow in an annular sector duct is solved by expansions of eigenfunctions in the radial direction and boundary collocation on the straight sides. The method is efficient and accurate. The flow field for slip flow differs much from that of no-slip flow. The Poiseuille number increases with increased inner radius, opening angle, and decreases with slip.展开更多
A numerical study is presented for the fully developed two-dimensional laminar flow of viscous incompressible fluid through a curved square duct for the constant curvature δ = 0.1. In this paper, a spectral-based com...A numerical study is presented for the fully developed two-dimensional laminar flow of viscous incompressible fluid through a curved square duct for the constant curvature δ = 0.1. In this paper, a spectral-based computational algorithm is employed as the principal tool for the simulations, while a Chebyshev polynomial and collocation method as secondary tools. Numerical calculations are carried out over a wide range of the pressure gradient parameter, the Dean number, 100 ≤ Dn ≤ 3000 for the Grashof number, Gr, ranging from 100 to 2000. The outer wall of the duct is treated heated while the inner wall cooled, the top and bottom walls being adiabatic. The main concern of the present study is to find out the unsteady flow behavior i.e. whether the unsteady flow is steady-state, periodic, multi-periodic or chaotic, if Dn or Gr is increased. It is found that the unsteady flow is periodic for Dn = 1000 at Gr = 100 and 500 and at Dn = 2000, Gr = 2000 but steady-state otherwise. It is also found that for large values of Dn, for example Dn = 3000, the unsteady flow undergoes in the scenario “periodic→chaotic→periodic”, if Gr is increased. Typical contours of secondary flow patterns and temperature profiles are also obtained, and it is found that the unsteady flow consists of single-, two-, three- and four-vortex solutions. The present study also shows that there is a strong interaction between the heating-induced buoyancy force and the centrifugal force in a curved square passage that stimulates fluid mixing and consequently enhance heat transfer in the fluid.展开更多
Combined effects of centrifugal and coriolis instability of the flow through a rotating curved duct with rectangular cross section have been studied numerically by using a spectral method, and covering a wide range of...Combined effects of centrifugal and coriolis instability of the flow through a rotating curved duct with rectangular cross section have been studied numerically by using a spectral method, and covering a wide range of the Taylor number ?for a constant Dean number. The rotation of the duct about the center of curvature is imposed in the positive direction, and the effects of rotation (Coriolis force) on the flow characteristics are investigated. As a result, multiple branches of asymmetric steady solutions with two-, three-and multi-vortex solutions are obtained. To investigate the non-linear behavior of the unsteady solutions, time evolution calculations as well as power spectrum of the unsteady solutions are performed, and it is found that the unsteady flow undergoes through various flow instabilities in the scenario “chaotic?→ multi-periodic?→ periodic?→ steady-state”, if Tr is increased in the positive direction. The present results show the characteristics of both the secondary flow and axial flow distribution in the flow.展开更多
A depth understanding of fluid flow past a curved duct having rectangular cross-section with different aspect ratios(l)are essential for various engineering applications such as in chemical,mechanical,biomechanical an...A depth understanding of fluid flow past a curved duct having rectangular cross-section with different aspect ratios(l)are essential for various engineering applications such as in chemical,mechanical,biomechanical and bio-medical engineering.So highly ambitious researchers have given significant attention to study new characteristics of fluid flow in a curved duct.The flow characterization in the rectangular duct has been studied over a wide range of numerical and selective experimental studies.However,proper knowledge with the effects of Coriolis force for different aspect ratios is important for better understanding of the transitional behaviour and the subsequent heat generation,which is required to improve further.The purpose of this study is to reveal insight into the transitional flow pattern and heat transfer in a curved rectangular domain.The Navier-Stokes equations are solved using the spectral method,while the Crank-Nicolson method is used to solve the energy equation.An in-house FORTRAN code is developed to get the numerical solution.For post-processing purposes,Tecplot-360 and Ghost-script tools are used.The present study exposes development of Dean vortices that affect heat generation as well as thermal