【作者】 时骏祥；

【导师】 王建华；

【作者基本信息】 中国科学技术大学 ， 工程热物理， 2009， 博士

【摘要】 随着空天技术的不断发展,更加高效的强化冷却技术已经成为工程热物理学科及航天航空研究领域里的热门课题。因为,要改进空天飞行器或者战斗机的性能,要提高燃气轮机的工作效率或者战略弹道导弹的飞行速度,通常面临着如何提高结构抗高温能力的问题。虽然以多孔介质为载体的发散冷却,因其高效率的冷却方式,受到越来越多的关注与研究,然而它在实验方法和基础理论方面值得研究的问题还有很多。其中,一些是传统的多孔介质内传热传质问题需要深入研究,例如:数学模型和边界条件的确定;液体冷却介质在微孔中的流动、吸热、相变问题。此外,过去对应发散冷却特性的研究大多局限于分析单个参数对冷却效率的影响,而没有根据实际应用综合起来分析。随着人们对发散冷却系统的优化设计要求的提高,如何用最少的冷却介质达到最高的冷却效率?如何用最佳的多孔材料组合满足结构更高的热防护需求?诸如此类问题是发达国家面临的基础科学问题,也给我们的科学研究带来新的机遇和挑战。本文以理论分析为基础、数值模拟为工具,针对发散冷却基础理论问题开展一些探索性研究。局部热非平衡发散冷却模型的边界条件:首先将多孔介质热边界条件分为固体-固体、固体-流体两类;其中固体-流体又可以分为多孔介质内部流场与外部流场方向平行和垂直两类。发散冷却过程热端边界条件就属于固体-流体-垂直这类,此类边界条件在学术界一直有很多争议。本文通过一维、稳态、热非平衡模型的解析解,比较五种不同的边界条件下的计算结果。这五种边界条件来自于不同的研究文献,既有针对固体-流体-平行的,也有针对固体-流体-垂直的。在给定的参数下,本文从冷却物理意义出发,给出了两个判断边界条件是否合理的标准:1)热端表面流体温度与固体温度的变化趋势是否合理;2)冷却效率与冷却介质注入量的关系是否合理。通过分析,确定了在这五种边界条件中,哪一些是完全不合理的,哪一些是无条件适用的,哪一些需要在一定条件下才可以用。这一研究澄清了长久以来学术界在对发散冷却过程进行数值模拟时,关于热非平衡边界条件的争议,对于我们的研究工作也有很好的借鉴作用。发散冷却过程中冷却介质与外部流场的耦合作用:过去,对于发散冷却的研究往往侧重于多孔骨架内部流场与表面温度,而忽略了发散冷却过程中,冷却介质进入热端边界层对外部流场同样有着非常重要的影响。本文研究了伴有发散冷却的二维多孔平板层流边界层流动及换热特性,用稳态的数值模拟方法,耦合分析了外部热流与内部冷却流相互作用下的传热传质特性,并分析了几个重要的无量纲参数(Re,Pr,V)对多孔介质平板热表面温度分布的影响。液体发汗冷却过程中的相变现象研究:由于气态冷却介质吸热能力的局限及人们对更高冷却效率的追求,液体冷却介质具有很高的使用价值和广阔的应用背景,因为液体冷却介质具有高相变潜热、小储存空间、输送能耗低等特点。本文采用了局部热非平衡与混合模型相结合的方法LTNE-TPMM(Local Thermalnon-Equilibrium—Two Phase Mixture Model),描述了液体冷却介质相变发生的物理过程,并成功地进行了一维、稳态、液体相变冷却现象的数值模拟。同时,通过对模型中各参数的分析,发现了一个十分有趣的现象:在多孔骨架内部,在液相转向两相混合物时,冷却介质的温度可能高于固体温度。这与以气体为冷却介质的发散冷却的结论是不同的。尽管这一结论仍需要实验来进一步的论证,但无疑对今后进一步实验研究散冷却特性提出了一个有趣的课题。发散冷却系统设计中冷却介质注射量控制及双层材料结构的优化问题:本文就发散冷却系统优化问题做了一些探索性研究,主要包括两个部分。1)用一维、可压缩、非稳态、局部热非平衡模型,数值研究了瞬态冷却过程及最小冷却介质注射量的依赖参数。这样做的目的是:在确保推进器不被烧蚀的前提下,充分发挥材料本身的抗高温能力,以最小的冷却介质注射量作为发散冷却系统的设计目标。结果表明:瞬态冷却过程研究十分重要,因为尽管当冷却过程达到稳态以后热端温度在烧蚀点以下,但在进入稳态之前可能已经发生烧蚀。2)采用双层多孔介质发散冷却结构,以遗传算法为优化工具,以热端表面最低温度为优化目标,优化双层多孔介质结构参数为参数,分别用冷却介质流量(或压力损失)、结构总重量和成本为约束条件,找出可行的最优双层发散冷却结构设计参数。结果验证了这种结构设计优化的可行性以及遗传算法作为优化算法的有效性,这些研究对于工程应用有很好的指导作用。发散冷却中的逆问题:从多孔介质内部温度可测量的角度出发,采用热非平衡模型及共轭梯度求解方法,成功地推算出发散冷却过程中热端承受的热流密度,并获得了非常好的计算结果。由于发散冷却过程特殊性,其逆问题也与传统换热过程的逆问题有不同之处:1)局部热非平衡模型,要求流—固两相耦合求解;2)高温、高压下流体的可压缩性和多孔骨架的变物性必须考虑。因此,这一研究扩展了传热学中逆问题的应用范围,而关于流体可压缩性与固体变物性的讨论以及求解敏感系数应该注意事项的分析方法,为今后研究类似换热系统逆问题提供有参考价值的借鉴。更多还原

