Smoke Flow And Thermal Performance Of Chinese Kangs

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.