enhancement in the flow with underlying the flow controlling parameters,the Dean number(Dn),the Grashof number(Gr)and the Taylor number(Tr).Time-dependent results followed by phase spaces show that transient flow undergoes in the scenario‘chaotic→multi-periodic→periodic→steady-state’generating 2-to 8-vortices for the periodic/multi-periodic flow at 2000≤Tr≤2205 for l=2,whereas similar sort of flow is observed in the range of 3100≤Tr≤3195 for l=3.More complicated 4-to 13-vortex solutions are obtained for the chaotic flow regime at l=2 in the range of 0≤Tr<2200 and at l=3 in the range of 0≤Tr<3100.The chaotic flow that occurs at the certain range of Tr proficiently intensifies the heat transfer than the unperturbed,periodic or multi-periodic flow.The overall investigation reveals that in the rotating duct,the temperature-influenced buoyancy compulsion and centrifugal-coriolis joint forces are dominant,influencing the characteristic of the fluid and thus optimizing the transfer of heat.The present investigation will contribute to enhancing the understanding of fluid flow and heat transfer of internal heating/cooling/gas turbines,electric generators,biological systems,and some separation processes.展开更多
Due to widespread applications of the bent ducts in engineering fields such as in chemical,mechanical,bio-mechanical and bio-medical engineering,scientists have paid considerable attention to invent new characteristic...Due to widespread applications of the bent ducts in engineering fields such as in chemical,mechanical,bio-mechanical and bio-medical engineering,scientists have paid considerable attention to invent new characteristics of fluid flow in a bent duct(BD).In the ongoing study,a spectral-based numerical technique is applied to explore flow characteristics and energy distribution through a loosely bent square duct(BSD)of small curvature.Flow is accelerated due to combined action of the non-dimensional parameters;the Grashof number Gr(=1000),the curvatureδ(=0.001),and the Prandtl number Pr(=7.0)over a wide domain of the Dean number 0<Dn≤5000.Fortran code is developed for the numerical computations and Tecplot software with Gost Script and Gost View is used for the post-processing purpose.The numerical study investigates steady solutions(SS)and as a result,a structure of six-branches of SSs composed of 2-to 6-vortex solutions is obtained.Then oscillating behavior with flow transition is discussed by obtaining timedependent solutions followed by power-spectrum analysis.Results showthat the trend of unsteady flow(UF)undergoes in the sequence‘steady-state→multi-periodic→steady-state→chaotic→multi-periodic→chaotic’,if Dn is increased.Asymmetric 2-to 4-vortex solutions are obtained for UF.Convective heat transfer(CHT)is then examined obtaining temperature gradients and energy contours,and it is found that CHT is significantly enhanced by the secondary flow(SF).The present study reveals that the role of secondary vortices over heat transfer(HT)is highly significant and HT occurs substantially for the chaotic solutions.Finally,for the interest of validation,the present numerical result is compared with the previously published experimental outcomes,and a good agreement is remarked.展开更多
In this study,a large eddy simulation(LES)for fully-developed turbulent flows through a duct of regular-polygon cross-section using the immersed boundary(IB)method is performed.In case of the turbulent flow through th...In this study,a large eddy simulation(LES)for fully-developed turbulent flows through a duct of regular-polygon cross-section using the immersed boundary(IB)method is performed.In case of the turbulent flow through the square duct,though there are some disagreements of the mean quantities related with the streamwise velocity among the present LES,the previous direct numerical simulation(DNS)and the LES without the IB method,and the present LES can reproduce the secondary flow of the DNS and LES.The LES result for ten types of regular-polygon duct shows that the secondary-flow speed decreases as the number of sides of the regular polygon n increases and that the secondary flow in case of the regular icosagon duct disappears like the turbulent pipe flow.In case of low n,the behavior of the turbulent structures near the side center is different from that near the vertex.展开更多
