【作者】 王磊；

【导师】 赵福云；

【作者基本信息】 武汉大学 ， 热能工程， 2020， 博士

【摘要】 由自然对流或混合对流驱动的室内空气流动及传热传质过程是电子器件冷却、室内空气环境营造、多功能型建筑围护结构设计等工程实际中普遍存在的基础共性问题。目前室内空气对流输运研究主要针对唯一稳态解的情形,而作为典型受限空间内部流动的室内空气对流输运系统具有强烈的非线性特征,其当前状态有时与过去经历有关,即存在滞后效应。这意味着相同物理边界及控制参数下,室内可以存在若干组不同的空气、热及气态组分（CO₂、水蒸气、VOCs等等）稳态输运结构,即室内空气对流输运多解行为。理解和模拟此类室内空气对流多组稳态解行为,可以在设备散热除湿的协同强化、室内热与气态污染物的高效去除等工程应用方面起到积极的促进作用。此外,异性介质部分填充室内空气对流输运问题广泛存在于设备散热强化、多功能型建筑围护结构设计、太阳能集热器等工程应用之中。相比于固体介质,多孔介质具有更广域的室内空气掺混及热输运调节能力,现有研究针对多孔介质层状部分填充室内空气对流输运问题,多孔介质离散化部分填充室内空气对流输运问题还有待深入研究。本文从热质源复杂分布情形下室内空气对流多组稳态解现象、多孔介质离散化部分填充室内空气对流耦合输运问题等方面开展了相关基础研究工作。首先,本文通过理论分析和数值模拟探究了热质源复杂分布情形下室内空气对流多组稳态解现象,主要工作如下:通过理论分析及数值模拟阐述了室内空气对流多组稳态解的内在机理及实现方法;基于滞后效应提出了参数连续法以探究室内空气对流输运模式及热质输运能力与控制参数之间的多解演化规律,给出了上行解分支、下行解分支及静止解分支的概念及定义。探讨了多重离散热源作用下室内空气自然对流输运过程存在的多组稳态解。通过参数连续法研究了热源强度比、Rayleigh数、热源尺寸、热源偏心距及腔室倾斜角与室内空气自然对流输运模式及各热源冷却效果之间的多解演化规律。研究发现可以通过初始条件控制以调节离散热源系统内各热源冷却效果。提出了参数极限值法以揭示滞后效应的有效作用范围,以便尽可能快速地实现所期望的室内空气对流输运模式;揭示了室内空气对流输运多解行为的瞬态演化过程,并发现了冷羽流在室内空气对流输运模式转变过程中的主导作用。研究了多重离散热源作用下室内空气混合对流输运过程存在的多组稳态解。分析了置换送风模式和混合送风模式下Reynolds数、Richardson数以及离散热源布置方式对室内空气对流输运结构及各热源冷却的影响。通过施加不同的初始条件,混合送风模式下室内空气对流输运可以由沿左侧壁面向下发展的强迫对流主流及其右侧逆时针涡旋控制,也可以为右偏转强迫对流主流及其包含顺时针涡旋控制。该研究可以为多个发热设备组成系统的冷却强化提供一定理论指导。揭示了等温等浓度、等热质通量及混合热湿源等不同情形下室内空气双扩散自然对流输运过程存在的多组稳态解。通过参数连续法分析了浮升力比、湿源位置及Rayleigh数与室内空气双扩散自然对流输运模式及热湿输运能力之间的多解演化关系。对于等温等浓度、等热质通量及恒热流等浓度这些情形,可以通过初始条件设置以实现不同除湿或者散热需求的室内空气流动及热湿输运模式。该研究可以为设备散热除湿的增益强化、室内热与气态污染物的高效去除提供理论指导。最后,本文揭示了离散化布置多孔材料对室内空气对流输运的调节作用,给出了适用于流体区域及多孔介质区域的热函数定义式。首先模拟分析了离散化布置多孔介质对室内空气自然对流及热输运的影响。结果表明,离散化布置多孔介质可以在极大范围内调节室内空气自然对流及其传热效率,存在最佳的多孔介质布置参数以最大化或最小化室内自然对流传热速率;多孔介质布置参数对平均传热效率的影响非常依赖Darcy数;给出了适用于不同多孔介质布置形式的平均热输运速率与各控制参数之间的统一关联式。而后研究了离散化布置多孔介质对室内空气混合对流及其热输运的影响。研究发现附着于加热壁面的多孔介质导致室内出现逆温分布,进而诱导微弱二次涡旋的出现;附着于底部壁面的多孔介质极大增强中心区域的流体掺混;给出了适用于不同多孔介质附着位置的平均传热速率与控制参数之间的统一型关联式。更多还原

