随活塞同步振动下纳米流体的强化传热特性

The heat transfer enhancement characteristics of nanofluids under the condition of synchronous vibration with piston

内燃机工作过程中,燃烧产生的热能一部分传给燃烧室部件,传给燃烧室部件中的热能有一半以上传给活塞,由于内燃机热负荷不断提高,必须要对活塞进行有效冷却。当活塞功率密度超过0.3kW/cm2时,必须采用冷却油腔进行冷却。为揭示纳米流体在冷却油腔内的流动和传热特性,对不同种类纳米流体在定常和振动状态下直圆管中的物性、摩擦阻力和传热特性进行了实验研究。研究发现:未施加换热腔振动时,在纯净水中添加纳米颗粒后摩擦阻力系数在层流流域内略有上升,湍流范围内几乎没有变化,但是传热效果却大大增强,最优强化在层流-湍流转捩点附近出现且随着纳米体积浓度和导热系数的增大而增大。对换热腔施加随活塞同步振动后,传热强化与振动频率成正比、与雷诺数成反比;用纳米流体代替传统流体后传热效果大大增强,同时还发现纳米流体种类对强化效果影响显著。

In the whole working process of combustion engine,heat energy is produced and part of which transfers to combustion chamber.More than half of that energy transfers to the piston.Therefore,piston needs to be cooled efficiently with increase of thermal loading of combustion engine.When the power density of the piston exceeds 0.3kW/cm2,the piston must be cooled by cooling chamber.In order to reveal the flow and heat transfer characteristic of nanofluids in cooling chamber,experiments about physical properties,friction drag and heat transfer rates with different types of nanofluids were carried out in a straight circular pipe under steady and vibration state.It is found that,in the steady state,the friction factor of low mass concentration nanofluids is slightly increased for laminar flow by adding nanoparticles into purified water,while almost has no change for turbulent flow.However,the coefficient of heat transfer is enhanced.It is also found that the optimal enhancement appears nearby the turning point of laminar-turbulent flow and increases as mass concentration of nanofluids and coefficient of heat conductivity increase.When heat transfer chamber vibrates synchronously with piston,heat transfer enhancement coefficient is proportional to vibrational frequency and inversely to the Reynolds number.Using the nanofluids instead of purified water,heat transfer is significantly enhanced and the type of nanofluid has a major impact on the heat transfer effect.