Abstract:

To solve the problems such as low suction superheat of the evaporator, droplet entrained in the sight glass and insufficient cooling capacity caused by the maldistribution flow of R507A refrigerant on the tube side of the dry evaporator in the ultra-low temperature screw chiller unit, based on the refrigeration cycle system process flow, the spatiotemporal evolution mechanism of gas-liquid flow in the evaporator tube side was constructed and elaborated. Based on the gas-liquid flow mechanism model, the spatial distribution of refrigerant in the pre-optimization evaporator were assessed via computational fluid dynamics (CFD), clearly revealing that the centrifugal effect at the inlet diversion elbow and the swirling effect inside the branch pipes were key factors leading to refrigerant flow maldistribution in the heat exchange tubes. Based on these diagnostic findings, three improved inlet piping structure schemes incorporating T-shaped inlet diversion pipe were proposed. Numerical simulation analysis demonstrated that the T-shaped structure effectively mitigated refrigerant maldistribution within the tubes, and the scheme 3 was identified as the optimal configuration. Based on this, the effect of different flow directions, different inlet branch pipe spacings, and boundary conditions (maximum load, minimum load, maximum pressure difference, minimum pressure difference) on the uniformity of refrigerant distribution in scheme 3 was further studied. Analysis revealed that flow direction had a minimal effect on the degree of maldistribution in scheme 3, whereas inlet branch pipe spacings exerted a more significant effect on refrigerant maldistribution at the tube bundle ends compared to the middle region. After optimization, the degree of maldistribution of scheme 3 was significantly reduced by 50.01% compared with that before optimization, and the unevenness under the boundary conditions was much lower than that of the model before optimization. This optimization scheme had been verified through unit rectification and experimental tests: under nominal operating conditions, the optimized dry evaporator achieved an approximate 6.95% increase in cooling capacity, a 7.23℃ rise in suction superheat, a 8.11% improvement in coefficient of performance (COP). The tests were conducted under the boundary conditions, and no abnormal phenomena were observed, which effectively solved droplet entrained problem at the evaporator outlet. This scheme offers a valuable engineering insight for optimizing evaporator performance and resolving critical issues like droplet entrained.

Key words: dry evaporator, inlet tube structure, gas-liquid flow, numerical simulation, degree of irregularity

摘要：

为解决超低温螺杆冷冻水机组中干式蒸发器管侧R507A制冷剂分配不均匀导致的蒸发器吸气过热度低、视镜带液及制冷量不足等问题，基于制冷循环系统工艺流程，构建并阐述了蒸发器管侧制冷剂的气-液两相时空演变流动机理。基于气-液流动机理图，通过计算流体动力学（computational fluid dynamics，CFD）评估了优化前干式蒸发器内制冷剂的空间分布，明确揭示了蒸发器入口分流弯管的离心效应及支管内部的旋流效应是导致换热管内制冷剂分配不均的关键因素。基于上述诊断结果，提出了3种包含T形入口分流管的进口管路优化方案，并进行数值模拟分析与对比。结果表明，T形结构能有效改善换热管内制冷剂的不均匀度，且方案3被确定为最优方案。在此基础上进一步研究了不同流动方向、不同入口分流支管间距及边界工况（最大负荷、最小负荷、最大压差、最小压差）等因素对方案3制冷剂分配均匀性的影响。分析发现，流动方向对方案3的不均匀度影响较小，而入口分流支管间距对制冷剂不均匀度的影响表现为两端区域显著、中间区域较小。经优化，方案3的不均匀度较优化前降低了50.01%，且边界工况下的不均匀度均远低于优化前模型。该优化方案经机组整改及实验测试验证，在名义工况下，优化后的干式蒸发器相比优化前制冷量提升约6.95%，吸气过热度提升7.23℃，制冷系数（coefficient of performance，COP）提高8.11%，并针对优化后的边界工况进行了测试，均无异常现象，并且有效解决了蒸发器出口带液问题。该方案对优化蒸发器性能、解决吸气带液等问题具有重要的工程指导意义。

关键词: 干式蒸发器, 入口管路结构, 气液两相流, 数值模拟, 不均匀度