摘要
炕作为中国最古老的供暖方式至今仍被广泛使用。2004年底全国约有6700万铺炕,使用人口达1.75亿。在建筑中使用炕供暖可利用生物质能源来改善室内热环境并减少商品能源消耗。目前,炕的设计主要基于民间经验,缺少理论基础支持。炕在实际应用中仍存在室内空气污染严重、炕表面热舒适性差等问题,导致炕有逐渐被使用商品能的供暖方式(如土暖气、家用锅炉等)替代的趋势,长此下去这将加重我国农村能源供应负担。目前北方农村能源消耗占全国农村总能源消耗量的56%,且超过80%的农村能源消耗用于采暖需求。考虑到炕的地域分布广、应用数量大、使用时间长、搭建经验丰富、文化底蕴深厚等特点,对炕进行改进或创新是我国北方农村建筑供暖技术转换和更新的关键。如何合理地设计和使用炕以提高其热性能需要深入地研究和探讨,从而为炕的工程设计提供科学依据。本文对此进行了实验研究和理论分析,其主要的学术贡献如下：
首先,设计和搭建炕实验平台,对典型炕的热性能进行实验研究,科学地认识了炕的传热机理及烟气流动特性,系统地分析了炕的得热、蓄热和散热性能。研究发现,炕烟道内烟气流动状态为紊流,炕体外表面散热以辐射方式为主,炕表面温度随着时间变化波动大,在空间分布上存在炕头表面温度过热和炕梢表面温度过冷问题。
其次,针对炕在实际使用过程中常出现的倒烟回流问题,建立了炕的传热与烟气流动宏观描述模型,探究了在风压和热压共同作用下炕的烟气流动规律,讨论了炕烟气流量的多解性、稳定性和跳跃性,从理论上解释了炕在实际运行过程中出现的倒烟回流现象,提出了炕烟气正向流的初步设计方法和解决烟气倒流的措施。
再次,为了改善炕表面的热舒适性,分析了在连续加热和间歇加热条件下炕的蓄热特性,对影响炕板温度的各因素进行了敏感性分析,得出了炕板时间常数和加热方式是影响炕表面温度波动的主要因素,并提出了将炕表面温度控制在舒适范围内的方法。同时,采用数值模拟方法研究了不同烟道布置形式对炕表面温度分布均匀性的影响,对选定的烟道布置形式进行了改进。结果表明,调整烟道出口处挡烟板的高度比β=0.25和在烟道进口处增加平面尺寸为0.5m×0.5m、倾斜角γ=5°的挡烟板可以有效地改善炕表面温度的均匀性。
最后,建立了炕建筑动态热过程计算模型,分别开发出了面向研究分析用的炕建筑动态热过程模拟程序和便于工程应用的DeST-k软件包。选取一典型炕住宅进行案例分析,从炕的运行方式、炕板热容量、烟道布置形式等方面对炕的热性能进行优化设计；同时,在此基础上定量分析了炕在不同气候和建筑热工特性等条件下的全年运行效果。结果表明,通过合理地设计炕的结构和控制炕的运行方式可以保证炕表面温度满足热舒适性要求：适当地提高建筑围护结构的热工特性可以充分发挥炕对建筑的供暖效果：在选取的北方8个代表城市的气候条件下,使用炕采暖可以使建筑热负荷降低50%-80%。
综上所述,本文工作为中国炕的热性能研究及其应用提供了科学依据和理论基础。
Chinese kangs, the most ancient heating system in China, are still widely used today. There were about 67 million kangs used by 175 million people in 2004. A kang utilizes the sustainable biomass energy for space heating to improve indoor thermal environment and reduce commercial energy consumption. However, the existing design of kangs is largely based on the accumulation of craftsman experience and there is a lack of scientific studies and engineering design guidelines. Several problems are found in the use of kangs, such as serious indoor air pollution and poor thermal comfort, which results in a gradual replacement of kangs with the heating technologies using commercial energy. This will add a heavy burden to the rural energy supply of China. The rural energy consumption of Northern China represents 56% of total energy use in rural China, and more than 80% of rural northern energy consumption is used for home heating. While considering the scope of wide usage, large quantity, long history, abundant experience and great culture accumulations of Chinese kangs, the improvements or innovations of the traditional kangs are crucial for the future transition and new heating technologies for rural residences in northern China. How to design and use the kang scientifically to improve its thermal performance needs a detailed study, which can form a basis for engineering design. In the thesis, the following work was accomplished by both experimental studies and theoretical analysis.
Firstly, an experimental study on thermal performance of a full-scale typical elevated kang was carried out to understand the smoke flow and heat transfer mechanisms of the kang and analyze the heat charge, thermal storage and heat discharge of the kang system. It is found that the smoke flow in the kang is turbulent. The radiative heat plays the major role in the heat release from the surfaces of kang body. The surface temperature of the kang upper plate fluctuates a lot with time and seems to be overheated at the kang head and undercooled at the kang tail.
Secondly, a macroscopic smoke flow and heat transfer model of an elevated kang was developed to deal with the backflow problems usually appeared when the kang works, which can be used to explore the kang smoke flow under the combined thermal buoyancy and wind force. The solution multiplicity, stability and switching of the smoke mass flow rates are discussed. This study explains the smoke backflow phenomenon, and presents a preliminary smoke flow design method to ensure upward flows and also proposals to eliminate smoke backflows for the kang.
Thirdly, in order to improve thermal comfort related to the outside surface of the upper kang plate, the thermal storage performances of the elevated kang under both continuous and intermittent firings are investigated. It is obtained that the time constant of the kang plate and the firing pattern are the key parameters that affect the magnitudes and fluctuations of the outside surface temperature of the upper kang plate by a parameter sensitivity analysis. A control method for the outside surface of the upper kang plate within the thermal comfort range is proposed. In addition, the effect of the kang flue layout on the outside surface temperature uniformity of the upper kang plate is investigated by numeric simulations, and the specified flue layout is improved. The results indicate that the outside surface temperature uniformity of the upper kang plate can be greatly improved by regulating the ratio of the smoke stopper height to the flue heightβ=0.25 and adding a 0.5m×0.5m panel with the incline angleγof 5°at the inlet of the kang flue.
Finally, a dynamical thermal process model of a home with a kang is developed, which is implemented in both a computer program of the dynamical thermal process model of a kang building for academic use, and a commercial software package DeST-k (original package developed by Tsinghua University) for engineering use. A typical kang residence in northern China is selected for a case study to optimize thermal performance of the kang in the aspects of firing pattern, thermal capacity of the kang plate and the kang flue layout. In addition, the annual practical effects of the optimal kang under different insulation levels of building envelope and climatic conditions are also discussed. The simulated results show that the thermal comfort requirement of the outside surface of the upper kang plate can be met by a proper design of the kang construction and control of the firing pattern. The better the insulation level of building envelope or the room air tightness is, the greater the indoor air temperature rise and the building heating load decrease are by the kang heating. In the climates of the selected eight cities in northern China, use of a kang for home heating can reduce by 50%-80% of building heating load.
In summary, the studies in this thesis have provided some scientific basis and theories for further study on the thermal performance and the application of Chinese kangs.
引文
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