【作者】 邵莉；

【导师】 潘继红；

【作者基本信息】 山东大学 ， 热能工程， 2009， 博士

【摘要】 螺旋管因其具有传热效率高、结构紧凑和加工制作方便等优点,在高效蒸汽发生器和冷却器、核反应堆、电站锅炉、船舶动力、石油化工、航天航空、微电子器件冷却、先进燃料电池系统冷却、食品制药、以及制冷与低温技术等领域得到了非常广泛的应用。螺旋管内的两相流与传热由于离心力和二次流的作用而比直管内的要复杂的多,其水动力学和传热特性与机理研究是当前两相流与传热领域的重要课题之一。流体在卧式螺旋管内流动方向的连续变化使得作用在流体上的重力和离心力的大小、方向和作用机制都不断发生变化,致使其两相流与传热现象更为复杂。已有卧式螺旋管内两相流与传热的研究大多是以空气—水或水—水蒸汽为工质的,而对环境友好的新制冷剂R134a在卧式螺旋管内的两相流与传热特性与机理的研究却鲜有报道。因此,本文的主要目的是在设计建设两相流与传热实验台基础上,对R134a在卧式螺旋管内流动沸腾的流型、阻力与传热特性进行系统的研究,建立流动与传热过程的流型图,得到沸腾过程中管壁温度的变化特性,发展流动阻力与传热系数计算的经验关联式,并对卧式螺旋管内的凝结传热特性进行研究。利用透明螺旋管实验段对流动沸腾过程的流型进行了可视化研究,并对各种流型的形成及其转变特性进行了机理分析。在得到了文献中常见的泡状流、弹状流或塞状流、分层流、波状分层流、块状流以及环状流外,还观察到了两种新流型,本文分别称之为“波环状流型”和“超大气弹流型”。波环状流型出现在较高质量流速、低干度条件下上升段中波状分层流向环状流之间的过渡过程:超大气弹流则出现在相同条件下下降段中分层流和环状流之间的过渡过程。由于卧式螺旋管的上升段和下降段之间流型存在较大的差别,因此,基于实验数据和Kattan流型图,分别发展了R134a卧式螺旋管内流动沸腾上升段和下降段的流型图。对卧式螺旋管内的流动沸腾阻力特性进行了研究,得到了工质的干度、质量流速和压力等参数对两相摩擦阻力压降系数φ₁₀²的影响关系。系统压力和工质干度对φ₁₀²的影响比较明显,φ₁₀²随着干度的增加而增加,随着压力的增加而减小。质量流速对φ₁₀²也有一定的影响,φ₁₀²随质量流速的增加稍有增加。通过实验数据的回归分析得到了流动沸腾摩擦阻力压降的计算关联式,最大偏差为19.7%,有95.3%的实验数据分布在拟合公式的±15%误差范围内。对卧式螺旋管内流动沸腾的壁温分布特性进行了研究与机理分析,得出了卧式螺旋管沿管长和横截面圆周方向的壁温分布规律。螺旋管壁面温度沿管长呈逐渐降低的趋势,沿横截面圆周方向,最外侧壁温最低,最内侧壁温最高。而且在环状流区域,螺旋管顶部截面的内外侧壁面之间的温差比底部截面的内外侧温差要大。对卧式螺旋管内流动沸腾的传热特性进行了系统的研究和机理分析,得出了局部传热系数沿管长的变化规律,获得了工质的干度、质量流速、压力以及热流密度和螺旋管结构等参数对局部传热系数的影响关系。沸腾传热系数沿管长总体呈逐渐升高的趋势,传热系数随着干度、热流密度和质量流速的增加而增加,低干度区质量流速对传热系数的影响较小,随着干度的增加质量流速对传热系数的影响愈加显著;在高热流密度、小质量流速的条件下,高干度区传热出现恶化,传热系数有所减小;系统压力和螺旋管曲率对传热系数的影响不大,传热系数随系统压力和曲率的增大稍有增加。通过对实验数据的回归分析,发展了本实验参数范围内R134a卧式螺旋管内流动沸腾传热系数的计算关联式,最大偏差为22.1%,有90.3%的实验数据分布在拟合公式的±15%误差范围内。对卧式螺旋管内的凝结传热特性进行了研究,获得了工质的干度、质量流速和冷凝温度对平均凝结传热系数的影响关系。工质的干度和质量流速对平均凝结传热系数的影响较大,传热系数随着干度和质量流速的增加而增加;低干度区质量流速对传热系数的影响较小,随着干度的增加,质量流速对传热系数的影响愈加显著:螺旋管与直管内凝结传热特性的比较表明,螺旋管的平均凝结传热系数比直管的要大,低干度、小质量流速时螺旋管的凝结传热效果更好。总之,本文在R134a卧式螺旋管内的两相流与传热的流型、流动阻力和沸腾传热特性等方面都取得了具有一定创新性的研究成果,为制冷空调领域已有设备的更新改造和新设备的开发设计提供了急需的基础数据和设计依据,为新型螺旋管传热器的设计开发奠定了基础。更多还原

