【作者】 田华；

【导师】 马一太；

【作者基本信息】 天津大学 ， 工程热物理， 2010， 博士

【摘要】 随着国际社会对臭氧层破坏和全球变暖等环境问题日益关注,制冷剂减量及替代成为制冷热泵行业的研究热点。本文针对制冷剂替代提出两种方案,即减量延续技术和永久性替代,并针对两种方案的关键技术进行了理论和试验研究。本文通过大量的数据调研,提出了在大中型冷水热泵机组中采用降膜式蒸发方式比目前广泛应用的满液式蒸发减少了30%的制冷剂充灌量,具有重大的应用前景。基于此,对满液式蒸发和降膜式蒸发建立换热模型并进行模拟计算,结果表明:随着热流密度和蒸发压力的增大,满液式蒸发和降膜式蒸发的换热系数是逐渐增大的。而对于降膜式蒸发,液膜雷诺数也是重要的影响因素,随着液膜雷诺数的增大,存在干涸现象出现的临界值,在临界值出现之前换热系数逐渐增大,超过临界值则保持不变。本文建立水平单管满液式蒸发和降膜蒸发试验台,通过对两种换热方式进行换热试验研究,结果验证了理论分析的合理性,同时发现在相同工况下,降膜式换热系数要高出满液式的6.3%。本文根据试验数据提出了新的换热关联式,与大量试验数据保持在13%以内的误差,具有较高的准确性。为了直接观测到两种换热模式的换热机理,本文设计了可视化试验台。通过对满液式沸腾现象的观测和分析发现,汽泡在加热壁面上的汽化核心分为稳定型和不稳定型;汽泡从在壁面上出现到脱离壁面的汽泡生长时间大约在4ms-5ms之间;汽泡脱离直径大约在1.4-1.8mm之间;同时发现,降膜式沸腾产生的汽泡尺寸、汽泡脱离液态制冷剂的时间要小于满液式;降膜式沸腾对流现象和沸腾扰动现象更明显,但当热流密度较大时,容易出现干涸现象。在永久性替代方案中,本文提出采用CO₂双级循环,并就关键部件CO₂双级滚动活塞压缩机进行了设计和加工。对双级循环的分析表明,在相同工况下,双级循环效率比单级循环的效率高出12%～25%。本文对双级压缩机进行了动力学和有限元分析,结果表明:在压缩机开始排气时,各部件的受力情况最恶劣,低压级压缩机的变形量大于高压级压缩机,尤其是低压级偏心段是变形最大的地方,为优化设计提供参考依据。而对双级压缩机的温度场模拟表明,由于双级压缩中间冷却使得排气温度不高,压缩机机体温度均不超过60℃。为了对加工的CO₂双级滚动活塞压缩机进行测试,本文设计了压缩机测试试验台,并分别对低压级压缩机、高压级压缩机和双级压缩机进行测试,结果表明:压缩机运行平稳,其中低压级单独运行效率为35%,高压级为45%～75%。为进一步提高效率,提出了改进方案。更多还原

【Abstract】 As the international community pays more and more attention to the ozone layer depletion and global warming, refrigerant decrease and substitute is becoming a focus in research of the refrigeration and heat pump industry. This thesis mainly brings out two methods of substituting refrigerant: the decreasing & technology and permanent substitutes. We have also made some theoretical and experimental study on the key technology of these two methods.According to a large number of statistics, we have found that adopting the falling film evaporation in large and medium-sized heat pump systems reduces refrigerant charge by 30% compared with the widely used flooded evaporation. So the falling film evaporation has good prospects of application. Based on this, we established a heat transfer model and did simulation calculation on the flooded evaporation and falling film evaporation. And the results show that the heat transfer coefficient of the flooded evaporation and falling film evaporation increases with the rising heat flux and evaporation pressure. However, for the dry falling film evaporation, film Renault number is an important factor. As the film Renault number increases, there will be a critical value for the dry-out phenomena, and the heat transfer coefficient increases before reaching the critical value and then it will stay unchanged thereafter. We built up a test-bed for the horizontal single tube flooded and falling film evaporation. Through heat transfer experimental research on the two types of heat transfer, the results show that the theoretical analysis is rational and we also find that the heat transfer coefficient of falling film evaporation is 6.3% higher than the flooded type under the same conditions. In addition, this thesis gives a new correlation of heat transfer that is within a 13% deviation from the numerous data and is relatively accurate.In order to directly observe the heat transfer mechanism of the two heat transfer models, we made the visual test-bed. Through observing and analyzing the flooded boiling phenomena, we find that bubbles of the nucleus of boiling on the heating surface can be stable and unstable; the lasting time from the bubble occurring to the bubble departing the wall is about 4ms-5ms; the bubble diameter while departing is between 1.4-1.8mm; flooded boiling generates larger bubbles than the falling film boiling and the bubbles need more time to depart the surface; the convection and boiling interruption phenomena in falling film boiling is more evident, but the dry-out tends to occur when the heat flux is large.In the permanent substitute scheme, this thesis adopted the two–stage CO₂ cycle and came up with the designing and manufacturing of the key parts of the two-stage CO₂ rolling piston compressor. Analysis of the two-stage cycle shows that the efficiency of the two-stage cycle is 12-25% higher than the single stage cycle. This thesis made dynamic analysis and finite-element analysis on the two-stage compressor and the results show that when the compressor starts to exhaust, every part suffers the largest intensity of bearing and the distortion of low-pressure compressor is worse than the high-pressure, especially the eccentric part of the low pressure, which offers references for the optimization design. The simulation on the temperature of two-stage compressor indicates that the exhaust temperature is not high because of the intermediate cooling of the two-stage compression and the temperature of the compressor body is lower than 60℃.In order to test the CO₂ two-stage rolling piston compressor, we designed the test-bed for the compressor and did tests for the low pressure stage, high pressure stage and two-stage compressor. The test results show that the compressor operates smoothly and the efficiency of the low pressure stage and high pressure stage is 35% and 45⁷5% respectively. Further more, we brought out the implement plan to increase the efficiency.更多还原