【Abstract】 With the development of the aero space technology,a highly efficient cooling technology has been gradually become a hot topic in the research area of engineering thermalphysics and astronavigation.In order to meet the requirements of the improving aero space vechicle performance and increasing the gas turbine efficiency of fighter aircraft,enhancing the capacity of anti-high temperature and antioxidant of hot components is an essential problem.Although the transpiration cooling with porous media has been more and more attented and studied due to its high cooling effectiveness,there are many problems in the investigations of experimental methods, theoretical models and numerical approaches.Except the traditional heat and mass transfer problems,there are the contentions of mathematical models and boundary conditions,limitations of phase-change problem of liquid coolant within porous media,and so on.In addition,the most past investigations on transpiration cooling were limited within the effect of individual parameter on cooling efficiency,and not integrated with the acual application together.With the increasing requirement of the cooling efficiency,how to achieve the highest cooling efficient using the least coolant consumption? how to meet the requirement of these thermal protections using a reasonable combination of two porous material layers? Such as basic scientific issues are that the researchers in developed countries have to face,and at the same time; these issues provide to our scientific researchers with new opportunities and challenges.Based on the theoretical analysis and numerical simulation,this dissertation will present some explorative investigations on the basic problems of transpiration cooling.Boundary conditions of local thermal non-equilibrium model:Firstly the thermal boundary conditions of porous media can be divided into two types:at solid-solid interface and at solid-fluid interface.The solid-fluid boundary conditions can be also divided into two categories:parallel and vertical relations between internal and external fluid flow fields.The boundary conditions of the transpiration cooling process are usally in solid-fluid-vertical type;such boundary conditions have been controversial in the academic community for a long time.To compare and discusse five kinds of different boundary conditions(BCs),an analytical solution of a steady and one-dimensional problem of transpiration cooling described by a local thermal non-equilibrium(LTNE) model is presented in this work.The influence of the five BCs on temperature field and cooling effectiveness is discussed using the analytical solution.Two physical criteria:1) if the analytical solution of coolant temperature may be higher than hot gas temperature in a steady state,2) if the variation trend of thermal effectiveness with coolant mass flow rate at hot surface is reasonable,are used to estimate the five BCs.Through the discussions,it is confirmed which BCs in all cases are usable,which BCs under certain conditions are usable,and which BCs are thoroughly unreasonable.Coupled effect of cooland injection and external hot flow in transpiration cooling process:In the past studieds,the investigations on transpiration cooling were generally focused on the internal coolant flow filed and hot surface temperature,but an important effect of the coolant flow on the external hot flow field was neglected.In this work,heat transfer characteristics in the laminar boundary layer with transpiration cooling function are numerically analyzed by an integral method of coolant injection and thermal boundary layer.The effects of coolant injection ratio,the Re and Pr numbers of the exterior hot flow on the temperature at porous plate surface are discussed.Phase change problem in transpiration cooling with liquid coolant:Because of the limitation of heat-absorbing capacity of gaseous coolant and the requirement of a higher cooling efficiency,liquid coolant as a potential candidate has a higher value in thermal protection application,due to its large latent heat of phase transition, smaller storage space,lower power comsumption of transport.This dissertation will present a numerical method to simulate the transpiration cooling processes with coolant phase change within porous matrix.This method is based on coupling Local Thermal Non-Equilibrium(LTNE) with Two-Phase Mixture Model(TPMM),using the LTNE to describe the heat exchange process,and using the TPMM to describe the liquid coolant phase change within the matrix.The effects of thermal conductivity, porosity,and sphere diameter of the porous matrix on the temperature and saturation distributions within the matrix are analyzed using the LTNE-TPMM.Through this work,an inverse phenomenon with the foregone investigations on the transpiration cooling without coolant phase change is discovered,namely in two-phase region, coolant temperature may be higher than solid temperature.It is clear,this inversion can be captured only by the LTNE-TPMM.Although requiring verification by an experiment or explanation from a view point of micro-scale in the future,this inversion phenomenon leads to an interesting research topic.Minimum coolant injection rate and optimized structure design of two layered porous media:This dissertation presents some groping investigations on the optimized design of transpiration cooling systems.The optimization includes two parts:1) A numerical investigation on transient process of transpiration cooling,and the control parameters of the lowest coolant mass flow rate are carried out using a compressible,unsteady and local thermal non-equilibrium model.The numerical investigation shows that it is important to study transient cooling process,because the porous matrix could be ablated before the cooling process achieves to a steady state, though the temperature in steady stat may be lower than the melting point.2) An optimization structure design of diffusion cooling with two-layer porous matrixes is presented.Coolant is injected into the porous structure under a certain differential pressure over the porous matrix with a total thickness.With the variation of the porous material,porosity and thickness ratio of two layers,the lowest temperature at the hot surface is the ultimate target of the optimization which satisfying the global constraint conditions of weight and cost,etc.The genetic algorithm is used to find the feasible optimization design with the analytical solution of one-dimensional LNTE model.The results indicate that this method of optimization is effective,and several similar optimal designs are found under the different constraints.Inverse problem of transpiration cooling:In this work,an inverse problem of transient transpiration cooling is investigated in detail.The heat flux to hot surface, which is dependent on time or space,is estimated according to the temperatures measured by thermal sensors.The conjugate gradient method(CGM) is applied to solve this inverse problem.Through the CGM,satisfactory results can be obtained. This investigation expands the application area of inverse problem.Because the characteristics of transpiration cooling are differect from the tranditional inverse problems:1) Local thermal non-equilibrium model requires fluid-solid couple solving; 2) The compressibility and variable thermal properties must be considered under a large temperature gradient and high pressure.Therefore,this work provides a useful reference for the future research on inverse problem.更多还原