Dissipative Particle Dynamics Study Of Congjugate Heat Transfer And Its Algorithm On Outflow Boundary Conditions

Numerical simulation is an important way to study fluid flows and heat transfer.In recent years,micro and nano fluidic devices have been widely adopted in Medical Science,Bio-engineering and Chemistry Engineering.The accompanying micro-and nano-scale fluid flows have also become a very hot research topic due to their enormous significance either in science or engineering.Compared with fluid flows based on continuity assumption,micro-and nano-scale fluid flows behave with its own diversities.On the one side,fluid flows are influenced by velocity-slip,capillary effects,fluctuations,rarefaction effects,and so on;On the other hand,the kinds of fluid may be very complicated,such as DNA,blood cells,suspended matter,macromolecular,and so on.In some extreme cases,micro-and nano-scale fluid flows are often affected by chemical reaction and other physical fields like electrical fields and magnetic fields.For the above reasons,methods based on solving Navier-Stoke equations become unsuitable for simulations of micro-and nano-scale fluid flows.A multi-scale numerical method is highly demanded.Dissipative particle dynamics is full of potential for these small-scale fluid flows.Although it does not solve the Navier-Stoke equations,it is able to simulate macro-,mesoscopic-,micro-scale fluid flows simultaneously.Therefore,this article study both micro-scale and macro-scale fluid-solid conjugate heat transfer problems using dissipative particle dynamics;it also deals with the boundary conditions of open flows,which are a drawback and difficult problem for particle-based method like dissipative particle dynamics.Firstly,this article systematically introduces the foundation of dissipative particle dynamics in terms of simulations of fluid flows and heat transfer,which forces on the explanations of governing equations.Besides,this article explains to us the programming procedures of dissipative particle dynamics including how to choose parameters,how to deal with basic boundary conditions and how to integrate the governing equations.In this section,a new scheme to compute the corresponding properties of fluids from the mesoscopic parameters is proposed.Meanwhile,a comparative study between dissipative particle dynamics and smoothed particle hydrodynamics is made through a simulation of the unsteady Poiseuille flow.Fluid-solid conjugate heat transfer is a common phenomenon in the thermo-fluid engineering fields.And conjugate heat transfer can be viewed as the ‘fourth-type heat transfer boundary conditions'.This is because the temperatures and fluxes at the fluidsolid interface cannot be prescribed before simulations but they are a part of the simulations.Different conjugate heat transfer problems may be at different scales.For example,micro-channel forced convection need to consider the heat conduction in the walls,which is one of the micro-scale conjugate heat transfer problems.However,the influence of conducting body on natural convection belongs to the group of macro-scale conjugate heat transfer problems.Past studies on conjugate heat transfer use finite volume method,finite difference method and lattice Boltzmann method.This article firstly adopts dissipative particle dynamics to study conjugate heat transfer problems.Based on the idea of simulating conjugate heat transfer using finite volume method,a dissipative particle dynamics algorithm for fluid-solid conjugate heat transfer is proposed.Then,microchannel conjugate heat transfer and natural convection in a square with one or four heat conducting bodies inside are simulated.In the former case,if the ratio of heat conductivities between solid and fluid is high enough,we can simply ignore the thickness of walls.In the latter case,the value of heating source and the ratio of heat conductivities between solid and fluid have different influences on velocity fields and temperature fields.When the amplitude of heating source reaches a certain value,an extra circular cell appears near the right side of conducting body.And if the ratio increases,the Nusselt number in the wall with high temperature will decrease.How to deal with the open channel flow is both a fundamental and difficult problem for dissipative particle dynamics.While studies on micro-and nano-channel flows are of high value,existing research on channel flows using dissipative particle dynamics adopts periodic boundary conditions to evade the dealing with of fluid inflow and outflow boundary conditions.This has limited the application of this method greatly.On the other hand,many applications of dissipative particle dynamics on heat transfer problems have concentrated on problems in closed cavities.In contrast,little attention has been paid to heat transfer in channels,which is due to the lack of efficient schemes for inflow and outflow boundary conditions.So this article summarizes past schemes and proposes a simple but efficient algorithm for inflow and outflow boundary conditions for dissipative particle dynamics based on past works.Two cases like developing flows in a channel and back-step flows are used to check the correctness of this algorithm.Then,using this algorithm,slip flows in hydrophobic channels and conjugate heat transfer in channels with fluid inflow at uniform velocities are simulated.In the former case,it is found that hydrophobicity has no effects on length of entrance region.And slip lengths decrease firstly and then remain unchanged along with flow directions.In the latter case,averaged flow rates affect temperature distributions and fluid will be heated more quickly as the ratio of heat conductivities between solid and fluid increases.Dissipative particle dynamics is a reliable multi-scale numerical method.It has good efficiency and accuracy in simulations of macro-scale problems.Meanwhile,it poses advantages by not solving the Navier-Stoke equations for fluid flows;this particle-based method is also easy to model complex fluids like blood cells,DNA and other mesoscopic fluids.Contents of this article concentrate on proposing new algorithms for dissipative particle dynamics and thus will improve the application ability of this method.