低流阻高传热性能宽流道板式换热器研究

发布时间：2018-05-15 09:45

  本文选题：宽流道 + 板式换热器　； 参考：《西安建筑科技大学》2015年硕士论文

【摘要】：随着我国经济快速发展,对能源需求量逐年上升。传统的燃煤锅炉不仅不可避免的产生大量的烟尘,造成雾霾等空气污染,并向大气中排放大量温室气体,而且热能级不匹配,煤是高品位能源,燃烧时温度可达上千度,而建筑物供暖需求只需要维持18?C。二级出水属于低品位能,环境友好,水量巨大,蕴含了巨大的能量,能够满足供暖需求。热泵一般采用直接提取和间接提取二级出水热量,而在直接提取过程中由于二级出水水质复杂容易腐蚀热泵机组,对机组长期运行不利;在间接提取中,目前运行的换热器中普遍存在换热系数不高、污垢生长过快或者运行费用偏高,没有按照污垢生长的特点进行运行。污水厂出水虽然达到了二级出水排放标准,但是取水口水质并非十分干净,比如用水高峰期污水量超过污水厂处理能力,以及下雨天导致污水量猛增,污水厂将采取直接溢流排放的措施,在这些情况下排放水中大尺度杂物都会增加。为了使系统长期运行,保护热泵机组,主要对间接式系统进行研究,对于二级出水这种污杂物水质不稳定的特点,对板式换热器采用宽流道的结构形式,可以有效防止阻塞情况的发生,并通过结构优化达到高传热的目的。首先,本课题对宽流道板式换热器进行建模,使用计算流体软件FLUENT进行模拟,采用平均努谢尔数的方法来衡量换热效果,验证了模拟结果的可靠性,采用雷诺数分离法拟合出相关传热准则方程和欧拉方程。其次,通过研究二级出水生物黏膜的组成和特性,对换热器污垢生长影响因素进行分析,将生物膜污垢分为生长期和稳定期。在生物膜增长期,由污垢积聚预测模型得出污垢热阻生长的渐近型方程;在生物膜稳定期,拟合了污垢厚度和流速的关系,以此可以求出相关流速下的污垢热阻值。从板式换热器的传热原理和热力计算原理出发,推导了换热器面积计算、对数平均温差、总传热系数以及流动阻力的计算公式,结合污垢热阻渐近型方程,开发了二级出水换热器设计软件,该软件能够对换热器污垢生长进行预测。最后,利用数值模拟对换热器结构进行优化分析,综合传热和流动特性27mm的流道间距比较好;运用三项指标对换热器不同板间距受污垢的影响进行评价,结合污垢生长特点得出如下结论:二级出水侧最佳运行流速为0.5~0.6m/s之间;通过变化蒸发器出水温度,对比各项参数可知,当蒸发器出水温度为8℃时,三项指标更好。
[Abstract]:With the rapid development of our economy, the energy demand is increasing year by year. Traditional coal-fired boilers not only inevitably produce a large amount of smoke and dust, resulting in air pollution such as haze, but also emit a large amount of greenhouse gases into the atmosphere, and the heat level is not matched. Coal is a high-grade energy source, and the temperature of combustion can reach thousands of degrees. Building heating needs only need to be maintained at 18 C. The secondary effluent belongs to low grade energy, environment friendly, huge water quantity, contains huge energy, can meet heating demand. Heat pump generally uses direct extraction and indirect extraction of secondary effluent heat, but in the process of direct extraction, because of the complex water quality of secondary effluent is easy to corrode the heat pump unit, it is unfavorable for the unit to run for a long time. At present, the heat transfer coefficient is not high, the fouling growth is too fast or the running cost is too high, so the heat exchanger does not run according to the characteristics of fouling growth. Although the effluent from the wastewater treatment plant has reached the secondary effluent discharge standard, the water quality at the water intake is not very clean. For example, the amount of sewage at the peak of water use exceeds the treatment capacity of the wastewater treatment plant, and the amount of sewage water increases sharply during rainy days. The plant will take direct overflow measures, in which large-scale debris will increase. In order to make the system run for a long time and protect the heat pump unit, the indirect system is mainly studied. It can effectively prevent congestion and achieve high heat transfer through structural optimization. First of all, the model of the plate heat exchanger with wide channel is modeled, and the simulation is carried out by using the computational fluid software FLUENT. The method of average Nuschel number is used to measure the heat transfer effect, and the reliability of the simulation results is verified. The related heat transfer criterion equation and Euler equation were fitted by Reynolds number separation method. Secondly, by studying the composition and characteristics of biological mucosa in secondary effluent, the factors affecting the fouling growth of heat exchanger were analyzed, and the biofilm fouling was divided into growth period and stable period. In the period of biofilm growth, the asymptotic equation of fouling thermal resistance growth is derived from the fouling accumulation prediction model, and the relationship between fouling thickness and flow velocity is fitted in the stable period of biofilm, and the fouling thermal resistance value under correlation velocity can be obtained. Based on the heat transfer principle and thermodynamic calculation principle of plate heat exchanger, the calculation formulas of heat exchanger area, logarithmic mean temperature difference, total heat transfer coefficient and flow resistance are derived, combined with the asymptotic equation of fouling thermal resistance. The design software of secondary effluent heat exchanger is developed, which can predict the fouling growth of heat exchanger. Finally, the structure of heat exchanger is optimized and analyzed by numerical simulation. The flow channel spacing of 27mm is better than that of heat transfer and flow characteristics, and the influence of fouling on different plate spacing of heat exchanger is evaluated by using three indexes. Combined with the characteristics of fouling growth, the following conclusions are drawn: the optimal operating velocity of the secondary effluent side is between 0.5~0.6m/s and the three indexes are better when the effluent temperature of the evaporator is 8 鈩，

本文编号：1891946