【作者】 李帅；

【导师】 周杰； 龚思福；

【作者基本信息】 重庆大学 ， 工程（建筑与土木工程）（专业学位）， 2019， 硕士

【摘要】 强化换热管在石油、化工、冶金、动力等领域的应用极为广泛,换热性能的每一点提高都意味着较大的经济与社会效益。三维内肋换热管是目前使用比较广泛的一种高效强化换热管,三维内肋圆管由于肋片的扰动作用使得管内换热效果较光管相比有明显提升,椭圆管较圆管相比具有管外的迎风面积小、结构紧凑、尾部形成的旋涡小、管内换热效果有一定提升等诸多优点。本文考虑将三维内肋圆管与椭圆管的优点相结合,提出了三维内肋椭圆管,并以拟合出三维内肋椭圆管管内努塞尔数和摩擦系数的准则关系式、分析五个因素(长短轴比、肋高、肋宽、肋间距、肋密度)分别对换热与压降特性产生的影响以及得到管内强化换热机理为目标,对三维内肋换热管管内换热与压降特性进行实验和数值模拟研究,主要内容和结论如下:通过实验对比分析三维内肋椭圆管和三维内肋圆管管内的换热与压降特性,结果表明:在相同肋参数条件下,三维内肋椭圆管的平均热力性能系数为1.428,三维内肋圆管的平均热力性能系数为1.409,均大于光管圆管的热力性能系数,说明三维内肋椭圆管管内换热效果要好于三维内肋圆管。由于三维内肋椭圆管采取肋叉排的形式,不同肋参数对换热影响的差异较大,本文借助正交试验选出三维内肋椭圆管的最佳几何参数,结果表明:当肋参数范围为1.0≤a/b≤2.0,0.05385≤h/d≤:0.09217,0.01923≤b/d≤0.03687,0.06731≤Pt/d≤0.1152,32≤z≤56,雷诺数范围为15000<Re<40000,Pr=0.7时,五个因素对热力性能系数影响的主次顺序为:肋高H>肋密度Z>肋宽B>长短轴比a/b>肋间距Pt,最优因素参数组合方案为:长短轴比为2、肋高4mm、肋宽1.4mm、肋间距3.5mm、肋密度56。肋高和肋密度对换热管性能影响的敏感性较大。通过数值计算分析几何结构单一因素变化对换热效果产生的影响并拟合出三维内肋椭圆管管内努塞尔数和摩擦系数的准则关系式,为实际工程提供一定的计算依据。三维内肋换热管是通过增强流体的湍动和扩大管内换热面积来达到强化换热的目的,通过分析管内温度场和速度场,阐述了三维内肋椭圆管管内的强化换热机理:三维内肋管管内的肋片和椭圆形管道引起的二次流改变了管内速度场和温度场的分布,同时也改变了管内局部对流换热系数的分布,减小了管内速度场和热流梯度场之间的夹角,达到强化换热的效果。借助场协同理论分析了沿程变化的cos θ、dT/dy和dT/dy· cos θ对换热产生的影响,发现等速线上dT/dy· cos θ的分布与局部对流换热系数的分布基本一致,说明换热效果受到传热温差、温度边界层厚度以及速度矢量与温度梯度夹角等三者的协同作用所控制。本文试探性提出换热管管内换热面积扩展比的计算公式,并简要分析了面积扩展比与热力性能系数之间的关系。更多还原

【Abstract】 Enhanced heat exchanger tubes are widely used in petroleum,chemical,metallurgical,power and other fields.Every improvement of heat transfer performance means greater economic and social benefits.Three-dimensional inner-fin heat exchanger tube is a kind of high-efficiency heat transfer enhancement tube which is widely used at present.Because of the disturbance of fins,the heat transfer effect of three-dimensional inner-fin circular tube is much better than that of smooth tube Compared with circular tube,the elliptical tube has many advantages,such as smaller windward area outside the tube,compact structure,smaller vortices formed at the tail,and certain heat transfer effect in the tubeConsidering the advantages of three-dimensional inner-ribbed circular tube and elliptical tube,a three-dimensional inner-ribbed elliptical tube is proposed in this paper.By fitting the relationship between Nusselt number and friction coefficient in three-dimensional inner-ribbed elliptical tube,the influence of five factors(length-to-length axis ratio,rib height,rib width,rib spacing and rib density)on heat transfer and pressure drop characteristics is analyzed,and the mechanism of heat transfer enhancement in tube is obtained.For the purpose,the heat transfer and pressure drop characteristics of three-dimensional inner fin heat exchanger tubes were experimentally and numerically simulated.The main contents and conclusions are as follows:The heat transfer and pressure drop characteristics of three-dimensional inner-ribbed elliptical tube and three-dimensional inner-ribbed circular tube are compared and analyzed by experiments.Under the same rib parameters,the average thermal performance coefficient of three-dimensional inner-ribbed elliptical tube is 1.428,and that of three-dimensional inner-ribbed circular tube is 1.409,which are higher than that of smooth tube.It shows that the heat transfer effect of three-dimensional inner-ribbed elliptical tube is better than that of three-dimensional inner-ribbed circular tube.Because the three-dimensional elliptical tubes with inner ribs adopt the form of cross-row,the influence of different rib parameters on heat transfer is quite different.In this paper,the optimal geometric parameters of three-dimensional elliptical tubes with inner ribs are selected by means of orthogonal experiment.The results show that:when the rib parameter range for 1.0≤a/b≤ 2.0,0.05385 ≤h/d ≤0.09217,0.01923 ≤ b/d ≤0.03687,0.06731 ≤ Pt/d ≤0.1152,Pr=0.7,32 ≤ z ≤56,15000<Re<40000,the order of five factors affecting the thermal performance coefficient is:rib height H>rib density Z>rib width B>rib width B>long-short axis ratio a/b>rib spacing Pt,and the optimal combination scheme of The ratio of long axis to short axis is 2,the height of ribs is 4 mm,the width of ribs is 1.4 mm,the spacing of ribs is 3.5 mm,and the density of ribs is 56.The rib height and density are sensitive to the performance of heat exchanger tubes.The influence of single factor change of geometrical structure on heat transfer effect is analyzed by numerical calculation,and the relationship between Nusselt number and friction coefficient in three-dimensional elliptical tube with inner ribs is fitted,which provides a reference for practical engineering.The purpose of three-dimensional inner-fin heat exchanger tube is to enhance heat transfer by enhancing turbulence of fluid and enlarging heat transfer area in tube.By analyzing temperature field and velocity field in tube,the mechanism of enhanced heat transfer in three-dimensional inner-fin elliptical tube is expounded.The secondary flow caused by fins and elliptical tubes in three-dimensional inner-fin tube changes the distribution of velocity field and temperature field in tube,and also changes the distribution of temperature field in tube.The distribution of local convective heat transfer coefficient reduces the angle between the velocity field and the heat flow gradient field in the tube,and achieves the effect of heat transfer enhancement.The effects of cos 0,dT/dy and dT/dy ·cos θ on heat transfer are analyzed by means of field synergy theory.It is found that the distribution of dT/dy· cos θ on the isovelocity line is basically consistent with the distribution of local convective heat transfer coefficient.It shows that the heat transfer effect is controlled by the synergistic effect of heat transfer temperature difference,temperature boundary layer thickness and the angle between velocity vector and temperature gradient.This paper tentatively puts forward a formula for calculating the area expansion ratio of heat exchanger tubes,and briefly analyses the relationship between the area expansion ratio and the thermal performance coefficient.更多还原