内燃机活塞内冷油腔内两相流的流动与换热机理研究

发布时间：2018-06-28 16:14

本文选题：内冷油腔 + 两相流　；参考：《山东大学》2017年博士论文

【摘要】：随着内燃机强化程度的不断提高,活塞热负荷问题凸显。解决活塞热负荷问题已成为内燃机强化提高的瓶颈问题。采用活塞强化冷却技术是解决该问题最有效的手段之一。目前,内燃机活塞普遍采用内冷油腔结构,冷却油与空气形成的两相流在油腔里不断振荡可以形成强化换热的效果。因此,内冷油腔内两相流流动和传热已成为近年来研究的热点,但目前缺乏机理研究。本文致力于活塞的传热系统研究,利用计算流体学基础和活塞温度场有限元分析系统,在大量试验数据的基础上创新性的提出了油腔中两相流循环特性的若干假设,并建立了喷嘴的喷油模型;通过开发专门的瞬态机油打靶可视化试验平台,将试验结果结合有限元模拟、CFD模拟、动力学分析及推导计算等方法确定出两相流体传热的初始条件和边界条件,提出了油腔内两相流振荡流动特性的主要影响因素,建立了带有修正项的对流换热准则关联式,揭示了内冷油腔内两相流的流动与换热的机理,从而为活塞冷却系统的设计优化提供了直接可靠的计算依据,并为发动机性能的提升奠定了理论基础。论文的主要研究工作如下:1.内冷油腔冷却喷嘴射流特性研究通过搭建喷嘴喷流试验台,对不同喷油压力和喷油温度时的冷却喷嘴喷流进行了试验研究,得出了内冷油腔进油口捕捉率与喷嘴出口到喷流截面距离之间的关系。研究发现:喷油压力越大,喷油温度越高,喷油油束的发散角越大,从而影响内冷油腔的回流量。另外根据试验数据计算结果显示,冷却喷嘴喷流为层流射流。在试验研究的基础上对喷嘴喷流进行仿真计算分析,探讨喷嘴喷流压力、喷流速度和喷流距离、喷流半径等之间的关系。模拟结果显示:喷口附近核心区速度的最大值在偏移中心0.2-0.6mm的地方,且随着压力增加,核心区速度最大值的速度梯度增大;随喷流截面和喷口距离的增加,截面轴心速度的作用越来越小,喷流径向截面速度的衰减越来越平缓,且喷流截面速度随喷流径向截面位置的改变,其分布规律具有相似性;流体喷入空气后,距离喷嘴0～28.6mm内,喷流速度随距离增加而逐渐衰减,随着射流的进一步发展,在距离喷口 28.6mm附近,机油与空气进行的质量和能量剧烈交换,截面轴心速度衰减加剧,造成喷流速度急剧下降。另外,研究显示喷嘴喷油量对内冷油腔环形腔内的静态填充有很大的影响。最后,建立了冷却液由喷嘴喷出到油腔入口段的集束层流非淹没射流模型,得到了流速、流量、喷油压力、喷嘴半径以及扩展角在各截面处的关系以及截面轴心速度与喷流距离等的变化规律。2.内冷油腔内两相流动态特性的试验研究为了揭示各因素对两相流流型的影响程度及相应的流型转换机制,采用试验手段对两相流的流动形态进行了探讨。考虑活塞运动对流动形态的影响,对内冷油腔内两相流流动形态的形成及转变进行理论分析,提出了面积覆盖率的概念,建立了两相流流动形态的预测模型,并对内冷油腔内两相流的传热强度进行判定。开发了一种动态可视化打靶试验台,可以实时监测内冷油腔内流体的流动形态。通过拍摄设备直接观测内冷油腔内两相流的流动形态,对比出不同转速、喷流压力、喷流温度、静态填充率、内冷油腔大小和截面形状等条件下内冷油腔内两相流流动形态的变化。结果显示:发动机转速较低时,内冷油腔内两相流以波状流为主,随着发动机转速增加,“液塞”现象越明显,发动机转速越高,机油振荡越剧烈,内冷油腔内两相流流动形态越复杂;喷油温度的不同直接导致了机油粘度的改变。随着机油粘度增大,内冷油腔内流型转变加速,使得内冷油腔内出现“液塞”现象所需的发动机转速降低;内冷油腔的截面形状对其运动形态影响较大,腰形内冷油腔内的两相流分布比较规律,内冷油腔上行时,右侧的机油比较早的撞向油腔顶部,下行时,则是左侧比较早的撞向底部,椭圆形内冷油腔内的机油,形成“液塞”趋势的位置比较多,而水滴形内冷油腔下行时,大多数循环中都是靠近内冷油腔进出油口的机油先向下形成“液塞”,从而中间形成一个比较大的空气区。相比之下,喷油压力对油腔内的流动影响可忽略不计。综合来看,内冷油腔腔内两相流流动形态的主要影响因素为填充率、油腔形状和发动机转速。3.内冷油腔内两相流动态特性的数值研究通过CFD计算模型对内冷油腔内两相流的流动进行仿真计算,对比了不同发动机转速、喷流压力、喷流温度、内冷油腔大小和截面形状等条件下,内冷油腔内两相流的面积覆盖率及传热特性,仿真结果与试验结果吻合良好。结果表明:内冷油腔上下壁面的传热系数随曲轴转角的变化规律相反,内外壁面的传热规律则呈现一致性;喷油温度不同,机油的黏温特性不同,从而影响内冷油腔内的面积覆盖率及其传热特性;转速不同,机油在内冷油腔内的振荡强度不同,从而改变内冷油腔内的湍流强度及其变传热特性;油腔结构不同,直接影响机油填充率及其在往复运动时对油腔壁面的冲刷程度,从而影响换热效果。4.内冷油腔综合传热模型的建立从工程应用的角度出发,结合试验研究和数值模拟结果,利用管内强制对流基础关联式,在努谢尔特、普朗特和雷诺准则的基础上运用最小二乘法拟合出带有修正项的准则关联式,建立了瞬时对流传热系数的预测模型。模型建立过程中发现:对于内燃机活塞内冷油腔,发动机额定转速内随着发动机转速的提高,流体的粘性底层厚度减小;不同缸径任意发动机转速时,粘性底层厚度都大于内冷油腔管壁的粗糙度;相同发动机转速时,随缸径增加,粘性底层厚度减小;雷诺数和普朗特数乘积的自然对数值仅和发动机转速有关,而且随其增加而增大。通过数值法对假设条件、忽略因素、非稳定性等进行影响程度分析,并通过有限元分析结合硬度塞测温试验对关联式计算得到的传热系数进行误差分析,确保计算方法、结果的准确性和适用性。结果显示,内冷油腔传热系数预测模型可以有效预测不同发动机转速、缸径、机油温度时活塞内冷油腔内流体的对流传热系数,可为活塞内冷油腔的设计提供理论基础。结合试验研究和数值模拟结果还可以得知,内冷油腔往复运动时,不同曲轴转角时的传热系数可以在此关联式的基础上通过壁面的面积覆盖率进行修正。5.内冷油腔的换热特性对活塞可靠性的影响结合硬度塞测温试验,利用有限元分析软件、疲劳分析软件和动力学分析软件,模拟内冷油腔对活塞热负荷的影响,分析探讨了油腔位置对活塞热负荷的影响,内冷油腔的设置对活塞二阶运动的影响,以及镶圈内冷一体新型活塞结构对活塞强度的影响。冷却效果证明内冷油腔的使用可大大降低整个活塞的温度,而且在结构强度允许的范围内,内冷油腔在活塞头部的位置越高,活塞头部冷却效果越好,热负荷越低。研究还发现,内冷油腔的使用可降低活塞运行过程中的变形量,并减小活塞与缸套之间的作用力,从而明显改善活塞的摩擦磨损、侧向力及裙部压力等。但是,活塞的二阶运动平稳性会有所降低,同时也会增大活塞的敲击噪声,因此还需要对活塞的裙部型线进行相应的优化。对新型内冷油腔结构的研究发现,镶圈内冷一体的结构能更好的避免应力集中现象,既可以通过内冷油腔降低整个活塞的热负荷,又可以通过使用镶圈来提高环槽的耐磨性以及第一环槽和燃烧室的强度。综合以上研究可以发现,内冷油腔的传热效率影响了活塞的热负荷,内冷油腔内两相流的流动形态与其传热规律有密切联系。发动机转速、喷油压力、喷油温度、内冷油腔截面大小等影响因素直接或者间接的影响了两相流的流动形态,两相流的流动形态直接反映了两相流的分布,决定了液相对内冷油腔的有效冲刷面积,表征了传热的强度大小。另外,由于模拟计算中没有考虑轴向平面中气液界面的变化,忽略了气液两相流交替振荡带走的热量,因此数值模拟结果与本文所总结关联式的结果相比偏低。
[Abstract]:With the increasing enhancement of internal combustion engine, the problem of piston heat load is highlighted. To solve the problem of piston heat load has become the bottleneck of strengthening the internal combustion engine. The piston intensive cooling technology is one of the most effective methods to solve this problem. The continuous oscillation of two phase flow in the oil chamber can form the effect of enhanced heat transfer. Therefore, the flow and heat transfer of the two phase flow in the internal cooling oil have become a hot spot in recent years. But there is a lack of mechanism research at present. This paper is devoted to the research of the heat transfer system of the piston, and the finite element analysis system of the computational