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

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