【作者】 鲁红亮；

【导师】 陈焕新；

【作者基本信息】 华中科技大学 ， 制冷及低温工程， 2010， 博士

【摘要】 平行流冷凝器换热系数高、结构紧凑、制冷剂充灌量少,且其铝质结构能够大量替代铜材,是最有前途的换热器形式之一。目前平行流冷凝器主要应用于汽车空调中,将其用于家用空调可以降低能耗,／提高空调效率,但也会面临工况改变、系统匹配和结霜除霜等一系列问题。本研究获得“基于平行流换热器的节能型低成本家用空调的开发及其产业化(2007A090302115)”及“微(小)通道换热器及其空调系统关键技术产业化(2007241)”资助,对平行流冷凝器的热分布特性和流量分配特性进行了系统研究。基于有限体积法原理建立了平行流冷凝器稳态分布参数模型,包括过热气体、气液两相和过冷液体三种模块,每一模块通过ε-NTU迭代求解热量与压降,经实验结果证实该模型是有效的。采用该仿真模型分析了现有6流程平行流冷凝器中制冷剂传热系数、压力、温度等的热分布特性,统计分析了换热面积与冷凝热量的关系：第一流程的换热面积权重为28.2%,冷凝热权重为34.2%,而第六流程则分别为7.69%和2.27%,可见前者单位换热面积的热利用效率比后者高；扁管总数不变时,分别设计了8个典型流程方案和7个典型扁管方案并进行比较,从理论上论证了从第一流程至第六流程扁管数逐渐增加的方案不如逐渐减少的方案,并非第一流程扁管数越多、最后一个流程扁管数越少冷凝器性能越好,流程数并非越多越好,以六流程、三流程为宜,此时换热面积的变化与制冷剂的物态相互匹配,强化了传热。试制了含6流程40根扁管平行流冷凝器的窗式空调样机,并在焓差实验室提供的标准工况T1,T3和最大工况T3下分别进行了实验研究：与原来采用翅片铜管的样机相比,T1工况下采用平行流冷凝器的空调样机制冷量提高了4.27%,而能效比则提高了12.32%,达到了2.28,这说明采用微通道冷凝器能够增加空调容量,降低耗功,提高能效。选择6个流程中的19根扁管布置T型表面热电偶得到了冷凝器中制冷剂的温度分布：工况T1,T3和最大工况T3下,同一流程各扁管间的最大入口温差存在于第一流程,分别为5.36℃,6.35℃,8.49℃；而最大出口温差存在于第五流程,分别为10.04℃,8.72℃,8.18℃,该温度分布表明单相和两相流体在同一流程并列扁管中温度分布都存在不均匀现象,同一流程各扁管的入口制冷剂温度随着制冷剂在竖直集管中的向下流动而降低,其不均匀性与制冷剂流量分配不均是相互关联的。制冷剂在集管和同流程多并列扁管之间的分流与汇流,是热力学第二定律涉及到的典型不可逆过程,尤其是两相流不仅有流量分配还涉及到不同相的分离问题,其作用机理和耦合规律十分复杂,文中总结归纳了影响两相制冷剂在扁管中流量分配的因素：结构特征、运行工况以及二者的耦合作用。基于可压缩流体模型,根据流体网络理论建立了气态氮气在单流程41根扁管的流量分配仿真模型：扁管流量呈抛物线型分布,、两侧大中间小,第41根扁管流量最大,达到了0.3493g/s,是平均值0.29242g/s的1.2倍,而接近于管网正中间的第21根扁管流量最小,为0.2636g/s,仅为最大值的75.5%,该结果与Yin的实验结果是一致的。基于均相流模型,建立了两相制冷剂在9根扁管中流量分配的预测模型,得到了呈抛物线型的流量分布曲线和分配规律,为揭示建立相应的工程抑制机制提供了有力的理论工具。现场测试表明多联机所连接的多台室内机容量是相互影响且不均匀的,这些室内机及其连接管道构成一个复杂的异程式制冷剂流体网络,这就导致了制冷剂流量在各室内机之间分配的不平衡。在考虑了连接管段阻力的条件下,根据流体网络理论建立了由5台室内机及连接管管网的仿真模型：将该管网中的每一个部件包括两相及单相流部件都视为一个流阻,按照管网拓补结构以并联、串联和混联的方式连接各个部件而构成管网。仿真结果表明：在总制冷剂质量流量改变时,越靠近该流体管网中心位置的室内机,其流量越接近设计值,而两端的室内机有最大的不平衡率；还提出了综合分流不平衡率(IDDR)来评价其管网的总不平衡程度。更多还原

