生物磁流体传热的耗散粒子动力学模拟

Dissipative particle dynamics simulation of biomagnetic fluid flow and heat transfer

耗散粒子动力学(dissipative particle dynamics,DPD)是一种基于粒子的拉格朗日型介观方法,对生物磁流体等复杂流体流动与传热问题具有较大优势.如果考虑铁磁流体动力学(ferrohydrodynamic,FHD)效应和磁流体动力学(magnetohydrodynamic,MHD)效应,生物磁流体的流动传热问题非常复杂,因而引起了众多学者的关注.本文提出了一种简单可行的方案建立了DPD流体介观参数与真实流体宏观输运系数之间的联系.在此基础上,采用DPD方法模拟了半环形区域内生物磁流体的流动与传热问题并且考虑了磁数和哈特曼数对区域内温度场和流场的影响.结果表明,随着磁数的增加,热磁对流会产生3个热羽流和4个漩涡.此外,哈特曼数的增加会引起热源表面大部分区域的局部怒塞尔数的显著下降.

Dissipative particle dynamics is a Lagrangian particle-based mesoscopic technique with higher computation efficiency than microscopic numerical methods, originally conceived as a hybrid of lattice gas automata（LGA）, Brownian dynamics（BD） and molecular dynamics（MD）. As an improved version of this method, the energy-conservative dissipative particle dynamics（eDPD or DPD＋E） allows for temperature-variation in fluid system and has great potential to solve hydrodynamics problem involving heat transfer in complex fluids such as biomagnetic fluid, ferrofluid, eletrorheological fluid and so on. Considering the both effects of ferrohydrodynamic and magnetohydrodynamic, the hydrodynamics behavior of biomagnetic fluid is rather complicated and has received considerable attention from various researchers. In this work, the fluid flow and heat transfer in a semi-annulus filled with biomagnetic fluid in presence of an external magnetic field is investigated systematically by the eDPD method for different conditions. The flow is steady, two-dimensional, laminar and incompressible. To begin with, a simple and practicable strategy is proposed to matchup the mesoscopic parameters in eDPD model with the transport coefficients of real fluid because the eDPD formlation has no direct relation to transport coefficients. In detail, the viscosity and thermal diffusivity for the eDPD fluid are calculated by numerically simulating the isothermal Poiseuille flow and two-dimensional heat conduct with uniform heat generation, respectively. To study the magnetostatic interactions regardless of the induced magnetic field, a model concerned with the magnetic field is introduced. The Kelvin force and Lorentz force on DPD particles are computed in the same way as the buoyance force; the heat source induced by the external magnetic is replaced by imposing an external heat flux on the DPD particles. Then the eDPD model is first validated against the finite volume method（FVM） results for the natural convection at a given Rayleigh number and Prandtl number to access the accuracy of the eDPD simulations and it is found that the eDPD model predict the temperature and flow fields throughout the convection domains properly. Finally the effects of Magnetic number and Hartmann number, which arise from ferrohydrodynamics and magnetohydrodynamics respectively, have been examined and presented via temperature isotherms and streamlines. The obtained results simply reveal that the thermomagnetic convection is very different from buoyance-driven natural convection. Three thermal plume and four vortexes appears over the hot wall and heat transfer drop a lot near the middle zone as Magnetic number increases. It can be also found that the local Nusselt number near the hot wall significantly decrease as Hartmann number increases.