【作者】 张艾萍；

【导师】 徐志明；

【作者基本信息】 华北电力大学 ， 热能工程， 2013， 博士

【摘要】 本文根据对流换热场协同理论和超声学理论,对污垢及除垢超声波对换热器内对流换热场协同的影响和超声波防除垢效果的影响因素及除垢效果的评价等问题进行了研究。首先分析了污垢的特性及其对传热和对流换热场协同的影响；其次,为了提高超声波防除垢效果,根据多普勒频移原理,建立了超声波在流动液体中传播的数学模型,并求出描述超声波声压分布的解析解。利用数值模拟软件,对超声波在各种管型中传播特性进行数值模拟,得出超声波的声压分布及其频响特性,从而分析了影响除垢超声波传播特性和空化的因素。在此基础上,利用数值模拟,研究了超声空化对换热管内速度场与热流场协同的影响。为了优化超声波防除垢仪的运行,首次提出防除垢效果实时在线评价方法,并定义了评价指标。最后,对数值模拟结果进行了实验验证,并根据所得研究成果,提出了优化超声波防除垢仪设计和运行的构想,为后续研究指明了方向。根据场协同原理,应用数值计算方法分别对不同管型内由CaCO3污垢对换热过程的影响进行了数值模拟,并对计算结果进行了细致的分析,对每种管型做出了客观的评价。计算结果表明,各种管型的污垢对速度矢量与温度梯度的协同程度影响不同,其中,波纹管的协同角β随污垢厚度的增加而减小,在结垢状态下波纹管的管内换热能力有所增强。而其它管型的协同角β随污垢厚度的增加而增大,协同程度变差。所有管型的速度与压力梯度的场协同角θ随污垢厚度增加均增大,压损增加。交叉缩放管的速度与温度梯度的协同角β要比其它强化换热管型小,其换热效果要强于其它管型；并且其速度与压力梯度的协同角θ受污垢的影响也较小,从综合换热性能系数来看,交叉缩放管的综合换热性能系数最高,所以交叉缩放管在高效低阻强化换热领域有很大的发展前景。根据多普勒频移原理,对静止的波动方程进行改进,建立了超声波在流动液体和粘滞性液体中传播的数学模型,并求出解析解。求解结果表明,当换热介质流动速度不大时,声压衰减幅度不大。当介质流速增大时,相应的单位体积内的能量密度减小,防垢效果减弱。液体的粘滞性是超声波在换热介质中传播衰减的主要原因。粘滞性的增加,使介质的声吸收系数增大,使超声波吸收的速度加快,减小了超声波传播的距离,降低超声波的防除垢效果。利用数值模拟软件,对超声波在各种管型中传播特性进行数值模拟,得出超声波的声压分布,从而得到强化管对超声波防除垢的影响。利用数值模拟研究了超声空化对换热管内速度场与热流场协同的影响,得出不同结构形式管道的空化数是不同的,圆管的空化数最大,然后依次为横肋管、波纹管,最后为横纹管。场协同数与空化数存在对应的函数关系,流体场协同数随着空化数的增加而增加。超声空化后流体速度与温度梯度的场协同数要比未空化时的场协同数减小结合传热理论,首次提出了一种可以实时在线评价超声波防除垢效果的方法,并定义了评价指标,对影响评价指标的因素进行了分析和修正,使修正后的评价系数能够在线评价换热器在变工况条件下的除垢效果。建立超声波防除垢实验台,通过实验测试了流速和温度对超声波传播特性的影响,实验结果表明：超声波在水中传播的最佳介质温度为55℃,超声波在换热器内的有效作用距离约为2m。更多还原