Half-ducted fan and ducted fan have been designed and numerically analyzed for investigating the radial flow effect on the overall performance and the three dimensional flow field in design. Based on quasi-three dimen...Half-ducted fan and ducted fan have been designed and numerically analyzed for investigating the radial flow effect on the overall performance and the three dimensional flow field in design. Based on quasi-three dimensional flow theory, the meridional flow was calculated by adopting the radial balance equations, while the calculation of the blade to blade flow was obtained by 2D cascade data with the correction by a potential flow theory. Two types of axial flow fan were designed. One is the full ducted case as if it was in the straight pipe and another is the half-ducted case with the radial inflow and outflow. The previous experimental results of authors were used to decide the inclinations of both the inflow and outflow. And the circular arc blade with equal thickness was adopted. The numerical results indicate that both of the designed fans can reach the specified efficiency and also the efficiency surpasses more than 11%. Furthermore, the static pressure characteristic of half-ducted fan is much better than that of ducted fan. In comparison of the three dimensional internal flow of these two fans, the improvement of the flow angle at inlet and outlet, the distributions of velocity in the flow field and the pressure distributions on the blade surfaces can be achieved more successfully in accordance with the design intension on consideration of flow angle in design. The conclusion that half-ducted design with considering radial inflow and outflow is feasible and valid in comparison with ducted design for axial flow fans has been obtained at the end of the paper.展开更多
A study on the half-ducted axial flow fan designed by a diagonal flow fan design method was conducted. The rotor which has NACA65 blades was designed, calculated numerically, manufactured and tested experimentally. As...A study on the half-ducted axial flow fan designed by a diagonal flow fan design method was conducted. The rotor which has NACA65 blades was designed, calculated numerically, manufactured and tested experimentally. As a result of the design and CFD, the meridional streamline and three distributions of the meridional, tangential and radial velocity at inlet and outlet go well as designed values of the half ducted fan. On the other hand, the values of the meridional velocity and the tangential velocity are little smaller than the design values at the hub side of the radial distribution. The improvement of the design is prospected for this point, that is, the approach between the design value and the actual flow is prospected if the tangential velocity is assigned small at hub and is assigned large at the tip so as to accord the actual flow in the vortex design of the rotor blade. Then the designed half-ducted rotor with four NACA65 blades was fabricated by a three-dimensional printer and tested in the wind tunnel in order to validate the half-ducted design method. For the comparison between the design values and the experimental values at the design flow rate coefficient of φ = 0.264, the experimental values of the pressure rise coefficient ψ and the efficiency η are rather small than the design values, while the experimental value of the torque coefficient τ is almost the same as the design value. However, the experimental value of approximately 0.45 of the maximum efficiency is comparably large value considering for the limitation of the situation of half-ducted. For the comparison between the experimental values and the CFD values at φ = 0.264, the CFD values are almost the same values as the experimental values for all the values of ψ, τ and η. In addition, the tendencies of the CFD values when the flow rate coefficient changes are almost similar as the experimental tendencies, though the flow rate coefficient for the CFD values when ψ or η takes the peak value shifts toward larger flow rate. For the case at rotor outlet at φ = 0.264, two values of the meridional velocity and the tangential velocity are larger than the design values at the tip side of the radial distribution.展开更多
文摘An electrohydrodynamic (EHD) method, which is based on glow discharge plasma, is presented for flow control in an S-shaped duct. The research subject is an expanding channel with a constant width and a rectangular cross section. An equivalent divergence angle and basic function are introduced to build the three-dimensional model. Subsequently, the plasma physical models are simplified as the effects of electrical body force and work (done by the force) on the fluid near the wall. With the aid of FLUENT software, the source terms of momentum and energy are added to the Navier-Stokes equation. Finally, the original performance of three models (A, B and C) is studied, in which model A demonstrates better performance. Then EHD control based on model A is discussed. The results show that the EHD method is an effective way of reducing flow loss and improving uniformity at the duct exit. The innovation in this study is the assessment of the EHD control effect on the flow in an S-shaped duct. Both the parametric modeling of the S-shaped duct and the simplified models of plasma provide valuable information for future research on aircraft inlet ducts.
