共查询到19条相似文献,搜索用时 125 毫秒
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粘土空心砖结构优化的数值模拟 总被引:1,自引:0,他引:1
本文采用有限容积法对50种具有不同孔数及排列方式的240×115×90粘土空心砖进行了当量导热系数的三维数值模拟研究.模拟中考虑了粘土空心砖的孔内表面辐射、孔数及排列方式以及室内外温差对其当量导热系数的影响.模拟结果表明:孔内表面辐射对当量导热系数有不可忽略的作用,孔数及排列方式直接影响当量导热系数的值,存在具有最低当量导热系数的粘土空心砖结构.在该结构下空心砖的当量导热系数几乎不随室内外温差的变化而变.模拟结果对粘土空心砖的结构选型、工程设计及建筑节能具有重要的意义. 相似文献
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纳米流体中悬浮的纳米颗粒可以增强其导热性能已经得到广泛认可,然而纳米流体颗粒增强传热的机理目前尚不清楚.研究表明,纳米颗粒的聚集是纳米流体导热系数增大的重要机制,而且纳米颗粒聚集的形态对纳米流体的导热系数有重要影响,但是目前的导热系数模型大多是建立在Maxwell有效介质理论的"静态"和"均匀分散"假设基础上.本文用平衡分子动力学模拟Cu-Ar纳米流体,采用Green-Kubo公式计算导热系数,采用Schmidt-Ott关系式计算不同聚集形态下的分形维数.对比导热系数与分形维数可以发现:在相同体积分数下,较低的分形维数会有更高的导热系数,分析了分形维数与导热系数的定量关系.此外,通过径向分布函数可以看出纳米颗粒紧密聚集与松散聚集的差异,基液分子在纳米颗粒附近的纳米薄层中处于动态平衡状态.研究结果有助于理解纳米颗粒聚集形态对导热系数的影响机理. 相似文献
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分形多孔介质导热数值模拟分析 总被引:4,自引:0,他引:4
本文基于分形理论构造了不同类型的分形结构来模拟多孔介质,采用有限容积法对其导热问题进行了数值模拟计算,详细分析了基质导热系数、孔隙流体导热系数、孔隙率等因素对其有效导热系数的影响规律.结果发现有效导热系数与基质导热系数、孔隙流体导热系数大致分别成幂函数关系,有效导热系数与孔隙率大致成指数函数关系.并把数值模拟分析结果与文献中的经验公式进行了对比分析,为其提供了数值验证. 相似文献
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多孔介质中的输运过程,如导热、渗流过程,关注的是热量从高温壁面穿过介质到达低温壁面、流体从多孔介质的边界沿孔隙流到另外一端的过程。此类现象可归结为载流子在多孔介质通道(基质或孔隙)中沿外部势差方向的运动过程。多孔介质通道具有分形特征,可以采用分形维数来描述其通道的通透性。本文基于现象的相似性特征,提出并发展了粒子在多孔介质中的方向随机行走模型,用粒子在基质中的方向随机行走过程来模拟真实的热流传输过程;根据分形统计规律得到粒子方向随机行走分形谱维数,并用其描述基质结构的连通性和方向性。研究结果表明,在孔隙率相同情况下,粒子在基质中的方向随机行走分形谱维数与有效导热系数大小有相同的变化趋势。 相似文献
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含湿建筑材料的导热系数 总被引:13,自引:1,他引:12
本文报道了用常功率平面热源法测定含湿加气混凝土砌块导热系数和导温系数的试验结果。发现实测到的总当量导热系数λ随含湿量的增加而单调地递升,名义导温系数α随含湿量的变化曲线却在低含湿量区出现明显的拐点和极小值。文中,对含湿建筑材料在温度梯度下的湿—热迁移机理作了初步探讨,并给出了一种半经验方法,提供了实用的算图(图8)和进行温度修正的式(14)和(15),可由式(13)从实测的总当量导热系数λ中扣除湿迁移影响,由此得到的真导热系数λ_0和含湿量之间呈现线性关系(图7)。 相似文献
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A multidimensional, finite element analysis (FEA) for the freezing, holding, rewarming and heating processes of biological tissues during the cryosurgery process of the new Combined Cryosurgery/Hyperthermia System is presented to theoretically test its validity. The tissues are treated as nonideal materials freezing over a temperature range, and the thermophysical properties of which are temperature dependent. The enthalpy method is applied to solve the highly nonlinear problem. It was found that when the same boundary condition and the same target tissue presented, the novel Cryosurgery/Hyperthermia System could supply the target tissue an approximative cooling rate, a much lower minimal temperature, a much greater warming rate, and a much greater thermal gradients compared with that of the simplified Endocare system. The numerical simulation indicates that the novel combined cryosurgery and hyperthermia system can provide an excellent curative effect in the corresponding cryotherapy. And the most attractive feature of this FEA framework is that it can be easily mastered by the surgeon without in-depth theory of heat transfer to analyze the cryosurgery process beforehand due to the friendly GUI (graphical user interface) of Ansys software. 相似文献
