【作者】 宋艳；

【导师】 姚杨；

【作者基本信息】 哈尔滨工业大学 ， 供热、供燃气、通风及空调工程， 2008， 博士

【摘要】 在目前常用的浸没式换热器和壳管式换热器不能较好的满足使用要求的情况下,提出在污水源热泵中使用淋激式换热器。由于污水水质的特殊性,应对污水源热泵中与污水接触的换热器结构型式等问题进行深入研究。并将带该淋激式换热器的污水源热泵系统应用于哈尔滨某制药厂中,回收处理后污水中的热能加热原生污水,以满足该制药厂污水生物处理对水温的要求,保证低温环境下污水生物处理的正常进行,这是寒冷地区寒冷月份污水处理厂及其它污水处理场合的实际需要。本文针对淋激式换热器和带该换热器的处理后污水-原生污水热泵系统,主要的研究内容和研究成果如下:(1)建立水平管降膜流动和换热的数学模型,并对模型方程进行变量转换和无量纲化,将由于液膜厚度变化导致的不规则求解域转换为规则的矩形求解域,给出该数学模型的数值解法,编制计算机程序,进而对淋激式换热器的降膜流动特性,传热特性以及稳定特性进行研究。(2)考虑管间流动形态对液膜厚度的影响,分析三种情况下的液膜厚度分布差异,以及管间距影响。分析在污水源热泵的使用工作范围内,最有可能的是膜状布液降膜流动。膜状布液降膜是管两端液膜厚,中间薄,且液膜厚度变化受管间距影响较小。进行无量纲液膜厚度的拟合关系研究,与其他文献中有关膜厚实验拟合式对比,并通过天津大学的水平管降膜实验验证。(3)研究了污水液膜内速度分布,温度分布,及Nu的变化情况,并对可能影响液膜速度分布、温度分布和换热的因素及影响规律进行分析。喷淋密度和粘度是影响液膜速度的关键因素,保证一定喷淋密度的条件下,基本工况下大于0.148kg/(m.s),淋激式换热器在处理高粘度液体时,仍可以达到0.3m/s以上的平均速度,而且达到这样的理想流速无需外加动力,比壳管式换热器节省动力消耗。管径对平均速度影响不大,但管径的减小使液膜容易产生波动,液膜稳定性降低。液膜内温度分布,越靠近壁面处的液膜温度沿圆周变化越明显,越远离壁面,液膜温度趋于一致。引入无量纲降膜换热系数Nu,进行拟合关系研究,并与其他文献中实验拟合式对比。对液膜的稳定性,包括波动特性和破裂特性等进行分析。在相同接触角下,流量越大,越不易破裂;在相同热流密度下,接触角越大,越容易破裂。换热管材对液膜稳定性也有影响,钛优于不锈钢,不锈钢优于紫铜。并提出目前预防破裂的可行措施。(4)建立淋激式换热器分布参数模型及整个系统模型,预测该处理后污水-原生污水热泵在该制药厂工况下的运行情况,对三级淋激式换热器的换热情况作详细模拟分析,并考察污垢热阻对污水源热泵系统性能的影响趋势和影响程度。在污垢热阻较小时,三级壳管式蒸发器的传热系数大于三级淋激式冷凝器,但随着污垢热阻的增大,壳管式换热器传热系数的降低比淋激式换热器快,减小的幅度比淋激式大。因此淋激式换热器比壳管式换热器更适合于结垢严重的原生污水换热使用。(5)提出淋激式换热器强化传热方法,用异型管(椭圆管)来代替圆管。对椭圆管上液膜的速度分布、温度分布,以及无量纲液膜厚度和无量纲降膜换热系数Nu等进行研究。研究发现随着形状因子a/b增大,平均液膜厚度减小,Nu增大,但膜厚减小,Nu增大的趋势逐渐减小,在基本工况下,a/b大于1.8以后,Nu随a/b的变化已不明显。a/b=2的椭圆管的高降膜换热系数区占半周长的65%,而圆管的高降膜换热系数区只占半周长的48%。开发管型,就是使相同圆周的管子上,具有较高传热膜系数的区域所占比例较大。建议选用形状因子a/b大于1小于1.8的椭圆管,能起到较显著的强化传热的作用。本课题研究是“十一五”国家科技支撑计划项目六“水源地源热泵高效应用关键技术研究与示范”(2006BAJ01A06)的一部分。本文工作对深入认识淋激式换热器降膜流动换热,以及在污水源热泵中切实使用淋激式换热器提供理论指导,并对推广巧用污水源热泵,在诸如药厂、啤酒厂、医院、游泳馆等特殊场合因地制宜地就地回收污水热能,并就地使用提供有益参考和借鉴。更多还原