fluid foundation and the piston temperature field. On the basis of the experimental data, some assumptions about the circulation characteristics of the two phase flow in the oil cavity are put forward, and the injection model of the nozzle is established. By developing a special transient oil shooting visual test platform, the experimental results are combined with the finite element simulation, the CFD simulation, the dynamic analysis and the deduction calculation to determine the heat transfer of the two-phase fluid. The initial conditions and boundary conditions are the main factors affecting the oscillation flow characteristics of the two phase flow in the oil cavity. The correlation formula of the convective heat transfer criterion with the correction term is set up. The mechanism of the flow and heat transfer of the two phase flow in the internal cooling oil cavity is revealed, which provides a direct and reliable basis for the design and optimization of the piston cooling system. The main research work of the motive performance is laid out. The main research work of the paper is as follows: 1. the study of the jet characteristics of the cooling nozzle of the inner cooling oil cavity is carried out by setting up the nozzle jet test bed. The experimental research on the jet flow of the cooling nozzle at different injection pressure and injection temperature has been carried out, and the capture rate of the inlet of the inner cooling oil cavity and the nozzle exit to the spray are obtained. It is found that the greater the fuel injection pressure, the higher the injection temperature, the greater the divergence angle of the fuel injector, and the reflux of the inner cooling oil cavity. In addition, the results of the experimental data show that the jet of the cooling nozzle is a laminar jet. The simulation results show that the maximum velocity at the core area near the nozzle is in the location of the center 0.2-0.6mm of the offset center, and the velocity gradient of the maximum velocity of the core region increases with the increase of pressure, and the axial velocity of the cross section and the nozzle distance increase with the increase of the jet section and the nozzle distance. The effect of the jet is getting smaller and smaller. The velocity attenuation of the radial section of the jet is becoming more and more slow, and the velocity of the jet section is similar to that of the radial section of the jet. After the fluid is injected into the air, the jet velocity decreases gradually with the distance from 0 to 28.6mm. With the further development of the jet, the velocity of the jet is 2. In the vicinity of 8.6MM, the mass and energy exchange between the oil and the air, the attenuation of the axial velocity of the section intensifies, causing a sharp drop in the velocity of the jet. In addition, the study shows that the injection of the nozzle has a great influence on the static filling in the annular cavity of the inner cold oil cavity. Finally, the cluster laminar flow of the cooling fluid from the nozzle to the inlet section of the oil chamber is established. Inundated jet model, the relationship between flow rate, flow rate, injection pressure, nozzle radius and expansion angle at each section, and the variation of the axial velocity and jet distance in the section of the section.2. are studied in order to reveal the influence of each factor on the flow pattern of the two phase flow and the corresponding flow pattern converter. The flow morphology of two phase flow is discussed by means of experimental method. Considering the effect of piston motion on the flow pattern, the formation and transformation of two phase flow patterns in internal cooling oil are theoretically analyzed. The concept of area coverage is put forward, a prediction model for the flow shape of two phase flow is established, and the two phase flow in internal