【Abstract】 Helical pipes are widely used in a variety of practical applications,such as in steam generators and coolers,nuclear reactors,power plant boilers,ship power equipment,petrochemical industry,aviation electronic device cooling,advanced cooling system for fuel cells,food and pharmaceutical,as well as refrigeration and cryogenics,due to their high efficiency in heat transfer,compact in volume,and easiness in manufacturing.Since the centrifugal forces and the secondary flow resulted from the curved structure,the two-phase flow and heat transfer in helical pipes is much more complex than that in the straight pipes.As a result,the two-phase flow and heat transfer in helical pipes is still an unsolved important problem in the two-phase flow area. Because the magnitude,direction,and working mechanism of the gravitational and centrifugal forces continuously vary due to the continuous variation of flow directions in the horizontal helical pipes,the two-phase flow and heat transfer in the straight helical pipe is much more complex than that in the vertical helical pipes.To date, most of the research activities have been conducted on the gas-water and water-steam two-phase flows in the horizontal helical pipes,not much effort has been devoted to the two-phase flow and heat transfer characteristics and mechanism of environmentally friendly refrigeration of R134a in the horizontal helical pipes. Therefore,the primary objective of this thesis is to establish a new experimental setup for investigating the two-phase flow and heat transfer characteristics in the horizontal helical pipe.The visualization experiment is performed for developing the flow pattern figures.The two-phase flow and boiling heat transfer experiments for R134a in the horizontal helical pipe are conducted for determining the flow frictional pressure drops and boiling heat transfer characteristics,and new correlations for predicting the frictional pressure drops and boiling heat transfer characteristics for R134a are developed based on the obtained experimental data.In addition,the condensation heat transfer characteristics for R134a in the horizontal helical pipe is experimentally determined. The flow patterns for two-phase flow in the horizontal helical pipe are obtained through visualization experiment and the mechanism for the flow pattern forming and change features are analyzed.Two new flow patterns,which are tentatively named the "wave annular flow" and "super slug flow ",are observed,in addition to the well-known flow patterns,such as the bubbly flow,plug slug flow,stratified flow, wave stratified flow,dispersed flow,annular flows.Since the flow patterns are different in the rising section and the declining section,two different flow pattern figures are proposed for the rising and declining sections,respectively.The flow frictional pressure drops are experimentally determined for the two-phase flow of R134a in the horizontal helical pipe.The effect of the vapor quality, mass flowrate,and system pressure on the flow frictional factorφ₁₀² is obtained.The flow frictional factorφ₁₀² increases distinctly with increase in the quality and decreases in the system pressure,and increases lightly with increase in the mass flowrate.The new correlation is developed for predicting the flow frictional factors through the regression analysis on the test data with a maximum deviation of 19.7% under an uncertainty of±15%and the confidence level of 95.3%.The wall temperature distribution during flow boiling of R134a in the horizontal helical pipe is experimentally determined along with a mechanism analysis.The experimental result show that the wall temperature continuously decreases along the pipe length during the flow boiling of R134a in the horizontal helical pipe.The highest and lowest temperature occurred at the inner and outside of the cross sections, respectively,and the difference in temperature of the top is higher than that of the bottom cross section.The flow boiling heat transfer characteristics of R134a in the straight helical pipe is experimentally determined and the mechanism is analyzed.The effects of vapor quality,mass flowrate,pressure,and heat flux on the local boiling heat transfer coefficients are obtained.The local boiling heat transfer coefficients of R134a in the horizontal helical pipe continuously increases along the pipe length and with increases in quality,heat flux,and mass flowrate.The effect of mass flowrate on the heat transfer coefficients is more distinct in the higher quality range than in the lower quality range.The new correlation is developed for predicting the local boiling heat transfer coefficients through the regression analysis on the test data with a maximum deviation of 22.1%under an uncertainty of±15%and the confidence level of 90.3%.The condensation heat transfer characteristics for R134a flowing in the horizontal helical pipe is experimentally investigated.The effect of vapor quality, mass flowrate,and average condensing temperature on the average condensation heat transfer coefficients is determined.The vapor quality and mass flowrate have a distinct effect on the average condensation heat transfer coefficients.The coefficient increases with increase in the mass flowrate,and the effect of mass flowrate on the coefficient increase with increase in the quality.The experimental results show that the average condensation heat transfer coefficient for R134a in the horizontal helical pipe is larger than that in the straight tubes.In summary,some innovative research results have been achieved in the flow patterns,flow frictional factors,and heat transfer characteristics for flow boiling of R134a in the horizontal helical pipe.The research work can provide useful knowledge and design basis for retrofitting the present systems and developing new equipment and systems for refrigeration and air-conditioning applications.It also has potential application for the development of innovative helical pipe heat exchangers.更多还原