【作者】 梁冰；

【导师】 明平剑；

【作者基本信息】 哈尔滨工程大学 ， 动力工程与工程热物理， 2025， 博士

【摘要】 在船用柴油机功率不断突破的背景下,燃烧室内的高温高压环境对活塞的热管理提出了更为严格的要求。活塞振荡冷却技术可以有效控制活塞温度,对于维持发动机的稳定性和延长其使用寿命至关重要。在活塞内冷油腔中,气液两相流动随着活塞的周期性往复运动与腔壁进行热交换,有效带走热量。目前,船用十字头型低速柴油机组合活塞振荡冷却系统中气液两相流动特性复杂、冷却机制尚不明确。本文通过试验研究和数值模拟方法,对船用十字头型低速柴油机活塞冷却系统中振荡条件下组合活塞内冷油腔的气液两相流动与传热机理进行深入研究。主要研究工作如下:搭建了船用十字头型低速柴油机组合活塞内冷油腔的可视化试验台,该试验台复刻了组合活塞内冷油腔的主要特征,并采用十字头型低速机特殊的伸缩套管的供油方式模拟实际工况,集成必要的系统以准确复现柴油机组合活塞内冷油腔的往复运动环境。试验通过高速摄像捕捉了全冲程下组合活塞内冷油腔的气液两相流型变化规律,深入分析了转速和填充率对各腔室气液两相流动特性的影响,通过各腔的主要流型特征明确了内腔的振荡效应和外腔的旋流效应,揭示了组合活塞的多种冷却机制及其多腔室的结构特征产生的耦合效应。建立了带有伸缩套管的船用十字头型低速柴油机组合活塞振荡冷却系统的数值模型,分析了供油方式及透气孔设计对气液两相混合的关键作用及其对壁面传热的影响规律。进一步分析了填充率和转速变化对组合活塞内冷油腔的油相分布、速度场和温度场的影响规律,并以液膜覆盖量化了流体流动特性对各腔室壁面传热的影响,探讨了射流冲击和旋流效应对传热效率的提升作用。提出了以活塞头速度公式为基础的简化方法,结合加载动量源项和模块化方法,将带有伸缩套管的组合活塞振荡冷却模型有效为组合活塞头模型、内腔模型、射流模型和多腔模型。通过对各腔模型的结构因素影响研究,明确了构建多腔传热响应面模型的关键结构参数,为组合活塞内冷油腔多目标优化提供了重要参考。此外,研究了雷诺数、普朗特数、脉动数和连杆比等参数对各腔模型传热的影响,揭示组合活塞内冷油腔的换热机理。基于响应面-遗传算法的多目标优化设计方法,对组合活塞内冷油腔的传热性能进行优化研究,使用更为准确的活塞热边界条件,以活塞稳态温度场的最高温度、最大温度梯度和第一环槽温度为目标函数,采取带精英策略的非支配排序的遗传算法,分别采用VIKOR、TOPSIS和熵值法三种决策方法对Pareto最优解集进行筛选。优化结果表明,活塞头最高温度下降了22.62K,第一环槽温度下降了12.79K,最高温度梯度下降了39.05K/m,总换热量增加了18.68%,实现了组合活塞振荡冷却系统的综合传热性能优化,为低速机组合活塞设计提供了理论支撑。更多还原

【Abstract】 With the continuous increase in the power output of marine diesel engines,the high-temperature and high-pressure environment within the combustion chamber has imposed stricter requirements on piston thermal management.Oscillating cooling technology is crucial for maintaining engine stability and extending its service life by effectively controlling piston temperature.However,the characteristics of two-phase flow and cooling mechanisms in the oscillating cooling systems of marine crosshead low-speed diesel engine pistons remain unclear due to their complexity.This study investigates the two-phase flow and heat transfer mechanisms in the piston cooling system under oscillating conditions using experimental and numerical simulation methods.The main research work is as follows:A visualization test bench for the internal cooling oil cavity of a combined piston in a marine crosshead low-speed diesel engine was developed.The bench accurately replicated the main features of the cooling oil cavity and adopted the special telescopic tube oil supply method of the crosshead low-speed engine to simulate actual operating conditions.By integrating necessary systems,the bench precisely reproduced the reciprocating motion environment of the cooling oil cavity.High-speed cinematography was employed to capture the gas-liquid two-phase flow pattern changes throughout the entire stroke,revealing the main flow characteristics of each cavity and the coupling effects of the multi-cavity structure.The study also analyzed the effects of rotational speed and filling rate on the gas-liquid two-phase flow characteristics,clarifying the oscillation effect in the inner cavity and the swirl effect in the outer cavity.These findings provide a theoretical basis for optimizing the cooling system design of combined pistons and enhancing the thermal management of marine low-speed diesel engines.A numerical model of the oscillating cooling system for the combined piston with a telescopic tube was established.The model analyzed the key roles of the oil supply method and the vent hole design in gas-liquid mixing and their impact on wall heat transfer.The effects of filling rate and rotational speed on the oil distribution,velocity field,and temperature field within the cooling cavity were further investigated.The influence of fluid flow characteristics on wall heat transfer in each cavity was quantified using liquid film coverage,and the effects of jet impact and swirl on heat transfer efficiency were explored.A simplified method based on the piston head velocity formula was proposed,combined with the loading of momentum source terms and modular methods,to effectively decompose the oscillating cooling model with a telescopic tube into sub-models of the piston head,inner cavity,jet,and multi-cavity.The key structural parameters for constructing a multi-cavity heat transfer response surface model were identified through the analysis of structural factors in each cavity model,providing important references for multi-objective optimization of the cooling cavity.Additionally,the effects of Reynolds number,Prandtl number,pulsation number,and connecting rod ratio on heat transfer in each cavity model were studied,revealing the heat transfer mechanism of the combined piston cooling cavity.A multi-objective optimization design method based on response surface methodology and genetic algorithms was employed to optimize the heat transfer performance of the piston cooling oil cavity.The highest temperature,maximum temperature gradient,and first ring groove temperature were used as objective functions.An elitist non-dominated sorting genetic algorithm was applied,and the Pareto optimal solution set was screened using VIKOR,TOPSIS,and entropy weight methods.The optimization results showed a 22.62 K reduction in the highest piston head temperature,a 12.79 K decrease in the first ring groove temperature,a 39.05K/m reduction in the highest temperature gradient,and an 18.68%increase in total heat transfer.This study achieved comprehensive optimization of the composite piston oscillating cooling system,providing theoretical support for the design of low-speed diesel engine pistons.更多还原