cooling oil cavity is carried out. The heat transfer intensity is determined. A dynamic visual target test bench is developed to monitor the flow pattern of the internal cooling fluid in real time. The flow pattern of the two phase flow in the internal cold oil cavity is observed directly by the shooting equipment, and the different rotational speeds, the jet pressure, the jet temperature, the static filling rate, the size of the inner cooling oil cavity and the shape of the cross section are compared. When the engine speed is low, the two phase flow in the inner cooling oil is mainly wave flow. As the engine speed increases, the more obvious the "liquid plug" phenomenon is, the higher the engine speed, the stronger the oil oscillation, the more complicated the flow pattern of the two phase flow in the inner cooling oil cavity; the injection temperature is not good. The same directly leads to the change of oil viscosity. As the viscosity of the oil increases, the flow pattern transformation in the inner cooling oil is accelerated and the engine speed of the engine is reduced in the inner cooling oil cavity. The shape of the inner cooling oil cavity has a great influence on its motion shape, the distribution of the two phase flow in the inner cold oil cavity is relatively regular, and the inner cooling oil cavity is in the inner cold oil cavity. When you go up, the oil on the right side hits the top of the oil chamber earlier, and when down, the oil in the oval inner cold oil is in the bottom, and the position of the "liquid plug" is much more in the oval inner cold oil. And when the water droplet shaped inner cold oil chamber goes down, most of the cycles are down to form "liquid" by the oil in and out of the oil cavity near the inner cold oil cavity. In contrast, the effect of the injection pressure on the flow in the oil cavity is negligible. In a comprehensive view, the main influence factor of the two-phase flow pattern in the inner cooling chamber is the filling rate. The numerical study of the dynamic characteristics of the two phase flow in the inner cold oil cavity of the engine speed.3. is studied by CF. The D model is used to simulate the flow of two phase flow in the inner cooling oil cavity. The area coverage and heat transfer characteristics of the two phase flow in the internal cooling oil chamber are compared with the different engine speed, jet pressure, jet temperature, inner cooling oil cavity size and section shape. The simulation results are in good agreement with the experimental results. The results show that the inner cooling oil cavity is on the inner cooling oil cavity. The heat transfer coefficient of the lower wall is contrary to the changing law of the crankshaft angle, and the heat transfer law of the inner and outer surfaces is consistent, and the injection temperature is different and the viscosity of the oil is different, which affects the area coverage and the heat transfer characteristics of the inner cooling oil. The rotational speed is different and the oscillation intensity of the inner oil inside the oil inner cooling oil is different, thus changing the internal cooling oil. The turbulence intensity and its variable heat transfer characteristics in the cavity, the structure of the oil cavity is different, which directly affects the oil filling rate and the erosion degree of the oil chamber wall in the reciprocating movement, thus affecting the heat transfer effect of the integrated heat transfer model of the.4. inner cold oil cavity from the angle of engineering application, combining the experimental research and numerical simulation results, using the internal force. On the basis of Nusselt, Prandt and Reynolds criterion, the convective formula is fitted by the least square method. The prediction model of the instantaneous convective heat transfer coefficient is established. The model is found in the process of the