风冷冰箱具有大容量、自动除霜等诸多优势，其中单蒸发器三温(冷藏室、冷冻室和变温室)风冷冰箱具有结构简单、多温区的优点，而且变温室可根据不同需求在冷藏与冷冻温区之间灵活转换，已成为风冷冰箱的主流型式之一。单蒸发器三温风冷冰箱中，冷藏室和变温室的送风由各自的风门控制，研究并优化风门控制策略对于冰箱节能意义重大。此外，由于三个温区回风温、湿度存在较大差异，会诱发蒸发器表面非均匀结霜现象，需要深入研究以改善其结霜／除霜性能，降低冰箱能耗。因此，本文搭建单蒸发器三温风冷冰箱试验台，分别研究风门控制策略对冰箱性能的影响、蒸发器表面霜层累积过程和分布特征以及蒸发器除霜特性，并建立风冷冰箱蒸发器变风量结霜模型，分析蒸发器换热面积一定时翅片分布对其结霜性能的影响规律。 实验对比了冷藏、变温风门独立控制和耦合控制对风冷冰箱能耗的影响。独立控制时，两个风门打开过程重叠时间少，甚至交替打开，导致风门打开总时长较大。风门打开时冷藏室和变温室回风温度较高，造成蒸发温度和蒸发器出风温度升高，一方面增大了压缩机功率，另一方面，蒸发器出风温度升高导致冷冻室温度不降反升。因冷冻室温度控制压缩机启停，其不降反升会延长压缩机运行时间。因此，风门打开时间长会导致较大的冰箱能耗。耦合控制时两个风门完全同步打开，风门打开总时长比独立控制时缩短了16.7％。因而，风门耦合控制使压缩机平均功率和开机率分别降低了1.9％和1.3％，冰箱的能耗降低了3.1％。 实验研究了冷藏、变温、冷冻三股回风非均匀局部混合及压缩机启动后蒸发器内制冷剂两相区发展对蒸发器表面霜层非均匀分布的影响。三股温、湿度不同的回风非均匀局部混合诱发了蒸发器表面横向霜层不均匀分布，且中间区域霜层较厚，原因在于三股回风在该区域混和后相对湿度较大，甚至达到过饱和状态，加速了霜的累积；压缩机启动后，制冷剂两相区从蒸发器进口逐渐向出口发展，期间蒸发器进口区域热流密度大于管程中后部，因而进口管排表面更易累积霜，再加上风冷冰箱处在周期性启停的工作状态，从而导致蒸发器进口区域霜层较厚。 实验研究了蒸发器表面霜层非均匀分布下单段电加热除霜过程中的融霜不同步问题。结果表明，采用单段底置加热器除霜时，融霜存在“先下后上”的不同步现象，除霜过程中融霜迟滞时间(即顶部霜层相比底部霜层完全融化的延迟时间)达20.46 min，约占总除霜时间的59.8％；除霜终止时冷冻室箱温回升较大，由-20.4℃上升到了-8.8℃，造成箱体内有较多的热量残留，导致除霜后制冷恢复阶段的能耗较大。 实验研究了采用两段加热器改善融霜同步性时，加热器功率分配对蒸发器除霜性能的影响规律。对比单段加热器(功率为180W)，若两段加热器总功率不变(功率分配为130W／50W)，由于每段加热器功率的降低，其表面温度下降，导致顶部管排融霜迟滞时间延长了15.7％，冷冻室箱温回升增大了2.1℃；若两段加热器总功率增加(功率分配为180W／60W)，顶部管排融霜迟滞时间缩短了10.7％，冷冻室箱温回升下降1.6℃，而且，其除霜及恢复阶段总能耗与一段加热器基本相同。由此可见，蒸发器底部加热器功率不变，额外增加中部加热器功率是改善除霜同步性，减少冷冻室箱温回升的有效方法。 建立风冷冰箱蒸发器变风量结霜模型，数值研究了换热面积一定时翅片分布对蒸发器结霜性能的影响。蒸发器表面结霜过程中，翅片较密管排空气侧压降大，对总压降的增加起主导作用；此外，由于蒸发器风机风量在翅片较密管排影响下衰减较快，而翅片较疏管排空气侧换热系数的增加不能弥补风量衰减对换热量造成的影响，导致翅片较疏管排的换热量在结霜初期几乎不会出现上升的趋势，诱发了蒸发器换热量提前衰减，需要缩短除霜间隔时间以恢复蒸发器换热能力。减少翅片较密管排上翅片数并等量增加翅片较疏管排翅片数后，各管排空气侧压降增幅的差异减小，相同运行时间的总压降减少了7％。从而，蒸发器换热量相同的情况下，除霜间隔时间延长8.18％。因此，风冷冰箱蒸发器换热面积一定时，以各管排空气侧压降趋近等幅增加为原则优化翅片布置，可实现空气侧总压降相同时延长结霜运行时间的目的，从而改善蒸发器结霜性能，降低除霜频次。 关键词：风门耦合控制；霜层分布；除霜性能；翅片分布 论文类型：应用基础

 The air-cooled refrigerator has many merits including large volume and automatic defrost, among which the single-evaporator three-temperature type, with a refrigerting compartment (RC), a freezing compartment (FC) and a varibale temperature compartment (VC), is popular due to simple structure and multi-temperature zones. Besides, the VC could achieve an internal tempertaure within the range from 5℃ to -18℃ flexibly. In a single-evaporator three-temperature air-cooled refrigerator, the supply-air to the VC and the RC is controlled by respective air-dampers. The air-damper control strategy optimization is of great significance for the refrigerator energy saving. Meanwhile, the temperature/humidity of the return air from three compartments differs remarkably and will induce nonuniform frost formation on evaporator surface, necessitating further investigation to improve evaporator frosting/defrosting performance. Accordingly, the test bench for the single-evaporator three-temperautre refrigerator was built, the effect of air-damper control strategy on refrigerator performance and its improvement was studied, and frost formation on evaporator surface and the consequential defrost performance was also investigated. Moreover, the variable air flow rate frosting model of air-cooled refrigerator evaporator was established to analyze the effect of fin distribution on evaporator frosting performance. The effect of RC and VC air-damper control strategies on the refrigerator energy consumption was investigated, including independent and coupling control. When controlled indepedently, the RC and VC air-damper opened with less overlap time, even opened alternatively. As a result, the overall air-damper opening time was extended. During either air-damper opening, the RC/VC return-air with higher temperture would raise the evaporating temperature and then the evaporator supply-air temperature. On the one hand, the compressor power input would increase with the increase in evaporating temperature. On the other hand, the evaporator supplr-air temperature even exceeded the average value in the FC, causing the FC temperature to rise instead of declining. Since the start-up and shut-down of the compressor were controlled by FC temperture, the increase in FC temperature would extend the compressor-on time. As a consequence, the longer air-damper opening time would lead to the increase in the refrigerator energy consumption. The coupling control shortened the overall air-damper opening time by 16.7% compared with the independent control since the RC and VC opening process overlapped with each other. Consequently, the coupling control of two air-dampers dropped the average compressor power input and the compressor-on time ratio by 1.9% and 1.3%, respectively. Finally, the energy consumption of the refrigerator was reduced by 3.1%. Effects of several factors on nonuniform frost distribution on the evaporator surface were investigated experimentally, including nonequilibrium