【作者】 王淑香；

【导师】 徐进良；

【作者基本信息】 华北电力大学 ， 可再生能源与清洁能源， 2014， 博士

【摘要】 螺旋管因其传热效率高、结构紧凑、加工方便等优点,在高效蒸汽发生器和冷却器、电站锅炉、核反应堆、石油化工、以及制冷空调等领域得到广泛应用。自然工质CO2作为CFCs和HCFCs的替代工质已成为人们研究的热点,在制冷、热泵及低温余热发电系统中,CO2跨临界循环系统具有广阔的应用前景。研究螺旋管内CO2的流动与换热问题对于新型高效换热器的设计与应用具有重要的科学意义和实用价值。在充分考虑超临界以及亚临界状态下、尤其是准临界温度附近C02特殊的热物性变化基础上,本文自行设计并搭建了高压C02流动传热实验研究平台,通过热平衡、阻力与换热校正,验证了实验系统的可靠性,可以在宽广参数范围内深入开展螺旋管内CO2流动换热与阻力特性研究。在螺旋管内超临界压力C02传热特性研究方面,采用低压大电流加热方式,研究了恒热流密度条件下螺旋管内超临界压力C02的传热特性,获取了运行压力、质量流速和热流密度对换热特性的影响规律。结果表明沿程换热系数总体呈先上升后下降的趋势且极大值发生在主流平均温度小于准临界温度而壁温大于准临界温度条件下。通过理论分析,深入揭示了螺旋管中超临界压力C02热边界层发展、工质变物性以及浮升力和离心力的耦合作用规律,并对不同实验工况下的沿程及周向壁温和传热系数分布特性给出机理解释。基于实验获取的2346个实验数据,以准临界温度为界限,分别拟合出适用于低焓值和高焓值区间螺旋管内超临界CO2换热实验关联式。对螺旋管内超临界压力CO2的阻力特性开展研究,分析改变质量流速和热流密度时,摩擦压降、加速压降、重位压降以及摩擦压降与总压降比值的变化情况。发现摩擦因子随着热流密度呈先缓慢降低后升高的趋势。深入揭示浮升力对螺旋管内超临界压力下C02阻力特性影响机理,并提出基于实验数据的摩擦压降关联式。为揭示CO2在螺旋管内流动沸腾时的换热机理,对不同压力以及不同加热功率时,沿管长和横截面周向壁温分布规律进行研究。深入分析CO2流动沸腾时的干度、质量流速、系统J压力以及热流密度对局部沸腾换热的影响。发现螺旋管内壁温周向分布不均匀,指出浮升力和离心力的相对大小是导致周向壁温分布差异的主要原因。局部平均换热系数随热流密度以及进口压力的增加而显著增加,但增大质量流速对换热系数的影响不大,表明核态沸腾是CO2在螺旋管内流动沸腾的主要传热模式而强制对流效应较弱；发现了随着热流密度增加所引起的核态沸腾强度变化以及干涸和再润湿使得换热系数随出口干度的变化可分为三种变化趋势。并发展了本实验参数范围内C02立式螺旋管内流动沸腾传热系数计算关联式。研究了C02在螺旋管内的沸腾阻力特性,结果表明,亚临界压力C02在螺旋管内的摩擦压降随热流密度的升高而增大,而在不同加热功率下随质量流速会有不同变化趋势,整体呈上升而在小功率或较高质量流速出现下降趋势。压力升高时液相和气相之间的密度比减小,进而导致两相相对速度降低、摩擦压降减小。通过对实验数据回归分析,得到全液相摩擦乘子两相阻力关系式。本研究将为高效换热器的发展以及制冷空调领域相关设备的更新换代与新设备的设计开发提供参考依据。更多还原

【Abstract】 Helically coiled tubes are widely used in a variety of practical applications such as steam generators and coolers, power plant boilers, nuclear reactors, petrochemical industry as well as refrigeration and air conditioning systems, due to their practical importance of high efficiency in heat transfer, compactness in structure and easiness in manufacturing. The investigation on natural refrigerant of carbon dioxide as an alternative refrigerant for CFCs and HCFCs was turned out to be a hot research topic, transcritical carbon dioxide cycle has a promising prospect of application in the areas of refrigeration, heat pump and low-grade waste recovery. The research on flow and heat transfer characteristics for CO2flowing through helically coiled tube has important scientific and practical significance for the design and application of new efficient heat exchangers. In full consideration of physical properties for CO2at supercritical and subcritical pressures especially near the pseudo-critical temperature, an experimental setup for CO2flow and heat transfer investigation was built up and the reliability of the setup was verified by flow resistance, heat transfer coefficients and thermal balance calibrations in this paper, experiments can be widely conducted on flow and heat transfer characteristics for CO2in the helically coiled tube in this experimental setup.Experimental investigation was conducted on the heat transfer characteristics of CO2at supercritical pressures under the uniform heat flux boundary conditions, the influences of operating pressure, mass flux and heat flux on heat transfer characteristics were investigated experimentally. It was found that the heat transfer coefficients first increased and then decreased. The maximum values of heat transfer coefficients always occurred at the temperature range of Tb<Tpc<Twi. By theoretical research, the coupling effects of thermal boundary layer developing, thermo-physical property variation, buoyancy and centrifugal forces have been disclosed, the mechanism explanation of heat transfer coefficients and temperatures distribution along the flow direction was given at different experimental conditions. Bases on the2346data obtained in the present experimental study, two correlations of Nusselts number which are suitable for different temperature regions have been proposed.Research about the flow resistance for CO2heated at supercritical pressures in the helically coiled tube was conducted. Effects of mass flux and heat flux on the variations of frictional pressure drop, acceleration pressure drop, gravitational pressure drop and the ratio of the frictional pressure drop to the total pressure drop were identified and analyzed. The friction factor was found to decrease gradually and then increase with heat flux. The mechanism of buoyancy force effect on the flow resistance is analyzed. A new correlation of friction factor is presented based on the experimental data.As for flow boiling heat transfer of CO2through the helically coiled tube, experiments were conducted on inner wall temperature distributions along the flow direction and the circumference at different pressures and different heat fluxes. The effects of vapor quality, mass flux, heat flux and operating pressure on heat transfer coefficient distribution were discussed. The experimental results show that inner wall temperature distributions along the circumference were non-uniform, and it was pointed out that the relative significance of the buoyancy force and centrifugal is the main reason for this phenomenon. The local average heat transfer coefficient increases with increasing heat flux and inlet pressure, but the increment of mass flux has no effect on the heat transfer, suggesting that the nucleate boiling is the dominant mechanism while the forced convection effect is weak. The intensity of nucleate boiling changes with increasing heat flux and the variation of heat transfer coefficient with outlet vapor quality can be divided into three different trends, which is induced by the alternative of wall dry-out and rewetting. A new correlation of local average heat transfer coefficient has been proposed based on the data points.Research on the flow resistance for CO2flow boiling at the subcritical pressure in the helically coiled tube was studied. The results show that the frictional pressure drop for flow boiling increases with increasing heat flux, it shows different trends with the mass flux at different heating power, the results show totally increasing trend but decreasing at low heat flux or high mass flux. The frictional pressure drop decreased with increasing operating pressure because of the ratio of density between liquid and gas decreasing. A new correlation of two-phase frictional pressure drop is presented based on the experimental data.This research work can provide useful knowledge and a reference for the development of high efficiency heat exchanger and improvement of the equipment for refrigeration and air-conditioning applications.更多还原