【Abstract】 The indoor air flow,heat and airborne species transports driven by free convection or mixed convection are fundamental problems that are associated with many practical engineering applications such as the cooling of electronic device,control of indoor air environment and multifunctional building envelope design.The past studies are mainly devoted to the situation of the unique solution on indoor air convective transport.Indoor air convective transport,regarded as an internal fluid flow in a typically confined space,presents intensively nonlinear characteristics,and its current state would be affected by the experience,namely hysteresis effect,which indicates that following identical physical boundary conditions and operating parameters,multiple steady-state solutions,i.e.,no less than two steady-state patterns of indoor air convective transport could be found.This phenomenon could be helpful for the collaborative enhancement of heat dissipation and moisture removal from electronic equipment,and the efficient dismissal of indoor heat and airborne pollutants.In addition,a variety of practical engineering applications,such as the cooling performance enhancement of equipment,multifunctional building envelope design,and solar collectors,are relevant to the fluid flow and heat transfer of air convective transport in cavities partially filled with heterogeneous media.Porous media presents a better performance in control of indoor air mixing and convective heat transfer than those of solid one.The literature survey is confined to convective heat transfer in enclosures partially filled with porous layers.The effort should be done to reveal the intervention characteristics of discrete porous insertions on indoor air convective transport.A few fundamental investigations have been conducted in this dissertation,which have been utilized to illustrate the multiple steady-state solutions of indoor air convection under the complicate distribution of heat and species sources,and the effects of discrete porous insertions on air convective transport in cavities.Firstly,multiple steady-state solutions of indoor air convective transport under the complicate distribution of multiple discrete heat and mass sources have been comprehensively studied through theoretical analysis and numerical simulation in this dissertation.The main outcomes are presented as follows,The intrinsic fluid dynamic characteristic and the realization principle,concerning multiple steady-state solutions of indoor air convective transport,have been demonstrated via theoretical analysis and numerical simulation.According to hysteresis phenomenon,a universal parameter continuation method has been proposed to investigate the effects of the governing parameters on the multiple steady-state solutions of the indoor air convection,and heat and mass transfer rates,where three solution branches have been defined,so-called upward solution branch,downward solution branch and rest solution branch.Multiple steady-state solutions of natural convection in enclosures imposing multiple discrete heat sources have been numerically studied.The effects of heat flux ratio,Rayleigh number,size and eccentricity of the right heater,and inclination angle on multiple steady-state solutions of the natural convective transport,and the cooling performances of discrete heat sources have been illustrated through the proposed parameter continuation method.The simulation results show that the desired cooling performance of a single heater from multiple discrete heat sources could be controlled through the initial condition.A parameter extremum method has been proposed to evaluate the effective domain of hysteresis effect,which aims to achieve the desired indoor air convective structure as quickly as possible.The transient evolution process of indoor air convective structure between different solution branches has been explored,and the cold plume has been found to play the dominant role in the transition between different solution branches.Non-unique steady-state solutions of mixed convection in enclosures subjected to multiple discrete heat sources have been studied,where both displacement flow scheme and mixing flow scheme have been considered.This study has been devoted to revealing effects of the Reynolds number,the Richardson number and the distribution of discrete heat sources on both the indoor air convective transport and the cooling performance of each heater from multiple discrete heat sources.Under the mixing flow scheme,there are two typical patterns of mixed convective transport in enclosures by imposing different initial conditions,in which one of them is that forced convection mainstream develops downward along the left sidewall while a counterclockwise vortex appears at the right of mainstream,and the other one is that the right-deflecting forced convection mainstream encircles the clockwise vortex.This study could be beneficial for enhancing cooling performance of multi-heating equipment system.Multiple steady-state solutions of indoor air double-diffusive natural convection have been numerically analyzed,where different situations of discrete heat and moisture sources have been considered,including constant but different levels of temperature and concentration,and constant but different levels of heat and mass fluxes,and mixing types of heat and mass sources.The effects of the buoyancy ratio,the position of the moisture source,the Rayleigh number on the double-diffusive natural convective structure,and the heat and moisture transfer rates have been analyzed by the proposed parameter continuation method.Results show that as for the situations concerning constant but different levels of temperature and concentration,and constant but different levels of heat and mass fluxes,and constant heat flux and invariant concentration,the expected indoor air convective structure,and the heat and mass transfer rates could be obtained through imposing the initial condition.This research could be helpful for synergistically enhancing the heat dissipation and dehumidification of equipment,and achieving the efficient removals of heat and gaseous pollutants in buildings.Finally,effects of discrete porous insertions on indoor air convective transport have been explored.And the heatfunction,being suitable for porous region and fluid region,has been established.Initially,the fluid flow and heat transfer of natural convection in enclosures with discrete porous insertions attached to the hot wall has been numerically studied.Numerical results illustrate that the indoor airflow structure and the associated heat transfer rate could be varied extensively upon obeying different designs of porous media.Moreover,there exist optimal morphologies of porous media to maximize or minimize the heat transfer rate.Furthermore,the effects of morphology of porous media on the average Nusselt number heavily depend on Darcy number.And the unified correlation of the average Nusselt number,as a function of various governing parameters,has been proposed.Afterwards,mixed convection heat transfer inside lid-driven cavities under different arrangements of porous media has been studied.Numerical results show that porous insertions uniformly mounted on the hot wall could contribute to the presence of inversion layer,which further results in the secondary vortex while those placed at the bottom could be helpful for greatly intensifying the mixing effect of fluid that lies in the core region.And the identical form of correlations of the average Nusselt number has been proposed,which could be suitable for different arrangements of porous insertions.更多还原