【Abstract】 The paper studied mainly fouling and descaling ultrasorlic influence heat convection of pace synergism in convection heat exchange, and the factors which affects ultrasonic anti descaling effect and the question of descaling effective evaluation based on principles of field synergy and ultrasonic theory.Firstly, analyzing the characteristics of fouling and it affects heat transfer and heat convection of pace synergism.Secondly, according to the Doppler frequency shift principle, in order to improve the ultrasonic anti-descaling effect, establishing the spread of mathematical model of ultrasonic in flowing liquid to find out the analytical solution for distribution of sound pressure. The propagation characteristic of ultrasonic in all kinds of tube type was acquired by using numerical simulation software. It was concluded that the distribution of sound pressure and frequency response characteristics, and analyzing the factors that affect the propagation characteristic of descaling ultrasonic and cavitation, then, for further study, the ultrasonic cavitation effect on the velocity and heat flow of field synergy heat exchange tube. In order to optimize the operation of the ultrasonic anti-descaling instrument, the author put forward real-time online evaluation method of anti-descaling effect and defines the evaluation index for the first time. Finally, the numerical simulation results validated by experiment, and according to the research results, the paper puts forward optimizing instrument design and operation of conception for ultrasonic anti descaling, and pointed out the direction for follow-up research.According to the principles of field synergy, heat transfer process was affected by different tubes which scale CaCO3dirt by using numerical simulation method, and the results of calculation made a detailed analysis and objective evaluation. The result of calculation indicates that the velocity vector and the temperature gradient of the synergy degree were affected by different tube. The collaborative angle B of corrugated pipe decreased with the increases of dirt thickness, and the bellows tube heat exchange capacity and the heat exchange capacity of corrugated pipe increased in the fouling state. However, the collaborative angle β for other type tube increased with the thickness of fouling and the synergy degree become poor. The synergy Angle θ of velocity and pressure gradient increased with fouling thickness and the loss of pressure for all types of tube. Although the collaborative angle B of speed and temperature gradient for alternating axis tubes was smaller than other heat exchange tube, the heat transfer effect was stronger than other tube type and the collaborative angle θ of velocity and pressure gradient were less influenced by fouling. From the view of integrated heat transfer coefficient performance alternating axis tubes, the performance of comprehensive heat exchange coefficient was highest. Alternating axis tubes had great prospects on the high efficiency and low resistance enhanced heat transfer area.According to the Doppler frequency shift principle, wave equation on the stationary was improved, then established spreading mathematical model of ultrasonic flow in liquid and viscous liquid, and calculated the analytic solution. The results showed that acoustic attenuation amplitude was relatively small when the heat transfer medium flow rate was relatively small. When the flow velocity increased, the energy density per unit volume will be smaller and anti-scaling effect will be weaker. Liquid viscosity is the main reason for ultrasonic attenuation in heating medium. Viscosity increasing made the increase of absorption coefficient and the speed of ultrasonic absorption, and made the reduction of ultrasonic wave propagation distance and ultrasonic anti-fouling effect. Using numerical simulation software, the ultrasonic propagation characteristics in a variety of tube were numerically simulated that the distribution of sound pressure and the effect of enhanced tube of ultrasonic anti-fouling can get.We can get cavitation effect on velocity and temperature of field synergy in the heat exchanger by using numerical simulation. Simulation results showed that cavitation coefficient was different while difference of structure forms pipe. Cavitation coefficient of round tube was maximum, transverse tube’s coefficient was greater than the bellow, and coefficient was minimum for transversely ridged tube. Field synergy coefficient and cavitation coefficient existed a corresponding function that synergy coefficient in the fluid field increased with increase of cavitation coefficient. Field synergy coefficient of fluid velocity and temperature gradient after ultrasonic cavitation was smaller than the untreated.The thesis proposes the method that can realize real-time online evaluation for ultrasonic anti-fouling effect combined heat transfer theory, and defined the evaluation index. Evaluation indexes which were analyzed and modified made the coefficient can use to online evaluation of the descaling effect when heat exchanger worked in variable conditions.The ultrasonic anti-fouling experimental station is established. In the ultrasonic antifouling experiment station, the effect of temperature is studied on ultrasonic propagation character of descaling.The results show that the best temperature is55℃, ultrasonic wave propagation in the water.And ultrasonic effective action range is2m in heat exchanger.更多还原