【Abstract】 With compactactness, high-efficiencey, less refrigerant charge and totally aluminum rather than copper, microchannel heat exchanger is one of the most promising heat exchangers in future. However, the microchannel heat exchanger previously used for automobile air conditioner must be redesigned as operating condition, system match and frost and defrost change for household air conditioner. The present research was supported by the cooperation project in industry, education and research of Guangdong province and Ministry of Education of P.R.China (Grant No.2007A090302115) and (Grant No.2007Z41) to investigate thermophysical properties and flow distribution of microchannel condenser.According to s-NTU and finite control volume, a steady distributed mathematical model was developed to simulate heat transfer and pressure drop of air and refrigerant through louver fin and microchannel, and the simulated result agreed well with experiment. It is the key to the overall performance of microchannel condenser whether the flowing area of tubes and passes matches with refrigerant condensation.15 configuration schemes with different pass and tube setup were compared to obtain the optimum configuration for microchannel condenser.A 6kW window type air conditioner prototype with 6-pass microchannel condenser was built and investigated experimentally under T1 standard, T3 standard and T3 maximum operating condition. The result showed that under T1 standard operating condition the new developed prototype performed better than the one with traditional finned copper round tube heat exchanger:254W about 4.27% higher in refrigerating capacity,204W less in input power and 12.32% higher to 2.28 in EER, which revealed superior performance of the prototype. Testing refrigerant temperature distribution instead of mass flow of tubes was proposed to evaluate the thermal performance of microchannel condenser in operating condition. Both near ends of 19 parallel flat tubes were set 38 T-type surface thermocouples to collect the temperature distribution. The experiment showed that the temperature distribution was uneven both in single phase and in two-phase:inlet refrigerant temperature of each horizontal tube in the same pass decreased along refrigerant flow in the vertical header. For T1 standard, T3 standard and T3 maximum operating condition, the maximum inlet temperature differences between the tubes of the same pass were 5.36℃,6.35℃, 8.49℃in the 1st pass while the maximum outlet temperature differences were 10.04℃, 8.72℃,8.18℃in 5th pass.The flow distribution between multiple parallel tubes and header is typically inreversible and difficult to understand especially phase separation involved in two phase flow. Many different factors were summarized as three types:operating condition, structural parameter and the match of the above two. In the assumption of homogeneous flow, a mathematical model based on fluid network theory was developed to predict flow distribution and phase separation in 9 flat tubes and their connecting headers on the second pass of the microchannel condenser. The simulated mass flow rate distribution in 9 tubes is parabolic and approaches to even distribution when inlet quality comes to the median 0.45 from both directions.The field test on GMV-R620W4/A with 16 indoor machine units installed in Hubei Chinese Medical Hospital showed that cooling capacity was uneven for indoor machine. It was supposed to be refrigerant mass flow maldistribution in the multi-connected airconditioning unit’s fluid network. Fluid network theory was employed to develop a model by a series of fluid circuits of different refrigerating components including single-phase pipes and two-phase electronic expansive valve (EEV), evaporator, pipes in parallel or series. Then a particular iterating control algorism was developed to overcome the nonlinearity of fluid resistance and to distribute refrigerant mass flow in proportion to corresponding fluid resistance until the pressure of fluid network achieves a balance. It was found that the closer the indoor unit is to the centre of fluid network, the less its mass flow deviates from nominal value, and the units at both two poles of the pipe network are the ones with maximum disequilibrium.更多还原