摘要
随着经济的飞速发展,我国能源需求量迅猛增长,能源供应不足已成为制约我国经济社会发展的一大重要因素,这对我国的节能减排事业提出了更高要求。工业生产中的热能,约50%以低品位余热的形式直接排放,既浪费能源又污染环境。作为电能消耗大户,铝电解过程中约30%的输入能量最终以低温烟气(100-200℃)的形式经烟管汇总、收尘后直接排放。回收和利用这部分热能进行发电对铝电解行业的节能减排具有重要的现实意义。
本文以湖南省高新领域工业支撑科技计划重点项目“低温烟气余热发电关键技术研究”为依托,采用数值模拟方法,对铝电解低温烟气余热发电系统中翅片管式换热器的传热性能进行了深入研究。
本文的主要研究内容与结论如下：
(1)以大型计算流体动力学商业软件FLUENT为平台,建立了翅片管传热的计算模型。在建模时,经过综合比较,截取单元管长进行计算,并将管内工质对流换热与管外烟气对流换热进行耦合计算,从而避免了以往计算中将管壁设置为恒热流或恒壁温边界条件时,所造成的与管内外实际对流换热情况存在的差异。
(2)对管外烟气及管内液态与气态工质在不同流速与温度时的换热特性进行了数值模拟,结果表明：随着管外烟气和管内工质流速的增大,其各自的平均换热系数增大,换热得到加强；而当温度提高时,管外烟气与管内液态工质换热系数增大,但因为受到物性参数的影响,气态工质换热系数随温度的提高而有所下降。计算域内烟气侧换热系数在50-130W／(m2·K)之间,工质侧换热系数在190-1900W／(m2·K)之间。
(3)沿用实验研究的思路,将各工况下的数值模拟结果整理成Nu数的形式,分别得到了烟气和工质的换热特性曲线,并通过最小二乘法原理拟合得到了二者的换热准则方程式。同时,搭建了小型实验平台,采用传热有效度——传热单元数方法,通过编制计算程序对单元翅片管进行迭加,从而实现了数值模拟结果与试验热交换器测试结果的检验,结果表明,计算值与实验值的最大相对误差不超过20%,数值模拟结果可靠。
With the rapid development of economy, the energy demand is increasing surprisingly in China. Energy short supply has been one of the principal elements restricting the development of economy and society, which makes energy-saving and emission-reduction becomes an urgent task. In industrial processes, almost 50% of thermal energy transforms into low-grade waste heat discharged directly, which leads to energy waste and environmental pollution. In aluminum electrolysis enterprises, the main energy consumers, nearly 30% of input energy transforms into low temperature (100-200℃) flue waste heat and is discharged after the dust collection process. Recycling and utilizing those waste heat energy to generate power is of great significance for energy-saving in the aluminum electrolysis industrials.
Based on the program "The research of key technologies in the power generation system using low temperature flue waste heat" supported by the high-tech plan in the industrial filed in Hunan province, the thermal performance of finned tube heat exchanger was studied by using the numerical simulation method in this dissertation.
The main contents and conclusions are as following:
(1) The calculation model of heat transfer in the finned tube heat exchanger was established by using the computational fluid dynamics software-FLUENT. In present model, comparisons were performed synthetically and finned tube with unit length was chosen to be calculated. In order to avoid the difference between the actual situation and the situation in which the constant heat flux boundary condition or constant wall temperature boundary condition was set on the tube wall, the coupling calculation method was applied to investigate the convective heat transfer performance of waste flue outside the tube and working fluid inside the tube.
(2) The influences of temperature and velocity on thermal performance were researched among waste flue, liquid and gaseous working fluid. The results of numerical simulation show that as the velocity increased, for waste flue and working fluid, the average surface heat transfer coefficient increases and heat transfer performance is promoted. As the temperature increases, the average surface heat transfer coefficient of waste flue and liquid working fluid increase, however, that of gaseous working fluid decreases due to the difference property parameters between the liquid and gas. In the calculation zone, the average surface heat transfer coefficient on waste flue side ranges from 50 to 130 W/(m2·K), and ranges from 190 to 1900 W/(m2·K) on working fluid side.
(3) According to experimental study, the result data of numerical simulation under various operating conditions is usually arranged into the form of Nu. The characteristic curve of heat transfer was obtained and dimensionless equations of heat transfer were fitted by least square method. An experimental platform was built and a calculation program was programmed based onε-NTU method. Finned tube units were accumulated using programming, by which numerical solutions and experimental conclusions were compared. Conclusions show the max relative error of results between program and experiment is below 20%. The results show the reliability of the numerical simulation.
引文
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