【Abstract】 Fouling occurs when sewage goes through the conventional submerged-tube heat exchanger and the shell and tube heat exchanger. This in turn, can lead to a decrease in the overall heat transfer coefficient and efficiency of heat exchanger. Due to the deficiencies, a spray heat exchanger was developed for Sewage Source Heat Pump System (SSHPS). The optimal structure of heat exchanger was discussed in consideration of the abundant effects caused by the impurities and the fouling of the sewage. Then, the SSHPS with the spray heat exchanger was used in a pharmaceutical manufacturer in Harbin. The heat of the treated sewage was taken to increase the temperature of the raw sewage for the biological treatment in low-temperature environment. It was considered as an effective and practical technique for the sewage treatment in cold region and in winter.In this paper, the spray heat exchanger and the treated sewage - raw sewage heat pump system with the spray heat exchanger were discussed. Firstly, a numerical model to predict the gravity-driven flow of liquid films in horizontal tube and the thermal characters were developed in the surface-fitted coordinate systems. The methods of the variable transform and the dimensionless function were used. The irregular solving domain, resulted from the changes of the liquid film thickness, was transformed to the regular rectangle solving domain by the methods. The two-dimension model of the exchanger was solved numerically through MATLAB codes. Moreover, the characters of the flow and heat transfer of the falling film were discussed.Secondly, the impact of the flow patterns and the tube’s placement and spacing on the liquid film thickness distribution were studied. The film-distribution was the main state in the liquid film flow. The thicknesses of the liquid film reached the maximum on the top and bottom of the tube and reached the minimum on the middle region of tube surface. Furthermore, the results shows that the horizontal spacing maked less impact on the thickness of the liquid film. The improved predictor on dimensionless thickness of the liquid film was derived. The results by correlating on data value showed in close agreement with the experimental data of the experiment on the falling film of the horizontal tube obtained by Tianjin University and another reported experimental data.Thirdly, the flow velocity and temperature distribution and the Nusselt number variation of the liquid film were obtained. Moreover, the parameters impacted on the flow velocity, the temperature and the heat transfer were discussed. The spray density and viscosity were the key factors associated with the flow velocity of the liquid film. With a constant spray density of 0.148kg/(m.s), the mean velocity of the liquid film can reach the optimal value of 0.3m/s as the spray heat exchanger used the high-viscosity fluid without the external power pumping. It was an important technique for reducing the energy consumption and cost on pumping in contrast to the shell and tube heat exchanger. The diameter of the tube had less impact on the mean velocity of the liquid film. However, the reduction of the diameter of the tube resulted in decrease of the flow stability and the fluctuation of the liquid film. The temperature of the liquid film varied violently with the circular angle near the surface of the tube, and the temperature tended to be accordance near the free surface of the liquid film. The dimensionless Nusselt Number and the improved correlating equation were used to predict the heat transfer of the liquid film. Moreover, the stability, the fluctuation and the fracture character of the liquid film on the tube were discussed. At the same contact angle, the larger the flux of the sewage was, the harder the liquid film was fractured. At the same heat transfer, the bigger contract angle was, the easier the liquid film was fractured. The material of the tube also related to the stability of the liquid film: The tube made of the titanium was superior in stability to the stainless steel and the red copper. This in turn, the methods to improve the stability for the liquid film on the tube were derived.Fourthly, the distributed parameter model for spray heat exchanger and heat pump system were developed to predict the performance of the SSHPS of the pharmaceutical manufacturer in Harbin. The thermal performance of the three-level spray heat exchanger was predicted and studied. Moreover, the impact of the thermal resistance of fouling occurred on the thermal performance of SSHPS was discussed. As less fouling occurred, the overall heat transfer coefficient of three-level shell and tube heat exchanger was even larger than the three-level spray heat exchanger. However, Effects of fouling were more abundant in the shell and tube heat exchanger, than in the spray heat exchanger. The increment of thermal resistance caused much more decrease of the overall heat transfer coefficient of the shell and tube heat exchanger than the spray heat exchanger. So did the speed of the reduction. When compared versus each other, the spray heat exchanger was generally more efficient than the shell and tube heat exchanger etc using heat from the sewage, especially the raw sewage with the impurities.Fifthly, the circular tube was replaced with the elliptical tube to improving the heat transfer coefficient in the spray heat exchangers. The flow velocity distribution, the temperature distribution, the dimensionless thickness of the liquid film and the dimensionless Nusselt Number based correlating equation for the elliptical tube were obtained. The results showed that the increment of shape factor of a/b resulted in the decrease of mean thickness of the liquid film and the increment of the Nusselt Number. The decrease of the thickness of the liquid film caused the increment of the Nusselt Number trended to slow down smoothly. At the condition of a/b>1.8, there were less variation of the Nusselt Number with the value of a/b. The area with high film heat transfer coefficient in the elliptical tube was 65% of the half-perimeter of the elliptical tube at a/b=2. However, the area with high film heat transfer coefficient in the circular tube was only 48% of the half-perimeter of the circular tube. With the same perimeter, the more area with high film heat transfer coefficient in a tube leaded to the better thermal performance for the improvement of heat transfer. Therefore, it was recommended for improving the heat transfer of the exchanger that the optimal shape factor was from 1 to 1.8 and the form of the tube was ellipse.Our research was one of the sub-projects of Study on the key technique and demonstration of high-efficiency water source heat pump system, supported by National Eleventh-Five Year Technological Supporting Projects from China Ministry of Science and Technology (MOST) and coded as 2006BAJ01A06. The technique is also useful for analyzing and improving the heat transfer of falling film on the spray heat exchanger, for optimizing the performance of the SSHPS and for using the heat of the sewage in beer factory, hospital, natatorium etc.更多还原