【作者】 韩晓东；

【导师】 汤勇；

【作者基本信息】 华南理工大学 ， 机械制造及其自动化， 2016， 博士

【摘要】 在全球气候不断暖化的背景下,以太阳能热水作为驱动能源的太阳能吸收式制冷系统由于其具有季节匹配性好、环保和节能的特点而受到广泛的研究。然而相比于压缩式制冷系统,太阳能吸收式制冷系统性能系数(COP)值相对较低,能源转换效率不高,而且由于太阳能在时间与空间上的分布不均匀性以及能量密度低的特点,太阳能热水温度时常无法达到机组所需最低驱动热源温度,使得太阳能吸收式制冷系统性能容易受到天气因素的影响,这些因素在很大程度上制约了太阳能吸收式制冷系统的应用与推广。基于此,本文以实现太阳能吸收式制冷系统在低品位太阳能热水驱动下实现高效制冷为目标展开研究。在低品位太阳能热水驱动下,发生器内冷剂水蒸发能力不足是太阳能吸收式制冷系统制冷能力低的主要原因。提高冷剂水蒸发能力,即强化冷剂水的相变传质过程,需要通过提高传质驱动力和降低传质阻力两个方面进行。冷剂水的传质驱动力来源于太阳能热水对发生器内溴化锂水溶液所传递的热能,传质阻力则由溴化锂水溶液本身物性和发生器内部的真空度所决定。传统的被动式强化传质与传热手段对于低品位热能驱动的溴化锂水溶液的热质传递过程的强化作用相对有限,因此本文提出了基于超声波强化的主动式热质传递强化方法。本文通过建立太阳能吸收式制冷系统的热力学模型,研究了太阳能吸收式制冷空调的热质传递特性和系统运行特性,并通过太阳能吸收式制冷系统的?分析和热力学完善度分析研究了其性能评价方法,为改善和优化机组性能提供了理论依据和评价指标。通过实验研究了超声波对溴化锂水溶液的传质强化机理及其应用于吸收式制冷机组的可行性。研究了超声波换能器与汽液分界面的距离、超声波功率等在不同驱动热源温度时对溴化锂溶液的冷剂水强化传质的影响。研究表明对于加热热源温度在65℃-80℃时,超声波对冷剂水传质的强化率可达到0.6至0.2之间;对于功率和频率一定的超声波,换能器与传质界面间存在一个最优距离使得超声波对于冷剂水传质的强化率最高。超声波可以显著减小溴化锂水溶液气液分界面处的传质阻力,有效提高发生器内冷剂水的传质速率。通过实验研究了超声波作用于不同结构的加热管壁面时对溴化锂水溶液沸腾传热过程的影响,并对溴化锂水溶液在过冷沸腾和饱和沸腾过程中的超声波强化传热过程进行系统的研究,分析了超声波强化沸腾传热机理。研究表明超声波对于沸腾传热过程起阻碍作用还是强化作用取决于加热壁面处蒸汽泡的初始半径及超声波的功率和频率等多种因素的综合作用,这也是不同研究文献中出现超声波对于沸腾传热过程起到强化或是抑制作用结论不同的原因之一。研究发现通过选择适当频率的超声波在饱和沸腾下其对传热过程的负面影响有限,甚至可以促进发生器内的传热过程。最后根据吸收式制冷系统的热力学分析模型,使用MATLAB/SIMULINK数值计算与仿真软件对太阳能吸收制冷系统进行了仿真研究。并根据实验所获得超声波对溴化锂溶液热传递过程的实验数据,仿真研究了超声波对整个吸收式制冷机组制冷能力和制冷性能的影响。并在实验研究与系统仿真的基础上设计基于超声波强化的太阳能吸收式制冷样机,该样机比传统太阳能吸收式制冷系统在低品位太阳能热水驱动时系统COP有了显著的提升。更多还原

【Abstract】 With the successive warming of the Global climate,solar absorption refrigeration system gets more extensive research,because of its good seasonal matching,environmental friendly and energy saving.However,the solar energy absorption refrigeration system’s COP value is relatively low,compared to the compression refrigeration system.Besides,because of the uneven distribution of solar radiation in time and space and the character of low energy density,the performance of solar absorption refrigeration system is easily influenced by weather factors.These factors largely restrict the application and popularization of solar energy absorption refrigeration system.So this paper investigates how to realize the solar absorption refrigeration system,driven by low grade solar water,to run efficiently.While driven by low grade solar hot water,the insufficient of refrigerant water evaporation capacity in generator is the main reason why the capacity solar absorption refrigeration system is low.In order to improve evaporation capacity of refrigerant water,i.e.,strengthen the phase change mass transfer process,the mass transfer driving force need to be increased and the mass transfer resistance need to be reduced.Refrigeration water’s mass transfer driving force comes from the heat which the solar hot water transmits to lithium bromide solution in generator,and the resistance of mass transfer is determined by lithium bromide solution itself and generator’s internal pressure.And the traditional passive strengthening method of heat and mass transfer isn’t very effective to the lithium bromide solution driven by low grade heat.So this paper presents an active strengthening method of heat and mass transfer basing on ultrasonic vibration.This paper presented a model for the thermodynamic analysis of solar absorption refrigeration system,studied the heat and mass transfer characteristics and the operating characteristics of the solar absorption refrigeration system,analyzed the performance evaluation method with exergy and thermodynamics perfection,the above contents provides the theoretical basis and evaluation index to improve and the performance of the system.This paper studied the mechanism of mass transfer enhancement of the refrigerant water in lithium bromide solution,and verified the feasibility of ultrasonic enhancement.The ultrasonic transducer was placed at different heights from the surface,and the lithium bromide solution was heated to different temperature,then the influence of ultrasound on mass transfer was researched.The study shows when the heat source temperature varies from 65℃ to 80℃,the enhancement ratio of mass transfer can reach 0.2 to 0.6.For the determinate power and frequency ultrasonic,transducer has an optimal distance so that the mass transfer enhancement rate of refrigerant water is the highest with ultrasound.Ultrasound can greatly reduce the mass transfer resistance at the gas-liquid interface of the lithium bromide solution,which effectively improves the mass transfer rate of refrigerant water in generator.This paper experimental studied the ultrasonic effect on heat transfer of copper tubes with different surface structures in lithium bromide solution pool boiling region(including sub-cooled boiling and saturated boiling),and investigated the mechanism of heat transfer enhancement by ultrasonic wave.The research shows that whether the effect of ultrasound on boiling heat transfer process is positive or negative is determined by the initial radius of bubble and the power and frequency of ultrasonic wave.The study shows the negative effect of the ultrasound on the heat transfer is limited for the proper ultrasonic frequency,and even can promote the heat transfer in generator.According to the thermodynamic analysis model of the absorption refrigeration system,this paper makes the simulation study of the solar absorption refrigeration system using MATLAB/SIMULINK numerical calculation and simulation software.And basing on the experiment data,this paper studied how ultrasound influenced the cooling capacity and cooling performance of absorption chiller.Finally,this paper designed a solar absorption refrigeration prototype basing on the experimental research and system simulation,and the experiment results showed that the prototype has a more significant improvement in cooling performance than the traditional solar absorption refrigeration system in low grade heat source.更多还原