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The heating effect of urethral warming during cryosurgery has been investigated theoretically, via heat transfer simulations. Two warmer configurations have been considered: (i). the clinically available urethral warmer, which has a configuration of a counter flow fluid heat exchanger; (ii). a newly designed urethral warmer, based on a temperature controlled electrical heater, termed a "cryoheater". A dramatic effect of thermal resistance to heat transfer through the heat exchanger wall has been identified, which is absent in the cryoheater. It follows that the cryoheater is expected to be more efficient in generating an unfrozen region around the urethra. It is shown that the conventional heat exchanger may fail to prevent freezing around the urethra in a significant number of prostate cases, depending on the layout of cryoprobes around the urethra. On the other hand, clinical reports exist which suggest that the heat exchanger improves in many cases the outcome of cryosurgery, in terms of long term complications. It is speculated in the current report that the cryoheater can further improve the outcome of cryosurgery, by providing protection from freezing in a wider range of cases. It is suggested that a future study be conducted to examine the correlation between the layout of cryoprobes and surgical outcome. 相似文献
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A model is presented to simulate the cooling processes during tumor cryosurgery with different kinds of flows through the cryoprobe. The heat flux between the cryoprobe wall and the tumor, the heat transfer coefficient under different inflow conditions are obtained numerically. The impact of the inlet mass flow rate, gas volume fraction on these parameters is investigated. It is found that the heat transfer coefficient decreased significantly when inflow changed from two-phase annular flow to droplet flow, and to gas flow. The inlet gas volume fraction and flow velocity only significantly affect the freezing ability of the probe when the inflow is gas or in droplet phase. Simulation of the tumor temperature profiles under different flow conditions show that the heat transfer coefficient is a crucial parameter in temperature prediction during cryosurgery. Results indicate that when the cryoprobe wall is assumed at a constant temperature conventionally, the cooling effect could be overestimated. It would be more reasonable to use the constant wall heat transfer coefficient to simulate the cooling progress under a specific flow. 相似文献
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蔬菜种子干燥中种皮的效应与优化传热传质机理 总被引:4,自引:1,他引:3
1前言含水率大的种子在贮存过程中极易发生劣变,使发芽力与发芽势降低山。种子干燥是一个复杂的传热传质过程,而且还涉及到生命力保存的问题。虽然种子的形态存有差异,但是种子的基本结构都是一致的。一般都由胚、胚乳和种皮三部分组成。种皮对种子具有保护与协调的功能,其透气性与透水性决定了种皮是种子与周围环境进行质量与能量交换的必经之路。为此本文综合种子的生物生理特性研究了蔬菜种子干燥中种皮的作用,并结合种皮的生物学功能探讨了优化传热传质的机理。2种皮对干燥的影响实验装置见文献[2],采用在线测量的方法可连续检… 相似文献
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在血管壁施加第三类边界条件是计算血液与组织间对流换热的一种近似计算方法.为分析其可行性,用有限元数值模拟方法计算血管分支结构中血液与组织的对流换热,得到不同流速和半径下分支血管内血液的截面平均Nu数沿管长的变化曲线.结果表明,血管树中分支血管的Nu数变化幅度不大,且趋于稳定值的速度很快.以相同边界条件下包含简单血管系统的舌体为例,分别用近似方法和完全耦合计算方法,进行血液流场和舌体温度场模拟.通过比较计算结果,得出两种方法得到的温度场分布趋势基本相同;用完全耦合计算方法得到的舌体温度略高于用近似方法得到的舌体温度,两者差值小于0.2℃. 相似文献