基金Project supported by the National Natural Science Foundation of China(Nos.11672164 and41831278)the Taishan Scholars Project Foundation of Shandong Province of China
文摘A numerical analysis is presented for the oscillatory flow of Maxwell fluid in a rectangular straight duct subjected to a simple harmonic periodic pressure gradient.The numerical solutions are obtained by a finite difference scheme method.The stability of this finite difference scheme method is discussed.The distributions of the velocity and phase difference are given numerically and graphically.The effects of the Reynolds number,relaxation time,and aspect ratio of the cross section on the oscillatory flow are investigated.The results show that when the relaxation time of the Maxwell model and the Reynolds number increase,the resonance phenomena for the distributions of the velocity and phase difference enhance.
基金supported by the National Natural Science Foundation of China(11371141 and 11871218)Science and Technology Commission of Shanghai Municipality(STCSM)under Grant No.18dz2271000
文摘We show existence of time-periodic supersonic solutions in a finite interval, after certain start-up time depending on the length of the interval, to the one space-dimensional isentropic compressible Euler equations, subjected to periodic boundary conditions. Both classical solutions and weak entropy solutions, as well as high-frequency limiting behavior are considered. The proofs depend on the theory of Cauchy problems of genuinely nonlinear hyperbolic systems of conservation laws.
基金supported by National Natural Science Foundation of China(No.51876221,51776225)High-end Foreign Expert Introduction Project(G20190001270,B18054)。
文摘The flow field in a semi-circular duct is simulated by Large Eddy Simulation(LES)and its particle field is simulated by Lagrange particle tracking method.Reynolds number Reb(based on bulk velocity and hydraulic diameter)is 80,000 and Ret(based on friction velocity and hydraulic diameter)is 3528.Particle diameter dpis chosen as 10,50,100,500 mm corresponding to St as 0.10,2.43,9.72,243.05.The results show that the intensity of the secondary flow near the ceiling is less than that near the floor because the ceiling is curved and able to inhibit the secondary flow.It is found that the difference between the semicircular duct and the square duct is that the secondary flow in a corner of the semi-circular duct is not symmetrical along the diagonal although they have the same generation mechanism.Regarding the particles,small particles(dp≤10 mm)are found to uniformly distribute in the duct,while large particles(dp≥50 mm)preferentially distribute in the corner and floor center.The maximum particles(dp=500mm)fall on the floor quickly and their dispersion mainly depends on the secondary flow near the floor.Particle deposition in the corner depends on particle size due to the effect of secondary flow and gravity.The effect of lift force on particles becomes more significant for 50 and 100 mm particles in comparison with other smaller particles.In the end,the effect of secondary flow is found to be more significant to dominate particle behavior than that of flow fluctuation.
基金partly supported by a grant from the Center of Excellence on Modeling and Control Systems (CEMCS) of Ferdowsi University of Mashhad,Iran
文摘The objective of this work is to estimate the accuracy of a predicted velocity profile which can be gained from experimental results, in comparison with the exact ones by the methodology of entropy generation. The analysis is concerned with the entropy generation rate in hydrodynamic, steady, laminar, and incompressible flow for Newtonian fluids in the insulated channels of arbitrary cross section. The entropy generation can be calculated from two local and overall techniques. Adaptation of the results of these techniques depends on the used velocity profile. Results express that in experimental works, whatever the values of local and overall entropy generation rates are close to each other, the results are more accuracy. In order to extent the subject, different geometries have been investigated. Also, the influence studied, and the distribution of volumetric geometries is drawn. of geometry on the entropy generation rate is local entropy generation rate for the selected geometries is drawn.
