【作者】 游永华；

【导师】 黄素逸； 范爱武；

【作者基本信息】 华中科技大学 ， 工程热物理， 2013， 博士

【摘要】 管壳式换热器广泛应用于我国各个工业生产部门,强化其传热速率可以有效提高能源利用效率,进而缓解能源供需矛盾。本文从数值模拟和实验测量两方面入手,分别就提高管侧和壳侧热流体综合性能,开展管壳式换热器单相强化传热研究,主要研究工作情况如下：(1)采用数值方法从提高综合性能指标和减小熵产(数)两个方面研究一种新型管内插物,即锥形扰流片,层流条件下的强化传热性能。研究表明锥形片内插物可以有效强化管内层流换热,同时流动阻力也有一定幅度增加。与错排锥形片相比,顺排锥形片对管内流体的扰动较强烈,传热速率增加较多,综合性能较好。顺排锥形片强化管各雷诺数平均的Nu数比光管最多强化了4.51倍,平均f因子增加比为2.31-14.77,综合性能指标PEC值在1.17到2.97之间。锥形片内插物结构参数,特别是锥形片张角和间距,对强化管热流体性能影响较大,倾角30°、张角60°、无量纲间隙0.10和片距1.5是本次研究得到的最佳内插物参数组合。熵产计算表明在层流条件下,粘性熵产和传热熵产都随着强化管Re数的增加而加大,后者数值远大于前者,在整个不可逆损失中占有主导地位。锥形片张角较大、无量纲间隙和片距较小时,强化管的熵产较小,不可逆损失较少,从有用能损失最小的角度看综合性能较优。(2)采用实验测量和数值计算两种方法系统研究了一种新型管束支撑元件,即三叶孔支撑板,湍流条件下的壳侧强化传热性能。实验研究表明三叶孔板可以有效强化壳侧传热性能,并得到了三叶孔板换热器壳侧流动、传热性能的试验准则关系式。三叶孔板换热器实验样机在大Re数下传热速率较高,然而它的综合性能随Re数的增加而变坏,壳侧组合参数Nu/△p在16kPa-1～34kPa-1范围之间。数值研究表明三叶孔板形成的射流和强烈回流,可以冲刷管壁,提高流体湍流强度水平,这加速了主流流体混合,同时使边界层厚度减小,最终有效强化了换热管外表面的传热速率。就本文所研究的案例来看,改变三叶孔板板距对壳侧强化传热性能的影响不是很明显,但对流动阻力增加的影响较大。板距较大时,壳侧压力损失较小,因而综合性能较优。四叶孔板换热器壳侧表面传热系数h仅比三叶孔板有少量增加,而压力损失Δp增加却很多,单位压力损失的表面传热系数(h/△p):和单位泵功传热能力(hA/W)分别只有后者的68%和10%。(3)采用数值模拟和实验验证相结合的方法研究花隔板换热器壳侧湍流强化传热性能。数值研究借助多孔介质的概念,引入分布阻力和分布热源,建立了基于空隙率和渗透率的管壳式换热器流动传热性能计算的数值模型。数值模型对花隔板换热器进行了计算,壳侧主要参数的模型预测值与实验值的最大相对误差在14%范围内。本文还利用计算得到的速度温度云图,分析了花隔板强化壳侧传热的物理机制,并与折流板换热器的预测流场温度场进行了比较。折流板换热器虽然表面传热系数较大,但壳侧流动阻力增加更多,壳侧综合性能指标h/Δp只有花隔板换热器的82%。本文创新点主要体现在以下两个方面：(1)从不同角度系统研究了一种新型管内插物(锥形扰流片)和两种新型管束支撑元件(三叶孔支撑板和花隔板)强化传热性能。包括采用不同评价体系(热力学第一定律与第二定律评价方法),应用不同研究方法(数值模拟与实验研究),调查强化元件结构参数的影响,分析强化传热物理机制(场协同角计算与流场-温度场云图分析)等。(2)灵活应用各种CFD建模方法开展强化传热研究,以满足不同对象不同计算工作量和计算精度要求。本文采用的数值建模方法大致包括单元流道方法、流固热耦合方法、周期性模型方法、基于多孔概念的简化方法、UDF(S)、并行计算、批处理、单层和两层k-ε湍流模型等。更多还原

