【作者】 王烨；

【导师】 王良璧；

【作者基本信息】 兰州交通大学 ， 环境工程， 2011， 博士

【摘要】 自然对流是由于流体的温度差或浓度差引起了密度差,在重力或其它力场作用下形成升浮力而产生的一种流体流动和对流传热现象。封闭腔中自然对流由于在建筑节能、电子元器件的冷却、太阳能集热器设计、核反应器设计等领域中的普遍应用而受到了广泛关注。在过去的几十年中,为了深入理解封闭腔中自然对流流动与传热过程,人们开展了大量的实验和数值研究工作,但是封闭腔中湍流自然对流数值分析的结果与实验结果之间依然存在较大差异。随着计算机能力和计算方法的发展,数值分析方法越来成为不可缺少的研究手段。因此,进一步对封闭腔内湍流自然对流模型进行研究非常必要。已发表的研究结果表明无论用高雷诺数κ-ε模型还是低雷诺数κ-ε模型对封闭腔内湍流自然对流问题进行计算时所得结果与实验结果之间均存在较大差异。本文针对这一现状开展了一系列工作。1.为了获得数值结果和实验结果产生较大差异的原因,对自然对流和强迫对流从动量传递与热量传递之间的比拟关系方面进行了分析。发现自然对流流动边界层与温度边界层之间不存在比拟关系,从而不能像强迫对流一样用αt=νt/σt封闭能量方程中的-ρcpuiT。2.提出了一求解封闭腔湍流自然对流换热的高低混合κ-ε模型,将由该模型所得结果与实验数据进行了对比。结果表明该模型在对湍流自然对流换热的预测精度上虽达到一定改善,但还需要进一步提高。3.为了对高低混合κ-ε模型进行进一步改进,提出修正湍流普朗特数方案σt=1+Pr,并形成湍流修正κ-ε模型。将该模型所得结果与实验数据进行了对比,发现用修正κ-ε模型在Ra>109条件下获得封闭腔内空气湍流自然对流换热的平均Nu数与实验值之间的相对偏差平均值降低到了8.43%,同时在108≤Ra≤109时所得结果与其它文献中的数值结果很接近。4.将修正κ-ε模型用于兰州地区住宅供暖换热器传热特性分析。在满足室内供暖温度要求的情况下,获得了外墙类型、室外环境条件、换热器面积、邻室传热等因素对换热器传热性能的影响规律、换热器表面温度、换热器提供的热量、室内流场、温度场等特征。更多还原

【Abstract】 Acted by field force like gravity force, natural convection is induced by the buoyancy force resulting from temperature difference or concentration difference. It is a fluid flow and heat transfer phenomenon commonly occurring in many cases.Natural convection in enclosures has been received considerable atteinstruments, solar energy collectors design and nuclear reactor design. Significant numbers of experimental and numerical works had been carried out in the past decades with an attempt to deeply understand the turbulent flow and heat transfer in enclosures. However, the discrepancies between the numerical results and the experimental results of the turbulent natural convection in enclosure are still significant. With developing of computer power and numerical method, the numerical analysis, as a study method, becomes more and more indispensable. So, it is worth making more efforts to study turbulent model for natural convection in enclosures.A great numbers of the published investigations on turbulent natural convection in enclosures indicate that the discrepancy between the numerical results and the experimental results is significant using both the high-Reynolds-number k-ε model and the low-Reynolds-number k-ε model. Based on this situation, a series of works have been done in this dissertation:1. To find the reason that the discrepancy exists between the numerical results and the experimental results, the differences of forced convection and natural convection are analyzed from the aspect of the analogy between momentum transfer and heat transfer. According to the analysis, it is found that the definition of, at=vt/σt, should not be used to enclose the turbulent heat flux,-ρcpuT’in energy equation because the analogy between flow boundary layer and thermal boundary layer does not exist in natural convection.2. A composited k-ε model to resolve the turbulent natural convection heat transfer in enclosures is proposed. The composited k-ε model is used to numerically analyze the characteristics of fluid flow and heat transfer of natural convection in an air-filled enclosure. The numerical results are compared with the published experimental results and the numerical results, respectively. The results indicate that the composited k-ε model has a higher prediction accuracy than other models in some aspects, but the composited k-ε model should be further improved.3. To improve the accuracy of the composited k-ε model, turbulent Prandtl number is modified into σt=1+Pr, and then a revised turbulent model is formed. The revised turbulent model is used to analyze the characteristics of turbulent natural convection in enclosure and the results are compared with the published experimental results and numerical results, respectively. The comparisons indicate that the revised k-ε model can accurately predict the heat transfer characteristics of the hot and cold wall under the condition of Ra>109. Especially, when109<Ra≤1014, the averaged relative error between the numerical results and the published experimental results is equal to8.43%, which is much lower than any numerical results reported. In addition, on the condition of108≤Ra≤109, the numerical results of the revised k-ε model are in good agreement with the results reported in the published literatures.4. The revised k-ε model is used to numerically analyze the heat transfer characteristics of a heat exchanger used in a typical room for heating in Lanzhou region. More details of the effects of outerwall types, outerdoor environment conditions, heat exchanger surface area, heat transfer between neighbour rooms on the heat transfer characteristics of the heat exchanger, the surface temperature of the heat exchanger and the heat quantity supplied by the hat exchanger, the flow field and temperature field of the room arc obtained.更多还原