文摘The effects of the aspect ratio on unsteady solutions through the curved duct flow are studied numerically by a spectral based computational procedure with a temperature gradient between the vertical sidewalls for the Grashof number 100 ≤ Gr ≤ 2 000. The outer wall of the duct is heated while the inner wall is cooled and the top and bottom walls are adiabatic. In this paper, unsteady solutions are calculated by the time history analysis of the Nusselt number for the Dean numbers Dn = 100 and Dn = 500 and the aspect ratios 1≤γ≤ 3. Water is taken as a working fluid (Pr =7.0). It is found that at Dn = 100, there appears a steady-state solution for small or large Gr. For moderate Gr, however, the steady-state solution turns into the periodic solution if γ is increased. For Dn = 500, on the other hand, it is analyzed that the steady-state solution turns into the chaotic solution for small and large Gr for any γ lying in the range. For moderate Gr at Dn = 500, however, the steady-state flow turns into the chaotic flow through the periodic oscillating flow if the aspect ratio is increased.
文摘In this paper,to simulate the three dimensional turbulent flow in suddenly expanded rectangular duct numerically,the SIMPLEC algorithm is employed to solve the incompressible Navier-Stckes equation with k-εturbulence model.The numerical resulis show well the three dimensional turbulent flow field in the rectangular duct behind the sudden expansion cross-section. and agree.fairly well with the experimental result of the length of the main circumfluence.The numerical method of this paper can be applied to numerical analysis of this kind of turbulent flow.
文摘One-dimensional analytical theory is developed for supersonic duct flow with variation of cross section, wall friction, heat addition, and relations between the inlet and outlet flow parameters are obtained. By introducing a self- similar parameter, effects of heat releasing, wall friction, and change in cross section area on the flow can be normalized and a self-similar solution of the flow equations can be found. Based on the result of self-similar solution, the sufficient and necessary condition for the occurrence of thermal choking is derived. A re- lation of the maximum heat addition leading to thermal choking of the duct flow is derived as functions of area ratio, wall friction, and mass addition, which is an extension of the classic Rayleigh flow theory, where the effects of wall friction and mass addition are not considered. The present work is expected to provide fundamentals for developing an integral analytical theory for ramjets and scramjets.
文摘The fully developed slip flow in an annular sector duct is solved by expansions of eigenfunctions in the radial direction and boundary collocation on the straight sides. The method is efficient and accurate. The flow field for slip flow differs much from that of no-slip flow. The Poiseuille number increases with increased inner radius, opening angle, and decreases with slip.
文摘A numerical study is presented for the fully developed two-dimensional laminar flow of viscous incompressible fluid through a curved square duct for the constant curvature δ = 0.1. In this paper, a spectral-based computational algorithm is employed as the principal tool for the simulations, while a Chebyshev polynomial and collocation method as secondary tools. Numerical calculations are carried out over a wide range of the pressure gradient parameter, the Dean number, 100 ≤ Dn ≤ 3000 for the Grashof number, Gr, ranging from 100 to 2000. The outer wall of the duct is treated heated while the inner wall cooled, the top and bottom walls being adiabatic. The main concern of the present study is to find out the unsteady flow behavior i.e. whether the unsteady flow is steady-state, periodic, multi-periodic or chaotic, if Dn or Gr is increased. It is found that the unsteady flow is periodic for Dn = 1000 at Gr = 100 and 500 and at Dn = 2000, Gr = 2000 but steady-state otherwise. It is also found that for large values of Dn, for example Dn = 3000, the unsteady flow undergoes in the scenario “periodic→chaotic→periodic”, if Gr is increased. Typical contours of secondary flow patterns and temperature profiles are also obtained, and it is found that the unsteady flow consists of single-, two-, three- and four-vortex solutions. The present study also shows that there is a strong interaction between the heating-induced buoyancy force and the centrifugal force in a curved square passage that stimulates fluid mixing and consequently enhance heat transfer in the fluid.
