【作者】 朱国梅；

【导师】 刘何清； 郝小礼；

【作者基本信息】 湖南科技大学 ， 供热、供燃气、通风及空调工程， 2008， 硕士

【摘要】 氟利昂类制冷剂具有良好的热力性能、输运性能和安全性能,以及与制冷设备中的润滑油和材质具有良好的兼容性,使其在制冷和空调领域得到非常广泛的应用。但是氟利昂的泄露导致臭氧层空洞和温室效应。鉴于此,专家们提倡使用自然工质作为制冷剂,重新启用了二氧化碳作为制冷剂。1992,前制冷协会主席Lorentzen分析了二氧化碳作为制冷剂采用跨临界循环方式的可行性。目前,这种循环方式已成为国内外研究的重点和热点,且采用二氧化碳跨临界循环方式已经在汽车空调、热泵领域得到了应用。但是对于CO₂在空调系统中和热泵过程中的换热机理尚未清楚。尤其是二氧化碳达到超临界状念时,对流换热机理非常复杂,即使在相同的工况之间,一个微小变化也会导致二氧化碳流体对流换热的差异。本文以超临界二氧化碳为研究对象,对其在水平单管和竖直单管中向下流动（顺流）和向上流动（逆流）做了数值模拟研究。主要研究内容有以下几个方面:（1）分析影响超临界二氧化碳换热的因素有质量流、流动方向、管径、重力等等,但归根到底是由于流体物性参数变化引起的,因此采用了Refprop7.0计算了二氧化碳在相同压力7.5MPa下,不同温度下CO₂流体的比热、黏度、导热系数和密度。（2）把计算参数值拟合回归成曲线函数导入CFD-Fluent模拟,采用定壁温冷却的对流换热情形,在整个模拟过程中主要考虑了重力、流体入口温度、不同单管管径和不同质量流入口4种影响因素对超临界二氧化碳在单管中流动换热的影响。（3）计算出超临界CO₂流体对流换热的平均努谢尔数和不同管长与管径比处局部努谢尔数,比较努谢尔数的大小,得出在相同条件下的最佳流体入口温度、最佳单管管径和最佳质量流以及得出超临界CO₂流体在冷却对流换热的强烈程度与流体特征温度的关系;同时计算局部努谢尔数与三种关联式计算值相比得,陶宇兵推荐的关联式与文中的计算值误差较小。（4）水平单管换热比竖直单管换热剧烈;竖直单管顺流换热比逆流换热剧烈。更多还原

【Abstract】 As a natural and environmentally favorable fluid, the refrigerant carbon dioxide has been revived due to its good thermal properties recently. In recent years, lots of theoretical analysis and experimental studies have been proved that the transcritical carbon dioxide cycle has a widely future and the automobile air conditioning refrigeration and heat pump systems used CO₂ as working fluid at home and abroad, but also have some difficulties to conquer. Therefore the dissertation principally presents the heat convection of supercritical carbon dioxide flowing in horizontal and vertical single tube with downward and upward flowing cooled by wall. The major researches are summarized as follows:（1） In the dissertation, SC-CO₂ thermal properties and the related charts are systematically described firstly. The main effects SC-CO₂ heat convection are flow direction, mass flow, diameter and gravity force and so on, ultimately its physical parameters affected it, such as density, viscosity, thermal conduction and specific heat. Therefore Refprop7.0 used in the calculation of the carbon dioxide’s parameters under the same pressure of 7.5 MPa.（2） The parameter values are curve fitting using Matlab, and then compiled function to UDF, added into CFD-Fluent. In the whole simulated process, main consideration is four kinds of influence factors on the supercritical carbon dioxide in the single-tube heat convection, including gravity, inlet fluid temperature, different single-tube diameters and different quality affected it, and using fixed-wall temperature （T=300k） as cooling condition.（3） Calculated mean Nu_m and local Nu_b, which at the same ratio of the length and diameter, of supercritical CO₂ flowing in single tube. Under the same conditions we can know the optimum inlet fluid temperature, the best single-tube’s diameter and the prime mass flowing from comparing with Nu_m. We can also know the relation between heat convection intensity and distinction temperature. At the same time calculated Nu_b values compared to three correlations, it presents the correlation which is recommended by Tao-yubing is anastomosed best.（4） The intense of heat convection in horizontal single-pipe is best among vertical single-tube, and intense of heat convection in downward flow condition is better than upward flow condition.更多还原