【作者】 王艳；

【导师】 俞坚；

【作者基本信息】 北京工业大学 ， 热能工程， 2009， 博士

【摘要】 吸收式制冷技术作为一种以热能为驱动力、对臭氧层无破坏作用的制冷方式,近年来越来越受到工业界及相关科研工作者的重视。太阳能吸收式制冷因为可有效地利用低品位热源和可再生能源,成为了近年来吸收式制冷技术研究的重点。但由于受热源温度的限制,太阳能吸收式制冷系统的制冷系数不能得到有效提高,从而得到广泛应用。为了提高太阳能吸收式制冷系统的性能,本文从溴化锂溶液的特性着手,根据旋流理论在国内首次提出了利用流体的旋转运动,降低吸收式制冷系统发生器内溴化锂溶液的蒸发压力,从而降低溶液的蒸发温度,在不增加外界热源的情况下,增加用于制冷循环的冷凝蒸气量,提高吸收式制冷系统制冷效率的方法。根据旋流理论提出了一种新型的用于吸收式制冷循环的双室涡旋发生器。吸收式制冷循环是利用相变过程伴随的吸、放热来获取低温,以消耗热能为动力的制冷方式。吸收式制冷循环中工质的化学和热物理性质对系统性能起着关键性作用。为了更好地研究溴化锂吸收式制冷系统,提高系统的性能,建立了溴化锂溶液和水蒸气随压力、温度和浓度的热物性参数方程。同时分析了采用本文所提出的双室涡旋发生器的小型太阳能吸收式制冷系统的热质平衡特性。为了得到最优的双室涡旋发生器结构,提高太阳能吸收式制冷系统的效率,建立了一种具有切向入口的涡旋发生器结构,使流体通过切向入口进入到发生器内产生旋转运动。采用流体动力学软件FLUENT模拟了不同结构的涡旋发生器内流体的流动及传热特性。模拟结果表明:流体通过切向入口进入到涡旋发生器后,产生了强烈的旋转运动,形成了以中部为核心的Rankin组合涡。在发生器内,流体的压力呈抛物线分布规侓，在中心处，压力最小魈逶诜⑸髂诘难沽Γ?着流体入口速度的增大而减小;随着入口喷嘴尺寸减小,发生器内的压力减小,从而有效地降低了发生器内溴化锂溶液的蒸发温度,形成有利于溴化锂溶液蒸发的环境。通过数值模拟可知:利用流体的旋转运动,可有效地降低涡旋发生器内的蒸发压力,从而降低进入到发生器中溴化锂溶液的蒸发温度,提高热源的可利用温差。在不改变外界热源质量与数量的前提下,达到增加用于制冷循环的冷凝蒸气量,提高系统制冷系数的目的。根据数值模拟计算结果设计了一种由圆锥体和圆柱体组成的双室涡旋发生器。双室涡旋发生器的锥角为20°,流体的入口采用与圆柱体相切的三段式渐缩喷嘴保证流体切向进入到发生器内产生旋流运动。双室涡旋发生器由高压发生室和低压发生室组成。低压发生室利用流体的旋转运动降低溴化锂溶液的蒸发压力,高压发生室用来对产生的冷凝蒸气进行压力恢复。介绍了采用双室涡旋发生器的吸收式制冷系统实验装置的循环流程以及实验装置中各设备的选型及参数,为小型太阳能吸收式制冷系统双室涡旋发生器流体特性的研究提供实验平台。通过实验研究了双室涡旋发生器内流体的流动及传热特性。实验研究结果表明:流体在双室涡旋发生器中产生强烈的旋转运动,有效地降低发生器中部的发生压力,提高了溶液的可利用温差,产生更多用于制冷循环的冷凝蒸气量。采用双室涡旋发生器的吸收式制冷系统,其COP随着入口温度的增加而增大。当溶液入口温度达到90℃时,其COP值达到0.83,比传统吸收式制冷系统的COP值高22%。采用双室涡旋发生器的吸收式制冷系统由于可以在低温情况下实现高效制冷,因此可以有效地利用太阳能、废热、地热等低品位热源,解决能源利用以及单效吸收式制冷系统由于热源温度低,系统性能较低的问题。更多还原