文摘Combined effects of centrifugal and coriolis instability of the flow through a rotating curved duct with rectangular cross section have been studied numerically by using a spectral method, and covering a wide range of the Taylor number ?for a constant Dean number. The rotation of the duct about the center of curvature is imposed in the positive direction, and the effects of rotation (Coriolis force) on the flow characteristics are investigated. As a result, multiple branches of asymmetric steady solutions with two-, three-and multi-vortex solutions are obtained. To investigate the non-linear behavior of the unsteady solutions, time evolution calculations as well as power spectrum of the unsteady solutions are performed, and it is found that the unsteady flow undergoes through various flow instabilities in the scenario “chaotic?→ multi-periodic?→ periodic?→ steady-state”, if Tr is increased in the positive direction. The present results show the characteristics of both the secondary flow and axial flow distribution in the flow.
文摘A depth understanding of fluid flow past a curved duct having rectangular cross-section with different aspect ratios(l)are essential for various engineering applications such as in chemical,mechanical,biomechanical and bio-medical engineering.So highly ambitious researchers have given significant attention to study new characteristics of fluid flow in a curved duct.The flow characterization in the rectangular duct has been studied over a wide range of numerical and selective experimental studies.However,proper knowledge with the effects of Coriolis force for different aspect ratios is important for better understanding of the transitional behaviour and the subsequent heat generation,which is required to improve further.The purpose of this study is to reveal insight into the transitional flow pattern and heat transfer in a curved rectangular domain.The Navier-Stokes equations are solved using the spectral method,while the Crank-Nicolson method is used to solve the energy equation.An in-house FORTRAN code is developed to get the numerical solution.For post-processing purposes,Tecplot-360 and Ghost-script tools are used.The present study exposes development of Dean vortices that affect heat generation as well as thermal enhancement in the flow with underlying the flow controlling parameters,the Dean number(Dn),the Grashof number(Gr)and the Taylor number(Tr).Time-dependent results followed by phase spaces show that transient flow undergoes in the scenario‘chaotic→multi-periodic→periodic→steady-state’generating 2-to 8-vortices for the periodic/multi-periodic flow at 2000≤Tr≤2205 for l=2,whereas similar sort of flow is observed in the range of 3100≤Tr≤3195 for l=3.More complicated 4-to 13-vortex solutions are obtained for the chaotic flow regime at l=2 in the range of 0≤Tr<2200 and at l=3 in the range of 0≤Tr<3100.The chaotic flow that occurs at the certain range of Tr proficiently intensifies the heat transfer than the unperturbed,periodic or multi-periodic flow.The overall investigation reveals that in the rotating duct,the temperature-influenced buoyancy compulsion and centrifugal-coriolis joint forces are dominant,influencing the characteristic of the fluid and thus optimizing the transfer of heat.The present investigation will contribute to enhancing the understanding of fluid flow and heat transfer of internal heating/cooling/gas turbines,electric generators,biological systems,and some separation processes.
文摘Due to widespread applications of the bent ducts in engineering fields such as in chemical,mechanical,bio-mechanical and bio-medical engineering,scientists have paid considerable attention to invent new characteristics of fluid flow in a bent duct(BD).In the ongoing study,a spectral-based numerical technique is applied to explore flow characteristics and energy distribution through a loosely bent square duct(BSD)of small curvature.Flow is accelerated due to combined action of the non-dimensional parameters;the Grashof number Gr(=1000),the curvatureδ(=0.001),and the Prandtl number Pr(=7.0)over a wide domain of the Dean number 0<Dn≤5000.Fortran code is developed for the numerical computations and Tecplot software with Gost Script and Gost View is used for the post-processing purpose.The numerical study investigates steady solutions(SS)and as a result,a structure of six-branches of SSs composed of 2-to 6-vortex solutions is obtained.Then oscillating behavior with flow transition is discussed by obtaining timedependent solutions followed by power-spectrum analysis.Results showthat the trend of unsteady flow(UF)undergoes in the sequence‘steady-state→multi-periodic→steady-state→chaotic→multi-periodic→chaotic’,if Dn is increased.Asymmetric 2-to 4-vortex solutions are obtained for UF.Convective heat transfer(CHT)is then examined obtaining temperature gradients and energy contours,and it is found that CHT is significantly enhanced by the secondary flow(SF).The present study reveals that the role of secondary vortices over heat transfer(HT)is highly significant and HT occurs substantially for the chaotic solutions.Finally,for the interest of validation,the present numerical result is compared with the previously published experimental outcomes,and a good agreement is remarked.