local mixing of return air from the RC, the VC and the FC, and the development of refrigerant two-phase zone in the evaporator after compressor start-up. Horizontal nonuniform frost distribution on evaporator surface was induced by nonequilibrium local mixing of return air from three compartments, and much more frost accumulated in the middle area. The reason was that the local relative humidity was the largest because three streams of return air mixed there, even oversaturated before drawing to the evaporator for cooling, which facilitated frost formation there. Besides, the evaporator was in short of refrigerant feeding when the compressor initially started up, and the two-phase refrigerant gradually developped from the inlet to the oule of the evaporator, during which the heat flux of inlet area was greater than that of outlet area. Hence, the evaporator inlet tube was easy to be frosted. Since the compressor of the refrigerator was in the working state of periodical on/off condition, the frost formation on the inlet tube surface would inevitably be much thicher after long-term operation. The evaporator defrosting performance with single electric heater was studied experaimentally under the condition of frost maldistribution on evaporator surface. Results show that there existed a mismatch between frost distribution and defrost heat dissipation when the single heater was below the evaporator. Hence, the time lag between the frost on the top melt completely behind that on the bottom lasted 20.46 min, accounting for 59.8% of the total defrosting time. Meanwhile, the FC tempertaure was elevated from -20.4℃ to -8.8 ℃, leaving much residual heat after defrosting and necessitating more energy consumption to remove the residual heat during the recovery stage. The effect of power distribution of two heaters on the evaporator defrosting performance was studied experaimentally, and two heaters were equiped at the bottom and in the middle of the evaporator, respectively. Compared to the original single heater with 180 W at the bottom of the evaporator, two heaters with 50 W for the middle one and 130 W for the bottom one led to lower surface temperature due to reduced power input for each heater, thus resulting in extended defrosting time and increased FC tempertaure rise. By contrast, two heaters with 60 W for the middle one and 180 W for the bottom one decreased the time lag between the frost on the top melt completely behind that on the bottom by 10.7%, and reduced FC temperature rise by 1.6℃. Experimental results show that the 180/60 W power distribution for two heaters would not increase the overall energy consumption including that for defrosting directly and for recovery indirectly. Hence, the bottom heater should be allocated with the same power input of the original single heater and the middle heater with additional power input when using two heaters to improve evaporator defrosting performance and reduce FC temperature rise. The variable air flow rate frosting model of the evaporator in refrigerator was established, and the effect of fin distribution on evaporaotr frosting performance was numerically analyzed with the constant heat transfer area. Results show that during frosting, the tube with the highest number of fins subjected to the largest air side pressure drop, which dominated the variation in the overall pressure drop. Meanwhile, the air flow rate of the evaporator fan reduced significantly with the increase in pressure drop. This caused the fact that the increase in heat transfer coefficient of the tube with less fins could hardly make up the decline of the heat transfer capacity that induced by the decrease in air flowrate. Hence, the capacities of the tube with less fins would not be enhanced during the initial frosting stage. Consequently, the decline of the evaporator capacity would occur in advance, necessitating the decrease in time interval between adjacent defrosting action to avoid the over-deterioration in the evaporator capacity. The overall pressure drop reduced by 7% with the same operation time when reduce the number of fins in the denser fin tube and increasing the same number of fins in the sparse fin tube. Meanwhile, the defrosting interval could be extended by 8.18% under the premise of the same heat exchange rate. Therefore, when the heat transfer area of the evaporator kept constant, the fin distribution could be optimized based on the same increase in air side pressure drop but the maximum operation time, so as to improve the evaporator frosting performance and reduce the defrosting frequency. KEY WORDS: Air-damper coupling control; Frost distribution; Defrosting performance; Fin distribution TYPE OF DISSERTATION: Application Fundamentals