Wake Evolution From Magnetic Obstacle And Its Effects On Flow Control Around Bluff Obstacle And Heat Transfer

Electrically conducting fluid flow under an external non-uniform local magnetic field will be retarded by the Lorentz force. The non-contact force has been widely utilized in the industrial applications including metallurgy(such as electromagnetic stirring) and flow measurement(such as Lorentz force velocimetry). In this thesis, the characteristics of flow and heat transfer of electrically conducting fluid under non-uniform local magnetic field were investigated and the vortices structure of magnetic obstacle, wake evolution and the influence on flow and heat transfer were studied numerically and compared with existent experimental results. Furthermore, a magnetic obstacle was set upstream to investigate its impact on flow around a bluff obstacle and convection heat transfer. The detailed content and results are listed as follows:Firstly, the characteristics of flow and heat transfer of an electrically conducting fluid flow under a non-uniform local magnetic field in a rectangular duct were investigated by numerical simulation. When the magnetic constrainment factor(κ), interaction parameter(N) and Reynolds number(Re) were appropriate, the flow shows different regimes: one pair of inner magnetic vortices between the magnetic poles(N=9,Re=100 and κ=0.4) or steady six-vortex pattern involving inner magnetic vortices, connecting vortices and attached vortices(N=9,Re=400 and κ=0.4), respectively, which is similar to the flow around a bluff obstacle, such as a circular cylinder, called a magnetic obstacle. If the interaction parameter and Reynolds number are fixed, the steady vortical flow will be replaced by the bifurcate flow as magnetic constrainment factor increases. As Reynolods number increases(such as Re=500), the attached vortices will come off the magnetic obstacle alternately, and form the von Karman vortex street in the wake of magnetic obstacle. As blockage ratio(β) increases the vortex shedding frequency increases, and the connecting vortices may be disappear when β>0.2. If the duct spanwise width(Ly) is enough(such as β≤0.2), vortex shedding frequency is almost not influenced by κ, N and Re, and the connecting vortices are stable. The wake instability of magnetic obstacle is more favorable on the heat transfer than the stable vortical flow. The heat transfer increases as the κ increases, and decreases as the β increases while flow is instability.(The reviewers of ASME Journal of Heat Transfer and International Communications in Heat and Mass Transfer evaluate that this kind of flow has been analyzed in the past but still is far from being completely understood. In particular, to the best of my knowledge, the heat transfer problem of this flow has not been analyzed before, so that results of this paper are valuable.)Secondly, the effects of Reynolds number, separation ratio(ξ) and interaction parameter on the flow around a row of four magnetic obstacles, interaction of two continuous magnetic obstacles and convection heat transfer were investigated numerically. The vortex structures and the wake evolution of magnetic obstacles are similar to that of single magnetic obstacle as Re increases. The interaction between wake vortical flows decreases as Re decreases and ξ increases. When the interaction is weak, the vortex shedding mode is similar to that of single magnetic obstacle. If the inertia force is weak(such as Re=600), the wake of magnetic obstacles will become stable with an increasing in N. In contrast, the wake vortex of two in-between magnetic obstacles will overspread and disorder the wake region. Consequently, there is no dominant frequency observed in the corresponding power spectra at Re=900. In comparison with single magnetic obstacle, multiple magnetic obstacles can effectively enhance the convection heat transfer. Moreover, the increment of heat transfer increases as N and Re increase, and increases as ξ decreases. For fixed Re and N, the total heat transfer performance has a optimum value at ξ=2.(The reviewers of International Communications in Heat and Mass Transfer evaluate that the authors present an interesting study of flow and heat transfer in a duct with an array of magnetic obstacles(side-by-side) for various separations among them. The paper provides useful information related to the dynamic and thermal behavior of the flow.)Thirdly, the effect of magnetic obstacle on the characteristics of flow and heat transfer around a circular or square cylinder was investigated, and their difference was also analyzed. Two flow patterns without(the cavity mode) and with vortex shedding(the wake impinge mode) from the magnetic obstacle occur. The cavity mode occurs at small spacing ratio, and as spacing ratio increases the wake impinge mode occurs. The magnetic obstacle can effectively reduce the drag coefficient of circular cylinder and square cylinder for two modes, and the optimum condition of the drag reduction is the cavity mode. Tthe additional pressure drop penalty incurred by the magnetic obstacle is influenced by obstacle shape. For the square cylinder, the drag reduction is more than pressure drop penalty. Because the convection heat transfer between windward surface and low temperature fluid decreases, the total heat transfer of circular and square cylinder is weak than that of single cylinder.