engine piston internal cooling oil chamber and engine speed in the rated speed of engine. The viscous bottom thickness of the fluid is reduced, and the viscous bottom thickness is greater than that of the inner cooling oil chamber when the engine speed is at any cylinder diameter. When the engine speed is the same, the thickness of the viscous bottom decreases with the increase of the cylinder diameter; the natural pair value of the product of Reynolds number and Prandt number is only related to the engine speed, and increases with the engine speed. The influence degree of the hypothesis, the neglecting factor and the non stability is analyzed by the numerical method. The error analysis of the heat transfer coefficient calculated by the correlation method is carried out by the finite element analysis combined with the hardness test. The results show the prediction model of the heat transfer coefficient of the inner cooling oil cavity. The convective heat transfer coefficient of the internal cooling fluid in the inner cooling oil of the piston can be predicted effectively, which can provide a theoretical basis for the design of the internal cold oil cavity of the piston. On the basis of the area coverage of the wall surface, the influence of the heat transfer characteristic of the.5. inner cold oil cavity on the piston reliability is combined with the test of the hardness test. The effect of the internal cooling oil cavity on the piston heat load is simulated by the finite element analysis software, the fatigue analysis software and the dynamic analysis software, and the position of the oil cavity to the piston is analyzed and discussed. The influence of the heat load, the effect of the internal cooling oil chamber setting on the piston two order movement and the influence of the inner cooling piston structure on the piston strength. The cooling effect proves that the use of the inner cooling oil chamber can greatly reduce the temperature of the whole piston, and the higher the position of the inner cooling oil cavity is in the piston head within the allowable range of structural strength. The better the effect of the piston head cooling, the lower the heat load. It is also found that the use of the inner cooling oil cavity can reduce the deformation of the piston and reduce the force between the piston and the cylinder, thus obviously improving the friction and wear of the piston, the lateral force and the skirt pressure. However, the two order motion stability of the piston will be reduced, at the same time. It also increases the percussion noise of the piston, so it is necessary to optimize the skirt profile of the piston. In the study of the new type of inner cooling oil cavity structure, it is found that the inner cooling structure can better avoid the stress concentration phenomenon, which can reduce the heat load of the whole piston through the inner cooling oil cavity, and can improve the ring by using the inlaid ring. It is found that the heat transfer efficiency of the inner cooling oil cavity affects the thermal load of the piston, and the flow pattern of the two phase flow in the inner cooling oil cavity is closely related to the heat transfer law. The flow morphology of the two phase flow is affected or indirectly. The flow pattern of the two phase flow directly reflects the distribution of the two phase flow, which determines the effective scour area of the liquid relative to the inner cold oil cavity and characterizing the intensity of the heat transfer. In addition, the gas liquid two-phase flow is ignored in the simulation calculation, and the gas liquid two phase flow is ignored. For the heat taken by the oscillation, the numerical simulation results are lower than those obtained by the conclusion in this paper.
【学位授予单位】：山东大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TK401

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