纳米流体强化内燃机活塞冷却油腔传热的基础研究

发布时间：2018-03-19 18:43

本文选题：纳米流体　切入点：多相流模型　出处：《大连理工大学》2015年博士论文　论文类型：学位论文

【摘要】：随着发动机强化程度的不断提高,活塞承受的热负荷急剧增加。单纯增大或改进活塞振荡冷却油腔的结构,已经不能满足一些工况下的散热需求,因此需要选择一种换热性能更好的冷却工质。纳米流体具有十分优异的传热特性,如果能够将其作为振荡冷却油腔的工作介质,则可以有效降低活塞的整体温度。然而在流动切向往复振荡作用下,冷却油腔内部的机油和空气形成的气液两相混合物,与壁面冲击传热的过程十分复杂。涉及到气液两相流体的撞击、分离、再附着等振荡混合过程,其机理还没有完全搞清楚。现阶段还没有一套较为完整的理论,对这种状态下气液两相流的流动与传热过程进行准确地描述。当采用纳米流体作为活塞冷却油腔的换热介质后,此时的气液两相流(空气和机油)变成了固气液三相流(纳米颗粒、空气、机油),流动与换热过程更加复杂,从而限制了纳米流体在活塞冷却油腔中的应用。为此,有必要对纳米流体常规条件下的流动与传热特性进行深入探索,通过基础研究来揭示纳米流体强化传热的物理机制,完善纳米流体的数学物理模型。在此基础上,进一步考察冲击射流和往复振荡条件下,纳米流体的流动与传热特性,最后再将其应用于活塞振荡冷却油腔的散热过程。本文采用实验与数值模拟相结合的手段对纳米流体进行研究,从微流动的角度解释了纳米流体强化换热的物理机制,解决了纳米流体强化活塞冷却油腔传热的基础问题,丰富了往复振荡条件下气液两相流以及固气液三相流强化传热的理论体系,具有较为重要的理论研究意义和工程实际应用价值。具体的研究工作如下：1.从固液两相的动量守恒方程出发,推导了适用于纳米颗粒的虚拟粘度模型。该模型精度明显好于基于高浓度、大尺度粒子流的经验公式和基于分子运动论的推导公式,可以为纳米流体的Eulerian模型提供参考；2.通过比较不同模型,考察了纳米流体的湍流流动特性。结果表明：除了物性参数的改变外,相间的速度滑移所引起的脉动速度、雷诺应力和小尺度涡量的增加,以及纳米颗粒的迁移效应所引起的物性参数随空间的非均匀性分布,也是纳米流体强化传热的重要因素；3.考察了不同喷射距离条件下,纳米流体的射流冲击换热特性。探讨了不同湍流模型、壁面函数和多相流模型的预测能力,分析了纳米颗粒加入后对基础流场的影响；4.建立了与流动方向垂直的往复振荡作用下,气液两相流(空气+基础流体)和固气液三相流(纳米颗粒+空气+基础流体)在活塞冷却油腔内部与壁面传热的数理模型。发现壁面剪切应力、流体平均速度和纳米颗粒浓度共同决定了振荡过程的换热效果。并定量提出了往复振荡条件下,强化换热的判定准则；5.将纳米流体应用于实际内燃机活塞的散热过程,研究了不同种类、不同浓度纳米流体在冷却油腔内部的瞬时流动和换热特性。采用纳米流体作为活塞冷却油腔的换热介质后,可以有效降低活塞的整体热负荷。以体积分数为1%和2%的铜-机油纳米流体为例,相对于传统机油,一个振荡周期内的平均对流换热系数分别提高了24.93%和35.79%。
[Abstract]:With the continuous improvement of the degree of enhancement of the engine, the heat load increases dramatically. The piston structure simply increase or improve the piston oscillation cooling oil chamber, has been unable to meet the cooling demand of some conditions, therefore need to select a better heat transfer performance of refrigerant cooling. Nano fluid has excellent heat transfer properties, if it can be as the working medium oscillating cooling oil cavity, it can effectively reduce the overall temperature of the piston. However in tangential flow reciprocating oscillation, gas-liquid two-phase mixture cooling oil chamber inside the oil and air formation, and wall impingement heat transfer is very complex. Involves gas-liquid separation, impact the reattachment of oscillation mixing process, its mechanism is not entirely clear. At present there is not a complete theory, on the flow and heat transfer of gas-liquid two-phase flow in this condition To accurately describe the process. When using nano fluid as a heat transfer medium piston cooling oil chamber, the gas-liquid two-phase flow (air and oil) into a liquid solid three-phase flow (nano particles, air, oil), flow and heat transfer process is more complicated, which limits the application of nanofluids in piston cooling oil in the cavity. Therefore, it is necessary to conduct in-depth exploration of the flow and heat transfer characteristics of nanofluids under normal conditions, the physical mechanism of heat transfer enhancement of nanofluids to reveal through basic research, improve the physical and mathematical model of nano fluids. On this basis, further investigation of impinging jet and vibration conditions, flow and heat transfer characteristics of nano the fluid, the heat transfer process of its application in piston cooling oil chamber. By combining experimental and numerical simulation method for the study of nano fluids, from micro flow angle To explain the physical mechanism of nano fluid heat transfer enhancement, solve the basic problem to strengthen the piston cooling oil chamber heat transfer nanofluids, enrich the gas-liquid two-phase flow under the conditions of the theoretical system of reciprocating oscillation and heat transfer enhancement of solid gas liquid three-phase flow, and has important theoretical significance and practical value in engineering application. The main research is as follows 1.: from the momentum equation of solid-liquid two-phase of virtual viscosity model applicable to nano particles was deduced. The accuracy of the model is significantly better than the empirical formula based on high concentration, large scale particle flow and based on the formula of molecular motion theory, can provide a reference for the Eulerian model of nanofluids; 2. through the comparison of different models. Study the turbulent flow characteristics of nanofluids. The results showed that: in addition to physical parameters change, fluctuating velocity and slip velocity caused by the Reynolds stress Increased stress and small scale vortex, non uniform distribution of physical parameters caused by the effect of nano particles and migration with the space, it is also an important factor in nano fluid heat transfer enhancement; 3. were investigated under the condition of jet distance, jet impingement heat transfer characteristics of nanofluids. Discussed the different turbulence model, forecast the ability of wall function and multiphase flow model, analyzes the influence on flow field based nano particles; 4. built reciprocating oscillation perpendicular to the direction of flow, gas-liquid two-phase flow (air + fluid) and liquid solid three-phase flow (air + + nano particle based fluid) in the mathematical model of heat transfer of piston the internal cooling oil chamber and the wall surface. It is found that the wall shear stress, flow velocity and the concentration of nano particles determines the effect of heat transfer and the reciprocating oscillation process. The oscillation conditions are presented quantitatively, heat transfer enhancement judgment 5. set standards; nanofluids applied to internal combustion engine piston cooling process of different kinds and different concentrations of nanofluids in instantaneous internal cooling oil cavity flow and heat transfer characteristics. The use of nano fluid as a heat transfer medium piston cooling oil cavity, can effectively reduce the overall low heat load by volume of the piston. Fraction 1% and 2% copper oil nano fluid as an example, compared with the traditional oil, the average convection of an oscillation period of heat transfer coefficient was increased by 24.93% and 35.79%.

【学位授予单位】：大连理工大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TK403

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