磁热场协同作用下的传热传质强化

通过三维非稳态动量和能量方程的模拟,对圆管通道内空气的磁热风作用机理进行了分析。得出了无量纲长径比为 20、30、62和100,带电线圈和管道的直径比从2到200时磁场密度的梯度的影响规律。磁场与温度场的协同作用改变了速度和温度边界层的分布,存在最优条件下的强化传热、传质效果。

Directional growth behavior of a(Al) dendrites during concentration-gradientcontrolled solidification process in static magnetic field

The large and small sized Cu（solid）/Al（liquid） couples were prepared to investigate the directional growth behavior of primary a（Al） phase during a concentration-gradient-controlled solidification process under various static magnetic fields（SMFs）.The results show that in the large couples,the α（Al） dendrites reveal a directional growth character whether without or with the SMF.However,the 12 T magnetic field induces regular growth,consistent deflection and the decrease of secondary arm spacing of the dendrites.In the small couples,the α（Al） dendrites still reveal a directional growth character to some extent with a SMF of ≤5 T.However,an 8.8 T SMF destroys the directional growth and induces severe random deflections of the dendrites.When the SMF increases to 12 T,the a（Al） dendrites become quite regular despite of the consistent deflection.The directional growth arises from the continuous long-range concentration gradient field built in the melt.The morphological modification is mainly related to the suppression of natural convections and the induction of thermoelectric magnetic convection by the SMF.

Effects of static magnetic fields on melt flow in detached solidification

A series of three-dimensional numerical computations were conducted to understand the effects of different static magnetic fields on thermal fluctuation and melt flow during the detached solidification of CdZnTe. Numerical calculations were carried out by three different configurations of magnetic field: without magnetic field, with an axial magnetic field (AMF) and with a cusp-shaped magnetic field (CMF). The results reveal that the magnetic fields can effectively suppress the melt flow and thermal fluctuation and the suppression effect of the AMF is stronger than that of the CMF. Besides, the physical mechanism of thermocapillary?buoyancy convection instability was discussed and the effects of magnetic field on the critical Marangoni number were also obtained.

Effects of geometric parameters and axial magnetic field on buoyant-thermocapillary convection during detached solidification

In order to understand the effect of geometric parameters and axial magnetic field on buoyant-thermocapillary convection during detached solidification, a series of threedimensional numerical simulations were conducted by the finite-difference method. The results indicate that the stable flow is observed when the Marangoni number （Ma） is small; however, when the value of Ma increases and exceeds a threshold value, the stable steady flow transits to be unstable flow. As the height of the melt increases, the flow is enhanced at first and then gets weakened. As the width of gap decreases gradually, the strength of flow is enhanced. The approach of using axial magnetic field is an effective way to suppress the buoyant-thermocapillary convection. As the magnetic field strength increases, the inhibition is enhanced. The critical Marangoni number increases slightly with a greater melt height, a narrower width of gap, and a more strength of magnetic field.

A nanofluid MHD flow with heat and mass transfers over a sheet by nonlinear boundary conditions: Heat and mass transfers enhancement

In this paper,we have numerically examined the steady boundary layer of a viscous incompressible nanofluid and its heat and mass transfers above a horizontal flat sheet.The boundary conditions considered were a nonlinear magnetic field,a nonlinear velocity and convection.Such nonlinearity in hydrodynamic and heat transfer boundary conditions and also in the magnetic field has not been addressed with the great details in the literature.In this investigation,both the Brownian motion and thermophoretic diffusion have been considered.A similarity solution is achieved and the resulting ordinary differential equations (nonlinear) are worked numerically out.Upon validation,the following hydrodynamic and heat and mass transfers parameters were found:the reduced Sherwood and Nusselt numbers,the reduced skin friction coefficient,and the temperature and nanoparticle volume fraction profiles.All these parameters are found affected by the Lewis,Biot and Prandtl numbers,the stretching,thermophoretic diffusion,Brownian motion and magnetic parameters.The detailed trends observed in this paper are carefully analyzed to provide useful design suggestions.

Calculation of optimal current density of double side permanent magnet linear synchronous motor

The use of design method considering a coil temperature to maximize the thrust density of a double side coreless permanent magnet linear synchronous motor(PMLSM) was presented.The optimal current density where the coil temperature reaches an allowable temperature with heat analysis was applied to a magnetic circuit design.Changing optimal current density is verified whenever the design parameters of the motor are altered.The design parameters of the motor were applied to thrust calculation.In this way,the optimal model,which is a reversal of the existing design method,is deduced.The results were compared with the experimental data to verify their validity.When the convection heat transfer coefficient is applied to other models,the results of the analysis and test values show good concordance.The method proposed has some limitations.

Combined Effect of Magnetic Field and Thermal Dispersion on a Non-Darcy Mixed Convection

This paper is devoted to investigate the influences of thermal dispersion and magnetic field on a hot semi-infinite vertical porous plate embedded in a saturated Darcy-Forchheimer-Brinkman porous medium. The coefficient of thermal diffusivity has been assumed to be the sum of the molecular diffusivity and the dynamic diffusivity due to mechanical dispersion. The effects of transverse magnetic field parameter (Hartmann number Ha), Reynolds number Re (different velocities), Prandtl number Pr (different types of fluids) and dispersion parameter on the wall shear stress and the heat transfer rate are discussed.