文摘In this study,a large eddy simulation(LES)for fully-developed turbulent flows through a duct of regular-polygon cross-section using the immersed boundary(IB)method is performed.In case of the turbulent flow through the square duct,though there are some disagreements of the mean quantities related with the streamwise velocity among the present LES,the previous direct numerical simulation(DNS)and the LES without the IB method,and the present LES can reproduce the secondary flow of the DNS and LES.The LES result for ten types of regular-polygon duct shows that the secondary-flow speed decreases as the number of sides of the regular polygon n increases and that the secondary flow in case of the regular icosagon duct disappears like the turbulent pipe flow.In case of low n,the behavior of the turbulent structures near the side center is different from that near the vertex.
文摘Half-ducted fan and ducted fan have been designed and numerically analyzed for investigating the radial flow effect on the overall performance and the three dimensional flow field in design. Based on quasi-three dimensional flow theory, the meridional flow was calculated by adopting the radial balance equations, while the calculation of the blade to blade flow was obtained by 2D cascade data with the correction by a potential flow theory. Two types of axial flow fan were designed. One is the full ducted case as if it was in the straight pipe and another is the half-ducted case with the radial inflow and outflow. The previous experimental results of authors were used to decide the inclinations of both the inflow and outflow. And the circular arc blade with equal thickness was adopted. The numerical results indicate that both of the designed fans can reach the specified efficiency and also the efficiency surpasses more than 11%. Furthermore, the static pressure characteristic of half-ducted fan is much better than that of ducted fan. In comparison of the three dimensional internal flow of these two fans, the improvement of the flow angle at inlet and outlet, the distributions of velocity in the flow field and the pressure distributions on the blade surfaces can be achieved more successfully in accordance with the design intension on consideration of flow angle in design. The conclusion that half-ducted design with considering radial inflow and outflow is feasible and valid in comparison with ducted design for axial flow fans has been obtained at the end of the paper.
文摘A study on the half-ducted axial flow fan designed by a diagonal flow fan design method was conducted. The rotor which has NACA65 blades was designed, calculated numerically, manufactured and tested experimentally. As a result of the design and CFD, the meridional streamline and three distributions of the meridional, tangential and radial velocity at inlet and outlet go well as designed values of the half ducted fan. On the other hand, the values of the meridional velocity and the tangential velocity are little smaller than the design values at the hub side of the radial distribution. The improvement of the design is prospected for this point, that is, the approach between the design value and the actual flow is prospected if the tangential velocity is assigned small at hub and is assigned large at the tip so as to accord the actual flow in the vortex design of the rotor blade. Then the designed half-ducted rotor with four NACA65 blades was fabricated by a three-dimensional printer and tested in the wind tunnel in order to validate the half-ducted design method. For the comparison between the design values and the experimental values at the design flow rate coefficient of φ = 0.264, the experimental values of the pressure rise coefficient ψ and the efficiency η are rather small than the design values, while the experimental value of the torque coefficient τ is almost the same as the design value. However, the experimental value of approximately 0.45 of the maximum efficiency is comparably large value considering for the limitation of the situation of half-ducted. For the comparison between the experimental values and the CFD values at φ = 0.264, the CFD values are almost the same values as the experimental values for all the values of ψ, τ and η. In addition, the tendencies of the CFD values when the flow rate coefficient changes are almost similar as the experimental tendencies, though the flow rate coefficient for the CFD values when ψ or η takes the peak value shifts toward larger flow rate. For the case at rotor outlet at φ = 0.264, two values of the meridional velocity and the tangential velocity are larger than the design values at the tip side of the radial distribution.