【Abstract】 Heat exchangers are widely applied in a variety of industrial fields. Heat transfer enhancement improves their thermal efficiencies effectively, and energy consumptions are decreased. As a result, the imbanance between the supply and demand is eased up. In the current study, numerical simulation and experimental investigation are conducted to research the single-phase thermal augmentation for shell-and-tube heat exchangers, and special attentions are paid to the improvement of overall performances on the shell and tube sides. The main contents of present work are listed as below:1. A new type of tube insert, i.e., conical strip inserts, is numerically studied for the laminar heat transfer enchancement on the tube side. Larger and minimal entropy generation are adopted as the performance evaluation criteria. Investigation results demonstrate that conical strip inserts can enhance the heat transfer rate effectively, while the flow resistance increases as well. Compared with strip inserts of staggered alignment, non-staggered ones result in larger increment of heat transfer rate amd better PEC value. The maximum averaged Nu ratio between enhanced tube and smooth tube is equal to4.51, while the dimensionless PEC lies in the range between1.17and2.97. Compared with slant angle and strip-wall gap, the geometry angle and strip pitch have larger influence on the thermo-hydraulic performances of enhanced tubes, and the parameter combination of slant angle30°, geometry angle60°, dimensionless strip-wall gap0.10and strip pitch1.5generates the best PEC value in the current investigation.Entropy generation computation indicates that both viscous and heat transfer entropy generations increase with Re, and the latter has much larger value than the former and dominates the irreversible loss of convection heat transfer. The larger the geometry angle, or the smaller the strip-wall gap and strip pitch, the smaller the entropy generation of enchanced tube is, and thus the better the overall performance is from the viewpoint of minimal exergy loss.2. Experimental measurement and numerical simulation are successively performed to investigate the shell-side heat transfer enhancement of heat exchangers with trefoil-hole baffles in the turbulent regime. Experimental results demonstrate that trefoil-hole baffles could enhance the heat transfer rate effectively, and experimental correlations of pressure loss and heat transfer rate are obtained, which indicate that the shell-side overall performance of heat exchanger with trefoil-hole baffles becomes worse at larger Re, despite the heat transfer rate has larger value under such conditions. The combined parameter Nu/△p varies between16kPa-1and37kPa-1in the investigated Re range.Numerical study is conducted for the mechanism of heat transfer enhancement on the shell side, as well as for the effects of baffle pitch and tube arrangement on the thermo-hydraulic performances. Numerical results demonstrate that trefoil-hole baffles generate jet flow and intensive recirculation, which can wash the tube wall and increase the turbulence intensity level substantially. Thus, the bulk temperature becomes more uniform, and thermal boundary layer is decreased. As a result, the heat transfer rate is considerably enhanced, meanwhile the flow resistance rises as well. For the investigated cases, larger baffle pitch can decrease the increment of flow resistance effectively, while the heat transfer rate is still notably enhanced. Thus larger pitch facilitates to obtain better overall performance. However, baffle pitch is confined by the strength and stiffness of tubes. Fourfoil-hole baffles generate slightly larger convective heat transfer coefficients (h), while the increments of flow resistance (Ap) are much larger. The combined paramenters of h/△p and hA/W of fourfoil-hole baffles are equal to about68%and10%of the counterparts of the trefoil-hole baffles, respectively.3. Flower baffles are numerically studied for the shell-side turbulent thermal augmentation of heat exchangers, and numerical model is constructed based on the stolen concepts of volumetric porosity and surface permeability, as well as the introduction of distributed flow resistance and heat source. The numerical model is validated by the experimental data of a heat exchanger with flower baffles, and the maximum relative deviations of pressure loss and convection heat transfer coefficient are less than14%. With the numerical results, the mechanism of shell-side heat transfer enhancement of heat exchanger with flower baffles is revealed and comparisons of flow and temperature fields with the counterparts of segmental-baffle heat exchanger are conducted.Main innovative points of the present research lie in below: 1. One new type of tube insert (i.e., conical strip insert) and two new types of tube bundle supports (including trefoil-hole baffle and flower baffle) are investigated for the thermal augmentation on the tube and shell sides of shell-and-tube heat exchangers, respectively. And varied viewpoints are adopted in the research, such as evaluation criterion of the first law of thermodynamics vs that of second law; numerical study vs experimental investigation; research on the effects of geometrical parameters vs analysis of thermal augmentation mechanism, etc.2. A variety of CFD techniques are applied in the current research on the thermal augmentation of heat exchangers, in more details, including unit channel method, periodical modeling method, conjugate heat transfer, simplified modeling method based on the stolen concept of porous media, user-defined-function, parallel-computing, batch processing, single-layer and two-layer κ-ε turbulent models, etc.更多还原