Mechanism Of Boiling Heat Transfer Enhancement In Nanofluids By Molecular Dynamics Simulation

Good cooling performance of cooling water cavity of cylinder head has significant effect on the reliability,economical efficiency and power performance of internal combustion engine.Forced convection heat transfer is the main heat transfer in the cooling water cavity,whereas in the valve bridge of cylinder head with the highest temperature and great heat flux,boiling heat transfer is the main heat transfer.In recent years,due to the good heat transfer performance of nanofluids,researchers have tried to enhance the heat transfer in the cooling water cavity using nanofluids.So far,the enhancement of nano fluids on forced convective heat transfer has been widely reported and confirmed,however,there are still contradictions and controversies on the results of boiling heat transfer of nano fluids.Most of the traditional study methods speculate the mechanism of boiling heat transfer in nanofluids from the macroscopic scale,which is lack of theoretical basis.Therefore,new approach for the microscopic study of the boiling heat transfer of nanofluids is in badly need.Molecular dynamics(MD)simulation can be used to accurately describe the structural features of nanofluids from the microscopic view.MD method can be used to separate out the factors in the boiling heat transfer of nanofluids and study them respectively,thereby revealing the mechanism of boiling heat transfer effectively,which provides a theoretical foundation for the application of nanofluids on the heat transfer enhancement in the internal combustion engine.In this paper,the suspended and deposited nanoparticles in the base liquid were firstly dispersed,and MD method was employed to investigate the effects of suspended and deposited nanoparticles on pool boiling respectively.Then,the dispersed nanoparticles were integrated to study the pool boiling behaviors of nanofluids.Furthermore,referring to the sub-cooled flow boiling in the,valve bridge of cylinder head,the flow boiling behaviors of nanofluids were investigated.Finally,the mechanisms of nanofluids in the improvement of boiling heat transfer and the feasibility of its application in the cooling water cavity were revealed.The main research contents were described as follows:(1)In pool boiling,the effects of suspended nanoparticles on boiling heat transfer were investigated.Simulation models were established to investigate the influences of far-surface and near-surface nanoparticles on explosive boiling and evaporation.Two surface conditions were considered:smooth surface and uneven surface.Then,the effects of heating temperature and nanoparticle's type,size and concentration on boiling heat transfer were studied.Results showed that boiling heat transfer was enhanced by suspended nanoparticles,and the increment in near-surface nanoparticles was larger than that in far-surface nanoparticles.The far-surface nanoparticle moved with fluids,and its micromotion enhanced the heat transfer inside fluids at the evaporation stage.However,the near-surface nanoparticle was absorbed on the non-evaporated liquid film and did not move with fluids,but its temperature quickly reached the heating temperature,which became a new heating source to transmit heat to fluids.(2)In pool boiling,the effects of deposited nanoparticles on boiling heat transfer were investigated.Simulation models were established to investigate the effects of deposited nanoparticles on boiling heat transfer.Firstly,the droplet contact angle was investigated.Results showed that the contact angle was reduced by deposited nanoparticles,and thus the effects of deposited nanoparticles on the surface wettability were initially verified.Then,the effects of deposited nanoparticles on boiling heat transfer were studied.Results showed that fluids responded to explosive boiling faster due to the deposited nanoparticles,which indicated that the time of bubble nucleation was advanced leading to the enhancement of thermal convection near the surface.Moreover,the surface wettability and the thermal matching at the solid-liquid interface were improved by deposited nanoparticles,and thus the heat transfer efficiency between the surface and fluids was increased.Furthermore,surfactants had significant influence on the wettability of nanoparticles,and results showed that the enhancement of boiling heat transfer was increased with the increasing wettability of deposited nanoparticles.(3)The boiling behaviors of nanofluids were investigated,including saturated pool boiling,saturated flow boiling and sub-cooled flow boiling.In saturated pool boiling and flow boiling,the effects of flow velocity and heating temperature on boiling heat transfer were investigated.Results showed that nanofluids exhibited a faster response to explosive boiling and a larger heat flux than the base fluid.With an increase in heating temperature,the enhancement of heat transfer was increased by nanofluids,and the increment in flow boiling was larger than that in pool boiling.Furthermore,the effect of flow velocity on flow boiling reduced with the increasing heating temperature,but the deposition of near-surface nanoparticles could be avoided by increasing the flow velocity appropriately.Due to the sub-cooled flow boiling in the valve bridge of cylinder head,the sub-cooled flow boiling behaviors of nanofluids were studied.Taking water as the base liquid and referring to the temperature in the valve bridge of cylinder head,the research scope of heating temperature was further expanded.Results showed that heat flux reached the CHF with the increasing heating temperature,and the CHF of nanofluids increased about 70%.However,for the application of nanofluids in internal combustion engine,it is necessary to ensure that nanofluids can enhance boiling heat transfer,which depends on the relationship between the deterioration effect of nanoparticle sediments on heat transfer and other enhancement effects on heat transfer.In the cooling water cavity,due to the flow velocity and surfactants,nanoparticles do not deposite onto the surface continually so the thickness of sediments will not continue to increase.Compared with other enhancement effects,the deterioration effect of thermal resistance of sediments on heat transfer is relatively small in the total heat exchange amount,and thus nano fluids can enhance the boiling heat transfer in the cooling water cavity.Therefore,the application of nanofluids in the cooling water cavity of internal combustion engine is feasible.