【作者】 於仲义；

【导师】 肖波； 袁旭东；

【作者基本信息】 华中科技大学 ， 供热、供燃气、通风及空调工程， 2008， 博士

【摘要】 土壤源热泵系统利用埋设于地下的换热器与土壤交换热量,提升了可再生浅层地热能的能量品位,被称为是21世纪一项最具发展前途的空调新技术。制约土壤源热泵技术发展的因素很多,其中地埋管换热器传热一直是该技术的研究关键之一。如果对地埋管热量传递过程特性认识不全面,盲目设计地埋管结构和尺寸会导致地下换热性能严重下降,空调系统的能耗增高,甚至完全瘫痪,这势必给新技术的推广带来巨大阻碍。本文首先从与地埋管换热过程密切关联的土壤特性以及需要向土壤排热或取热的地埋管换热负荷特性着手,着重分析了土壤热特性、地下水渗流特性以及不同地区、不同建筑的换热负荷分布特性和规律,提出了描述地埋管热量在土壤中传递的两种主要机制和表征换热负荷的三个重要特性。这些特征直接决定了地埋管的换热性能,换热器能否长期保持良好的换热状态,同时也决定了土壤源热泵系统保持多年运行的可持续性。通过理论分析,本文建立了两种传热机制下地埋管换热器传热过程的数学物理模型,借助于边界条件和初始化条件体现出地埋管动态换热负荷特性。采用有限体积法对求解域进行整场离散、整场求解。依据土壤源热泵设计和运行所关注的进出口温度以及换热可持续性,引入参数地埋管换热器能效系数和换热效用时间,对地埋管换热能效特性和最大可用温差的可持续换热时间进行量化分析。根据优化的无穷远边界值对地埋管换热器传热过程进行动态数值模拟,分析了有无地下水渗流土壤中地埋管换热器换热性能的差异,并讨论了不同的地埋管结构和尺寸、土壤热状态、分层特性、换热负荷特性以及运行方式条件下地埋管传热特性和效用时间变化规律,且针对土壤分层特性提出了区段换热理论及其动态迁移特性,从而为地埋管换热器的优化设计及空调系统参数匹配提供了理论与技术支持。根据地埋管换热器传热模型建立了现场实验平台,利用现场实验测试验证本文所建立的地埋管换热器数学模型及其实现过程的正确性、可靠性,并从实验的角度验证和分析进水流体温度、流速以及运行方式对土壤温度场和地埋管换热器传热特性的作用规律,为土壤源热泵的工程实践提供基础数据储备。鉴于地埋管周围土壤温度的模拟值与实测值,在地埋管循环流体与周围土壤换热过程中,土壤温度在轴向方向上变化不大,因而可采用二维传热模型分析钻井外土壤传热。基于此,本文采用地埋管传热多极理论模型并结合热阻网络平衡原理建立了垂直U型地埋管的热量传递解析模型。该模型将地埋管与周围土壤传热区域分为钻井内外两部分,钻井内为稳态传热,采用多极理论U型管模型,钻井外为瞬态传热,采用柱热源模型,并考虑了地埋管内循环介质轴向变化。在一定的换热时间下,利用该模型分析地埋管传热特性,具有较高的可靠性和较少的计算工作量。本论文的研究将使地埋管换热器的设计以及工程实践更为合理化,为推广和发展土壤源热泵这一极具节能与环保潜力的浅层地热能利用技术提供了理论基础和支撑。更多还原

【Abstract】 For utilizing heat exchangers buried in the ground to transfer heat with the soil and upgrade the tenor of the renewable geothermal energy in shallow layer, a ground source heat pump (GSHP) system is known as one of air conditioning techniques which have the greatest developmental future in the 21th century. It is a matter of common observation that the development of GSHP technique is restricted by a lot of factors, in which the heat transfer of a ground heat exchanger (GHE) is the key of research on GSHP techniques at all times. Due to lacking in an comprehensive acquaintance with the heat transfer process, the arbitrary design for the constructure and size of GHEs would lead to the serious drop in performance of heat exchangers, and the increase in energy consumption of the whole air conditioning system, to top it all, going so far as do the system paralysis, all which should bring great barriers of the promotion and popularization of the new technology.The desertation first launched on the soil trait and the commutative load rejected to or extracted from the soil which were tightly related with the heat transfer of geothermal heat exchangers. The soil thermal property and ground water seepage flow behavior as well as the load distribution in various regions and buildings were analyzed intensely. Two primary heat transfer manners describing the heat quantity transport in the soil and three important characteristic representing the thermal load property were put forward. These traits directly determined the heat transfer performance of GHEs and whether or not the heat exchangers could keep the good performance in the long run in the limited buried places, which in turn decided the years of the sustained and effective operation of GSHP systems.Through theoretical analysis, the mathematical and physical model of a vertical U-tube GHE under two kinds of heat transfer manners was established in which the thermal load characteristic of GHEs can be incarnated with the help of boundary conditions and initialization conditions. For such kind of coupled heat transfer problem, the methodology of a straightforward discretization and an overall solution utilizing the finite-volume method in all mediums zones, was adopted in this paper. In view of the inlet and outlet temperature and heat transfer continuable feature of GHEs focused in the design and operation of GHSP systems, the energy efficiency coefficient and heat transfer endurance were introduced to quantify the energy efficiency performance and the endurance period according to maximal available temperature difference between the inlet and outlet in the U-tube heat exchanger. The performance of GHE in soil with and without groundwater flow was analyzed firstly through dynamic numerical simulation of heat transfer process between the soil and the working fluid by the use of the optimized infinite boundary size. Basing on the model, the varied regularity of energy efficiency performance and heat transfer endurance with the conditions including the different configuration and dimension of the U-tube heat exchangers, the soil thermal behavior, stratified feature in the soil, thermal load characteristic and operational mode were discussed. Regional heat transfer theory and migratory property under the condition of dynamic heat transfer capacity in U-tube heat exchangers were brought forward in the thesis. It supplies theory support and technology accumulation for the optimal design of the GHE and the parametric matching of air conditioning systems.According to the heat transfer model of GHE, an experimental facility in situ for measuring the performance of exchangers was upbuilt. Through being compared with experiments in testing boreholes, the accuracy and reliability combined with its implementation in the mathematic model of GHE was validated. From the point of view of the experiments, the influences rules of the inlet water temperature, the flow velocity and operational mode on the temperature field in the surrounding soil and heat transfer characteristic of GHE were verified and analyzed. Thus all above can make contribution the basic data for engineering practice in GSHP systems.In the light of the simulative and experimental testing value in the soil surrounding the GHE, the minor temperature of the soil in the axial direction should be found during the heat transfer between the circulating fluid in the U-tube and the soil. So the soil heat transmission outside the borehole can be analyzed by two dimensional heat transfer model. On the basis of that, a new heat transfer analytical model considering the axial flowing based upon a multipole theory model combined with the thermal resistance network balance principle was developed. The heat transfer region was divided into two parts at the borehole wall. Both steady and transient heat transfer methods were used to analyze the transmission of heat inside and outside borehole respectively. As for inside borehole, multipole theory model in the U-tube was used to solve the heat transfer problems. As for outside borehole, constant heat flux cylindrical source model was used to calculate the borehole wall temperature. To a certain heat transfer duration, adapting this model to analyze the heat transfer performance can not only afford high reliability but also need a minor calculating work capacity.The research in this desertation can make the design and engineering practice of GSHP more reasonable. Furthermore, it can also provide theoretical base and technique support for the popularization and development of applications in the GSHP system utilizing shallow statum geothermal energy which have great energy saving and environmental protection advantaged potentiality.更多还原