【Abstract】 As one method of refrigeration, which is driven by heat energy and has no damage on the ozone layer, absorption refrigeration technology has attracted more and more attention from industry and related researchers. Solar absorption refrigeration has become the researching emphases of absorption refrigeration because which can use the low-grade heat source or the renewable energy. But the improvement of the coefficient of performance (COP) of the solar refrigeration system is limited due to the lower temperature of the heat source, thus can not be widely used. In this paper, according to the vortex flow theory, a new dual chamber vortex generator model is suggested to improve the COP of the absorption refrigeration system. The vortex generator uses the rotating flow of the fluid to reduce the evaporation pressure and temperature at the generator. The main purpose is that at no additional external heat source to increase the refrigerant vapor and improve the COP of the solar absorption refrigeration system.Absorption refrigeration cycle is the use of phase change accompanied by the absorption process, to obtain low-temperature heat and consume the thermal energy for cooling. In the absorption cycle, the chemical and physical properties of the working pair play a key role for the system performance. In the paper, the relationship of the pressure, temperature and concentration of lithium bromide solution has been set up. At the same time, the heat and mass balance of the small-scale solar absorption system which uses the dual chamber vortex generator is analyzed.A new generator model which has a tangential inlet is set up. The optimum structure of the dual chamber vortex generator has been obtained through using the CFD software to simulate the flow and heat characteristics of different structure vortex generator. The results of the study showed that fluid entered the generator through the tangential inlet produced a strong rotation and the Rankin vortex at the center of the generator. The pressure distribution is parabola regulation. At the center the pressure is lower than around of the generator. The pressure decreased with the fluid inlet velocity increased and the inlet diameter decreased. According to the characteristic of the Lithium Bromide solution the evaporation temperature decreased in the generator with the fluid inlet velocity increased and the inlet diameter decreased. A low pressure region is created because of the vortex flow which will help the evaporation of Lithium Bromine solution at low temperature. The available temperature difference of lower-temperature heat source from an external resource increased to generate more refrigerant vapor and improve the COP of the solar absorption chiller.A novel dual chamber vortex generator which consisted of a lower chamber and upper chamber is designed in according to the numerical simulation. The lower chamber is composed of a cylinder and a cone part whose conic angle is 20°with a tangential inlet. The tangential inlet is composed of three parts to ensure the rotating flow of the fluid in the generator. The upper chamber is a cylinder with a central inlet at the bottom and two outlets at the side. Due to the rotating flow, the pressure was reduced toward the central portion of the lower chamber. And the refrigeration vapor which produced in the lower chamber came back the condensing pressure in the upper chamber. In order to investigating the flow and heat transfer characteristic, a small solar absorption chiller testing system is set up. Main four flow loops of the experiment system and the parameters of apparatus are introduced. In the thesis a novel dual chamber vortex generator was investigated under different temperatures and solution mass flow rates. The experiments and analysis results showed that the fluid enters the vortex generator to create a strong rotation.The lower pressure developed in the lower chamber could help the evaporation in the generator. The higher the pressure difference, the lower the saturated temperature which can utilize heat source to generate more refrigerant. And the higher the inlet temperature, the larger the available temperature difference. The higher the inlet temperature, the higher the vapor mass flow rate and the evaporation ratio. As solution mass flow increases, the evaporation ratio decreases and the vapor mass flow rate increases The COP increases as the solution inlet temperature increases. When the inlet temperature is 90℃, the COP can reach 0.83, which is higher than that of a conventional absorption chiller by 22%. The experiment research has depicted that the COP can be improved much higher by using the novel dual chamber vortex generator in the solar absorption chiller.更多还原